Utilization of Calcium Acetate and Calcium Magnesium Acetate for

440

Energy & Fuels 1999, 13, 440-448

Utilization of Calcium Acetate and Calcium Magnesium Acetate for H2S Removal in Coal Gas Cleaning at High Temperatures J. Adańez,* F. Garcıá-Labiano, L. F. de Diego, and V. Fierro Instituto de Carboquı´mica (CSIC), P.O. Box 589, 50080 Zaragoza, Spain Received June 18, 1998

The effectiveness of calcium acetate (CA) and calcium magnesium acetate (CMA) as feasible H2S capture agents was tested in an entrained flow reactor at temperatures up to 1100 °C. Both sorbents have the ability to form highly cenospheric oxide particles on heating to high temperatures. The thin, porous walls of the cenospheres make these sorbents a very attractive material to sulfur removal due to their high utilization. Sulfur capture up to 90%, near the thermodynamic equilibrium, was achieved with dry powder injection of CA and CMA at furnace gas temperatures of about 1000 and 800 °C, respectively, a Ca/S molar ratio of 2, and a residence time of 0.8 s. Although more expensive, the sulfur capture effectiveness of these sorbents was superior to any natural calcium-based sorbent. Moreover, because of the high porosities exhibited by CA and CMA, the sulfided sorbents can be easily stabilized and converted into an inert material before disposal. These qualities make them very attractive materials to remove H2S at high temperatures in the reducing atmospheres typical of the integrated gasification combined cycle (IGCC) processes.

Introduction Among the advanced power generation options for coal utilization, the integrated gasification combined cycle (IGCC) systems offer increased efficiency and the greatest ability to meet the stringent environmental emission limits.1 This technology features complete conversion of coal to fuel gas followed by gas cleaning and a combined cycle gas turbine/steam turbine system for power generation. Among the gas cleaning steps of this process, desulfurization is one of the most important relating to use of the gas turbine. It must be taken into account that the sulfur contained in coal reacts in the reducing atmosphere of a gasifier, generating H2S and COS. Since a majority of the sulfur compounds are removed from the coal gas before it is further fired, undesirable gaseous emissions in the final flue gas can be quite low. Desulfurization at temperatures above 700 °C would make a substantial contribution in improving the thermal efficiency of electric power generation in IGCC systems.1,2 Different natural calcium-based sorbents have been used in the past as potential desulfurization sorbents at high temperatures and coal gasification conditions.3 Limestones, dolomites, and calcium hydroxides are the most common calcium-based sorbents used in these applications because they have the advantage of being cheap, abundantly available, and commonly used as bulk chemicals. Sulfur removal around 70% was (1) Takematsu, T. Coal Gasification for IGCC Power Generation. IEA Coal Res. 1991, 37, 15-24. (2) Thambimutu, K. V. Gas Cleaning for advanced coal-based power generation. IEA Coal Res. 1993, 53, 97-113. (3) Adańez, J.; Garcıá-Labiano, F.; de Diego, L. F.; Fierro, V. Energy Fuels 1998, 12, 726-733.

obtained with some of these sorbents at a calcium to sulfur molar ratio of 3. However, if the gases obtained in the IGCC process have to be used in a gas turbine, sulfur removal near 100% would be desirable. Moreover, these sorbents are not easy to stabilize by typical oxidation methods due to the difficulty of reacting the inner part of the sorbents with the oxygen to form stable CaSO4.4,5 There are other calcium-based sorbents that are more expensive than the natural ones but with excellent results in sulfur removal at oxidizing conditions.6-8 These are calcium acetate (CA) and the calcium magnesium acetate (CMA). CA and CMA can be prepared by reacting acetic acid with lime or dolomite lime, respectively. Because acetic acid is currently produced from petroleum and natural gas, the cost of CA and CMA is still high. However, several studies have been carried out to obtain acetate derived from renewable organic wastes or biomass so that the cost of these sorbents can be drastically reduced.9 CA and CMA decompose around 380-400 °C and produce acetone and carbon dioxide, leaving behind (4) Abbasian, J.; Rehmat, A.; Banerjee, D. D. Ind. Eng. Chem. Res. 1991, 30, 1990-1994. (5) Torres-Ordon˜ez, R. J.; Longwell, J. P.; Sarofim, A. F. Energy Fuels 1989, 3, 506-515. (6) Levendis, Y. A.; Zhu, W.; Wise, D. L.; Simons, G. A. AIChE J. 1993, 39, 761-773. (7) Steciak, J.; Levendis, Y. A.; Wise, D. L. AIChE J. 1995, 41, 712722. (8) Shuckerow, J. I.; Steciak, J. A.; Wise, D. L.; Levendis, Y. A.; Simons, G. A.; Gresser, J. D.; Gutoff, E. B.; Livengood, C. D. Resour. Conserv. Recycl. 1996, 16, 15-69. (9) In Calcium Magnesium Acetate. An Emerging Bulk Chemical for Environmental Applications; Wise, D. L., Levendis, Y. A., Metghalchi, M., Eds.; Elsevier: New York, 1991; Vol. 2, Chapters 15-18, pp 319504.

10.1021/ef9801367 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/08/1999

Utilization of Ca(CH3COO)2 and CaMg(CH3COO)6

Energy & Fuels, Vol. 13, No. 2, 1999 441

CaCO3 or CaCO3 and MgO, respectively.

Ca(CH3COO)2 f CaCO3 + CH3COCH3

(1)

CaMg2(CH3COO)6 f CaCO3 + 2MgO + 3CH3COCH3 + 2CO2 (2) The acetone further decomposes at higher temperatures to allene, CH2dCdCH2, or into hydrocarbon radicals. The residual CaCO3 starts to decompose around 700 °C to CaO and CO2, although the temperature and calcination rate depends on the CO2 partial pressure in the gas phase. The sulfidation takes place later by reaction of the CaO with H2S at reducing conditions existing in the gasifier.

CaO + H2S f CaS + H2O

(3)

The magnesium oxide does not react with H2S to any significant degree because the reaction is not thermodynamically favored under gasification conditions and acts inert. However, the MgO formation by MgCO3 decomposition produces a higher porosity in the sorbent and improves the intraparticle diffusion of the gaseous reactants. At temperatures below that of the decomposition of CaCO3 to CaO at the operating pressure, the direct sulfidation of CaCO3 is still possible, although the reaction rate is slower than the reaction with CaO.10

CaCO3 + H2S f CaS + CO2 + H2O

(4)

CA and CMA have the unique property to calcine, forming highly cenospheric particles, with thin and porous walls, resembling “popcorn”-like structures. The high porosities exhibited by these particles greatly reduce the importance of the limiting resistance in gassolid reactions and increase the sorbent utilization within a typical furnace conditions at high temperatures and low residence times. Furthermore, the product CaS has to be converted into inert CaSO4 before disposal because H2S is released from the reaction of CaS and water. The conversion of CaS to CaSO4 is expressed by the following reaction:

CaS + 2O2 f CaSO4

(5)

The stabilization of the CaS product could be prevented by pore plugging due to the higher molar volume of the CaSO4 (52.2 cm3 mol-1) with respect to the molar volume of CaS (28.9 cm3 mol-1). The high porosity exhibited by these sorbents can help the stabilization with air or oxygen of the sulfided sorbents proceeding from CA and CMA. Values as high as 77% have been reported by Levendis et al.6 and Steciak et al.11 for the porosity in the particle shell, excluding the central void of the cenospheres. As a consequence, as the cost of CA production is reduced by using renewable organic wastes or biomass as starting material and the cost of the waste generation increases, the potential of CA and CMA becomes economically more favorable. Injection of sorbents can be allocated in the postgasification zone. Selection of the optimum place for injection is dictated by kinetic and thermodynamic (10) Yrjas, P.; Iisa, K.; Hupa, M. Fuel 1996, 75, 89-95. (11) Steciak, J.; Levendis, Y. A.; Wise, D. L.; Simons, G. A. J. Environ. Eng. 1995, 595-604.

reasons. Temperature, residence time, and H2S concentration influence the effectiveness of the sorbent. Moreover, it is not possible to reach H2S concentrations below that corresponding to the thermodynamic equilibrium at the operating conditions. Although there are some works reported in the literature on the effectiveness of calcium acetate (CA) and calcium magnesium acetate (CMA) to reduce SO2,6 NOx,12 dual SO2-NOx,7,11 and even other vapor emissions after coal combustion, little is known about the effectiveness for H2S removal at reducing conditions. In this work, the effect of the main variables affecting H2S removal was studied in an drop-tube reactor at temperatures from 700 to 1100 °C, using both CA and CMA. This will allow us to identify optimum temperatures and residence times for the sorbent injection. In addition, to better understand the sulfidation process of these sorbents, the evolution of the pore-size distribution with the progress of the reaction was studied by analyzing the pore structure of partially reacted samples by N2 physisorption, scanning electronic microscopy (SEM), and energy-dispersive X-ray (EDX) techniques. Experimental Section The testing of the effectiveness of CMA and CA for H2S removal was conducted in a high-temperature drop-tube reactor, as shown in Figure 1. The desired reactant gas mixture was prepared by blending N2, CO2, CO, and H2, which were measured and controlled by specific mass flow controllers. The H2O content was obtained by complete evaporation of a constant flow rate of water supplied by a peristaltic pump. The H2S, measured and controlled by its specific mass flow controller, was added in a zone near the gas inlet of the reactor. A gas composition of 3.5% CO2, 5% CO, 5% H2, 0-1.5% H2S, 2-8% H2O, and N2 for the balance was used for the experimental testing. This composition is different from those obtained in the entrained flow gasifiers with very high CO (60%) and H2 (30%) contents, although it maintains the main characteristics of the process, the CO2 content, that affects both the sorbent calcination and sintering rates. The H2O content was taken as those corresponding to the water-gas shift reaction equilibrium at the reaction temperature, with values between 2.3% at 700 °C and 7.4% at 1100 °C. Personal detectors of CO and H2S were used during experimentation to alert the workers for possible leaks in the system. The sorbent powders were introduced at the top of the furnace using a volumetric syringe-type feeder with an entraining system for the solids. The synthetic gas and the sorbent were introduced at the top of the reactor and passed downward through a 4 cm i.d. × 200 cm Kanthal APM reactor tube. Two thermocouples located 40 and 160 cm from the top were used for measuring the temperature in the reaction zone. The gas flow rate varied to control the residence time from 0.5 to 1.2 s, at temperatures from 700 to 1100 °C. After reaction, the exhaust gas and the reacted solids leave the drop-tube reactor at the bottom. A sample of the exhaust gas was continuously collected and analyzed in a Varian 3400CX GC equipped with a thermal conductivity detector (TCD) and a sulfur-specific flame photometric detector (FPD) in series. Before the sorbent injection, the gas composition was analyzed using TCD and FPD detectors to determine CO2, CO, H2, N2, and H2S concentrations. During the experiment, gas samples were injected automatically every 1.5 min and only FPD detector outputs were considered, resulting in a nearcontinuous monitoring of H2S concentration. (12) Steciak, J.; Zhu, W.; Levendis, Y. A.; Wise, D. L. Comb. Sci. Technol. 1994, 102, 193-211.

442 Energy & Fuels, Vol. 13, No. 2, 1999

Ada´ nez et al.

Figure 2. Pore area distribution of CA and CMA samples.

Figure 1. Experimental setup. Sulfur removal, SR, was determined through the variation of the H2S concentrations obtained during the experiment as

SR )

Cin - Cout × 100 Cin

(6)

where Cin was the concentration at the inlet and Cout the concentration at the outlet during the experimental work. The sorbents were collected in a high-efficiency cyclone electrically heated at 250 °C to avoid hydration of the sulfured lime. Finally, the exhaust gas was mixed with natural gas to improve its combustion characteristics, and then it was burnt in a flare. Further details of the experimental system, in particular the gas analysis method and experimental procedure, are described elsewhere.3 Sorbent Characterization. The reacted sorbents were analyzed by different methods to determine their physicochemical characteristics. The surface area of the particles was measured by N2 physisorption at 77 K in a Micromeritics ASAP-2000. Pore-size distributions in a range from 2 to 200 nm in diameter were also obtained using the BJH data inversion algorithm. Moreover, to better understand the effect of the porous structure on the sulfidation reaction, samples were dispersed on a double-sided sticky graphite tab and carbon-coated using a Fisons CA508 Carbon Evaporator for SEM observation and EDX analysis. A Zeiss DSM 942 SEM apparatus equipped with secondary and backscattered electron detectors and a Bewindow EDX detector (Oxford Link-Isis) for qualitative analy-

sis from carbon on and for semiquantitative analysis from sodium on were used. Materials. Two different sorbents were used herein for comparative purposes: CA (Ca(CH3COO)2) and CMA (CaMg2(CH3COO)6). Reagent-grade CA was purchased from Panreac, and CMA of 95% purity was obtained from Cryotech Deicing Technology. The materials were sieved, and CA and CMA powder in size ranges from 100 to 200 µm was used to test the effectiveness of both sorbents as H2S capture agents at high temperatures. When CA or CMA are injected into a zone of high temperature, highly cenospheric particles are formed. The physical structure of both sorbents exhibited notable differences. Figure 2 shows the pore area distribution of calcined CMA and CA sorbents obtained by N2 physisorption at 77 K and using the BJH data. Two distinct ranges of pore size can be appreciated: one between 2 and 20 nm and the second between 20 and 200 nm. CA has a large concentration of mesopores around 10 nm and also exhibited a minor pore area of macropores. In contrast, CMA exhibited a lower amount of pores under 20 nm and more pores above 20 nm. The surface area is estimated to be that of the particle shell, excluding the central void of the cenospheres, whose contribution in area is negligible. To detect whether the two pore-size ranges corresponded to different decomposition processes, a calcination test was conducted in CO2 atmosphere to avoid calcination of the CaCO3 formed from CA and CMA. As can be appreciated in Figure 2, pores below 20 nm seem to be due to the decomposition of CaCO3 for both sorbents and those above 20 nm correspond to the decomposition of the organic matter and to the MgCO3 calcination in CMA, which were created simultaneously in the formation of the cenospheres. The greater amount of organic acetate in CMA compared with CA could be responsible for the higher amount of mesopores above 20 nm and macropores in the first sorbent, although not all the surface will be active for reaction, because MgO does not react with H2S.

Results and Discussion The sulfidation of small particles at high temperatures is a complex process involving multiple mechanisms and changes inside the sorbent particles. The main variables affecting the sorbent sulfidation were studied through the knowledge of the sulfur removal and the evolution of the sorbent pore structure under different conditions. A temperature of 1000 °C, 5000 vppm of H2S, a Ca/S molar ratio of 1, and a residence time of 0.8 s were used as basic operating conditions. Effect of the Ca/S Molar Ratio. It is well-known that the Ca/S molar ratio is a basic parameter in the process that determines the final sulfur removal in the reactor. Moreover, it affects the operating cost of the whole process and the amount of residue generated,



Figure 3. Effect of Ca/S molar ratio on sulfur removal (1000 °C, 0.8 s, 5000 vppm of H2S).

which have to be stabilized before disposal. It must be taken into account that for the same Ca/S molar ratio, the amount of solid introduced in CMA is about 3 times that required by CA, due to the presence of the inert MgO. Figure 3 shows sulfur removal, SR, obtained in the reactor as a function of the Ca/S molar ratio for the two sorbents studied at 1000 °C and a 0.8 s residence time. The experiments at a Ca/S ratio of 1 were repeated 3 times, resulting in a (4% scatter in the data. It is obvious that an increase in the Ca/S molar ratio produced an increase in the sulfur removal. It can be seen that the most effective sorbent at this temperature was CA, reaching efficiencies as high as 90% at a Ca/S molar ratio of 2, near the thermodynamic equilibrium conditions (H2Oeq ) 5.9 vol %, H2Seq ) 188 vppm, SReq ) 96.2%). The effectiveness of CMA was found to be lower at these conditions, reaching a sulfur removal of 59% at that Ca/S molar ratio. It must be kept in mind that it is not possible to exceed the H2S concentration corresponding to the thermodynamic equilibrium. The equilibrium constant, K, and its temperature dependence for the sulfidation reaction are described by the following equations2

K ) [H2O]/[H2S] log10 K )

3519.2 - 0.268 T

(7) (8)

where [H2O] and [H2S] are the gas-phase molar fractions and T is the temperature in K. For example, for a water content of 3.5% typical of dry-feeding entrained bed gasifiers, the H2S concentration in equilibrium varies between 34 vppm at 800 °C and 111 vppm at 1000 °C. These values accomplish the requirements of gas turbines used in IGCC processes, which accept H2S concentrations of about 100-150 vppm of H2S.13 Sulfur removal to reach H2S concentrations near to those of the thermodynamic equilibrium would be expected during CA injection at the outlet of the entrained bed gasifiers at 1000 °C with a Ca/S molar ratio of 2 or even lower. The higher degree of turbulence in actual reactors is expected to enhance H2S removal beyond that reported herein, where the experiments were conducted in laminar flow.12 (13) Ben-Slimane, R.; Hepworth, M. T. Energy Fuels 1995, 9, 372378.

Different limestones, dolomites, and calcium hydroxides have been used in the past as desulfurization agents at reducing conditions and high temperatures in drop-tube reactors.3 Sulfur retentions up to 70% with calcium to sulfur molar ratios of 3 were obtained in the best operating conditions, and further desulfurization is necessary if the gases are going to be used in a gas turbine. In contrast, the sulfur removal obtained with CA and CMA highly exceed those reached with any other calcium-based sorbent, and it would be possible to attain the requirements of gas turbines without any further cleaning. To understand these good results, an analysis of the sorbent particles by SEM photomicroscopy was carried out. When CA or CMA are injected into a zone of high temperature, highly cenospheric particles are formed. Figure 4 shows different aspects of cenospheres obtained from CA and CMA injection. The global geometry and particle size can be appreciated in Figure 4a. Samples were homogeneous, and the particles were quite fragile, especially those obtained from CA. The particle sizes of the cenospheres were measured through the use of these SEM photographs with a known magnification. Despite the use of the same particle size in the raw sorbent injected into the reactor, +100-200 µm, important differences in size were noted between CA and CMA residues. This observation is in fair agreement with the results reported by Steciak et al.,12 who found different sizes in the residues depending on the chemical composition of the sorbent. The mean volume of cenospheres, assuming they are spherical particles, from CMA was smaller than that obtained from CA and slightly higher (∼168 µm) than the unreacted CMA particles. In contrast, CA tended to form residues much larger than the raw sorbent, with a mean size in volume of 480 µm, although particles up to 600 µm were found. According to Steciak et al.,12 this increase in size may be due to the fusion of several particles during volatile evolution and/or calcination. Pores and blowholes created during devolatilization were evident in the cenospheres from CA, although they were not detected in the CMA, as can be appreciated in the SEM photograph of Figure 4a and in more detail in the single particles of Figure 4b. The pores and blowholes may enhance the utilization of the calcium oxide formed during devolatization by facilitating H2S diffusion inside of the sorbent and could be responsible for the higher utilization of the CA with respect to the CMA. To better understand the internal structure of the cenospheres, some particles were cracked, and the result is shown in Figure 4c for CA and CMA. The high water solubility and, especially, the extreme fragility of CA residues made the use of conventional polishing techniques for the observation of particle cross-sections impossible. Thus, small amounts of samples were put on top of a resin sample holder, embedded in a drop of epoxy resin, left overnight, and then fractured in order to obtain a fresh surface in which well-preserved particle cross-sections could be observed. A cut previously made in the sample holder in a direction crossing the hardened resin drop (but not reaching it) ensured that the fracture affected the region containing the sample, so that a statistically significant number of cross-sections could be obtained. The backscattered electron signal


Ada´ nez et al.

Figure 4. (a) SEM photographs of CA and CMA particles after sulfidation at 1000 °C, 0.8 s, and 5000 vppm of H2S, (b) high magnification SEM of a particle, and (c) cracked-open particle to reveal interior voids and spongelike structure.

provided sufficient phase contrast between sample and resin for the recognition and image processing of particle cross-sections. The structure so obtained for the CA sample is a typical example of cenospheric particles with a spongelike wall porosity. As can be seen in Figure 4c, the wall thickness is quite homogeneous, with values from 2 to 5 µm. Because the heterogeneous reaction of CaO with H2S depends on the available surface area of the solid, the high porosity of the external walls allow a high utilization of the sorbent. To better understand the relation between the sulfidation process and pore structure of the cenospheres,

chemical characterization of the samples by EDX was performed. A sample of CA sulfided at 1000 °C, 0.8 s, and 5000 vppm of H2S was partially crushed and examined for the presence of Ca and S in the external and internal walls of the particle. The results of the EDX shown in Figure 5 point to the conclusion that sulfidation is higher in the external walls than in the internal ones. This idea was also confirmed by SEM. Figure 6 shows the SEM photograph of an external and an internal wall of the previous sample. It can be seen how the surface of the external wall (Figure 6a) is completely covered by “bumps”. According to Attar and Dupuis14


Figure 5. XRD analysis of the external (s) and internal (‚‚‚) walls of sulfided CA.

Figure 6. SEM microphotography of the (a) external and (b) internal walls of sulfided CA.

and Fenouil and Lynn,15 the bumps are believe to be small crystals of CaS formed at high temperatures. This structure was not found in the internal parts of the cenospheres, as can be seen in Figure 6b, which leads to the conclusion that the sulfidation here is lower than in the most external part of the sorbent. Similar results, even with sharper differences in sulfidation between the external and internal walls, were found for the sulfided samples from CMA due to the more compact aspect of these cenospheres without large blowholes. (14) Attar, A.; Dupuis, F. Ind. Eng. Chem. Process Des. Dev. 1979, 18, 607-618. (15) Fenouil, L. A.; Lynn, S. Ind. Eng. Chem. Res. 1995, 34, 23242333.


Figure 7. Thermogravimetric analysis of CA and CMA samples at 850 °C: calcination in nitrogen atmosphere, sulfidation in reducing atmosphere with 10000 vppm of H2S, and oxidation with air.

The lower sulfidation of the internal walls, even in the case of the CA with large a pore mouth and blowholes, is due to several reasons. On one hand, calcination of CaCO3 has a large effect on the global sulfidation process of the cenospheres because although the reaction between H2S and CaCO3 is thermodynamically possible, the reaction rate is much lower than that with the CaO.10 Thermogravimetric analysis of sulfided CA and CMA samples indicated the presence of CaCO3 at typical operating conditions. The amount of CaCO3 decreased with increasing residence time and temperature, although values up to 30% were found after reaction at 1100 °C and a 1.2 s residence time. Similar results were found by Steciak et al.11 in their study of SO2 removal by sorbent injection at high temperatures, who reported values between 30% and 45% after reaction at 1150 °C. The lower calcination conversions found in the samples from CMA could explain the lower utilization of this sorbent during the injection in the drop tube. On the other hand, due to the high reactivity of the CaO, most of the H2S reacts with the external surface of the particles. This fact prevents the existence of H2S in the internal parts of the cenospheres during the short residence times of the sorbents used herein. As a consequence, a lower reaction rate and, thus, a lower sulfidation conversion is obtained in the internal parts of the particles. Another important aspect related to the use of sorbents for H2S removal is the stabilization before disposal of the solid residue generated. The typical stabilization method consists of converting the unstable CaS into an inert CaSO4 residue. However, the stabilization of the CaS product could be prevented by the pore plugging due to the higher molar volume of the CaSO4 (52.2 cm3 mol-1) with respect to the molar volume of the CaS (28.9 cm3 mol-1), as normally occurring for the typical calciumbased sorbents used for H2S removal at high temperatures. To test the ability of these sorbents for stabilization, although the conditions are different from the existing ones during sorbent injection, some tests were carried out in a thermobalance. Figure 7 shows the thermograms corresponding to the processes of calcination, sulfidation, and oxidation of both CA and CMA. Cenospheres previously obtained in the drop-tube reactor in CO2 atmosphere were totally calcined in the thermobalance at 850 °C. Thermal decomposition of CaCO3 to give CO2 and CaO is evident in the figure.


Ada´ nez et al.

Figure 8. Effect of temperature on sulfur removal (Ca/S ) 1, 0.8 s, 5000 vppm of H2S).

Figure 9. Effect of temperature on BET specific surface area of the sorbents (Ca/S ) 1, 0.8 s, 5000 vppm of H2S).

The differences in weight-loss percentages were due to the presence of inert MgO in the CMA sample. The CaO was later sulfided at a constant temperature of 850 °C in a reducing atmosphere similar to that used in the drop-tube experiments. A total sulfidation conversion of the samples was obtained with both sorbents. The change from a reducing to an oxidizing atmosphere produced a very quick oxidation of the sample, reaching complete conversion from CaS to CaSO4 both in the case of CA and CMA. The high porosity exhibited by these sorbents allows an easy and quick stabilization of the residues, and thus an inert material can be obtained at the end of the process. Effect of Temperature. The increase in the inlet temperature of the fuel gas to the turbine has important economic consequences. Overall cycle efficiency improvements of 2-3% have been reported changing from cold to hot cleaning techniques.1,2 The sorbent injection location needs to be chosen at a temperature where the CaS would be stable and as high as possible to obtain high thermal efficiencies. In this work, temperatures between 700 and 1100 °C were used. Figure 8 shows sulfur removal obtained for the different sorbents working with different reactor temperatures and 5000 vppm of H2S, a Ca/S molar ratio of 1, and a residence time of 0.8 s. H2S removal reached a maximum of about 55% at 1000 °C for CA and decreased slightly at higher temperatures. In contrast, this maximum is displaced toward lower temperatures for CMA, reaching similar values to CA of about 54% at 800 °C. A maximum in sulfur removal was also found at this temperature by Steciak et al.7 in their experiments of dual SO2-NOx removal by CMA injection in coal boilers. The CA and CMA utilization obtained means that sulfur removal near the thermodynamic equilibrium can be reached with calcium to sulfur molar ratios about 2, as experimentally verified. From a practical point of view, the suitability to use CA or CMA would depend on the tolerance of the gas turbine to be used. If the gas turbine accepts higher concentrations of H2S, the use of CA (maximum SR at 1000 °C) would be desirable because the higher temperature of the gas feeding to the turbine allows a higher efficiency of the process. If the tolerance of the turbine is more restrictive, the use of CMA (maximum SR at 800 °C) would be desirable because the H2S concentration corresponding to the thermodynamic equilibrium is lower, as it is obvious from eqs 7 and 8. However, in this case there is a loss

of efficiency due to the lower temperature used during gas cleaning. It must be pointed out that temperature affects the calcining process, thus the pore structure and the surface area of the sorbents exposed to the H2S are different at each temperature. Moreover, temperature also affects the sulfidation reaction rate. The final SR obtained by the sorbent will depend on a combination of these two mechanisms. The sulfidation reaction of cenospheres derived from CA and CMA seems to be strongly affected by the calcination process. TGA analysis of partially sulfided samples indicated the presence of CaCO3 and CaS and small amounts of CaO. Because of the high surface area and porosity developed during acetate pyrolysis and CaCO3 calcination, the CaO formed is very reactive. The CaO sites, accessible by pores and blowholes, react almost instantaneously with the bulk H2S. At temperatures above 1000 °C, the effects of sintering decrease the CaO reactivity. This conclusion is supported by measured BET surface areas, which are shown in Figure 9. It can be observed how SBET increased with temperature and exhibited a peak around 1000 °C for CA and around 900 °C for CMA. Specific surface areas from CMA were always higher than CA, although not all the surface was active for the sulfidation reaction because of the presence of inert MgO sites. Especially interesting were the samples of CMA sulfided at 700 °C, which exhibited an intense yellow color. Calcination experiments carried out at similar conditions to the sulfidation except for the presence of H2S demonstrated that calcination conversion was near zero. However, sulfur removal around 30% was obtained in the drop-tube reactor. Chemical analyses of the samples were performed to possibly detect elemental sulfur, as found by Shuckerow et al.8 in the pyrolysis of CMA in a gas stream containing sulfur dioxide and low oxygen concentration. However, no elemental sulfur was detected in the analyses. The conclusion is that the direct reaction of H2S with CaCO3 is quite important in the case of CMA sulfidation. Pore-size distribution of sulfided CA and CMA samples was also analyzed as a function of temperature. Figure 10 shows the BJH pore-area distribution of sulfided CA and CMA sorbents at different temperatures. The sorbent pore-size distribution depends on the equilibrium between the area generated by calcination and the area disappearing from sulfidation and sintering. The



Figure 11. Effect of residence time on sulfur removal (1000 °C, Ca/S ) 1, 5000 vppm of H2S).

Figure 10. Effect of temperature on the pore area of the sulfided sorbents (a) CA, and (b) CMA (Ca/S ) 1, 0.8 s, 5000 vppm of H2S).

filling of the smaller pores, under 4 nm, with CaS produces a displacement of the peak toward pores of higher size with increasing temperature in CA samples. In contrast, a decrease in the pore-size distribution was found in CMA in the pore range between 2 and 20 nm, but no pore dissappearance was found in any case. This was due to the presence of inert MgO that avoided the pore plugging during the reaction. Because a lower sorbent utilization was reached with CMA at temperatures above 1000 °C, the reduction of surface area detected at these temperatures was mainly due to sintering effects. Effect of Residence Time. The high sulfidation rate observed at high temperatures in IGCC systems allows gas cleaning by sorbent injection at short residence times. Different reaction times between 0.5 and 1.2 s were used by varying the linear gas velocity at 1000 °C, 5000 vppm of H2S, and a calcium to sulfur molar ratio of 1. Figure 11 shows the sulfur removal obtained for CA and CMA. A slight increase in SR was observed when increasing the residence time from 0.8 to 1.2 s. It must be taken into account that during this time, several processes have taken place in the sorbent: acetate pyrolysis, calcination of MgCO3 and CaCO3, sintering, and sulfidation. The final sulfur removal would be the result of these different factors affecting the sulfidation reaction. Acetate pyrolysis and MgCO3 calcination, in the case of CMA, were complete, even at the lowest reaction time tested, as verified through TGA analyses of the samples. Calcination of the CaCO3 increased with increasing residence time, although it was not complete even in the longer experiments at 1.2 s, for both sorbents. Solid sintering produces a decrease in the reacting surface area although it increases the

pore size, improving the H2S diffusion rate inside the particle. On the other hand, the filling of pores with the CaS product causes the H2S diffusion rate inside the particle to fall. The combined effect of the above processes give a slight increase in sulfur removal with increasing residence time. The slightly higher sulfur removal detected at 0.5 s with respect to the experiments at 0.8 s can be due to secondary experimental effects. In fact, a better dispersion of the sorbent particles in the background gases was observed during the shorter residence-time experiments due to the higher gas flow rate used. Similar behavior was found by Steciak et al.7 in their experiments on sorbent injection in oxidizing conditions. Effect of H2S Concentration. The H2S concentration in the reactor will depend on the sulfur content of the coal used in the gasifier. Concentrations up to 1.5 vol % H2S can be found using coals with a high sulfur content. In the experimental work, two different H2S concentrations, 5000 and 8000 vppm, were used. An increase in the H2S concentration gave a small increase in SR due to the increase in the reaction rate by the higher H2S concentration. Effect of Sorbent Particle Size. Particle size and pore structure are important factors affecting the sulfidation reaction of the sorbent, especially at the extreme conditions existing during their injection into the gasifier. It is well-known that the heterogeneous reaction of CaO with H2S depends on the available surface area of the solid. Particle size and pore structure influence the calcination rate and gas diffusion into the particle, which represents an important resistance to the global sulfidation reaction rate. Thus, small particles and high porosities are desirable to obtain high utilization of the sorbent. To study the effect of this parameter, three particle sizes of CA were injected into the reactor: 10-100, 100200, and 200-300 µm. Figure 12 shows the sulfur removal during the dry-sorbent injection for the three particle sizes used. A reduction in sulfur removal was found for the highest particle sizes, although no significant effect was detected for particles below 200 µm. However, it must be indicated that the sizes of the cenospheres obtained were much bigger than the size of the raw CA, in some way due to the fusion of several particles during cenospheres formation. In this work, cenospheres quite similar in size (mean size in volume about 480 µm) were obtained by dry-sorbent injection


Figure 12. Effect of raw particle size on sulfur removal for CA (1000 °C, Ca/S ) 1, 0.8 s, 5000 vppm of H2S).

of CA with a different initial size, which could explain the similar SR obtained with the particles of 10-100 and 100-200 µm. The above results have shown that the external wall of the cenospheres is the zone of highest sulfidation. Therefore, the size of the cenosphere should be a very important parameter in the sulfidation reaction. Especially interesting would be the possibility of obtaining cenospheres of a very small size, although this is only possible during wet-spraying of the sorbents. The high solubility of these compounds provides the possibility of injecting these sorbents by wet-spraying aqueous solution, and it is possible to achieve sorbents with virtually any size and porosity.16 However, according to Levendis et al.,6 the wall structure and thickness, rather than the particle size, are the important parameters in assessing the utilization of the cenospheric sorbent particles. Conclusions The effectiveness of calcium acetate and calcium magnesium acetate as feasible H2S capture agents was tested in a drop-tube reactor at temperatures from 700 to 1100 °C and simulating the gas conditions existing (16) Simons, G. A. Resour. Conserv. Recycl. 1992, 7, 161-170.

Ada´ nez et al.

in a gasifier. Their ability to form cenospheric particles with high porosity and surface area make them very attractive materials to remove H2S during their injection, at high temperatures, in the reducing atmospheres typical of the integrated gasification combined cycle processes. The sulfur capture effectiveness of these sorbents was shown to be superior to any other natural calcium-based sorbent. H2S capture up to 90%, near the thermodynamic equilibrium, was achieved with dry powder injection of CA and CMA at furnace gas temperatures of about 1000 and 800 °C, respectively, a Ca/S molar ratio of 2, and a 0.8 s residence time. However, because of the hard conditions of the test at high temperature and low residence time, the internal parts of the sorbent remained unreacted. The sorbent utilization finally reached is the result of the simultaneous effects of calcination, sintering, H2S diffusion, and sulfidation reaction. Thus, sulfur removal is expected to be enhanced during the CA and CMA injection in industrial reactors with respect to that obtained in this work due to the higher degree of turbulence existing. The temperature and Ca/S molar ratio have shown to be the most important parameters affecting sulfur removal; the H2S concentration, residence time, and particle size of raw sorbent are less important. Moreover, thermogravimetric analyses of sulfided cenospheres from CA and CMA have shown that the residue generated with these sorbents can be easily stabilized and converted into an inert material before disposal. The results showed herein have demonstrated that both CA and CMA are excellent sorbents to be used for gas cleaning in IGCC processes. Acknowledgment. This research was carried out with financial support from the Comisioń Interministerial de Ciencia y Tecnologıá (Project AMB 95-0784) and the European Coal and Steel Community (Project 7220-ED/76). The authors thank Dr. Diego Alva´rez for his assistance with the SEM-EDX techniques. EF9801367

Utilization of Calcium Acetate and Calcium Magnesium Acetate for

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