Energy Fuels 2011, 25, 647–655 Published on Web 01/20/2011
Conversion of Sulfur during Pulverized Oxy-coal Combustion Daniel Fleig,* Klas Andersson, Filip Johnsson, and Bo Leckner Division of Energy Technology, Department of Energy and Environment, Chalmers University of Technology, SE-412 96 G€ oteborg, Sweden Received September 29, 2010. Revised Manuscript Received December 17, 2010
On the basis of experiments in the Chalmers 100 kWth oxy-fuel test facility, this study presents an analysis of sulfur chemistry of pulverized lignite combustion, comparing oxy-fuel and air-fired conditions. Four test cases were investigated: an air-fired case, two oxy-fuel cases with dry recycling (30 and 35 vol % O2), and one oxy-fuel case with wet recycling (43 vol % O2 on a dry basis). The amounts of sulfur in the flue-gas, ashes, and condensed water from the condenser were quantified, and a sulfur mass balance was established. The composition of the ashes and the ash-forming matter in the fuel was analyzed. The ashes were investigated by X-ray diffraction, while the size of fuel and ash particles was determined by laser diffraction. In general, the results show that the lignite has a high sulfur self-retention by ash, especially in oxy-fuel combustion. The experiments also show that the conversion of fuel S to SO2 from oxy-fuel combustion is around 35% lower compared to air-fired conditions, whereas the flue-gas concentration of SO2 is higher in oxy-fuel combustion because of the absence of air-borne nitrogen.
Reaction 1 takes place during the heating phase of the fuel, and reaction 2 takes place during the char combustion. Sulfur
dioxide (SO2), for example, formed according to reaction 3, is the thermodynamically favored sulfur oxide at high temperatures (>1000 °C) and oxygen-rich conditions. Under substoichiometric conditions in the flame, hydrogen sulfide (H2S) is also formed in the gas or is released during devolatilization from organic sulfur compounds. However, when the oxygen is in excess, with quantities typical of pulverized-coal combustors, the outlet concentration of H2S is negligible. At lower temperatures, the equilibrium shifts toward sulfur trioxide (SO3), but the formation of SO3 is very slow, and the resulting concentration of SO3 is several orders of magnitude lower than that of SO2 in the emitted gas. In air-fired pulverized-coal combustion, typically 0.1-1% of SO2 is oxidized to SO3.6 At temperatures below 500 °C, SO3 reacts with water vapor (H2O) in the flue-gas, forming gaseous sulfuric acid (H2SO4), which can cause low-temperature corrosion if it condenses on metal surfaces. At low temperatures, adsorption of SO3/ H2SO4 by particles is efficient. For example, the alkalinity of the fly ash from combustion of sub-bituminous coals, commonly having a low sulfur content, is often sufficiently high to adsorb nearly all H2SO4(g).7 Sulfur can also remain in the minerals without being released. SO2 can react during char combustion with alkaline earth and alkali metals (Mg, Ca, Na, and K) to form sulfates. Simultaneously, the decomposition of sulfates may take place.4 In oxy-fuel combustion, the SO2 concentration is significantly higher than in air-fuel combustion because of the recirculation of flue-gas. This favors sulfation and stabilizes the sulfates formed.8-10 Sulfur retention also depends upon fuel-specific characteristics, such as coal-particle size and the
*To whom correspondence should be addressed. Telephone: þ46-(0)31-772-1453. E-mail: [email protected]
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(6) Brady, J.; Holum, J. Fundamentals of Chemistry; John Wiley and Sons: New York, 1988. (7) Srivastava, R. K.; Miller, C. A.; Erickson, C.; Jambhekar, R. Emissions of Sulfur Trioxide from Coal-Fired Power Plants; POWERGEN International: Orlando, FL, 2002. (8) Cheng, J.; Zhou, J.; Liu, J.; Zhou, Z.; Huang, Z.; Cao, X.; Zhao, X.; Cen, K. Prog. Energy Combust. Sci. 2003, 29, 381–405. (9) Liu, H.; Okazaki, K. Fuel 2003, 82, 1427–1436. (10) Liu, H.; Katagiri, S.; Okazaki, K. Energy Fuels 2001, 15, 403–412.
Introduction Oxy-fuel combustion is one of the most promising carbon dioxide (CO2)-capture technologies because, to a large extent, it builds on commercial components, thus lowering the risk of investment. In oxy-fuel combustion, oxygen (O2) is used instead of air in the combustion process to generate a flue-gas with a high concentration of CO2. A large amount of flue-gas is recycled to replace the absent nitrogen to avoid excessive combustion temperatures. As a result, the combustion takes place in an atmosphere with a high concentration of combustion products. Sulfur (S) is introduced by coal in the form of sulfides, organic sulfur compounds, sulfates, and traces of elemental sulfur.1 The sulfur content and the way sulfur is bound vary with the type of coal and depend upon the age and location of the coal source.2 During devolatilization, the main part of the organic sulfur compounds and sulfides is released to the gas phase. Sulfates, generally present in low quantities in coal,2,3 are only released during char combustion.4 Pyrite (FeS2) is usually the main fraction of sulfides in coal.1,3 It is released according to the following reactions:5 2FeS2 f 2FeS þ S2
2FeS f 2Fe þ S2
S þ 2O2 f 2SO2
r 2011 American Chemical Society
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way sulfur, alkali, and alkaline earth metals are bound. Calcium (Ca) has a dominant role in sulfur self-retention by ash; the Ca/S molar ratio in the coal is, therefore, an important factor,11,12 along with the reactivity of Ca present in the coal. Calcium in coals can be found as calcite (CaCO3), dolomite [CaMg(CO3)2], organic Ca, and Ca in clays and silicates. Ca present as CaCO3, common in coals,13 reacts to lime (CaO) during combustion if the temperature is sufficiently high and the CO2 partial pressure is sufficiently low, according to CaCO3 f CaO þ CO2
show lower Most oxy-fuel combustion experiments SO2 emissions than under air-fired conditions. Generally, these results are based on gas-phase measurements. However, Kiga and Takano21 also measured the mass flows of sulfur in the ashes, although their sulfur mass balance was not completely closed. Stanger and Wall24 present significantly reduced SO2 emissions for oxy-fuel compared to air-fuel conditions but without any major differences in the sulfur content of the ashes. They state, without providing experimental data on deposits, that the missing sulfur in oxy-fuel combustion might be the result of a higher deposition rate of S compounds compared to air firing. Obviously, there is a need to clarify the fate of sulfur in combustion facilities operating under oxy-fuel conditions. In the present paper, the behavior of sulfur during pulverized-coal combustion is discussed in detail. The objective is to identify and quantify the sinks of sulfur in a 100 kWth oxy-fuel test unit with lignite as fuel. Furthermore, the differences in sulfur behavior between air- and oxy-fuel combustion are highlighted.
CaO, also formed from organic Ca and dolomite, reacts with SO2 into CaSO4.14 2CaO þ 2SO2 þ O2 f 2CaSO4
At temperatures below 900 °C (around 800 °C in air combustion) and at atmospheric pressure, calcination (reaction 4) does not take place in oxy-fuel combustion because of the high CO2 partial pressure;15 direct sulfation of CaCO3 is favored (similar to the situation in pressurized air combustion16). 2CaCO3 þ 2SO2 þ O2 f 2CaSO4 þ 2CO2
Experimental Section Flue-gas measurement in the Chalmers 100 kWth oxy-fuel test unit was combined with fuel and ash analyses with the aim to investigate the mass balance of sulfur. The sulfurous combustion products were evaluated in the ash, flue-gas, and condensed water of the test unit. The sulfur sinks were quantified, and the mass balances from air- and oxy-fuel combustion were compared. The composition of ash-forming matter and the particle-size distributions of the fuel and ashes were analyzed. Test Conditions and Sampling Procedure. The Chalmers 100 kWth oxy-fuel test unit is outlined in Figure 1 (a more detailed schematic can be found in a previous paper25). Tests were performed during air- and oxy-fuel combustion, the latter with dry and wet flue-gas recycling. Gas composition was measured upstream of the flue-gas condenser (flue-gas composition) and, in the oxy-fuel cases, downstream of the O2 mixing point as well (feed-gas composition; see Figure 1). The flue-gas concentrations of CO2, O2, and SO2 in Table 1 were taken as average concentrations of sampling sequences, typically over a 10-20 min duration. The O2 concentration in the feed gas was 34.9 and 29.7% in the oxy-fuel cases with dry recycling, hereafter referred to as OF35 and OF30, respectively. In the oxy-fuel case with wet recycling, the O2 concentration in the oxidizer was 42.6% on a dry basis (around 32% O2 on a wet basis), referred to as OF43w. The test duration was calculated on the basis of the amount of coal used in the experiment measured by the gravimetric feeding system. The peak combustion temperature in the OF30 case was from previous measurements estimated to be about 150 °C higher than in the air-fired case (peak temperature about 1200 °C for the air-fired case25). In the OF35 and OF43w cases, this difference was expected to be in the range of 200-300 °C.
In pulverized oxy-coal combustion, the temperature is sufficiently high for calcination.17 CaO, initially formed from CaCO3, has a highly porous structure; however, the CaO particles can sinter at the high temperatures occurring in pulverized-coal combustion. Additional formation of CaSO4 fills the pores and reduces the theoretical sulfation potential of CaO particles. In general, the sulfation efficiency is better in small CaO particles than in large ones. Oxy-fuel combustion favors the calcination/sulfation process of CaCO3, not only because of higher SO2 concentration but also because of the longer calcination time compared to air firing.17,18 Because the calcination of CaCO3 particles is slower in a CO2-rich environment, calcination takes place more simultaneously with sulfation than in a N2-rich environment and CO2 produced during calcination makes the developing CaSO4 layer more porous and brittle.17 The desulfurization efficiency of limestone in a CO2/O2 atmosphere at 1200 °C was, therefore, found to be still higher than the desulfurization efficiency of limestone in an air-combustion atmosphere at 900 °C (the residence time was 3 s).17 The optimal desulfurization temperature of limestone is higher in a CO2/O2 atmosphere (1050 °C) than in air (900 °C).17 Furthermore, Chen et al.18 observed that CaO, calcined in a CO2/O2 environment, has a less specific surface area but larger pore diameter than CaO calcined in air. The larger pore diameter also increases the sulfation efficiency of CaO because of reduced pore filling and plugging. Hence, the desulfurization efficiency in oxy-fuel combustion is significantly higher than in air firing.
(19) Woycenko, D.; Ikeda, I.; van de Kamp, W. L. Technical Report IFRF Doc F98/Y/1; International Flame Research Foundation (IFRF): Livorno, Italy, 1994. (20) Croiset, E.; Thambimuthu, K. V. Fuel 2001, 80, 2117–2121. (21) Kiga, T.; Takano, S.; Kimura, N.; Omata, K.; Okawa, M.; Mori, T.; Kato, M. Energy Convers. Manage. 1997, 38, 129–134. (22) M€ onckert, P.; Dhungel, B.; Kull, R.; Maier, J. Impact of combustion conditions on emission formation (SO2, NOx) and fly ash. Proceedings of the 3th International Energy Agency (IEA) Greenhouse Gas R&D Programme (IEA GHG) International Oxy-Combustion Workshop; Yokohama, Japan, 2008. (23) Tan, Y.; Croiset, E.; Douglas, M. A.; Thambimuthu, K. V. Fuel 2006, 85, 507–512. (24) Stanger, R.; Wall, T. Prog. Energy Combust. Sci. 2011, 37, 69–88. (25) Andersson, K.; Normann, F.; Johnsson, F.; Leckner, B. Ind. Eng. Chem. Res. 2008, 47, 1835–1845.
(11) Kasbohm, J. Zur thermischen Schwefelfreisetzung aus Braunkohlen. Dissertation, Universit€ at Greifswald, Greifswald, Germany, 1988. (12) Sheng, C.; Xu, M.; Zhang, J.; Xu, Y. Fuel Process. Technol. 2000, 64, 1–11. (13) Cooper, B. R.; Ellingson, W. A. The Science and Technology of Coal and Coal Utilization; Plenum Press: New York, 1984; pp 21-25. (14) Borgwardt, R. H. Environ. Sci. Technol. 1970, 4, 59–63. (15) Wang, C.; Jia, L.; Tan, Y.; Anthony, E. J. Fuel 2008, 87, 1108– 1114. (16) Yrjas, P.; Iisa, K.; Hupa, M. Fuel 1995, 74, 395–400. (17) Chen, C.; Zhao, C. Ind. Eng. Chem. Res. 2006, 45, 5078–5085. (18) Chen, C.; Zhao, C.; Liang, C.; Pang, K. Fuel Process. Technol. 2007, 88, 171–178.
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Figure 1. Schematic of the sampling positions in the Chalmers 100 kWth oxy-fuel test unit. Letters denote measurement positions: A, ash; W, water; G, gas; T, temperature. Table 1. Experimental Conditions in the Test Cases measured flue-gas concentrations (on a dry basis) test case
test duration (h)
O2 in feed gas (vol %, dry)
heat input (kW)
m_ fuel (kg/h)
air firing OF35 OF43w OF30
9.1 9.6 11.5 11.0
21 34.9 42.6 29.7
76 76 76 76
13 13 13 13
16.0 90.9 90.8 91.4
3.3 6.9 7.3 5.7
550 1870 1955 1917
Table 2. Analyses of the Fuel ultimate (wt %, daf)a
proximate (wt %, as received) coal used in the test case
volatiles (wt %, daf)
air firing OF35 OF43w OF30
10.7 10.6 10.5 9.9
4.38 4.56 4.75 5.18
84.9 84.8 84.7 84.9
61.0 58.8 57.7 59.3
67.2 67.0 66.8 66.5
5.36 5.37 5.39 5.5
0.87 0.81 0.82 0.69
0.90 0.90 0.88 0.93
25.7 25.9 26.1 26.4
daf = dry and ash-free basis. b By difference.
The sulfur release from the lignite under isothermal conditions in an O2 atmosphere was investigated at different temperatures in a previous study,26 in which small samples (around 1 g) were burned in an oven and the SO2 concentration in the flue-gas was measured. At 600 °C, around 50% of fuel S was released as SO2, and at 1200 °C, around 80% was released. The S release from the lignite strongly depends upon the temperature, as anticipated.11 For a general evaluation of sulfur self-retention by ash, the composition of the ash-forming matter in the fuel was analyzed. Figure 2 shows average mass fractions of the main ash-forming matter in the fuel (except for O, S, and Cl) based on two samples, taken from the lignite used in the OF35 and OF43w cases. The total fraction of the main ash-forming matter was 2.7% of the dry fuel mass. For a general estimation of the potential for sulfur self-retention by ash, the molar ratios of Ca/S, Mg/S, 2K/S, and 2Na/S were of interest. The amounts of K and Na were slight in the coal. The Ca/S molar ratio was 1, and the Mg/S molar ratio was 0.5. Thus, theoretically, the fuel contains enough alkaline elements to capture all sulfur. The ashes accrued during the experiments contained various amounts of S. Because of this fact, the elemental composition of the bottom, cyclone, and filter ashes was analyzed to investigate how the Ca and Mg are distributed among the ashes. The ashes were studied through X-ray diffraction using a Philips X-ray diffractor with a Cu KR X-ray tube to determine how sulfur and alkaline earth and alkali metals are bound in the ashes. Only crystalline phases with at least 2% of ash mass could be detected. The fuel and ash particle sizes may affect the potential for sulfur self-retention. The specific surface area increases with a
Beside online gas analyses of SO2, O2, and CO2, the temperature before and after each component (see Figure 1) was continuously measured and the ashes were sampled at different locations. The condensed water, denoted further as condenser water, was sampled in the flue-gas condenser. It was necessary to estimate the ash quantities at each location because of different S mass fractions. The bottom and cyclone ashes were weighed. Minor ash flows, which could not be quantified in the system, for example, ashes found in the flue-gas cooler, fabric filter, reactor cooling tubes (deposits), and reactor walls, were subsumed within residual ash as the difference between the weighed ashes and the ash content of the fuel. Fuel, Ash, and Water Analyses. The S concentration in the ashes was analyzed according to the Swedish standard SS 187186, with an accuracy of (5% for the range surveyed. A small ash sample was heated in an oven in an O2 atmosphere to at least 1400 °C, and the amount of SO2 released was measured. German predried lignite with low ash content was used as fuel. The proximate and ultimate analyses of the fuel batches used in each test case are given in Table 2. The S mass fraction in the combustibles (YS) is an average value based on five separate analyses (with a variation in S content between 0.86 and 0.94%). The total content of sulfur in the fuel was determined according to the Swedish standard SS 187177, with an accuracy of (5% for the range examined. The ash mass fraction of the fuel as received (Yash) is an average value of at least two fuel analyses for each test case. The ash mass content of the fuel was determined according to the Swedish standard SS 187157, for which a combustion temperature of 815 °C was applied. In the OF30 test case, the coal derived from a separate coal delivery and had slightly higher S and ash contents than the coal used in the other three test cases.
(26) Fleig, D.; Normann, F.; Andersson, K.; Johnsson, F.; Leckner, B. Energy Procedia 2009, 1, 383–390.
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Figure 2. Composition of the main ash-forming matter in the fuel, except for S, O, and Cl. The circle represents 2.7% of dry fuel mass.
decreased particle size, and a higher sulfur self-retention can be expected. The size of the fuel and ash particles was determined by a Malvern Mastersizer 2000 laser diffraction particle-size analyzer with a Hydro 2000S sample dispersion unit. Figure 3 shows the particle size distribution of the fuel, having a mean size (on the basis of mass) of d(0.5) = 40 μm. Except for the S content of fuel, ash, and flue-gas, the amount of sulfur was also measured in the condenser water through inductively coupled plasma-optical emission spectrometry (ICP-OES). Because the temperature in the flue-gas cooler affects the absorption of SO2, the temperature of the flue-gas was maintained at 20 °C downstream of the flue-gas cooler in all test cases. Sulfur Mass Balance. The flue-gas, fuel, and ash analyses were used as input to the sulfur mass balance in each test, with the exception of the OF30 case that was excluded because no ash samples were taken from the filter and cooler. The global sulfur balance in the unit is derived by the following equation: m_ S, fuel ¼ m_ S, gas þ m_ S, ash þ m_ S, water
Figure 3. Particle size distribution of the fuel.
where Yi is the mass fraction of element i in the lignite (Table 2), Mi is the corresponding molar mass, and XO2 is the molar fraction of excess O2 in the flue-gas (Table 1). Leakage and impurities were taken into account by a molar leakage fraction, Xleak (estimated at 2% in the experiments based on the CO2 content of the flue-gas). In oxy-fuel cases OF35 and OF43w, the carrier gas contained a CO2 flow of 37 mol/h out of a gas cylinder, which was included in the term n_ cgCO2 in eq 11. In the OF30 case, flue-gas was used as the carrier gas. In the OF35, OF43w, and OF30 cases, 4331, 4227, and 4731 ppm, respectively, represent the volume concentrations XSO2,max. For the air-fired case, a volume concentration XSO2,max of 823 ppm, was determined by the following relationship:
The S flow carried by the fuel, m_ S,fuel, was calculated from Tables 1 and 2 by the equation below m_ S, fuel ¼ YS m_ fuel ð1 - Yash - Ymoisture Þ
XSO2 , max ¼
YS MS YS MS
YC YN þM þ 2M þ C N
n_ cgCO2 m_ fuel
þ Xleak - Xleak 2
1 þ 1 - OX2O
YC þM C
YO - 2M O
YH þ 4M H
0 B @1 þ
XO2 1 - XO2
0:79 1 þ 0:21
0:79 1 þ 0:21
þ YS, residual ash m_ residual ash
where m_ residual ash includes all ashes not quantified in the system, such as ashes found in the flue-gas cooler, fabric filter, reactor tubes, and walls. The S content of the residual ash (YS, residual ash) was presented as a range of S contents analyzed in the filter and cooler ashes. The total flow of ash (m_ ash) was in each case calculated from the fuel analysis. The flows of the bottom and cyclone ashes were estimated by weighing. Because the flows of total, bottom, and cyclone ashes were known, the residual ash flow was calculated from the following equation:
YS þ 0:79 0:21 MS
m_ S, ash ¼ YS, bottom ash m_ bottom ash þ YS, cyclone ash m_ cyclone ash
was calculated as the ratio of the measured molar fraction of SO2 in the flue-gas on a dry basis (XSO2) and the maximal theoretical molar fraction if all fuel S were converted to SO2, on a dry basis (XSO2,max). The flue-gas volume flow may vary somewhat with the conversion of fuel S to SO2, but this was neglected in eq 10. For oxy-fuel combustion, XSO2,max was calculated by the equation XSO2 , max ¼
YN þ 2M N
· The total sulfur mass flow in ash (mS,ash) is the sum of all sulfur ash flows
XSO2 XSO2 , max
YC þM C
was expressed as the resulting conversion of fuel S to SO2 (CSO2). The conversion CSO2 ¼
where YS is the mass fraction of sulfur in the fuel. The S flow in the flue-gas (m_ S,gas) is represented by SO2. The quantitative influence of SO3 on the mass balance of sulfur proved insignificant in previous studies, in which the gas-phase combustion chemistry was modeled.26,27 Most SO3 formed during combustion is captured by the fly ash or dissolved in the condenser water. Therefore, the mass flow of S in the gas phase m_ S, gas ¼ CSO2 m_ S, fuel
m_ residual ash ¼ m_ ash - m_ bottom ash - m_ cyclone ash
The S flow lost in the condenser (m_ S,water) was determined by an analysis of the condenser water. A similar loss was also expected in the Peltier cooler used to dry the extracted gas before entering it into gas analysers. Therefore, m_ S,water was included in all test cases.
(27) Fleig, D.; K€ uhnemuth, D.; Normann, F.; Andersson, K.; Johnsson, F.; Leckner, B. VDI-Ber. 2009, 2056, 289–294.
m_ S, water ¼ YS, water m_ water 650
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Figure 5. Ash mass flows. The residual ash includes all ashes not found as bottom ash or cyclone ash.
Figure 4. Conversion of fuel S to SO2 versus the measured SO2 concentration in the flue-gas on a wet basis. Table 3. Sulfur Content and Mass Flow of Sulfur in the Condenser Water test case
m_ water (g/h)
YS,water (wt %)
m_ S,water (g/h)
percentage of fuel S (%)
air firing OF35 OF43w OF30
5100 6400 6400 6400
0.010 0.018 0.025 0.020
0.51 1.15 1.60 1.28
0.5 1.2 1.7 1.3
Table 4. Mass Fraction of Sulfur in the Asha measured mass fraction of S in the ash (%) test case
air firing OF35 OF43w
6.6 6.5 6.8
3.0 3.6 3.7
15.1 8.1 and 16.8 12.4 and 17.9
9.0 9.3 9.9
a The S content in the residual ash, YS,residual YS,cooler ash and YS,filter ash.
Figure 6. Resulting S mass flows in the ash. An error bar is given for the residual ash, indicating the range of variation in the S mass fraction (Table 4).
is represented by
If there is no difference in the sulfur balance because of inaccuracies in measurements or operation, the sulfur selfretention by ash (RS) can be calculated from RS ¼
m_ S, ash m_ S, water ¼ 1 - CSO2 m_ S, fuel m_ S, fuel
Results Figure 4 shows the SO2 concentration in the flue-gas on a wet basis and the conversion of fuel S to SO2 (CSO2) for the four test cases investigated. The conversion was calculated using eqs 10-12. The measured SO2 concentration on a dry basis (shown in Table 1) was more than 3 times higher in the oxy-fuel cases compared to that of the air-fired case. This was because of the lower flue-gas flow compared to air firing. The SO2 concentration on a wet basis in the oxy-fuel cases using dry recycling was higher than with wet recycling because of the lower amount of water vapor in the flue-gas during dry recycling. The conversion of fuel S to SO2 was significantly lower in the oxy-fuel cases than in the air-fired case; the SO2 emission (mg/MJ fuel input) from the oxy-fuel cases was reduced by about 35% compared to the air-fired case. Table 3 shows the measured S content of the condenser water (YS,water) and the corresponding S flow (m_ S,water). The amount of sulfur in the condenser water was slight, and most
Figure 7. Sulfur mass balance. The value of 100% represents the sulfur introduced by the fuel (see analyses in Table 2), and the bar represents the sulfur sinks. An error bar is given for the residual ash, indicating the range of variation in S mass fraction (Table 4).
of the sulfur not present in the flue-gas ought to be found in the ashes. The S flow in the water was higher in the oxy-fuel cases than in the air-fired case because of the higher SO2 concentration and probably owing to a higher SO3 concentration. The main part of SO3 should be removed, at the latest, in the condenser. An additional observation was that the 651
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Figure 8. Metal and metalloids in (a) bottom ash OF43w, (b) cyclone ash OF43w, and (c) ash from the fabric filter, average sample (from all test cases after completion of the tests). The mass in panels a, b, and c is represented by 50.8, 58.6, and 49.1% of ash, respectively. The remaining mass in the respective cases is mainly made up of oxygen and sulfur.
amount of S in the condenser water was higher in the wet oxyfuel case than in the dry cases. Table 4 shows the measured S mass fractions in the ashes taken from the bottom of the furnace, the cyclone, the flue-gas cooler, and the filter. From the flue-gas cooler, one sample was taken in the air-fired case and two samples from different positions in the OF35 and OF43w cases. The S fraction in the ashes of the flue-gas cooler varied significantly, while it was more uniform in the other ashes (bottom, cyclone, and filter). In the cyclone ash, the S fraction was about half of that of the bottom ash. The filter ash contained about 50% more sulfur than the bottom ash. All ashes (and deposits) not collected as bottom or cyclone ashes were considered as residual ash. A range in S fraction was defined by the values measured in the cooler and filter ashes, constituting the main part of the residual ash. Other residual ashes, such as ash from the reactor walls, were difficult to relate to individual test cases and were, therefore, not continuously sampled. These samples from the reactor wall and from deposits on the cooling tubes inside the furnace showed a S fraction of about 14%, which is high compared to the fractions in the bottom and cyclone ashes. This relatively high S content will be further discussed below. Figure 5 shows the measured flows of bottom and cyclone ashes, in addition to the resulting flows of residual ashes. The flow of cyclone ash was slightly lower, and the flow of the bottom ash was slightly higher in the oxy-fuel cases, as opposed to the air-fired case because of the lower volume flow of gas in the oxy-fuel cases decreasing the efficiency of the cyclone. In addition, the flow of residual ash was higher in the oxy-fuel cases, owing to the lower cyclone efficiency and the higher ash content of the fuel. Figure 6 shows the corresponding S flows in the ashes. The range of S flows in the residual ashes is presented in Table 4 for both minimal and maximal S contents. The S flow in the ash was higher in the oxy-fuel cases than in the air-fired case because of a slightly higher average sulfur content in the ash and higher amount of bottom and residual ashes. The impact of the flow of residual ash on the sulfur balance was considerable because of the high S fractions in the residual ashes. Figure 7 gives the sulfur mass balance. The S flow input (by the fuel), set to 100%, was 96.9 g/h in the OF43w case and 99.5 g/h in the air-fired and OF35 cases. The fraction of fuel S found in the flue-gas (Figure 7) is equal to the conversion of fuel S to SO2 presented in Figure 4. The sulfur lost with the condenser water has only a small influence on the mass balance. The error bars belonging to the sulfur in ash indicate the variation of S in the residual ashes. The sum of the estimated outlet S flows was lower than the S flow input (by the fuel) in all
test cases, as seen in Figure 7. The difference is 6-9% for the air-fired case, 21-27% for the OF35 case, and 8-17% for the OF43w case. The S fraction differed in the bottom, cyclone, and residual ashes. It was, consequently, worth examining whether the fractions of Ca, Mg, Na, and K show a co-variation with the S content. For example, Fuertes et al.28 determined there to be a correlation between the contents of S and Ca in the ashes accrued under fluidized-bed combustion (FBC) conditions. In all test cases of this study, the content of different metals and metalloids was determined for the bottom and cyclone ashes, while that of the residual ash was represented by a sample taken from the fabric filter after all tests had been completed. Thus, this value can be regarded as a rough average of all test cases. The content of different metals and metalloids in the bottom ashes was quite similar in all cases, with no significant difference between oxy-fuel and air-fired conditions. The different cyclone ashes showed a similar elemental composition as well. The results of the OF43w case are presented to illustrate the findings. Figure 8a shows the main metals and metalloids in the bottom ash, and Figure 8b shows the main metals and metalloids in the cyclone ash, while Figure 8c illustrates the main metals and metalloids in a sample taken from the fabric filter. The mass fractions of Ca, Mg, Al, and Fe were always higher in the cyclone ashes in contrast to the bottom ashes. The bottom ashes showed higher mass fractions of S and Si than the cyclone ashes. A correlation between the S content and the contents of the other elements in the bottom and cyclone ashes could not be found. The Ca fraction in the filter ash was relatively high, considering that the S fraction was nearly 10% and that S is bound to CaSO4. The fractions of K and Na were considerably higher in the filter ash; however, they could theoretically only be combined with about 4% of the sulfur. The bottom ash analysis of the OF30 case by X-ray diffraction showed the presence of anhydrite (CaSO4), lime (CaO), sodium oxide (Na2O), potassium oxide (K2O), small amounts of periclase (MgO), and magnesioferrite (MgFe2O4). The cyclone ash from the OF30 case showed the same compounds, except magnesioferrite and potassium oxide, while in the samples taken from the fabric filter, CaSO4 and apatite [Ca5(PO4)3(OH,F,Cl)] were detected. CaO was not detected in the filter ash. To summarize, the results show that S was bound as CaSO4 in the ashes. It is unclear whether minor quantities of sulfur were also captured by Mg, Na, and K, (28) Fuertes, A. B.; Artos, V.; Pis, J. J.; Marban, G.; Palacios, J. M. Fuel 1992, 71, 507–511.
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Figure 9. Particle size of the (a) bottom ash from the OF43w case, (b) cyclone ash from the OF43w case, and (c) ash from the fabric filter (average sample from all test cases after completion of the tests).
formed. Fuertes et al.28 observed an increase of the sulfur content with a decrease in the particle size of ashes generated under FBC conditions; as a consequence, the sulfur content was high in the fly ash. This corresponds well to the results presented here; the lighter and smaller particles of the filter ash contained more sulfur than the larger and denser particles of the cyclone ash.
Table 5. Bulk Density of the Ashes and the Theoretical Specific Surface Area for Spherical and Non-porous Ash Particles
bulk density (measured) (kg/m3) average particle density (assumed) (kg/m3) specific surface area (calculated) (m2/kg)
bottom ash OF43w
cyclone ash OF43w
Discussion Three main questions will be discussed: (1) Why is sulfur missing in the mass balance? (2) Why does oxy-fuel combustion emit less SO2/MJ fuel input than air-fuel combustion? (3) How and where is sulfur captured by the ash in the process? The sum of the outlet flows of sulfur from water, ash, and gas was 6-27% lower than the fuel S flows (inlet flows). The differences in the S balance are considered acceptable because the balance is a result of several measurements and assumptions, some important aspects of which are discussed below. The gas analysis represents a conventional technique [validated with a Fourier transform infrared (FTIR) instrument without drying the flue-gas], and the flue-gas flow can be estimated with good accuracy. Therefore, the gaseous flow of sulfur was determined with satisfactory precision. The amount of sulfur caught in the condenser water was also determined with good accuracy but is minor and insignificant to the total sulfur balance. Therefore, the remaining sulfur should be present in the ash (deposits included). The flow of residual ash has a significant impact on the S balance because of the high S fraction of these ashes. The flow of residual ash was calculated by the ash content of the fuel as measured in the laboratory. However, this ash content may differ from the amount of ash generated in the combustion unit, mainly because of a different combustion temperature and atmosphere. For example, the sulfur content in the ash produced in the laboratory differs from that in the ashes from the combustion unit. A change in the S content of the ash causes a change in the amount of ash itself.30 Furthermore, the ash content from the fuel analysis was different in each case. If, instead, an average ash fraction was used for the air, OF35, and OF43w cases, the difference in the S balance would decrease somewhat in the air-fired case and increase in the OF43w case, whereas it would be the same in the OF35 case. A further
because the measurement method could only detect crystalline phases. The particle size distribution of the ashes was determined to investigate if a correlation exists between the sulfur concentration in the ashes and the particle size or rather the specific surface area of the ash particles. Figure 9 shows the particle size distribution for bottom ash (panel a) and cyclone ash (panel b), both sampled under OF43w conditions. Figure 9c shows the particle size distribution of an average sample of filter ash taken after completion of all test cases. The bottom and cyclone ashes from the other cases were quite similar and are, consequently, not further discussed. The average particle diameter [d(0.5)] of the bottom ash (16.7 μm) was slightly larger than the average particle diameter of the cyclone ash (14.2 μm), and the average particle diameter of the filter ash was small compared to that of the other ashes (4.1 μm). The size distribution of the particles in the bottom ash was wider than that of the cyclone ash. Table 5 gives the bulk density of the ashes. The filter ash had the lowest bulk density, and the cyclone ash had the highest bulk density because of the mode of operation of a cyclone. As an illustration, the specific surface area is calculated, assuming that the particles are spherical without any porous structure and with an average density of 1000 kg/m3. The calculated specific surface area of the bottom ash, shown in Table 5, is slightly larger than that of the cyclone ash. The filter ash has the highest specific surface area and also the highest S content. In their study of Kolubara lignite, Grubor and Manovic29 found that ash, formed from coal particles of a lesser density, contained more sulfur than ash formed from coal particles with a higher density because calcium was more active in coal particles of a lesser density. Organically bound Ca in the fuel is most reactive during combustion, and small CaO particles are
(30) Rees, O. W.; Shimp, N. F.; Beeler, C. W.; Kuhn, J. K.; Helfinstine, R. J. Circ.-Ill. State Geol. Surv. 1966, 396, 1–10.
(29) Grubor, B.; Manovic, V. Energy Fuels 2002, 16, 951–955.
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source of error was the determination of sulfur in fuel and ash, which implied relatively high uncertainty. During the initial hours of operation, the flue-gas is cooled somewhat more, because of the heat transfer to the cold combustor walls. This is expected to reduce the efficiency of the cyclone, and initially, more ash is transported to the bag filter, while the temperature in the combustion zone will only be slightly affected. Despite the uncertainty of the measurements, the conclusion is clear: the ash captures more sulfur in oxy-fuel combustion than in air-fuel combustion, although the increase of S contained in the ash does not perfectly match the reduced content of S in the flue-gas, especially not in the OF35 case. The main reason for the lower SO2 emission (mg/MJ fuel input) or lower conversion of fuel S to SO2 (Figure 4) in oxyfuel combustion compared to air-fuel combustion is the higher SO2 concentration, which favors the formation of sulfates.8-10 Liu et al.,10 for example, noted that formed CaSO4 decomposes less at a higher SO2 concentration. The same should be valid for the sulfates in the fuel. This may also explain the higher SO2 emissions in the wet oxy-fuel case, where the SO2 concentration was lower than in the dry oxyfuel cases (Figure 4). The formation of sulfates in the oxy-fuel cases was probably also improved by the longer residence time of the gases in the furnace compared to the air-fired case. Another reason for the lower SO2 emission in oxy-fuel combustion is the possibility of improved sulfation of CaO in a CO2 atmosphere.17 Nevertheless, all bottom ashes showed similar contents of sulfur, with differences possibly caused by the uncertainty of the measurements. A plausible reason is that there is a limit to sulfur capture, which was already reached for all bottom ashes because of the lengthy exposure time of the bottom ashes in the flue-gas at high temperatures. Another possibility of lower conversion of fuel S to SO2 in oxy-fuel combustion would occur at an unsatisfactory conversion of the coal. However, the combustion temperature in the oxy-fuel cases was higher than in the air-fired case, and the amount of unburned fuel in the ashes was lower. For example, in the bottom ash of the OF35 case, the unburned amount was 0.1% of ash mass, as compared to 0.4% in the airfired case. It is possible that the conversion to SO2 increases in oxy-fuel cases with even higher combustion temperatures because of the decomposition of sulfates and the sintering of CaO. For example, the OF35 case already showed a slightly higher SO2 emission than the OF30 case. The last question about how and where sulfur is captured by the ashes during the process is the most difficult one to answer and requires additional work. Sulfur is released as gaseous compounds during combustion, mainly in the form of SO2. Simultaneously, SO2 is captured during char combustion by alkaline earth and alkali metals. Also, some fuel S may remain in the ash without being released, for example, sulfur already available as sulfate in the fuel. SO2 can react in the furnace with CaO to form CaSO4. Both compounds were detected by X-ray diffraction analysis of the ashes (except for the filter ash, where no CaO was detected). Generally, smaller particles with a larger surface area per unit mass capture more SO2 than larger particles, where the blockage of pores during CaSO4 formation tends to inactivate more material than in smaller particles. In addition, the temperature in pulverized coal combustion is high enough to make sintering possible, something which may destroy the pore structure of the CaO formed. Such destruction of the pore structure also affects the larger particles more than the smaller particles. A possible reason for the higher sulfur content in the bottom ash than in
the cyclone ash was that the bottom ash had a slightly larger surface area (Table 5). Another important reason might be that the bottom ash was exposed to the flue-gas during the entire experiment, whereas the cyclone ash was separated from the flue-gas. The bottom ash in the furnace remained at a temperature suitable to capture SO2. The ash samples taken from the reactor wall showed high sulfur content (14% in mass), which indicates that sulfur self-retention by ash mainly takes place inside the furnace. The significantly higher S content of the ash collected from the reactor wall compared to the bottom ash can be explained by the smaller particle size and lengthy exposure time (the furnace walls were not cleaned between the test runs). A high CO2 concentration can cause carbonation of CaO; CaCO3 is formed at temperatures lower than 900 °C.15 White deposits were found on the walls of the flue-gas cooler (inlet temperature of 500 °C) after the oxy-fuel experiments, whereas no white deposits were found after the air experiments. The white deposits were probably a mixture of CaCO3 and CaSO4. The SO2 concentration was measured up- and downstream of the cyclone and downstream of the cooler and fabric filter, respectively (see also Figure 1) to examine if the ashes captured SO2 even after the combustion was completed. In this test, a water-cooled probe with a ceramic filter was used to protect the gas analysis instruments from particles. No significant changes in the SO2 concentrations were measured before and after each component. One might argue that the ash layer captured on the ceramic filter could influence sampling, compensating for the processes occurring inside the cyclone, flue-gas cooler, and fabric filter. However, it is not plausible that the fraction of sulfur captured in the filter cake should always be the same as in the cyclone, flue-gas cooler, and fabric filter. Consequently, it can be expected that SO2 reacted with alkaline earth and alkali metals mainly inside the furnace. Conclusions The fate of sulfur during air-fuel and oxy-fuel combustion of pulverized coal was investigated by experiments in a 100 kWth test unit fired with lignite. The conversion of fuel S to SO2 was lower in oxy-fuel combustion than in air-fuel combustion, which led to about 35% lower SO2 emission under oxy-fuel conditions for the fuel investigated. About 67% of the fuel S was converted to SO2 under air-fired conditions, while only about 43% was converted during oxy-fuel combustion. On the other hand, the SO2 concentration was more than 3 times higher during oxy-fuel combustion because of the lower flue-gas flow compared to air firing. The higher SO2 concentration in oxy-fuel combustion was accompanied by higher sulfur self-retention by ash. The lignite used showed high selfretention, also a consequence of the low coal rank. Although the S content of the ashes increased during oxy-fuel conditions, the low conversion of fuel S to SO2 in oxy-fuel firing was not reflected by the same amount of sulfur in the ashes, because the mass balances were not entirely closed. Possible reasons for the discrepancies were uncertainties in the determination of the content of sulfur in the fuel and ashes, the amount of total ash, and the startup time of each experiment. Nevertheless, the experiments showed significantly higher sulfur self-retention by ash in oxy-fuel than in air-fired conditions. This study only considers one fuel, a lignite, and is not extensive enough to predict the behavior of other coal types, for example, anthracite coal. However, it can be expected that fuels with high sulfur self-retention under air-fired conditions 654
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m_ S,gas = sulfur mass flow in the flue-gas (g/h) m_ S,water = sulfur mass flow in the condenser water (g/h) n_ cgCO2 = molar flow of the carrier gas CO2 (mol/h) RS = sulfur self-retention by ash XCO2 = molar fraction of carbon dioxide in the flue-gas Xleak = estimated air leakage molar fraction in the flue-gas XO2 = molar fraction of oxygen in the flue-gas XSO2 = molar fraction of sulfur dioxide in the flue-gas XSO2,max = molar fraction of sulfur dioxide in the flue-gas if all fuel S were converted to SO2 Yash = ash mass fraction after combustion of the fuel (as received), laboratory conditions YC = mass fraction of carbon in the fuel (daf) Ycombustibles = mass fraction of combustibles in the fuel (as received) YH = mass fraction of hydrogen in the fuel (daf) Ymoisture = mass fraction of water in the fuel (as received) YN = mass fraction of nitrogen in the fuel (daf) YO = mass fraction of oxygen in the fuel (daf) YS = mass fraction of sulfur in the fuel (daf) YS,bottom ash = mass fraction of sulfur in the bottom ash YS,cyclone ash = mass fraction of sulfur in the cyclone ash YS,cooler ash = mass fraction of sulfur in the cooler ash YS,filter ash = mass fraction of sulfur in the filter ash YS,residual ash = mass fraction of sulfur in the residual ash
show similar or improved sulfur self-retention during oxy-fuel combustion for both wet and dry recycling. It can be concluded that the reduced SO2 emission during oxy-fuel combustion is, indeed, a benefit, although the correspondingly high SO2 concentration imposes an increased risk for highand low-temperature corrosion, induced by sulfur compounds. It is, therefore, important to consider where and how flue-gas desulfurization takes place in the process. Acknowledgment. The financial support by Vattenfall AB is gratefully acknowledged.
Nomenclature CSO2 = conversion of fuel S to sulfur dioxide (ratio of SO2/ fuel S) d(0.5) = mass mean particle diameter (μm) Mi = molar mass of species i (kg/mol) m_ ash = total ash mass flow (g/h) m_ bottom ash = bottom ash mass flow (g/h) m_ cyclone ash = cyclone ash mass flow (g/h) m_ fuel = fuel mass flow (kg/h) m_ residual ash = residual ash mass flow (g/h) m_ water = condenser water mass flow (g/h) m_ S,ash = total sulfur mass flow in the ash (g/h) m_ S,fuel = sulfur mass flow in the fuel (g/h)