coal selection criteria

8/12/2019 Coal Selection Criteria

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Coal Selection Criteria for Industrial PFBC Firing

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COOPERATIVE RESEARCH CENTRE FOR COAL IN

SUSTAINABLE DEVELOPMENT

COAL SELECTION CRITERIA FOR INDUSTRIAL PFBC

FIRING

PROJECT 3.2

by

John F. Stubington

Valmaiwati Budijanto

School of Chemical Engineering and Industrial ChemistryUniversity of New South Wales, Sydney 2052, Australia

(March 2003)


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ABSTRACT

Pressurized Fluidized Bed Combustion (PFBC) is one of the clean coal technologies.There are several PFBC plants operating all over the world. As this technology is

relatively new, some problems were encountered during the plants operation. These

include combustion inefficiency, bed agglomeration, cyclone clogging, filter blockage,

gas turbine and in-bed heat exchanger tube erosion and corrosion. In this report, we have

focussed only on those aspects of the problems which were coal-related, since those

aspects affect coal selection for PFBC.

Combustion inefficiency was mainly caused by unburnt char elutriation from the bed. For

Australian export coals, it was found that unburnt char elutriation was related to the ratio

of Telovitrinite : Inertinite. For a wider range of coal rank, there was generally a decrease

in combustion efficiency with increasing rank, but this generalisation did not always

predict coal performance in commercial PFBC plants. Hence, petrographic analysis is

preferred for bituminous and sub-bituminous coals. A Telovitrinite : Inertinite ratio 1200oC.

During combustion, iron contained in the coals was oxidized and decomposed, causing

fouling and deposit formation. Low iron content coals were recommended to be used tominimize deposit formation.

Two solutions to filter blockage problems were to use ash for maintaining bed inventory

and to use coals with high Al2O3and SiO2contents in their ash, which agglomerated to

larger ash particles. The recommended method to overcome filter blockage is to allow

larger particles into the filter which form a layer of cake on the filter surface instead of


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penetrating into it. Cyclone plugging was due to the same properties of coal which caused

sticky ash material.

Gas turbine blades erosion was due to fine quartz particles while corrosion is due to fine

ash particles and corrosive compounds of sulfur, alkali and alkaline earth elements

contained in the coals. To reduce erosion and corrosion it was recommended to use coals

with low quartz, sulfur and alkali contents.

Another part of PFBC plant which experienced erosion and corrosion is the in-bed heat

exchanger tubes. In preventing such erosion and corrosion, at low temperature it is

important to apply thermal spray coatings. For high temperature, the tube materials

should have sufficient erosion and corrosion resistance due to the formation of hard oxide

scale on the surfaces.

Pollutant emissions need to be regulated to achieve sustainable environment control.

These emissions were mainly influenced by the operating conditions rather than the coal

properties.


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TABLE OF CONTENTS

PageAbstract i

Table of Contents iii

List of Tables iv

1. Introduction 1

2. Industrial PFBC Plants 2

3. Problems in PFBC Plants and Their Solutions 3

3.1 Combustion Efficiency 3

3.1.1 Elutriation of Unburnt Carbon 3

3.1.2 Other Combustion Efficiency Considerations 4

3.2 Bed Agglomeration 4

3.3 Ash Deposits 7

3.4 Cyclone Plugging 8

3.5 Filter Blockage 9

3.6 Erosion & Corrosion 10

3.6.1 Gas Turbine Erosion 11

3.6.2 Gas Turbine Corrosion 12

3.6.3 In-bed Heat Exchanger Tubes Erosion 12

3.6.4 In-bed Heat Exchanger Tubes Corrosion 13

3.7 Environmental Performances 13

4. ABB Carbons Process Test Facility (PTF) 16

4.1 Combustion Efficiency 16

4.2 Sulfur Retention 174.3 NOxEmissions 18

4.4 N2O Emissions 18

5. Conclusions 19

6. Acknowledgments 22

7. References 23


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LIST OF TABLES

PageTable 1. Fuels Tested in PTF. 16

1. INTRODUCTION

Coal fed power plants are the most widespread choice to produce electric power, as coal

deposits are abundant and spread all over the world. In addition, the price of coal is

relatively stable. Nevertheless, its carbon dioxide (CO2) emission per unit calorific value

is among the greatest of fossil fuels. Hence it is essential to develop a competent coal

utilization technology that maximizes the plant thermal efficiency while keeping the

emission of CO2and other non-environmental friendly emissions (SOx, NOx, etc) at their

minimums.

Pressurized Fluidized Bed Combustion (PFBC) is one of several clean coal technologies.

Besides being thermally efficient, it requires low capital and operating costs and has the

potential to be a competitive source of low cost generation when using low to medium

sulfur content coals (Stubington 1997).

However, some problems have arisen in commercial operation of PFBC plant, including

elutriation of unburnt carbon, bed agglomeration, cyclone plugging, and gas turbine blade

and in-bed heat exchanger tube erosion. These are the problems that are associated

mainly with the coal used. Therefore, there is a need to carefully select the coal fired to

minimize or eliminate these problems.


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2. INDUSTRIAL PFBC PLANTS

In Japan, the Tomatoh-Atsuma plant was installed by MHI for Hokkaido Electric to

produce 85 MWe since 1998. The 350 MWe Karita plant, which was built by ABB

Carbon and IHI for Kyushu Electric Power, has been operating since October 1999. Also

in Japan, Babcock-Hitachi built a 250 MWeOsaki plant for Chugoku Electric Power that

has been operated commercially at full load since December 2000 and another 250 MW e

unit is scheduled to start operating in 2008. In Sweden, ABB Carbon built the Vrtan

plant for Stockholm Energi with a total output of 135 MWe. Another plant in Europe,

Escatrn, which produces 79.5 MWeoutput, was constructed by ABB Carbon and Spain

B&W for Endesa. The Tidd demonstration plant was completed by ABP (a joint venture

between ABB and B&W) for AEP, powering Ohio with 75.6 MWe, but is now shut

down. ABB Carbon has built another PFBC plant at Cottbus in Germany which is

operating to produce a total output of 75.6 MWe.

One difference between the plants is the coal used. The coals fired in Japanese power

plants are mainly Australian bituminous coals while Escatrn is using lower rank Spanish

black lignite. Another difference is the coal feeding system. Tidd, Osaki and Karita use

the slurry feeding system, for which the coal and limestone are mixed with water and

then pumped by several positive displacement pumps to the PFBC. On the other hand,

Tomatoh-Atsuma adopted the dry coal-limestone feed system due to its high efficiency

and reliability. After the coal is pressurized in a lock-hopper-system, it is supplied to the

PFBC through a distribution hopper and supply tubes (Koshimizu 1998).

One major difference between the three plants in Japan is the hot gas cleaning system.

Tomatoh-Atsuma applies a combination of cyclones and ceramic filter for cleaning the

stack gases prior to entering the conventional gas turbine. The ceramic filter allows

increased gas turbine efficiency and consequently cycle efficiency. On the other hand,

Karita and Osaki rely only on cyclones to clean the hot gases. However, they are using

special ruggedized gas turbines which are able to tolerate the low quantity of fine

particles which escape from the cyclone.


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3. PROBLEMS IN PFBC PLANTS AND THEIR SOLUTIONS

3.1 Combustion Efficiency3.1.1 Elutriation of unburnt carbon

Recent research discovered that unburnt char elutriation was the major disadvantage of

using one Australian black coal, causing combustion inefficiency in PFBC plants. The

elutriated fine char particles may pass through the cyclone and be caught in the filter cake

on the ceramic filter, giving rise to the sticky ash problems. The combustion of the

unburnt char increases the cake temperature, contributing to the stickiness and causing

damage to the filter (Stubington, Wang et al. 1998).

Combustion-enhanced attrition was found to be the dominant mechanism generating

elutriable char particles (Wang and Stubington 2002). Unburnt char elutriation is directly

related to combustion efficiency and is defined as the percentage of the elemental carbon

in the coal fed that was collected in the cyclone and measured as the loss on ignition of

the cyclone fines (Wang and Stubington 2001). For a standardized test in the bench-scale

PFBC, it is predicted using the following correlation (Wang and Stubington 2001):

Char Elutriation = 3.26 (Telovitrinite/Inertinite)0.4045

(%) (R2= 0.74) (Eq. 1)

High telovitrinite content contributed to a high unburnt carbon elutriation while coal with

low inertinite content (mature coals) exhibited light-up problems (Palit and Mandal

1995). Coals with higher telovitrinite/inertinite (and higher unburnt char elutriation)

exhibited greater swelling during devolatilization in PFBC, producing chars with larger

pores from which more fine char particles were generated by attrition (Wang andStubington 2001). A coal with unburnt char elutriation of less than 1.7% was found to be

satisfactory, while char elutriation above 4.2% was unsatisfactory. Coal with char

elutriation between 1.7 4.2% could not yet be categorized due to insufficient data and

should be considered unsatisfactory until further research revealed appropriate data

(Wang and Stubington 2001).


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Tomatoh-Atsuma was using the coals Fuel Ratio (Fixed Carbon/Volatile Matter) as the

parameter for predicting coal combustion performance in the furnace. However,

occasionally contradictory results had been encountered. The CCSD research discussed

above found that the elutriated unburnt carbon correlated with the ratio of

Telovitrinite/Inertinite rather than with of Fuel Ratio. This research had helped Tomatoh-

Atsuma in solving its problems (Wang 2002).

3.1.2 Other Combustion Efficiency Considerations

The major factor causing combustion inefficiency is mostly unburnt carbon elutriation,

caused by attrition of the char particles in the fluidized bed and hence affected by the char

structure formed during devolatilization. Earlier work reported that other factors affected

the combustion efficiency, including coal rank or volatile content, coal reactivity,

swelling, fragmentation and calorific value. One previous study concluded that a lower

coal rank or a higher volatile content increased the combustion (Laughlin and Sullivan

1997). An increase in pressure resulted in reduction of the volatile transport rate from

inner pore to outer surface and thus decreased the coal volatile yield (Laughlin and

Sullivan 1997). Char reactivity increased with increasing oxygen and alkaline oxide

contents and porosity. It also increased with decreasing rank and mean vitrinite

reflectance (Laughlin and Sullivan 1997). An increase in char reactivity increased the

combustion efficiency, but char reactivity was not an important consideration for high

pressure conditions. Generally, high volatile bituminous coals performance was less

sensitive towards changes in chemical kinetics. Lower coal calorific value and higher ash

and sulfur contents increased the inefficiencies (Huang, McMullan et al. 2000). Although

no correlation between Crucible Swelling Number (CSN) and combustion efficiency was

developed, it was shown that an increase in CSN decreased the combustion efficiency for

Taiheiyou and Lithgow data in the Wakamatsu plant (Misawa 2000).

3.2 Bed Agglomeration

Another major issue in PFBC plant is bed agglomeration or sinter egg formation. These

agglomerates are bed particles which are fused together around a hollow core that

originated from coal paste lumps (Zando and Bauer 1994). Escatrn, Vrtan, Tidd,

Tomatoh-Atsuma, Wakamatsu and Karita encountered this problem. At Escatron,


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sintering caused several boiler stops. Tidd experienced bed agglomeration only when it

was operating at full load and over 815oC. Bed agglomerations were indicated by uneven

bed temperatures, decaying bed density and reduction in the heat absorbed (Scott and

Carpenter 1996).

After being analyzed by SEM, EDAX and XRD, it was found that the agglomerate

consisted of fine particles of SiO2and Al2O3in the ash. These particles stick together in

the presence of CaO (from the bed particles) to form Ca2Al2SiO7glass (Ishom, Harada et

al. 2001). The oxides adhered to the surface of the combusting coal. Fine ash and more

CaO deposited on the agglomerate forming a bigger agglomerate. Bed agglomerates

formed when the temperature was below 1300oC, possibly around 1100

oC where

particles in the agglomerate started to deform even if the whole grain melted at 1300oC

(Ishom, Harada et al. 2001).

The causes of these sinter accumulations were poor fuel splitting resulting in large paste

lumps in the bed, insufficient fluidizing velocity and localized high feed concentration at

full bed height (Zando and Bauer 1994). Failure in the fuel feeding system, e.g. blockage,has also led to an agglomeration problem. To achieve a finer fuel splitting, it was

necessary to increase the paste moisture content. However, this could only be done at the

expense of reduced thermal efficiency. Installation of more air nozzles improved the bed

fluidization. Decreasing the bed particle size and operating in the turbulent regime could

also help the fluidization.

Inadequate fuel distribution, which was caused by bed defludization, could increase the

unburnt carbon elutriation, gas temperature (due to post combustion of unburnt elutriated

char) and SOx emission (Wang 2002). Karitas measures to solve these problems were

decreasing the top limestone particle size from 6 mm to 2 mm, adding more fluidizing

gas nozzles to improve fluidization in the bottom area and reducing the operating

pressure (Wang 2002). Another problem faced by Karita was that it could not operate at

pressures above 1.2 MPa, which caused bed agglomeration for some coals. Karita is now

operating at about 80% load, with an operating pressure below 1.1 MPa (Wang 2002).


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Blockage of fuel feeding lines has been noted in Wakamatsu. This could be resolved by

improving the coals particle size distribution and equipment modifications (Sakanishi

1995). Such a problem was also reported in Osaki, where their fuel nozzle was clogged

several times by foreign material in the raw coal and coal lumps. As a countermeasure, a

reducer in front of the nozzle cut-off valve was installed (Matsumoto and Kawahara ).

Swelling coals are sticky and they could stick the surrounding bed particles together

forming agglomerates (Palit and Mandal 1995). Therefore, it was advised to use coals

with low crucible swelling number (CSN) or non-caking coals.

Bed agglomeration was also encountered in plants that used dry coal feed, such as

Tomatoh-Atsuma, instead of slurry feeding system. The temperature of the combustion

domain near the fuel nozzle outlet induced the agglomeration. A low ash fusion

temperature generated agglomeration. The Tomatoh-Atsuma plant selects coals based on

the iron content, coals with an iron content of 7% or more will have low ash melting

point (Kazuhiro 2002). Karita requires their coals to contain less than 7% Fe2O3and to

have an ash fusion temperature higher than 1200

o

C. If coals with low ash fusiontemperature are used, the bed temperature has to be kept below the ash fusion

temperature to prevent agglomeration (Palit and Mandal 1995).

Bed agglomeration is caused by amorphous clay mineral fragments and alkali species

adhering to sorbent and chars surfaces (Steenari, Lindqvist et al. 1998). Inside the

agglomerates, the chars are still burning, causing high temperature and reducing

conditions. Steenari et al. found that reducing conditions in the bed caused sintering

through reaction in the CaS-CaSO4system and through eutectic melting of silicate-iron

mixtures (Steenari, Lindqvist et al. 1998). An increase in the coals clay content increased

the viscosity of the paste (Wright, Clark et al. 1991). Less agglomeration was found when

using dolomite instead of limestone as the sorbent. The reason was that dolomite contains

a higher quantity of MgO which raised the ash fusion temperature of the CaO-MgO-

Al2O3 (Marocco and Bauer 1993). Improved bed mixing and fluidization was observed

by using finer dolomite (


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Earlier PFBC research showed that Australian coals are superior, in terms of bed

agglomeration, due to their high ash fusion temperature (Stubington 1997). However this

is no longer an advantage if the bed is operated at a higher Ca:S ratio than is required for

sulfur capture in order to maintain the bed inventory.

Greater concerns exist when combusting low rank coals (sub-bituminous coals and

lignites). Although they have high combustion efficiencies their high alkali content

caused sintering and fouling problems, compared with combusting bituminous coals

(Sondreal, Jones et al. 1993). Again, by using low-alkali coals, this problem could be

reduced. Pressure, steam and hot spots in the bed also promoted sintering. Low bed

temperature (


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Deposit formations on heat exchanger surfaces had been reported at Vartan, Tidd and

Escatron (Steenari, Lindqvist et al. 1998). Furthermore, deposit formation and fouling

were also found on cyclone surfaces and other parts of the flue gas ducts.

The key element in fouling is iron. FeS2, which is the most common iron mineral pyrite,

decomposed and oxidized during combustion (Steenari, Lindqvist et al. 1998). In

reducing conditions, mixtures of FeS and FeO are formed. FeO and other iron-rich oxides

react with kaolinite and quartz to form molten products at temperatures between 900

1000oC (Steenari, Lindqvist et al. 1998). To reduce ash deposition and fouling, it is

advised to use coals with low iron content.

3.4 Cyclone Plugging

Cyclones play a significant role in ensuring the survival of the gas turbine, especially

when ceramic filter tubes are absent. The gas exhaust from the cyclones has to be

sufficiently clean to minimize the turbine blade erosion. Osaki had encountered cyclone

plugging, causing them to suspend their operation for inspection and it was found that the

plugging was due to the properties of the coal which produced sticky ash material.

The cyclone plugging in Tidd was due to the high coal and sorbent elutriation rates,

maldistribution of ash loading to individual cyclones and undersized ash removal system

(M.Marrocco and al. 1991). In addition, Tidd had experienced cyclone fires. The majority

of the fires occurred in the lower part of the cyclones. They happened because of carbon

carryover to the cyclones, which was due to operation at low loads where combustion

efficiency and low bed particle residence time had significant impacts (M.Marrocco and

al. 1991).

The same problem was reported at the Wakamatsu plant in Japan and was solved by

improving the coal particle size distribution (Sakanishi 1995). The Ca:S molar ratio was

increased way above the requirements for SO2control to reduce the fly ash stickiness and

to maintain bed inventory.


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Furthermore, the major reason causing several operational shutdowns of the Escatrn

plant in Spain was cyclone ash extraction system blockage (Scott and Carpenter 1996).

Sintered material (agglomerates) deposited on the cyclone walls and in the ash extraction

system. Increasing the coal feed rate to increase the production of steam increased the

bed height and the flow of particulates to the cyclone. This led to more agglomeration

which blocked the cyclone. Moreover, the complex design of cyclones with many ducts

and flow direction changes further intensified the plugging. Modifications to the cyclone

ash removal system have reduced the problem (Martinez and Menendez 1995), (Martinez

and Menendez 1994).

3.5 Filter Blockage

This problem is only faced by PFBC plants which depend on the ceramic filter for

secondary hot gas clean-up prior to the gas turbine inlet, an example of such plants is

Tomatoh-Atsuma in Japan. This problem involved filter blockage, filter breakage, gas

leakage and fires, attributed to temperature effects, hydrodynamic effects, mechanical

effects, filter material effects, sorbent properties/reactions, ash composition effects and

volatilisation / condensation of alkalis (Stubington 1997). Most of them have been solved

but the problem is being investigated further to improve the understanding of ash

chemistry.

Finer ash particles penetrate into the filter, causing filter blockage. This ash was

described as sticky due to its tendency to stick on the filter surface and it could not be

removed by cleaning. It led to unstable pressure drop across the filter cake (Stubington,

Wang et al. 1998). Excessive deposits could lead to filter breakage. Larger ash particles

in the exhaust gas flow to the filter reduced the blockage, thus easing the cleaning of

filter cake. This solution was demonstrated at Wakamatsu.

Elutriated material from the attrition of limestone bed particles contained calcium

compounds that could form low melting point eutectics which decreased the ash fusion

temperature of material accumulated in the filter cake. A higher Ca:S ratio was necessary

to maintain the bed height for low-sulfur Australian coals. This neutralized the high ash


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fusion temperature advantage of Australian coals (Stubington, Wang et al. 1998). The use

of dolomite sorbent instead of limestone could also raise the melting point of alkali

eutectics in the filter cake (Stubington 1997). Another solution was to use coal ash

instead of limestone for maintaining bed height. High ash coals were found to build bed

height faster (Sudhakar Gupta, Mandal et al. 1995). This method was not very effective

in capturing sulfur, however this should not be a major problem as Australian coals

produce low level of sulfur emissions (Peeler, Lane et al. 1990). Alternative methods to

maintain bed height, such as zero-stage cyclone or selection of coal with appropriate ash

particle size distribution, have been investigated. They should be encouraged to maintain

the advantage of Australian coals.

Experiments with two Australian coals were conducted at Wakamatsu. Ashes from one

caused high stationary pressure drop in the ceramic tube filter (Iwamoto, Ishom et al.

2001). This coal was found to produce very fine ash (


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Operating procedures: start-up, shut-down and load-following procedures affected theconcentration and temperature profiles which must be compatible to materials

limitations.

3.6.1 Gas Turbine Blade Erosion

Gas turbine blade erosion and corrosion is an acute problem in PFBC plants, decreasing

the turbine efficiency and blades durability and increasing the risk of turbine operation

(Li, Chuming et al. 1991). Although the hot gas exhausted from the furnace has been

desulfurized and cleaned, a certain quantity of corrosives and particulates entering the gas

turbine is inevitable. The erosion rate was found to increase substantially when cyclone

clogging occurred (Li, Chuming et al. 1991).

Ash particles may erode the turbine blades. The main component of the ash that is

responsible for the erosion is fine quartz particles. Quartz particles are very hard and

angular, so that the very fine particles passing through the cyclones are abrasive to the

metal blades. Most of the large quartz particles are removed by the cyclone, only the fine

particles (


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less free SiO2. Also, a maximum of 10% ash contained in the coal is needed to obtain an

excellent result. Karita requires the soot and dust concentration at the gas turbine inlet to

be 840 mg/Nm3or less.

3.6.2 Gas Turbine Blade Corrosion

Corrosion is due to fine ash particles and corrosive compounds of sulfur, alkali and

alkaline earth elements contained in the coals (Li, Chuming et al. 1991). As mentioned

before, cyclones are not 100% effective and hence some particulates and corrosive

compounds managed to escape and enter the gas turbine especially when the cyclones

were clogged.

Metal corrosion occurs as a result of complex chemical reactions at high temperature.

Sulfur (SO2and SO3) and alkali (Na2O and K2O) react to form alkali sulfates with low

melting points (Li, Chuming et al. 1991). Deposition of such sulfates in their molten state

act as an adhesive to stick the micro particles on the blades, promoting complex chemical

reactions forming low melting point complexes impairing the oxide protection on the

blade thereby exfoliating the metal surface by gas and particles flow (Li, Chuming et al.1991).

To minimize the total alkali release from dolomites, an extremely pure metamorphic

dolomite (e.g. Kaiser Dolowhite) may be used. Alkali removal sorbents, such as

emathlite, have been tested for PFBC application. Up to a temperature of 1200oC, either

a packed bed of emathlite was placed after the cyclone or small emathlite particles were

injected directly into the combustion products prior to the cyclone entrance to control the

alkali to acceptable level (Newby, Keairns et al. 1989). To reduce this type of corrosion,

it is recommended to operate the plant with coal that is low in sulfur and alkali, such as

Australian black coals.

3.6.3 In-bed Heat Exchanger Tubes Erosion

This type of erosion was experienced by Wakamatsu plant. The erosion mechanism is

complicated due to the high operating temperature and the following interaction of


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oxidation and erosion (Tsumita, Namba et al. 1997). In preventing such a phenomenon, at

low temperature it is important to apply thermal spray coatings. For high temperature, the

tube materials should have sufficient erosion and corrosion resistance due to the

formation of hard oxide scale on the surfaces (Tsumita, Namba et al. 1997). Abrasion and

leakage was found in Osakis boiler tube. The cause of this problem was believed to be

solidified fluid deposits which remained on tubes in the furnace. When high velocity fluid

flowed, abrasion progressed ten times faster. Modifications in the shutdown procedure

were proved to prevent such occurrence (Matsumoto and Kawahara ).

3.6.4 In-bed Heat Exchanger Tubes Corrosion

Some researchers found that the presence of ash in the bed accommodated the

competition between gaseous halide carrier and solid alumino-silicate for corrosion. The

kinetics of this competition were controlled by the location of chlorine and alkali release

from the coal, re-trapping of alkali may occur if the bed is relatively high (~ 10 ft)

(Keairns, Alvin et al. 1977). The same researchers discovered that more alkali will be

released at lower pressure (Keairns, Alvin et al. 1977). Decreasing the bed temperature

will reduce the alkali emission, but at the expense of reduced efficiency.

When alumino-silicate was present, in sufficient concentration, it would reduce the alkali

emissions from the bed by forming feldspars (Keairns, Alvin et al. 1977). Alumino-

silicate, as a getter, is cheap and effective over a wide range of temperature. It is an

attractive option in reducing alkali emissions, however neither its capacity nor

concentrations were known in order to design an effective alkali suppression stage

(Keairns, Alvin et al. 1977). In addition, its presence could cause plant deterioration

through erosion.

3.7 Environmental Performances

Most PFBC plants do not encounter any environmental problems since PFBC is already

an environmentalal friendly technology. Nevertheless, the local Environmental Protection

Agency (EPA) sets the pollutant emission regulation to achieve sustainable environment

control.


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Tomatoh-Atsuma has to limit its SOxemission below 94 ppm, NOxemission below 98

ppm and dust emission below 28 mg/Nm3. It successfully operates under normal

conditions while producing 10 ppm SOx, 40 ppm NOxand less than 10 mg/Nm3

dust. In

accomplishing this excellent result, coal with 0.9% sulfur and 1.6% nitrogen was fired

and a Ca:S ratio of 3-6 was used (Koshimizu 1998).

In Fukuoka, Japan, the emissions of SOx should be 76 ppm or less, NOx should be a

maximum of 60 ppm and the concentration of soot and dust at the stack outlet should not

exceed 30 mg/Nm3. To meet these requirements, Karita is firing coals which contain

1.0% or less sulfur and a maximum of 55% volatile matter.

The Osaki plant faces more stringent NOx and particulates emission regulations. The

maximum permissible emission limit for SOxis the same as Karita (76ppm), 19ppm for

NOxand 9 mg/Nm3

for particulates. The harsh regulations were not a problem for Osaki

as its technology enabled operation at full load while producing only 7.0 ppm SOx, 17.8

ppm NOxand 3.5 mg/Nm

3

particulates.

A correlation was developed to predict the emission of NOx from bench and pilot scale

PFBC and it was found that pressure had no influence on the emission of NO x (Newby,

Keairns et al. 1989):

NOx= 12.25 exp(2827/T) [O2]0.24

Xn0.44

Y-0.1

(ppmv) (Eq. 3)

where T = bed temperature (K)

[O2] = volume percent oxygen in the combustion products

Xn = weight percent nitrogen in the coal

Y = concentration of SO2in the combustion products (ppmv)

In contrast, other researchers found that NO emissions decreased with increasing pressure

and increasing Ca:S ratio (Nagel, Spliethoff et al. 1999). At pressures above 4 bar and

with extra sorbent feed, NO emissions reduced with increased temperature. On the other

hand, N2O emissions were independent of pressure and sorbent added, but instead


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depended on CO emissions (i.e. carbon conversion). Higher oxygen partial pressure

resulted in more complete combustion and hence lower CO emissions. The overall NOx

emissions were lower in PFBC than in AFBC (Nagel, Spliethoff et al. 1999).

Abe et al. measured emissions from the Wakamatsu demonstration plant and concluded

that the cyclone gas temperature (Tc) controlled the emissions of CO, N2O, NOxand SO2

under stationary conditions (Abe, Sasatsu et al. 1999). Analyses found that N2O and SO2

emissions were more dependent on gas temperature (Tc) compared with CO and NOx

emissions (Tc1/2

). Bed temperature also had some role in explaining the spikes of SO2

and N2O emissions during partial load (Abe, Sasatsu et al. 1999). Sudden changes in bed

temperature due to changes in combustion (e.g. increase in coal load) may decrease the

oxygen concentration in the burner zone thus increasing NO2 and SO2 concentrations.

The following series of ASHTR

equations could be used to estimate the concentrations of

exhaust gases under PFBC operations (Abe, Sasatsu et al. 1999):

PCO= PO21/2

x exp (13.431 x 103/Tc 21.562) (R

2= 0.9794) (Eq. 4)

Calculated NOxconversion = ([O2]/3.5)1/2

x {[F1/(1+F1) F2/(1+F2)]} (Eq. 5)

F1= PO21/2

x 5.00 x 10

-4

x exp (0.21 x 10

3

/Tc) (Eq. 6)F2= PO2

1/2x 4.57 x 10

-7x exp (12.1 x 10

-3/Tc) (Eq. 7)

NOx= NOxconversion x input [N] x 22.4/gas flow rate x 106 (ppm) (Eq. 8)

It was found that maximum reduction of NOxemissions (up to 70%) could be achieved

when the bed was operated at the stoichiometric air ratio (Hippinen, Lu et al. 1993). Air

staging was only useful in reducing the emissions if it changed the temperature

distribution of the reactor, as NOx is highly dependent on reactor temperature. Sulfur

retention efficiency decreased when operating the bed with primary air ratio below 1

(Hippinen, Lu et al. 1993). Air staging could also cause increased emissions of CO and

unburnt carbon in the fly ash, thus reducing the combustion efficiency although fly ash

recycling had been implemented. This problem could be prevented by operating at higher

temperature or by using secondary air pre-heating which facilitated the production of

NOx(Hippinen, Lu et al. 1993).


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4. ABB CARBONS PROCESS TEST FACILITY (PTF)

ABB Carbon built a 1 MWeProcess Test Facility (PTF). Although the test rig was small

in size, it used process parameters (temperature, pressure, bed height and excess air)

which were the same as full-scale plant (Andersson, Bergqvist et al. 1999). The fuels

tested in the test rig are shown in Table 1. The results obtained from these PTF tests will

be discussed below.

Table 1. Fuels Tested in PTF (Andersson, Bergqvist et al. 1999).

Origin of Fuel Fuel TypeVolatiles, w-%

dry ash free

Ash, w-%

dry

S, w-%

as fired

LHV, MJ/kg

as fired

Vietnam Anthracite 5.4 7.0 0.31 32.5

United States Green delayed petcoke 10.3 1.3 5.65 34.1

United

Kingdom

Low volatile

bituminous17.2 20.0 1.06 27.3

South Africa /

Italy

Medium volatile

bituminous + lignite35.9 16.4 2.85 24.9

PolandHigh volatile

bituminous34.9 10.5 0.78 28.9

AustraliaHigh volatile

bituminous34.7 16.5 0.56 27.8

ChinaHigh volatile

bituminous42.6 27.7 1.52 21.5

Spain Black lignite 63.6 28.5 8.60 17.8

Germany Brown coal 56.7 5.4 0.66 17.9

Estonia Oil shale 79 58.3 0.52 8.4

Israel Oil shale 100 60 2.8 4.5

4.1 Combustion Efficiency

Combustion efficiency was calculated from the mass balance. CO emissions were low

(


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Bergqvist et al. 1999). Petcoke, which has a lower volatile content than low volatile

bituminous coal, had higher combustion efficiency. This might be due to its oil refinery

origin. Oil shales also had high combustion efficiency despite their low heating values

and high ash contents (Andersson, Bergqvist et al. 1999). Excess air increased

combustion efficiency but it also reduceed the power output because airflow depended on

the gas turbine compressor capacity.

4.2 Sulfur Retention

Sulfur retention was also calculated from the mass balance. In this PTF, SO2 emission

was measured at the outlet of the primary cyclone, and thus correlated only to the sulfur

capture in the bed and freeboard (Andersson, Bergqvist et al. 1999). This is important

since earlier research showed that owing to its huge size, the PTF hot gas filter played a

significant role in capturing sulfur. This was also experienced by the Tidd plant (Mudd

and al. 1993).

The general rules in choosing the appropriate sorbent type, composition and size

distribution were (Andersson, Bergqvist et al. 1999):

1. Coals with high ash contents should use finer sorbent. As these coals reduced the bedmaterial residence time, using larger sorbent particles that only stay in the bed for a

short period is a waste.

2. When firing low ash or low sulfur content coals, it is preferred to use slightly coarsersorbent particles to maintain bed inventory and optimize bed quality and heat transfer.

There are cases where bed maintenance is superior to sulfur retention requirements

thus Ca:S ratio could be very high (e.g. Wakamatsu).

3. High sulfur content coals should be fired with rather fine sorbent disregarding the ashcontent. This method is used to obtain high sorbent flow, hence guaranteeing the bed

quality.

This test was conducted using standard sorbent with an average particle size of 0.7mm.

After undergoing tests, it was found that oil shales did not need additional sorbent for

complete desulfurization as they contained enough calcium. Sorbent feed for brown coal


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was mainly for the purpose of bed maintenance instead of sulfur retention as its ash

contains desulfurizing components (Andersson, Bergqvist et al. 1999). Modifications in

sorbent size distribution or type had proved to improve the desulfurization significantly.

Other factors which influenced SO2 emissions were bed temperature and excess air.

Increases in bed temperature and excess air improved desulfurization (Andersson,

Bergqvist et al. 1999).

4.3 NOxEmissions

NOx emissions depended on the fuel type, fuel nitrogen content, bed material

composition, temperature, etc. However, the parameter which had the strongest influence

was excess air ratio during combustion. An increase in excess air produced more NOx.

The formation of NOx involves complicated interactions between all the parameters

stated previously (Andersson, Bergqvist et al. 1999).

4.4 N2O Emissions

N2O emissions were mainly influenced by bed, freeboard and gas path temperatures. The

higher the temperature, the less N2O was emitted. There was some influence of coal type

but this effect was inferior to the temperature effect. At an operating temperature of 860

oC, the emissions of N2O were less than 20 ppm (Andersson, Bergqvist et al. 1999).


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5. CONCLUSIONS

Combustion inefficiency was one of the potential problems faced by PFBC plants. It was

mainly caused by unburnt char elutriation. For the relatively narrow range of coal rank of

Australian export coals, unburnt char elutriation from PFBC correlated with the coals

petrographic composition, specifically with the ratio Telovitrinite : Inertinite. This effect

was attributed to the highly swelling Telovitrinite generating larger diameter pores in the

devolatilised char, allowing greater combustion-enhanced attrition from the pore mouths

on the char surface. A Telovitrinite : Inertinite ratio below 0.200 would be satisfactory

and a Telovitrinite : Inertinite ratio above 1.871 would indicate an unsuitable coal for

PFBC firing.

Other factors reported to affect PFBC combustion efficiency include Coal reactivity,

volatile content, swelling, fragmentation and calorific value. These factors were studied

over a wider range of coal rank, indicating that combustion inefficiency increased with

coal rank. However, the general correlation with coal rank did not always predict

commercial-scale PFBC performance, so the correlation (Eq. 1) with petrographic

analysis is recommended for assessing sub-bituminous and bituminous coals.

Bed agglomeration or sinter egg formation occurred at Escatrn, Vrtan, Tidd, Tomatoh-

Atsuma, Wakamatsu and Karita. The coal-related factor which caused bed agglomeration

was the ash fusion temperature. Low ash fusion temperature generated agglomeration.

Despite their high combustion efficiencies, low rank coals contain high levels of alkali

that caused agglomeration problems. Two of the Japanese commercial plants firing

Australian export coals specify < 7% Fe2O3in the coal ash and one also specifies an ash

fusion temperature > 1200oC. However, since this problem still limits the maximum

output from the Karita plant, it warrants the further research being conducted in CCSD.

Another problem in PFBC plants was fouling and deposit formation. The key element

responsible for this was iron, which decomposed and oxidized during combustion. Coals

with low iron content are advised to minimize this problem.


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Cyclones play a significant role in ensuring the survival of the gas turbine, especially

when ceramic filter tubes are absent. The gas exhaust from the cyclones has to be

sufficiently clean to minimize the turbine blade erosion. In Osaki, it was found that

cyclone plugging was due to the same properties of coal which caused sticky ash

material, as described below.

Filter blockage is a problem faced by PFBC plants which rely on the ceramic filter for

secondary hot gas clean-up prior to the gas turbine inlet. Serious filter blockages could

lead to filter breakage and fires. Finer ash particles penetrated into the filter, blocking the

pores. This ash was sticky, tending to stick on the filter surface, and could not be

removed by cleaning. One coal-related solution to this problem was to use coal ash for

maintaining bed inventory. Another method was to use coals with higher Al2O3and lower

SiO2 contents in their ash, which agglomerated to larger ash particles, thus preventing

filter blockage. However, the recommended method to overcome filter blockage is to

allow larger particles into the filter. These larger fly ash particles do not penetrate into the

ceramic filter material, but form a cake on the surface which can be cleaned reliably.

Erosion and corrosion of gas turbine blades by coal ash particles are potentially acute

problems in PFBC plants especially in plants which do not employ ceramic tube filters

but only cyclones for hot gas particulate cleaning. In these plants, erosion and corrosion

rates were found to increase substantially when cyclone clogging occurred. The main ash

component that is responsible for the erosion is fine quartz particles. Most of the large

quartz particles are removed by the cyclone, so that only the fine particles (


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Erosion and corrosion of the in-bed heat exchanger tubes also occurred. These problems

were not coal-related. A solution for erosion at low temperature is to apply thermal spray

coatings to the tubes and at high temperature the tube material should have sufficient

erosion and corrosion resistance due to the formation of a hard oxide scale on the surface.

Most PFBC plants do not encounter any environmental problems since PFBC is already

an environmentally friendly technology. However, they need to obey the stringent

emission regulations set by the local EPA. The pollutant reduction methods are primarily

related to the operating conditions of the plant, rather than to coal properties. Karita fires

coals with sulfur content 1% and volatile matter 55%.


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6. ACKNOWLEDGMENTS

We wish to acknowledge the financial support for this paper of the CRC for coal in

Sustainable Development, which is funded by the CRC Program of the Commonwealth

of Australia. We would like to acknowledge the significant contributions to this work by

Dr. Alan Wang, who visited the Japanese PFBC plants in 2002 and provided the latest

information from them.


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