Energy & Fuels 1999, 13, 585-591
585
Behavior of Chloride during Coal Combustion in an
AFBC System
Wei Xie, Wei-Ping Pan,* and John T. Riley
Materials Characterization Center, Department of Chemistry, Western Kentucky University,
Bowling Green, Kentucky 42101
Received July 15, 1998
Two 1000-h burns were conducted with the 12-in. (0.3 m) laboratory AFBC system at Western
Kentucky University. Operating conditions similar to those used at the 160-MW AFBC system
at the TVA Shawnee Steam Plant located near Paducah, KY, were used. A low-chlorine (0.012%
Cl and 3.0% S) western Kentucky No. 9 coal and a high-chlorine (0.28% Cl and 2.4% S) Illinois
No. 6 coal were used in this study. Four different metal alloys [carbon steel C1020 (0.18% C and
0.05% Cr), 304 SS (18.39% Cr and 8.11% Ni), 309 SS (23.28% Cr and 13.41% Ni), and 347 SS
(18.03% Cr and 9.79% Ni)] were exposed uncooled in the freeboard area at the entrance to the
convective pass, where the metal temperature was approximately 900 K. A two-phase investigation
was carried out in order to study the fate of chlorine during coal combustion in an AFBC system
and to study the susceptibility of boiler components to corrode in combustion gases containing
hydrogen chloride. As determined from emission and ash studies, the temperature in an AFBC
system plays a key role in the retention of chloride, which is more favorable at low operating
temperatures. A small amount of scale failure was observed on the other three samples in both
test runs. On the basis of the SEM-EDS mapping results, there was no localized chloride
distribution observed on the surface of the coupons, either in the scale failure area nor on the
rest of the metal part. Some trace amount of chloride was found but was evenly distributed on
the surface of the coupons. There was no concentration of chloride on the spot of scale failure.
The scale failure might be due to sulfur attack and/or the effect of erosion. Further study with
higher chlorine-content coals for more conclusive information is needed.
Introduction
The occurrence of furnace wall and superheater
corrosion in fluidized-bed combustor systems has caused
some operational and economic concerns.1 It is generally
accepted that chlorine may play a role in this corrosion.
To predict the performance of high-chlorine coals in
these combustion systems, it is necessary to have a
better understanding of the different corrosion mechanisms in which chlorine and sulfur may be involved.2
It is also important to evaluate the critical point of coal
chlorine content which may cause initial corrosion.
To better understand the combustion behavior of
chloride during coal combustion in an AFBC system, a
comprehensive research project was performed at Western Kentucky University. The project concentrated on
the effect of coal chloride content on HCl emission
reduction, on the effect of chloride on the absorption of
SOx emissions, on the effect of operating parameters on
the distribution of chloride in the fly ash and bed ash,
(1) Minchener, A. J.; Lloyd, D. M.; Stringer, J. The Effect of Process
Variables on High-Temperature Corrosion in Coal-Fired Fluidized Bed
Combustors. In Corrosion Resistant Materials for Coal Conversion
Systems; Meadowcroft, D. B., Manning, M. I., Eds.; Applied Science
Publishers: New York, 1983; Chapter 15, p 299.
(2) Mayer, P.; Manolescu, A. V.; Thorpe, S. J. Influence of Hydrogen
Chloride on Corrosion and Corrosion-Enhanced Cracking Susceptibility
of Boiler Construction Steels in Synthetic Flue Gas at Elevated
Temperature. In Corrosion Resistant Materials for Coal Conversion
Systems; Meadowcroft, D. B., Manning, M. I., Eds.; Applied Science
Publishers: New York, 1983; Chapter 5, p 87.
and on the effect of high-temperature corrosion for
different metals in fluidized beds during coal combustion.
The laboratory-sized atmospheric fluidized-bed combustor (AFBC) at Western Kentucky University (WKU)
was designed to serve as a flexible research and
development facility to evaluate combustion performance and to estimate the effects of flue gas emissions.
The WKU AFBC system was configured to simulate the
160 MW AFBC system at TVA’s Shawnee Steam Plant
near Paducah, KY, and the operating conditions for the
WKU system are similar to those used in the Shawnee
Plant’s system.
The chlorine content of coal varies from just a few
parts per million to thousands of parts per million.
Emissions of chloride from coal-fired plants can range
from 50 to several thousand parts per million, depending on the original concentration in the coal, the type
of combustor, and any pollution control equipment
installed. It has been estimated that 94% of the chloride
in coal is volatilized during combustion, generally being
emitted as gaseous HCl.3 In an AFBC system, limestone
may be able to capture the chlorine. Limestone degenerates to CaO, and the CaO reacts with HCl to produce
CaCl2.4 In an AFBC system, capture of chloride by
limestone in the combustion zone depends on the
(3) Shao, T. M.S. Thesis, Western Kentucky University, 1994.
(4) Heidbrink, J. M.S. Thesis, Western Kentucky University, 1996.
10.1021/ef9801569 CCC: $18.00 © 1999 American Chemical Society
Published on Web 02/26/1999
586 Energy & Fuels, Vol. 13, No. 3, 1999
temperature of the zone and the ratio of calcium-tosulfur. A study by Liang and others5 showed that
chloride capture has a large variation with temperature
moving from a low of 18% gaseous HCl at 700 °C to 99%
HCl at 950 °C. The resulting product is almost entirely
in the form of liquid CaCl2. Munzner and Schilling6
studied the effect of limestone in a bench-scale AFBC
system and showed that a greater recapture of chloride
occurred with larger excesses of limestone or when the
Ca/S ratio was greater than 2.
The combustion of coal in a fluidized-bed combustor
with a limestone bed is one method of controlling sulfur
emissions. With increasing use of high-chloride coal for
combustion in an AFBC system, the addition of limestone may help reduce chloride emission. Therefore, in
situ sulfur and halogen capture by limestone is the
major advantage of fluidized-bed combustion. It is
possible that chlorine might also affect the detailed
chemistry of the processes of sulfur capture in fluidizedbed combustors7,8 and also might play a role in sulfur
deposition in ashes.
One of the main goals for this project is to evaluate
the critical point where the coal chloride content may
cause initial corrosion. The chloride content of most
coals burned in U.S. coal-fired boilers has recently been
limited to less than about 0.3% by weight to avoid
aggravation of the potential for fireside corrosion.
However, no formal report has been published to support this limit, although there have been several studies
conducted on British coals which indicate that aggressive fireside corrosion can be correlated to the use of
high-chlorine coal.
Experimental Section
Two 1000-h burns were conducted with the 0.3-m (12-in.)
laboratory AFBC system at Western Kentucky University. A
schematic diagram of the AFBC system as used for the tests
and the full description of the AFBC system (Figure 1) have
been previously presented by Pan and co-workers.9,10 The
combustor’s operating parameters (air/water flow, coal/lime
feed, bunker weight, temperatures, and pressure) are controlled and logged to file with a Zenith 150 MHz computer
utilizing the LABTECH software version 3.0. During combustion runs, any needed changes in the parameters can easily
be entered into the computer by accessing the correct control
screen and making the necessary corrections on line.
A 1000-h burn was done with a low-chlorine (0.012% Cl and
3.0% S) Western Kentucky No. 9 coal (WKU 95011), which is
the same type of coal as that supplied to the TVA plant in
1993. A second 1000-h burn was conducted with a high(5) Liang, D. T.; Anthony, E. J.; Leowen, B. K.; Yates, D. J.
Proceedings, 11th International Conference on FBC, Montreal, Canada,
April 21-24, 1991; Vol. 2, pp 917-922.
(6) Munzner, H.; Schilling, D. H. Proceedings, 8th International
Conference on FBC, Houston, TX, March 18-21, 1985; Vol. III, pp
1219-1226.
(7) Johnson, I.; Lence, J. F.; Shearer, J. A.; Smith, G. W.; Swift, W.
M.; Treats, F. G.; Turner, C. B.; Jonke, A. A. Support Studies in
Fluidized-Bed Combustion; Argonne National Laboratory Quarterly
Report ANL/CEN/FE-79-8, 1979.
(8) Griffin, R. D. A New Theory of Dioxin Formation in Municipal
Solid Waste Combustion. Chemosphere 1986, 15.
(9) Orndorff, W. W.; Su, S.; Napier, J.; Bowles, J.; Li, H.; Li, D.;
Smith, J.; Pan, W.-P.; Riley, J. T. Studies with a Laboratory Atmospheric Fluidized Bed Combustor. Proceedings, 12th International Coal
Testing Conference, Cincinnati, OH, 1996; pp 67-75.
(10) Pan, W.-P.; Riley, J. T. Behavior of Chlorine during Coal
Combustion in an AFBC System; Final Report to Electric Power
Research Institute (W09002-13), 1997.
Xie et al.
Figure 1. Schematic diagram of Western Kentucky University’s AFBC system.
Table 1. Proximate and Ultimate Analysis Valuesa for
the Coals and Limestones Used in the Study
95011
95031
KY limestone
moisture
ash
Volatile Matter
Fixed Carbon
Proximate Analysis (%)
10.07
8.32
9.37
10.78
43.34
37.21
47.29
52.02
0.19
57.93
18.90
22.98
Ash
Carbon
Hydrogen
Nitrogen
Sulfur
Oxygen
Chlorine (ppm)
BTU/lb
Ultimate Analysis (%)
9.37
10.78
74.08
72.16
5.08
4.82
1.54
1.54
3.2
2.38
6.72
7.57
118
3065
3203
12842
57.93
11.18
0.16
0.00
0.00
30.73
36
n/a
a Moisture is as-received; all other values are reported on a dry
basis.
chlorine (0.28% Cl and 2.4% S) Illinois No. 6 coal (WKU 95031).
Analytical values for the coals, as well as the limestone from
Kentucky Stone in Princeton, KY, are given in Table 1. The
coal and limestone both were air-dried before being crushed
to -4 mesh (4.75 mm). The limestone was also used as the
bed material in the AFBC system. During combustion runs,
limestone was fed into the system at a constant rate. The
summary of steady-state run conditions for both test runs are
shown in Table 2.
Four different metal alloys (carbon steel C1020, 304 SS, 309
SS, and 347 SS) were studied in this project. Elemental
compositions of these alloys are given in Table 3. Each metal
coupon had a 5.08 cm outside diameter and was 0.3175 cm
thick with a 1.587-cm diameter hole in the center. A set of
metal coupons was placed at 3.35 m above the fuel injection
port, which is 10 cm below the convective pass heat exchange
tubes. The coupons were held in place by a machinable
tungsten rod (powder metallurgically prepared) and separated
by ceramic mounts. Two sets of each coupon (eight total) were
used in each run. Each specimen within a group was rotated
as to position in an array every 250 h during the test burn.
The temperature near the metal coupons was approximately
Behavior of Chloride During Coal Combustion in an AFBC System
Energy & Fuels, Vol. 13, No. 3, 1999 587
Table 2. Summary of Steady-State Run Conditions
primary zone
gas velocity (ms-1)
0.33 (298 K)
1.24 (1120 K)
limestone feed rate
(kg h-1)
Ca/S molar ratio
10.44
(for coal 95031)
2.21
(for coal 95031)
3.1
8.41
(for coal 95011)
2.21
(for coal 95011)
3.1
bed temp
in 0.56 m
in 0.97 m
in 1.90 m (probe 1)
in 2.60 m (probe 2)
in 3.30 m (probe 3)
Temperature (K)
1113
1108
1073
943
958
843
1173
1168
1133
1013
1023
923
fuel feed rate (kg h-1)
1223
1218
1193
1078
1088
978
Table 3. Analytical Composition (Percent by Weight) of
Four Alloy Metals
element
C
Cr
Cu
Mn
Mo
Ni
P
S
Si
Fe
others
C1020
0.180
0.050
0.450
0.010
0.020
0.008
0.005
0.024
balance
0.051
C304
C309
C347
0.05
18.39
0.41
1.84
0.36
8.11
0.032
0.001
0.46
balance
N-0.06
0.058
23.28
0.90
1.78
0.140
13.41
0.017
0.011
0.44
balance
N-0.059
CO-0.03
0.045
18.03
0.14
1.20
0.17
9.79
0.018
0.014
0.52
balance
N-0.029
CO-0.21
900 K. The coupons were weighed before and after the run
and examined using SEM-EDS. The procedure for cleaning
the specimens was to blow deposits off using compressed
nitrogen until a constant weight was reached. The SEM
analysis was performed using a JEOL JSM-5400 SEM. Attached to the SEM for energy-dispersive X-ray analysis (EDS)
was a KEVEX Sigma 1 system with a Quantum detector for
elemental analysis down to carbon. The following instrument
operating parameters were used for the SEM/EDS analysis:
electron beam energy, 20 keV; working distance, 24 mm;
sample tilt angle, 0°. A total of eight spots (35X) on each
specimen were analyzed. The concentration of S, O, Cr, Cl,
Ca, Mn, Ni, Fe, Na, Mg, Al, Si, and K were determined. The
highest and lowest sulfur content spots are presented and
discussed in the next section.
Small pieces (2 cm × 0.3 cm) were cut from specimens using
a LECO VC-50 precision diamond saw. The small pieces were
mounted using a LECO PR-32 mounting press (pressure: 4
psi, 20 min heating, and 40 min cooling, Lucite powder was
used). The mounted specimens were ground and polished using
a LECO AP-60 8-in. Grinder/Polisher, using three different
SiC grit sizess400 PSA (600 s), 600 PSA (300 s), and 800 PSA
(300 s)sat 300 rpm and 70 lbs of pressure. The specimens were
polished with 1-µm diamond compound/red wool felt cloth/oil
microid diamond compound extender (120 s, 300 rpm and 65
lbs). The mounted specimens (cross section) were examined
using SEM/EDS. The results obtained by EDS analysis were
corrected by computer for corrections such as adsorption,
fluorescence, and atomic weight.
Results and Discussion
A two-phase investigation was carried out in order
to study the fate of chlorine during coal combustion in
an AFBC system and to study the susceptibility of boiler
components to corrode in combustion gases containing
hydrogen chloride.
Effects of Temperature. The effect of temperature
on the emission of chloride is illustrated in Figure 2.
Figure 2. Effect of temperature on the emission of hydrogen
chloride at different heights above the fuel injection port.
Figure 3. Effect of bed temperature on the concentration of
sulfur in the fly ash of the AFBC system.
Figure 4. Effect of bed temperature on the chloride content
in the bed ash of the AFBC system.
More HCl was observed when the temperature was
raised. The capture of HCl by limestone is more difficult
than the capture of SO2.10 Also, the reaction between
HCl and CaO is more favorable at lower temperatures.5
The increased concentration of HCl with increasing
distance above the fuel injection ports might result from
secondary combustion (the secondary air was injected
0.8 m above the setter plate) occurring in the freeboard
region.
The effects of bed temperature on the chlorine contents in fly ash and bed ash are shown in Figures 3 and
4. It is obvious that the chlorine content in the source
coal is a decisive factor in the distribution of chloride
588 Energy & Fuels, Vol. 13, No. 3, 1999
Xie et al.
Figure 5. Effect of coal type on the emission of sulfur dioxide
at different temperatures in the AFBC system.
in the ash. The low-chlorine coal (95011) released less
hydrogen chloride during combustion. As a result, there
were almost no temperature effects on the absorption
of HCl in both the fly ash and bed ash from the
combustion of this coal. For the high-chlorine coal
(95031), both Figures 3 and 4 show that chloride
retention is more favorable at low temperatures.11,12
Effects of Coal Chlorine Content on the Retention of SO2. Figure 5 shows the results of sulfur dioxide
emission from tests with two different coals (95011 and
95031). It is obvious that efficient sulfur capture is
obtained between 1120 and 1170 K for both coals. The
lower sulfur oxide emission for coal 95031 observed at
the higher temperature may be due to the effect of high
chlorine content.13 It may also be due to the interaction
between SO2 and HCl in the oxygen-rich conditions.14
Several reactions may be involved. In the oxygen-rich
conditions and high temperature, HCl can react with
oxygen to form Cl2, a reaction known as the Deacon
Reaction.
4HCl + O2 f 2Cl2 + 2H2O
Figure 6. Effect of coal type on the emission of hydrogen
chloride at different temperatures in the AFBC system.
Figure 7. Effect of the Ca/S ratio on the sulfur content in
the fly ash.
(1)
When SO2 is presented via coal sulfur combustion, an
interesting and important reaction is one in which SO2
may be attacked by Cl2 to form SO3 and HCl:
Cl2 + SO2 + H2O f 2HCl + SO3
(2)
Therefore, more SO2 can be converted to SO3 and then
be absorbed by CaO. SO2 absorption may also be due to
the formation of voids, allowing the diffusion of HCl and
SO2 toward the inside of limestone particles.15
Effect of Coal Type and Ca/S Ratio. There is good
agreement between HCl emission and the chlorine
(11) Julien, S.; Brereton, C. M. H.; Lim, C. J.; Grace, J. R.; Anthony,
E. J. Fuel 1996, 75 (4), 1655.
(12) Bramer, E. A. Flue Gas Emission from Fluidized Bed Combustion. In Atmospheric Fluidized Bed Coal Combustion; Elsevier: The
Netherlands, 1995.
(13) Johnson, I.; Lence, J. F.; Shearer, J. A.; Smith, G. W.; Swift,
W. M.; Treats, F. G.; Turner, C. B.; Jonke, A. A. Support Studies in
Fluidized-Bed Combustion; Argonne National Laboratory Quarterly
Report ANL/CEN/FE-79-8, 1979.
(14) Xie, W.; Han, W.; Pan, W.-P.; Riley, J. T. The Effect of Halides
on Emissions from Atmospheric Fluidized Bed Combustion. Presented
at the 83th Kentucky Academy of Science Meeting, Morehead, Kentucky, 1997.
(15) Matsukata, M.; Takeda, K.; Miyatani, T.; Ueyama, K. Simultaneous Chlorination and Sulphation of Calcined Limestone. AIChE
J. 1996, 51 (11), 2529-2534.
Figure 8. Effect of the Ca/S ratio on the chloride content in
the fly ash.
content in the coals, as illustrated in Figure 6. In other
words, the coal with higher chlorine content showed
greater emissions of HCl.
Figures 7 and 8 show the effect of the Ca/S ratio on
the sulfur and chloride retention in ash. One can see
from these figures that the Ca/S ratio has more influence on the sulfur retention for the high-sulfur-content
coal (95011). With an increase in the Ca/S ratio, the
sulfur content in fly ash increased. Also, it can be seen
that there is little effect of the Ca/S ratio on the chlorine
content in fly ash. On the other hand, Figure 8 shows
that the Ca/S ratio is more important for the capture of
chloride compared to sulfur for the high-chlorine-content
coal (95031). It is assumed that the HCl is probably
Behavior of Chloride During Coal Combustion in an AFBC System
Energy & Fuels, Vol. 13, No. 3, 1999 589
Figure 9. Effect of time of exposure to AFBC combustion
gases from low-chlorine coal on the weight of metal coupons.
Figure 10. XRD data for alloy 304 before and after two test
runs for coals 95011 and 95031.
captured in the low bed temperature region when the
flue gas is passing through the heat exchange tube
region because the reaction between HCl and CaO is
more favorable at the lower temperature.11,12 The
composition of the fly ash (collected from the wet
cyclone) in both cases is Ca(OH)2, which is due to CaO
reacting with water, CaSO4, CaCO3, and CaCl2. This is
the only place CaCl2 was identified in this study.
Effect of Chlorine Content on Corrosion. The
C1020 specimen was cracked after 250-h of operation
in both test runs. Figure 9 illustrates the weight change
data for the alloys in the first 1000-h burn with the lowchlorine coal. The type 347 specimen showed the highest
weight gain among the other three samples. However,
alloy 304 showed the most oxide scale (∼20 µm) on the
surface, followed by alloy 347 (∼12 µm), while alloy 309
(∼10 µm) showed the least oxide scale on the surface.
These observations are based on the color (reddish) and
SEM studies.10 The outer scale was chiefly hematite
(Fe2O3, red-brown oxide, dA ) 2.70, 2.52, 1.69, 1.84),
which was identified using X-ray diffraction spectroscopy, as illustrated in Figure 10. The inner layer was
enriched in chromium oxide. Figure 11 shows line scans
of elements (Fe, Cr, and Ni) present in the cross-section
of alloy 347. Internal precipitates in the metal just below
the metal-oxide interface, sometimes in alloy grain
boundaries, had a globular appearance, which is typical
of chromium sulfide, but EDS results showed neither
Cr nor S (or anything else) to be especially concentrated
in them. Also, a few spot analyses were made to
determine whether any of the internal precipitates
Figure 11. Scale formed on alloy 347 after 1000 h of exposure
at 1144 K to combustion gases from low-chlorine coal. (a) Fe
X-ray line scans, (b) Cr X-ray line scans, (c) Ni X-ray line scans.
present in alloys were chromium sulfide, and the
mapping results showed no sulfur enrichment with
chromium. However, the results showed sulfur was
somewhat enriched with calcium. Ni contained in alloys
seems to provide no help with regard to corrosion
prevention, since it was consumed in the outer layer.
590 Energy & Fuels, Vol. 13, No. 3, 1999
Very similar results were observed in the case of alloy
specimens 304 and 347.
Slight oxide spallation was observed in all three
alloys. The degree of spallation for the three alloys
followed the order 304 > 347 > 309 (the least). This is
the reason for the fluctuation of weight change shown
for alloy 304. The weight gain is due to oxidation, and
the weight loss is due to oxide spallation. The two
reactions compete with each other. The spallation might
be due to the cooling effect (cooling of the samples from
the reaction temperature)16 or sulfur attack and the
effect of erosion.17,18 The EDS results for alloy 347
indicated that the amount of sulfur and calcium on the
surface increased with burning time. These results may
be due to the presence of deposits on the surface of the
coupons. The concentration of oxygen on the coupons
increased with time of combustion but leveled off after
500 h. The results for iron are opposite from the oxygen
results. These results may indicate the formation of
oxide scale followed by the spallation of oxide.
There was no chloride identified on the surface of any
metals that were exposed during the combustion of the
low-chlorine coal. Fly ash passed through the specimens
during the entire run. Thus, the effect of erosion should
also be taken into consideration.10 Alloy 309 is the best
corrosion-resistant material (less oxide scale and scale
spallation) among the three alloys tested under our
experimental conditions. The performance of alloy 309
may be due to the high amount of chromium in the
material.
Figure 12 shows the weight changes for the three
alloys in the second 1000-h burn with the high-chlorine
coal (0.28% Cl). Weight gains were observed for alloys
309 and 347 before 500 h. In the case of the 304 alloy,
the weight remained almost constant in the first stages
of the test burn. There was no chloride (EDS results)
observed on the surface of coupons before 500 h. A small
amount of oxide spallation was observed on all three
samples, which is similar to the results obtained with
the low-chlorine coal in the first 1000-h test.
On the basis of the EDS results, the elemental
distribution on the surface presents a similar trend with
the first test runs for all three alloys. However, weight
loss was observed in all three coupons after 500 h.
Chloride was identified on the surface of the coupons.
The higher chloride (around 1 wt %) contents were
observed after the first few cleaning (by blowing with
N2) cycles. The chloride content on the surface of the
coupon decreased to about 0.1% after cleaning was
complete. This may indicate that most chloride exists
in the ash deposits. Ash deposits on the gas heat
exchange tubes (average temperature around 770 K),
which were just a few inches above the uncooled
coupons, were collected at different locations (bottom,
middle, top, right, and left tubes). The sulfur and
chloride distributions in the deposits on the heat
(16) Verma, S. K. Oxidation/Sulfidation Behavior of Ferritic 446,
Austenitic 310, and Cobalt-Base Alloy 6B in High-Temperature
Exposures in A Coal Gasification Atmosphere. Proceedings, Symposium
on Corrosion in Fossil Fuel Systems; Wright, I. G., Ed.; The Electrochemical Society, Inc., 1983; Vol. 83-5, pp 172-201.
(17) Wright, I. G.; Stringer, J. Fly Ash Erosion of Boiler Convection
Banks. Presented at EPRI/Florida Power and Light Seminar on Use
of Coal in Oil-Design Utility Boilers, Dec 2-4, 1980, Section 5, 1-18.
(18) Tabakoff, W.; Kotwal, R.; Hamed, A. Erosion Study of Different
Materials Affected by Coal Ash Particles. Wear 1979, 52, 161-73.
Xie et al.
Figure 12. Effect of time of exposure to AFBC combustion
gases from a high-chlorine coal on the weight of metal coupons.
Figure 13. Chloride and sulfur concentrations at different
locations on the heat exchange tubes in the AFBC combustor.
exchange tubes are shown in Figure 13. Most of the
chloride was deposited on the outside tubes (bottom, left
and right). As for sulfur in the deposits, the same results
were not observed, since sulfur was evenly distributed
in the deposits on the tubes. This may indicate that the
outside tubes may be more easily attacked (if any) by
chloride than the inner and top tubes.
On the basis of the mapping results, chloride was
evenly distributed on the surface of the metal coupons.
There was no significant precipitate of chloride in the
outer and inner scale or the cross section of the coupons.
There was no significant difference in the morphologies
of cross sections of coupons exposed in the two different
runs. There was no evidence to indicate chloride was
the cause of any corrosion problem. However, there was
more scale spallation observed in the high-chlorine coal
test run than was observed in the low-chlorine coal test
run. This may be due to the higher erosion rate
occurring in the case of high-chlorine coal. The higher
erosion rate was due to the higher ash contents (10.78%
vs 9.37%) and the higher coal feeding rate (10.44 vs 8.41
Kg/h, to keep the Ca/S ratio ) 3) in the high-chlorine
coal. This is evidence to suggest that sulfur and chlorine
may both enhance attack on the metal coupons, but the
data are not conclusive.
Conclusions
Considering the results of this study, some observations and conclusions that can be made are as follows.
The capture of HCl by limestone is more difficult than
Behavior of Chloride During Coal Combustion in an AFBC System
the capture of SO2. There is no significant change in
the emission of HCl when the Ca/S ratio is varied. The
bed temperature in an AFBC system plays a key role
in the retention of sulfur and chloride in ash. When the
bed temperature is too high, less SO2 is absorbed in the
bed ash and more is absorbed in the fly ash. The chloride
content of a coal is an important factor in the retention
of chloride in the ash. Chloride retention in both the
fly ash and bed ash is more favorable at low operating
temperatures. When the sulfur or chlorine content in
coal reaches a certain point, the Ca/S ratio in the
combustion mixture will be an important factor in the
absorption of SO2 and HCl. In the combustion of a highchlorine coal, more sulfur is captured in the bed ash
than in the combustion of a low-chlorine coal. Of the
four alloys tested, the high-chromium (∼23%) 309 alloy
steel forming Cr2O3 on the surface is the most resistant
to corrosion of the four materials tested. Scale spallation
Energy & Fuels, Vol. 13, No. 3, 1999 591
of alloy coupons was observed in both 1000-h test burns
with low- and high-chlorine coals. The second test burn
with the high-chlorine coal showed more scale spallation
than that obtained with the first run with the lowchlorine coal. This increase in spallation may be due to
erosion by the higher ash content of the high chlorine
coal. There is no conclusive evidence to indicate chlorine
may cause corrosion problems under our experimental
conditions.
Acknowledgment. The financial support for this
work received from the Electric Power Research Institute (Contract No. W09002-13) and the technical assistance from the Illinois Clean Coal Institute is gratefully acknowledged.
EF9801569