coal selection criteria for industrial pfbc firing project 3.2 - CCSD

“Coal Selection Criteria for Industrial PFBC Firing”
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 Chemistry
University of New South Wales, Sydney 2052, Australia
(March 2003)
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ABSTRACT
“Coal Selection Criteria for Industrial PFBC Firing”
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 <
0.200 is recommended for satisfactory PFBC performance.
Low ash fusion temperature generated agglomeration. Despite their high combustion
efficiencies, low rank coals contain high alkali that caused agglomeration problems. Two
of the Japanese commercial plants firing Australian export coals specify < 7% Fe2O3 in
the coal ash and one also specifies an ash fusion temperature > 1200 o C.
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 to
minimize deposit formation.
Two solutions to filter blockage problems were to use ash for maintaining bed inventory
and to use coals with high Al2O3 and SiO2 contents 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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“Coal Selection Criteria for Industrial PFBC Firing”
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
“Coal Selection Criteria for Industrial PFBC Firing”
Abstract
Page
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 Carbon’s Process Test Facility (PTF) 16
4.1 Combustion Efficiency 16
4.2 Sulfur Retention 17
4.3 NOx Emissions 18
4.4 N2O Emissions 18
5. Conclusions 19
6. Acknowledgments 22
7. References 23
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LIST OF TABLES
“Coal Selection Criteria for Industrial PFBC Firing”
Page
Table 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 CO2 and 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
“Coal Selection Criteria for Industrial PFBC Firing”
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 MWe Osaki plant for Chugoku Electric Power that
has been operated commercially at full load since December 2000 and another 250 MWe
unit is scheduled to start operating in 2008. In Sweden, ABB Carbon built the Värtan
plant for Stockholm Energi with a total output of 135 MWe. Another plant in Europe,
Escatrón, which produces 79.5 MWe output, 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 Escatrón 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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“Coal Selection Criteria for Industrial PFBC Firing”
3. PROBLEMS IN PFBC PLANTS AND THEIR SOLUTIONS
3.1 Combustion Efficiency
3.1.1 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 (%) (R 2 = 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 and
Stubington 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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“Coal Selection Criteria for Industrial PFBC Firing”
Tomatoh-Atsuma was using the coal’s 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). Escatrón, Värtan, Tidd,
Tomatoh-Atsuma, Wakamatsu and Karita encountered this problem. At Escatron,
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“Coal Selection Criteria for Industrial PFBC Firing”
sintering caused several boiler stops. Tidd experienced bed agglomeration only when it
was operating at full load and over 815 o C. 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 SiO2 and Al2O3 in the ash. These particles stick together in
the presence of CaO (from the bed particles) to form Ca2Al2SiO7 glass (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 1300 o C, possibly around 1100 o C where
particles in the agglomerate started to deform even if the whole grain melted at 1300 o C
(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). Karita’s 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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“Coal Selection Criteria for Industrial PFBC Firing”
Blockage of fuel feeding lines has been noted in Wakamatsu. This could be resolved by
improving the coal’s 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% Fe2O3 and to
have an ash fusion temperature higher than 1200 o C. If coals with low ash fusion
temperature 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-CaSO4 system and through eutectic melting of silicate-iron
mixtures (Steenari, Lindqvist et al. 1998). An increase in the coal’s 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 (

“Coal Selection Criteria for Industrial PFBC Firing”
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 (

“Coal Selection Criteria for Industrial PFBC Firing”
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 –
1000 o C (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 SO2 control to reduce the fly ash stickiness and
to maintain bed inventory.
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“Coal Selection Criteria for Industrial PFBC Firing”
Furthermore, the major reason causing several operational shutdowns of the Escatrón
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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“Coal Selection Criteria for Industrial PFBC Firing”
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 (

“Coal Selection Criteria for Industrial PFBC Firing”
• Operating procedures: start-up, shut-down and load-following procedures affected the
concentration 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 (

“Coal Selection Criteria for Industrial PFBC Firing”
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/Nm 3 or 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 (SO2 and 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 1200 o C, 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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“Coal Selection Criteria for Industrial PFBC Firing”
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 Osaki’s 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). Aluminosilicate,
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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“Coal Selection Criteria for Industrial PFBC Firing”
Tomatoh-Atsuma has to limit its SOx emission below 94 ppm, NOx emission below 98
ppm and dust emission below 28 mg/Nm 3 . It successfully operates under normal
conditions while producing 10 ppm SOx, 40 ppm NOx and less than 10 mg/Nm 3 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/Nm 3 . 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 SOx is the same as Karita (76ppm), 19ppm for
NOx and 9 mg/Nm 3 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 NOx and 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 NOx (Newby,
Keairns et al. 1989):
NOx = 12.25 exp(2827/T) [O2] 0.24 Xn 0.44 Y -0.1 (ppmv)
where T = bed temperature (K)
[O2] = volume percent oxygen in the combustion products
Xn = weight percent nitrogen in the coal
(Eq. 3)
Y = concentration of SO2 in 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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“Coal Selection Criteria for Industrial PFBC Firing”
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, NOx and SO2
under stationary conditions (Abe, Sasatsu et al. 1999). Analyses found that N2O and SO2
emissions were more dependent on gas temperature (α Tc) compared with CO and NOx
emissions (α Tc 1/2 ). 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 ASH TR equations could be used to estimate the concentrations of
exhaust gases under PFBC operations (Abe, Sasatsu et al. 1999):
PCO = PO2 1/2 x exp (13.431 x 10 3 /Tc – 21.562) (R 2 = 0.9794) (Eq. 4)
Calculated NOx conversion = ([O2]/3.5) 1/2 x {[F1/(1+F1) – F2/(1+F2)]} (Eq. 5)
F1 = PO2 1/2 x 5.00 x 10 -4 x exp (0.21 x 10 3 /Tc) (Eq. 6)
F2 = PO2 1/2 x 4.57 x 10 -7 x exp (12.1 x 10 -3 /Tc) (Eq. 7)
NOx = NOx conversion x input [N] x 22.4/gas flow rate x 10 6 (ppm) (Eq. 8)
It was found that maximum reduction of NOx emissions (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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“Coal Selection Criteria for Industrial PFBC Firing”
4. ABB CARBON’S PROCESS TEST FACILITY (PTF)
ABB Carbon built a 1 MWe Process 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 Type
Volatiles, 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
bituminous
17.2 20.0 1.06 27.3
South Africa /
Italy
Poland
Australia
China
Medium volatile
bituminous + lignite
High volatile
bituminous
High volatile
bituminous
High volatile
bituminous
35.9 16.4 2.85 24.9
34.9 10.5 0.78 28.9
34.7 16.5 0.56 27.8
42.6 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
(

“Coal Selection Criteria for Industrial PFBC Firing”
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 bed
material 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 coarser
sorbent 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 ash
content. 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
Page 21

“Coal Selection Criteria for Industrial PFBC Firing”
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 NOx Emissions
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
o
C, the emissions of N2O were less than 20 ppm (Andersson, Bergqvist et al. 1999).
Page 22

5. CONCLUSIONS
“Coal Selection Criteria for Industrial PFBC Firing”
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 coal’s
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 Escatrón, Värtan, 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% Fe2O3 in the coal ash and one also specifies an ash
fusion temperature > 1200 o C. 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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“Coal Selection Criteria for Industrial PFBC Firing”
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 Al2O3 and 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 (

“Coal Selection Criteria for Industrial PFBC Firing”
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%.
Page 25

6. ACKNOWLEDGMENTS
“Coal Selection Criteria for Industrial PFBC Firing”
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.
Page 26

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