coal selection criteria for industrial pfbc firing project 3.2 - CCSD
coal selection criteria for industrial pfbc firing project 3.2 - CCSD
coal selection criteria for industrial pfbc firing project 3.2 - CCSD
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“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
COOPERATIVE RESEARCH CENTRE FOR COAL IN<br />
SUSTAINABLE DEVELOPMENT<br />
COAL SELECTION CRITERIA FOR INDUSTRIAL PFBC<br />
FIRING<br />
PROJECT <strong>3.2</strong><br />
by<br />
John F. Stubington<br />
Valmaiwati Budijanto<br />
School of Chemical Engineering and Industrial Chemistry<br />
University of New South Wales, Sydney 2052, Australia<br />
(March 2003)<br />
Page 1
ABSTRACT<br />
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
Pressurized Fluidized Bed Combustion (PFBC) is one of the clean <strong>coal</strong> technologies.<br />
There are several PFBC plants operating all over the world. As this technology is<br />
relatively new, some problems were encountered during the plants’ operation. These<br />
include combustion inefficiency, bed agglomeration, cyclone clogging, filter blockage,<br />
gas turbine and in-bed heat exchanger tube erosion and corrosion. In this report, we have<br />
focussed only on those aspects of the problems which were <strong>coal</strong>-related, since those<br />
aspects affect <strong>coal</strong> <strong>selection</strong> <strong>for</strong> PFBC.<br />
Combustion inefficiency was mainly caused by unburnt char elutriation from the bed. For<br />
Australian export <strong>coal</strong>s, it was found that unburnt char elutriation was related to the ratio<br />
of Telovitrinite : Inertinite. For a wider range of <strong>coal</strong> rank, there was generally a decrease<br />
in combustion efficiency with increasing rank, but this generalisation did not always<br />
predict <strong>coal</strong> per<strong>for</strong>mance in commercial PFBC plants. Hence, petrographic analysis is<br />
preferred <strong>for</strong> bituminous and sub-bituminous <strong>coal</strong>s. A Telovitrinite : Inertinite ratio <<br />
0.200 is recommended <strong>for</strong> satisfactory PFBC per<strong>for</strong>mance.<br />
Low ash fusion temperature generated agglomeration. Despite their high combustion<br />
efficiencies, low rank <strong>coal</strong>s contain high alkali that caused agglomeration problems. Two<br />
of the Japanese commercial plants <strong>firing</strong> Australian export <strong>coal</strong>s specify < 7% Fe2O3 in<br />
the <strong>coal</strong> ash and one also specifies an ash fusion temperature > 1200 o C.<br />
During combustion, iron contained in the <strong>coal</strong>s was oxidized and decomposed, causing<br />
fouling and deposit <strong>for</strong>mation. Low iron content <strong>coal</strong>s were recommended to be used to<br />
minimize deposit <strong>for</strong>mation.<br />
Two solutions to filter blockage problems were to use ash <strong>for</strong> maintaining bed inventory<br />
and to use <strong>coal</strong>s with high Al2O3 and SiO2 contents in their ash, which agglomerated to<br />
larger ash particles. The recommended method to overcome filter blockage is to allow<br />
larger particles into the filter which <strong>for</strong>m a layer of cake on the filter surface instead of<br />
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“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
penetrating into it. Cyclone plugging was due to the same properties of <strong>coal</strong> which caused<br />
sticky ash material.<br />
Gas turbine blades erosion was due to fine quartz particles while corrosion is due to fine<br />
ash particles and corrosive compounds of sulfur, alkali and alkaline earth elements<br />
contained in the <strong>coal</strong>s. To reduce erosion and corrosion it was recommended to use <strong>coal</strong>s<br />
with low quartz, sulfur and alkali contents.<br />
Another part of PFBC plant which experienced erosion and corrosion is the in-bed heat<br />
exchanger tubes. In preventing such erosion and corrosion, at low temperature it is<br />
important to apply thermal spray coatings. For high temperature, the tube materials<br />
should have sufficient erosion and corrosion resistance due to the <strong>for</strong>mation of hard oxide<br />
scale on the surfaces.<br />
Pollutant emissions need to be regulated to achieve sustainable environment control.<br />
These emissions were mainly influenced by the operating conditions rather than the <strong>coal</strong><br />
properties.<br />
Page 3
TABLE OF CONTENTS<br />
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
Abstract<br />
Page<br />
i<br />
Table of Contents iii<br />
List of Tables iv<br />
1. Introduction 1<br />
2. Industrial PFBC Plants 2<br />
3. Problems in PFBC Plants and Their Solutions 3<br />
3.1 Combustion Efficiency 3<br />
3.1.1 Elutriation of Unburnt Carbon 3<br />
3.1.2 Other Combustion Efficiency Considerations 4<br />
<strong>3.2</strong> Bed Agglomeration 4<br />
3.3 Ash Deposits 7<br />
3.4 Cyclone Plugging 8<br />
3.5 Filter Blockage 9<br />
3.6 Erosion & Corrosion 10<br />
3.6.1 Gas Turbine Erosion 11<br />
3.6.2 Gas Turbine Corrosion 12<br />
3.6.3 In-bed Heat Exchanger Tubes Erosion 12<br />
3.6.4 In-bed Heat Exchanger Tubes Corrosion 13<br />
3.7 Environmental Per<strong>for</strong>mances 13<br />
4. ABB Carbon’s Process Test Facility (PTF) 16<br />
4.1 Combustion Efficiency 16<br />
4.2 Sulfur Retention 17<br />
4.3 NOx Emissions 18<br />
4.4 N2O Emissions 18<br />
5. Conclusions 19<br />
6. Acknowledgments 22<br />
7. References 23<br />
Page 4
LIST OF TABLES<br />
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
Page<br />
Table 1. Fuels Tested in PTF. 16<br />
1. INTRODUCTION<br />
Coal fed power plants are the most widespread choice to produce electric power, as <strong>coal</strong><br />
deposits are abundant and spread all over the world. In addition, the price of <strong>coal</strong> is<br />
relatively stable. Nevertheless, its carbon dioxide (CO2) emission per unit calorific value<br />
is among the greatest of fossil fuels. Hence it is essential to develop a competent <strong>coal</strong><br />
utilization technology that maximizes the plant thermal efficiency while keeping the<br />
emission of CO2 and other non-environmental friendly emissions (SOx, NOx, etc) at their<br />
minimums.<br />
Pressurized Fluidized Bed Combustion (PFBC) is one of several clean <strong>coal</strong> technologies.<br />
Besides being thermally efficient, it requires low capital and operating costs and has the<br />
potential to be a competitive source of low cost generation when using low to medium<br />
sulfur content <strong>coal</strong>s (Stubington 1997).<br />
However, some problems have arisen in commercial operation of PFBC plant, including<br />
elutriation of unburnt carbon, bed agglomeration, cyclone plugging, and gas turbine blade<br />
and in-bed heat exchanger tube erosion. These are the problems that are associated<br />
mainly with the <strong>coal</strong> used. There<strong>for</strong>e, there is a need to carefully select the <strong>coal</strong> fired to<br />
minimize or eliminate these problems.<br />
Page 5
2. INDUSTRIAL PFBC PLANTS<br />
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
In Japan, the Tomatoh-Atsuma plant was installed by MHI <strong>for</strong> Hokkaido Electric to<br />
produce 85 MWe since 1998. The 350 MWe Karita plant, which was built by ABB<br />
Carbon and IHI <strong>for</strong> Kyushu Electric Power, has been operating since October 1999. Also<br />
in Japan, Babcock-Hitachi built a 250 MWe Osaki plant <strong>for</strong> Chugoku Electric Power that<br />
has been operated commercially at full load since December 2000 and another 250 MWe<br />
unit is scheduled to start operating in 2008. In Sweden, ABB Carbon built the Värtan<br />
plant <strong>for</strong> Stockholm Energi with a total output of 135 MWe. Another plant in Europe,<br />
Escatrón, which produces 79.5 MWe output, was constructed by ABB Carbon and Spain<br />
B&W <strong>for</strong> Endesa. The Tidd demonstration plant was completed by ABP (a joint venture<br />
between ABB and B&W) <strong>for</strong> AEP, powering Ohio with 75.6 MWe, but is now shut<br />
down. ABB Carbon has built another PFBC plant at Cottbus in Germany which is<br />
operating to produce a total output of 75.6 MWe.<br />
One difference between the plants is the <strong>coal</strong> used. The <strong>coal</strong>s fired in Japanese power<br />
plants are mainly Australian bituminous <strong>coal</strong>s while Escatrón is using lower rank Spanish<br />
“black lignite”. Another difference is the <strong>coal</strong> feeding system. Tidd, Osaki and Karita use<br />
the slurry feeding system, <strong>for</strong> which the <strong>coal</strong> and limestone are mixed with water and<br />
then pumped by several positive displacement pumps to the PFBC. On the other hand,<br />
Tomatoh-Atsuma adopted the dry <strong>coal</strong>-limestone feed system due to its high efficiency<br />
and reliability. After the <strong>coal</strong> is pressurized in a lock-hopper-system, it is supplied to the<br />
PFBC through a distribution hopper and supply tubes (Koshimizu 1998).<br />
One major difference between the three plants in Japan is the hot gas cleaning system.<br />
Tomatoh-Atsuma applies a combination of cyclones and ceramic filter <strong>for</strong> cleaning the<br />
stack gases prior to entering the conventional gas turbine. The ceramic filter allows<br />
increased gas turbine efficiency and consequently cycle efficiency. On the other hand,<br />
Karita and Osaki rely only on cyclones to clean the hot gases. However, they are using<br />
special ruggedized gas turbines which are able to tolerate the low quantity of fine<br />
particles which escape from the cyclone.<br />
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“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
3. PROBLEMS IN PFBC PLANTS AND THEIR SOLUTIONS<br />
3.1 Combustion Efficiency<br />
3.1.1 Elutriation of unburnt carbon<br />
Recent research discovered that unburnt char elutriation was the major disadvantage of<br />
using one Australian black <strong>coal</strong>, causing combustion inefficiency in PFBC plants. The<br />
elutriated fine char particles may pass through the cyclone and be caught in the filter cake<br />
on the ceramic filter, giving rise to the sticky ash problems. The combustion of the<br />
unburnt char increases the cake temperature, contributing to the stickiness and causing<br />
damage to the filter (Stubington, Wang et al. 1998).<br />
Combustion-enhanced attrition was found to be the dominant mechanism generating<br />
elutriable char particles (Wang and Stubington 2002). Unburnt char elutriation is directly<br />
related to combustion efficiency and is defined as the percentage of the elemental carbon<br />
in the <strong>coal</strong> fed that was collected in the cyclone and measured as the loss on ignition of<br />
the cyclone fines (Wang and Stubington 2001). For a standardized test in the bench-scale<br />
PFBC, it is predicted using the following correlation (Wang and Stubington 2001):<br />
Char Elutriation = <strong>3.2</strong>6 (Telovitrinite/Inertinite) 0.4045 (%) (R 2 = 0.74) (Eq. 1)<br />
High telovitrinite content contributed to a high unburnt carbon elutriation while <strong>coal</strong> with<br />
low inertinite content (mature <strong>coal</strong>s) exhibited light-up problems (Palit and Mandal<br />
1995). Coals with higher telovitrinite/inertinite (and higher unburnt char elutriation)<br />
exhibited greater swelling during devolatilization in PFBC, producing chars with larger<br />
pores from which more fine char particles were generated by attrition (Wang and<br />
Stubington 2001). A <strong>coal</strong> with unburnt char elutriation of less than 1.7% was found to be<br />
satisfactory, while char elutriation above 4.2% was unsatisfactory. Coal with char<br />
elutriation between 1.7 – 4.2% could not yet be categorized due to insufficient data and<br />
should be considered unsatisfactory until further research revealed appropriate data<br />
(Wang and Stubington 2001).<br />
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“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
Tomatoh-Atsuma was using the <strong>coal</strong>’s Fuel Ratio (Fixed Carbon/Volatile Matter) as the<br />
parameter <strong>for</strong> predicting <strong>coal</strong> combustion per<strong>for</strong>mance in the furnace. However,<br />
occasionally contradictory results had been encountered. The <strong>CCSD</strong> research discussed<br />
above found that the elutriated unburnt carbon correlated with the ratio of<br />
Telovitrinite/Inertinite rather than with of Fuel Ratio. This research had helped Tomatoh-<br />
Atsuma in solving its problems (Wang 2002).<br />
3.1.2 Other Combustion Efficiency Considerations<br />
The major factor causing combustion inefficiency is mostly unburnt carbon elutriation,<br />
caused by attrition of the char particles in the fluidized bed and hence affected by the char<br />
structure <strong>for</strong>med during devolatilization. Earlier work reported that other factors affected<br />
the combustion efficiency, including <strong>coal</strong> rank or volatile content, <strong>coal</strong> reactivity,<br />
swelling, fragmentation and calorific value. One previous study concluded that a lower<br />
<strong>coal</strong> rank or a higher volatile content increased the combustion (Laughlin and Sullivan<br />
1997). An increase in pressure resulted in reduction of the volatile transport rate from<br />
inner pore to outer surface and thus decreased the <strong>coal</strong> volatile yield (Laughlin and<br />
Sullivan 1997). Char reactivity increased with increasing oxygen and alkaline oxide<br />
contents and porosity. It also increased with decreasing rank and mean vitrinite<br />
reflectance (Laughlin and Sullivan 1997). An increase in char reactivity increased the<br />
combustion efficiency, but char reactivity was not an important consideration <strong>for</strong> high<br />
pressure conditions. Generally, high volatile bituminous <strong>coal</strong>s per<strong>for</strong>mance was less<br />
sensitive towards changes in chemical kinetics. Lower <strong>coal</strong> calorific value and higher ash<br />
and sulfur contents increased the inefficiencies (Huang, McMullan et al. 2000). Although<br />
no correlation between Crucible Swelling Number (CSN) and combustion efficiency was<br />
developed, it was shown that an increase in CSN decreased the combustion efficiency <strong>for</strong><br />
Taiheiyou and Lithgow data in the Wakamatsu plant (Misawa 2000).<br />
<strong>3.2</strong> Bed Agglomeration<br />
Another major issue in PFBC plant is bed agglomeration or sinter egg <strong>for</strong>mation. These<br />
agglomerates are bed particles which are fused together around a hollow core that<br />
originated from <strong>coal</strong> paste lumps (Zando and Bauer 1994). Escatrón, Värtan, Tidd,<br />
Tomatoh-Atsuma, Wakamatsu and Karita encountered this problem. At Escatron,<br />
Page 8
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
sintering caused several boiler stops. Tidd experienced bed agglomeration only when it<br />
was operating at full load and over 815 o C. Bed agglomerations were indicated by uneven<br />
bed temperatures, decaying bed density and reduction in the heat absorbed (Scott and<br />
Carpenter 1996).<br />
After being analyzed by SEM, EDAX and XRD, it was found that the agglomerate<br />
consisted of fine particles of SiO2 and Al2O3 in the ash. These particles stick together in<br />
the presence of CaO (from the bed particles) to <strong>for</strong>m Ca2Al2SiO7 glass (Ishom, Harada et<br />
al. 2001). The oxides adhered to the surface of the combusting <strong>coal</strong>. Fine ash and more<br />
CaO deposited on the agglomerate <strong>for</strong>ming a bigger agglomerate. Bed agglomerates<br />
<strong>for</strong>med when the temperature was below 1300 o C, possibly around 1100 o C where<br />
particles in the agglomerate started to de<strong>for</strong>m even if the whole grain melted at 1300 o C<br />
(Ishom, Harada et al. 2001).<br />
The causes of these sinter accumulations were poor fuel splitting resulting in large paste<br />
lumps in the bed, insufficient fluidizing velocity and localized high feed concentration at<br />
full bed height (Zando and Bauer 1994). Failure in the fuel feeding system, e.g. blockage,<br />
has also led to an agglomeration problem. To achieve a finer fuel splitting, it was<br />
necessary to increase the paste moisture content. However, this could only be done at the<br />
expense of reduced thermal efficiency. Installation of more air nozzles improved the bed<br />
fluidization. Decreasing the bed particle size and operating in the turbulent regime could<br />
also help the fluidization.<br />
Inadequate fuel distribution, which was caused by bed defludization, could increase the<br />
unburnt carbon elutriation, gas temperature (due to post combustion of unburnt elutriated<br />
char) and SOx emission (Wang 2002). Karita’s measures to solve these problems were<br />
decreasing the top limestone particle size from 6 mm to 2 mm, adding more fluidizing<br />
gas nozzles to improve fluidization in the bottom area and reducing the operating<br />
pressure (Wang 2002). Another problem faced by Karita was that it could not operate at<br />
pressures above 1.2 MPa, which caused bed agglomeration <strong>for</strong> some <strong>coal</strong>s. Karita is now<br />
operating at about 80% load, with an operating pressure below 1.1 MPa (Wang 2002).<br />
Page 9
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
Blockage of fuel feeding lines has been noted in Wakamatsu. This could be resolved by<br />
improving the <strong>coal</strong>’s particle size distribution and equipment modifications (Sakanishi<br />
1995). Such a problem was also reported in Osaki, where their fuel nozzle was clogged<br />
several times by <strong>for</strong>eign material in the raw <strong>coal</strong> and <strong>coal</strong> lumps. As a countermeasure, a<br />
reducer in front of the nozzle cut-off valve was installed (Matsumoto and Kawahara ).<br />
Swelling <strong>coal</strong>s are sticky and they could stick the surrounding bed particles together<br />
<strong>for</strong>ming agglomerates (Palit and Mandal 1995). There<strong>for</strong>e, it was advised to use <strong>coal</strong>s<br />
with low crucible swelling number (CSN) or non-caking <strong>coal</strong>s.<br />
Bed agglomeration was also encountered in plants that used dry <strong>coal</strong> feed, such as<br />
Tomatoh-Atsuma, instead of slurry feeding system. The temperature of the combustion<br />
domain near the fuel nozzle outlet induced the agglomeration. A low ash fusion<br />
temperature generated agglomeration. The Tomatoh-Atsuma plant selects <strong>coal</strong>s based on<br />
the iron content, <strong>coal</strong>s with an iron content of 7% or more will have low ash melting<br />
point (Kazuhiro 2002). Karita requires their <strong>coal</strong>s to contain less than 7% Fe2O3 and to<br />
have an ash fusion temperature higher than 1200 o C. If <strong>coal</strong>s with low ash fusion<br />
temperature are used, the bed temperature has to be kept below the ash fusion<br />
temperature to prevent agglomeration (Palit and Mandal 1995).<br />
Bed agglomeration is caused by amorphous clay mineral fragments and alkali species<br />
adhering to sorbent and chars surfaces (Steenari, Lindqvist et al. 1998). Inside the<br />
agglomerates, the chars are still burning, causing high temperature and reducing<br />
conditions. Steenari et al. found that reducing conditions in the bed caused sintering<br />
through reaction in the CaS-CaSO4 system and through eutectic melting of silicate-iron<br />
mixtures (Steenari, Lindqvist et al. 1998). An increase in the <strong>coal</strong>’s clay content increased<br />
the viscosity of the paste (Wright, Clark et al. 1991). Less agglomeration was found when<br />
using dolomite instead of limestone as the sorbent. The reason was that dolomite contains<br />
a higher quantity of MgO which raised the ash fusion temperature of the CaO-MgO-<br />
Al2O3 (Marocco and Bauer 1993). Improved bed mixing and fluidization was observed<br />
by using finer dolomite (
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
Earlier PFBC research showed that Australian <strong>coal</strong>s are superior, in terms of bed<br />
agglomeration, due to their high ash fusion temperature (Stubington 1997). However this<br />
is no longer an advantage if the bed is operated at a higher Ca:S ratio than is required <strong>for</strong><br />
sulfur capture in order to maintain the bed inventory.<br />
Greater concerns exist when combusting low rank <strong>coal</strong>s (sub-bituminous <strong>coal</strong>s and<br />
lignites). Although they have high combustion efficiencies their high alkali content<br />
caused sintering and fouling problems, compared with combusting bituminous <strong>coal</strong>s<br />
(Sondreal, Jones et al. 1993). Again, by using low-alkali <strong>coal</strong>s, this problem could be<br />
reduced. Pressure, steam and hot spots in the bed also promoted sintering. Low bed<br />
temperature (
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
Deposit <strong>for</strong>mations on heat exchanger surfaces had been reported at Vartan, Tidd and<br />
Escatron (Steenari, Lindqvist et al. 1998). Furthermore, deposit <strong>for</strong>mation and fouling<br />
were also found on cyclone surfaces and other parts of the flue gas ducts.<br />
The key element in fouling is iron. FeS2, which is the most common iron mineral pyrite,<br />
decomposed and oxidized during combustion (Steenari, Lindqvist et al. 1998). In<br />
reducing conditions, mixtures of FeS and FeO are <strong>for</strong>med. FeO and other iron-rich oxides<br />
react with kaolinite and quartz to <strong>for</strong>m molten products at temperatures between 900 –<br />
1000 o C (Steenari, Lindqvist et al. 1998). To reduce ash deposition and fouling, it is<br />
advised to use <strong>coal</strong>s with low iron content.<br />
3.4 Cyclone Plugging<br />
Cyclones play a significant role in ensuring the survival of the gas turbine, especially<br />
when ceramic filter tubes are absent. The gas exhaust from the cyclones has to be<br />
sufficiently clean to minimize the turbine blade erosion. Osaki had encountered cyclone<br />
plugging, causing them to suspend their operation <strong>for</strong> inspection and it was found that the<br />
plugging was due to the properties of the <strong>coal</strong> which produced sticky ash material.<br />
The cyclone plugging in Tidd was due to the high <strong>coal</strong> and sorbent elutriation rates,<br />
maldistribution of ash loading to individual cyclones and undersized ash removal system<br />
(M.Marrocco and al. 1991). In addition, Tidd had experienced cyclone fires. The majority<br />
of the fires occurred in the lower part of the cyclones. They happened because of carbon<br />
carryover to the cyclones, which was due to operation at low loads where combustion<br />
efficiency and low bed particle residence time had significant impacts (M.Marrocco and<br />
al. 1991).<br />
The same problem was reported at the Wakamatsu plant in Japan and was solved by<br />
improving the <strong>coal</strong> particle size distribution (Sakanishi 1995). The Ca:S molar ratio was<br />
increased way above the requirements <strong>for</strong> SO2 control to reduce the fly ash stickiness and<br />
to maintain bed inventory.<br />
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“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
Furthermore, the major reason causing several operational shutdowns of the Escatrón<br />
plant in Spain was cyclone ash extraction system blockage (Scott and Carpenter 1996).<br />
Sintered material (agglomerates) deposited on the cyclone walls and in the ash extraction<br />
system. Increasing the <strong>coal</strong> feed rate to increase the production of steam increased the<br />
bed height and the flow of particulates to the cyclone. This led to more agglomeration<br />
which blocked the cyclone. Moreover, the complex design of cyclones with many ducts<br />
and flow direction changes further intensified the plugging. Modifications to the cyclone<br />
ash removal system have reduced the problem (Martinez and Menendez 1995), (Martinez<br />
and Menendez 1994).<br />
3.5 Filter Blockage<br />
This problem is only faced by PFBC plants which depend on the ceramic filter <strong>for</strong><br />
secondary hot gas clean-up prior to the gas turbine inlet, an example of such plants is<br />
Tomatoh-Atsuma in Japan. This problem involved filter blockage, filter breakage, gas<br />
leakage and fires, attributed to temperature effects, hydrodynamic effects, mechanical<br />
effects, filter material effects, sorbent properties/reactions, ash composition effects and<br />
volatilisation / condensation of alkalis (Stubington 1997). Most of them have been solved<br />
but the problem is being investigated further to improve the understanding of ash<br />
chemistry.<br />
Finer ash particles penetrate into the filter, causing filter blockage. This ash was<br />
described as sticky due to its tendency to stick on the filter surface and it could not be<br />
removed by cleaning. It led to unstable pressure drop across the filter cake (Stubington,<br />
Wang et al. 1998). Excessive deposits could lead to filter breakage. Larger ash particles<br />
in the exhaust gas flow to the filter reduced the blockage, thus easing the cleaning of<br />
filter cake. This solution was demonstrated at Wakamatsu.<br />
Elutriated material from the attrition of limestone bed particles contained calcium<br />
compounds that could <strong>for</strong>m low melting point eutectics which decreased the ash fusion<br />
temperature of material accumulated in the filter cake. A higher Ca:S ratio was necessary<br />
to maintain the bed height <strong>for</strong> low-sulfur Australian <strong>coal</strong>s. This neutralized the high ash<br />
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“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
fusion temperature advantage of Australian <strong>coal</strong>s (Stubington, Wang et al. 1998). The use<br />
of dolomite sorbent instead of limestone could also raise the melting point of alkali<br />
eutectics in the filter cake (Stubington 1997). Another solution was to use <strong>coal</strong> ash<br />
instead of limestone <strong>for</strong> maintaining bed height. High ash <strong>coal</strong>s were found to build bed<br />
height faster (Sudhakar Gupta, Mandal et al. 1995). This method was not very effective<br />
in capturing sulfur, however this should not be a major problem as Australian <strong>coal</strong>s<br />
produce low level of sulfur emissions (Peeler, Lane et al. 1990). Alternative methods to<br />
maintain bed height, such as zero-stage cyclone or <strong>selection</strong> of <strong>coal</strong> with appropriate ash<br />
particle size distribution, have been investigated. They should be encouraged to maintain<br />
the advantage of Australian <strong>coal</strong>s.<br />
Experiments with two Australian <strong>coal</strong>s were conducted at Wakamatsu. Ashes from one<br />
caused high stationary pressure drop in the ceramic tube filter (Iwamoto, Ishom et al.<br />
2001). This <strong>coal</strong> was found to produce very fine ash (
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
• Operating procedures: start-up, shut-down and load-following procedures affected the<br />
concentration and temperature profiles which must be compatible to materials<br />
limitations.<br />
3.6.1 Gas Turbine Blade Erosion<br />
Gas turbine blade erosion and corrosion is an acute problem in PFBC plants, decreasing<br />
the turbine efficiency and blades durability and increasing the risk of turbine operation<br />
(Li, Chuming et al. 1991). Although the hot gas exhausted from the furnace has been<br />
desulfurized and cleaned, a certain quantity of corrosives and particulates entering the gas<br />
turbine is inevitable. The erosion rate was found to increase substantially when cyclone<br />
clogging occurred (Li, Chuming et al. 1991).<br />
Ash particles may erode the turbine blades. The main component of the ash that is<br />
responsible <strong>for</strong> the erosion is fine quartz particles. Quartz particles are very hard and<br />
angular, so that the very fine particles passing through the cyclones are abrasive to the<br />
metal blades. Most of the large quartz particles are removed by the cyclone, only the fine<br />
particles (
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
less free SiO2. Also, a maximum of 10% ash contained in the <strong>coal</strong> is needed to obtain an<br />
excellent result. Karita requires the soot and dust concentration at the gas turbine inlet to<br />
be 840 mg/Nm 3 or less.<br />
3.6.2 Gas Turbine Blade Corrosion<br />
Corrosion is due to fine ash particles and corrosive compounds of sulfur, alkali and<br />
alkaline earth elements contained in the <strong>coal</strong>s (Li, Chuming et al. 1991). As mentioned<br />
be<strong>for</strong>e, cyclones are not 100% effective and hence some particulates and corrosive<br />
compounds managed to escape and enter the gas turbine especially when the cyclones<br />
were clogged.<br />
Metal corrosion occurs as a result of complex chemical reactions at high temperature.<br />
Sulfur (SO2 and SO3) and alkali (Na2O and K2O) react to <strong>for</strong>m alkali sulfates with low<br />
melting points (Li, Chuming et al. 1991). Deposition of such sulfates in their molten state<br />
act as an adhesive to stick the micro particles on the blades, promoting complex chemical<br />
reactions <strong>for</strong>ming low melting point complexes impairing the oxide protection on the<br />
blade thereby exfoliating the metal surface by gas and particles flow (Li, Chuming et al.<br />
1991).<br />
To minimize the total alkali release from dolomites, an extremely pure metamorphic<br />
dolomite (e.g. Kaiser Dolowhite) may be used. Alkali removal sorbents, such as<br />
emathlite, have been tested <strong>for</strong> PFBC application. Up to a temperature of 1200 o C, either<br />
a packed bed of emathlite was placed after the cyclone or small emathlite particles were<br />
injected directly into the combustion products prior to the cyclone entrance to control the<br />
alkali to acceptable level (Newby, Keairns et al. 1989). To reduce this type of corrosion,<br />
it is recommended to operate the plant with <strong>coal</strong> that is low in sulfur and alkali, such as<br />
Australian black <strong>coal</strong>s.<br />
3.6.3 In-bed Heat Exchanger Tubes Erosion<br />
This type of erosion was experienced by Wakamatsu plant. The erosion mechanism is<br />
complicated due to the high operating temperature and the following interaction of<br />
Page 16
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
oxidation and erosion (Tsumita, Namba et al. 1997). In preventing such a phenomenon, at<br />
low temperature it is important to apply thermal spray coatings. For high temperature, the<br />
tube materials should have sufficient erosion and corrosion resistance due to the<br />
<strong>for</strong>mation of hard oxide scale on the surfaces (Tsumita, Namba et al. 1997). Abrasion and<br />
leakage was found in Osaki’s boiler tube. The cause of this problem was believed to be<br />
solidified fluid deposits which remained on tubes in the furnace. When high velocity fluid<br />
flowed, abrasion progressed ten times faster. Modifications in the shutdown procedure<br />
were proved to prevent such occurrence (Matsumoto and Kawahara ).<br />
3.6.4 In-bed Heat Exchanger Tubes Corrosion<br />
Some researchers found that the presence of ash in the bed accommodated the<br />
competition between gaseous halide carrier and solid alumino-silicate <strong>for</strong> corrosion. The<br />
kinetics of this competition were controlled by the location of chlorine and alkali release<br />
from the <strong>coal</strong>, re-trapping of alkali may occur if the bed is relatively high (~ 10 ft)<br />
(Keairns, Alvin et al. 1977). The same researchers discovered that more alkali will be<br />
released at lower pressure (Keairns, Alvin et al. 1977). Decreasing the bed temperature<br />
will reduce the alkali emission, but at the expense of reduced efficiency.<br />
When alumino-silicate was present, in sufficient concentration, it would reduce the alkali<br />
emissions from the bed by <strong>for</strong>ming feldspars (Keairns, Alvin et al. 1977). Aluminosilicate,<br />
as a getter, is cheap and effective over a wide range of temperature. It is an<br />
attractive option in reducing alkali emissions, however neither its capacity nor<br />
concentrations were known in order to design an effective alkali suppression stage<br />
(Keairns, Alvin et al. 1977). In addition, its presence could cause plant deterioration<br />
through erosion.<br />
3.7 Environmental Per<strong>for</strong>mances<br />
Most PFBC plants do not encounter any environmental problems since PFBC is already<br />
an environmentalal friendly technology. Nevertheless, the local Environmental Protection<br />
Agency (EPA) sets the pollutant emission regulation to achieve sustainable environment<br />
control.<br />
Page 17
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
Tomatoh-Atsuma has to limit its SOx emission below 94 ppm, NOx emission below 98<br />
ppm and dust emission below 28 mg/Nm 3 . It successfully operates under normal<br />
conditions while producing 10 ppm SOx, 40 ppm NOx and less than 10 mg/Nm 3 dust. In<br />
accomplishing this excellent result, <strong>coal</strong> with 0.9% sulfur and 1.6% nitrogen was fired<br />
and a Ca:S ratio of 3-6 was used (Koshimizu 1998).<br />
In Fukuoka, Japan, the emissions of SOx should be 76 ppm or less, NOx should be a<br />
maximum of 60 ppm and the concentration of soot and dust at the stack outlet should not<br />
exceed 30 mg/Nm 3 . To meet these requirements, Karita is <strong>firing</strong> <strong>coal</strong>s which contain<br />
1.0% or less sulfur and a maximum of 55% volatile matter.<br />
The Osaki plant faces more stringent NOx and particulates emission regulations. The<br />
maximum permissible emission limit <strong>for</strong> SOx is the same as Karita (76ppm), 19ppm <strong>for</strong><br />
NOx and 9 mg/Nm 3 <strong>for</strong> particulates. The harsh regulations were not a problem <strong>for</strong> Osaki<br />
as its technology enabled operation at full load while producing only 7.0 ppm SOx, 17.8<br />
ppm NOx and 3.5 mg/Nm 3 particulates.<br />
A correlation was developed to predict the emission of NOx from bench and pilot scale<br />
PFBC and it was found that pressure had no influence on the emission of NOx (Newby,<br />
Keairns et al. 1989):<br />
NOx = 12.25 exp(2827/T) [O2] 0.24 Xn 0.44 Y -0.1 (ppmv)<br />
where T = bed temperature (K)<br />
[O2] = volume percent oxygen in the combustion products<br />
Xn = weight percent nitrogen in the <strong>coal</strong><br />
(Eq. 3)<br />
Y = concentration of SO2 in the combustion products (ppmv)<br />
In contrast, other researchers found that NO emissions decreased with increasing pressure<br />
and increasing Ca:S ratio (Nagel, Spliethoff et al. 1999). At pressures above 4 bar and<br />
with extra sorbent feed, NO emissions reduced with increased temperature. On the other<br />
hand, N2O emissions were independent of pressure and sorbent added, but instead<br />
Page 18
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
depended on CO emissions (i.e. carbon conversion). Higher oxygen partial pressure<br />
resulted in more complete combustion and hence lower CO emissions. The overall NOx<br />
emissions were lower in PFBC than in AFBC (Nagel, Spliethoff et al. 1999).<br />
Abe et al. measured emissions from the Wakamatsu demonstration plant and concluded<br />
that the cyclone gas temperature (Tc) controlled the emissions of CO, N2O, NOx and SO2<br />
under stationary conditions (Abe, Sasatsu et al. 1999). Analyses found that N2O and SO2<br />
emissions were more dependent on gas temperature (α Tc) compared with CO and NOx<br />
emissions (α Tc 1/2 ). Bed temperature also had some role in explaining the spikes of SO2<br />
and N2O emissions during partial load (Abe, Sasatsu et al. 1999). Sudden changes in bed<br />
temperature due to changes in combustion (e.g. increase in <strong>coal</strong> load) may decrease the<br />
oxygen concentration in the burner zone thus increasing NO2 and SO2 concentrations.<br />
The following series of ASH TR equations could be used to estimate the concentrations of<br />
exhaust gases under PFBC operations (Abe, Sasatsu et al. 1999):<br />
PCO = PO2 1/2 x exp (13.431 x 10 3 /Tc – 21.562) (R 2 = 0.9794) (Eq. 4)<br />
Calculated NOx conversion = ([O2]/3.5) 1/2 x {[F1/(1+F1) – F2/(1+F2)]} (Eq. 5)<br />
F1 = PO2 1/2 x 5.00 x 10 -4 x exp (0.21 x 10 3 /Tc) (Eq. 6)<br />
F2 = PO2 1/2 x 4.57 x 10 -7 x exp (12.1 x 10 -3 /Tc) (Eq. 7)<br />
NOx = NOx conversion x input [N] x 22.4/gas flow rate x 10 6 (ppm) (Eq. 8)<br />
It was found that maximum reduction of NOx emissions (up to 70%) could be achieved<br />
when the bed was operated at the stoichiometric air ratio (Hippinen, Lu et al. 1993). Air<br />
staging was only useful in reducing the emissions if it changed the temperature<br />
distribution of the reactor, as NOx is highly dependent on reactor temperature. Sulfur<br />
retention efficiency decreased when operating the bed with primary air ratio below 1<br />
(Hippinen, Lu et al. 1993). Air staging could also cause increased emissions of CO and<br />
unburnt carbon in the fly ash, thus reducing the combustion efficiency although fly ash<br />
recycling had been implemented. This problem could be prevented by operating at higher<br />
temperature or by using secondary air pre-heating which facilitated the production of<br />
NOx (Hippinen, Lu et al. 1993).<br />
Page 19
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
4. ABB CARBON’S PROCESS TEST FACILITY (PTF)<br />
ABB Carbon built a 1 MWe Process Test Facility (PTF). Although the test rig was small<br />
in size, it used process parameters (temperature, pressure, bed height and excess air)<br />
which were the same as full-scale plant (Andersson, Bergqvist et al. 1999). The fuels<br />
tested in the test rig are shown in Table 1. The results obtained from these PTF tests will<br />
be discussed below.<br />
Table 1. Fuels Tested in PTF (Andersson, Bergqvist et al. 1999).<br />
Origin of Fuel Fuel Type<br />
Volatiles, w-%<br />
dry ash free<br />
Ash, w-%<br />
dry<br />
S, w-%<br />
as fired<br />
LHV, MJ/kg<br />
as fired<br />
Vietnam Anthracite 5.4 7.0 0.31 32.5<br />
United States Green delayed petcoke 10.3 1.3 5.65 34.1<br />
United<br />
Kingdom<br />
Low volatile<br />
bituminous<br />
17.2 20.0 1.06 27.3<br />
South Africa /<br />
Italy<br />
Poland<br />
Australia<br />
China<br />
Medium volatile<br />
bituminous + lignite<br />
High volatile<br />
bituminous<br />
High volatile<br />
bituminous<br />
High volatile<br />
bituminous<br />
35.9 16.4 2.85 24.9<br />
34.9 10.5 0.78 28.9<br />
34.7 16.5 0.56 27.8<br />
42.6 27.7 1.52 21.5<br />
Spain Black lignite 63.6 28.5 8.60 17.8<br />
Germany Brown <strong>coal</strong> 56.7 5.4 0.66 17.9<br />
Estonia Oil shale 79 58.3 0.52 8.4<br />
Israel Oil shale 100 60 2.8 4.5<br />
4.1 Combustion Efficiency<br />
Combustion efficiency was calculated from the mass balance. CO emissions were low<br />
(
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
Bergqvist et al. 1999). Petcoke, which has a lower volatile content than low volatile<br />
bituminous <strong>coal</strong>, had higher combustion efficiency. This might be due to its oil refinery<br />
origin. Oil shales also had high combustion efficiency despite their low heating values<br />
and high ash contents (Andersson, Bergqvist et al. 1999). Excess air increased<br />
combustion efficiency but it also reduceed the power output because airflow depended on<br />
the gas turbine compressor capacity.<br />
4.2 Sulfur Retention<br />
Sulfur retention was also calculated from the mass balance. In this PTF, SO2 emission<br />
was measured at the outlet of the primary cyclone, and thus correlated only to the sulfur<br />
capture in the bed and freeboard (Andersson, Bergqvist et al. 1999). This is important<br />
since earlier research showed that owing to its huge size, the PTF hot gas filter played a<br />
significant role in capturing sulfur. This was also experienced by the Tidd plant (Mudd<br />
and al. 1993).<br />
The general rules in choosing the appropriate sorbent type, composition and size<br />
distribution were (Andersson, Bergqvist et al. 1999):<br />
1. Coals with high ash contents should use finer sorbent. As these <strong>coal</strong>s reduced the bed<br />
material residence time, using larger sorbent particles that only stay in the bed <strong>for</strong> a<br />
short period is a waste.<br />
2. When <strong>firing</strong> low ash or low sulfur content <strong>coal</strong>s, it is preferred to use slightly coarser<br />
sorbent particles to maintain bed inventory and optimize bed quality and heat transfer.<br />
There are cases where bed maintenance is superior to sulfur retention requirements<br />
thus Ca:S ratio could be very high (e.g. Wakamatsu).<br />
3. High sulfur content <strong>coal</strong>s should be fired with rather fine sorbent disregarding the ash<br />
content. This method is used to obtain high sorbent flow, hence guaranteeing the bed<br />
quality.<br />
This test was conducted using “standard” sorbent with an average particle size of 0.7mm.<br />
After undergoing tests, it was found that oil shales did not need additional sorbent <strong>for</strong><br />
complete desulfurization as they contained enough calcium. Sorbent feed <strong>for</strong> brown <strong>coal</strong><br />
Page 21
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
was mainly <strong>for</strong> the purpose of bed maintenance instead of sulfur retention as its ash<br />
contains desulfurizing components (Andersson, Bergqvist et al. 1999). Modifications in<br />
sorbent size distribution or type had proved to improve the desulfurization significantly.<br />
Other factors which influenced SO2 emissions were bed temperature and excess air.<br />
Increases in bed temperature and excess air improved desulfurization (Andersson,<br />
Bergqvist et al. 1999).<br />
4.3 NOx Emissions<br />
NOx emissions depended on the fuel type, fuel nitrogen content, bed material<br />
composition, temperature, etc. However, the parameter which had the strongest influence<br />
was excess air ratio during combustion. An increase in excess air produced more NOx.<br />
The <strong>for</strong>mation of NOx involves complicated interactions between all the parameters<br />
stated previously (Andersson, Bergqvist et al. 1999).<br />
4.4 N2O Emissions<br />
N2O emissions were mainly influenced by bed, freeboard and gas path temperatures. The<br />
higher the temperature, the less N2O was emitted. There was some influence of <strong>coal</strong> type<br />
but this effect was inferior to the temperature effect. At an operating temperature of 860<br />
o<br />
C, the emissions of N2O were less than 20 ppm (Andersson, Bergqvist et al. 1999).<br />
Page 22
5. CONCLUSIONS<br />
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
Combustion inefficiency was one of the potential problems faced by PFBC plants. It was<br />
mainly caused by unburnt char elutriation. For the relatively narrow range of <strong>coal</strong> rank of<br />
Australian export <strong>coal</strong>s, unburnt char elutriation from PFBC correlated with the <strong>coal</strong>’s<br />
petrographic composition, specifically with the ratio Telovitrinite : Inertinite. This effect<br />
was attributed to the highly swelling Telovitrinite generating larger diameter pores in the<br />
devolatilised char, allowing greater combustion-enhanced attrition from the pore mouths<br />
on the char surface. A Telovitrinite : Inertinite ratio below 0.200 would be satisfactory<br />
and a Telovitrinite : Inertinite ratio above 1.871 would indicate an unsuitable <strong>coal</strong> <strong>for</strong><br />
PFBC <strong>firing</strong>.<br />
Other factors reported to affect PFBC combustion efficiency include Coal reactivity,<br />
volatile content, swelling, fragmentation and calorific value. These factors were studied<br />
over a wider range of <strong>coal</strong> rank, indicating that combustion inefficiency increased with<br />
<strong>coal</strong> rank. However, the general correlation with <strong>coal</strong> rank did not always predict<br />
commercial-scale PFBC per<strong>for</strong>mance, so the correlation (Eq. 1) with petrographic<br />
analysis is recommended <strong>for</strong> assessing sub-bituminous and bituminous <strong>coal</strong>s.<br />
Bed agglomeration or sinter egg <strong>for</strong>mation occurred at Escatrón, Värtan, Tidd, Tomatoh-<br />
Atsuma, Wakamatsu and Karita. The <strong>coal</strong>-related factor which caused bed agglomeration<br />
was the ash fusion temperature. Low ash fusion temperature generated agglomeration.<br />
Despite their high combustion efficiencies, low rank <strong>coal</strong>s contain high levels of alkali<br />
that caused agglomeration problems. Two of the Japanese commercial plants <strong>firing</strong><br />
Australian export <strong>coal</strong>s specify < 7% Fe2O3 in the <strong>coal</strong> ash and one also specifies an ash<br />
fusion temperature > 1200 o C. However, since this problem still limits the maximum<br />
output from the Karita plant, it warrants the further research being conducted in <strong>CCSD</strong>.<br />
Another problem in PFBC plants was fouling and deposit <strong>for</strong>mation. The key element<br />
responsible <strong>for</strong> this was iron, which decomposed and oxidized during combustion. Coals<br />
with low iron content are advised to minimize this problem.<br />
Page 23
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
Cyclones play a significant role in ensuring the survival of the gas turbine, especially<br />
when ceramic filter tubes are absent. The gas exhaust from the cyclones has to be<br />
sufficiently clean to minimize the turbine blade erosion. In Osaki, it was found that<br />
cyclone plugging was due to the same properties of <strong>coal</strong> which caused sticky ash<br />
material, as described below.<br />
Filter blockage is a problem faced by PFBC plants which rely on the ceramic filter <strong>for</strong><br />
secondary hot gas clean-up prior to the gas turbine inlet. Serious filter blockages could<br />
lead to filter breakage and fires. Finer ash particles penetrated into the filter, blocking the<br />
pores. This ash was sticky, tending to stick on the filter surface, and could not be<br />
removed by cleaning. One <strong>coal</strong>-related solution to this problem was to use <strong>coal</strong> ash <strong>for</strong><br />
maintaining bed inventory. Another method was to use <strong>coal</strong>s with higher Al2O3 and lower<br />
SiO2 contents in their ash, which agglomerated to larger ash particles, thus preventing<br />
filter blockage. However, the recommended method to overcome filter blockage is to<br />
allow larger particles into the filter. These larger fly ash particles do not penetrate into the<br />
ceramic filter material, but <strong>for</strong>m a cake on the surface which can be cleaned reliably.<br />
Erosion and corrosion of gas turbine blades by <strong>coal</strong> ash particles are potentially acute<br />
problems in PFBC plants especially in plants which do not employ ceramic tube filters<br />
but only cyclones <strong>for</strong> hot gas particulate cleaning. In these plants, erosion and corrosion<br />
rates were found to increase substantially when cyclone clogging occurred. The main ash<br />
component that is responsible <strong>for</strong> the erosion is fine quartz particles. Most of the large<br />
quartz particles are removed by the cyclone, so that only the fine particles (
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
Erosion and corrosion of the in-bed heat exchanger tubes also occurred. These problems<br />
were not <strong>coal</strong>-related. A solution <strong>for</strong> erosion at low temperature is to apply thermal spray<br />
coatings to the tubes and at high temperature the tube material should have sufficient<br />
erosion and corrosion resistance due to the <strong>for</strong>mation of a hard oxide scale on the surface.<br />
Most PFBC plants do not encounter any environmental problems since PFBC is already<br />
an environmentally friendly technology. However, they need to obey the stringent<br />
emission regulations set by the local EPA. The pollutant reduction methods are primarily<br />
related to the operating conditions of the plant, rather than to <strong>coal</strong> properties. Karita fires<br />
<strong>coal</strong>s with sulfur content ≤ 1% and volatile matter ≤ 55%.<br />
Page 25
6. ACKNOWLEDGMENTS<br />
“Coal Selection Criteria <strong>for</strong> Industrial PFBC Firing”<br />
We wish to acknowledge the financial support <strong>for</strong> this paper of the CRC <strong>for</strong> <strong>coal</strong> in<br />
Sustainable Development, which is funded by the CRC Program of the Commonwealth<br />
of Australia. We would like to acknowledge the significant contributions to this work by<br />
Dr. Alan Wang, who visited the Japanese PFBC plants in 2002 and provided the latest<br />
in<strong>for</strong>mation from them.<br />
Page 26
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