846
Ind. Eng. Chem. Res. 1997, 36, 846-853
Kinetic Study of High-Temperature Removal of H2S by Novel Metal Oxide Sorbents Enrique Garcı´a,† Cristina Cilleruelo,† Jose´ V. Ibarra,*,† Miguel Pineda,‡ and Jose´ M. Palacios‡ Instituto de Carboquı´mica, CSIC, Poeta Luciano Gracia 5, 50015 Zaragoza, Spain, and Instituto de Cata´ lisis y Petroleoquı´mica, CSIC, Campus Universidad Auto´ noma, Cantoblanco, 28049 Madrid, Spain
The behavior of different mixed oxides, including zinc titanates (ZT) and zinc ferrites modified with CuO (ZFC) or TiO2 (ZFT), as hot gas desulfurizing sorbents was investigated. The sorbents were prepared by calcination at 650 °C of a mixture of bulk oxides in three different stoichiometries in order to form new phases and modify their textural properties. Tests of stability against reduction were obtained by thermoprogrammed reduction, and kinetic studies of the sulfidation reaction were carried out in a thermobalance in the temperature range of 550-650 °C. Kinetic parameters of the intrinsic reaction were obtained assuming a grain model. The sulfidation behavior of the sorbents as extrudates was investigated in a fixed-bed reactor in terms of breakthrough curves. Fresh and sulfided samples were characterized by Hg porosimetry, X-ray diffraction, and SEM-EDX. The study shows that the addition of TiO2 or CuO to zinc ferrite based sorbents calcined at 650 °C has little effect on the stability against reduction but markedly influences their textural properties. The stabilizing effect of Ti is observed in samples calcined at higher temperature or in non-iron-containing sorbents. The calculated kinetic constants indicate that the Zn content and the incorporation of Cu have an enhancing effect on the kinetics of the sulfidation process. Including H2 in the feed gas decreases the reactivity and increases the activation energy. Extrudated sorbents showed a good performance as desulfurizing agents and maintained the H2S concentration in the outlet gas below 20 ppm. ZT sorbent exhibited a poor efficiency, which makes the addition of Ti questionable. Introduction The use of fossil fuels for power generation is expected to increase in the coming years because of industrialization and population growth. Current methods for the conversion of chemical to electrical energy use using relatively old technologies are relatively inefficient and environmentally pollutant. The two main objectives of emerging technologies for power generation plants are the reduction of energy costs and pollutant emissions. New technologies which are reaching the commercial stage include the integrated gasification combined cycle (IGCC) and the gasifier-molten carbonate fuel cells (MCFC). For these technologies, a highly efficient sulfur removal from several thousand parts per million (ppm) down to ∼1 ppm for fuel cell power plants or ∼100 ppm for gas turbines is needed (Lew et al., 1989). Commercial desulfurization processes are usually based on liquid scrubbing at or below ambient temperatures, resulting in a reduction thermal efficiency loss as well as the need for expensive wastewater treatment. In order to achieve maximum thermal efficiency, the fuel gas should be desulfurized at temperatures approaching that at which it leaves the gasifier. Recent developments in hot-gas desulfurization have focused on regenerable solid mixed-metal oxide sorbents to remove reduced sulfur species (H2S, COS, etc.) from coal gasification. Westmoreland and Harrison (1976) reported the results of a thermodynamic screening of the high-temperature desulfurization potential of 28 * Author to whom all correspondence should be addressed. Fax: +34-76-733318. E-mail:
[email protected]. † Instituto de Carboquı ´mica. ‡ Instituto de Cata ´ lisis y Petroleoquı´mica. S0888-5885(96)00194-7 CCC: $14.00
single-metal oxides by using the free-energy minimization method. Zinc ferrite has been the most studied sorbent in the last years (Focht et al., 1988; Gupta et al., 1992; Woods et al., 1991). However, zinc ferrite has several unsolved problems: (i) When the H2S concentration in the outlet gas from the reactor becomes high, the sorbent conversion is only 40-50% of the theoretical. (ii) Under the reducing operation conditions zinc ferrite decomposes into the individual oxides. Fe2O3 is easily reduced to the metal Fe0, and ZnO, above 650 °C, is reduced to metallic Zn and volatilized. (iii) Because of the stability of zinc sulfate, regeneration requires a higher temperature than sulfidation with subsequent efficiency loss and irreversible damage by thermal sintering. To improve the zinc ferrite performance, different metal oxides, such as Zn, Cu, Al, Ti, Fe, Co, Mo, and V have been assayed (Tamhankar et al., 1986); however, their role in the sulfiding process is not well understood. One of the most promising among the zinc-based sorbents was zinc titanate (Mojtahedi et al., 1994; Lew et al., 1992; Gupta and Gangwal, 1993). TiO2 was found to stabilize the ZnO phase and increase the limit temperature of operation up to 700 °C (Flytzani-Stephanopoulos and Sarofim, 1990); however, the effect on the sulfidation was not adequately studied. The aim of the paper is to evaluate the performance of mixed-metal oxide sorbents, zinc titanates (ZT) and copper (ZFC) or titanium (ZFT) containing modified zinc ferrites, in the sulfidation process in a fixed-bed reactor. The characterization of the fresh and sulfided sorbents using different techniques and the kinetic study undertaken in a thermobalance allow a better knowledge of the involved process to be gained and help in the development of new sorbents. © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 847 Table 1. Designation and Crystalline Phases Identified by XRD in Fresh Sorbents XRD analysis sample ZFT ZFT (1100 °C) ZFC ZT
stoichiometry
before calcination
after calcination
1:0.2:0.8 1:0.5:0.5 1:0.8:0.2 1:0.2:0.8 1:0.5:0.5 1:0.8:0.2 0.2:1:0.8 0.5:1:0.5 0.86:1:0.14 2:3 1:2 0.8:1
ZnO, R-Fe2O3, anatase ZnO, R-Fe2O3, anatase ZnO, R-Fe2O3, anatase ZnO, R-Fe2O3, anatase ZnO, R-Fe2O3, anatase ZnO, R-Fe2O3, anatase ZnO, R-Fe2O3, tenorite ZnO, R-Fe2O3, tenorite ZnO, R-Fe2O3, tenorite ZnO, anatase ZnO, anatase ZnO, anatase
ZnO, R-Fe2O3, anatase ZnO, R-Fe2O3, anatase ZnO, R-Fe2O3, anatase spinel (ZnFeTiO4, ZnFe2O4), rutile spinel (ZnFeTiO4, ZnFe2O4) spinel (ZnFeTiO4, ZnFe2O4) (Cu,Zn)Fe2O4, R-Fe2O3 (Cu,Zn)Fe2O4, R-Fe2O3 (Cu,Zn)Fe2O4, R-Fe2O3 anatase, R-Zn2TiO4, ZnO anatase, R-Zn2TiO4, ZnO anatase, R-Zn2TiO4, ZnO
Experimental Section Sorbents. The designations of the studied sorbents are shown in Table 1. All samples were prepared from commercial oxides (Merck, reagent grade) by calcination at 650 °C for 16 h. Additionally, sorbents of series ZFT were also calcined at 1100 °C to form spinel phases that were not formed at 650 °C. Within every series sorbents with three different stoichiometries were prepared in order to change the chemical species and/or the textural properties in the fresh sorbents. The samples were powder of particle size 50 µm. For testing in the fixed-bed reactor, samples ZFC (0.86:1:0.14), ZFT (1:0.8:0.2), and ZT (0.8:1) were also prepared as extrudates. A mixture of pure oxides in the convenient concentrations was homogenized in a planetary ball mill for 1 h. When water was added to the homogenized powder, a consistent slurry was used to obtain cylindrical extrudates, 5 mm diameter × 5 mm length, that were dried at 110 °C overnight and calcined at 950 °C. Characterization Techniques. Textural properties were measured by Hg intrusion porosimetry with a Micromeritics pore sizer 9310 instrument up to a final pressure of 2.1 × 107 Pa which allows filling of pores down to 6 nm diameter. X-ray diffraction patterns were recorded with a Seifert 3000 diffractometer using nickelfiltered Cu KR radiation. Morphological studies and element distributions were carried out with SEM-EDX equipment consisting of an ISI DS-130 scanning electron microscope coupled to an Si(Li) X-ray detector and a PGT SUN Sparcstation 5 for energy-dispersive X-ray analysis. The thermoprogrammed reduction (TPR) experiments were undertaken in a thermobalance Cahn 2000 using a gas flowrate of 50 cm3/min of H2 25 vol % in He at a heating rate of 5 °C/min. Kinetic Measurements. Kinetic experiments of sorbent reduction and sulfidation were carried out in a Setaram TA92 thermogravimetric analyzer (TGA). The reaction conditions were set so that the overall reaction rate would be controlled by the chemical reaction as explained below. Kinetic experiments were performed isothermally at 550, 600, and 650 °C with a nonreducing gas A containing 0.5% H2S in N2 and a reducing gas B with 0.5% H2S and 10% H2 in N2. Gas flow rates were set by mass flow controllers (Brooks). The effect of the gas flow rate on the reaction rates was previously examined in the 50-300 mL/min range, and the results (not shown) indicated that above 100 mL/min the gas flow rate has no effect on the reaction rate of sulfidation. Similar results were obtained for reduction. This indicates an absence of external masstransfer resistance above that flow rate, and thus a total flow rate of 300 mL/min was fixed in all experiments.
The intrinsic rate constants of sulfidation were calculated according to the grain model (Szekely et al., 1976; Tamhankar et al., 1981). In this model the solid particle is considered to be composed of small but dense grains, and each grain reacts according to the shrinkingcore model. The examination of sorbent particles under the microscope showed irregular forms by cylindrical rather than spherical, and thus the following expression was used to estimate the grain radius (rg):
rg )
2 Sg[wodr + (1 - wo)di]
(1)
In the absence of external mass-transfer resistance, the overall rate of noncatalytic gas-solid reaction is affected by chemical reaction and pore diffusion. Usually with small particles, the chemical reaction is likely to control the overall rate. The effect of particle diameter on the reaction rate was also studied in this work, and the results (not shown) indicated the absence of pore diffusion control below a particle diameter of 100 µm. Therefore, a particle diameter of 50 µm was chosen in order to work in conditions of chemical control. This is a particular case of the grain model in which diffusion through the interstices among the grains presents a negligible resistance and thus the reactant concentration is uniform throughout the solid. In these conditions the conversion-time relationship is given by the expression:
t ) t1F1
(2)
For cylindrical grains the expressions of t1 and F1 are (Levenspiel, 1988):
t1 )
Csorg bk(Cao - Cco/K)
F1 ) 1 - (1 - X)1/2
(3) (4)
The solid molar concentration, Cso, is calculated by the expression:
Cso )
wo(1 - ) M[wo/dr + (1 - wo)/di]
(5)
the conversion, X, by
X) and the porosity, , by
W - W0 Wmax - W0
(6)
848 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 Table 2. Textural Properties of Fresh Sorbents sample ZFT ZFT (1100 °C) ZFC ZT
stoichiometry
Vp (cm3/g)
rp (nm)
S (m2/g)
1:0.2:0.8 1:0.5:0.5 1:0.8:0.2 1:0.2:0.8 1:0.5:0.5 1:0.8:0.2 0.2:1:0.8 0.5:1:0.5 0.86:1:0.14 2:3 2:1 0.8:1
0.37 0.30 0.30 0.21 0.29 0.28 0.35 0.38 0.36 0.50 0.36 0.42
100 102 83 33 56 75 96 76 27 28 80 89
8 6 7 13 10 8 7 10 25 35 10 9
H2S Retention in a Fixed Bed. The sulfidation performance of selected sorbents as extrudates (5 × 5 mm) was investigated, in terms of breakthrough curves, in a 2.3 cm i.d. fixed-bed quartz microreactor. The sulfidation experiments were carried out at atmospheric pressure, 600 °C, and a space velocity of 683 h-1 (STP) corresponding to approximately 2 cm of bed height. The composition of the reactant gas was 0.5% H2S, 10% H2, and 15% H2O(v) in N2. Gas analyses at the inlet and outlet were performed by mass spectrometry using a quadrupole mass analyzer (Spectramass). Results and Discussion
Figure 1. SEM-EDX line profiles of sorbents of ZFT 1100 series: (A) ZFT (1:0.8:0.2), (B) ZFT (1:0.5:0.5), (C) ZFT (1:0.2:0.8).
) Vpdp
(7)
If the sulfidation reaction is considered to be irreversible, eq 3 simplifies to:
t1 )
Csorg bkCao
(8)
The apparent kinetic constant, k′, was calculated from the slope of plots F1 vs t using eq 2. The intrinsic reaction constant, k (k ) k′rg), was determined by taking into account the radius of the grain (rg) which can be approximately calculated from eq 1.
Characterization of Fresh Sorbents. The crystalline chemical species in the fresh samples, before and after calcination, identified by XRD are shown in Table 1. In calcined samples spinel-type compounds are usually found as major phases. Within each series the variation in the concentration of the component oxides is not evidenced by significant changes in the X-ray patterns because no new chemical phases are formed; it is only reflected in changes in the relative peak intensity as a consequence of the phase abundance. In sorbents of series ZFC the spinel compound corresponds to ZnFe2O4 (franklinite). In fact, Cu can enter the spinel lattice in substitutional positions with reference to Zn within a broad range of composition yielding negligible structural changes. In sorbents of series ZFT calcined at 650 °C, results in Table 1, column 4, indicate that the temperature of calcination was too low to form spinel-type compounds. These calcined samples are mostly a mixture of pure oxides as before calcination. If the temperature of calcination is increased up to 1100 °C (Series ZFT 1100), the spinel phase is formed for all compositions, but, even so, a part of Ti remains as rutile. The conclusion is that Fe and Ti are competitive in entering the ZnO lattice to form spinel structures. If Fe is present, Ti has little chance to enter the lattice in order to stabilize the structure against reduction. At the same time Ti also delays the formation of ZnFe2O4. In the absence of Fe (sorbents of Series ZT) the chance of Ti entering the lattice of ZnO increases and R-Zn2TiO4 becomes the major phase even at a relatively low temperature of calcination; however, a part of Ti is found as an inactive phase (anatase). In this series the stabilizing effect of Ti is expected to be higher than that in series ZFT (especially those calcined at 650 °C) but lower than that deduced from the high concentration. SEM-EDX line profiles show that all the studied sorbents are homogeneous at micrometer scale because the particle size of the chemical species identified by XRD is very small ( ZT > ZFT. The sulfidation in the presence of a reducing gas (H2) leads to lower kinetic constants and higher activation energies due, in this case, to the reaction taking place on more reduced species of iron, besides zinc losses through reduction and subsequent vaporization. H2S Retention in a Fixed Bed. The breakthrough curves of the H2S retention obtained in a fixed-bed reactor for the three extrudate sorbents are shown in Figure 8. The profile for all curves is similar; i.e., the H2S concentration in the outlet gas is very low (less than 10 ppm) because the spatial velocity is very low and no axial dispersion occurs. After a certain elapsed time, the breakthrough takes place and the H2S concentration increases steeply and tends to reach the H2S concentration at the inlet gas of the reactor. The breakthrough time, arbitrarily defined as the time at which the H2S concentration in the outlet gas is 100 ppm, differs widely for the studied samples. The values of 4056 min for ZFT, 4344 min for ZFC, and 2622 min for ZT samples agree reasonably well with the expected time for an ideal sorbent, taking into account the active oxide concentration of the sorbent, the bed mass, the flow rate, and the H2S concentration in the feed gas for the ZFT and ZFC sorbents. However, it is approximately half of that theoretically calculated for the ZT sorbent. In order to explain the different behavior exhibited by the studied sorbents in the reactor, a characterization of the sulfided extrudates was carried out. Characterization of Sulfided Extrudates. The textural properties of the sulfided samples are shown
Figure 7. Weight ratio changes for the sulfidation of ZFC sorbents in two steps: reduction (10% H2) followed by sulfidation (0.5% H2S).
a fixed-bed reactor. In this case, the time to reach total conversion (Figures 4 and 5) slightly increases if H2 is included in the feed gas for iron-containing sorbents (ZFT and ZFC) and is unaltered for sorbent ZT. In sorbent layers placed at the top bed of a reactor, reduction occurs before sulfidation. In order to simulate this situation and obtain kinetic parameters of the two processes independently, experiments of reduction were carried out with a gas of composition 10% H2 in N2 followed by sulfidation with a gas 0.5% H2S in N2 at three temperatures (Figures 6 and 7). It can be observed that the reduction curves have two zones of different slope associated with two processes. The first one is fast and little dependent on the temperature and can be associated with the reduction of iron and copper oxides. The second is slower and highly temperature dependent and must be associated with the ZnO reduction and volatilization. In agreement with TPR experiments, results from thermobalance indicate that neither Cu nor Ti in samples calcined at 650 °C affects the stabilization of zinc ferrites. As far as sulfidation is concerned, the conversion curves shown on the right-
Table 5. Kinetic Constants of the Reduction and Sulfidation Processes of the Sorbents first step: reduction sample
stoichiometry
ZFT
1:0.5:0.5
ZFC
0.86:1:0.14
ZTa
0.8:1
a
temp (°C)
k′ (°C)
550 600 650 550 600 650 550 600 650
0.7 1.1 1.4 1.4 2.0 3.1
k
(×10-5
cm
0.2 0.4 0.5 0.2 0.3 0.4
There is not noticeable weight loss by reduction.
s-1)
A
(s-1)
1096 1096 1096 2321 2321 2321
second step: sulfidation Ea (cal/mol) 12 179 12 179 12 179 12 259 12 259 12 259
k′
(s-1)
24.1 26.0 30.5 35.0 41.4 43.9 21.1 28.9 32.2
k (×10-5 cm s-1)
A (s-1)
Ea (cal/mol)
9.0 9.7 11.4 13.1 15.5 16.4 8.9 12.2 13.6
213 213 213 290 290 290 1107 1107 1107
3614 3614. 3614 3460 3460 3460 6472 6472 6472
852 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997
Extrudated sorbents showed a good behavior in H2S retention in a fixed bed by being able to achieve very low emission levels. The stabilizing effect of Ti addition to extrudated ZT sorbents is questionable in tests of performance carried out in a fixed-bed reactor because the efficiency is considerably reduced. Acknowledgment This work was carried out with financial support from the European Coal and Steel Community (ECSC Project No. 7220-EC/027). E.G. also thanks IBERDROLA for a grant. Figure 9. Local sorbent conversion across a diameter of sulfided extrudates obtained by SEM-EDX point analysis of sulfur. Table 6. Textural Properties of Sulfided Sorbents sample
stoichiometry
Vp (cm3/g)
rp (nm)
S (m2/g)
ZFT ZFC ZT
1:0.5:0.5 0.86:1:0.14 0.8:1
0.20 0.20 0.20
151 104 204
3 4 2
in Table 6. Sulfided sorbents show, in relation to fresh samples in Table 2, a marked reduction of the surface area and pore volume which can imply the appearance of diffusional problems at high sorbent conversions. The XRD study of sulfided samples reveals the presence of ZnS (wurtzite, sphalerite) and FeS (pyrrhotite) as sulfided compounds and TiO2 (anatase) as the nonsulfided compound. The average conversion of the sulfided extrudates, calculated on the basis of sulfur analysis and the active metal oxides present in the fresh samples, is lower than 1 in all cases. These results suggest that the diffusional resistances may prevent the sorbent from reaching total conversion. Figure 9 shows profiles of the sorbent conversion across a sulfided extrudate diameter obtained by quantitative point SEM-EDX analysis of sulfur. It can be observed that there are not significant differences in S distribution across the particle diameter, but in Ti-containing sorbents the mean conversion was very low (0.4). According to this study, the presence of Ti to modify the zinc ferrites as hot-temperature sorbents is questionable because the stabilizing effect on ZnO also detrimentally affects sulfidation. In addition, these sorbents, especially as extrudates, are prone to create inaccessible pores as the sorbent conversion becomes high, which reduces the sorbent efficiency. These findings suggest, thus, that the addition of TiO2 to zinc ferrite based sorbents has to be studied in greater depth in order to improve the reactivity and diffusional aspects of the modified sorbents. Conclusions The addition of TiO2 or CuO to zinc ferrite based sorbents calcined at 650 °C (ZFT and ZFC) does not affect the thermal stability against reduction but markedly influences their textural properties. A stabilizing effect of Ti was observed in no-iron-containing sorbents (ZT) or calcined at higher temperature (ZFT 1100). The kinetic constants, calculated by assuming a grain model, show that the zinc content and the incorporation of copper have a positive effect on the kinetics of the sulfur retention process at high temperature. The sulfidation in the presence of reducing gas (H2) leads to lower kinetic constants due, in this case, to the reaction taking place on more reduced species of iron, besides zinc losses caused through reduction and subsequent vaporization.
Nomenclature b ) stoichiometric coefficient of reactant solid, dimensionless Cao ) bulk concentration of gaseous reactant, mol cm-3 Cso ) concentration of solid reactant, mol cm-3 dr ) true density of solid reactant, g cm-3 di ) true density of the inert, g cm-3 dp ) particle density, g cm-3 k ) intrinsic reaction rate constant, cm/s k′ ) apparent kinetic constant, s-1 K ) equilibrium constant for the reaction, dimensionless M ) molecular weight of the reactant, g mol rg ) grain radius, cm Sg ) specific surface area of the fresh particle, cm2 g-1 t ) reaction time, s t1 ) time for complete conversion, s Vp ) pore volume, cm3 g-1 wo ) weight fraction of solid reactant in the fresh sorbent, dimensionless W ) mass sample at time t, g W0 ) initial mass sample, g Wmax ) mass sample at complete conversion, g X ) extent of reaction, dimensionless Greek Letters ) particle porosity
Literature Cited Cilleruelo, C.; Garcı´a, E.; Moliner, R.; Ibarra, J. V.; Pineda, M.; Palacios, J. M. Hot gas desulfurization using zinc-ferrite regenerable sorbents. In Coal Science; Pajares, J. A., Tasco´n, J. M. D., Eds.; Elsevier: Amsterdam, 1995; Vol. II, pp 1883-1886. Flytzani-Stephanopoulos, M.; Sarofim, A. F. Hot gas desulfurization by zinc oxide-titanium dioxide regenerable sorbents. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1990, 35 (1), 7785. Focht, G. D.; Ranade, P. V.; Harrison, D. P. High temperature desulfurization using zinc ferrite reduction and sulfidation kinetics. Chem. Eng. Sci. 1988, 43, 3005-3013. Gupta, R.; Gangwal, S. K. Long-term testing of zinc titanate for desulfurization of hot coal gas in a fluidized-bed reactor. AIChE Annual Meeting, Symposium on Gas Purification, St. Louis, MO, Nov 1993; Paper 130c. Gupta, R.; Gangwal, S. K.; Jain, S. C. Development of zinc ferrite sorbents for desulfurization of hot coal gas in fluid bed reactor. Energy Fuels 1992, 6, 21-27. Levenspiel, O. Chemical Reaction Engineering; Reverte: Barcelona, 1988. Lew, S.; Jothimurugesan, K.; Flytzani-Stephanopoulos, M. Hightemperature H2S removal from fuel gases by regenerable zinc oxide-titanium dioxide sorbents. Ind. Eng. Chem. Res. 1989, 28, 535-541. Lew, S.; Sarofim, A. F.; Flytzani-Stephanopoulos, M. The reduction of zinc titanate and zinc oxide solids. Chem. Eng. Sci. 1992, 47, 1421-1431.
Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 853 Mojtahedi, W.; Salo, K.; Abbasian, J. Desulfurization of hot coal gas in fluidized bed with regenerable zinc titanate sorbents. Fuel Process. Technol. 1994, 37, 53-65. Pineda, M.; Fierro, J. L. G.; Palacios, J. M.; Cilleruelo, C.; Ibarra, J. V. Kinetic behavior and reactivity of zinc ferrites for hot gas desulfurization. J. Mater. Sci. 1995, 30, 6171-6178. Szekely, J.; Evans, J. W.; Sohn, H. Y. Gas solid reactions; Academic Press: New York, 1976. Tamhankar, S. S.; Hasatani, M.; Wen, C. Y. Kinetic studies on the reactions involved in the hot gas desulfurization using a regenerable iron oxide sorbent. Chem. Eng. Sci. 1981, 36, 1181-1191. Tamhankar, S.; Bagajewicz, M.; Gavalas, G.; Flytzani-Stephanopoulos, M. Mixed-oxide sorbents for high-tempoerature removal of hydrogen sulfide. Ind. Eng. Chem. Process Des. Dev. 1986, 25, 429-436.
Westmoreland, P. R.; Harrison, D. P. Evaluation of candidate solids for high-temperature desulfurization of Low-BTU gases. Environ. Sci. Technol. 1976, 10, 659-665. Woods, M. C.; Gangwal, S. K.; Harrison, D. P.; Jothimurugesan, K. Kinetics of the reactions of a zinc ferrite sorbent in hightemperature coal gas desulfurization. Ind. Eng. Chem. Res. 1991, 30, 100-107.
Received for review April 2, 1996 Revised manuscript received November 11, 1996 Accepted November 14, 1996X IE960194K X Abstract published in Advance ACS Abstracts, January 1, 1997.
