BIOMASS AS FUEL FOR REHEATING

FURNACES – FOCUSING ON

IMPURITIES

Siri Andersson

Emilia Hamedi

2014

KTH Material science and engineering

2

Abstract

An important problem today is the large amount carbon dioxide emissions. Research is carried out to

investigate if it is possible to replace the fossil fuels with biomass based fuels. This paper will

investigate a research done in order to find out and evaluate the effect of biomass-based fuels in

reheating furnaces at steel plants. There are already a lot of studies on renewable fuels and alternative

fuels. The focus will be on how the alkali metals in the syngas affect the steel in a reheating furnace.

The result from this paper showed that there are potentials for replacing fossil fuels with biomass

based fuels, but there are some important aspects to consider. The main difference between biomass

based fuels and fossil fuels is the energy density, which means a larger amount of biomass will be

needed to deliver the same amount of energy as oil, natural gas or LPG (gasol). Another disadvantage

with biomass based syngas is the impurities and alkali metals which can affect both the plant and the

steel products. As a result of this, pretreatment of biomass is often done before it is used. Depending

on the requirement of purity, biomass can be added directly to the heating furnace or added to a

gasifier that creates syngas. The syngas contains impurities from the biomass which can lead to

problems for the equipment and the product. Due to that, syngas is normally pretreated and cleaned to

get the best quality. The cleaning part is usually done through hot- or cold gas cleaning.

Results from the experiment showed that it is possible to lower the tar concentration through preheated

air inserted from the bottom of the gasifier. Also reduction of particles could be done in the syngas

with electrostatic separation due to the size of the particles are lower than 30μm.

There is a problem with measuring the ash concentration after combustion with syngas. Future work

should investigate if it is possible to improve this measurement technologies and the economic cost for

replacing fossil fuels with biomass fuel.

Keywords: Biomass, syngas, reheating furnace and impurities.

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Table of contents

Abstract ....................................................................................................................................... 2

Table of contents .......................................................................................................................... 3

1.0 Introduction ............................................................................................................................ 5

1.1 Aim of the project ................................................................................................................ 5

1.2 Background ......................................................................................................................... 6

1.2.1 Biomass ........................................................................................................................ 6

1.2.2 Biogas from algae ......................................................................................................... 6

1.2.3 Syngas .......................................................................................................................... 7

1.2.4 Alkali metals ................................................................................................................. 7

1.2.5 Reheating furnace .......................................................................................................... 8

1.2.6 Investigating potential problems and solutions of renewable fuel use in steel reheating

furnace .................................................................................................................................. 9

2.0 Method ................................................................................................................................. 10

2.1 Literature review ............................................................................................................... 10

2.2 Experiment ........................................................................................................................ 10

2.3 Fuel ................................................................................................................................... 11

2.4 Procedure of the experiment ................................................................................................12

3.0 Results ...................................................................................................................................12

3.1 Review: Impurities in biomass .............................................................................................12

3.2 Review: Pretreatment of biomass ......................................................................................... 13

3.2.1 Torrefaction .................................................................................................................14

3.2.2 Pyrolysis ......................................................................................................................16

3.3 Review: Impurities in syngas ............................................................................................... 17

3.3.1 Sulfur .......................................................................................................................... 17

3.3.2 Nitrogen ...................................................................................................................... 17

3.3.3 Particles ...................................................................................................................... 18

3.3.4 Alkali compounds ....................................................................................................... 18

3.3.5 Hydrogen Chloride .......................................................................................................19

3.3.6 Tars (Condensable Hydrocarbons) .................................................................................19

3.4 Review: Pretreatment of syngas .......................................................................................... 20

3.4.1 Hot gas cleanup ........................................................................................................... 20

3.4.2 Cold gas cleanup ......................................................................................................... 23

3.5 Review: Gasification technology for syngas ........................................................................ 26

3.5.1 Fixed bed gasification .................................................................................................. 26

3.5.2 Fluidized bed gasification ............................................................................................ 26

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3.5.3 Entrained flow gasification .......................................................................................... 26

3.6 Review: Reactions of steel surface during combustion ......................................................... 26

3.6.1 Thermodynamic analysis of corrosion problems ............................................................ 27

3.6.2 Formed ash from different fuels during combustion ....................................................... 29

3.7 Review: Monitoring of alkali species concentration ............................................................. 30

3.7.1 Excimer laser induced fragmentation fluorescence ......................................................... 30

3.7.2 Plasma Excited Alkali Resonance Line Spectroscopy .................................................... 30

3.7.3 Surface Ionization ....................................................................................................... 30

3.8 Experiment: composition, tar content and particle levels ....................................................... 32

3.8.1 Syngas composition ..................................................................................................... 32

3.8.2 Particles ...................................................................................................................... 33

4.0 Discussion ............................................................................................................................ 34

5.0 Conclusions .......................................................................................................................... 36

6.0 Future work .......................................................................................................................... 36

6.0 Acknowledgement ................................................................................................................. 36

7.0 References ............................................................................................................................ 37

7.0 Appendix .............................................................................................................................. 40

7.1 Fuel .................................................................................................................................. 40

7.2 High temperature agent gasification. ................................................................................... 40

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1.0 Introduction

The European Commission has come up with a climate and energy target called 20-20-20 which

means that until 2020 decrease of greenhouse gas emission by 20% compared to 1990, at least 20% of

the energy should be renewable and energy efficiency should increase with 20%. For the European

Union to achieve its national target of 20% renewable energy, it has been national policy for each

country, depending on the position and potential. It varies from 10% in Malta to 49% in Sweden to

jointly come down to 20%. [1]

During the UN climate conference in 2010, it was determined that all industrial countries should

develop national long-term plans to achieve low carbon dioxide (CO2) emissions. The EU

Commission has come up with a target to reduce EU emissions until 2050 by 80-95%. For Sweden,

the long term goal is reducing net greenhouse gas emissions by 100% till 2050. [2]

The steel industry in Sweden contributes to a large amount of carbon dioxide emission. In 2010, the

steel plants accounted for 9.6 % of the carbon dioxide emission. That represented 6Mton carbon

dioxide during 2010. By replacing fossil fuels with renewable fuels, it is possible to reduce the

emission of carbon dioxide. [3]

1.1 Aim of the project

This work is carried out in order to fulfill the requirements of Bachelor of Science degree. The purpose

of this project is to investigate a research done in order to find out and evaluate the effect of biomass-

based fuels in reheating furnaces at steel plants. There are already a lot of studies on renewable fuels

and alternative fuels, however much less on the application within the steel industry. This work will

focus on how the alkali metals in the syngas affect the steel in a reheating furnace. This will be done

by reviewing what is happening with alkali and contaminants during gasification. An additional aim is

to see if it is possible to reduce the alkali-related issues in the furnace and if it is possible to develop

existing methodologies and technologies for controlling and monitoring alkali in the gas phase inside a

gasifier or in fuel supply. The following aspects were mainly considered:

Ashes/impurities in biomass

Ashes/impurities in syngas

Measure ash and substances/impurities in syngas

To find out the reactions that can take place using phase diagram

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1.2 Background

1.2.1 Biomass

Biomass is all kind of living organisms or biological dead materials that are originated in the nature; it

can be planted energy crops, residues from forest industries or burnable waste. By different methods

such as combustion or processing of biomass, biomass can be converted into heat and electricity. This

means that oil and coal can be replaced. [4]

Biomass such as energy crops contains lignocellulose, but also certain alkali metals such as sodium

(Na) and potassium (K), since potassium is important while growing. During combustion, these can

affect the properties of the steel because the alkali metals normally contribute to corrosion and

slagging as it forms compounds. Since the alkali in biomass is bound to simple functional groups or

exist as simple functional salts, they are easily released into the gas. [5]

1.2.1.1 Bioenergy crops

A bioenergy crop is a crop that is planted with the only purpose of producing fuel. To not compete

with food production the cellulose based energy crops is usually chosen hence it do not require as high

energy as food crops, and therefore does not require as much fertilizer inputs. From cellulose based

energy crops, pellets, synthesis gas and liquid fuel can be produced. In the future, it could be an

alternative for reheating furnaces in the steel industry. Since there are many different kinds of

bioenergy crops with different environmental profiles we have decided to focus on salix which is

suitable to cultivate in the Swedish climate. [6]

1.2.1.2 Salix

Salix is a renewable energy crop that is planted for the production of different fuels. The steel

industry's energy demand for reheating furnaces today is 3.33 TWh. In order to get 3.33 TWh from

salix in form of synthesis gas, 83.000 hectares needs to be planted. This is equal to 2.87% of Sweden

and corresponds to the area of Dalarna (a landscape in Sweden).

Carbon neutrality is achieved only when the same amount of forest that is harvested is regrowth and

also that the same amount of felled trees does not exceed the number of annually planted.

1.2.2 Biogas from algae

Biogas is a gas produced by digestion of organic biodegradable materials such as food wastes, manure

and algae but also produced sugar beets. The gas is obtained by fermentation and contains manly

methane (CH4), carbon dioxide (CO2), and small amounts of hydrogen sulfide (H2S). The table 1

below shows the composition of a typical raw biogas. [7]

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Table 1: Typical composition of biogas.

Component CH4 CO2 N2 H2 H2S Low Heating Value

(Pure CH4)

Value 50-75% 25-50% 0-10% 0-1% 0-3% 35,9 MJ/Nm

Since the abundance of algae in the sea and lakes have to be taken care of, some researches in

producing biogas from algae have been done. There is potential for biogas production from algae, but

the techniques need to be developed further to reduce the nitrogen (N) content, formed ammonia

(NH3), and how to handle digestion residues that has high cadmium (Cd) content. The gas has low

flame temperature since it contains carbon dioxide (CO2), but it can still be used in gas appliances and

for processing heat. In order to also achieve an acceptable calorific value, the gas must be cleaned

from carbon dioxide and other impurities.

If the clean gas is compressed, it can be used in vehicles and also replace natural gas. Based on an

environmental perspective, biogas can be considered carbon neutral if only the products from the

digestion process are considered. A life cycle analysis perspective normally shows that greenhouse gas

emission is reduced by less than 100%. It is also profitable if losses during upgrading and purification

are minimized. [8] [9]

1.2.3 Syngas

Syngas or synthesis gas is a gas produced by a chemical process. Gas extracted from feedstock that

contains carbon of natural origin, is called natural gas. Syngas from biomass feedstock can be

produced by a method called gasification

Syngas is a mixture of hydrogen (H), carbon monoxide (CO) and sometimes methane (CH4) as well as

inert gases like carbon dioxide (CO2) and nitrogen (N). The starting material and gasification

technology will affect the composition. A typical range of syngas composition can be 20-40% H, 35-

40% CO, 25-35% CO2, 0-15% CH4 and 2-5% hydrocarbons.

The reaction in gasification can take different forms but the process is defined by high temperature and

adding small amounts of oxygen. This makes the carbon containing material react with oxygen, steam

and carbon dioxide to generate carbon monoxide and hydrogen, which are the major components of

syngas composition. Syngas can be used in different applications; it can be refined into synthetic

petroleum or hydrogen, to create electricity or to use as a fuel. [10] [11] [7]

1.2.4 Alkali metals

Alkali metals are very reactive since they only have one valence electron. This means that alkali

metals just have to give away one electron to be stable. This is the reason way alkali metals are really

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reactive and are never found in the nature in elemental forms. Due to that they are normally stored in

paraffin oil or mineral oil. [12] [13] [14]

Alkali metals react with water, halogens and many other compounds. Further down in the group of

alkali metals in the periodic system, the reaction with water occur at a higher rate and becomes more

explosive. When the alkali metals react with water, are alkaline hydroxides formed which are very

corrosive. If the alkali metals instead react with halogens, alkali metal halides forms, which are ionic

crystalline compounds called salts and these are soluble in water. [13] [15] [15] [16]

As mentioned before, alkali metals can be stable by losing one valence electron to achieve noble gas

state. They are then forming cations and have a +1 oxidation state, this is the easiest and most common

way for alkali metals to form ions and be stable.

One exception is the alkalides, who contain alkali metals in a -1 oxidation state and forms anions.

These compounds are unstable and very unusual. They exist because they have filled S-subshells

which make them more stable. They have some chemical properties similar to electrides, who has

electrons acting as anions and forms salts. [17] [18] [19] [20]. Combustion of fuels which contains

alkali metals will lead to coatings, corrosions and also decrease the thermal conductivity, this is not

desirable. [21]

1.2.5 Reheating furnace

A reheating furnace is needed because the entire steel production is not at the same location. When the

cool steel has arrived to the steel plant, it must be reheated for annealing and subsequent process,

hence it has to be warmed up first. Heating of the steel slabs take place in three steps, preheating,

heating and homogenization (also called soaking), this is done to homogenize the temperature of the

steel. Using convection and radiation from the burner and the walls of the furnace, heat is transferred

to the steel and homogeneous temperature throughout the steel is achieved after a certain time. Since

the formability in the rolling process depends on the final temperature it can vary for different grades

of steel and products. The steel must be suitable for plastic deformation in hot rolling process hence

the target temperature usually is between 1100-1300 °C. A reheating furnace in steel plants is shown

in figure 1. [7] Fuel supply

Figure 1. Reheating furnace [8]

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As an example, the furnace 301 at SSAB (Borlänge, Sweden) is used, see figure 2 for at heat balance.

The furnace operates at the maximum capacity of 128 MW and is divided into 8 different zones. The

heat delivered to the steel differs in each zone and therefore the amount of required fuel varies

accordingly. In order to achieve the capacity needed, the fuel will be preheated. The heat flux of each

component is shown in figure 2, where furnace is schematically shown as a system. The furnace has

recently been revamped to use natural gas instead of LPG (gasol).

Figure 2: Reheating furnace 301 at SSAB [23]

Each type of fuel has a calorific value (heating value), using a fuel with a low calorific value requires a

greater amount of fuel which leads to heat losses of the furnace will become greater. Hence the

calorific value has to be considered when the furnace total efficiency is determined. No matter the type

of fuel used, the final temperature must be achieved for the steel slabs. [22]

1.2.6 Investigating potential problems and solutions of renewable fuel use in steel reheating

furnace

Earlier studies have shown that it might be possible to use wood powder produced from wood pellets

to use in a reheating furnace. But there is some huge difference between firing a wood powder

compared to fossil fuels. One of the huge differences is the large presence of wood ash that emerges

during firing.

In one experiment pellets from north Europe was used with an ash content of max 0.4%. The ash was

mixed with powdered iron oxide from a low carbon steel alloy after a heating cycle at 1250˚C. To see

if there would be a reaction between steel and ash, another test was made by putting ash on various

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steel alloys at 1150˚C for 1h. A similar test was also done were pellet ash was mixed with alumina

silicates holders and a refractory furnace brick for 4h at 1200˚C. The result was that the ash gave a

glassy face on the surface of the furnace brick but not on the alumina. The pure scale did not melt

either. This means that when considering direct firing with wood pellets, the composition of the

refractory is important.

From stem wood, a clean wood powder is obtained but to decrease the process cost, cheaper bi-

product biomass like bark, branch, crown and stumps could be used. These bi-products lead to a lot of

ash. At high furnace temperature, the alkali metals are in the gas-phase which means that the majority

will likely pass through without reacting.

This means that the ash from wood products can lead to problems in the furnace. To overcome these

problems, one way is to use wood products at low temperatures. This will decrease the problem with

molten and volatile phase. Another way is to gasify the solid fuels first to get a relatively clean syngas.

This will lower the amount of impurities in comparison with solid fuels but there can still be some

impurities in the gas like tar, sulphur compounds, nitrogen containing compounds, particles, halogen

species and alkali metal species. [23]

2.0 Method

2.1 Literature review

An extensive literature study has been done on biomass and syngas for use in reheating furnaces to

investigate whether it is possible to replace fossil fuels with biomass. The focus has been on what

contaminants biomass and syngas contains as well as the opportunities to pretreat and clean biomass

and syngas for further improvement. Studies from the literature review has also been done to see what

effects impurities has on the steel surface in reheating furnace and what damage these contaminants

can lead to for the steel.

2.2 Experiment

There are several challenges when processing solid biomass in to syngas that will reduce the

effectiveness of energy conversion. An experiment was done to see how biomass fuel performed

during air/steam gasification in an updraft gasifier with high temperature. In figure 3 below, the high

temperature agent gasification is described schematically with the main parts as; preheater, gasifier,

feed hopper, and combustion chamber.

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Figure 3: High temperature agent gasification.

Biomass is transported from the feed hopper into the gasifier using a screw that brings biomass

forward. In the bottom of the gasifier is preheated air inserted from the preheater, this means that the

biomass and the hot gas flow will move in different directions. The gas is moving upwardly in the

gasifier and carried out at the top through a tube down to the combustion chamber and is burned. The

small particles that remain after the reaction is carried out through a grate and are placed besides. The

advantage with this process is that steam can be supplied to the feed stream if it is necessary to

improve the steam gasification. [24]

2.3 Fuel

In the experiment was hydrothermal carbonized biomass (biocoal) used as fuel. The table 2 shows the

composition of substance, moisture content and ash content of the biocoal used in the gasification

process and the differences between biocoal and raw biomass such as wood chips.

Table 2: Differences between biocoal and raw biomass

Content Biocoal, CH1,267O0,182 Raw biomass, CH1,789O0,633

Moisture content at 105°C (%) 10.6 82.2

Ash cont. at 550°C (% dry) 6.7 4.6

Low heating Value (MJ/kg dry) 28.9 19.2

Carbon C (% dry) 66.3 45.6

Hydrogen H (% dry) 7 6.8

Nitrogen N (% dry) 3.7 4.2

Oxygen O (% dry) 16.1 38.5

Sulphur S (% dry) 0.12 0.33

Biocoal has a higher value of carbon content due to the pretreatment. [24]

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2.4 Procedure of the experiment

By using the preheater, the gasification agent was heated up to 1000 °C before the biomass was added

to the gasifier. Feeding through the top was commenced as a high enough temperature was reached.

The syngas composition was then monitored with a Gas Chromatography, GC. Inside the reactor,

thermocouples of type S was placed to measure the temperature. The pressure drop was monitored

with digital manometer placed over the grate in the gasifier. The gas was then cooled down and

analyzed before it was emitted. [24]

3.0 Results

3.1 Review: Impurities in biomass

Biomass contains different impurities such as alkali metals (the chemical composition is shown for dry

agrol and salix in table 3) and ash components such as calcium oxide (CaO), potassium oxide (K2O),

magnesium oxide (MgO), silicon dioxide (SiO2), and sulfur trioxide (SO3). During gasification, alkali

metal compounds will primarily react in contrast to other ash components. Since calcium (Ca),

magnesium (Mg) and silicon (Si) have a high ionization numbers in oxide form, they are not so

reactive and they will sink to the bottom of the gasifier as mineral components. It is different for the

alkali metals potassium (K) and sodium (Na), since in oxide form, they have an ionization number of

only one, which means they do not have strong ionic bonds and will therefore react as gases and

vaporize in the gasification. The impurities can then condense and evaporate at temperatures above

800°C.

In some types of biomass, such as straw and energy crops is chlorine (Cl) presented, which together

with the alkali metals can form to alkali chlorides such as potassium chloride, KCl.. Potassium

chloride coating can be measured and mitigated by corrosion-inhibiting additives such as sulfation.

Chlorine can also react with hydrogen and convert to hydrogen chlorine during gasification. Alkali

chlorides can lead to sticky and corrosive deposits on for example heat exchange surfaces. Since the

alkali in biomass is more mobile compared with alkali in coal, it can be easily released into the gas

phase. The alkali can even be bounded to simple functional groups or exist as a functional salts so the

eutectic sodium and potassium can evaporate at temperatures above 700°C. If the gas is not cooled, the

vaporized alkali metals will be left in the gas. [25]

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Table 3: Chemical composition of biomass for agrol and salix (dry) (%) [26]

3.2 Review: Pretreatment of biomass

The reason for pretreating biomass is to make properties that are suited for further processing and

sometimes transportation. The most important goals with pretreatment are the cost effectiveness and

thermochemical properties that are needed. Pretreatment methods can be divided into three different

categories: biochemical, mechanical and thermochemical which are shown in figure 4. The choice of

pretreatment method depends on the final bio-fuel product. Since the focus is on syngas, torrefaction

and phyrolysis will be described below. [27]

Impurities Agrol Salix

Al <0.005 0.037

Ba 0.001 0.001

Ca 0.078 0.47

Fe 0.004 0.034

K 0.044 0.25

Mg 0.017 0.049

Mn 0.012 0.007

Na 0.008 0.021

Si 0.009 0.22

Ti 0.02 0.012

C 51 50.3

Cl 0.005 0.007

S <0.002 <0.002

O 38.2 37.4

N 0.15 0.069

H 6.26 6.17

Water (%) 8 8

Ash (%) 0.144 2.517

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Figure 4: Illustration of upgrading technologies for different production of bio-fuel. [28]

3.2.1 Torrefaction

Torrefaction is a process which can be considered as uncompleted pyrolysis. During this pretreatment

process the biomass is heated to temperatures between 200-300 °C in an inert atmosphere. Physical

and chemical properties changes during the process. The fiber structure changes which facilitates

grindability. The chemical changes leads to a higher amount of carbon content, lower oxygen and

hydrogen content which improves the heating value of the material. The final product of the

torrefaction process is solid particles which are more spherical, hydrophobic, and easy to process. [26]

3.2.1.1 Phases in the torrefaction process

The different stages in the torrefaction phase are:

1. Heating of the biomass to 100 °C until water starts to evaporate.

2. Drying step after which the temperature increases up to 200 °C, although internal temperature

does not change.

3. Post-drying, where water and other volatiles matter are removed.

4. The temperature increases to above 200 °C and degradation of hemicellulose occurs and the

dry mass loss occurs.

5. The torrefied material is cooled down. [27]

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3.2.1.2 Difficulties of torrefaction

The energy and mass yield are depending on the temperature, heating rate and torrefaction time. If

they changes it will affect the process condition and the energy and mass yield will vary. After

torrefaction up to 80% of the initial mass is retained and the corresponding energy is up to 90% of the

original value. That means that mass loses will be bigger than energy loses. As mentioned before, the

torrefaction temperature is 200-300 °C and throughout this range different phases of degradation will

take place. Below 250 °C, hemicellulose and lignin will be the most reactive, which starts to gasify

and carbonize. Gasification and carbonization of the lignin will be limited since it is the most thermo-

stable component. The biomass will lose more hydrogen and oxygen than carbon, if the torrefaction

time is increased. This means that the LHV (low heating value) could increase from 17-19 MJ/kg up to

19-23 MJ/kg, since the energy density in solid mass is related to amount of carbon. The following

graphs (figure 5, 6 and 7) shows that the thermochemical properties are depending on torrefaction time

and temperature. [27]

Figure 5: Effect of torrefaction temperature on moister content of whet straw at various times.

Figure 6: Effect of torrefaction temperature on the heating value of wheat straw at various times.

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Figure 7: Effect of torrefaction temperature on the weight loss of wheat straw at various times.

3.2.2 Pyrolysis

Heating of organic material or biomass at temperatures above 450 °C is called pyrolysis in absence of

oxygen.

The equation 1 shows combustion of biomass into different products:

( ) ( ) eq. 1

There are two types of pyrolysis, fast- and slow pyrolysis where final temperature and heating rate will

determine thermochemical and physiochemical properties of the final products. Depending on the

process conditions (heating time, residence time and final temperature) the final product will be

different, it might be gaseous (syngas), liquid (pyrolysis oil) or solid char. Degradation of cellulose is

an important factor during pyrolysis to get a solid charcoal with a carbon content of 85-90% with low

oxygen and hydrogen amount. Table 4 below shows characteristics of some pyrolysis processes. [27]

Table 4: Affecting parameters (final temperature, heating rate and residence time)

Pyrolysis Process Residence Time Heating Rate Final

Temperature (°C)

Products

Carbonization Days Very low 400 Charcoal

Conventional 5-30 min Low 600 Char, bio-oil, gas

Fast <2 s Very high ~500 Bio-oil

Flash <1 s High <650 Bio-oil,chemicals, gas

Ultra-rapid <0,5 s Very high ~1000 Chemical, gas

Vacuum 2-30 s Medium 400 Bio-oil

Hydropyrolysis <10 s High <500 Bio-oi

Methanopyrolysis <10 s High >700 Chemicals

17

Other parameters then those above, such as the pressure and the ambient gas composition will also

affect the final pyrolysis product. The different temperature ranges affect different constituents of

biomass. At 130-180 °C, hemicellulose will decompose and produce condensable and non-

condensable gaseous and less amount of char compared to lignin and cellulose. Therefore the amount

of char that is produced from the pyrolysis is depending on the amount of each constituent. Even the

particle size has a significant role during the pyrolysis process since the heat transfer and pyrolysis

kinetics are depending on the surface area of a particle.

3.3 Review: Impurities in syngas

The amount of impurities varies heavily and depends on the feedstock impurities and the syngas

methods, these are shown in table 5 below. It generally includes sulfur and nitrogen compounds,

particles, alkali compounds, hydrogen chloride and condensable hydrocarbons. [28]

Table 5: Common feedstock impurity levels. Moisture-free in percent by mass.

Impurity Wood Wheat straw Coal

Sulfur 0.01 0.2 0.1-5

Nitrogen 0.25 0.7 1.5

Chlorine 0.03 0.5 0.12

Ash: K2O

SiO2

Cl

P2O5

0.04

0.08

0.001

0.02

2.2

3.4

0.5

0.2

1.5

2.3

0.1

0.1

3.3.1 Sulfur

Sulfur impurities arise mostly as hydrogen sulfide (H2S) with minor content of carbonyl sulfide

(COS). Depending on the feedstock the concentration of hydrogen sulfide can vary from 0.1 mL/L to

30mL/L. Coal contains much more sulfur than biomass. Sulfur in coal can be as much as 50 g/kg

which is a lot more compared to biomass that only contains 0.1-0.5 g/kg. There are some forms of

biomass that can come up to 1 g/kg.

The problem with sulfur is that sulfur compounds corrode metal surfaces. The catalyst used to upgrade

or clean syngas can be deactivated because of sulfur, just a small amount of sulfur is enough, which is

why sulfur is required to be removed to a level of parts per billion.

3.3.2 Nitrogen

In syngas, most nitrogen impurities occur as ammonia (NH3) with a lesser amount as hydrogen

cyanide (HCN). Nitrogen is released through gasification and combustion from heterocyclic aromatic

compounds in the feedstock or from protein structure. How much ammonia and hydrogen cyanide are

18

released depends on physical properties like particle size, intrinsic properties like nitrogen content and

on the process situation. From primary reactions of biomass and from secondary gas phase reactions

ammonia can be created. The concentration of ammonia and hydrogen cyanide in the secondary

reaction increases when the temperature increases from feedstock conversion. This leads to hydrogen

cyanide will change to ammonia when residence time and H2 are increased.

Ammonia concentrations of several weight percent can be the result of nitrogen in the biomass

feedstock. At normal gasification temperatures, 2/3 of this ammonia can decompose to molecular

nitrogen, at this time the concentration of ammonia in syngas is approximately a couple of hundreds or

a few thousand parts per million., This is a low value which, even then can lead to problems for the

catalyst, sometimes used to improve the syngas quality.

3.3.3 Particles

The particle size in syngas varies from 1 µm to 100 µm and the compositions can vary a lot depending

on the feedstock impurities and process. The main part of the particles consist of inorganic compound,

which means alkali metals, alkali earth metals, iron, magnesium and silica. They can also consist of

remaining solid carbon from biomass during gasification. It is important that the particles in syngas are

removed, in most cases the applications require particle removal by 99%. This is because of particles

cause corrosion, erosion and fouling.

3.3.4 Alkali compounds

Alkali metals, such as potassium and sodium, and alkaline earth metals are found in many gasification

feedstocks. The concentrations of alkali metals are normally much higher in biomass than in coal.

Higher levels of alkali metals are found in herbaceous biomass and higher level of alkaline earth

metals are found in wood biomass.

Alkali metals are very reactive and are more reactive than alkaline earth metals, which make alkali

metals more problematic than alkaline earth metals in syngas applications. At temperatures above 600

°C some alkali compounds can melt or vaporize in the reactor. This means that they can leave the

reactor as aerosols or vapors, normally as hydroxides, sulfates and chlorides. The problem with alkali

compounds when they are transported out of the reactor is that they can cause corrosion and

significant fouling in downstream process.

In gasification-based systems there not only biomass is contributing to alkali metal impurities. There

are some catalyst, used to reduce the impurities in the syngas and to change the constituents of the

syngas, based on alkali metals and transition metals. These metals together with sodium and potassium

from the biomass can lead to ash fouling and corrosion when they first are vaporized in high

temperature areas in the system and then condense in the cooler areas.

19

Alkali metal impurities can lead to problems in gasification systems in the way that they can result in

sintering and slagging of ash. This is the reason why it is so important to avoid alkali metals

impurities. Another reason is that catalysts are very sensitive to alkali metals and can be poisoned

easily by the alkali metals in the biomass. The need of reduction of alkali metals varies from a few

grams per kilo down to a few micrograms per kilo.

3.3.5 Hydrogen Chloride

Chlorides in syngas usually take the form of hydrogen chloride, HCl. In biomass will the chlorine

occur as alkali metal salts and react easily in high temperature environment through vaporization with

water vapor from hydrogen chloride. Compared to other impurities in the syngas are the

concentrations of chloride relatively low but their can still leads to huge problems in the material.

In the gas phase can hydrogen chloride also react with other impurities and create more compositions

as sodium chloride (NaCl) and ammonium chloride, NH4Cl. When sodium chloride and ammonium

chloride condense in cooler downstream equipment they can lead to fouling and create deposits.

Another problem with chloride impurities are that they can cause poisoning of catalyst used for

methanol and ammonia.

3.3.6 Tars (Condensable Hydrocarbons)

Condensable hydrocarbon i.e. tars are composed of condensable organic compounds and is proceed

from highly oxygenated compounds of moderate molecular weight to heavy, highly reduced

compounds. The compounds of tars are divided in three groups; the primary, secondary and tertiary.

Primary tars are organic compounds from coal or biomass. Secondary tars gets when the temperature

and residence time increases. Tertiary tars get when the temperature increases even more and the

reaction time increases. Because of feedstock composition and processing conditions can

thermochemical transformation process generate up to thousand different tar species. It is hard to

define what tar consists of because of the complex chemical nature, it is even hard to analyzing and

collecting tar because of this. That is why intergovernmental has come up with a definition of tar as

“all hydrocarbons with molecular weights greater than that of benzene” as a measurement between

researchers.

Organic compounds results in impurities in the synthesized product so it is preferred to remove or

decomposition all organic compounds. One way to reduce tar yields is to increase the temperature and

reaction time but this leads to more heavy hydrocarbons. Even if it would be best to remove all tar, it

is an easier way to just remove enough tar for its dew point to be less than the minimum temperature

in the gas steam. [28]

20

3.4 Review: Pretreatment of syngas

Depending on the application, the required purity of the gas varies, hence the gas cleaning is not

always necessary. If the gas is going to be combusted directly when it is produced, gas cleaning is not

necessary. If the gas is produced to be transferred to other plants it must be cooled before storing (and

sometimes pressurized). The gas usually passes through purification where condensable compounds

are removed and the gas is cooled at room temperature. The purification system is divided in different

parts such as cyclone, scrubbers, and different types of filters.

Since the syngas sometimes is used in turbines and fuel catalysts, high demands for gas purification is

normal. There are two different pretreatment techniques, hot gas- and cold gas cleanup, depending on

pressure and temperature. [7]

3.4.1 Hot gas cleanup

In the beginning the main reason for hot gas cleanup was to decrease the maintenance of the

equipment that was used during syngas combustion. Later, the environmental standards became

tougher and the emissions to the environment needed to decrease, which also led to improved syngas

quality. More benefits of hot gas cleanup are better efficiency, better syngas conversion with fewer

byproducts and reduced waste streams. Hot gas cleaning is used in pressurized gasification processes

where temperature exceeds 500 °C and pressure is between 15-25 bars. The disadvantage of this

technique is that it is expensive. [7] [29]

3.4.1.1 Sulfur

Sulfur dioxide (SO2) and hydrogen sulfide (H2S) are regular compounds of sulfur, to reduce sulfur

contents at high temperatures, focus is on those two compounds. Many applications require low levels

of sulfur to avoid equipment failure and it is then highly recommended to focus on reducing hydrogen

sulfide. Even if not all of the combustion applications require low concentrations of sulfur, sulfur

emissions to the environment needs to be minimized. One more advantage of reducing sulfur from

syngas (in the form of H2S), compared to reduce sulfur from flue gases (in the form of SO2) is that it is

more economical.

Through adsorption it is possible for gas to combine with solid materials either physically or

chemically. There are three steps in a sulfur adsorption process which are reduction, sulfidation and

regeneration. The first step in this process is reduction and is a preparation step where solid sorbent is

reduced for chemical adsorption with sulfur. In the second step, sulfidation, sulfur normally reacts

with metal oxides and creates a metal sulfur compound, such as zink sulfide (ZnS) and iron (II) sulfide

(FeS). In the last step, regeneration, an original oxide sorbent and a sulfur dioxide gas are produced

through regeneration. This step is very important since regeneration has the ability to reduce

impurities in the material and to reduce waste streams.

21

3.4.1.2 Nitrogen

Many syngas applications require low concentrations of ammonia (NH3), as low as parts per million

(ppm) levels. To reach this, it is more important for nitrogen compounds to decompose to ammonia,

instead of direct removal from the gas. It depends on during gasification of biomass, ammonia is

released but the ammonia dose not decomposes as fast as it needs to achieve the low concentrations.

This can be done either with thermal catalytic decomposition or catalytic oxidation. Through thermal

destruction of oxide gas containing ammonia it is possible to reduce the production of nitrogen

monoxide (NO) and N2O by ammonia, decomposing to N2, H2 and NOx. But they will still form, even

though the stability of carbon dioxide (CO2) and N2 are stronger. By using selective oxidation the

unwanted effects on methane (CH4) such as carbon monoxide (CO) and hydrogen (H) compositions

can be reduced, which otherwise would occur by simple oxidation of syngas.

3.4.1.3 Particles

In the last 30 years, high temperature particle cleanup has provided significant improvements to

syngas applications. The most common techniques for hot gas particle cleanup are; internal separation,

barrier filtration, and electrostatic interaction.

3.4.1.3.1 Internal separation

Internal separation works by heavier solids separating from lighter gases through the use of mass and

acceleration principles of separation. The most common way to do this is by using a cyclone, which is

also the oldest method for solid separations. There are other methods such as dust agglomerators and

impact separators but they are not as common. It takes long time for small particle to settle by gravity,

but by using a cyclone the time is reduces, due to centripetal acceleration. Figure 8 shows how a

cyclone works. The gas stream will pass through a double vortex which means an outer and an inner

vortex. At first, the outer vortex forces the particles outwards and downwards and by internal forces

are the particles separated from the vapor. Afterwards, the gas will change direction and go through

the inner vortex and be forced upwards, before it leaves the arrangement through a vortex finder.

Figure 8: Example of a typical cyclone

22

3.4.1.3.2 Barrier filtration

Particles can be reduced by filters through four different mechanisms or a combination of those. One

of the most common methods is called barrier filter. It takes place by a gas stream passing around

fibers, granules or through a porous solid. Figure 9 shows the four different mechanisms; diffusion,

internal impaction, direct interception and gravitational settling.

Figure 9: Filtration mechanisms: A) diffusion, B) internal impaction, C) direct interception, D)

gravitational settling.

If the particles deviate from the gas streamlines and collide with the filter, they can be collected by a

combination of diffusion, internal impact and gravitational settling and in that way be reduced. If

particles in another way follows the gas streamline can they be reduced by direct interception, the

criteria for this is that the streamline passes the filter close enough. When it comes to porous materials

it is possible to reduce some particle larger than a specific size.

3.4.1.3.3 Electrostatic separation

Via electrostatic separation it is possible to separate the particles from the gas, which is a very

effective method. The method is up to 100 times stronger on particles, less than 30 μm, compared to

the force of gravity. Because of differences in the dielectric properties between particle and gas

molecules, it is possible to separate those from each other by particle charged by a strong electric field.

3.4.1.3.4 Additional particle removal technologies

Another technique to reduce particle levels, is the turbulent flow precipitator, TFP. This technology is

similar to the others but have some significant advantages. It can operate at high temperatures and is

very effective when it comes to reducing particles, larger than 0.5 μm. For particle size between 2 μm

and 3 μm, the removal efficiency is as high as 99.8%

TFP works in that way that it divides the gas stream in to two areas, one area with turbulent flow and

one with stagnant flow. These two areas are beside each other and the turbulent flow area leads the gas

flow through the equipment mean while the other area creates settlings.

23

3.4.1.4 Alkali

The two most common ways to reduce alkali concentrations in syngas are removal by condensation

with other impurities or by hot adsorption on solid sorbents. Alkali vapors nucleate and agglomerate

when the temperature on the gas goes down to under alkali condensation points. It leads to formation

and addition of particles to the gas stream. It is important that the temperature is lower than 600° to

effectively decrease that the alkali vapor passes the equipment for particle reduction. Contrariwise

solid sorbent can be used in different temperatures and forms of alkali. This means that combustion

and fuel cell applications can be used.

3.4.1.5Chlorine

Biomass contains chlorine and during some circumstances are salts and hydrogen chloride (HCl)

formed. To reduce the hydrogen chloride during hot gas cleaning, the most common way is to use a

sorbent of alumina, carbon or alkali oxides, sometimes the sorbent even reduces alkali. This is done in

order to reduce gaseous hydrogen chloride effectively, at a temperature between 500-550° by

adsorption which then transports the hydrogen chloride to a solid surface, usually this lead to salt

formation.

3.4.1.6 Tars (Condensable Hydrocarbons)

To reduce the gas from condensable hydrocarbons, i.e. tars, four different ways is possible: thermal

cracking, catalytic cracking, non-thermal plasmas and physical separation. In the product gas there

will always be some tar but if the gasification temperature increases, the tar concentration decreases.

Only at chemical equilibrium will tar not be expected, even at low temperatures. For catalytic

cracking, thermal cracking and non-thermal cracking, researchers are trying to reach chemical

equilibrium by increasing the residence time of tar to achieve degradation. The last basic way to

reduce tar levels in the gas, is physical separation. At first, the product gas is cooled to liquid from

condensable vapors, and is then removed by physical means. Which method to use to, depends on

what the intended use of the gas is supposed to be. [29]

3.4.2 Cold gas cleanup

If the gasification instead occurs at atmospheric pressure, cold gas purification is preferable with

turbines and fuel catalysts in mind [7]. Cold gas cleanup is normally categorized as a “wet” method

compared to hot gas cleanup, which is a “dry” method. This due to the use of different temperatures,

the hot gas cleanup process occurs at a very high temperature while the temperature for cold gas

cleanup can vary between -62 °C for chilled methanol to 100 °C for condensed water. Cold gas

cleanup also normally uses liquid adsorbents to reduce the impurities. The disadvantage with these

first cold gas cleanup techniques is that it leads to decreased overall plant effectivity, but it will still

be an important technique in the future that will be used due to proven reliability and high efficiency.

[29]

24

3.4.2.1 Sulfur

The most common processes for low temperature reduction of sulfur are chemical, physical or mixed

chemical/physical solvent processes. In addition, there are also other ways of sulfur reduction, for

example chemical redox and biological processes. In the chemical solvent process, the weak chemical

bonds created between an amine compounds and an acid gas (CO2 and H2S are normally used),

through the use of a liquid solvent. First, acidic gases are extracted by an amine in an absorber. Later a

sorbent will recycle to the absorber by stripping, which is a process that removes components from a

liquid stream through a vapor stream, to create a concentrated acid gas stream. In the physical process,

methanol (CH4O) and dimethyl (C2H6S) are used as solvents. Compared to the chemical process the

advantages of the physical process are minimal heating requirement, minimal solvent loss and high

loadings capacity. The physical process is also good in other sulfur recovery processes, due to the

ability to remove carbonyl sulfide (COS) and hydrogen sulfide (H2O) without removing large volumes

of other acids gases such as carbon dioxide (CO2). The advantages with acid gas scrubbing processes

are the high effectiveness and selectivity, on the other hand the disadvantages are high capital and

operating costs.

3.4.2.2 Nitrogen compounds

Nitrogen impurities such as ammonia (NH3) and hydrogen cyanide (HCN) are normally removed with

water absorption due to high water solubility of ammonia, even condensed water vapor can remove

nitrogen compounds when they condense. Depending on feedstock and the upstream processing it is

possible to reduce ammonia to levels of picoliter per liter. It is possible to increase the purity of syngas

further by other gases in the syngas, such as carbon dioxide (CO2) and sulfur dioxide (SO2). These

gases affect the absorption of ammonia in aqueous scrubbing medium. For example carbon dioxide

will have an effect on both the acid gas and the ammonia to assume aqueous phase and in that way

increase the purification in the syngas.

3.4.2.3 Particles

To remove particles, water is used as a "wet scrubbing" process. "Wet scrubbing" is a very simple and

effective method, highly developed in the industry and is divided in six categories; spray scrubbers,

wet dynamic scrubbers, cyclonic spray scrubbers, impactor scrubbers, venture scrubbers, and

electrostatic scrubbers. These categories are ranked in order of effectiveness with spray scrubbers

being most effective to electrostatic scrubbers being least effective. Particles can be removed with

internal forces in the most basic processes where the effectiveness increases when the particle size

increases from 3μm. It normally leads to higher energy consumption when the effectiveness of particle

removal increases.

25

3.4.2.4 Alkali compounds

Alkali impurities can be reduced in the same manner as tars (mentioned further down in the report)

and particulates. It is done by alkali vapor condensing and agglomerating into small particles or

bonding with tars. At 300 °C most of the alkali components condense through steam and are then

removed in connection with the removal of tars and particulates by wet scrubbers. It is also possible to

pretreat the alkali components, unlike tar and particulate matter, by reducing the alkali from the raw

feedstock.

It is very frequent that alkalis are reduced by water washing or leaching. The reason is, in biomass,

most of the alkali is water soluble and up to 95 % of feedstock are water soluble or in ion exchanged

forms. Emissions of alkali can also be reduced by washing with acid instead of water. It improves the

reduction of alkali from 30% with water to 70% with acid.

3.4.2.5 Chlorine

In syngas chlorine (Cl) can exist in two ways, such as particles of ammonium chloride (NH4Cl) or as

gaseous hydrogen chloride (HCl). Ammonia chloride are formed when hydrogen chloride and

ammonia (NH3) react with each other during the gasification process because both molecules adopt

gas phase before the syngas is cooled to about 300° C. To remove chlorine wet scrubbing is a common

method, which is also very effective on removing tar, alkali and particles. It can be done either through

precipitation of ammonium chloride salts or through absorption of hydrogen chloride vapor.

Ammonia chloride particles can grow under the gas flow and adversely affect the equipment, therefore

gasification system should maintain a temperature above 300 °C, so that purification of gas can be

completed.

In the wet scrubbing, the formation of ammonium chloride is limited since cooling occurs very fast.

The advantage with the wet scrubber technique is that the process effectively reduces both forms of

chloride in gas stream. But both these processes can harm the equipment and reduce the effectiveness

since they create acidic compounds and filter cake.

3.4.2.6 Tars (Condensable Hydrocarbons)

Even though several tar compounds are non-soluble in water, tars can be removed by wet scrubbing

process. This is done by condensing vaporized tar into aerosols, which can be absorbed easily into the

water droplets when temperature is high enough during the wet scrubbing process. The wet scrubber

will then retain impurities of tar, which later is separated from the water in a settling tank. The water

can then be used over again in the scrubber. The effectiveness of the wet scrubber will in the long run

decrease when the compounds of soluble tars accumulate. Before the waste water is released to the

environment, it must first be treated with biological or/and chemical waste-water processes. [29]

26

3.5 Review: Gasification technology for syngas

Depending on the original composition of the biomass, gasification process and technology used in the

preparation, the composition of the final gas will vary. Then different types of syngas can be divided

into different categories depending on calorific value. Existing techniques that is used are fixed bed

gasification, fluid bed gasification and entertained flow gasification.

3.5.1 Fixed bed gasification

Fixed bed includes updraft, downdraft and cross draft, depending on where the biomass is supplied

and the reactive agent. This gasification technology involves indirectly preheated through downstream

gasification.

3.5.2 Fluidized bed gasification

Fluidized bed gasification includes bubbling fluid bed and circulating fluidized bed. Dried biomass is

directly supplied into the bed material and the gasification reactions will then take place, the

gasification has no exact reactions zones. The bed material such as sand, ash and finer mineral

products act as heat transfer medium to the fuel. [7]

3.5.3 Entrained flow gasification

This gasification process can produce higher amount of H2 and CO gases due to this process works at

higher temperatures and shorter residence time. [30]

3.6 Review: Reactions of steel surface during combustion

As mentioned before the reheating furnace is divided in three different zones; preheating, heating and

homogenization. Preheating of the slabs occurs, flame ash settles on the steel. After 2 - 2.5 hours occur

scale formation which occurs at sufficiently high temperatures (around 800 ˚C) the gas starts reacting

with the steel to form oxide. Iron diffuses upward to form iron oxide which settles on the top layer.

The oxide layer on the steel slab consists of three different oxides, iron (II) oxide (FeO), iron (III)

oxide (Fe2O3) and iron (II, III) oxide (Fe3O4). These oxides occur as the minerals wüstite, hematite and

respectively magnetite. If the system is in equilibrium the scale would be composed entirely of one

oxide, depending on the temperature. Equilibrium is not the case in reality, and the scale is composed

of several oxides such as FeO and Fe3O4. This is caused by mass transfer a limitation (slow diffusion)

which is controlled by Fick's first law, see equation 2. [25]

eq.2

Where J is the flow, D is the diffusion coefficient, c is the concentration and x position.

Oxide formations on steel slabs are also controlled by the temperature gradient, gas composition and

residence time. During the heating process, impurities such as alkali metals and gas components can

react with the steel surface. Impurities on the steel surface can either react before or during the scale

27

formation. It can also evaporate since the temperature is high. Since there are several different

components present, it is hard to predict the complex reactions. It will vary with local conditions, but

possible reactions can be:

Reactions between impurities and the formation of new, unforeseen minerals

Phase transition of impurities at different temperature zones

Reactions with Fe2O3/Fe3O4/FeO and impurities which results in complex Fe-minerals

Diffusion of impurities to extensive layer and inner zone along with corrosion of steel

Occurrence of substances with low melting point

Possible effects can be:

Promotion of scale formation and diffusion of Fe

Reduced heat transfer to the steel

Altered oxide formation locally on substance surfaces

Micro structural transformation due to physical and chemical processes

3.6.1 Thermodynamic analysis of corrosion problems

Thermodynamic calculations make it possible to study the equilibrium and to see which diffusion

controlled reactions will take place. A thermodynamic analysis from earlier study on the steel surface

has been done in order to investigate the effect of alkali metals (sodium and potassium) and chloride.

3.6.1.1 Analysis 1: Low temperature

The heating phase is the first step when the alkali oxides and alkali chlorides been deposited on steel

substances but not formed scale. Calculations show that at about 800 ˚C, the kinetics is so fast that the

scale is formed. Small amounts of solid chloride-compounds (with Na, K, or Fe) are formed probably

because of low hydrogen chloride (HCl) concentration which operate reaction 1 to the right. (R may

be: Na, Fe, or K)

RCl (g) + H2O = ROH (g) + HCl (g) (1)

3.6.1.2 Analysis 2: High temperature

The formation of different compounds may affect the scaling properties. Hence a calculation of alkali

oxides and alkali chlorides influences when Fe2O3 scaling have been done, in order to see how the

scale is affected by the various ash materials.

28

Reactions between Fe2O3, Na2O, and K2O (alkali oxide):

Interaction between Fe2O3 and K2O is not very strong, there is no formation of new compounds. But

over 1027 °C will reactions between Na2O and Fe2O3 occur and form Na2Fe2O4.

Na2O + Fe2O3

Na2Fe2O4 (2)

3.6.1.3 Analysis 3: Multi-component interaction at high temperature

The systems are complex, where mineral from casing liners and fluid bed material may occur. To take

into account, other components at high temperature and their effect on the ash equilibrium with Fe2O3,

selected components have been analysed in this step. Also analyses in order to illustrate the effect of

introducing other minerals in equilibrium with alkali and scale have been done.

Fe2O3-K2O and Fe2O3-KCl systems and interaction with Al2O3:

To be able to remove potassium oxide (K2O) and potassium chloride (KCl) from the two different

systems in the syngas, an aluminium silicate that neutralizes alkali substances can be added to the

system. Analysis shows that if small amounts of silicon dioxide (SiO2) and aluminium oxide (Al2O3)

are added to the system, aluminium silicate (2KAlSiO4), potassium aluminate (2KAlO2) and

potassium silicate (K2SiO3) can be formed. The reactions will then be:

K2O + Al2O3 + 2SiO2 = 2KAlSiO4 (3)

K2O + Al2O3 = 2KAlO2 (4)

KAlSiO4 = KAlO2 +SiO2 (5)

K2O + SiO2 = K2SiO3 (6)

Because of the high temperature is KAlSiO4 decomposed in reaction 5. Silicon dioxide (SiO2) and

aluminium oxide (Al2O3) contribute to bind potassium chloride (KCl) in potassium silicates.

3.6.1.4 Analysis 4 Vaporization

Vaporization of alkali in alkali oxides and alkali chlorides which starts at 827 ºC and end at 1077 ºC

are analysed. The analysis showed that potassium chloride (KCl) and sodium chloride (NaCl) vaporize

slowly at temperatures above 727 ºC and faster at 927 ºC. Potassium (K) and sodium (Na) react with

chloride (Cl) and hydroxide (OH) then vaporize by forming potassium chloride, sodium chloride and

ROH (R = K or Na). [31]

29

3.6.2 Formed ash from different fuels during combustion

Biomass contains salts in soluble form which are ash-forming elements but during combustion it will

occur as bounded mineral matter. To understand the ash behaviour and their relations with the ash

analysis, related problem in the process and equipment have been studied. The results are shown in a

three phase diagram below in figure 10.

Figure 10: properties of the ash and slag, depending on the composition

Critical components given as assembled components CaO+ MgO, K2O+Na2O and P2O5+SiO2. Red

dashed area (P2O5+SiO2) indicates large risk of slag formation. The acidic particles which can have

low melting temperature are founded here.

In the violet dashed area (K2O+Na2O) are fine particle (< 1μm) formation extensive, where the

components are in gas phase and condenses when the exhaust gas cools. The amounts of fine particles

are related to the alkali contents in the fuel. The particle emission increases downward in the phase

diagram. Particle formation depends on the ratio between potassium/silicon (K/Si) and

potassium/phosphor (K/P), silicon and phosphor can affect potassium to bond with the bottom ash, and

in that way reduce the tendency to form particles.

Low risk of slag formation and decreasing tendencies of particle formation in the direction of

CaO+MgO components is green marked. Particles in this area pass through the furnace as a dry and

non-sticky dust. During the gasification phosphor will be separated as solid phosphates and should not

exist in the furnace. Ash composition then will be in the upper left area of the phase diagram.

Analysis from the ash contents showed that if potassium and SiO2/P2O5 increases for ashes containing

silicates and phosphates will the melting temperature decrease which can lead to higher risk of slag

formation. [25] [32]

30

3.7 Review: Monitoring of alkali species concentration

In combustion of solid fuels such as biomass, the hot flue gas is usually contaminated with various

alkali species. As mentioned before, this causes problems such as corrosion on various system

components, resulting in e.g. ashes on the final products and reduced heat transfer efficiency. Alkali

emissions also affect the energy and mass flow in furnaces. To control this kind of problems and

protect plant equipment from corrosion and attrition, there must be able to measure the level of where

the alkali species and alkali related components occur. The maximum allowed value is 0.1-1 ppm (wt

%) or below. There are different methods of measuring the alkali concentration and one of the most

common in situ measurements of various alkali species is continuous on-line. By using an excimer

laser induced fragmentation fluorescence (ELIF) and surface ionization (SI), a real time monitoring

can be measures of the flue gases from the solid fuel combustion in fluidized bed (FB) gasification of

biomass. By using ELIF and SI it is possible to measure the alkali species by investigating ELIF

signals from the flame vaporizers as a function of energy density of the laser. During the investigation

could alkali species continuously with sub- ppb sensitivity and time resolution down to seconds be

measured [33].

3.7.1 Excimer laser induced fragmentation fluorescence

The method is based on fragmentation of alkali molecules that are presented in the flue gas. An 193nm

excimer laser is used, that produces alkali atoms such as sodium and potassium which fluorescence

(emitted radiation of an absorbed wavelength) at 580-770 nm. Using a laser beam to fragment the

alkali compounds with excitation of the free alkali metal atoms. By fluorescence measurements can

the concentrations be determined. The laser power is sensitive for detection of alkali particles. [33]

3.7.2 Plasma Excited Alkali Resonance Line Spectroscopy

A plasma method called PEARLS has proven stable alkali detection and can be used for detection of

alkali compound in vapor phase and bound to other particles. The method is based on thermal

excitation of alkali atoms in hot plasma where the compounds are decomposed and the concentration

of potassium or sodium is measured by emission spectroscopy or optical absorption. [33]

3.7.3 Surface Ionization

The method is based on using the SI instrument that uses a hot platinum (Pt) surface for the ionization

of alkali metals in the gas stream. The technique can be employed for counting alkali-containing

particles. A molecule or an atom that is adsorbed by a hot surface can be desorbed in ionic or in

natural form. [33] The possibility of ionic and natural fluxes form is expected by equation 3.

( )

eq. 3

31

Where or n+/n0, represents the ratio of natural and ionic fluxes from the surface, g+ /g0 is the

statistical sum ratio of ions and naturals (g+/g0=1/2 for alkali metals) and e, ,IP,kB and T represents

elementary charge, surface work function, ionization potential, Boltzmann’s constant, and absolute

temperature, respectively.

The ionization probability possibility that an adsorbed species will desorbed in ionic form is

predicted by:

( ) eq. 4

The ionization potential is larger than the surface works function for the elements that are adsorbed on

a metal surface. Then will be close to zero (since <<1), and the emission of naturals are depended

on emission of ions hence will emission of ions will dominate over emission of naturals.

This application is developed for alkali metal detection which is based on desorption of Na+ and K

+

ions when compounds containing sodium and potassium will strike a hot platinum surface. Alkali

metals ionization potential (IP) is presented in table 6.

Table 6: Ionization potential of alkali

Na K

IP 5,14 eV 4,34ev

β 0,89 >0,99

5,5 eV 5,5 eV

Platinum Surface temperature 1227° 1227°

These properties of SI phenomenon make it possible to design selective and sensitive alkali detection

instrument based on surface ionization. If larger alkali salt metals strike the hot metal surface, the

ionization process will become longer and more complicated since the particles must first decompose

before alkali metals can undergo ionization at the hot surface. Studies have shown that the ionization

efficiency will decrease with particle size above a particle diameter of 0,01μm which depend on type

of alkali salt and surface temperature. The figure 11 below show the SI instrument. [33]

32

Fig 11: The heated sampling line is used for extracting flue gas to the SI detector consisting of a

filament and an ion collector. [33]

3.8 Experiment: composition, tar content and particle levels

An experiment from 2014-04-07 was done in the reheating furnace with wood chips. Unfortunately a

problem occurred during the run which resulted in no valuable data for the experiment could be used.

Instead data from an earlier experiment form 2013-09-25 with biocoal was used and the syngas

content and the tar composition was analysed. See Appendix A for photos of the fuel and the reheating

furnace. [24]

3.8.1 Syngas composition

Figure 12 describes the amount of common compositions in syngas and also hydrocarbons with low

molecular weight, in this case called tars since they still can affect the steel surface. In table 7 are the

syngas containing components of tar with high molecular weight summarized. [24]

Figure 12: Syngas composition varying with time

0

5

10

15

20

25

13:12:00 14:24:00 15:36:00 16:48:00

Ga

sco

mp

osi

tio

n (

mo

l %

)

Time

Syngas composition

H2

CH4

CO

CO2

33

Table 7: Composition of high molecular weight tar [24]

Component 20130925

ER 0,038

H2O/C 3,99

Benzene 10,76

Toluene 15,11

m/p-Xylene 2,14

o-Xylene 2,91

Indan 0,69

Indene 2,06

Naphthalene 3,37

2-Methylnaphthalene 1,36

1-Methylnaphthalene 1,54

Biphenyl 0,34

Acenaphthylene 1,42

Acenaphthene 0,85

Fluorene 0,35

Phenanthrene 0,68

Anthracene 0,24

Fluorantene 0,39

Pyrene 0,39

Total tar (mg/Nm3) 44,58

Tar yield (mg/kg dry fuel) 103,87

Tar dew point (0C) 49,1

3.8.2 Particles

The particle size and the amount of particles in the syngas depend on each other and are shown in

figure 13 below. The amount particles are highly reduced when the size of the particle increases. Table

8 summarize the results of particles measurement and are shown below. [24]

34

Figure 13: Shows the amount of particles varying with particle size. [24]

Table 8: Particulate content [24]

Equivalence ratio (mol/mol)

0,038

H2O/C

3,99

Total particles (mg/Nm3)

1100,16

Particles yield (mg/kg dry fuel)

2563,37

4.0 Discussion

The benefits of bio fuels compared to that of fossil fuels is that they are regarded as renewable and

carbon-neutral energy source because the carbon dioxide that is released during combustion is

recycled through the process of photosynthesis, which means it is a renewable resource.

There are also disadvantages of increased biomass use. Energy crops give higher subsidies than food

products, which may in the future lead to higher prices for food products. It can also affect

biodiversity as biomass plantations require large land areas at the cost of forests. Therefore, the

production and cultivation of biomass should occur in a controlled and sustainable manner.

When comparing the result from the experiment with the other gasification technologies there is a

large difference between the amounts of tar. The experiment was done with a pretreatment technology

called hydrothermal carbonization. This is a method for wet biomass to improve the solid fuels and

0,000

200,000

400,000

600,000

800,000

1000,000

1200,000

0,000 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000

dm

/dlo

g(D

p)

[mg

/Nm

³]

Dp [µm]

Particle size

35

then be used in a high temperature agent. The difference here is that preheated air where inserted in the

gasifier before the biomass where added. This will lower the tar composition in the syngas. Since it

also is possible to add steam to the feed stream, it is possible to decrease the tar composition due to

steam reforming reactions. Due to this, the tar level is very low after the experiment compared with

other gasification technologies and can be seen in table 9 below, were the tar concentration of different

gasification technologies are shown.

Table 9: Gasification technologies [30]

Gasification Tar (mg/Nm3)

Fixed bed

- Updraft 35 000

- Downdraft 500 – 1 000

Fluidized bed

- Bubbling 13 500

- Circulating Low

Entrained flow Almost tar free

To clean the syngas and reduce the particles, the best and most common technique is to use hot gas

cleanup sine it is more effective then cold gas cleanup due to the higher temperature. Since the

particles sizes from the experiment are up to 14 μm, it is possible to reduce the particle and clean the

syngas with electrostatic separation which is a very effective method and requires a particle size lower

than 30 μm. Electrostatic separation will in this case be the best method.

Some exact values for the amount of alkali metals could not be measured and cannot therefore be

compared with previous studies, as there are only few measuring methods and those are not well

proven. But earlier studies have shown that a lower combustion temperature will lead to a higher

amount of alkali metals remaining in the bottom ash. Since the alkali metals are very reactive,

reactions between alkali and other substances will occur.

The analysis from the reaction of steel surface during combustion are based on calculations from

equilibrium and are not completely reliable due to the reactions in actuality are controlled by diffusion.

36

5.0 Conclusions

It is possible to replace the fossil fuels with biomass fuels, but the heating value is lower for biomass

and contains more impurities which lead to more ash substances during combustion. Increasing the

purity of the biomass and syngas is there for necessary for both heating value and final product. To get

as clean syngas as possible, the best way is to first pretreat biomass by pyrolysis to reduce alkali

compounds. Preheated air in the gasifire reduces impurities in the syngas when biomass is transformed

into syngas. Also higher temperature will lower the impurities.

The syngas can be pretreated through hot gas cleanup which is the most effective cleaning method for

syngas to reduce the impurities. The final syngas can be more purified if it is cooled down before it is

used. Depending on the syngas application and the final produced product requires different levels of

purity. Every steps of purification are expensive and should not be used if it is unnecessarily.

Measure values of ash concentration was not done and can their fore not be compared to earlier

results. Today is it very difficult to measure the ash concentration and the technologies for these needs

to be developed further more. The ash compositions that can occur according to the phase diagram in

figure 10 will be K2O – CaO (MgO) – SiO2, K2O – CaO – P2O5 or K2O – MgO – P2O5.

6.0 Future work

Test of existing measurement methods on steel plant for investigating and development of the

methods.

Economic analysis and if it is possible to lower the cost.

More researches should be done to enhance the understanding and importance of biomass as

fuel instead of fossil fuels, to hopefully make it more profitably for the farmers to cultivate

salix.

6.0 Acknowledgement

We would like to thank our supervisor Pelle Mellin who has helped us through this project.

37

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40

7.0 Appendix

7.1 Fuel

Figure 16: Biocoal Figure 17: Wood chips

7.2 High temperature agent gasification.

Figure 18: Preheater Figure 19: Gasifire

41

Figure 19: Combustion chamber Figure 20: Feed hopper