tese nswparte 2

CHAPTER 1 – Introduction

The basic oxygen converter is the main process routes for making steel [1], which

requires a charge of molten iron. The blast furnace is the most important supplier of

molten iron for the steelmaking industry as it is the most efficient ironmaking process in

terms of high productivity, operational reliability and cost competitiveness [2].

However, the blast furnace is a major source of greenhouse gas emissions and also a

large energy consumer. In order to make the blast furnace more sustainable new

technologies [1,3] have been or are being implemented to improve furnace efficiency,

reduce energy consumption and lower the greenhouse gas emissions.

Coke is the most expensive raw material in the blast furnace and its production results

in high levels of greenhouse gas emissions [4]. One way to reduce coke consumption is

to inject supplementary fuels (coal, natural gas, oil) at the tuyere levels of the blast

furnace [5]. These supplementary fuels partially replace coke as a supplier of heat and

reductant gases. However, coke cannot be replaced entirely because it is still needed to

provide both mechanical support to the charge column and act as a permeable bed for

molten iron, slag and gases. Fuel injection implies an increase of the residence time of

coke in the furnace; the mechanical and chemical conditions that coke is subjected to

are much more severe and coke degradation increases[6,7]. Therefore coke quality must

change in order to address the requirements of blast furnace operation with low coke

rate [8].

The Low Temperature Compact Blast Furnace [3] and the Nitrogen Free Blast Furnace

under the European ULCOS (Ultra Low CO2 Emission Steelmaking) program [1,9] are

two new emerging technologies for blast furnace operation. The Low Temperature

Compact Blast Furnace aim is halving the energy consumption and increase blast

1

Chapter 1 – Introduction

furnace efficiency. The goal of the Nitrogen Free Blast Furnace is to drastically lower

greenhouse gas emissions.

The concept of the Low Temperature Compact Blast Furnace is based on lowering the

temperature of the thermal reserve zone. Yagi et al. [3] and Naito et al. [10] have shown

that lowering the temperature of iron oxide reduction from 1000ºC to 800-900ºC, using

a highly reactive coke, can improve reduction efficiency and effectively lower the

energy consumption. However, coke gasification at higher rates can lead to a reduction

in coke strength [3], lessening its ability to provide support for the ferrous burden and

permeability for gas and liquids.

The design of the N2 Free Blast Furnace is to operate nitrogen-free. The oxidizing gas is

a mixture of oxygen with carbon dioxide and carbon monoxide recycled from the

(filtered) furnace top gas. One advantage of this ‘oxy-firing’ is that oxygen levels can be

increased to approximately 35% without increasing flame temperatures excessively

because carbon dioxide has a higher heat capacity than nitrogen leading to reduced gas

flow in the furnace. However, because of the high carbon oxide levels in the gas stream,

the reaction conditions and thermal profile in the nitrogen-free furnace are expected to

be very different to those in conventional systems.

The fuel injection blast furnace and both Low Temperature Compact Blast Furnace and

the N2 Free Blast Furnace require cokes of specific quality for efficient operation of the

furnace. Although the operation of the blast furnace is defined by specific parameters

and quality of the raw materials for each technology, a high strength coke is a common

requirement for all these technologies. Strength is the most important quality of coke

because coke provides mechanical support to the charge column, influences gas

distribution in the shaft of the furnace and act as a permeable bed for liquids (molten

iron and slag) and gases.

A poor quality coke significantly impacts the blast furnace operation (Figure 1.1).

Cheng [11] and Gudeneau et al. [12] presented in their reviews the implications on the

blast furnace operation by using a poor quality coke:

2


� reduces permeability in the furnace due to reduction in the mean size of the coke

lumps and increases the size range of the coke lumps;

� changes the top temperature distribution by increasing the wall temperature which

increases the thermal load on the stack refractory;

� severely degrades the bosh and hearth regions triggering flooding;

� long casting times and cut tuyeres;

� produces thermal imbalance and instability in the furnace;

� increases the flue dust levels.

Coke is subjected to degradation as it descends in the furnace due to the action of

factors such as: coke gasification reactions, mechanical load, attrition, thermal stress,

alkali attack, and graphitization. Gasification is one of the most important factors in

coke degradation. At low temperatures (less than 1000ºC) gasification produces deeper

weakening of the coke lumps [5,13], which increases the formation of fines [14] and

produces fissures in the lumps making their fragmentation easier [7,15]. As temperature

increases gasification of the coke lumps occurs mostly at the surface, which increases

coke abradability, resulting in a reduction in size of the coke lumps [7,15,16].

3


Figure 1.1 The influence of coke quality in the blast furnace [11].

4


5

The coke gasification rate in the blast furnace is controlled by its intrinsic properties and

the operating conditions of the furnace [13,15,17,18,19], in terms of gas composition,

temperature and catalysts. A significant insight into coke gasification has been provided

by previous research. Coke properties such as coke microtexture, porosity and mineral

matter have been reported as indicators of coke reactivity [13,17,18,20,21,22,23,24,25].

Although there is some agreement in the literature in the way coke properties such as

coke microtexture and porosity affects gasification rate, there are still inconsistencies

and less investigated aspects in this area, such as the effect of the inherent catalytic

minerals in coke on reactivity. In order to prepare cokes of suitable quality for each new

technology of operation of blast furnace a very good understanding of the factors that

affect coke reactivity is required. As such, investigation of the less explored coke

properties and the assessment of their influence on coke reactivity are required so that

the most dominant factors affecting coke reactivity can be determined.

CHAPTER 2 – Literature Review

2.1 Blast furnace

2.1.1 Overview of the Blast Furnace process – coke importance in the furnace

The blast furnace is basically a counter-current reactor; raw materials are charged into

the furnace top, molten products are tapped from the bottom and gases pass from

bottom to the top of the furnace. The raw materials consist of ferrous materials (iron ore

as sinter or pellets), fuel (coke) and fluxes (limestone, dolomite). Other materials, like

coal, oil and natural gas may be co-injected with air through the tuyeres at the base of

the furnace. The reducing gases and heat required for the process are generated at the

bottom of the furnace by combustion of the fuels. These pass upward, counter-current to

the raw materials, exiting at the furnace top after imparting heat and enabling the

required chemical reactions on the raw materials. The main product of the blast furnace

is molten pig iron and the by-products are molten slag and gas. Iron is produced via

direct and indirect reduction of iron oxide. Coke is the main source of carbon, for the

direct reduction, and carbon monoxide, for the indirect reduction.

The blast furnace can be divided into six zones, from the relatively cool zone at the top,

to the intensely hot zone at the bottom: Granular zone (Solid zone), Cohesive zone

(Softening-Melting zone), Active Coke zone, Raceway, Hearth and Deadman (Stagnant

Coke zone) (Figure 2.1).

In the Granular zone the raw materials, layers of coke and iron ore, must ensure a

uniform distribution of the ascending gases. The temperature in this zone is up to

1000�C.

6

Chapter 2 –Literature Review

The Cohesive zone consists of alternate layers of coke and viscous, semi-fused mass of

slag and iron, through which the ascending gases are unable to flow. The permeable

coke layers, called coke slits, act as gas distributors and permit the gas to flow

horizontally through them. Since most of the gas has to pass through the coke slits, the

slits must be permeable and hence great importance is attached to the resistance of coke

to breakage.

The Active Coke zone contains loosely packed coke, feeding the raceway, and droplets

of iron and slag dripping into deadman and hearth. The temperature in this zone is about

1600�C.

The Raceway is the volume around the hearth periphery where coke is burned. The heat

produced by coke burning gives a flame temperature of 1800-2000�C.

The loose packed central coke column is called the Deadman zone. The solid coke

maintains an open bed through which the descending liquid iron and slag and ascending

reducing gases can pass. In this zone, for coke to ensure permeability, the coke lumps

must be of a specific mean size and size range and also have enough strength to resist

degradation [11].

The molten iron and slag are cast at regular intervals from the Hearth, where the

temperature is around 1500�C.

Based upon the above information the role of coke in the blast furnace can be grouped

into three categories: thermal, chemical and physical as described below:

The thermal role of coke is to provide heat for endothermic reactions and melting of

iron and slag.

The chemical role is to produce carbon monoxide for the reduction of the iron oxides

and to provide carbon for both direct reduction and iron carburisation.

The physical role of coke is: to provide a mechanical support to the charge column,

influence gas distribution in the shaft of the blast furnace and act as a permeable bed,

allowing metal and slag to pass down and the gases to pass up.

7


Figure 2.1 Schematic of the Blast Furnace.

8


2.1.2 Coke gasification in the blast furnace

Hot air is blown through the tuyeres into the blast furnace it reacts immediately with

coke to form carbon dioxide.

C + O2 CO2 Reaction 2.1

The reaction is strongly exothermic and supplies much of the energy necessary to

reduce iron oxides to metallic iron. Because carbon dioxide is unstable in the presence

of coke at temperatures over 1000�C it is converted to carbon monoxide [5,6] according

to the reaction below:

CO2 + C 2CO Reaction 2.2

This reaction is known as the Boudouard reaction, gasification reaction or solution loss

reaction. The Boudouard reaction provides the reducing agent for the iron oxides. The

tuyere gases, which consist mainly of carbon monoxide, nitrogen and small amounts of

steam [5,6], leave the combustion zone and pass up through the burden to the top of the

furnace.

Above the cohesive zone wustite (FeO) is reduced to iron by carbon (direct reduction) at

temperatures greater than 1000ºC (Reaction 2.3) and carbon monoxide (indirect

reduction) at temperatures lower than 1000ºC (Reaction 2.4) [5]. Carbon dioxide

generated by the indirect reduction gasifies the lumps of coke.

FeO + C Fe + CO Reaction 2.3

FeO + CO Fe + CO2 Reaction 2.4

Due to the endothermic nature of the direct reduction reaction and coke gasification

reactions the temperature of the ascending gases decreases. As the gases ascend in the

furnace, the concentration of carbon monoxide decreases because of indirect reduction

of wustite, which produces carbon dioxide. Another source of carbon dioxide is

9


reduction of magnetite (Fe3O4) and hematite (Fe2O3) by carbon monoxide in the Solid

zone of the blast furnace (Reactions 2.5 and 2.6, respectively).

Fe3O4 + CO 3FeO + CO2 Reaction 2.5

3Fe2O3 + CO 2Fe3O4 + CO2 Reaction 2.6

About 70% of the iron oxides are reduced by indirect reactions with carbon monoxide

and hydrogen (in a small contribution) before the cohesive zone (1250-1300�C);

whereas the remaining 30% of FeO is reduced via the direct reduction with coke within

the cohesive zone [14]. The gases that exit from the top of the conventional furnace are

mostly formed of 20-30% CO, 10-20% CO2 and the rest is nitrogen [5]. Biswas [5]

divided the blast furnace along its height into three main zones based upon the

temperature and type of reaction that occurs within these zones, namely Preheating

Zone, Thermal Reserve Zone and Direct Reduction and Melting Zone (Figure 2.2).

The total amount of solution loss in the blast furnace is mainly determined by the

carbon dioxide concentration which itself is controlled by the availability of oxygen

from iron oxides [15]. Goleczka and Tucker [26] and Barnaba [14] assumed that coke

gasification in the blast furnace due to solution loss is about 20-30% and 25%,

respectively.

Hatano et al. [16] created a mathematical model to find the temperature range at which

the solution loss reaction occurs in the blast furnace. They believe that the temperature

where gasification starts is affected by coke size, coke reactivity and the alkali content

in the coke ash. They consider the beginning temperature of the zone of solution loss

reaction is the temperature at which the reaction rate is 410-5 gg-1min-1. This

temperature increases with increasing coke size, decreasing coke reactivity and

decreasing content of alkali in the coke ash. The maximum temperature in the zone of

solution loss reaction is 1400ºC. Van der Velden et al. [18] and Hutny et al. [6]

assumed that the reaction of coke with carbon dioxide in conditions similar to those in

blast furnace begins at temperatures around 900�C and Gill et al. [25] believe that the

solution loss reaction becomes significant at a temperature range about 950-1100�C.

10


Figure 2.2 A theoretical diagram of temperature distribution of gas and solids along the height of the blast furnace and the chemical reactions within these zones [5].

11


The gasification rate increases with temperature from 1000-1200�C whether in pure

carbon dioxide or gas mixtures [7,18,26]. In pure carbon dioxide the rate of reaction

increases when the temperature is increased to 1300�C [16]. However, Van der Velden

et al. [18] observed that the gasification rate in gas mixtures similar to those present in

the blast furnace reached a maximum at 1200�C after wich the reaction rate decreased.

They concluded that the decrease of carbon dioxide concentration in the gas as the

temperature increases has a much stronger effect on the reaction rate than increasing

temperature in the furnace. Negro et al. [15] also considered that the total amount of

solution loss in the blast furnace is largely determined by the carbon dioxide

concentration which itself is controlled by the availability of oxygen from iron oxides.

2.2 Coke characterisation

Metallurgical coke can be described as a porous solid material comprising an organic

part, which is mainly carbon and small amounts of sulfur, nitrogen, hydrogen and

oxygen, and an inorganic part (approximately 10%). The organic part of the coke is

characterised by two properties namely microtexture and microstructure, which were

defined by Coin [27]:

Microstructure refers to the physical and spatial relation of the coke material,

that is, porosity, pore sizes, pore wall thickness etc., whereas Microtexture

refers to the nature of the carbon in the coke, its crystallite development, degree

of optical anisotropy etc. [classified by optical microscopy]

The characteristics of the organic and inorganic components of the coke will be

discussed in the following sections.

12


2.2.1 Coke microtexture

Microtexture is a description of the organic matter given by optical microscopy. Under

optical microscope examination, the coke microtexture is classified as anisotropic or

isotropic carbon. When isotropic carbon is exposed to polarised light it does not produce

variation in the wavelength of the reflected polarised light; it originates from coal

macerals which did not fuse during carbonisation process (fusinite and macrinite) and

fused vitrinite from low rank coals [27]. The anisotropic carbon causes variation in the

wavelength of the reflected polarized light and characterises the macerals fused during

carbonisation.

The anisotropic microtexture is itself classified by its size, shape and form of the

textural unit. Both the nomenclature and classification of the anisotropic carbon vary

between different laboratories (Figure 2.3). Since there is a large variation of the

definitions of anisotropy between different groups of researchers, it is important to be

aware of these differences when the results are compared.

The rank of the parent coal affects the microtexture of the product coke. Low rank coals

make cokes with predominantly isotropic microtexture. As the rank of the coal increases

different types of anisotropic microtexture such as fine, medium and coarse mosaic

units and flow types, gradually replace the isotropic microtexture in the coke [28,29].

Cokes made from high rank coals do not usually contain significant amounts of both

isotropic and fine mosaic microtextures [21,28]. The microtexture of metallurgical coke

is characterised by a high proportion of mosaic microtexture [30,31]. These

microtextural variations are illustrated in Figure 2.4.

Although optical microscopy is a widely used technique to characterize cokes it

nevertheless has limitations because of its low resolution (approximately 0.3μm [32]).

Transmission Electron Microscopy (TEM) is another imaging technique used for coke

microtexture characterisation. TEM examination supplements optical microscopy by

examination on a size scale not reachable by optical techniques (down to 0.8 nm) [33].

13


Figure 2.3 Nomenclature and classification of the anisotropic carbon by different laboratories [27].

14


(a) (b) (c)

Figure 2.4 Coke microtextures: a) Isotropic (I) and Very Fine Mosaic (VF); b) Very Fine Mosaic (VF) and Fine Mosaic (F) and c) Medium Mosaic (M) and Flow-like anisotropic microtexture (FA) [21].

A description of coal and coke microtexture using TEM was given by Oberlin and

Rouzaud in several studies [34,35,36]. They concluded that coke microtexture consists

of polyaromatic basic structural units (BSU) with size about 1 nm, formed by

polyaromatic layers (4 to 10 rings) isolated or stacked by 2 or 3. The BSUs are ordered

in stacked planes of the aromatic layers, named molecular orientation domains (MOD)

or local molecular orientation (LMO) [28,34]. A graphic representation of coke

microtexture is shown in Figure 2.5. Inside the molecular orientation domains (MODs)

the polyaromatic basic structural units (BSUs) are either misoriented or locally

orientated in parallel [37].

Coke microtexture is characterised by MODs of different sizes, varying from 5 nm to a

few micrometers [28]. The isotropic microtexture observed using optical microscopy is

discriminated by TEM into eight categories as a function of MOD size (Figure 2.6). The

optically anisotropic microtexture is also differentiated by TEM into crumpled lamellae

and planar lamellae. The quantification of coke microtexture is represented by a

frequency histogram of MODs categories.

15


Figure 2.5 Schematic presentation of MOD [38].

Figure 2.6 Classification of Molecular Orientation Domains [36].

16


X-ray diffraction is another technique that characterises the atomic level structure of the

carbonaceous materials. The X-ray diffraction (XRD) pattern of a carbonaceous

material such as coke shows diffuse peaks that correspond to (002), (100) and (110)

reflections of graphite and strong low-angle scattering [39]. The diffuse peaks of (002),

(100) and (110) reflections indicate the presence of small graphite-like domains [39].

The non-crystalline carbon (amorphous carbon) forms the background intensity of X-

ray diffraction pattern.

The crystalline structure of graphite consists of flat polycondensed aromatic layers,

which are known as lamellae, ordered in parallel to form a crystallite [36] (Figure 2.7).

The graphite crystallite is usually layered in the form ABABAB. The X-ray diffraction

(XRD) pattern of graphite is dominated by the position of the (002), (100) and (110)

reflections. The (002) reflection indicates the stacking of the aromatic layers and both

the (100) and (110) reflections correspond to the two-dimensional lattices of the

aromatic layers. The crystallographic parameters which can be determined using the

XRD pattern are both the height (Lc) and length (La) of the crystallite and the interlayer

spacing (d) (Figure 2.7). The crystallite height and the distance between lamellae in the

coke depend on the rank of the parent coal; as the coal rank increases the crystallite

height increases and the distance between lamellae decreases [36].

The mean crystallite size can be determined using the Scherrer equation [40]:

Bcos�K�Lc/a � Equation 2.1

Where K is a constant depending on the reflection plane (0.89 for (002) band and 1.84

for (110) band [41]), � is the wavelength of the incident radiation, B is the width of the

peak at half-maximum intensity and � is the peak position.

17


Figure 2.7 A schematic picture of a crystallite of graphite [42].

2.2.2 Coke microstructure

Cokes have pores with a wide range of sizes, from less than 1 nanometre to several

hundred microns [43]. A classification of the pore structure is given by Dubinin [44]:

micropores (the effective radii from 0.5-0.6 nm to 1.3-1.4 nm), mesopores (the effective

radii between 1.5-1.6 nm and 100-200 nm) and macropores (the effective radii over

100-200 nm). Total porosity includes the empty spaces left between the different carbon

microtextures (each MOD forms a pore wall – Figure 2.5) [37], large pores

(macropores) formed due to release of volatile matter during carbonisation and the

fissures produced by internal stress in the coke [23]. SEM images of cokes show that

pores greater than 0.1 - 5 microns are of different shapes such as circular, elliptical,

rectangular, triangular and slit-like pore sections [43]. Also blind pores have been

observed.

Coke microstructure is developed during the carbonisation. Its formation is affected by

the rank of the parent coal, fluidity, the amount of reactive macerals and coking rate

[45]. For instance, low rank coals (high-volatile coals) with good fluidity make cokes

18


with high porosity (55-60 vol%) and mean pores size larger than 200 μm, whereas

medium rank coals produce cokes less porous (<54%) and smaller pore size (about 125

μm) [46].

2.2.3 Mineral matter

The mineral matter content in coke is typically around 8-12% [47]. The mineralogical

composition of coke is different to that of the parent coal. During carbonisation some

minerals decompose and also reactions between minerals occur.

Minerals identified in metallurgical cokes by previous studies are quartz (SiO2), iron

oxides, mullite (Al6Si2O13), fluorapatite (Ca5(PO4)3F), pyrrhotite (Fe1-xS), brookite

(TiO2), anatase (TiO2), cristobalite (SiO2), alkali feldspars ((K,Na)(AlSi3O8)) and

aluminosilicates [23,48,49]. A recent study made on eleven metallurgical cokes from

different international sources identified the following additional minerals: akermanite

(Ca2MgSi2O7), anorthite ((Ca,Na)(Si,Al)4O8), calcium iron oxide (CaFe2O4), diopside

(CaMgSi2O6), fayalite ((Fe,Mg)2SiO4), gehlenite (Ca2Al2SiO7), metallic iron, oldhamite

(CaS) and rutile (TiO2) [50].

The aluminosilicates present in cokes are formed due to decomposition of clays such as

kaolinite, illite, montmorillonite and chlorite, during carbonisation [48,51]. Quartz,

fluorapatite, anatase and brookite are minerals that originate from the parent coals and

are relatively unaffected by the coking process. Alkali feldspars have been identified in

cokes by Mahoney et al. [48] and also in the bituminous coals [23] but they are affected

to some extent by the carbonisation process.

Some minerals have been identified to have an impact on the size of the anisotropic

microtexture formed in cokes. Gray and Champagne [52] and Gill et al. [25] observed in

some cokes that the anisotropic carbon around some clays and pyrite is of smaller size

than that in similar areas that lack minerals.

19


2.3 Coke gasification

2.3.1 Gas-solid reactions

Gasification of a porous particle involves several steps: transport of the reactant gas to

the particle surface, diffusion of the reactant inside the particle through the pores to the

reaction sites, reaction between gas and solid and elimination of the products [53,54,55].

The way that coke is gasified can be divided into three different ‘regimes’ depending on

the step that limits the reaction rate, namely, chemical kinetics (regime I), pore diffusion

(regime II) and gas phase mass transfer (regime III).

Regime I – Chemical kinetics

At low temperature the rate of chemical reaction is lower than the rate of diffusion of

the reactant gases on the external surface and through the pore structure of the particle.

The gasification rate is determined by the rate of chemical reaction on the coke surface

or the intrinsic reactivity. Intrinsic reactivity is the reaction rate per unit area of pore

surface without any mass transfer restriction. In regime I, the bulk density of the coke

decreases but the size of the particle remains constant during gasification. The

activation energy measured is the true activation energy [56].

The activation energy for the reaction of metallurgical coke with carbon dioxide under

chemically controlled conditions, measured in previous studies, were typically in the

range of 215-240 kJ/mol [56,57,58,59].

Regime II – Pore diffusion

As the particle temperature is increased gasification becomes limited by diffusion of the

reactant or product through the coke pore structure. The diffusion rate of gases is

influenced by bulk flow diffusion and pore size distribution [60]. In other words, the

collisions between gas molecules in the pores (molecular diffusion) or collisions of gas

molecules with the pore walls (Knudsen diffusion) affect the diffusion rate. The

activation energy decreases to half the true activation energy [56].

20


Regime III – Gas phase mass transfer

In this regime the physical process of mass transfer to the external surface of the particle

controls the reaction rate. The rate of conversion depends on particle size, gas

composition and a little on temperature. The slight dependence with temperature is a

result of low activation energy in this regime, which is close to zero. As the reaction

proceeds the coke particle becomes smaller and smaller but the density of the particle

remains constant.

An Arrhenius plot of the reaction rate as a function of temperature is used to indicate the

transition from a chemically controlled reaction to a gas phase mass transfer controlled

reaction (Figure 2.8). The zones I, II and III indicate the chemically controlled, pore

diffusion controlled and mass transfer controlled reactions. Two intermediary zones ‘a’

and ‘b’ shows the transition between ideal regimes, where both regimes I and II and

both regimes II and III coexist, respectively.

The transition temperature between regimes depends on the reaction parameters such as

gas flow rate, gas type, pressure and particle size, and some intrinsic coke properties

such as coke porosity (size and type of the pores) and the concentration of active sites of

coke particle [24,43,56,61,62]. For instance Regime I is favoured by low temperature,

low pressure, small particle size, low concentration of active sites, high gas flow rate

and high porosity of coke particle. Harris and Smith [59] concluded that at 800ºC

gasification with CO2 of coke with particle size between 0.2 and 2.0 mm and the

reactant gas parameters such as flow-rate and partial pressure ranging 500-1000 ml·min-

1 and 0.1-1 atm, respectively, occurs under Regime I conditions; the activation energy

measured for metallurgical coke was 216 kJ mol-1.

21


Figure 2.8 Ideal representation of the three controlling zones of carbon gasification, where Ea is the apparent activation energy and Et is the true activation energy [53].

2.3.2 Mechanism of reaction

The mechanism of the gasification reaction is based on the ability of carbon to remove

an oxygen atom from a carbon dioxide molecule and retain it on certain sites by

chemical bonding [61]. The oxygen is then released as carbon monoxide and a new

atom of carbon is exposed to carbon dioxide. It is generally agreed that coke

gasification reaction follow the oxygen-exchange mechanism [24,42,56,61]:

Cf + CO2 CO + C(O) Reaction 2.7

C(O) CO + Cf Reaction 2.8

22


where Cf is a free carbon active site, C(O) is the chemisorbed oxygen and i1, j1 and j3 are

the rates constants for forward and reverse reactions. The reversible reaction (Reaction

2.7) is a fast reaction [24]. The desorption step (Reaction 2.8) is slow and its rate

controls the overall of gasification reaction [24,63,64].

The product of reaction, carbon monoxide, has been identified by previous studies as an

inhibitor of gasification [61,65]. The retardation of the gasification rate occurs due to

chemisorption of carbon monoxide onto the active carbon sites.

The reaction rate (R) of the two stages process follows a Langmuir-Hinshelwood type

equation [24,56,61]:

� 2

2

32

1

1 COCO

CO

PkPkPk

R��

� Equation 2.2

k1 = i1�c, 3

12 j

jk � , 3

13 j

ik �

where k1, k2 and k3 are the rate constants, P refers to the partial pressures of CO and

CO2 and �c represents the total available active carbon sites. The rate constants are

directly related to the temperature. As temperature increases k1 increases and both k2

and k3 decrease [24,60]. The nature of the coke described by the active carbon sites and

impurities that catalyse or inhibit coke gasification are the coke intrinsic properties that

affect the rate constants [24].

Previous studies showed that only a part of the total surface of the pores participates in

the reaction, namely the active surface area. Ergun and Mentser [61] and Laurendeau

[56] defined the active sites as sites formed by irregularities of the surface which are

able to chemisorb a gas phase through electron transfer. Carbon edges, dislocations,

inorganic impurities and oxygen functional groups are considered active sites [56].

23


2.3.3 Reaction rate measurement

The gasification rate of carbonaceous samples is usually measured by the mass loss of

the sample during the reactivity test divided by the initial mass of sample [56,65]

(Equation 2.3):

dtdW

W1R

0

�� Equation 2.3

where, R is the reaction rate and W0 is the initial mass of the dry ash-free sample.

Radovic et al. [66] expressed the reaction rate as a function of the mass of carbon at

time t (instantaneous mass of carbon) (Equation 2.4):

dtdW

W1R �� Equation 2.4

where, R is the reaction rate and W is the mass of the dry ash-free sample at time t.

Carbon conversion is defined by the initial mass and the mass of sample at time t

(Equation 2.5):

0

0

WWW

X�

� Equation 2.5

where, X is the fractional carbon conversion and W0 is the initial mass of sample.

Equation 2.4 becomes:

dtdX

)X1(1R�

� Equation 2.6

The advantage of Equation 2.6 comparing to Equation 2.3 is that the former allows the

calculation of the reaction rate at any moment and it can be used to calculate intrinsic

reaction rate (the reaction rate at time t is divided by the total surface area at time t),

whereas the latter gives only the variation of carbon mass during the reactivity test.

24


2.4 Factors influencing coke gasification

Coke properties such as microtexture, microstructure and mineral matter are the main

factors that affect gasification rate. Coke properties depend on the properties of the

parent coal such as coal rank, maceral composition and mineral matter, and

carbonization conditions. In this section the effect of coke properties on gasification rate

and also the influence of coal properties and carbonization conditions on coke properties

will be presented.


Both isotropic and anisotropic coke microtextures react with carbon dioxide but they

react at different rates. Several studies indicate that isotropic microtexture is more

reactive with carbon dioxide than anisotropic microtexture [18,22,23,67,68,69,70].

Figure 2.9 shows coke microtexture after reaction with carbon dioxide. The isotropic

microtexture, which originated from inert macerals (see section 2.5.1), was more

affected by gasification than the anisotropic microtexture.

As shown in section 2.2.1, the anisotropic microtexture is classified as a function of the

size, shape and form of the textural unit. The reactivity of different classes of

anisotropic microtexture varies [21,30,31,45]. Flow type anisotropic microtexture and

coarse mosaic showed a strong resistance to carbon dioxide attack. The medium mosaic

was consumed in small proportions whereas the fine mosaic was the most reactive.

25


Figure 2.9 Gasification of coke microtexture [18].

This reduced reactivity of the anisotropic microtexture compared to the isotropic

microtexture may be explained by either a lower surface area of the carbon available for

reaction or lower intrinsic reactivity (the reaction rate per unit area of pore surface in the

absence of any mass transfer restriction [71]) of anisotropic carbon [31] due to a lower

number of active carbon sites. As the most reactive carbon atoms are located at the

edges of the lamellae not on the layer planes (basal planes) [53], the density of

accessible layer edges depend on MOD size. Therefore the smaller MOD size the

greater the free edge density [22,28,34,45]. Moreover the reactivity of the carbon active

sites located on different edges such as armchair and zig-zag is different [72].

Kashiwaya and Ishii [41] designed an experiment to observe the difference in reactivity

of carbon atoms located on the basal plane and the edges of the polyaromatic layers.

They measured the crystallite height (Lc) and crystallite length (La) of a metallurgical

coke at different temperatures under inert gas (Ar) and reactive gas (mixtures of Ar-CO-

CO2) (Figure 2.10). Crystallite height was affected only by temperature and no

significant difference between the Lc of both the annealed and reacted cokes, implying

that the reaction is very slow on the basal plane. The crystallite length of both annealed

and reacted cokes also showed an increase with temperature but the reacted coke had a

26


lower La than the annealed coke at similar temperatures, indicating that the reaction

occurs at the edges of the polyaromatic layers.

Figure 2.10 Crystallite size (Lc and La) of a metallurgical coke function of temperature under inert gas (Ar) and reactive gas (Ar-CO-CO2) [41].

Feng et al. [73] determined the crystallite size of a char sample during reaction with

carbon dioxide at constant temperature (800ºC) up to 90% carbon conversion.

Crystallite height did not change significantly below approximately 60% carbon

conversion but it decreased at greater conversion levels, whereas crystallite length

decreased during gasification, even at an early stage. They assumed that initially the

reaction occurs predominantly at the edges of the polyaromatic layers, which implies a

decrease of the crystallite length, and only at later stages of reaction the entire

polyaromatic layers are consumed.

Coke samples removed from the raceway region of the blast furnace showed that the

flow type anisotropic microtexture is selectively consumed [15,74,75]. This behaviour

may be explained by the alkali effect or abrasion. Flow type anisotropic microtexture

reacts readily with K and Na to form an “intercalated” compound due to the regular

27


arrangement of carbon layers creating micro-fissures which increases the reactivity with

carbon dioxide. The low mechanical strength of flow type anisotropic microtexture

enhances its abradability.

Kerkkonen [51] observed on cokes sampled from a quenched blast furnace that the

isotropic carbon gasified more in the lumpy zone, while mosaic carbon was removed in

a higher proportion in the raceway. He believed that the removal of the mosaic carbon is

due to evaporation of the silicates causing mechanical weakening of the carbon

structure.

2.4.2 Coke microstructure

Porosity of coke, defined by the volume percentage, morphology and size distribution of

the pores, has a major influence on coke strength. Coke strength decreases as the

porosity volume increases [7,31,46].

The rate of reaction of coke with carbon dioxide is determined not only by intrinsic

reactivity of carbon but also by pore accessibility [45,71]. Pore characteristics and pore

surface area play an important role when the reaction rate is slow (regime I conditions)

[76]. The reactivity increases as the surface area increases [45].

Szekely and Aderibigbe [60] observed the behaviour of pores during coke gasification

at 1000ºC and PCO2/PCO=0.5 (Figure 2.11). At early stages of reaction the size of pores

increases causing an increase in surface area. As gasification proceeds the pores are

further enlarged and coalesce resulting in a reduction of surface area. They observed the

reduction of surface area after approximately 40% carbon conversion. A similar

development of surface area (measured by nitrogen) of a coke sample during reactivity

test at 1100ºC and 100% CO2 was reported by Kawakami et al. [77], but the decrease of

surface area was observed as early as 25% conversion. The decrease of surface area at

different carbon conversion was probably due to different properties of the cokes used

in these studies and also the reaction conditions. Turkdogan et al. [43] believes that with

increasing reaction temperature surface area decreases due to incomplete internal

28


reaction. Szekely and Aderibigbe [60] also observed that the total pore volume for each

pore size from 10 nm to 100 μm increased during gasification up to approximately 50%

burn-off.

Figure 2.11 Variation of surface area (N2 adsorption) and porosity during gasification [60].

Patrick and Walker [46] correlated coke reactivity not only with porosity volume but

also with pore size, number of pores and pore wall size. They concluded that reactivity

increases with increasing volume of the pores, mean pore size and number of pores, and

decreasing pore wall size.

Vogt et al. [7] observed that coke gasification reaction controlled by kinetics affects

coke strength due to abrasion and also fragmentation of the coke lump as fissures are

opened during gasification. Van der Velden et al. [18] observed that the surface of the

cracks developed during further coking was gasified preferentially compared to the pore

surfaces. In addition, coke crushing may affect coke reactivity and stability by

introducing more fissures with reactive surfaces. Thus caution is recommended when

comparing reactivity of crushed coke to uncrushed coke.

29


Kerkkonen et al. [23] and Sakawa et al. [78] did not find a distinct connection between

porosity and reactivity of coke with carbon dioxide. This suggests that gasification is

probably more influenced by a certain range of pore size than the total porosity.

Kawakami et al. [77] concluded from the variation of the pore size distribution of a coke

during gasification that the reaction occurs mainly on the surface of the pores smaller

than 1 μm. Also, Kerkkonen et al. [23] found that the porosity was lowered by

increasing the content of inertinite in the parent coal. Moreover, Kerkkonen et al. [23]

and Sakawa et al. [78] observed an increase of the reaction rate with increasing

inertinite content, so they concluded that the rate was affected more by the amount of

inertinite in the parent coal than the porosity.

Total porosity and pore size distribution are usually determined using mercury

porosimetry, which measures the relationship between pressure and effective volume of

the sample and is used to determine porosity and pore size distribution. Pore surface

area is determined by both nitrogen and carbon dioxide adsorption onto the surface of

the sample. Nitrogen measures surface area of both mesopores and some micropores

[79] whereas carbon dioxide measures surface area of micropores [80] (see Chapter 4).

The total surface area is commonly used to normalise the reaction rate in order to

remove the effect of surface area, but several studies [66,77,81,82,83] have shown that

active surface, the surface area of the coke exposed to gas that actually reacts with

carbon dioxide, would be more appropriate. The ratio Active surface area/Total surface

area decreases as the carbon conversion increases up till 20%; after 20% carbon

conversion the ratio becomes constant [81,83]. However, there is no standard method

for determining the active surface area. The most commonly used methods for the

measurement of the active surface area, namely Gravimetric and Temperature-

programmed desorption (TPD) methods, introduce errors in determination of active

surface area due to the presence of some physisorbed oxygen. The physisorbed oxygen

reacts with carbon and the reaction product, carbon dioxide, also reacts with carbon

during its removal through the pores [82]. Also, the measurement of the active surface

area is influenced by different parameters such as temperature and pressure [84], which

makes difficult the comparison of the data between different studies. Moreover, the

30


determination of active surface area is suitable only for samples free of mineral matter

because minerals introduce active sites [66,72,85,86].

However, Miura et al. concluded [87] that carbon content and pore surface area do not

indicate the reactivity of char when catalysts are present. Kyotani et al. [88] also

observed no obvious correlation between surface area and chars reactivity. They believe

the effect of catalysis by mineral matter is more dominant than surface area.

2.4.3 Mineral matter

Minerals in coke can enhance coke degradation in different ways:

� Some of the minerals catalyse the gasification;

� The oxides in the mineral matter are reduced inside the coke lump by the available

carbon at high temperature (above 1400ºC) [25];

� Iron bearing minerals (such as hematite, siderite, pyrite), sulphur and MgO can

reduce the size of the anisotropic microtexture around them during coking [89];

� Large mineral particles increase the coke gasification rate by formation of cracks

around them during coking, allowing carbon dioxide to penetrate the coke pieces

more easily [23];

� The aluminosilicates increase their volume when they decompose to form slag; this

weakens the internal structure of the coke lumps in the active zone of the blast

furnace [51].

The ash yield from the proximate analysis of the coke was found to be a good indicator

of coke reactivity by Gill et al. [25] and Vogt et al. [67] in their study; coke reactivity

increased as ash content increased. However Duval et al. [22] did not observe any

influence of ash yield on coke reaction rate. They concluded that coke microtexture was

more dominant than the mineral impurities in controlling coke reactivity.

Ash composition has been considered an important parameter in coke reactivity

[20,25,70]. For instance total iron oxide from the ash analysis was considered a fairly

good predictor of coke reactivity [89] [13,90]; reactivity of the cokes increased as the

31


concentration of Fe2O3 in the coke increased. Calcium and potassium oxides are also

known to act as catalysts in the gasification process [18]. The catalytic activity of

different oxides is dissimilar. Gill et al. [25] classified the elements that affect

gasification as it follows: Si=Al < Mg < Fe < Ca < Na < K.

Kerkkonen et al. [23] and Samaras et al. [91] concluded that the elemental composition

of the ash cannot be used to predict the reactivity of coke, but minerals present in the

coke were considered more suitable indicators of coke reactivity than ash chemistry. For

instance transition metals, their oxides, alkali and alkaline earth metal compounds

exhibit catalytic activity during gasification [13,72].

Metallic iron is considered a very efficient catalyst of gasification [92,93]. During

gasification metallic iron in contact with carbon dioxide is oxidised. Reactivation of

metallic iron as a catalyst could be achieved by introducing hydrogen or carbon

monoxide for a specific length of time [92,93,94]. Figure 2.12 shows a substantial

increase of reaction rate after purging the sample with carbon monoxide and hydrogen

alternatively. Tanaka [95] also observed reduction of wustite to �-iron after 5 minutes of

treatment with CO at 780-800ºC of a char sample. However the catalytic effect of

metallic iron could not be revived beyond about 60% mass loss after the sample was

treated in 100% H2 [93].

32


Figure 2.12 Reactivation of metallic iron during gasification by CO and H2 [92].

Magnetite was not considered by Walker et al. [92] to be a catalyst, but Price et al. [89]

observed an increase in coke reactivity with increasing magnetite content. Also an in-

situ XRD study for the iron-catalysed CO2 gasification of carbon made by Ohtsuka et al.

[96] showed a high catalytic activity of magnetite; gasification was enhanced in the area

where the only iron compound present was magnetite.

Wustite exhibits a higher activity than magnetite for gasification with carbon dioxide

[96]. Walker et al. [92] identified wustite as a catalyst but with lower activity than

metallic iron.

Pyrrhotite was considered by Vandezande [97] to be a catalyst of the gasification

reaction. He observed in some cases using optical microscopy that pyrrhotite appear to

33


create tunnels through the coke particle. Hu et al. [98] showed that oxidation of

pyrrhotite under a carbon dioxide atmosphere produces iron oxide. The iron oxide form

depends on the oxygen partial pressure. It can be concluded that pyrrhotite catalyses

gasification due to formation of iron oxide. Moreover, Price et al. [89] performed a

reactivity rest on a coke made from a coal doped with pyrite. Pyrrhotite is the most

likely mineral formed from pyrite during carbonization although some metallic iron can

be also formed. The coke from the doped coal was more reactive than the untreated one.

Apatite [89], quartz [23] and feldspar [23,89] were found not to affect the gasification

reaction rate. Potassium and sodium bound in alkali feldspars did not show any catalytic

activity in the tests performed by Price et al. [89]. Fayalite is assumed not to catalyse

gasification. A mixture of coke with excess of SiO2 treated at 1000ºC converted iron

oxide to a form (most likely fayalite) that did not catalyse gasification [92].

The other minerals identified hitherto in the cokes such as mullite, akermanite,

anorthite, calcium iron oxide, diopside, gehlenite, oldhamite, rutile, brookite and anatase

have not been investigated for their potential to be catalysts, but they have been

expected to catalyse gasification.

Alkali metals such as potassium and sodium are associated with aluminosilicates in an

unexchangeable form and they are believed to be catalytically inactive [99]. Iron,

calcium, magnesium, potassium and sodium included in montmorillonite and illite in

coal have only a slight effect on coke reactivity and only then because they may release

some of these elements [23].

The amount of catalytic minerals is an important factor that determines coke reactivity.

Walker et al. [92] believe that even traces amounts of catalyst (less than 1 ppm) are able

to affect the reaction rate. The degree of dispersion of the catalyst is another important

factor that controls reactivity [72,100]. Lindert and Timmer [90] and Tanaka et al. [95]

observed an increase of coke reactivity to carbon dioxide as the dispersion of metallic

iron increased.

34


Another aspect of the behaviour of the catalytic minerals during gasification is sintering

[72]. During gasification, carbon is consumed around the catalytic mineral particles

which allow them to become mobile and agglomerate to form large particles. This

suggests that the activity of the catalyst diminishes.

Huang et al. [101] prepared chars from vitrinite- and inertinite-enriched fractions from a

low rank coal (34% volatile matter). The carbonised vitrinite-enriched fraction was

more reactive than the inertinite-enriched fraction but after demineralisation the former

was less reactive. The surface area of the micropores and mesopores of the inertinite-

enriched fraction was greater than that of the vitrinite-enriched fraction. Also no

anisotropy was observed in any of the samples. They concluded that the catalytic effect

of the mineral matter on the reaction rate is greater than the surface area. A similar

observation was made by Czechowski and Kidawa [102] in their study. They assumed

that the greater concentration of the elements Ca, K and Na in the ash of the carbonised

vitrinite-enriched fraction is responsible for its higher reactivity compared to that of the

inertinite-enriched fraction.

Mechanisms of catalysis

The catalyst may affect gasification in several ways [92]:

� It may affect both steps of the oxygen-exchange mechanism of the gasification

reaction by either changing the number of active sites or lowering the overall

activation energy of the reaction;

� It may induce pits in the carbon basal plane and expose additional edge planes for

reaction;

� It may bypass the oxygen-exchange mechanism of the gasification reaction

completely.

Two mechanisms of catalysis of the gasification reaction have been proposed: the

oxygen-transfer mechanism and the electron-transfer mechanism.

35


The oxygen-transfer mechanism also known as the ‘spill-over’ mechanism is the most

accepted [72,88,92,94]. The mechanism can be applied either for metallic iron or iron

oxide and proceeds in the following manner: carbon dioxide dissociates onto the

catalyst surface and the oxygen atom is chemisorbed on the catalyst surface, then the

oxygen atom is transferred to an adjacent carbon site. However, the mechanism may not

follow exactly the steps mentioned above [88]. The catalysis of carbon gasification by

both metallic iron and iron oxide is described by Reaction 2.9-11 [92] and Reaction

2.11-13 [88], respectively; where Cf represents the active carbon site.

x Fe + y CO2 Fex(O)y + y CO Reaction 2.9

Fex(O)y + y Cf x Fe + y C(O) Reaction 2.10

C(O) Cf + CO Reaction 2.11

Fe xOy + CO2 Fe xOy+1 + CO Reaction 2.12

Cf + Fe xOy+1 C(O) + Fe xOy Reaction 2.13

C(O) Cf + CO Reaction 2.11

The electron-transfer mechanism is based on the ability of transitional metals to accept

electrons and influence the distribution of � electrons in the aromatic layers [92]. Figure

2.12 shows two types of distribution of the � electrons may occur at a carbon active site.

The catalyst is believed to induce type (b) distribution (Figure 2.12b) of the � electrons,

which requires less energy to break the carbon-carbon bonds to release a CO molecule

implying a lower activation energy for desorption of carbon monoxide. The position of

the catalyst can be anywhere on the plane with the carbon active site.

36


(a) (b)

Figure 2.12 Distribution of � electrons in the aromatic rings (a) not affected by catalyst and (b) influenced by catalyst [92].

2.5 Factors influencing coke properties

Coke properties are affected by a number of factors, namely coal properties such as coal

rank, maceral composition and coal fluidity on the one hand and coke oven conditions

on the other hand. In this section the influence of coal properties and carbonization

conditions on coke properties will be discussed.

2.5.1 Coal properties

The coals most suitable for the preparation of metallurgical cokes are bituminous coals

characterized by a carbon content between 80-90% and vitrinite reflectance (R0 mean)

values between 0.6 to 1.6%.

37


2.5.1.1 Coal rank

The coal rank can be assessed according to the volatile matter yield from heated coal,

carbon content or vitrinite reflectance. The coal rank increases with increasing carbon

content, increasing vitrinite reflectance and decreasing volatile matter yield.

Coal rank is a major determinant of coke microtexture [21]. The coal becomes more

aromatic and more polymerised as the rank increases, therefore the size and content of

anisotropic carbon increases [29,31]. Hirsch [36] created a model of the coal structure

as function of rank, using X-ray diffraction (Figure 2.13). The model has three types of

structure namely ‘open structure’, ‘liquid structure’ and ‘anthracitic structure’. The

‘open structure’ is a very porous system occurring in low rank coals with carbon

contents of up to 85% (Figure 2.13 a). The lamellae are relatively randomly orientated

in all directions and are connected by amorphous material. The ‘liquid structure’ is

attributed to medium rank coals or bituminous coals (the carbon range from about 85 to

91%). The porosity of the ‘liquid structure’ is very low and the lamellae are better

orientated than in the previous structure forming crystallites of two or three combined

lamellae (Figure 2.13 b). The ’anthracitic structure’ is characteristic of high rank coals

with carbon contents of over 91%. The degree of lamellae orientation is the highest of

all structures and the amorphous material has disappeared (Figure 2.13 c). Takagi et al.

[103] have shown that the crystallite height (Lc) of a number of coals of different rank

(60-90 % C, dry ash free base) increased as the coal rank increased (Lc varied between

0.71-1.89 nm). Moreover, Lu et al. [104] reported an increase of both crystallite height

(Lc) and crystallite length (La) as the coal rank increased form high volatile bituminous

coal to semi-anthracite.

As coal rank increases the ordering in coke microtexture increases. Coals with ‘open

structure’ make poorly ordered cokes whereas cokes from coals with ’anthracitic

structure’ have the greatest ordering. Many studies have shown that coal rank is one of

the main factors influencing coke reactivity because coal rank has an important role in

the formation of coke microtexture [26,29,67,105]. Low rank coals make cokes with

small MOD with size about 5 nm, whereas cokes from high rank coals are characterised

by large MOD with size about 20 �m [67]. However, the carbon crystallite size (Lc and

38


La) of chars prepared from coals of different rank (81.9 – 91.3 % C, dry ash free basis)

showed little influence of coal rank, although the crystallite size of the parent coals was

significantly affected by the rank [106].

There are different opinions regarding the rank of the coals that make cokes with the

highest resistance to carbon dioxide attack. Toshimitsu et al. [107], Vogt et al. [67] and

Koba and Ida [68] consider coals with vitrinite reflectance approximately R0=1.4%

make cokes with minimum reactivity. But, Graham and Wilkinson [17] believe the

reactivity of coke to carbon dioxide is at a minimum when the parent coal has a vitrinite

reflectance about 1.25%.

Figure 2.13 Schematic representation of coal structure made by Hirsch [36].

39


2.5.1.2 Macerals

During coalification the original plant material is transformed into three main organic

groups of the coal defined by the optical microscopy as ‘macerals’, namely vitrinite,

liptinite (formerly called exinite) and inertinite. Van Krevelen [36] presented a brief

description of these macerals. He assumed that vitrinite is a product of coalification of

woody tissue. It acts as a binder surrounding the other macerals and mineral matter and

is very brittle [47]. Liptinite has a greater hydrogen content than vitrinite. It is the

maceral group that becomes the most fluid during the coking process. Macerals from the

inertinite group contain less hydrogen than vitrinite. During the carbonisation process a

small amount of the inertinite macerals fuse, the other part remains practically

unchanged. Inertinite is rich in carbon, poor in hydrogen and volatile matter, hard and

brittle [47].

During carbonisation macerals that are fusible like vitrinite, exinite and a part of the

inertinite form the ‘reactive maceral derived component’ (RMDC) in the coke. Inertinite

that does not fuse yields the ‘inert maceral derived component’ (IMDC). Fusible

macerals form anisotropic microtexture and small amounts of isotropic microtexture

whereas non-fusible macerals form isotropic microtexture only.

The distribution and size of macerals in coals control the development of coke

microtexture [17,21]. The inert maceral derived component must be in a certain

proportion so that strong coke cell walls can be formed. Coals containing fine inertinite

particles form coke with larger anisotropic microtexture and thicker walls than similar

rank coals with fewer or coarser inertinite particles. Coarse IMDC limits the anisotropy

size because the space between them is narrow and the domains cannot grow [52].

Homogenous distribution of macerals and minerals in coal form coke with a more

uniform structure than coals that have alternate layers of variable maceral concentration

[52].

40


Because fusibility of inertinite can change the microtextural composition of coke a

small number of researchers have tried to identify the factors that affect its fusibility and

classify the inertinites in terms of their ability to fuse.

Diessel [108] proposed a classification of inertinite by its behaviour during

carbonisation. The first category is of ‘highly reactive inertinite’, which produces cokes

with mosaic to flow anisotropy and variable pore size. The inertinite reflectance of this

class before carbonisation varies from 1.0 to 1.5%. ‘Moderately reactive inertinite’ is

the second class. The inertinite shows weak plasticity and the coke product has basic

anisotropy, small degassing pores and the inertinite reflectance before carbonisation

varies from 1.5 to 1.8%. ‘Non-reactive inertinite of small size’ is the third category. It

strengthens the coke due to a good integration into the reactive derived maceral

component (RDMC). ‘Non reactive inertinite of large size’ is the last category. Coke is

weakened by this material because of a poor integration into the reactive derived

maceral component (RDMC).

The inertinite reflectance that is the boundary between fusible and non-fusible inertinite

is shown in Figure 2.14; the letters E, V and I in the chart indicate reflectance area of

coal macerals exinite (liptinite), vitrinite and inertinite. Fusible inertinite has reflectance

less than the boundary reflectance. The boundary between fusible and non-fusible

inertinite is not well defined. Pearson [109], Barriocanal et al. [110] and Diessel [108]

correlated the amount of inert derived maceral components with coal rank. The

reflectance boundary moves to a greater value as the coal rank increases. Moreover,

Pearson [109] believes that the amount of vitrinite present may affect the fusibility of

inertinite. He assumed that the reflectance boundary of coals with the same rank varies

with vitrinite content; as the vitrinite content decreases the reflectance boundary moves

to the higher values. He also found that the fusibility of fusible inertinite could be

suppressed if the vitrinite content is very high.

41


Figure 2.14 A typical reflectance distribution of macerals in coking coal [108]

Sakawa et al. [78] found a good agreement between the content of inertinite in the

parent coals and coke gasification rate; as the inertinite levels increased the reaction rate

increased.

2.5.1.3 Coal fluidity

Coal fluidity affects coke reactivity because it controls the size and shape of coke

anisotropy [31] and the nature of interfaces between reactive and inert macerals [110].

42


Coal rank, maceral composition and coking rate determine coal plasticity [45]. Coal is

fluid over a limited temperature range during carbonisation. The initial softening

temperature and resolidification temperature describe the temperature range over which

the coal is fluid. The temperature of maximum fluidity and the range of temperature of

maximum fluidity increase with increasing rank [31]. The best coking coals have

optimum chemical reactivity and fluidity that ensure the growth of the anisotropic

domains. Low rank coals have solidification temperatures too low to allow growth of

the domain size. In very fluid systems, more than in normal coking coals the size of the

domains can be too large and the cokes produced do not have the necessary mechanical

and thermal resistance [111].

The mechanism of formation of coke microtexture is not similar for all coals. Fortin and

Rouzaud [32] described two mechanisms of formation of coke microtexture during

carbonisation that are a function of coal fluidity. One mechanism is for coals with high

fluidity and the other one for high rank coals of low fluidity. Medium rank bituminous

coals are characterized by higher plasticity than high rank bituminous coals. The high

plasticity of medium rank bituminous coals favours the reorientation of BSUs to form

MOD and the resultant coke mainly shows mosaic microtexture. High rank bituminous

coals show a pre-organized structure. Their limited plasticity allows only an

improvement of this structure developing a massive microtexture. Fortin and Rouzaud

[32] define massive microtexture as uniform anisotropy extended over the whole

particle.

Coal oxidation affects coal plasticity. Coal plasticity decreases as the oxidation time

increases [112]. Coal oxidation rate increases as the coal rank decreases. Oxidation of

low rank coals reduces the MODs in the coke because the amount of metaplast

decreases (see next section). The coke made from high rank coals contains very large

domains even if the coal plasticity is lowered by oxidation. These coals have initial

planar orientation and they are not affected by reduction of fluidity [38].

43


2.5.2 Carbonization process

During carbonisation moisture and volatile matter are released, the mineral matter is

transformed and both coke microtexture and structure are developed. The coke

microtexture and structure is the result of coal behaviour during the plastic stage.

During carbonisation, chemical and physical processes occur, such as chemical

transformations and orientation of BSUs. Different models have been used to explain

coal plasticity and coke structure development during the carbonisation process.

Fitzgerald and van Krevelen [36] introduced the “metaplast theory” to explain coal

plasticity and chemical transformations during the carbonisation process. They consider

the metaplast to be responsible for the plastic behaviour of coal. The process is

presented as occurring in three steps:

I. Coking coal Metaplast

II. Metaplast Semi-coke + Primary volatiles

III. Semi-coke Coke + Secondary gas

This hypothesis shows that the metaplast is formed by a depolymerisation process.

Metaplast is defined as an unstable plastic phase formed during the first stage of coal

pyrolysis. The maximum plasticity is considered to be the point at which the

concentration of metaplast is at a maximum [34]. The plastic phase occurs at

temperatures between 300 and 500�C [113].

The next stage consists of the cracking process; tar is evaporated and the non-aromatic

groups are split off [113]. The residual aromatic groups produce components less

polymerised than the coal and a large proportion of them are liquids at the pyrolysis

temperature. They form metaplast after their saturation with hydrogen, which is

generated by other aromatisation reactions [47]. The fluidity of the metaplast is

improved by increasing the content of hydrogen. Aromatisation and condensation are

the other reactions that take place during this stage. Aromatisation results from

44


dehydrogenation of saturated rings [47]. A scheme of cracking and aromatization

reactions is presented in Figure 2.15.

Figure 2.15 Reactions of cracking and aromatisation of coal components [47].

Large molecules are also formed by condensation reactions. An example of a

condensation reaction is presented below (Reaction 2.14):

R-OH + R’H R-R’ + H2O Reaction 2.14

where R and R’ are organic radicals. The metaplast solidifies to form anisotropic

ordered coke. In some cases coke formation is believed to proceed via liquid crystal

mesophase. Oxygen has an opposite role to that of the metaplast in the carbonisation

process. Oxygen acts as a cross-linking agent [114] as it induces condensation reactions

[34] lowering the fluidity of the coal.

45


The density of the semi-coke increases during the third stage because methane and

hydrogen (especially at higher temperature) are released. At the end of this stage coke is

produced [113].

Rouzaud [37] described not only the chemistry of the process, but also the physical

process of formation of the coke microtexture during carbonisation in his “two-

component structural model of coal” (Figure 2.16). In this model the orientation of

polyaromatic basic structural units (BSUs) was observed using transmission electron

microscopy (dark field mode).

Figure 2.16 Transformation of coal during pyrolysis; a) raw coal, b) plastic stage (~400-500�C), c) semi-coke (~ 500�C) and d) coke (~ 1000�C) [37].

46


He assumed that coal contains two components: a macromolecular network and a

molecular component. The macromolecular network is formed by BSUs in a random

distribution. These BSUs are bound to each other by either oxygen atoms (thick line) or

aliphatic bridges (zig-zag line) that act as cross-linkers and prevent the development of

large molecular orientation domains (MODs). The oxygenated groups (thin line) or

aromatic hydrogen (thin line) that is linked to BSUs do not affect their ordering. The

pores of the macromolecular network include the molecular component (dot), which is

composed of small hydrocarbon molecules that are more or less trapped by the

macromolecular network (Fig. 2.16a). Low rank coals contain large amounts of

oxygenated and aliphatic chemical groups, their content decreases with increasing coal

rank. High rank coals consist largely of molecular component and aromatic hydrogen.

At about 400ºC most of the bridges of the macromolecular structure are broken (Fig.

2.16b). Fragments of the macromolecular network and aliphatic bridges yield a new

‘macromolecular’ component, similar to the metaplast, where BSUs are free to reorient

and form a MOD. During coal pyrolysis a portion of the macromolecular component is

released as volatile matter and the remaining portion condenses. Cross-linking oxygen

and hydrogenated molecules play opposing roles. Hydrogenated molecules of the

macromolecular component reduce the viscosity of the fused coal and also retard cross-

linking reactions by donating hydrogen to free radicals. High hydrogen content

improves the plasticity and the BSU orientation [114].

At temperatures about 500ºC solidification occurs. A semi-coke is obtained after the

volatiles have been released (Fig. 2.16c).

From 500ºC to about 1000ºC significant changes in size and orientation of MODs do

not occur. At this stage only gases are released (Fig. 2.16d).

Deposition of hydrocarbon molecules from volatile matter obtained during the cracking

process produce a pyrolytic form of carbon [18,21] (Figure 2.17).

47


Figure 2.17 Deposition of pyrolytic carbon (P) [21].

Coke microtexture and structure can be changed if the operating conditions of the coke

oven (heating rate, coking temperature and pressure) or other parameters such as bulk

density of the charge, position in the oven and heat treatment of coal and coke are

modified. The effect of these parameters on coke microtexture and structure will be

presented in the next sections.

2.5.2.1 Heating rate

Fluidity is low at a low heating rate and increases with increasing heating rate [115].

High heating rate increases the concentration of metaplast and the MOD size increases

[34]; therefore coke anisotropy increases [20].

Mitchell et al. [21] carbonised two bituminous coals, one of low rank and the other one

of high rank, at different heating rates. The low rank coal carbonised at higher heating

48


rate produced a coke with lower content of isotropic carbon and greater proportion of

very fine and fine mosaic microtextures. The coke microtexture from the high rank coal

was also dependent on heating rate. The proportion of fine mosaic was greater and

lower for the flow-like anisotropic microtexture. This was explained by the shorter fluid

range and carbonization time.

Coke porosity was found to increase as the heating rate increases [52,116] because coal

swelling increases [36]. Nevertheless, the degree of increased porosity becomes smaller

at high heating rates [116].

2.5.2.2 Carbonization temperature

Coking temperature has a major influence on coke reactivity. It affects the degree of

ordering of the domains and porosity of coke. Increasing coking temperature produces

larger and more ordered BSUs and the carbon layer edges become less accessible to the

reactive gases [22]. The porosity of coke decreases as the temperature increases up to

about 800ºC then it becomes constant [116].

2.5.2.3 Bulk density

Increasing bulk density of the coal charge increases the apparent density of coke and

this implies a coke of lower porosity [7,17,26,52,70]. Two ways of increasing the bulk

density of the charge are stamping the coal [7] or preheating. The bulk density of a coal

charge also depends on moisture content and the grinding size of the charge. Graham et

al. [17] noticed that the density of the charge decreases until the moisture content is

10% and then rises with further increase in moisture content. Coal crushed more finely

amplified this effect above 10% moisture.

49


2.5.2.4 Position in the coke oven

Coke properties vary with the position of coke in the oven from top to the bottom and

from wall to centre. Coke reactivity decreases from top to the bottom of the oven. This

can be explained by bulk density in the oven, which increases with increasing depth

[117]. Reactivity of coke next to the oven wall is lower than that in the middle of the

oven [45]. This phenomenon is amplified when the heating rate is lower [20]. Two

explanations have been proposed for this. One is the heating rate of coal near the oven

walls is faster than its centre producing more anisotropic coke [45]. The other one is the

coke from the centre of the oven has a higher porosity than that from the wall [46].

2.5.2.5 Pressure

The amount of MOD classes eight to ten (see chapter 2.2.1) increases when the external

pressure increases. Pressure improves the fluidity and increases the capacity of the

metaplast to increase MODs. The MOD formation rate increases more quickly than the

diffusion rate because the pressure improves the confinement. This permits the

formation of larger domains than those obtained at atmospheric pressure [33].

2.5.2.6 Heat treatment

Heat treatment can be applied to the coal charge and also to the produced coke. The heat

treatment affects both coke porosity and microtexture. The pore diameter observed in

preheated coal charges was smaller and the pore-wall thickness was less than in the case

of wet charges [17]; in this case the difference could be attributed to pre-drying of the

coal prior to entry to the coke oven.

On reheating coke at 1400�C for two hours in inert atmospheres, it loses about 4-5%

weight due to the loss of volatile matter, sulfur and ash-coke reactions. Moreover, coke

structure is densified [26] as coke shrinking occurs. During heat treatment the mosaic

microtexture can become porous because of removal of sulfur and volatile material.

50


Legin-Kolar et al.[118] and Zamalloa et al. [119] observed an intensive structural

arrangement and the crystallites height started to grow in the annealed cokes above

1300ºC. Kashiwaya and Ishii [41] annealed coke above 1000ºC and observe in increase

not only in the crystallite height (Lc) but also in the crystallite length (La).

Iwakiri et al. [116] observed that increasing the temperature of heat treatment (900 –

1300ºC) of cokes reduces coke reactivity.

2.6 Methods of measuring coke reactivity

The most widely used test for assessing quality of metallurgical cokes is one developed

by the Nippon Steel Corporation (NSC). A 200g sample of coke with particle size

between 19 to 21 mm is placed into a reaction vessel, which is heated by an electrical

furnace, and the coke is reacted with carbon dioxide at a flow rate of 5 L/min for two

hours at 1100�5�C. After reaction, the Coke Reactivity Index (CRI) and Coke Strength

after Reaction (CSR) are measured. The Coke Reactivity Index is the percentage of

weight loss during the reactivity test (Equation 2.7):

CRI = 100 weightoriginal

in weight loss Equation 2.7

The coke sample from the reactivity test is used to determine the Coke Strength after

Reaction. The CSR is defined as the weight percentage of coke larger than 10 mm in

size after tumbling the reacted coke for 600 revolutions at 20 revolutions per minute

(Equation 2.8):

CSR = 100reactionafter weight

lingafter tumb mm 10 fraction ofweight

� Equation 2.8

Many studies have shown a good relationship between CRI and CSR; as CRI increases

CSR decreases [70,90,105]. Although the NSC test does not reproduce the blast furnace

51


conditions it gives information about coke quality and also a relatively good agreement

was observed between CSR and blast furnace permeability [120]. However the NSC test

cannot be used to determine the reaction kinetics during the test because it was not

designed to acquire data about the carbon loss during the test.

The most common techniques used to measure gasification reaction parameters use

either the thermogravimetric analyser (TGA) or the fixed-bed reactor (FBR). Each

apparatus has its shortcomings in determining some of the reaction parameters and these

must be considered in selection of the appropriate technique for an accurate

measurement of certain reaction parameters.

Thermogravimetric analyser (TGA)

Thermogravimetric analysers have been used in a series of studies [18] [24,60,76,77]

designed to measure reactivity under different specific conditions such as gas

composition, temperature, sample mass and particle size. Figure 2.8 shows a schematic

diagram of a TGA system used by Aderibigbe and Szekely [24] in their study. The

sample is placed in a basket suspended by an automatic recording balance. The basket is

located inside an electrical furnace and the reactant gas flows from the bottom to the top

of the furnace. Two thermocouples are located above and below the basket. The change

in mass of the sample during the reactivity test and the temperature are recorded.

The measurement of the reaction rate under kinetically controlled conditions using the

TGA could be affected to some extent by several factors:

� The flow of the reactant gas could affect to some degree the reading of the sample

mass because of the sensitivity of the balance;

� There could be some difference between the temperature of the sample and that

recorded by the thermocouple because the thermocouple cannot be placed inside the

sample and CO2 gasification is an endothermic reaction, which can lower the

temperature in the middle of the sample;

52


� A concentration gradient of the reactant and product gases can exist inside the

sample due to an inadequate gas flow, which is directly related to the mass of the

sample and particle size.

All the above mentioned factors have been previously identified as controlling the

reaction regime (see section 2.3.1). However, rigorous selection of reaction parameters

and conditions would minimise the effect of the above mentioned factors on the reaction

kinetics.

Figure 2.18 Schematic diagram of an experimental system using a thermogravimetric analyser [24].

Fixed-bed reactor (FBR)

A schematic diagram of a fixed-bed reactor is shown in Figure 2.19 [55]. The reactor is

placed into an electric furnace and the sample sits on a fixed holder. The reactant gas

passes through the sample bed from top to the bottom and a thermocouple measures the

temperature in the middle of the sample bed. The reaction rate is measured using the gas

53


flow-rate and the concentration of carbon monoxide in the exhaust gas. The design of

the fixed-bed reactor system has some advantages:

� The sample temperature measurement is accurate because the thermocouple is placed

in the middle of the sample bed;

� The gas flow is through the sample bed which allows quick removal of the gaseous

reaction product enabling conditions for regime I;

� The design of the system allows the measurement of the activation energy due to

possibility of quickly changing the reaction temperature.

Figure 2.19 Schematic diagram of a fixed-bed reactor system [55].

54


2.7 Chapter overview

Gasification is one of the factors that affect coke degradation in the blast furnace, which

in turn affects furnace performance. The temperature and gas composition change as

coke descends in the blast furnace. The mechanism of coke gasification also changes

with increasing temperature. At low temperatures the reaction takes place throughout

the particle and is chemically controlled, but at higher temperatures the reaction is

limited to the surface. Gasification rate is influenced by combined factors such as coke

reactivity and blast furnace conditions. The combined effect of these factors in the blast

furnace makes difficult the assessment of their effect on gasification rate.

Increasing temperature increases the reaction rate in laboratory conditions. The

concentration of carbon dioxide in the blast furnace gas decreases significantly at high

temperature. Diminishing carbon dioxide levels in the gas reduces the reaction rate. The

inhibition effect of carbon monoxide on gasification rate can also have a contribution in

lowering the reaction rate. Alkalis and some iron compounds from the burden are

known to catalyse the gasification reaction, mostly at the accessible surface of the coke

lumps.

Coke properties such as carbon microtexture, microstructure and the composition of

inorganic matter control coke reactivity. Different types of carbon microtexture react

differently with carbon dioxide. Flow type anisotropic microtexture and coarse mosaic

have a relatively strong resistance to carbon dioxide attack. Medium mosaic was

consumed more rapidly and fine mosaic, isotropic microtexture and inertinite were the

most reactive components. Gasification is assumed to occur mainly at the edges of the

molecular orientation domain (MOD) and is less likely on the basal planes. As the

MOD size decreases the density of the free edges on the pore surface increases which

would explain the greater reactivity of the isotropic microtexture than that of the

anisotropic microtexture.

Carbon crystallite height in chars and cokes as measured by the XRD was not affected

by gasification in the early stages of the reaction (approximately up to 60% carbon

conversion) but decreased at later stages, whereas crystallite length decreased as

55


gasification proceeded. However limited studies have investigated the effect of

gasification on carbon crystallite size in coke.

The access of carbon dioxide to the carbon active sites is through the pore network.

Particle surface area affects reaction rate when the reaction is chemically controlled

whereas pore size distribution is important at higher temperatures when the reaction is

limited by diffusion. During gasification with carbon dioxide the surface area of the

pores increases due to enlargement of the pores and then the pores coalesce at greater

conversion levels, which result in a decrease in surface area. Although it is generally

agreed that surface area increases as gasification proceeds in the early stages a good

relationship between surface area and the reaction rate was found only in some of the

studies.

The presence of mineral matter in coke was found to affect coke reactivity. Ash yield

and the elemental composition of the ash were considered in many studies as indicators

of coke reactivity. Several studies have found a good relationship between coke

reactivity and potassium, sodium, calcium and iron oxides from the ash analysis.

However oxides are not the only form of inorganic matter in coke, also other mineral

phases are present. There is currently limited information available in the literature

about the form of minerals present in the metallurgical cokes. Metallic iron, iron oxides

and pyrrhotite were the only catalysts present among the minerals identified in the

metallurgical coke by previous studies. The calcium, potassium and sodium minerals

identified so far in the coke do not catalyse gasification. Therefore using ash yield or

composition as indicator of coke reactivity should be reconsidered. The particle size and

dispersion of catalytic minerals are also of major importance in catalytic gasification.

The reactivity of coke increases as the catalyst particle size decreases and dispersion

increases. Quantitative analysis of the catalytic minerals identified in cokes and their

relationship with coke reactivity has not been previously investigated.

Although coke microtexture, porosity and catalytic minerals have been established as

major factors that influence coke reactivity by many studies, there is still uncertainty as

to the relative importance of these factors.

56


57

Coal properties such as rank, maceral composition, fluidity and mineral matter, and

carbonization conditions are the factors that determine the development of coke

properties. Macerals from high volatile coals that fuse during the carbonisation process

form coke with small size anisotropic microtexture. With increasing coal rank the size

of anisotropic microtexture increases. Based on this observation coal rank and maceral

composition have been used to predict coke reactivity. However the prediction was not

valid for all cases. Other factors such coal fluidity, mineral matter and carbonization

conditions could be the explanation for the poor relationship found between coke

reactivity and both coal rank and maceral composition.

CHAPTER 3 – Thesis Objectives

To make the blast furnace more sustainable the efficiency of the furnace operated under

current technologies must be improved and also new technologies must be considered.

In order to achieve this, a good understanding of coke degradation in the furnace is

required to be able to prepare cokes of suitable quality. Gasification has been identified

as an important factor that affects coke degradation in the furnace. Coke gasification is

controlled by coke properties and the blast furnace environment.

Many studies have investigated the effect of microtexture, surface area and to a lesser

extent mineral matter on coke reactivity. Although some agreement has been found

between coke properties and reactivity there are still inconsistencies in their

relationship. The aims of this project are to develop a fundamental understanding of the

effect of coke properties such as microtexture, surface area, carbon crystallite size (Lc

and La), and mineral matter on coke reactivity and to determine their relative

importance in the gasification process. Because coke properties are strongly related to

the properties of the parent coal, the influence of coal properties such as coal rank,

maceral composition and mineral matter on coke properties will be also investigated.

Of all the coke properties the influence of mineral matter on coke reactivity is least

understood. Although previous studies have concentrated on ash yield and elemental ash

composition it has been recognized that the forms of the mineral matter in the coke play

an important role in controlling coke reactivity. However, there is currently limited

information about the form and the relative concentration of minerals present in

metallurgical cokes. In this project the mineral forms present in coke will be identified

and their relative proportions will be determined. The influence of the catalytic minerals

present in the cokes on coke reactivity will be also investigated. Because mineral matter

transformation during gasification has been poorly investigated previously another

58

Chapter 3 – Thesis Objectives

59

objective will be to determine the effect of gasification on mineral matter, especially on

the catalytic mineral phases.

Coal rank, maceral composition and mineral matter determine coke properties.

Separating these factors and then determining their effect on the reaction rate will

improve the understanding of their effect on coke properties. A comprehensive analysis

regarding the effect of coal rank and maceral composition on coke properties and coke

reactivity will be carried out on carbonised maceral-enriched fractions prepared from

coals of different rank. To explain the mineral phases formed in cokes the mineralogy of

the parent coal will be identified.

Many blast furnace operators use parent coal properties such as rank, maceral

composition and to a lesser extent ash chemistry to predict coke reactivity. However,

the prediction of coke reactivity using these parameters has not been found valid at all

times. Therefore a better understanding of the importance of coal properties on coke

reactivity would improve the accuracy of the prediction of coke reactivity.

To summarise, the broad outlines of the project are as follows:

� Establish the effect of rank and maceral composition of the parent coal on coke

properties. Also, the influence of coke properties such as microtexture, surface area

and carbon crystallite size (Lc and La) on coke reactivity is investigated.

� Mineral matter characterisation (qualitative and quantitative) of the raw cokes and

their parent coals. The occurrence of catalytic mineral matter in the cokes is also

investigated.

� Establish the influence of the catalytic minerals present in cokes on coke reactivity.

� Investigate the effect of gasification on mineral matter in coke (qualitative and

quantitative). Also, the influence of gasification on catalytic mineral matter is

investigated.

CHAPTER 4 – Experimental

4.1 Parent coals

Nine Australian bituminous coals from New South Wales and Queensland having a

wide range of rank, maceral composition and mineral matter were selected to examine

the effect of coal properties on the gasification reaction rate of cokes produced from

these coals. The coals were sent externally (Amdel laboratories, Newcastle, NSW) for

chemical analyses, namely proximate analysis and ash composition. The petrographic

analysis was carried out at Commonwealth Scientific Industrial Research Organisation

(CSIRO) – Energy Technology laboratory. In Table 4.1 are listed the proximate

analysis, ash analysis and petrographic analysis.

60

Chapter 4 – Experimental

Table 4.1 Proximate, petrographic and ash analyses of the parent coals used for

preparation of cokes in this work.

Coal A B C D E F G H I Proximate Analysis (wt %, air-dried basis)

Moisture 1.9 2.4 2.5 2.4 2.7 1.4 1.1 1.5 1.5 Ash 6.2 5.6 7.7 7.10 9.1 7.0 9.8 9.7 9.6

Volatiles 34.0 28.9 26.2 22.3 22.8 21.3 20.2 20.2 17.6 Fixed Carbon 59.8 65.5 66.1 70.6 68.1 71.7 70.0 70.3 72.8

Coal Rank (%, mean maximum vitrinite reflectance in oil) R0 max. 0.95 1.00 1.05 1.18 1.19 1.27 1.29 1.40 1.61

Petrographic Analysis (vol %) Vitrinite 80.0 59.0 49.6 55.2 51.2 57.7 31.6 67.7 75.0 Liptinite 3.7 3.4 2.6 0.0 0.0 0.0 0.0 0.0 0.0 Inertinite 13.3 36.2 43.8 42.0 45.4 39.7 65.4 27.5 20.3

Mineral Matter 3.0 1.4 4.0 2.8 3.3 2.6 3.0 4.8 4.7 Ash Analysis (wt %)

SiO2 50.80 61.40 53.60 47.30 53.80 56.90 48.30 61.60 58.10Al2O3 37.9 28.3 28.4 36.3 33.1 18.3 37.9 28.1 27.4 Fe2O3 4.60 4.30 7.60 5.00 4.50 12.80 5.30 3.20 6.20 CaO 1.10 1.30 3.00 2.80 2.30 3.70 2.50 1.30 1.60 MgO 0.32 0.34 0.95 0.63 0.54 1.60 0.58 0.46 0.70 TiO2 1.90 1.50 1.40 1.90 1.50 1.10 1.40 1.40 1.60 Na2O 0.55 0.30 0.57 0.57 0.41 0.45 0.65 0.72 0.45 K2O 0.76 0.48 1.00 1.10 0.69 0.92 0.54 1.10 1.00 P2O5 0.74 0.79 1.70 1.80 1.50 1.30 1.90 0.84 0.80

Mn3O4 <0.02 <0.02 0.05 0.08 0.04 0.06 0.03 0.04 0.09 SO3 0.09 0.26 0.76 0.74 0.37 2.00 0.32 0.25 0.49

Cr2O3 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02CuO <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02

V2O5 0.09 0.05 0.05 0.05 0.04 0.04 0.02 0.05 0.07 ZnO 0.04 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02NiO <0.02 <0.02 <0.02 <0.02 0.03 <0.02 <0.02 <0.02 <0.02BaO 0.10 0.03 0.15 0.22 0.10 0.09 0.22 0.09 0.04 SrO 0.08 0.04 0.08 0.29 0.09 0.05 0.13 0.05 0.05 Total 99.15 99.19 99.39 98.86 99.07 99.39 99.87 99.28 98.67

61


4.2 Preparation of maceral-rich fractions

Inertinite- and vitrinite-rich fractions were prepared from five of the nine coals, namely

B, C, D, F and G. Liptinite was present only in two of the selected coals (B and C) in

low concentration (3.4% and 2.6%), consequently it was not considered in this work.

The coals were crushed to less than 1 mm size and the fraction 0.45 – 1.00 mm was

used for preparation of the maceral-enriched fractions.

The method used to separate macerals in this study was sink-float, using organic liquids

of different densities. This method is based on the different densities of the macerals.

Vitrinite density is lower than that of inertinite [36]. Solutions of different densities

were prepared for maceral separation. The solution is a mixture of a light density

component and a heavy density component prepared to have a specific density. Hexane

( = 0.66 g cm-3) was the light density liquid and the heavy density component was

perchlorethylene ( = 1.62 g cm-3).

Figure 4.1a depicts the maceral separation device. It comprises two glass flasks,

connected by a ground glass joint. The top flask has a funnel shape and its volume is

two litres. The top flask was designed with a Teflon stopcock and an inlet at its top. The

bottom flask is a one litre Erlenmeyer flask. About three litres of solution was necessary

to fill the flasks. The coal was poured into the top flask through the inlet using a funnel

and then stirred well. After stirring the stopcock was turned on to allow to the coal

particles, with greater density than that of the liquid, to settle into the bottom flask. The

coal particles with lower density than that of the solution went to the top. The separation

process lasted about 16 hours. After the separation had been completed the stopcock

was turned off and the flasks were disconnected. Beneath the top flask, still supported

by a metallic ring fixed on the stand, were placed a funnel with filter paper supported by

a Berzelius beaker, Figure 4.1b. The filter paper used was no.1 Whatman qualitative

filter paper. Then the stopcock was turned on and the flask was emptied. The coal

particles from the bottom flask were collected just by pouring the content of the flask

into a similar device like that described in Figure 4.1b. The coal fractions still in the

filter papers were dried in an unheated vacuum oven under nitrogen flow.

62


(a)

(b)

Figure 4.1 Schematic of the maceral separation device.

63


Petrographic characterisation was performed on the maceral enriched fractions mounted

in resin ( see section 4.5.1).

In order to decide the density range of vitrinite- and inertinite-rich fractions a test was

performed for each coal. 100 g of each coal was separated using seven solution

densities, namely 1.25, 1.30, 1.35, 1.40, 1.45, 1.50 and 1.55 g·cm-3. Figure 4.2 shows

the concentration of macerals and mineral matter of each separated fraction as a

function of density for coal F. This was a general trend for all five coals. Low density

fractions were rich in vitrinite and low in inertinite. The percentage of inertinite started

to increase significantly in fractions with density greater than 1.35 g·cm-3. The content

of mineral matter increased with increasing density. The density of the fraction richest

in vitrinite or inertinite was then selected for the bulk separation of the coals. About 1

kg from each coal underwent the sink-float process. A maximum 150 g of coal was used

for each batch of sink-float separation.

0

20

40

60

80

100

<1.25 1.25-1.30 1.30-1.35 1.35-1.40 1.40-1.45 1.45-1.50 1.50-1.55 >1.55

Density fraction (g cm-3)

Mac

eral

(%) Vitrinite

Inertinite

Mineral matter

Figure 4.2 The concentration of macerals and mineral matter in the separated fractions of coal F.

64


Table 4.2 shows the petrographic analysis of the vitrinite-rich fractions, intermediary

fractions and inertinite-rich fractions after the separation was completed. The

concentration of vitrinite in the vitrinite-rich fractions varied from 83.9 to 96.2 %. The

inertinite concentration in the inertinite-rich fractions was lower than the vitrinite

concentration in vitrinite rich fractions; it ranged between 72.1 -77.0 %. Along with the

vitrinite- and inertinite-rich fractions an intermediary fraction was produced, which had

greater density than vitrinite-rich fractions and lower density than the inertinite-rich

fractions. The intermediary fractions were used to prepare synthetic low ash coals

(SLACs) with maceral composition similar to that of the original coal but with less ash

yield. The intermediary fractions of coals B and G were called synthetic low ash coals

because they had the maceral composition similar to the coal source. The intermediary

fractions from coals C, D and F were richer in inertinite than the corresponding original

coal. In order to prepare the synthetic low ash coals the intermediary fractions were

mixed with vitrinite-rich fractions from the same coal in certain ratios. The ash content

of the vitrinite and inertinite rich fractions, the synthetic low ash coals and the original

coals from the proximate analysis are shown in Table 4.3.

65


Table 4.2 Petrographic analysis of the coal fractions and the original coal.

Vitrinite (vol. %)

Liptinite (vol. %)

Inertinite (vol. %)

Mineral Matter (vol. %)

Coal B Vitrinite-rich fraction 94.8 1.8 3.2 0.2 Intermediary fraction 59.3 3.4 36 1.1 Inertinite-rich fraction 20.5 3.3 73.9 2.3 Original coal 59.2 3.2 35.8 1.7 Coal C Vitrinite-rich fraction 85.2 1.5 12.3 1.0 Intermediary fraction 40.3 3 54.9 1.8 Inertinite-rich fraction 16.00 3.80 76.90 3.3 Original coal 51.2 2.7 42.8 3.3 Coal D Vitrinite-rich fraction 89.3 0.3 10.3 0.1 Intermediary fraction 37.2 0.1 60.3 2.4 Inertinite-rich fraction 24.3 0.3 72.1 3.3 Original coal 52.8 0.3 43.6 3.3 Coal F Vitrinite-rich fraction 96.2 0 3.4 0.4 Intermediary fraction 51.7 0 46.6 1.3 Inertinite-rich fraction 22.3 0 75.2 2.5 Original coal 57.3 0 40.0 2.7 Coal G Vitrinite-rich fraction 83.9 0 15.4 0.7 Intermediary fraction 33.8 0 64.2 2 Inertinite-rich fraction 20.9 0 77.0 2.1 Original coal 32.9 0 63.7 3.4

66


Table 4.3 Ash concentration in the vitrinite- and inertinite-rich fractions, the synthetic low ash coals and the original coals from the proximate analysis.

Ash, db (wt. %) – Proximate analysis B C D F G

Vitrinite-rich fraction 0.79 2.11 1.92 1.18 2.54 Inertinite-rich fraction 9.68 9.66 11.08 10.06 9.73

Synthetic low ash coal (SLAC) 4.06 5.04 5.25 4.95 6.57 Original coal 5.60 7.70 7.10 7.00 9.80

4.3 Coke preparation

4.3.1 The large scale coke ovens

The coals as received were crushed in a jaw crusher and sieved at 6 mm to provide a

product with a nominal particle size of less than 6 mm. Prior to carbonization the

crushed samples were stored in drums in nitrogen to prevent oxidation.

The 9 kg coke oven

A schematic of the 9 kg coke oven is shown in Figure 4.3. The coking vessel was a

cylindrical retort with a nominal capacity of 9 kg (189 mm diameter and 800 mm high)

heated by a furnace fitted with silicon carbide elements. The temperature of the furnace

was controlled by a programmable controller. The coal was packed into the retort in

four increments of 2.45 kg each. Each of the increments was tamped down to a pre-

determined height of 100 mm in order to achieve the desired bulk density of the charge.

The bulk density of the dried coal charge was around 850 kg m-3. A thermocouple was

placed at the geometric centre of the charge. The retort was closed using an end plate

67


and gasket. The end plate was designed with a gas off-take tube to remove the evolved

gases and tars during the carbonization process.

Figure 4.3 A diagram of the 9 kg coke oven.

The retort was introduced into the furnace after the furnace wall temperature had

reached 1050ºC. The temperature of the furnace dropped to approximately 700ºC due to

the cooling effect of charging the retort. From this temperature the furnace started to

heat the retort with a heating rate of 3ºC per minute. After the temperature of the centre

of the charge had reached 900ºC, the retort was held in the furnace for a further 50

68


minutes to raise the temperature up to 1050ºC in the centre of the charge. The total time

in the furnace was about 3 hours. The furnace and charge temperatures were recorded

during the carbonization process by a computer. The retort was then removed from the

furnace and allowed to cool in a nitrogen atmosphere. The proximate and ash analyses

of the cokes are presented in Table 4.4.

Table 4.4 Proximate and ash analyses of the cokes prepared in the 9 kg oven.

Coke A B C D E F G H I Proximate Analysis (wt %, air-dried basis)

Moisture 0.7 0.7 0.8 0.7 1.1 1.9 0.8 0.8 0.6 Ash 9.1 7.9 10.3 9.3 11.9 9.0 11.8 12.2 11.9

Volatiles 0.7 0.8 0.3 0.7 0.5 0.6 0.6 0.4 0.6 Fixed Carbon 90.2 91.3 89.4 90.0 87.6 90.4 87.6 87.4 87.5 Ash Analysis (wt %)

SiO2 51.50 62.00 54.50 48.10 54.30 58.00 48.00 61.30 56.90Al2O3 38.30 29.00 29.30 36.80 33.80 19.10 37.50 28.30 27.00Fe2O3 4.20 3.90 7.30 4.70 4.30 12.70 5.60 3.10 8.50 CaO 1.10 1.20 2.70 2.60 2.10 2.90 2.40 1.30 1.60 MgO 0.21 0.20 0.90 0.59 0.43 1.60 0.52 0.38 0.78 TiO2 1.80 1.50 1.40 1.80 1.40 0.99 1.40 1.40 1.40 Na2O 0.42 0.11 0.20 0.38 0.27 0.32 0.58 1.50 0.35 K2O 0.76 0.55 1.10 1.10 0.75 0.92 0.52 1.10 1.00 P2O5 0.72 0.74 1.60 1.80 1.40 1.20 1.80 0.84 0.71

Mn3O4 0.00 0.00 0.05 0.05 0.04 0.05 0.04 0.03 0.14 SO3 0.03 0.16 0.41 0.48 0.21 1.10 0.25 0.17 0.43

Cr2O3 0.00 0.02 0.00 0.04 0.00 0.17 0.00 0.02 0.00 CuO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

V2O5 0.06 0.03 0.03 0.05 0.05 0.03 0.02 0.05 0.06 ZnO 0.05 0.02 0.00 0.03 0.00 0.00 0.00 0.00 0.00 NiO 0.00 0.00 0.00 0.00 0.02 0.02 0.00 0.00 0.00 BaO 0.06 0.00 0.08 0.15 0.08 0.00 0.24 0.06 0.04 SrO 0.08 0.04 0.07 0.28 0.09 0.05 0.14 0.05 0.04 Total 99.29 99.47 99.64 98.95 99.24 99.15 99.01 99.60 98.95

69


The 400 kg coke oven

It is a 400 kg capacity, movable wall oven with a width of 0.450 m. The fixed and

moving walls are electrically heated. The nine test coals were each packed into one of

12 compartments (the remaining 3 compartments contained other test coals not

associated with this work). The 12 compartments were surrounded by a dummy layer of

moist coal placed at the bottom, ends and top of the 12 compartments. The mass charge

of each coal was 15.5 kg. The bulk density of the dried coal charge was 825 Kg m-3.

After the temperature in the centre of the charge had reached 900ºC, the charge was

held in the oven for another 4 hours and 15 minutes to reach 1039ºC in the centre. Then

the charge was pushed from the oven and quenched as monolith with water. The total

coking time was about 20 hours.

4.3.2 The 70 g coke oven

The maceral-enriched fractions prepared and the original coals were carbonised in a

small scale (70 g) oven. They were carbonized in two stages because the retort used in

the first stage was made of aluminium, which set the maximum temperature of

carbonisation to be 470ºC, since aluminium melts at 660 ºC. The semi-coke prepared in

the first stage was calcined to 1050ºC in a horizontal tube furnace.

Preparation of the semi-cokes

A diagram of the 70 g oven is presented in Figure 4.4a. The oven was a wire wound

vertical tube furnace with an internal diameter of 90 mm and a length of 300 mm. The

heating rate was controlled by a digital temperature controller. The oven was preheated

to 300ºC before the retort was introduced into the furnace. The furnace was continually

purged with nitrogen to avoid the oxidation of the sample. The retort was lined with

filter paper to prevent the plasticised coal adhering to it. Sub-samples of 70 g were

packed into the carbonization retort at a bulk density in the range 0.68 to 0.80 g cm-3.

An aluminium plunger was used to compact the sample into the retort. After the sample

70


was packed-in a steel disc was placed between the sample and the plunger. The steel

disc supported a ceramic sheath, which was inserted through both the static mass and

the aluminium plunger. A micrometer barrel and a linear transducer were placed on top

of the ceramic sheath. The linear transducer transmitted the contraction/dilatation of the

charge to a computer to be recorded. The role of the micrometer barrel is to measure the

dilatation of the charge if it exceeded the measuring range of the linear transducer

during carbonization. During the carbonization process the plunger supported a mass

which applied a static load of 10 kPa. Two thermocouples measured the temperature in

the geometric centre of the charge and the temperature of the retort. The temperatures

were recorded by a computer. After the temperature in the centre of the charge reached

300ºC the coal sample was heated up to 470ºC at a specific rate and held at this

temperature for 2 hours. Then the semi-coke produced was cooled under nitrogen flow.

The vitrinite-rich fractions swelled more than the coals and inertinite-rich fractions.

High swelling leads to high porosity in the carbonization product. Therefore, to control

the porosity in the coke, the heating rate of the vitrinite-rich fractions had to be lower

than the heating rate of the coals and the inertinite-rich fractions. The heating rate from

300 to 470ºC was 1ºC per minute for the original coals and the inertinite-rich fractions.

The vitrinite-rich fractions of coals B, C, D and F were carbonized at 0.1ºC per minute.

The vitrinite-rich fraction of coal G was heated up at even lower heating rate, 0.05ºC per

minute.

Preparation of calcined cokes

An electrically heated, horizontal tube furnace was used to calcine the semi-coke

samples (Figure 4.4b). The furnace consisted of an alumina furnace tube with an

alumina sheath inside the furnace tube. The thermocouples measured the temperature of

the furnace and the semi-coke samples within the alumina sheath. These two

temperatures were recorded by a computer during the calcination process. The semi-

cokes samples were put into alumina boats and inserted into the centre of the furnace at

ambient temperature. The open end of the alumina sheath was sealed with a gas tight

end cap. The alumina sheath was purged with high purity nitrogen. The furnace was

71


heated to 500ºC at a heating rate of 1ºC per minute, then at 10ºC per minute from 500ºC

to 1050ºC. The purge gas was changed from high purity nitrogen to ultra high purity

argon after the temperature reached 500ºC. At the completion of the calcining stage the

samples were cooled down inside the furnace under ultra high purity argon. Below

500ºC the purge gas changed back to high purity nitrogen.

72


(a)

(b)

Figure 4.4 A schematic diagram of (a) the 70 g coke oven and (b) the calcination furnace.

73


4.4 Coke reactivity test

The prepared cokes from both large and small ovens were then crushed to less than 1

mm and then the 0.6 – 1.0 mm fraction was used to carry out the reactivity test. Coke

specimens were dried at 105ºC overnight and then a 1.4 g specimen was taken for the

reactivity test.

4.4.1 Coke reactivity reactor system

A schematic diagram of the fixed-bed reactor system is provided in Figure 4.5. A

similar system was used by Harris and Smith [55] and Roberts [121] in their work. The

reactor system comprised a fixed bed of sample supported in a quartz reaction tube by a

sintered glass frit. The quartz tube was placed in an electrically heated furnace. The

temperature of the furnace was controlled by a Eurotherm temperature controller. The

reactant gas passed through the sample bed from top to the bottom at a flow rate of

approximately 0.750 L·min-1. A series of Brook mass-flow controllers controlled the

flow rate of the gas. The design of the reactor allows for a thermocouple to measure the

temperature in the centre of the sample bed. Carbon monoxide was the gas product of

the reaction. The concentration of carbon monoxide in the exhaust gas was measured

continuously using a non-dispersive infrared analyser (Horiba Model PIR 2000), and

recorded by a computer system. The temperature of the sample was also recorded. The

fixed-bed reactor system also allows the activation energy to be measured for every

experiment. In order to do this the power of the furnace was switched off and the

reaction rate was measured as the temperature decreased. The weight loss of the sample

was insignificant during this period (less than 1%). The repeatability error of the

experiments was less than 10%. The reaction rate was calculated using the

concentration of CO measured by the infrared analyser (see Section 4.4.2).

Coke gasification with carbon dioxide was performed in 100% CO2 anaerobic grade

(99.95%). Before entering into the reactor, the reactant gas (carbon dioxide) was passed

through an oxygen trap and a Drierite column to remove traces of oxygen and moisture,

respectively. The reaction temperature was selected on the basis of the reactivity of the

74


coke, so that the percentage of carbon monoxide in the outlet gas did not exceed 1%.

Low concentrations of carbon monoxide had negligible inhibition effect on the reaction

rate [55,65]. For these samples the temperature chosen ranged between 873ºC and

930ºC for the most reactive and the least reactive cokes, respectively. An example of

typical data obtained from the experiment, in which the reaction rate and activation

energy were determined, are presented in Figure 4.6.

Figure 4.5 A schematic diagram of the fixed-bed reactor system.

75


0

5

10

15

20

0 2 4 6 8 10 12 14 16 18

Carbon conversion (%)

App

aren

t rat

e (*

10-6

gg-1

s-1)

Reactor cooling

(a)

-13.0

-12.5

-12.0

-11.5

-11.0

-10.5

0.00083 0.00085 0.00087 0.00089 0.00091

1/T (K-1)

ln (R

ate)

(gg

-1s-1

)

(b)

Figure 4.6 Typical results from a FBR experiment; (a) Reaction rate versus carbon conversion and the region were the furnace was cooling down and (b) Arrhenius plot from the region were the furnace was cooling down.

76


4.4.2 Reaction rate calculation

The apparent reaction rate (a) was calculated according to Equation 4.1, where w is the

mass of sample remaining at time t.

��

��

dtdw

w1

a (g g-1 s-1) Equation 4.1

This method for calculating the mass loss during carbon dioxide gasification was based

on the concentration of the carbon-containing gas product (carbon monoxide) and the

gas flow rate. Carbon monoxide production (nCO) at a given instant was determined

according to the Equation 4.2:

RTF]CO[Pn CO

� (mol s-1) Equation 4.2

where P (atm) is the pressure of the gas in the reactor, [CO] is the concentration of

carbon monoxide at this instant, F (L s-1) is the measured gas flow rate, R (J K-1 mol-1)

is the universal gas constant and T (K) is the temperature of the gas. Using the carbon

monoxide production values from the IR analyser, the rate of carbon loss (r) at any

instant can be calculated using Equation 4.3:

212.01n

r CO � (g s-1) Equation 4.3

The sample mass loss (�m) to this time was given by Equation 4.4:

)tr(m t

0�� (g) Equation 4.4

Equation 4.1 can be rewritten as a function of the weight loss of the sample and rate of

carbon loss. Therefore, the apparent rate at any instant can be calculated by Equation

4.5:

77


mmr

0a �� (g g-1 s-1) Equation 4.5

where m0 is the initial weight of the sample. The apparent reaction rate is expressed in

grams of carbon reacted per gram of carbon remaining per second. In the calculation the

sample mass was ash-free.

4.4.3 Activation energy measurement

The activation energies were measured to provide information about reaction conditions

and to normalise the reaction rates of the cokes during the experiment to a selected

temperature. The reaction rate could be expressed as an Arrhenius type equation:

��

��

RTE

expA aa Equation 4.6

where Ea (kJ mol-1) is the activation energy, A (g g-1 s-1) is the pre-exponential factor, R

(J K-1 mol-1) is the universal gas constant and T (K) is the coke bed temperature. Taking

logarithms on both sides, Equation 4.6 can be rewritten as:

T1

RE

Alnln Aa �� Equation 4.7

The apparent activation energy was calculated from the slope of the straight line

obtained by plotting lna against 1/T (Figure 4.7b). The intercept of the line at 1/T = 0

(ln A) gives us the pre-exponential factor.

The reactivity of the cokes at a fixed temperature was calculated using Equation 4.8:

��

��

� �

��

��

� �

�

1

a

2

a

12

RTE

exp

RTE

exp Equation 4.8

78


where 2 is the reaction rate at 900ºC, 1 is the measured reaction rate, T2 is the selected

temperature (900ºC) and T1 is the sample temperature during the experiment. Equation

4.8 can be rewritten as:

��

��

��

��

21

a12 T

1T1

RE

exp Equation 4.9

4.5 Coke samples characterisation

4.5.1 Optical microscopy

Sample preparation

The samples were prepared as polished particulate blocks. Approximately 0.5 - 1.0 g

representative sample was placed into a silicone rubber mould (25mm × 25 mm × 10

mm deep). The Epoxy Resin LC - 191 was mixed with Hardner HY – 956 in a ratio 1:4

at 50ºC. A small amount of the mixture was poured into the mould to form a thick paste

consistency (about 3 mm deep) and then a 400 kPa pressure was applied for 16 hours.

The remaining volume of mould was filled with resin, labelled and again placed under

pressure to set. The block was polished in seven stages. In the first four stages was used

Struers Planopol 2/3 grinder and silicone carbide paper of different qualities P240,

P400, P800 and P1200. Struers Microcloth with 1 micron Linde C alumina, 0.05 micron

Linde B alumina and 0.04 micron Struers OPS colloidal silica on a Struers OP Chem

cloth were used for the last three stages.

79


Optical microscopy examination

A. Coal rank

Coal rank, expressed by its Maximum Reflectance (Rmax %), was measured with

reflected light microscope-photometer (Leitz-Orthoplan) – in a green light (546nm) on

Vitrinite (maceral Telocollinite). In this technique the intensity of the light reflected

from the polished surface of coal under examination is compared to the intensity of the

light reflected from the standard of known reflectance. Since coals of higher rank are

distinctly anisotropic its maximum and minimum reflectance can be determined in

polarized light. To measure the Maximum Reflectance the stage of the microscope was

rotated 360 degrees and maximum value found was recorded.

B. Coal maceral analysis

Maceral analyses of coals and their fractions as well as coke texture analysis were

performed using a Zeiss Universal optical microscope. The maceral samples were

analysed in a reflected plane polarised light using a 40x oil objective (with total

magnification of 500x). The proportions of each maceral were determined by a point

counting procedure. The stage of the microscope allowed advancing the specimen by

equal steps. Each component falling under the cross fitted in the eye piece of the

microscope was determined and the counts in each category were registered by an

automatic Swift point counter. The sample content was expressed in % volume and

based on minimum 500-point counts. The analysis was performed according to

Australian Standard (AS2856) based on ICCP guidelines and ISO7404.

C. Coke textural analysis

For the analysis of cokes was used an accessory full wave plate to achieve optimum

optical effect to discriminate between its textural components (the plate is placed

between the specimen and analyser to impart the interference colours generated during

the rotation of analyser). Figure 4.7 shows photomicrographs of different types of

microtexture in coke.

80


F

(a)

C

F A

I

(b)

Figure 4.7 Different type of microtexture shown by (a) coke C and (b) coke F; where FA is flow-like anisotropy, C is coarse mosaic, F is fine mosaic and I is isotropic.

81


4.5.2 Surface area

Micropore surface area of the raw and reacted cokes was measured using nitrogen and

carbon dioxide adsorption techniques. Nitrogen was adsorbed on the surface of the coke

particle at 77 K and carbon dioxide was adsorbed at 273 K. The micropore range

includes slit-shaped pores with size less than 0.6-0.7 nm and larger pores

(supermicropores) with size between 0.6-0.7 nm and 1.5-1.6 nm [80]. Only carbon

dioxide can penetrate the slit-shape micropores. Nitrogen at 77K has lower kinetic

energy than carbon dioxide at 273K; therefore the diffusion of nitrogen molecules into

the narrow microporosity is limited [122]. Nitrogen measures the surface of the

supermicropores and the mesopores with size between 1.5-150 nm [123]. This means

that nitrogen and carbon dioxide supplement each other and can provide more complete

information about the microporous structure.

A Micromeritics ASAP 2400 surface area analyser was used for these measurements.

Carbon dioxide and nitrogen were 99.995% purity. Prior to analysis, samples were

vacuum degassed, at 300°C, to an ultimate vacuum of less than 10 Pa. The BET

(Brunauer-Emmett-Teller) and Dubinin-Radushkevich equations were used to

determine the total surface area of the sample from nitrogen and carbon dioxide

isotherms.

4.5.3 Carbon structure

The raw and reacted coke samples were analysed using x-ray diffraction (XRD). The

samples before analysis were finely ground using a pestle mortar. The XRD analyses on

the cokes were run on a Philips PW1050 goniometer using CoK� radiation at 45kV.

Step scans were undertaken from 3 – 90º 2�, with a step interval of 0.04º 2� and 10

seconds count time per step.

Carbon crystallite size (Lc and La) was measured using the X-ray diffraction patterns.

The details of the carbon crystallite structure have been discussed in section 2.2.1. The

American Standard Test Method D 5187-91 was the procedure used to determine Lc. A

82


similar procedure was used to determine La. This technique allows measuring the

carbon crystallite height - Lc (002 band; 2� ~ 30º) and length - La (100 band; 2� ~ 51º).

The crystallite size was calculated using the Scherrer equation [40], which is:

Bcos�K�La/c � Equation 4.10

where, K is a constant depending on the reflection plane (K = 0.89 for the 002 band, and

K = 1.84 for the 100 band [41]); � is the wavelength of the incident X-rays (for cobalt

K� radiation, � = 1.7889�); B is the width of the corresponding band at half maximum

intensity and � is the peak position.

4.5.4 Qualitative and quantitative analysis of mineral phases

The mineral phases present in the raw and reacted cokes were identified using X-Ray

diffraction analysis. The carbon was removed from the mineral matter with minimal

alteration of the mineral species using radio-frequency oxygen plasma ashing at low

temperature (120ºC) – LTA (low temperature ashing) [124]. The XRD analysis of the

cokes was carried out on a Philips PW1050 goniometer using CoK� radiation at 45kV

and 30mA, with step scans from 3 – 90º 2�, a step interval of 0.04º 2� and a 10sec count

time per step.

Mineral phases identification was performed using Bruker Eva search/match software

(Figure 4.8). SIROQUANT™ [125], a personal computer quantitative X-ray diffraction

analysis software package, was used to quantify the minerals in the ash. The software

was developed by CSIRO and uses the full-profile Rietveld method of refining the

shape of a calculated XRD pattern against the profile of a measured pattern. In order to

calculate a synthetic XRD pattern for a mineral, SIROQUANT use the data from the

“hkl” file that contains the XRD pattern of each mineral. The total calculated pattern is

the sum of the calculated patterns of the individual phases. SIROQUANT overlays the

measured XRD pattern with the calculated pattern to calculate a synthesized pattern.

SIROQUANT also calculates a difference pattern, which displays the difference in

intensity between the measured and calculated patterns at every point. The fit of a

83


calculated pattern to the measured (observed) data is a figure of merit, Chi-squared (�²).

A value below 3.0 indicates a well refined pattern. Classical Rietveld procedures can

only be applied to crystalline phases. A procedure has been developed in SIROQUANT

that allows the determination of poorly crystalline and amorphous phases such as some

clays (amorphous melt in cokes) to be quantified through the use of observed (hkl) files.

A typical plot from SIROQUANT is shown in Figure 4.9. The mineral phase

quantification error is typically less than 0.3% in the LTA samples, which translates to

less than 0.03% on a coke basis.

Coke F

00-037-0477 (*) - Troilite-2H - FeS01-089-0552 (C) - Rutile, syn - Ti0.928O200-046-1045 (*) - Quartz, syn - SiO200-029-0725 (I) - Pyrrhotite-6T - Fe1-xS00-008-0464 (I) - Oldhamite, syn - CaS00-015-0776 (I) - Mullite, syn - Al6Si2O1301-073-1959 (C) - Magnesium Aluminum Oxide - MgAl2O400-036-0427 (*) - Jarosite, hydronian syn - (K,H3O)Fe3(SO4)2(OH)6

00-006-0696 (*) - Iron, syn - Fe01-070-1793 (C) - Iron Phosphate - FePO400-015-0876 (*) - Fluorapatite, syn - Ca5(PO4)3F01-087-0700 (C) - Diopside, syn - CaMg.74Fe.26(Si2O6)00-041-0224 (I) - Bassanite, syn - CaSO4·0.5H2O00-021-1272 (*) - Anatase, syn - TiO200-035-0592 (*) - Akermanite, syn - Ca2MgSi2O7File: lb3620.raw

Inte

nsity

(cou

nts)

0

200

400

600

800

1000

1200

1400

2 Theta

5 10 20 30 40 50 60 70 80 90

Figure 4.8 Identification of the mineral phases present in coke F using Bruker Eva search/match software.

84


Figure 4.9 A typical SIROQUANT plot, which shows the observed and calculated XRD patterns and the difference between them.

4.5.5 Visual analysis of mineral matter using FESEM

The distribution and association of the minerals in the raw and reacted cokes was

observed using scanning electron microscopy technique on polished blocks. The surface

of the specimens was made electrically conductive by the application of a thin coat of

carbon. The coating was few nanometres thick and did not interfere with the structure of

the specimen.

The field emission scanning electron microscope (FESEM) is essentially a traditional

scanning electron microscope, fitted with a field emission electron source, which

85


provides a very fine, but highly intense beam. This allows a higher resolution image

than with conventional SEM’s. The model used in this work was a Hitachi S4500 fitted

with an Oxford Isis energy dispersive x-ray analyser (EDS). This enables a semi-

quantitative microchemical analysis of materials in the samples. The EDS can produce

microchemical analysis of a particular area on a sample, or can yield image maps, which

are based on a pre-defined set of elements. The depth of analysis is limited to the

interaction volume of the electron beam (typically ~ 2-3 m). Typical SEM image and

EDS analysis are presented in Figure 4.10.

Figure 4.10 A typical SEM image (top) and EDS analysis (bottom).

86


87

4.5.6 Analysis of mineral matter in coal using QEMSCAN

QEMSCANTM [126] is an analytical technique which characterizes both coal and

associated mineral matter providing quantitative mineralogical, textural and chemical

data on a particle-by-particle basis. QEMSCAN creates digital images using both the

backscattered electrons (BSE) and energy dispersive X-ray signals (EDS) from a

scanning electron microscope (SEM). Each pixel of the digital images corresponds to a

mineral species or phase in a region under the electron beam. The identification of the

minerals is accomplished by comparing the collected X-ray spectrum with that of the

minerals, existent in a database, defined by their chemical composition.

CHAPTER 5 – Kinetics of the Coke – Carbon dioxide reaction

Previous studies have identified a number of factors that influence coke reactivity such

as coke microtexture (defined by optical microscopy – see section 2.2), carbon structure

(carbon crystallite size Lc and La – see section 2.2), microporosity and mineral matter.

Also, the rank and maceral composition of the parent coal have been reported to

determine coke properties and consequently coke reactivity. Due to this relationship

coal rank [13,20,67,90] and, to a lesser degree, maceral composition [78] have been

frequently used to predict coke reactivity. The chemistry of coke ash has been also used

to predict coke reactivity [13,20,25,67,90].

In order to determine the influence of the above factors on the reactivity of the cokes

investigated in this project, the measurement of reaction kinetics and a thorough

characterisation of the cokes and parent coals were required. The reaction rate was

measured under chemically control conditions where the reaction is not limited by either

pore diffusion or external mass transfer (see section 2.3.1). Also, the reaction inhibition

by the reaction gas product (CO) was minimised (see section 4.4.1).

In this chapter the reaction kinetics of nine metallurgical cokes with carbon dioxide is

reported and the relationship between coke reactivity and both coke and parent coal

properties is discussed. Also, changes in the coke properties during gasification are

investigated.

88

Chapter 5 – Kinetics of the Coke – Carbon dioxide reaction

5.1 Apparent reactivity experiments: Coke – Carbon dioxide reactions

Figure 5.1 shows the apparent reaction rates with CO2 (100%) of the nine cokes as a

function of carbon conversion. The apparent reaction rate was normalised to 900ºC

using the activation energy measured at the end of each experiment (see section 4.4.3).

The apparent reaction rate is reported as grams of reacted carbon per grams of

remaining carbon per second. A gradual increase of the apparent reaction rate with

increasing carbon conversion was observed at the initial stages of the reaction. The

reaction rate did not change significantly after 5 – 10 % carbon conversion for most

cokes (cokes A, D, E, H and I) whereas the reaction rate of the most reactive cokes,

namely coke F and coke C, still increased even after 15% carbon conversion.

The activation energy was derived from an Arrhenius plot of data collected during the

sample cooling at the end of the reactivity test. The Arrhenius plots are shown in Figure

5.2. The corresponding activation energies are presented in Table 5.1. All the activation

energies are in the range between 222 and 266 kJ mol-1.

0

10

20

30

40

50

60

0 5 10 15 20


App

aren

t rat

e (*

10-6

gg-1

s-1)

Coke F

Coke C

Coke D

Coke B

Coke G

Coke A

Coke E

Coke I

Coke H

Figure 5.1 The apparent reaction rate at 900ºC versus carbon conversion (100%

CO2).

89


-14

-13

-12

-11

-10

0.00080 0.00085 0.00090 0.00095 0.00100

1/T (K-1)

ln (R

ate)

(gg-1

s-1)

Coke F

Coke C

Coke D

Coke B

Coke G

Coke A

Coke E

Coke I

Coke H

Figure 5.2 Arrhenius plots obtained using data recorded during furnace cooling.

Table 5.1 The measured apparent activation energy of the reaction with carbon dioxide of the cokes and their measured temperature ranges.

Coke Activation energy (Ea) (kJ mol-1)

Burn-off (%)

Temperature range (K)

A 247 16.3 1203 – 1128 B 266 16.8 1193 – 1119 C 245 17.5 1166 – 1079 D 252 18.1 1189 – 1128 E 256 15.3 1191 – 1118 F 222 17.5 1143 – 1043 G 258 18.4 1191 – 1116 H 239 17.6 1204 – 1137 I 229 17.3 1203 – 1128

90


Verification of regime I conditions

It was essential to establish that the reactivity measurements were made under

conditions of chemical rate control, free of any physical limitations due to gas pore

diffusion and mass transfer. Under chemical control conditions, the reaction rate should

not be affected by sample particle size and gas flow rate. Previous studies by Harris and

Smith [55,59] showed that the reaction rate of coke with carbon dioxide at 800ºC and

890ºC respectively was not affected by coke particle size between 0.2 – 2.0 mm and the

flow rate of the reactant gas (100% CO2) between 500 – 1000 ml·min-1 at atmospheric

pressure. For this work the effect of the particle size and gas flow rate through the

reactor on the reaction rate was determined.

Table 5.2 lists the apparent reaction rates of cokes F and C at different gas flow rate and

particle size. Varying the size of the particles from 0.212 mm to 1.0 mm did not change

the reaction rate significantly. This means the partial pressure of the reactant gas could

be considered uniform through the coke particle. Also, changing the gas flow rate

through the sample bed did not change significantly the rate of reaction. It can be

concluded that the product gas (CO) was removed rapidly from the reaction site and did

not inhibit the reaction. Moreover, the relative density of all reacted samples showed a

linear relationship with burn-off (Figure 5.3), which means the particle size did not

change during reactivity test. This is a typical characteristic of chemically controlled

reactions (see section 2.3.1).

The activation energy values determined for the nine cokes were in the range 222–266

kJ mol-1 (Table 5.1). The magnitude of these numbers was consistent with the data

available in the literature for chemically controlled reaction rates: the activation

energies of the reaction of petroleum and metallurgical cokes with carbon dioxide

reported by Harris and Smith [59] and Pang et al. [58] were in the range 216-239 kJ

mol-1 and 215-240 kJ mol-1, respectively. The reactions were carried out at 800ºC and

900ºC, respectively; the range of temperature for the activation energy measurement

was not specified for these experiments. Laurendeau [56] reported the activation energy

of the reaction of coke with carbon dioxide measured by Blake in a previous study. In

this case the activation energy was 239 kJ mol-1 in the temperature range 850-900ºC.

91


Also, the activation energy measured by Kawakami et al. [57] for the reaction of coke

with carbon dioxide (regime I conditions) was 200 kJ mol-1, where the reaction

temperature was between 600 and 900ºC.

Table 5.2 The effect of particle size and flow rate on the reaction rate.

Coke C Coke F Particle size (mm)

Flow rate (ml/min) Rate*10-6

(g g-1 s-1) Rate*10-6 (g g-1 s-1)

0.600 - 1.000 750 24.2 36.4 0.425 - 0.600 750 25.8 36.4 0.212 - 0.425 750 22.6 35.3 0.600 - 1.000 850 25.7 35.0

0.6

0.7

0.8

0.9

1.0

0 5 10 15 20 25 30 35

Conversion (%)

Rel

ativ

e de

nsity

ABCDEFGHI

Theoretical line

Figure 5.3 Relative density as a function of burn-off; the theoretical line is the line expected if Regime I conditions were followed.

92


5.2 Coke surface area and intrinsic reactivity

he surface areas of the raw and corresponding reacted cokes (approximately 15%

n increase in surface area, measured by both nitrogen and carbon dioxide, was

he greater surface area measured by nitrogen than carbon dioxide of the reacted cokes

mesopores (as measured by N2 surface area) of the reacted cokes F and C.

T

carbon conversion) were measured using N2 and CO2 as adsorbate gases. Carbon

dioxide measures the surface area of the micropores (0.4 – 1.6 nm) and nitrogen

measures the surface area of larger micropores (supermicropores) with sizes between

0.6-0.7 nm and 1.5-1.6 nm and mesopores with sizes between 1.5-150 nm [123]. Figure

5.4 shows the N2 and CO2 surface areas of the raw and reacted cokes. Carbon dioxide

surface area was considerably greater than that measured using nitrogen, especially for

cokes F, C and H, and varied to a great degree from 2.3 to 64.5 m2g-1 whereas nitrogen

adsorption in these cokes was very low, always less than 0.6 m2g-1. Greater surface

measured by CO2 than N2 indicates that the micropore structure of the raw cokes

consisted mainly of narrow (slit-shaped) microporosity that was inaccessible to nitrogen

at 77K.

A

observed after reaction indicating an increase in micro and mesoporosity (figure 5.4).

The increase in nitrogen surface area of the reacted cokes was dramatic and for most of

the cokes it was greater than the carbon dioxide surface area after reaction. However,

the most reactive cokes F and C still had greater carbon dioxide surface area than

nitrogen surface area. This suggests that in these cokes the narrow microporosity was

still significant. Narrow microporosity could be present in the rest of the cokes but it

could not be differentiated because nitrogen surface area was greater than carbon

dioxide surface area.

T

indicates that as gasification proceeds microporosity is enlarged and then followed by

coalescence, increasing both the supermicroporosity and mesoporosity. Also,

gasification did not only enlarge the existing micropores but also generated new

microporosity, which was probably developed at the early stages [43]. This continual

regeneration of new, accessible microporosity explains the significantly greater surface

area of micropores (as measured by CO2 surface area) than both supermicropores and

93


0

20

40

60

80

100

N2 s

urfa

ce a

rea

(m2 g-1

)

Raw 0.24 0.32 0.56 0.44 0.31 0.39 0.49 0.56 0.48

Reacted 28.5 63.9 87.9 79.6 58.9 96.9 71.7 77.5 65.1

A B C D E F G H I

(a)

0

20

40

60

80

100

120

140

CO

2 su

rfac

e ar

ea (m

2 g-1)

Raw 3.4 2.3 13.4 6.0 8.8 64.5 8.5 12.8 4.5

Reacted 21.7 54.5 96.6 64.2 45.5 131.9 56.4 31.3 23.9

A B C D E F G H I

(b)

Figure 5.4 Total surface area of raw and reacted cokes (approximately 15%

burnout) (a) measured using N2 adsorption (b) measured using CO2 adsorption.

94


C

mesoporosity

ores, measured by mercury porosimetry for different type of carbons, including

ences in the apparent reactivity

f the cokes. Figure 5.5a shows that the initial apparent reaction rate was poorly related

oorly related to the reaction rate (Figure 5.5b). These relationships suggest that

losed microporosity and mesoporosity could have a role in microporosity and

formation. Turkdogan et al. [43] concluded that the volume of the closed

p

metallurgical coke, is approximately 25-50%. Although mercury porosimetry does not

measure the volume of micropores, they may be present in the cokes. During

gasification, any closed microporosity and mesoporosity would open gradually and

contribute to the total microporosity and mesoporosity.

The initial and final apparent rates should increase with increasing surface area if

surface area was a major factor responsible for the differ

o

to nitrogen surface area. Also, the relationship between carbon dioxide surface area and

initial apparent reactivity was poor. It is concluded that the surface area of accessible

microporosity and mesoporosity plays little role in determining reaction rate at initial

stages for the cokes used in this study reactivity.

At later stages of the reaction (15% burnout) the relationship between reaction rate and

carbon dioxide surface area was much improved whereas nitrogen surface area was still

p

although surface area of micropores and mesopores increased significantly during

reaction and any closed porosity is believed to become more accessible, the reaction

occurred mostly on the surface of the micropores, and the surface of the mesopores

contributed less to total reactivity.

95


F

C

HIGD

EB

A0

2

4

6

8

10

12

14

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7N2 initial surface area (m2g-1)

Initi

al a

ppar

ent r

ate

(*10

-6 g

g-1s-1

)

BG

IE

D

AH

C

F

0

2

4

6

8

10

12

14

0 20 40 60 8CO2 initial surface area (m2g-1)

Initi

al a

ppar

ent r

ate

(*10

-6 g

g-1s-1

)

0

(a)

ABE

I HG

D

C

F

0

10

20

30

40

50

60

0 20 40 60 80 100 120N2 final surface area (m2s-1)

Fina

l app

aren

t rat

e (*

10-6

gg-1

s-1)

F

C

D

EHA

I

GB

r2 = 0.92

0

10

20

30

40

50

60

0 50 100 150CO 2 final surface area (m2g-1)

Fina

l app

aren

t rat

e (*

10-6

gg-1

s-1)

(b)

igure 5.5 Dependence of apparent rates on total surface area measured using

both N2 and CO2 (a) initial apparent rate and (b) final apparent rate.

F

96


In order to remove the physical effect of surface area on the apparent rate, the intrinsic

te was calculated. The intrinsic rate was determined by dividing the apparent rate by

e surface area. Both carbon dioxide and nitrogen surface areas were used to determine

ra

th

the intrinsic rates at the initial and final (end of experiment) stages of the reaction. Table

5.3 shows the initial and final intrinsic rates and also the ratio (R) between the reaction

rate of the most reactive coke and the reaction rate of the least reactive coke. If the value

of R decreases after converting apparent to intrinsic rates, it suggests that surface area is

an important variable affecting the apparent rate. If R does not change, surface area is

less likely to be important. Normalising the initial apparent rates to both N2 and CO2

surface areas did not reduce significantly the range in reactivity of the cokes; the ratios

of the initial intrinsic rates calculated using both N2 and CO2 surface areas were 8.2 and

6, respectively. The final intrinsic rate determined using N2 surface area still showed

significant difference between them as R was 6.3. The difference between the final

intrinsic rates, calculated using CO2 surface area, was significantly reduced (R was

reduced from 6 to 2.4), which indicates a strong influence of surface area of the

micropores on the reaction rate. However, the difference in reactivity between cokes

could not be fully explained by the differences of the microporosity surface area (see

Chapter 9).

97


Table 5.3 The initial and final intrinsic rates of cokes with carbon dioxide measured using N2 and CO2 surface areas.

Intrinsic rate using N surface area Intrinsic rate using CO surface area

Coke 2(*10-6 g m-2 s-1)

2(*10-6 g m-2 s-1)

al Final Initial Final InitiA 5.19 0.30 0.37 0.44 B 6.26 0.17 0.96 0.22 C 12.09 0.30 0.56 0.31 D 8.35 0.17 0.68 0.24 E 9.63 0.12 0.39 0.18 F 26.90 0.44 0.18 0.36 G 7.05 0.14 0.46 0.21 H 3.27 0.07 0.16 0.19 I 4.60 0.09 0.56 0.29

Ratio 8.2 6.3 6.0 2.4 ‘Ratio’ represents the ratio betw ion rate of the most reactive and the reaction rate of the least reactive coke.

.3 Coal properties – Coal rank and Maceral composition

revious studies [13,20,67,90] have shown that coke reactivity decreases with

e formation of

oke microtexture [26,29,67,105]; as the coal rank increases the size of the anisotropic

rsion) apparent rates of the nine cokes was poor

igure 5.6a), although the anisotropy size of the cokes increased as the rank of the

een the react coke

5

P

increasing rank of the parent coal. Coal rank has an important role in th

c

microtexture of the coke increases.

The relationship between rank of the parent coal and both the initial and final

(approximately 15 % carbon conve

(F

parent coal increased as shown in section 5.5. The influence of coal rank on coke

properties and consequently on coke reactivity is believed to be overshadowed by other

more significant factors for these cokes.

98


0

10

20

30

40

50

02468

10

601214

ate

0.9 1.1 1.3 1.5 1.7Coal rank (R0, %)

Initi

al a

ppar

ent r

(*10

-6 g

g-1

s-1)

0.9 1.1 1.3 1.5 1.7Coal rank (R0, %)

Fina

l app

aren

t rat

e (*

10-6

gg-1

s-1)

(a)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.9 1.1 1.3 1.5 1.7Coal rank (R0, %)

Initi

al in

trin

sic

rate

(*

10-6

gm

-2s-1

)

0.0

0.1

0.2

0.3

0.4

0.5

0.9 1.1 1.3 1.5 1.7Coal rank (R0, %)

Fina

l int

rins

ic r

ate

(*10

-6 g

m-2

s-1)

(b)

0

5

10

15

20

25

30

0.9 1.1 1.3 1.5 1.7Coal rank (R0, %)

Initi

al in

trin

sic

rate

(*

10-6

gm

-2s-1

)

0.0

0.1

0.2

0.3

0.4

0.5

0.9 1.1 1.3 1.5 1.7Coal rank (R0, %)

Fina

l int

rins

ic r

ate

(*10

-6 g

m-2

s-1)

(c)

Figure 5.6 Relationship between the rank of parent coal (expressed as mean maximum vitrinite reflectance in oil) and (a) initial and final apparent rates, (b) initial and final in sic rates calculated using CO surface trin 2area and (c) initial and final intrinsic rates calculated using N2 surface area.

99


Although the

investigated

t in the parent coals was identified as a factor that influences the

action rate. Inertinite that does not fuse during carbonisation is the source of isotropic

ave found that both the rank and maceral composition of the parent

oals influence coke reactivity but their influence on coke reactivity was not evident for

influence of the rank of parent coal on the apparent reaction rate has been

in previous studies, the influence of the rank of parent coal on the intrinsic

reaction rate has not been previously reported. In this project it was investigated in order

to determine whether the poor relationship between the rank of the parent coal and

reaction rate was because of the differences in surface area. Figure 5.6b and 5.6c shows

the relationships between both initial and final intrinsic rates, measured using carbon

dioxide and nitrogen, and the coal rank. The intrinsic rates also showed a poor trend

with coal rank.

Inertinite conten

re

microtexture in the coke, which is the most reactive microtextural component [18,23].

Petrographic examinations of the reacted cokes showed that the non-fused inertinite was

the most reactive microtextural component (see section 5.4). Previous studies [78]

found a good relationship between the inertinite content of the parent coal and the

reaction rate; as the inertinite levels increased the reaction rate increased. However,

Figure 5.7 shows that no relationship was identified between both apparent and intrinsic

rates and inertinite content in the parent coal for either early or later stages of the

reaction. The influence of non-fused inertinite on the reaction rate is further discussed in

the next chapter.

Previous studies h

c

the series of cokes used in this study. Therefore, an in-depth study was required to

establish their importance on coke properties and coke reactivity. In the next chapter the

results of a study on maceral enriched fractions prepared from selected coals of different

rank is reported. This study was performed in order to establish the effect of rank and

maceral composition of the parent coals on coke properties and determine their relative

influence on coke reactivity.

100


02468

101214

10 20 30 40 50 60 70Inertinite (%)

Initi

al a

ppar

ent r

ate

(*10

-6 g

g-1

s-1)

0

10

20

30

40

50

60

10 20 30 40 50 60 70Inertinite (%)

Fina

l app

aren

t rat

e (*

10-6

gg-1

s-1)

(a)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

10 20 30 40 50 60 70Inertinite (%)

Initi

al in

trin

sic

rate

(*

10-6

gm

-2s-1

)

0.0

0.1

0.2

0.3

0.4

0.5

10 20 30 40 50 60 70Inertinite (%)

Fina

l int

rins

ic r

ate

(*10

-6 g

m-2

s-1)

(b)

0

5

10

15

20

25

30

10 20 30 40 50 60 70Ine rtinite (%)

Initi

al in

trin

sic

rate

(*

10-6

gm

-2s-1

)

0.0

0.1

0.2

0.3

0.4

0.5

10 20 30 40 50 60 70Inertinite (%)

Fina

l int

rins

ic r

ate

(*10

-6 g

m-2

s-1)

(c)

Figure 5.7 Relationship between inertinite content in the parent coal and (a) initiaand final apparent rates, (b) initial and final intrinsic rates calculated using CO2 surface area and (c) initial and final intrinsic rates calculated using N2 surface area.

l

101


5.4 Coke properties – Coke microtexture

al analysis of the nine r

ealed that the size of anisotropic

coal increased. Figure 5.8 shows

hotomicrographs of three of the cokes (coke B, coke D and coke F) which were

onversion) showed that isotropic microtexture was selectively consumed during

asification. Figure 5.9 shows photomicrographs of coke F raw and reacted (15% and

ble 5.4 The content of isotropic microtexture in the raw and 75% burnout cokes.

ke I

Microtextur aw cokes rev

microtexture increased as the rank of the parent

p

prepared from coals of different rank (R0max is 1.00%, 1.18% and 1.27%, respectively).

Coke B was characterized by fine mosaic anisotropy. Medium mosaic anisotropy was

typical for coke D whereas coarse mosaic anisotropy was formed in coke F (the

microtexture classification used was according to that presented by Marsh [31] in his

study). This analysis confirms the influence of rank of the parent coal on size of the

anisotropic microtexture formed in coke observed in previous studies (see section

2.5.1).

Microtextural examination performed on the raw and reacted cokes (15% and 75%

carbon c

g

75% burnout). Isotropic microtexture in the 15% reacted coke showed preferential

dissolution. In the 75% burnout coke isotropic microtexture presence was very rare.

Petrographic analyses also indicate a very low content of isotropic microtexture in the

75% burnout cokes (Table 5.4). These findings are in accordance with those reported in

previous studies (see section 2.4.1).

Ta

Coke A

Coke B

Coke C

Coke D

Coke E

Coke F

Coke G

Coke H

Co

(Vol %, mmf) Raw 13.9 27.2 36.2 29.1 29.3 25.6 32.6 21.2 17.7 75% burnout N/A 2.7 3.2 1.6 N/A 1.1 3.7 N/A 0.0 mmf – mineral matter free

102


(a)

(b)

(c) Figure 5.8 Microtexture of cokes made form coals of different rank (a) Coke B

(R0max–1.00%), (b) Coke D (R0max–1.18%) and (c) Coke F (R0max–1.27%); where F is Fine Mosaic, M is Medium Mosaic and C is Coarse Mosaic.

F

M

C

103


(a)

I

I

I

I

I

I

(b)

(c)

Figure 5.9 Microtexture of coke F during gasification (a) raw coke, (b) 15% burnout coke and (c) 75% burnout coke; where I indicates isotropic microtexture.

104


5.5 Coke properties – Carbon structure

Mean carbon crystallite size (L d La) of the raw and reacted cokes were calculated

using the procedure presented in section 4.5.3. Crystallite height (Lc) and width (La) of

the raw cokes varied from 1.46 to 1.67 nm and 3.60 to 4.18 nm, respectively (Table

5.5).

able 5.5 Crystallite size (Lc and La) of the raw cokes.

Coke A

Coke B

Coke C

Coke D

Coke E

Coke F

Coke G

Coke H

Coke I

c an

T

(n m)Lc 1.53 1.51 1.51 1.56 1.52 1.46 1.51 1.59 1.67La 4.02 3.73 3.82 3.87 3.69 4.18 3.82 3.77 3.60

Although the crystallite size (Lc and La) of the coals has been reported to increase as the

103,106], the crystallite size of the corresponding chars in the study

performed by Lu et al. [106] was not significantly influenced by the coal rank. The

influence of coal rank on the crystallite size of cokes has not been reported so far.

Therefore, the effect of rank of parent coal on crystallite size of the coke was

investigated. In this work the influence of rank of the parent coal on carbon crystallite

size (Lc and La) of the cokes was not significant, since a poor relationship was observed

between both Lc and La of the cokes with rank of the parent coals (Figure 5.10).

However, the maceral composition of the parent coals investigated here varied widely.

In order to separately investigate the influence of rank and maceral composition on coke

properties and reactivity, maceral-enriched fractions of coals of different rank were

repared (see next chapter).

coal rank increased [

p

105


Feng et al. [73] observed a decrease of the crystallite size (Lc and La) of a char, made

om a semi-anthracite, during gasification – La decreased from the beginning of fr

reaction – whereas Lc showed a significant decrease after approximately 60% carbon

conversion. In this project the effect of gasification on crystallite size was investigated

on cokes reacted with carbon dioxide to 15% and 75% burnout.

1.4

1.5

1.6

1.7

0.9 1.13.4

3.6

3.8

4

1.3

R0 max (%)

Lm

)

1.5 1.7

c (n

.0

4.2

4.4

1 1.3 5

R0 max (%)

)

igure 5.10 L and L of the raw cokes versus rank of parent coal (mean

hours had no significant effect on either Lc or La. Since the

okes were exposed to 1050ºC during carbonisation, this lack of change at 900ºC is not

unreasonable.

0.9 .1 1. 1.7

La (

nm

F c amaximum reflectance of vitrinite).

Coke gasification to high levels (75% carbon conversion) requires keeping the cokes for

at least 15 hours at 900ºC under a CO2 flow. In order to establish the effect of

gasification on carbon crystallite size the effect of temperature alone on crystallite size

had to be determined. Three cokes (A, C and F) were subjected to a thermal treatment

(annealing) for at least 15 hours at 900ºC under nitrogen. The Lc and La values of the

raw and the corresponding thermal treated cokes are presented in Table 5.6. Exposure of

these cokes to 900ºC for 15

c

106


Table 5.6 The Lc and La of the raw cokes and cokes annealed at 900ºC for 15 hours.

Coke Lc (nm) La (nm) Raw coke Annealed coke Raw coke Annealed coke

A 1.52 1.50 4.02 4.07 C 1.51 1.54 3.82 3.87 F 1.46 1.52 4.18 4.12

Figure 5.11a shows that crystallite height (Lc) changed during gasification. Lc showed a

slight increase at 15% carbon conversion but increased significantly at 75% burn-off,

and this pattern was observed for all of the cokes. Because temperature alone had no

effect on crystallite sizes, it is appropriate to conclude that the increase of crystallite

height is solely due to gasification.

Kashiwaya and Ishii [41] and Feng et al. [73] have found that at the early stages of

gasification (below approximately 30-40% conversion) gasification did not affect

ignificantly the Lc; the data from this study are consistent with these observations. At

garding Lc. Feng et al [73] noted a e of Lc at greater carbon

onversion levels, during char gasification with carbon dioxide and also during char

ombustion. However, Lu et al. [127] observed an increase of Lc during char

ombustion. The significant increase of Lc at high carbon conversion levels observed in

t of Lc because of the interference of the mineral matter peaks with the

arbon peak, which introduced errors into the fitting of the theoretical curve for La peak.

s

high carbon conversion levels, no consistent trends were reported by previous studies

significant decreasre

c

c

c

this work could be due to preferential consumption of small crystallites or merging of

neighbouring crystallites [127]. The selective consumption of the small crystallites

would produce an increase of the average crystallite height (Lc). The removal of the

disordered carbon between crystallites could result in a merger of the neighbouring

crystallites, which translates to a real growth in the crystallite height.

Crystallite length (La) showed little significant variation during gasification across the

cokes investigated (Figure 5.11b). The measurement error of La was relatively high

compared to tha

c

107


1.5

1.6

1.7

1.8

9

2.0

L c (n

m)

1.

Raw coke

15% burn-off

1.3

1.4

A B C D E F G H I

75% burn-off

(a)

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

A B C D E F G H I

La (

nm) Raw coke

15% burn-off

75% burn-off

(b)

Figure 5.11 Variation of (a) Lc and (b) La during gasification (the measurement

error bars are shown).

108


Because La did not change significantly during gasification in this work, and previous

studies [41,73] have reported a decrease of La during gasification, further investigations

were conducted in order to determine the importance of La on the reaction rate (see next

chapter).

In order to determine the influence of crystallite height (Lc) on the reaction rate, the

apparent and intrinsic rates at initial and final (approximately 15% burnout) stages were

plotted versus Lc. The influence of La on the reaction rate is not shown since no

significant change of La was observed during gasification (Figure 5.11b).

Increasing ordering of carbon structure leads to a lower reactivity [22]. The initial

apparent rates and the initial intrinsic rates, measured by carbon dioxide, were not

related to crystallite height (Lc) (Figure 5.12). The influence of other factors such as

catalytic mineral matter (see Chapter 8) rshadowed any influence of Lc on the

action rate.

A trend is observed between the Lc of the reacted cokes (approximately 15% burn-off)

and the final apparent rate; the most reactive cokes (F and C) had the lowest Lc and the

least reactive cokes (I and H) had the greatest Lc. The trend is not strong but it suggests

that Lc may affect to some extent the final reaction rate (15% burnout). Also, the trend

between Lc of the 15% burnout cokes and the final intrinsic rate (Figure 5.12b) is weak,

which indicates that Lc did not influence significantly the final reaction rate. A strong

influence of Lc on the reaction rate would show a strong relationship with the final

intrinsic rates (determined using CO2 surface area), since micropore surface area of the

reacted cokes was found to have a strong influence on the reaction rate (see section 5.2).

In the next chapter, a study on carbonised maceral-enriched fractions was conducted in

order to establish the influence of coal properties on crystallite size of carbonised

maceral-enriched fractions, and the influen on their reactivity.

ove

re

ce of crystallite size

109


BA

AE

12

E

DG

C

4

6

8

Initi

al a

ppar

ent r

at

H I

F

2

10

BG D

C

20

30

40

Fina

l app

aren

t rat

e(*

10-6

gg-1

s-1)

01.4 1.5 1.6 1.7

Initial Lc (nm)

e(*

10-6

gg-1

s-1)

H I

F

10

50

01.4 1.5 1.6 1.7 1.8

Final Lc (nm)

(a)

AG

B

D

F

E

I

H

C

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1.4 1.5 1.6 1.7

Initial Lc (nm)

Initi

al in

trin

sic

rate

(*

10-6

gm

-2s-1

)

G IE

DH

BC

F

A

0.0

0.1

0.2

0.3

0.4

0.5

1.4 1.5 1.6 1.7 1.8

Final Lc (nm)

Fina

l int

rins

ic r

ate

(*10

-6 g

m-2

s-1)

(b)

s vs. Lc and (b) initial and final intrinsic rates of the nine cokes, calculated using CO2 surface area, vs. Lc.

Figure 5.12 (a) Initial and final (15% burn-off) apparent rates of the nine coke

110


5.6 Coke properties – Ash composition

oke ash chemistry is believed to be strongly related to coke reactivity [20,25,70]. Total

iron oxide, calcium oxide, potassium oxide and sodium oxide from the ash analysis

were considered as reasonably good predictors of coke reactivity [89] [13,18,25,90].

Figure 5.13 shows the relationship between the initial and final apparent rates and

concentration of total iron, calcium, potassium and sodium in the nine cokes. A trend is

observed between total iron in coke and the initial apparent reaction rate. The

concentration of calcium, potassium and sodium in cokes showed a poor relationship

with initial and final apparent rates.

Iron, calcium, potassium and sodium occur in different mineral phases in cokes. For

instance, iron in the mineral matter in coke urs not only as oxide but also as metallic

iron [50] and as a component of other mineral phases such as pyrrhotite (Fe1-xS) [23]

and calcium ferrite (CaFe2O4) [50]. Some of the minerals identified in the cokes that

contain iron, calcium, potassium and sodium are not catalysts of gasification (see

section 2.4.3) and also those that are catalysts of gasification could have different

activity as catalysts. Therefore, it is more appropriate to identify the catalytic mineral

matter in the coke and then determine the relationship between the catalytic mineral

phases in cokes and reaction rate. The identification of the catalytic mineral phases in

the coke and their relationship with the gasification reaction rate are presented in

Chapters 7 and 8.

C

occ

111


BHA

E GDI

Cr2 = 0.50

0

2

4

6

8

0.0 0.5 1.0 1.5

Fe2O3 (% )

Initi

al a

ppar

en(*

10-6

gg-1

s

F10

t rat

-1)

12

e

A EB

HG

D

I

C

0

10

20

30

0.0 0.5 1.0 1.5

Fe2O 3 (%)

Fina

l app

aren

t(*

10-6

gg-1

s

F40

50

rat

e -1

)

H IA

BE

D G

0

2

4

0.0 0.1 0.2 0.3 0.4

CaO (% )

Initi

al a

(*10

-6

H

C6

ppa gg-1

F

8

10

12

rent

rat

e s-1

)

IA EB G

D

0

10

20

0.0 0.1 0.2 0.3 0.4CaO (% )

Fina

l ap

(*10

-6

C

F

30

40

pare

nt r

at g

g-1s-1

)

50

e

AGB

E I H

D

C

F

0

10

20

30

40

50

Fina

l app

aren

t rat

e (*

10-6

gg-1

s-1)

AB

G EI

D

H

C

F

0

2

4

6

8

10

12

0.0

Initi

al a

ppar

ent r

ate

(*10

-6 g

g-1s-1

)

0.1 0.2

K2O (% )

0.0 0.1 0.2

K2O (% )

EB I

D

A

GH

C

F

0

2

4

6

8

10

12

0.0 0.1 0.2

Na2O (% )

Initi

al a

ppar

ent r

ate

(*10

-6 g

g-1s-1

)

AE IB

DG

H

C

F

0

10

20

30

40

50

0.0 0.1 0.2

Na2O (% )

Fina

l app

aren

t rat

e (*

10-6

gg-1

s-1)

Figure 5.13 Initial and final apparent rates of the nine cokes against total iron, calcium, potassium and sodium concentration in the cokes.

112


5.7 Summary

In this chapter the effect of coke properties (coke microtexture, surface area, carbon

structure (Lc and La) and ash composition) and the parent coal properties (rank, maceral

composition) on reactivity of nine metallurgical cokes was investigated.

The apparent reaction rates of the nine cokes varied to a great extent from

approximately an order of magnitude between the highest and the lowest reactive cok

used in this study.

During gasification the micropore and mesopore surface area, measured by CO2 and N2,

increased dramatically. The CO2 surface area of the raw cokes and the 15% burnout

cokes ranged 2.3–64.5 m-1g and 21.7–131.9 m-1g, respectively. The N2 surface area of

the raw cokes was always less than 0.6 m-1g. The N2 surface area of the 15% burnout

cokes varied from 28.5 to 96.9 m-1g. The increase of both N2 and CO2 surface area of

the reacted cokes is consistent with the previous data reported by other studies.

The initial apparent rate was poorly related to both N2 and CO2 surface area, which

indicates that the surface area of the micropores and mesopores did not significantly

affect the reaction rate. A poor relationship was also observed between N2 surface area

of the 15% burnout cokes and apparent rate. However, the relationship between CO2

surface area and the apparent rate of the reacted cokes was strong. Micropore surface

area (CO2 surface area) appeared to affect significantly the reaction rate at about 15%

carbon conversion. Also, by calculating the intrinsic rates of the 15% burnout cokes

using micropore surface area (CO2 surface area) of the 15% burnout cokes, the variation

in the reaction rates of the cokes diminished significantly.

Although rank and to a less degree, maceral composition of the parent coals have been

reported as good indicators of coke reactivity, the apparent and intrinsic reaction rates

were poorly related to coal rank and maceral composition in this suite of cokes.

he size of the anisotropic microtexture increased as the rank of the parent coal

e

T

increased. Non-fused inertinite derived component was more reactive than the

113


anisotropic texture as optical microscopy analysis showed. These observations are in

greement with those made in previous studies.

k of the parent

oal. Previous studies also indicate no strong influence of coal rank on crystallite height

vious studies also reported no significant change in Lc during

asification at early stages, but it has been reported a decrease of Lc at high carbon

k trend

as observed between crystallite height (Lc) and intrinsic rate. Previous studies reported

nd carbon structure (Lc and

a)) and coke reactivity will be reported.

ate, but

alcium, sodium and potassium were poorly related to coke reactivity. The chemistry of

a

Carbon crystallite height (Lc) and length (La) of the raw cokes were in the range 1.46-

1.67 nm and 3.60-4.18 nm, respectively. Lc and La were not related to ran

c

(Lc). During gasification, crystallite height (Lc) slightly increased at the early stages of

reaction (15% burnout) but a significant increase was observed at high conversion levels

(75% burnout). Pre

g

conversion levels. Crystallite length (La) did not change significantly during

gasification. But, a decrease of La during gasification was observed in previous studies.

Crystallite height (Lc) of the raw cokes was poorly related to initial apparent and

intrinsic rates. At 15% burnout a trend was observed between crystallite height (Lc) and

apparent rate; the apparent reaction rate decreased as Lc increased. Also, a wea

w

a decrease in reactivity as the ordering of carbon structure increased. However, in this

study Lc did not show a strong effect on the reaction rate.

Coal macerals and rank of the parent coal appeared to have no effect on coke reactivity.

Also, the influence of carbon structure (Lc and La) on the reaction rate could not be

clearly established. In order to determine their relative importance on the reaction rate a

study was carried out on carbonised maceral-enriched fractions, which enabled partial

separation of some of the factors. In the next chapter the influence of coal macerals on

coke properties (in terms of microtexture, microporosity a

L

The total iron, calcium, potassium and sodium in coke were used as indicators of coke

reactivity in previous studies. A trend was observed between iron and reaction r

c

the ash yield is not considered a reliable indicator for coke reactivity, as iron, calcium,

potassium and sodium can be found in cokes in minerals that do not catalyse the

114


115

8).

gasification reaction. Therefore mineral characterisation and the influence of the

catalysts require investigation (see Chapter 7 and

CHAPTER 6 – Influence of coal macerals on coke properties

In this chapter the effect of maceral composition (see section 2.5.1.2) of the starting coal

on coke properties, including its reactivity, will be investigated. Five coals were

selected for further investigation of coal rank and maceral composition effect on coke

reactivity. The selection was based on the differences in rank and maceral composition

of the coals. The maceral-enriched fractions (inertinite and vitrinite) were prepared from

sub-samples of coals B, C, D, F and G. The maceral composition of the vitrinite-rich

fractions and inertinite-rich fractions is shown in Table 4.2. The mineral content in the

inertinite-rich fractions was almost similar to that in the original coals. But the vitrinite-

rich fractions had very low mineral matter content.

The maceral-enriched fractions obtained were carbonised in a 70 g oven. Also, sub-

samples of the original coals were carbonised in the same 70 g oven. The procedures for

maceral separation and carbonization were described in Chapter 4. Reactivity

measurements (using 100% CO2) were performed on all carbonised maceral-enriched

fractions and the cokes made from the original coals.

In this chapter, the inertinite that did not fuse during carbonisation process is referred as

either non-fused inertinite or inert maceral derived component (IMDC). Macerals that

fuse during carbonisation and form anisotropic microtexture are called reactive maceral

derived component (RMDC).

116

Chapter 6 – Influence of coal macerals on coke properties

6.1 Effect of carbonisation conditions on coke reactivity and coke properties

ue to the small quantity of maceral–enriched fractions a small oven was required for

eir carbonisation. In order to observe the effect of preparation conditions on both

roperties and reactivity of the coke, sub-samples of coals were also carbonised in the

on rate of the 70 g oven cokes (B,

, D and G) became greater than that of those made in the 9 kg oven. In order to explain

The amount of each maceral–enriched fraction prepared was several hundred grams.

D

th

p

small oven. Then the reactivity test was performed on these cokes. The reactivity of the

cokes prepared in the 70 g oven was compared to that of their corresponding cokes from

the 9 kg oven (Figure 6.1).

The cokes from 70 g oven were more reactive than those carbonised in the 9 kg oven,

with one exception, coke F, which showed similar reactivity for samples from both

ovens. At the initial stages small differences were observed between the reaction rates

of the cokes carbonised in the 70 g oven and that of their corresponding cokes from 9 kg

oven. But, as carbon conversion increased the reacti

C

the differences in reactivity of the cokes produced in different carbonisation conditions

the raw cokes were characterised by micropore surface area, microtexture and carbon

structure (Lc and La).

117


Coke B25

Coke C

0

5

10

15

0 2 4 6 8 10 12 14 16 18


App

aren

t ra

(*10

-6 g

g-1s

20te

-1)

70 g oven 9 kg oven

0

10

20

30

0 5 10 15 20


App

aren

t ra

(*10

-6 g

g-1s

40

50

te

-1)

70 g oven 9 kg oven

Coke D

0

5

10

15

20

25

0 5 10 15 20


App

aren

t rat

e (*

10-6

gg-1

s-1)

70 g oven 9 kg oven

Coke F

0

10

20

30

40

50

60

0 2 4 6 8 10 12 14 16 18


App

aren

t rat

e (*

10-6

gg-1

s-1)

70 g oven 9 kg oven

Coke G

0

5

10

15

20

0 4 8 12 16 2

C arbon con ve rsion (%)

App

aren

t rat

e (*

10-6

gg-1

s-1)

0

70 g oven 9 kg oven

C versus carbon conversion (100% CO2) of the cokes B, C, D, F and G prepared in the 70 g and 9 kg ovens.

Figure 6.1 The apparent reaction rate at 900º

118


6.1.1 Surface area and intrinsic reactivity

Table 6.1 shows the surface area, measured by CO2, of the raw cokes B, C, F, and G

prepared in the 70 g and 9 kg ovens. The influence of carbonisation conditions on the

micropore surface areas of cokes B and F were significant, but the micropore surface

areas of cokes C and G were only slightly affected by the carbonisation conditions.

Although carbonisation conditions influenced to some degree the micropore surface

area, the impact of carbonisation conditions on micropore surface area cannot be easily

predicted because the lack of systematic variation of micropore surface area with

carbonisation.

In order to determine the influence of micropore surface area on the reaction rate the

intrinsic reaction rates were determined (Figure 6.2). The initial intrinsic rates of the

cokes B, C and G prepared in the 70 g oven and their corresponding cokes made from 9

kg oven were different. Therefore, the differences in the microporosity surface area did

not account for the observed differences in reaction rate between the cokes B, C, and G

from 70 g oven and their corresponding cokes from 9 kg oven. Moreover, the initial

apparent rate of coke F prepared in both 70 g and 9 kg ovens was similar although coke

F from 70 g oven had a significantly smaller surface area than coke F made in the 9 kg

oven. It can be concluded that the apparent reaction rate was affected by factors other

than the surface area of micropores.

Table 6.1 Surface area of the raw cokes B, C, F and G carbonised in the 70 g and 9 kg ovens.

Coke Surface area (m2g-1)

70 g oven 9 kg oven B 10.4 2.3 C 11.0 13.4 F 14.1 64.5 G 11.1 8.5

119


0.0

0.4

0.8

1.2

nitia

l int

rins

ic r

ate

(*10

-6 g

m-2

s-1)

B C F G

I

70 g oven

9 kg oven

Figure 6.2 The initial intrinsic reaction rates of cokes B, C, F and G prepared in

the 70 g and 9 kg ovens.


Heating rate has a major effect on microtextural composition and the size of the

nisotropic texture of the product coke; both the amount of fused inertinite [21] and size

f anisotropy [20,34] in the product coke decrease as the heating rate decreases.

etrographic analysis showed that the amount of fused inertinite in the cokes from the 9

.3); inertin oals B and C did not fuse at all in the 70 g oven and a small

proportion of the inertinite in coals D and F fused in the 70 g oven.

a

o

P

kg oven was greater than that in the corresponding cokes made in the 70 g oven (Figure

ite in the c6

120


0

10

20

30

40

50

60

70

B C D F G

Iner

tinite

(vol

, %)

Coal

70 g oven

9 kg oven

Figure 6.3 Percentage of non-fused inertinite in the cokes B, C, D, F and G

prepared in the 70 g and 9 kg ovens compared to percentage of inertinite in the coal.

he anisotropic microtexture in the cokes from 70 g oven was smaller than that in the

re 6.4). The anisotropic microtexture of both cokes B and C

ade in the 70 g oven was mainly fine mosaic whereas the anisotropic microtexture

stic stage than that

xisting in the 9 kg oven. The influence of heating rate on coke microtexture, observed

ork, is in agreement with that reported by previous studies [20,21].

T

cokes from 9 kg oven (Figu

m

which characterise cokes B and C carbonised in the 9 kg oven was mostly medium

mosaic. Medium mosaic was the typical anisotropic microtexture observed in cokes F

and G carbonised in the 70 g oven whereas coarse mosaic and flow-like anisotropic

microtextures characterised cokes F and G prepared in the 9 kg oven.

The greater content of non-fused inertinite and smaller anisotropy formed in the 70 g

oven cokes is explained by the lower heating rate during the pla

e

in this w

121

Cha

pter

6 –

Influ

ence

of c

oal m

acer

als o

n co

ke p

rope

rtie

s

12

2

(a)

(c

)

(d

)

Figu

re 6

.4 M

icro

text

ure

of (a

) cep

ared

in th

e 70

g 9

kg

(bot

tom

) ove

ns; w

here

F is

fine

mos

aic,

M is

m

ediu

m m

osai

c, C

is c

oan

d FA

FAFA

M

MF

M

(b

)

oke

F an

d (d

) cok

e G

pr

is fl

ow-li

ke a

niso

trop

y.

M

F

(top

)

C

) cok

e B

, (b)

cok

e C

, (c

arse

mos

aic

and


Also, microtexture formation was not significantly affected during re-solidification after

the plastic stage, although different heating rates were used to carbonise the sem kes.

During the plastic stage of the carbonisation process in the 70 g oven the heating rate

was 1 ºC min-1 whereas the heating rate after 470ºC was much higher (10 º in-1),

even greater than that of the 9 kg oven (3 ºC min-1). Therefore microtexture

formation was mainly controlled by the heating rate during the plastic stage than that

after the plastic stage, which confirms the observations made in previous studies [37].

Marsh [31] and Mitchell and Benedict [21] showed that the non-fused inertinite is the

most reactive m xtural component with carbon dioxide, followed by very fine, fine,

medium and coarse mosaic microtexture. The greater reactivity of cokes B, C and G

prepared in the 70 g oven than that of their corresponding cokes made in the 9 kg oven

could be influenced to some degree by their greater content of no sed

derived component and finer anisotropic microtexture. However, both cokes F

carbonised in the 70 g and 9 kg oven had similar reactivity althoug e fo had

greater inertinite content and the smaller size of anisotropy. The influence of non-fused

inertinite component on coke reactivity cannot be clearly established from these data

and further characterisation is required (see section 6.2).


Carbon crystallite height (Lc) and length (La) of the raw cokes were d mined for the

cokes B, C, F and G prepared in both 70 g and 9 kg ovens (Figure 6.5). Carbonisation

conditions had a greater effect on Lc than on La. Cokes B, C and G produced in the 70 g

oven had smaller Lc than their corresponding cokes made in the 9 kg oven. Only coke F

made in the 70 g oven had slightly greater Lc than coke F prepar e 9

Smaller Lc of cokes produced in the 70 g oven could be explained by the lower heating

rate of carbonization process, which reduces coal fluidity [115] during the plastic stage

hindering carbon crystallite development [34].

i-co

C m

inertinite

rmer

kg oven.

the

n-fu

h th

eter

in th

icrote

ed

123


1.5

1.6

(nm

)L

c

1.3

1.4

70 g oven 1.36 1.37 1.54 1.40

9 kg oven 1.51 1.52 1.46 1.51

B C F G

4.6

3.8

4.2

4.4

La

)

4.0

(nm

3.2

3.4

3.6

70 g oven 3.85 3.96 3.76 3.53

9 kg oven 3.73 3.82 4.18 3.82

B C F G

Figure 6.5 Crystallite size (Lc and La) of the raw cokes B, C, F and G carbonised in the 70 g and 9 kg ovens (the vertical lines represent the measurement error bars).

124


Crystallite length (La) of cokes B and C from the 70 g oven was slightly greater than

eir corresponding cokes from the 9 kg oven whereas cokes F and G made in the 70 g

oven had slightly smaller La than their corresponding cokes from the 9 kg oven.

Considering the measurement error of La, the carbonisation conditions could not be

concluded to affect significantly the crystallite length (La) of the cokes prepared from

the same coal.

Crystallite height (Lc) may have some influence on reactivity of cokes B, C and G

prepared in the 70 g oven as their Lc compared to that of their corresponding cokes

carbonised in the 9 kg oven is lower. However, the influence of crystallite height on

coke reactivity cannot be established due to the small number of sample. The influence

of crystallite size on coke reactivity will be further discussed in section 6.2.

It can be concluded that the effect of low heating rate results in a decrease of the amount

f inertinite fused and smaller size of anisotropic microtexture. Also, the carbon

rystallite height (Lc) was smaller.

6.2 Effect of macerals on coke reactivity

A study by Huang et al. [101] on reactivity of carbonised vitrinite-rich and inertinite-

rich fractions showed that the carbonised vitrinite-rich fractions had lower reactivity

than their corresponding carbonised inertinite-rich fractions, which was explained by

the increased ordering of carbon structure (Lc) of the carbonised vitrinite-rich fractions.

They observed no influence of both micropore and mesopore surface area on reactivity

of the carbonised maceral-enriched fractions. However, Czechowski and Kidawa [102]

in some cases the reactivity of the carbonised vitrinite-

their corresponding carbonised inertinite-rich

e coals. They assumed that the reactivity of the carbonised vitrinite-rich

content of Na

th

o

c

and Huang et al. [101] found that

rich fractions was greater than that of

fraction for som

fractions was increased due to the greater content of catalyst, which was based on the

, K, Ca and Fe in the ash chemistry.

125


Sakawa et al. [78] concluded from the investigation of several coals of different rank

that the reactivity of carbonised fractions of consecutive densities, increased with

increasing both inertinite content and alkali index (B) of the fraction (the inertinite

content and the alkali index increased as the density fraction increased). However, they

could not differentiate between the influence of inertinite content and alkali index on

coke reactivity.

he data from the reactivity test showed that the carbonised inertinite-rich fractions

e from coal G at the early stages, but it increased

fter approximately 4% carbon conversion. Carbonised vitrinite-rich fractions were less

active than the cokes made from the original coals.

bonised vitrinite-rich fractions was

maller than that of their corresponding carbonised inertinite-rich fractions, because the

In this section the reactivity of the cokes prepared from the original coals in the 70 g

oven and their corresponding carbonized maceral enriched-fractions will be determined,

and also the influence of surface area and carbon structure (crystallite size) on reactivity

will be investigated.

T

were more reactive than the coke from the original coals prepared in the same oven

(Figure 6.6). Only the carbonised inertinite-rich fraction of coal G showed similar

reactivity with that of the coke mad

a

re

The size of the anisotropic microtexture of the car

s

vitrinite-rich fractions were carbonised at lower heating rate than inertinite-rich

fractions (to avoid swelling). The effect of heating rate on the size of anisotropic

microtexture was more significant for the maceral-enriched fractions from medium rank

coals than for those from low rank coals. The anisotropic microtexture of the vitrinite-

rich fraction B was predominantly very fine mosaic whereas the carbonised inertinite-

rich fraction B had mainly fine mosaic (Figure 6.7a). Vitrinite-rich fraction G was

characterized by fine mosaic microtexture whereas inertinite-rich fraction G was

characterized by medium mosaic microtexture (Figure 6.7b).

126


B

0

10

20

30

40

50

App

aren

t rat

e (*

10-6

gg-1

s-1)

C

10

20

30

40

50

60

App

aren

t rat

e (*

10-6

gg-1

s-1)

00 5 10 15


0 2 4 6 8 10 12 14 16 18


Inertinite Original Vitrinite


D F50

0

10

20

30

0 2 4 6 8 10 12 14 16


App

aren

t rat

(*10

-6 g

g-1s-1

40e )

01020304050

0 2 4 6 8 10 12 14 16


App

aren

t ra

(*10

-6 g

g-1s

6070

te

-1)

InerInertinite Original Vitrinite

tinite Original Vitrinite

G

0

5

10

15

20

0 2 4 6 8 10 12 14 16 18


App

aren

t rat

e (*

10-6

gg-1

s-1)


Figure 6.6 The apparent reaction rate of the cokes produced from the original coals (B, C, D, F and G) and their corresponding maceral-enriched fractions versus carbon conversion.

127


(a)

(b) Figure 6.7 Anisotropic microtexture of (a) carbonised inertinite-rich fraction B

(left) and carbonised vitrinite-rich fraction B (right) and (b) carbonised inertinite-rich fraction G (left) and carbonised vitrinite-rich fraction G (right); where VF is very fine mosaic (it has a uniform grey appearance), F is fine mosaic (it has a grey mottled appearance) and M is medium mosaic.

F

F

V F

M

F

V F

128


The initial apparent rates of the coke made from the original coal and its corresponding

carbonised vitrinite and inertinite-rich fractions were strongly influenced by the content

of non-fused inertinite; this pattern was observed for all cokes (Figure 6.8a-e).

However, no trend was observed between initial apparent rates and non-fused inertinite

content when data for all the cokes and maceral-enriched fractions were considered

(Figure 6.8f). This suggests that actually it is not the content of non-fused inertinite that

affects reaction rate but its intrinsic properties, which varies between cokes. A

further investigation was conducted on the carbonised inertinite-rich fractions properties

and their influence on reactivity and the results are presented in section 6.3.1.

not show any obvious influence on the

action rate. The reactivity of anisotropic microtexture has been reported [21,31] to

crease as its size decreases. Therefore, the presence of finer anisotropic microtexture

the carbonised vitrinite-rich fraction compared to that of the corresponding coke and

arbonised inertinite-rich fraction, should result in an increased reactivity of the

carbonised vitrinite-rich fraction, which would affect the relationship between reaction

rate and non-fused inertinite content observed for the cokes and maceral-enriched

fractions prepared from the same coal source (Figure 6.8a-e). The in nce of the size

of the anisotropic microtexture on the reaction rate will be discussed further in section

6.3.2.

the

The size of the anisotropic microtexture did

re

in

in

c

flue

129


B

r2 = 0.99

02468

10

0 20 40 60 80

Non-fused inertinite (vol, % )

Initi

al a

ppar

ent

rate

(*

10-6

gg-1

s-1)

C

r2 = 0.99

0369

1215

0 20 40 60 80


Initi

al a

ppar

ent

rate

(*

10-6

gg-1

s-1)

(a) (b)

D

r2 = 0.99

0

3

6

9

12

0 20 40 60 80


Initi

al a

ppar

ent

rate

(*

10-6

gg-1

s-1)

F

r2 = 0.98

0

6

12

18

24

0 20 40 60


Initi

al a

ppar

ent

rate

(*

10-6

gg-1

s-1)

(c) (d)

G

r2 = 0.92

0

1

2

3

4

0 20 40 6


Initi

al a

ppar

ent

rate

(*

10-6

gg-1

s-1)

0

All

0

6

12

18

24

0 20 40 60 80


Initi

al a

ppar

ent

rate

(*

10-6

gg-1

s-1)

(e) (f)

Figure 6.8 The initial apparent reaction rate of the cokes produced from the original coals (B, C, D, F and G) and their corresponding maceral enriched fractions versus their content of non-fused inertinite.

130


6.2.1 Surface area and intrinsic reactivity

The surface area of the carbonised original coals (B, C, F and G) and their

corresponding maceral-enriched fractions are shown in Table 6.2. The carbonised

vitrinite-rich fractions showed lower micropore surface area than their corresponding

carbonised inertinite-rich fractions. Figure 6.9 shows that generally, surface area

increased with the amount of non-fused e derived component. Huang et al. [101

surface area of micropores than that of the carbonised vitrinite-rich fractions. It can be

concluded that most of the microporosity in coke occurs in the non-fused inertinite

derived component and little is associated with the anisotropic microtexture. This

suggests that the increased microporosity of the non-fused inertinite can be one of the

factors that make the non-fused inertinite derived component more reactive than the

anisotropic microtexture; as greater surface is exposed to gasification.

Table 6.2 Surface area of the raw cokes made from the original coals (B, C, F and G) and their corresponding carbonised maceral-enriched fractions.

Coke Surface area (m2g-1)

inertinit ]

also observed that the carbonised inertinite-rich fractions were characterised by greater

Inertinite-rich fraction Original Vitrinite-rich fraction B 19.0 10.4 4.1 C 17.0 11.0 8.0 F 30.7 14.1 6.5 G 11.8 11.1 7.3

Since surface area of the micropores was shown to be a potential factor that influences

rate, the intrinsic rates of the cokes and their corresponding carbonised

inertinite-rich fractions were determined (Figure 6.9). Micropore

the reaction

surface

ces between reactivity of the carbonised

vitrinite and

area cannot explain entirely the differen

131


vitrinite-rich factions and their corresponding carbonised inertinite-rich factions, as the

itial intrinsic rates of the former were smaller than that of the latter. However, most of

in

the differences between reactivity of the cokes and their corresponding carbonised

inertinite rich-fractions appeared to be largely due to differences in micropores surface

area, as smaller differences were observed between their intrinsic rates than apparent

rates.

132


B

0.2

0.4

0.6

nitia

l int

rins

ic r

ate

(*10

-6 g

m-2

s-1)

B

0

5

10

15

20

Surf

ace

area

(m

2 g-1)

0 20 40 60 80

Non-fused inertinite (vol,% )

0.0

V O I

I

C

0

5

10

15

20

0 20 40 60 8


Surf

ace

area

(m

2 g-1)

0

C

0.0

0.2

0.4

0.6

0.8

1.0

V O I

Initi

al in

trin

sic

rate

(*

10-6

gm

-2s-1

)

F

0

10

20

30

40

0 20 40 6


Surf

ace

area

(m

2 g-1)

0

F

0.0

0.2

0.4

0.6

0.8

V O I

Initi

al in

trin

sic

rate

(*

10-6

gm

-2s-1

)

G

0

5

10

15

0 20 40Non-fused inertinite (vol,% )

Surf

ace

area

(m

2 g-1)

60

G

0.0

0.1

0.2

0.3

0.4

V O I

Initi

al in

trin

sic

rate

(*

10-6

gm

-2s-1

)

Figure 6.9 Surface area of the raw cokes made from the original coals (B, C, F and G) and their corresponding carbonised maceral-enriched fractions versus the concentration of non-fused inertinite (left-side charts). The right-side charts shows the initial intrinsic rates of the cokes made from the original coals (B, C, F and G) and their corresponding carbonised maceral-enriched fractions, where O is the coke made from the original coal, and V and I are the carbonised vitrinite- and inertinite-rich fractions.

133



Crystallite height (Lc) and length (La) were determined for the carbonised maceral-

enriched fractions and the cokes prepared from the original coals. The carbonised

inertinite-rich fractions, which contain high levels of non-fused inertinite, had lower Lc

than their corresponding carbonised vitrinite-rich fractions, which were characterised by

low levels of non-fused inertinite (Figur . It can be concluded that the non-fuse

inertinite has lower Lc than the reactive derived maceral component (the macerals that

fuses during carbonisation).

Crystallite height (Lc) increased with decreasing content of non-fused inertinite in the

cokes made from coals of higher rank (F and G) and their corresponding carbonised

maceral enriched fractions. However, the cokes from lower rank coals (B and C) had

greater Lc than their corresponding carbonised inertinite-rich fractions but similar Lc

with their carbonised vitrinite-rich fractions. This suggests that the non-fusible inertinite

in the parent coals affects the Lc of cokes made from higher rank coals more than that of

the cokes produced from lower rank coals. As the amount of non-fused inertinite

increases the Lc decreases because it produces a decrease of the average crystallite

height (Lc). Also, the non-fusible inertinite in the coals may affect the development of

the carbon crystallites during carbonisation, which can result in a real decrease of Lc,

more likely for the higher rank coals.

Crystallite height (Lc) of the vitrinite-rich fractions could be affected to some extent by

the heating rate during carbonisation as vitrinite-rich fractions were carbonised at lower

heating rate (0.1ºC min-1 for B, C and F and 0.05ºC min-1 for G) than both original coals

and inertinite-rich fractions (1ºC min-1) to avoid swelling during carbonisation. It has

been shown in section 6.1.3 that carbonisation conditions affected Lc; the crystallite

eight of three of four cokes prepared at higher heating (9 kg oven) was greater than

Therefore t

fractions m

e 6. 0) d 1

h

their corresponding cokes prepared at lower heating rate (70 g oven) (Figure 6.4).

he degree of ordering of carbon structure (Lc) of the carbonised vitrinite-rich

ay be lesser because of the lower heating rate.

134


BB1.5

1.1

1.2

1.3

1.4

0 20 40 60 80

Non-fused inertinite (vol,%)

Lc (

nm)

r2 = 0.91

02468

1.0 1.2 1.4 1.6

Lc (nm)

Initi

al a

ppa

rate

(*

10-6

gg-1

s

10

rent

-1)

CC1.5

1.1

1.2

0 20 40 60 80


Lc

1.3

1.4

(nm

)

0

5

1.1 1.2 1.3 1.4Lc (nm)

Initi

al a ra

(*10

-6

10

15

ppar

ent

te

gg-1

s-1)

F

1.2

1.4

1.6

1.8

0 20 40 60


Lc (

nm)

F

r2 = 0.99

05

10152025

1.2 1.4 1.6 1.8

Lc (nm)

Initi

al a

ppar

ent

rate

(*

10-6

gg-1

s-1)

G G

r2 = 0.95

1.2

1.4

1.6

1.8

0 20 40 6


Lc (

nm)

00

1

2

3

4

1.2 1.4 1.6 1.8

Lc (nm)

Initi

al a

ppar

ent

rate

(*

10-6

gg-1

s-1)

Figure 6.10 Crystallite height (Lc) of the raw cokes made from the original coals (B, C, F and G) and their corresponding carbonised maceral-enriched fractions versus the concentration of non-fused inertinite (left-side charts). The initial apparent rates versus Lc (right-side charts).

135


The Lc of the cokes B, F and G and their corresponding vitrinite and inertinite-rich

fractions showed a good relationship with initial apparent rate (Figure 6.10). The

reactivity of coke C was lower than that of its corresponding carbonised vitrinite-rich

fraction even though they have similar Lc. Although the relationship between the initial

apparent rate and Lc of the carbonised maceral-enriched fractions and coke made from

the same coal was good in some cases, no trend was observed between the initial

apparent rate and Lc when all the carbon nriched fractions and cokes were

considered (Figure 6.12). It can be concluded that the influence of Lc on the reaction

rate was overshadowed by other factors.

Crystallite length (La) of the coke and its corresponding carbonised vitrinite- and

inertinite-rich fractions showed no consistent trend with the content of non-fused

inertinite (Figure 6.11). Feng et al. [73] observed that the carbon structure became more

ordered in the vicinity of clays and iron s. Therefore, the crystallite length may

be influenced by the mineral matter present in the coke.

It has been reported that the gasification reaction occurs mainly at the edges of the

crystallites rather than on the basal planes [34,53]. Therefore the density of the free

edges increases as the length (La) decreases implying an increase of carbon reactivity.

In this study, the La was poorly related to the initial apparent rate (Figure 6.11). No

consistent trend was observed between the initial apparent rate and La of the carbonised

maceral-enriched fractions and coke from the same coal either. The poor relationship

could be explained by the reduced accessibility of the reactant gas to the edges of the

crystallites at the initial stages due to closed porosity [43] and/or presence of catalysts,

which can create pits in the carbon basal planes [92].

In conclusion, the reaction rate at the in ges was not significantly influenced by

either Lc or La.

ised maceral-e

par letic

itial sta

136


B

3.23.43.63.84.04.2

0 20 40 60 80


La (

nm)

B

02468

10

3.2 3.4 3.6 3.8 4.0

La (nm)

Initi

al a

ppar

ent

rate

(*

10-6

gg-1

s-1)

C C4.2

3.23.43.6

0 20 40 60 80


La (

n

3.84.0

m)

0

5

3.2 3.4 3.6 3.8 4.0 4.2

La (nm)In

itial

a ra(*

10-6

10

15

ppar

ent

te

gg-1

s-1)

F

3.2

F

0

4.4

3.43.63.84.04.2

0 20 40 60


La (

nm)

5101520

3.2 3.4 3.6 3.8 4.0 4.2

La (nm)

Initi

al a

ppar

enra

te

(*10

-6 g

g-1s-1

)

25t

G

3.63.84.04.2

(nm

)

G

3

4

l app

aren

t ra

te

-6 g

g-1s-1

)

3.00 20 40 60

Non-

3.23.4

fused inertinite (vol,%)

La

03.2 3.4 3.6 3.8 4.0

I

1

2

La (nm)

nitia

(*10

Figure 6.11 Crystallite length (La) of the raw cokes made from the original coals (B, C, F and G) and their corresponding carbonised maceral-enriched fractions versus the concentration of non-fused inertinite (left-side charts). The initial apparent rates versus La (right-side charts).

137


0

5

10

15

20

25

1.0 1.2 1.4 1.6 1.8

Lc (nm)

Initi

al a

ppar

ent r

ate

(*10

-6 g

g-1

s-1)

Figure 6.12 Initial apparent rate of th okes made from the original coals (BC, F and G) and their carbonised maceral-enriched fractions versus Lc.

6.3 Factors influencing rea of carbonised inertinite-rich

fractions and carbonised vitrinite-rich fractions

In the previous section (6.2) the influence of maceral composition on reactivity was

investigated. In order to understand the greater reactivity of carbonised inertinite-rich

fractions compared to that of the carbonised vitrinite-rich fractions, the influence of

their properties on the reaction rate are investigated.

6.3.1 Carbonised inertinite-rich fractions

Figure 6.13 sh

the carbonised

e raw c ,

ctivity

ows that the carbonised inertinite-rich fractions had different reactivity;

inertinite-rich fraction of coal F was the most reactive followed by that

138


of coal C. The reaction rate of carbonised inertinite-rich fractions increased gradually

during the reactivity test.

0

10

A

20

30

40

50

60

70

0 5 10 15


ppar

ent r

ate

(*10

-6 g

g-1s-1

) F

C

B

D

G

igure 6.13 The apparent reaction rate of the carbonised inertinite-rich fractions

nised inertinite-rich fractions of

oals B, C and D were similar, about 70%, whereas the percentage of non-fused

ertinite in the carbonised inertinite-rich fractions of both coals F and G was smaller,

e anisotropic microtexture varied between

arbonised inertinite-rich fractions. The anisotropic carbon in carbonised inertinite-rich

carbonised inertinite-rich fractions B and C (Figure 6.14).

Fagainst carbon conversion.

The reactivity of the carbonised inertinite-rich fractions varied to a great degree (Figure

6.13). The content of non-fused inertinite in the carbo

c

in

approximately 56% (Table 6.3). The size of th

c

fractions F and G was characterized mainly by medium mosaic whereas smaller size of

anisotropic microtexture, such as very fine and fine mosaic, was characteristic of the

139


Table 6.3 The percentage of non-fused inertinite in the carbonised inertinite-rich fractions.

B C D F G Non-fused inertinite derived

aceral component (vol. %) 68.0 71.0 69.3 56.4 56.0 m

Because the content of non-fused inertinite varied between the carbonised inertinite-rich

macerals, the influence of the content of non-fused inertinite on the initial apparent rate

was determined. A poor relationship is observed between the content of non-fused

inertinite and initial apparent rate (Figure 6.15a), which shows that the content of non-

fused inertinite has no dominating influence on the reaction rate.

The influence of the anisotropic microtexture in the carbonised inertinite-rich fractions

on the reaction rate was considered as the percentage of anisotropic microtexture was

significant (28-44 %). Coke reactivity has been reported to decrease as the size of the

anisotropic microtexture increased [21,31]. The size of anisotropic microtexture

d from the relationship between initial

al. A poor relationship is observed (Figure

microtexture of different sizes had no

n the reaction rate. The influence of the anisotropic microtexture on the

increases as the coal rank increases [26,105]. Therefore the influence of the anisotropic

texture on the reaction rate can be observe

apparent rate and rank of the parent co

6.15b), which suggests that the anisotropic

influence o

reaction rate is further discussed in section 6.3.2.

140


(a) (b)

(c) (d)

M

M

Figure 6.14 Microtexture of the carbonised inertinite-rich fractions of (a) coal B, (b) coal C, (c) coal F and (d) coal G; where VF is very fine mosaic (it has a uniform grey appearance), F is fine mosaic (it has a grey mottled appearance) and M is medium mosaic.

F

V F V F

141


C

DB

G

F

0

5

10

15

20

25

50 55 60 65 70 75

Non-fused inertinite (vol, %)

Initi

al a

ppar

ent r

ate

(*10

-6 g

g-1

s-1)

B

C

D

F

G

0

5

10

15

20

25

0.9 1.0 1.1 1.2 1.3 1.4

R0 (%)

Initi

al a

ppar

ent r

ate

(*10

-6 g

g-1

s-1)

(a) (b) Figure 6.15 Initial apparent rate of the carbonised inertinite-rich fractions versus

Surface area and intrinsic rates

The carbonised inertinite-rich fractions were characterized by the greatest micropore

surface area (CO2 surface area) compared to that of their corresponding carbonised

vitrinite-rich fractions and cokes (section 6.2.1). However, the surface area was poorly

related to the content of non-fused inertinite derived component (Figure 6.16). The

tinite-rich

were reduced when they were normalised to surface area, but there are still

icant differences between the intrinsic reaction rates (Figure 6.17). Micropore

lain completely the difference between the reaction rates of the

both non-fused inertinite and rank of the parent coal.

difference between the apparent reaction rates between the carbonised iner

fractions

signif

surface area does not exp

carbonised inertinite-rich fractions.

142


CB

F

G

05

1015

2025

3035

50 55 60 65 70 75


Surf

ace

area

(m2 g-1

)

Figure 6.16 Surface area measured by carbon dioxide against non-fused inertinite.

0

4

8

12

16

20

B C F G0.0

0.2

0.4

0.6

0.8

1.0

B C F G

Initi

al in

trin

sic

rate

(*10

-6 g

m-2

s-1)

Initi

al a

ppar

ent r

ate

(*10

-6 g

g-1s-1

)

Figure 6.17 Initial apparent and intrinsic rates of the carbonised inertinite-rich fractions.

143


Carbon structure (carbon crystallite size)

Figure 6.18 shows that the crystallite height (Lc) of the carbonised inertinite-rich

actions of coals F and G were similar, but greater than the crystallite height (Lc) of the

carbonised inertinite-rich fractions of coals B and C. The greater Lc could be due to the

presence of reactive maceral derived com isotropic microtexture). In the

previous section (6.2), it was shown that the carbonised vitrinite-rich fractions, which

are characterised by over 80% of reactive maceral derived component, had greater Lc

than the carbonised inertinite-rich fractions (Figure 6.10). Crystallite length (La) was

similar for carbonised inertinite-rich fractions for coals B, C and G. Only the carbonised

inertinite-rich fraction of coal F had greater La.

No trend was observed between initial apparent rate and either Lc or La (Figure 6.19).

Although the carbonised inertinite-rich fractions F and G had similar crystallite height

they had different reactivity. Also, the carbonised inertinite-rich fractions B and C had

different reactivity but similar Lc. A poor relationship was also observed between La and

reaction rate. Crystallite length (La) was similar for the carbonised inertinite-rich

fractions B, C and G but their reactivity was also different. It can be concluded that

neither Lc nor La influenced significantly the initial apparent rate.

fr

ponent (an

1.1

1.2

1.3

1.4

1.5

B C F G

Lc (n

m)

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

B C F G

La (n

m)

Figure 6.18 Crystallite size (Lc and La) of the carbonised inertinite-rich fractions B, C, F and G (the error bars are shown).

144


G

B

C

F

B

C

G

F

01.1 1.2 1.3 1.4 1.5

Lc (

5

10

15

20

25

nm)

Initi

al a

ppar

ent r

ate

(*10

-6 g

g-1s-1

)

03.2 3.4 3.6 3.8 4.0 4.2

5

10

15

20

25

La (nm)

Il a

ppar

ent r

ate

*10

-6 g

g-1

s-1)

igure 6.19 Initial apparent rate versus crystallite size (Lc and La) of the fractions.

bonised inertinite-rich fractions. The

arbonised inertinite-rich fractions contained, in most cases, a greater content of mineral

B, D, F and G were

uite similar (Figure 6.20). Their reaction rate initially increased with increasing carbon

onversion but after about 4-6% conversion the reaction rate did not change

nitia (

Fcarbonised inertinite-rich

In summary, the micropore surface area of the carbonised inertinite-rich fractions

influenced the reaction rate to some extent but it did not explain entirely the differences

between their reaction rates since differences between the intrinsic reaction rates were

still observed. Also, the carbon crystallite size (Lc and La) did not influence significantly

the reaction rate of the carbonised inertinite-rich fractions since a poor relationship was

observed between the apparent rate and both Lc and La. Therefore, other factors had a

stronger influence on the reactivity of the car

c

matter than the cokes made from the original coals. The influence of the catalytic

mineral matter on the reaction rate will be presented in Chapter 8.

6.3.2 Carbonised vitrinite-rich fractions

The reactivity of the carbonised vitrinite-rich fractions of coals

q

c

145


significantly. The carbonised vitrinite-rich fraction of coal C was the only one that

showed greater reactivity and the reaction rate increased gradually until the reactivity

test was completed.

0

2

4

6

8

10

12 gg-1

s

14

16

18

0 2 4 6 8 10 12 14 16


App

aren

t rat

e (*

10-6

-1)

C

F

B

D

G

Figure 6.20 The apparent reaction rate of the carbonised vitrinite-rich fractions against carbon conversion.

he size of anisotropic microtexture was smaller in the carbonised vitrinite-rich

actions made from high volatile coals (coal B and coal C) than that of the carbonised

itrinite-rich fractions made from medium volatile coals (coal F and coal G) (Figure

.21). The anisotropic microtexture of the carbonised vitrinite-rich fractions B and C

T

fr

v

6

was characterised mainly by very fine mosaic and low levels of fine mosaic. Mainly

fine mosaic and low levels of medium mosaic formed the anisotropic microtexture of

the carbonised vitrinite-rich fractions F and G. The content of anisotropic microtexture

in the carbonised vitrinite-rich fractions was very high, greater than 85% (Table 6.4).

146


147

(a) (b)

(c) (d)

Figure 6.21 Microtexture of the carbonised vitrinite-rich fractions of (a) coal B, (b) coal C, (c) coal F and (d) coal G; where VF is very fine mosaic (it has a uniform grey appearance) and F is fine mosaic (it has a grey mottled appearance).

V F

F

F

V F

FF


Table 6.4 The percentage of non-fused inertinite in the carbonised vitrinite-rich

fractions.

B C D F G

Inertinite (vol. %) 3.2 12.3 10.3 3.4 15.4

As discussed in the previous section, it is generally accepted that the reactivity increases

with decreasing the size of the anisotropic microtexture. However, the relationship

between initial apparent rate and the rank of parent coal was poor (Figure 6.22). This

indicates that the size of the anisotropic microtexture in the range very fine mosaic -

medium mosaic did not affect significantly the initial apparent rate.

In the previous section, it has been shown that the content of inertinite on the reaction

rate was more dominant than that of the anisotropic microtexture. The relationship

between initial apparent rate and the content of non-fused inertinite in the carbonised

vitrinite-rich fractions was poor, which suggests that non-fused inertinite derived

component did not affect the reaction rate (Figure 6.22).

G

FD

C

B

0

1

2

3

4

0.9 1.0 1.1 1.2 1.3 1.4

R0 (%)

Initi

al a

ppar

ent r

ate

(*10

-6 g

g-1

s-1)

C

DF

B G

0

1

2

3

nt r

s-1)

4

0 3 6 9 12 15 18

Non-fused ine rtinite (vol, %)

Initi

al a

ppar

e(*

10-6

gg

-1

Figure 6.22 Initial apparent rate of the carbonised vitrinite-rich fractions versus both rank of the parent coal and non-fused inertinite.

ate

148


Surface area and intrinsic rates

The surface area of micropores (CO2 surface area) did not vary significantly between

the carbonised vitrinite-rich fractions (Table 6.5). Moreover, the differences in apparent

reaction rates between the carbonised vitrinite-rich fractions were only slightly

diminished when the intrinsic rates are used for comparison (Figure 6.23). Therefore,

the micropore surface area of the carbonised vitrinite-rich fractions did not explain the

B C F G

differences between their reaction rates.

Table 6.5 Surface area, measured by CO2, of the carbonised vitrinite-rich

fractions.

Surface area (m2g-1) 4.1 8.0 6.5 7.3

4

0

1

B C F G

Init

2

3

ial a

ppar

ent r

ate

(*

10-6

gg-1

s-1)

0.0

B C F G

Init (*

Figure 6.23 Initial apparent and intrinsic rates of the carbonised

0.2

0.4

0.6

ial i

ntri

nsic

ra

10-6

gm

-2s-1

)

vitrinite-rich fractions.

te

149


Carbon structure (carbon crystallite size)

The effect of the rank of the parent coal on crystallite height (Lc) of the carbonised

vitrinite-rich fractions was significant (Figure 6.24). The carbonised vitrinite-rich

actions F and G, prepared from medium volatile bituminous coals, had considerably

reater Lc than the carbonised vitrinite-rich fractions B and C, made from high volatile

ituminous coals.

Although the rank of the parent coal showed its influence on the crystallite height (Lc)

of the carbonised vitrinite-rich fractions, it did not appear to affect the Lc of the cokes

prepared in the 9 kg oven (see section 5.5). The presence of inertinite in greater

proportion in the original coals than in the vitrinite-rich fractions may affect the

microtexture formation. Mitchell et al. [21] and Graham et al. [17] showed that the

distribution and size of macerals in coals controls the development of coke

microtexture. Gray and Champagne [52] concluded that coarse inertinite particles limit

the size of anisotropic microtexture because the space between the inertinite particles is

arrow and the molecular orientation domains cannot grow. Also, in section 6.2 it was

inite-rich f er rank (F and

poor relationship observed between the rank of the

arent coal and the Lc of the cokes was due to inertinite content that lowered the Lc of

ffect significantly La of the coke.

he influence of both crystallite height (Lc) and length (La) on the reaction rate was

oor (Figure 6.25). Although the carbonised vitrinite-rich fractions F and G had greater

c than the carbonised vitrinite-rich fraction B the differences between their reaction

fr

g

b

n

shown that the cokes made from the original coals had lower Lc than that the carbonised

ractions, particularly for the cokes made from coals of highvitr

G). It can be concluded that the

p

the cokes made from higher rank coal.

Crystallite length (La) of the carbonised vitrinite-rich fractions did not vary significantly

across the carbonised vitrinite-rich fractions from coals B, C and F, and it was slightly

greater for the carbonised vitrinite-rich fraction from coal G (Figure 6.24). It appears

that the rank of the parent coal does not a

T

p

L

rates were small.

150


The greater reactivity f the carbonised vit te-rich f ion C c ot be ex ined by

Lc, as the h fraction B with s r Lc had lower reaction rate.

lso, crystallite length (La) did not explain the increased reactivity of the carbonised

itrinite-rich fraction C because it was comparable to that of the carbonised vitrinite-

ch fractions B and F, which had lower reactivity. The increased reactivity of the

arbonised vitrinite-rich fraction C was due to its greater content in catalytic mineral

atter. This is further discussed in Chapter 8.

o rini ract ann pla

carbonised vitrinite-ric imila

A

v

ri

c

m

1.2

1.4

1.6

1.8

B C F G

Lc (n

m)

3.0

3.2

3.4

3.6

3.8

4.0

B C F G

La (n

m)

Figure 6.24 Crystallite siz

e (Lc and La) of the carbonised vitrinite-rich fractions.

C

F

1

2

3

ial a

ppar

ent r

ate

(*10

-6 g

g-1s-1

)

B G

4

Init

C

F2

3

ppar

ent r

ate

-6 g

g-1

s-1)

GB1

4

Initi

al a

(*10

01.3 1.4 1.5 1.6 1.7 1.8

Lc (nm)

03.2 3.4 3.6 3.8 4.0

La (nm)

Figure 6.25 Initial apparent rate versus crystallite size (Lc and La) of the carbonised vitrinite-rich fractions.

151


6.4 Summary

The properties of the cokes made in the 70 g oven were affected to some degree by the

lower heating rate used during carbonisation compared to that used for carbonisation of

the coals in the 9 kg oven. The extent to which inertinite fused decreased, which result

in greater content of non-fused inertinite in the cokes prepared in the 70 g oven, and the

size of the anisotropic texture was smaller. Previous studies also reported a decrease of

both percentage of inertinite that fuses and size of the anisotropic microtexture as

heating rate decreased. Also, the carbon crystallite height (Lc) was lower in the cokes

ade in the 70 g oven than in the 9 kg oven.

ing vitrinite-

ch fractions. The size of anisotropic microtexture of the vitrinite-rich fractions was

arbonised inertinite-rich fractions were characterised by much greater

icropore surface area than the vitrinite-rich fractions.

influence the reaction rate.

he reaction rate of the carbonised inertinite-rich fractions showed significant variation.

The differences in the reactivity of the carbonised inertinite-rich fractions was reduced

when the apparent rate was normalised by the micropores surface area. However,

crystallite height (Lc) showed no significant influence on the reaction rate. Therefore,

m

It was shown that the carbonised inertinite-rich fractions were far more reactive than the

carbonised vitrinite-rich fractions. Consistent differences were observed between the

properties of the carbonised inertinite-rich fractions and their correspond

ri

lower than that for the inertinite-rich fractions because of the lower heating rate used for

their preparation. The carbonised vitrinite-rich fractions had greater Lc than the

carbonised inertinite-rich fractions, which is consistent with data reported in previous

studies. Also, the c

m

The micropore surface area of the carbonised inertinite-rich fractions and their

corresponding carbonised vitrinite-rich fractions was found to affect to some extent the

reaction rate. Crystallite height (Lc) appeared to have some influence on the reaction

rate of the carbonised maceral-rich fractions prepared from the same coal, but when the

data for all the samples were considered together, a poor relationship was observed with

the reaction rate, which indicates that Lc did not

T

152


153

other factors had stronger influence on the reactivity of the carbonised inertinite-rich

fractions.

The carbonised vitrinite-rich fractions showed small variation between their reaction

rates for most of them (B, D, F and G). The variation of size of anisotropic microtexture

from very fine to medium mosaic did not affect the reaction rate of the carbonised

vitrinite-rich fractions B, D, F and G. The crystallite height (Lc) of the carbonised

vitrinite-rich fractions B, F and G, which was in the range 1.39–1.72 nm, did not

fluence significantly their reaction rates. The carbonised vitrinite-rich fraction C had

e

igh reactivity of the carbonised vitrinite-rich fraction C was due to its greater content

f catalytic mineral phases (see Chapter 8).

The poor relationship observed between Lc of the cokes and the rank of their parent coal

(see Chapter 5) is believed to be due to the presence of non-fusible inertinite in the

coals. The non-fusible inertinite in the medium volatile coals decreased significantly

the Lc of the cokes, compared to that of their corresponding carbonised vitrinite-rich

fractions, but had almost no influence on Lc of the cokes made from high volatile coals.

This suggests that the influence of the coal rank on crystallite height (Lc) of the cokes is

diminished by the presence of non-fusible inertinite in the parent coals.

study, the infl er on coke reactivity was also

Characterisation of mineral matter present in the cokes used in this study is

er 7 and the influence of the catalytic mineral matters on the reaction

te is presented in Chapter 8. Also, the transformations that occur in the mineral matter

in

significantly greater reactivity than the other carbonised vitrinite-rich fractions. Th

h

o

In order to explain the difference in reactivity across the cokes investigated in this

uence of the catalytic mineral matt

investigated.

presented in Chapt

ra

during gasification and also the importance of catalysts during gasification are presented

in Chapter 9.

tese nswparte 2

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