Ind. Eng. C h e m . Res 1991, 30, 199e1994
1990
Sulfation of Partially Sulfided Calcium-Based Sorbents J a v a d Abbasian* a n d Amir R e h m a t Institute of Gas Technology, 3424 S. State Street, Chicago, Illinois 60626
Daniel D. Banerjee Center for Research on Sulfur in Coal, Suite 200, Coal Deuelopment Park, P.O. Box 8, Carteruille, Illinois 62918
The solid waste materials from coal gasifiers that utilize limestone or dolomite for sulfur capture contain calcium sulfide, which is not stable and has the tendency to decompose when it contacts moisture in the air, releasing hydrogen sulfide to the atmosphere. The sulfide-containing wastes must be stabilized t o produce a solid product that is environmentally acceptable for disposal. The reaction of partially sulfided calcium-based sorbents with oxygen for the production of stable calcium sulfate was investigated. Tests were conducted in an ambient-pressure quartz thermogravimetric analyzer and a 2-cm-diameter quartz fluidized-bed reactor to determine the effects of the sorbent type (limestone or dolomite), the sorbent particule size, extent of the sulfidation, and the sulfation temperature on the rate and extent of the sulfation reaction. Analyses of the sulfated samples indicate that the maximum level of sulfation is higher for dolomite compared to limestone. The maximum level of sulfation is also higher for sorbents with lower levels of sulfidation or smaller particle size. Introduction The sulfur compounds present in coal are converted to hydrogen sulfide when the coal is gasified. To comply with the New Source Performance Standards (NSPS),a high fraction of the sulfur must be removed from the gas stream. Calcium-based sorbents such as limestone and dolomite are among the prime candidates for in-bed capture of sulfur from fuel gases. High efficiency advanced electric power generation systems using integrated coal gasification combined cycle (IGCC), or advanced/staged pressurized fluidized-bed combustion, require very low levels of H,S in the fuel gases. More than 90% of the H,S in the fuel gas can be removed in the gasifier by using a calcium-based sorbent such as limestone or dolomite. An external hot gas cleanup system using regenerable mixed metal oxides such as zinc ferrite is then used to further reduce the H,S content of the fuel gas to levels suitable for IGCC application. The removal of sulfur that takes place through the reaction of hydrogen sulfide with calcium oxide, calcium carbonate, or calcium hydroxide produces calcium sulfide. This compound is not stable and can react with the moisture in the air, releasing hydrogen sulfide to the atmosphere, and, therefore, is not suitable for direct disposal. Calcium sulfide, however, can be further reacted with oxygen (or air) to produce calcium sulfate, which is a stable and environmentally acceptable compound for disposal. Researchers (Abbasian et al., 1990a,b; Borgwardt and Roache, 1984; Kamath and Petrie, 1981; Ruth et al., 1972; Squires et al., 1971) in the field of chemical kinetics of limestone/dolomite reactions with hydrogen sulfide have already verified the potential use of these sorbents for sulfur capture. The reaction of calcined limestone/dolomite with H2S is very rapid, and the reaction almost reaches equilibrium. Based on equilibrium considerations, it is feasible to remove up to 90% of the sulfur and discharge it with the ash using in-bed calcium-based sorbents. A limited number of tests conducted at piiot-plant scale (Jones and Patel, 1985; Weldon et al., 1986; Keairns et al.. 1976) have verified the feasibility of this sulfur-removal method. The work in the area of desulfurization with calciumbased sorbents found in the literature does not sufficiently * To whom correspondence should be addressed. 0888-5885191/ 2630- 1990$02.50/0
Table I. Chemical Analyses of Sorbents analyses, w t 70 calcium magnesium potassium iron aluminum silicon strontium carbon dioxide oxygen (by diff) total
limestone 39.2 0.56 0.5 0.092 0.05 0.10 0.16 44.6 14.74 100.00
dolomite 22.15 13.65 0.5 0.135 0.07 0.275 0.005 48.1 15.11 100.00
dolomitic limestone 33.95 3.73 0.33 0.16 0.06 0.78 0.021 43.55 17.42 100.00
address the stabilization of partially sulfided Ca-based sorbent to produce an environmentally acceptable solid waste product. The limited experimental data on the sulfation of calcium sulfide that is reported in the literature (Abbasian et al., 1990a,b) indicate that calcium sulfide in the partially sulfided dolomite can be completely sulfated whereas in the case of limestone, only partial sulfation (less than 30% ) can be achieved. This is due to the presence of MgO in the calcined dolomite which does not react with H2Sand consequently with oxygen, providing better pore matrix for the diffusion of oxygen into the particle. The use of limestone rather than dolomite is generally desired because the higher fraction of calcium in the limestone compared to dolomite will require utilization of a smaller quantity of the sorbent for fuel gas desulfurization and will result in a smaller quantity of solid wastes for dosposal. The objective of this study was to determine the effects of operating variables on the stabilization of solid wastes from coal gasifiers (that utilize limestone or dolomite for sulfur capture) to environmentally acceptable solid products for final disposal. Experimental Section Sorbent Selection. Three calcium-based sorbents were selected for this study. The selection of these sorbents was primarily based on the calcium carbonate content. These sorbents are classified as limestone, dolomite, and dolomitic limestone. The sorbents were crushed and screened into narrow particle size ranges. Two cuts from each sorbent were selected for testing in this study. These selected cuts included coarse particles (0.07-cm diameter) 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 1991 COMWTER DATA A C O U I S I T I O N UNIT FILTER HOUS ING
NoOH LIQUID SCRUBBER
VENT
NaOH LIQUID SCRUBBER
PI TIC
Lp
FR I TTED OUARTZ DISC
,COMPUTER ACOUISITIO DATA N UNIT
- PRESSWE 1EL)ICATOR - TEWERATURE I I Q I C A T I N O CONTROLLER
Figure 1. Process flow diagram of quartz fluidized-bed unit.
and fine particles (0.02-cm diameter). Elemental analyses for both sorbents are given in Table I. The calcium content of the limestone corresponds to 98% calcium carbonate. The molar ratio of calcium to magnesium in dolomite is 0.98, indicating that both the limestone and dolomite are reasonably pure. The dolomitic limestone contained about 85% calcium carbonate and 14% magnesium carbonate, which corresponds to 66% limestone and 34% dolomite. Preparation of Sulfided Sorbents. Samples of both fine and coarse particles from each sorbent were calcined and then reacted with hydrogen sulfide in a 2-cm-diameter quartz fluidized-bed reactor. CaCO, = CaO + C02 (A) CaO + HzS = Cas
+ H20
(B)
A schematic diagram of the unit is presented in Figure 1. The unit essentially consists of a quartz fluidized-bed reactor that is externally heated by two electrical furnaces and equipment for feeding and measuring the flow rates of the gases, measuring and controlling the bed temperature, and monitoring the fluidized-bed pressure. The fluidized-bed distributor is made of a quartz fritted disk. This unit was used for the preparation as well as the stabilization of the sulfided sorbents. The sorbents (about 20 g) were calcined during the heatup period (30-980 "C) in a nitrogen atmosphere and then reacted with a reactant gas containing 5% hydrogen sulfide at 980 "C for a predetermined period to achieve the desired sulfidation levels of 33%, 6790, and 100%, which correspond to a Ca/S molar ratio of 3, 1.5, and 1 in the gasifier feed. The superficial gas velocities for the fine and coarse particles were 1 and 2 ft/s, respectively. The sulfided sorbents were analyzed. The measured levels of sulfidation of the sulfided samples are very close to the desired values as seen in Table 11. Sulfation of Sulfided Sorbents. Tests were conducted in an ambient-pressure thermogravimetricanalyzer (TGA) reactor unit to determine the optimum temperature for maximum conversion of calcium sulfide to stable cal-
Table XI. Results of Sulfidation Tests sorbent particle diam, cm limestone 0.02 limestone 0.02 limestone 0.02 limestone 0.02 limestone 0.07 limestone 0.07 limestone 0.07 dolomite 0.02 dolomite 0.02 dolomite 0.02 dolomite 0.07 dolomite 0.07 do1omite 0.07 dolomitic limestone 0.02 dolomitic limestone 0.02 dolomitic limestone 0.02 dolomitic limestone 0.07 dolomitic limestone 0.07 dolomitic limestone 0.07
% sulfidation
20
37 71 100 39 73 99 43 75 100 35 73 100 37
67 84 34 64 85
cium sulfate and to determine the rate and the extent of the sulfation reaction. Cas + 2O2 = CaCOl (C) A schematic diagram of the TGA unit used in this study is shown in Figure 2. The procedure for these tests included heating the sorbent (about 3 mg) in a nitrogen atmosphere to a predetermined temperature. At that point the gas mixture containing about 5% oxygen was allowed to flow past the sorbent while the change in the sample weight was continuously monitored. The gas flow rate in these tests was about 500 cm3/min. The sample was exposed to the reactant gas containing oxygen until the sulfided sorbent was converted to its peak value. The weight-versus-time data were translated into conversion-versus-time data. Conversion of calcium sulfide to calcium sulfate (reaction C) is calculated using the following formula: % CaS conversion = [(sample w t gain) X 32]/[(init sample wt) X (fraction of sulfur in the sample)(4 X IS)] X 100 (1)
1992 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991
I
t>
I
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t
--
PR PI
TE
-- PRESSURE REGULATOR PRE SSURE INDlCArOR
-
TEMPERATURE T I - TEMPERATURE TIC TEMPERATURE CONTROLLER TR TEMPERATURE
-pi
-
REACTANT GAS
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Figure 2. Process flow diagram of the ambient-pressure thermogravimetric (TGA)reactor unit.
76
IW
1 E
4
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mi^
j
6
4
TIME,
8
10
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Figure 3. Effect of temperature on stabilization of sulfided dolomite.
Figure 4. Effect of level of sulfidation on stabilization of sulfided dolomite.
where 32 is the atomic weight sulfur, 16 is the atomic weight of oxygen, and 4 is the number of oxygen atoms added to the sample during sulfation reaction. The sulfided sorbents were also reacted with oxygen in the 2-cm-diameter fluidized-bed reactor at similar conditions. The sulfated samples were analyzed to make a comparison between the TGA and the fluidized-bed results. Even though reaction C is highly exothermic, because of the small size of the samples and the apparatus, no significant temperature rise was observed during the sulfation tests in the TGA or the fluidized-bed tests.
was 43%. The results, as presented in Figure 3, indicate that the optimum temperature for maximum sulfation is in the range of 815-900 "C. Lower reaction rates at lower temperatures are expected because the reaction rate constant is lower at lower temperatures. The lower maximum levels of conversion at higher temperatures are probably caused by local sintering a t higher temperatures. Figure 4 shows the effect of the level of sulfidation on the sulfation (stabilization) of the sulfided dolomite sample. The dolomite samples with lower sulfidation levels react faster and are sulfated to a higher extent compared to samples with higher levels of sulfidation. Lower levels of sulfidation correspond to thinner layers of calcium sulfide. Because reaction C is probably diffusion limited, thinner layers of calcium sulfide should result in a higher reaction rate, which is consistent with the results seen in Figure 4. Similar results were observed for the limestone and dolomitic limestone that are presented in Figures 5 and 6, respectively. The results in Figure 5 indicate that
Results and Discussion A series of tests was conducted with fine dolomite particles to determine the optimum operating temperature for the sulfation reaction (stabilization). The range of temperatures in this series of tests was 650-980 "C. The level of sulfation of the samples used in this series of tests
Ind. Eng. Chem. Res., Vol. 30, No. 8,1991 1993
/
H
/
m-
37
I
J
TIME, m l n
Figure 5. Effect of level of sulfidation on stabilization of sulfided limestone.
Figure 8. Extent of sulfation of different Ca-based sorbents (coarse particles). 100 WLOMllE
i
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OF SUFIDATIffl, X
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-
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dolomitic limestone.
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Figure 6. affect of level of sulfidation on stabilization of sulfided
204
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Figure 7. Comparison of maximum level of sulfation in TGA and fluidized-bed units.
high levels of sulfation should not be expected for limestone particles that have sulfidation levels higher than 37% (Ca/S < 3). The extent of sulfation with the dolomitic limestone is between those of limestone and dolomite. A similar trend was observed for coarse particles. The sulfided samples of the three sorbents in two particle sizes were also reacted with oxygen in the ambientpressure fluidized-bed unit. The solid products from these tests were analyzed and the extent of sulfation was determined. A comparison of the maximum level of sulfation achieved in the two units with different samples is presented in Figure 7 indicating that the results obtained in
Figure 9. Extent of sulfation of different Ca-based sorbents.
fluidized-bed experiments are in very good agreement with the TGA test results. The maximum extent of sulfation as a function of the level of sulfidation (Ca/S ratio in the gasifier) and sorbent type (magnesium content) for the coarse and the fine particles is presented in Figures 8 and 9, respectively. The data in Figures 8 and 9 indicate that sulfided dolomite can be sulfated to levels approaching 100% when the Ca/S ratio in the gasifier is greater than 1.5. To achieve high levels of sulfation with sulfided limestone or dolomitic limestone, very high Ca/S ratio (Ca/S > 5 ) and small sorbent particles (particle diameter < 0.02 cm) should be used.
Conclusions and Recommendations The following conclusions and recommendations are based on the results of this study: The highest levels of sulfation of the sulfided calciumbased sorbents can be achieved in the temperature range of 815-900 'C. The rate and the extent of sulfation reaction are higher for samples with lower levels of sulfidation. The maximum level of conversion of calcium sulfide to stable calcium sulfate increases with an increase in the dolomite content of the sorbent. Sulfation testa should be performed at temperatures of above 980 "C to determine if calcium sulfide can be sintered to produce environmentally acceptable solid waste material. Sulfation tests should be performed at elevated pressure to determined the rate of reaction and the extent of sulfation at gasifier conditions.
Ind. Eng. C h e m . Res. 1991, 30, 1994-2005
1994
Registry No. H2S, 7783-06-4; dolomite, 16389-88-1.
Literature Cited Abbasian, J.; Rehmat, A.; Leppin, D.; Banerjee, D. D. Desulfurization of Fuels With Calcium-Based Sorbents. Fuel Process. Techno[. 1990a, 25, (1). Abbasian, J.; Rehmat, A.; Leppin, D.; Banerjee, D. D. An Advanced Coal GasificationJDesulfurization Process. Proceedings of the 25th IECEC, Reno, NV; AIChE: New York, 1990b. Borgwardt, R. H.; Roache, N. F. Reaction of H2S and Sulfur With Limestone Particles. Ind. Eng. Chem. Process. Des. Deu. 1984, 23. Jones, F. L.; Patel, J. G. Performance of Utah Bituminous Coal in the UGAS Gasifier. Presented at the Fifth EPRI Contractor's Conference on Coal Gasification, EPRI, Palo Alto, CA, 1985. Kamath, V. S.; Petrie, T. W. Rate of Reaction of Hydrogen Sulfide-Carbonyl Sulfide Mixture With Fully Calcined Dolomite. Enuiron. Sci. Technol. 1981, 15.
Keairns, D.; Newby, R.A,; O'Neill, E. P.; Archer, D. H. High Temperature Sulfur Removal System Development for Westinghouse Fluidized Bed Coal Gasification Process. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1976, 21, (9). Ruth, L. A,; Squires, A. M.; Graff, R. A. Desulfurization of Fuels With Half-Calcined Dolomite: First Kinetic Data. Enuiron. Sci. Technol. 1972, 6, (12). Squires, A. M.; Graff, R. A,; Pell, M. Desulfurization of Fuels With Calcined Dolomite. 1. Introduction and First Kinetic Results. Chem. Eng. Prog. Symp. Ser. 1971,67. Weldon, J.; Haldipur, G. B.; Lewandowski, D. A.; Smith, K. J. Advanced Coal Gasification and Desulfurization with Calcium Based Sorbents. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1986, 33, ( 3 ) .
Received for review February 1, 1991 Revised manuscript received April 24, 1991 Accepted May 9, 1991
Equation of State for Small, Large, Polydisperse, and Associating Molecules: Extension to Fluid Mixtures Stanley H. Huang and Maciej Radosz* Exxon Research and Engineering Company, Annandale, New Jersey 08801
Statistical associating fluid theory (SAFT)has been extended t o mixtures using rigorous statistical mechanical expressions for the hard-sphere, chain, and association terms. In contrast to previous equations of state, only the dispersion term requires mixing rules (averaging equations for SAFT parameters). We use two approaches t o deriving the mixing rules, a van der Waals one-fluid approximation (vdW1) rooted in conformal solution theory and a volume-fraction (vf) approximation similar to group-contribution equations of state. We test these approaches on 60 phase equilibrium data sets for asymmetric (small large) and associating binaries. While both sets of mixing rules, vdWl and vf, are found t o be adequate away from the critical region, the vf predictions are found t o be more accurate near the critical region.
+
Introduction Computer simulation plays a central role at different stages of process development. Such simulation sensitively depends on the quality of physical concepts underlying various process models. One of these concepts that is frequently invoked is to use an equation of state rooted in molecular thermodynamics to derive chemical potentials or fugacities needed to predict phase equilibria. For example, numerous single-stage and multistage, distillative and extractive separations are commonly predicted on the basis of vapor-liquid and liquid-liquid equilibria. To derive the chemical potentials for such predictions, one has to use an equation of state that is not only valid over the whole density range, from zero to the density of interest, but also applicable to the mixture of interest. We recently reported an equation of state for p u r e components (Huang and Radosz, 1990), based on the statistical associating fluid theory (SAFT). The reference part of SAFT includes the hard-sphere, chain, and association terms. The perturbation part accounts for relatively weaker, mean-field (e.g., dispersion and induction) effects. The reference part can be traced to the ideas and results of Wertheim (1984,1986) and Chapman et al. (1989, 1990), as described by Huang and Radosz (1990). The perturbation part is similar to that proposed initially by Alder et al. (1972) and also used by Chen and Kreglewski (1977) for pure compounds and by Simnick et al. (1979) for mixtures. The key result of our work on pure and 0888-5885191J 2630-1994$02.50f 0 0
polydisperse components was a practical, prototype equation of state with well-behaved, easy to estimate parameters. Our goal is to extend this equation of state to mixtures of small, large, chain, and associating molecules over the whole density range using only one binary adjustable parameter that is temperature independent. For reference, while specialized methods have been proposed for narrow groups of systems (for example, empirical activity coefficient models for associating molecules at low pressures, or cubic equations of state for small nonassociating molecules), there is no single approach available for mixtures of small and large, associating and nonassociating molecules, at low and high pressures. SAFT is unique in this respect. We will define SAFT in terms of the Helmholtz energy and present correlation results for low-pressure associating binaries away from the critical region and for high-pressure binaries near the critical region. In this work, we do not include systems containing cross associations, such as acids + water. These systems will be addressed later in the continuation of this work.
Equation of State The theoretical results underlying the equation of state are given in this section in terms of the residual Helmholtz energy am per mole, defined as am(T,Vm = aba(T,VJV) - aidralgu (T,V,N),a t the same temperature and density. AI1 other thermodynamic quantities can be derived fol1991 American Chemical Society
