Reduction of Sulfur Dioxide by Methane to Elemental Sulfur over

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Ind. Eng. Chem. Res. 1997, 36, 2128-2133

Reduction of Sulfur Dioxide by Methane to Elemental Sulfur over Supported Cobalt Catalysts Jing-Jiang Yu,† Qiquan Yu,‡ Yun Jin,‡ and Shih-Ger Chang* Energy and Environment Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720

Cobalt oxides supported on several types of carriers (silica, molecular sieves 5A and 13X, and γ-Al2O3) were evaluated for the reduction of sulfur dioxide by methane to elemental sulfur. Results showed that γ-Al2O3 is the most effective carrier and that, with a molar ratio of 2 to 1 of sulfur dioxide to methane in a feed gas, the sulfur yield reached a maximum value of 87.5% at a space velocity of 5000 h-1 and a temperature of 840 °C. X-ray diffraction results revealed that mixtures of cobalt oxide and cobalt sulfide components were observed. The effects of temperature, space velocity, and molar ratio of sulfur dioxide to methane in the feed on the activity of the cobalt catalyst supported on γ-Al2O3 were investigated. The effect of feed gas containing a hydrogen sulfide contaminant was also studied. A catalyst size-dependence study was performed to determine the influence of the internal diffusion. Introduction Sulfur dioxide in the atmosphere can cause acid rain. Processes have been developed to remove sulfur dioxide from the flue gas of power plants. Most of these processes capture sulfur dioxide and then convert it into waste material, which includes calcium sulfite and calcium sulfate. These waste materials require disposal. Because of the large amount of sulfur dioxide that is produced from power plants, the disposal of these solid wastes represents another environmental problem. There are several sulfur dioxide regenerable processes; some of them have been commercialized, while others are emerging. In these regenerable processes, sulfur dioxide from flue gas is first absorbed in an alkaline solution or adsorbed on a solid substrate and is subsequently desorbed to produce a stream of high-concentration sulfur dioxide. It is desirable to convert sulfur dioxide to elemental sulfur, which is very often the byproduct of choice. Sulfur dioxide can be reduced with methane to produce elemental sulfur at elevated temperatures according to a reaction with a stoichiometric molar ratio of 2 to 1 of SO2 to CH4:

2SO2 + CH4 f S2 + CO2 + 2H2O

(1)

This reaction was first studied by Young in 1915 (Young, 1915). In addition to the production of elemental sulfur, this reaction may produce several undesirable byproducts, such as hydrogen sulfide, carbonyl sulfide, carbon monoxide, and elemental carbon. A plant capable of producing 5 tons of elemental sulfur daily via the reduction of SO2 by natural gas was in operation in 1940 (Fleming and Fitt, 1950). This plant used a scheme that included processing steps to treat H2S and COS byproducts, in addition to a catalytic reactor for SO2 reduction. The requirement for treating the large amount of byproducts made this process economically unattractive. A different process was later developed which involved two steps: (1) reduction of a part of SO2 to H2S by * Corresponding author. † Present address: Beijing University of Chemical Technology. ‡ On leave from the Chemistry Department, Peking University, Beijing, China. S0888-5885(95)00575-6 CCC: $14.00

methane and/or low-value hydrocarbons; (2) H2S and remaining SO2 conversion to elemental sulfur in a multistage Clauss unit (Bierbower and Vansciver, 1974). Numerous research efforts have been carried out to develop a catalyst capable of achieving high-efficiency sulfur recovery. In such a way, the entire process for the conversion of SO2 to elemental sulfur can be accomplished in only one step. The catalytic effect of Al2O3 (Sarlis and Berk, 1988; Bobrin et al., 1989a), Bauxite (Helstrom and Atwood, 1978), Cr2O3/Al2O3 (Akhmedov et al., 1986), Co-Mo/Al2O3 (Mulligan and Berk, 1992), Fe-Cu-Cr (Nekrich et al., 1978), La2O3 (Bobrin et al., 1989b), and FeS/MoS2/WS2 (Mulligan and Berk, 1989) on SO2 reduction by CH4 has been studied previously. This paper addresses the performance of a cobalt catalyst in terms of the effect of carriers, H2S contaminant, catalyst sizes, and operating conditions. The phase transitions resulting from the catalytic reactions are also reported. Experimental Section A small fixed-bed quartz tubular catalytic reactor was used. The tubular reactor was fabricated from a 1.4cm-o.d., 1 mm wall thickness quartz tube. The entire reactor was mounted inside a tubular furnace. The reactor, which was 5 cm long, consisted of three zones. The inlet or the preheating zone (2.5 cm long) was packed with 20 mesh quartz chips, the reaction zone (1.5 cm long) was packed with 30-50 sieve-activated alumina catalyst particles, and the outlet zone (1 cm long) was packed with quartz chips (20 mesh), mainly for the purpose of supporting the catalyst, which sat on a perforated quartz plate with seven holes for the gas to exit. A thermocouple, reaching the center of the catalytic packing, provided measurements of the temperature of the catalytic reactions. The inlet and exit gases were analyzed by a gas chromatograph equipped with a thermoconductivity detector. A 2-m Porapak Q column (at 100 °C) was employed. The carrier gas, helium, flowed at a rate of 100 mL/min. Under these conditions, all of the major species involved (CH4, SO2, CO2, H2S, and COS) can be separated and determined quantitatively. The sulfur yield was determined by subtracting sulfur in H2S and COS produced from that in the initial SO2. This method of sulfur determination is considered satisfactory because more than 90% of © 1997 American Chemical Society

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Figure 1. X-ray diffraction pattern of Co3O4/γ-Al2O3 catalyst prior to reaction. ] marked by the spectrometer denotes the differentiation between signals and noises/base line.

Figure 2. X-ray diffraction pattern of Co3O4/γ-Al2O3 catalyst after reaction.

sulfur can be accounted for in a mass balance experiment where sulfur was collected and measured by weighing. The mass balance of carbon has been checked satisfactorily by measuring CH4, CO2, and COS. Catalysts were prepared by impregnating the carriers with solutions of cobalt nitrates, Co(NO3)2. A total of 8.73 g of Co(NO3)2‚6H2O (Aldrich, 98%), equivalent to 3 g of Co3O4, was dissolved in 30 mL of deionized water, to which was subsequently added 10 g of γ-Al2O3 (3050 mesh). The mixtures were stirred gently, to ensure a uniform impregnation, while evaporated at below 100 °C until dry. The solid mixtures were then heated to evaporate the crystalline water at 200 °C for 0.5 h, which were further heated to 400 °C for 0.5 h to form oxides by decomposition of nitrates. The catalysts were then activated at 820 °C for 4 h. The total weight of the catalysts was 12.9 g, corresponding to a ratio of active catalyst to carrier of about 0.3 by weight. Carriers evaluated included silica, molecular sieves 5A and

13X, and γ-Al2O3. The surface area of catalysts was measured by a Chromsorb apparatus. Results and Discussion 1. Characterization of the Catalysts. The surface area was 100 m2/g for Co3O4/γ-Al2O3 and 380 m2/g for Co3O4/SiO2. The X-ray diffractions of Co3O4/γ-Al2O3 before and after the reactions are depicted in Figures 1 and 2, respectively. The results indicated that Co3O4 (2θ 36.8, 65.3, 31.2) and γ-Al2O3 (2θ 45.7, 66.6, 36.7) (A. S. T. M. Powder Diffraction File No. 9-418; 10-425) were the only species detected before the reaction, while β-CoS1.035 (2θ 46.3, 54.9, 30.9), CoS2 (2θ 32.3, 36.8, 54.9), Co3O4, and γ-Al2O3 (A. S. T. M. Powder Diffraction File No. 25-1081; 41-1471;9-418; 10-425) were observed after the reaction. 2. The Effect of Carriers. The catalysts commonly employed for the reduction of SO2 by CH4 are of the

2130 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997

Figure 3. Effect of carriers on the conversion efficiency of SO2 as a function of temperature. Catalyst wt ) 1 g; S.V. ) 5000 h-1; SO2/CH4 ) 2.0; FCH4 ) 1333 mL/h; FSO2 ) 2667 mL/h. 0, Al2O3; ], SiO2; O, 13X; 4, 5A.

Figure 4. Effect of carriers on the yield of H2S (YH2S) as a function of temperature. Catalyst wt ) 1 g; S.V. ) 5000 h-1; SO2/CH4 ) 2.0; FCH4 ) 1333 mL/h; FSO2 ) 2667 mL/h. 0, Al2O3 %; ], SiO2 %; O, 13X %; 4, 5A %.

supported type (Akhmedov et al., 1986; Mulligan and Berk, 1992), and the reduction occurs according to reaction (1). The carriers employed in this study include silica gel, 5A molecular sieve, 13X molecular sieve and γ-Al2O3. The effects of carriers on SO2 conversion efficiency (C), H2S yield (YH2S), COS yield (YCOS), S2 yield (YS2), and product selectivity (SS2) have been investigated. The experimental results are shown in Figures 3-7, respectively. The SO2 conversion efficiency as a function of temperature for the cobalt catalyst supported on various carriers is shown in Figure 3. The results indicated that the SO2 conversion efficiency exhibited the following orders in the temperature ranges between 740 and 820 °C:

Figure 5. Effect of carriers on the yield of COS (YCOS) as a function of temperature. Catalyst wt ) 1 g; S.V. ) 5000 h-1; SO2/ CH4 ) 2.0; FCH4 ) 1333 mL/h; FSO2 ) 2667 mL/h. 0, Al2O3; ], SiO2; O, 13X; 4, 5A.

Figure 6. Effect of carriers on the yield of elemental sulfur (YS2) as a function of temperature. Catalyst wt ) 1 g; S.V. ) 5000 h-1; SO2/CH4 ) 2.0; FCH4 ) 1333 mL/h; FSO2 ) 2667 mL/h. 0, Al2O3; ], SiO2; O, 13X; 4, 5A.

γ-Al2O3 > SiO2 > 13X molecular sieve > 5A molecular sieve The SO2 conversion efficiency of the catalyst supported on γ-Al2O3 was significantly higher than that on the molecular sieves. In the temperature range of 700-820 °C, the C of the catalyst supported on γ-Al2O3, SiO2, 13X, and 5A increased from 54.1 to 87.9%, 18.6 to 40.9%, 22.0 to 29.8%, and 14.8 to 17.3%, respectively. Figures 4 and 5 show that the catalyst with 5A and 13X carriers did not produce byproducts, H2S and COS,

Figure 7. Effect of carriers on the sulfur selectivity (SS2) as a function of temperature. Catalyst wt ) 1 g; S.V. ) 5000 h-1; SO2/ CH4 ) 2.0; FCH4 ) 1333 mL/h; FSO2 ) 2667 mL/h. 0, SS2/Al2O3 %; ], SS2/SiO2 %; O, SS2/13X %; 4, SS2/5A %.

in the range of temperatures (700-820 °C) studied. The YH2S of the catalyst with SiO2 and γ-Al2O3 carriers increased with an increase in temperature (Figure 4). The YH2S of the catalyst with SiO2 carrier was undetect-

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Figure 8. Effect of the reaction temperature on C, YH2S, YCOS, YS2, and SS2 (carrier γ-Al2O3). Catalyst wt ) 1 g; S.V. ) 5000 h-1; SO2/CH4 ) 2.0; FCH4 ) 1333 mL/h; FSO2 ) 2667 mL/h. 0, C; ], YH2S; O, YCOS; 4, YS2; 9, SS2.

able (0%) between 700 and 780 °C; it reached 0.093 and 0.53% at 800 and 820 °C, respectively. The YH2S of the catalyst with the γ-Al2O3 carrier increased from 0.472 to 0.95% as the temperature was increased from 700 to 820 °C. The YCOS’s of the catalyst with SiO2 and γ-Al2O3 carriers at 820 °C were 1.83 and 0.4%, respectively (Figure 5). The SS2 of the catalyst with all four carriers studied was very large (>95%) below a temperature of 780 °C. However, the SS2 decreased gradually as the temperature was increased above 780 °C, which was due to an increase of H2S production along with an increase of temperature. The effect of temperature on the YS2 of the catalysts supported on various carriers is shown in Figure 6. At 820 °C, the YS2’s of the catalysts with carriers γ-Al2O3, SiO2, 13X, and 5A were 86.6, 38.5, 29.8, and 17.3%, respectively. The YS2 in the temperature range 740820 °C followed the following orders:

Co3O4/γ-Al2O3 > Co3O4/SiO2 > Co3O4/13X > Co3O4/5A It is obvious from the aforementioned results that the carrier of choice is γ-Al2O3. Figure 7 shows that all carriers achieve a selectivity of better than 94.2% elemental sulfur at temperatures between 700 and 820 °C. 3. Evaluation of the Co3O4/γ-Al2O3 Catalyst. Parametric studies on the performance of the Co3O4/γAl2O3 catalyst have been performed. Parameters studied included temperature, space velocity (S.V.), and the molar ratio (R) of SO2 to methane in the feed. The C increased rapidly with an increase of temperature between 720 and 840 °C (Figure 8). Both YH2S and YCOS exhibited a slight increase as the temperature rose above 780 °C. As a result, the selectivity of elemental sulfur decreased gradually with an increase of temperature beyond 780 °C. The sulfur yield reached 87.5% at 840 °C. The effect of space velocity is shown in Figure 9. When S.V. increased from 3750 to 10000 h-l, C decreased from 98.2 to 77.1%, YS2 from 95.0 to 76.0%, and YH2S from 3.03% to 0.198%. YCOS varied within a range of 0.145-0.827%, while SS2 increased from 96.8 to 98.7%.

Figure 9. Effect of space velocity on C, YH2S, YCOS, YS2, and SS2 (carrier γ-Al2O3). Catalyst wt ) 1 g; SO2/CH4 ) 2.0; 840 °C. 0, C %; ], YH2S %; O, YCOS %; 4, YS2 %; 9, SS2 %.

Figure 10. Effect of the molar ratio of SO2/CH4 on C, YH2S, YCOS, YS2, and SS2 (carrier γ-Al2O3). Catalyst wt ) 1 g; S.V. ) 5000 h-1; 840 °C. R ) FSO2/FCH4. Ftotal ) FSO2 + FCH4 ) 4000 mL/h. 0, C; ], YH2S; O, YCOS; 4, YS2; 9, SS2.

The effect of feed gas composition (i.e., R) on the catalyst performance is shown in Figure 10. As R was raised from 1.2 to 3.0, C decreased from 99.5 to 65.0%, YH2S from 77.5 to 1.56%, and YCOS from 8.28 to 0.781%, while SS2 increased asymptotically from 13.7 to 96.5%. These results indicate that when methane is in excess of that required (i.e., over the stoichiometric molar ratio), secondary reactions between sulfur and methane can take place to produce byproducts, H2S and COS (YH2S is larger than YCOS). Consequently, YS2 decreased with an increase of methane beyond the stoichiometric molar ratio. On the contrary, when methane was less than that required (i.e., less than the stoichiometric molar ratio), the conversion efficiency of SO2 decreased, as well as the production of elemental sulfur. At a stochiometric molar ratio of 2.0, YS2 reached a maximum value (87.5%), while C ) 91.3%, YH2S ) 2.31%, YCOS ) 1.49%, YS2 ) 87.5%, and SS2 ) 95.8%. The CH4 conversion can be calculated from the input flow rate of SO2 and CH4 (i.e., R) and the SO2 conversion efficiency. The performance as a function of temperature with a feed gas composed of slightly less methane than that required according to the stoichiometric molar ratio has been studied (Figure 11). As R shifted from 2.0 to 1.8,

2132 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 Table 1. Effect of Internal Diffusion on SO2 Reduction by CH4a

Figure 11. Effect of reaction temperature on YH2S at molar ratios R ) 2.0 and 1.8 (carrier γ-Al2O3). Catalyst wt ) 1 g; S.V. ) 5000 h-1. R ) 2: FSO2 ) 2667 mL/h; FCH4 ) 1333 mL/h. R ) 1.8: FSO2 ) 2571 mL/h; FCH4 ) 1429 mL/h. 0, YH2S/R ) 2.0; ], YH2S/R ) 1.8.

Figure 12. Effect of the addition of 10% H2S on YS2 and YH2S as a function of temperature (carrier γ-Al2O3). Catalyst wt ) 1 g; S.V. ) 5000 h-1; SO2/CH4 ) 2.4. FCH4 ) 1066.3 mL/h; FSO2 ) 2667 mL/ h; FH2S ) 266.7 mL/h. 0, YS2/R ) 2.0; ], YS2/10% H2S; O, YH2S/R ) 2.0; 4, YH2S/10% H2S.

an increase in YH2S was observed. The amount of this increase became larger with an increase in temperature. At an R of 2.0, YH2S increased slightly with temperature: from 0.338% (720 °C) to 2.31% (840 °C), while at an R of 1.8, YH2S increased abruptly from 0.717% (720 °C) to 9.79% (840 °C). However, the sulfur yield did not show a substantial change with a reduction of R from 2.0 to 1.8. YS2 at 720 °C was 60.0 and 59.9% with R of 2.0 and 1.8, respectively, while, at 840 °C, it was 87.5 and 86.9% with R of 2.0 and 1.8, respectively (Figure 9). These results compared favorably with those of other catalysts. A sulfur yield of 75% was reported (Aknmedov et al., 1986) with a catalyst of Cr2O3/γ-Al2O3 at 850 °C, while sulfur yields of 77.2 and 73.5% were obtained (Mulligan and Berk, 1992) at 725 °C with a catalyst of 15% Mo/γ-Al2O3 and (5% Co + 15% Mo)/ Al2O3, respectively. 4. The Effect of Hydrogen Sulfide. In some cases, the sulfur dioxide feed gas may be contaminated with gases such as hydrogen sulfide. Experiments were carried out to study the effect of hydrogen sulfide on the performance of the catalysts. The results (Figure 12) indicated that the hydrogen sulfide (10%) added to the feed gas could react with sulfur dioxide to form elemental sulfur and did not result in the accumulation

domain

t, °C

C, %

YH2S, %

YCOS, %

YS2, %

SS2, %

η

int dif kin int dif kin int dif kin int dif kin int dif kin int dif kin int dif kin

700 700 720 720 740 740 760 760 780 780 800 800 820 820

46.9 54.1 66.3 60.5 81.3 69.1 93.8 73.5 98.5 79.0 99.2 85.4 99.8 89.9

0.120 0.472 1.58 0.338 3.38 0.347 5.48 0.684 10.5 0.789 20.9 0.931 26.5 0.950

0.409 0 0.163 0.135 0.745 0.244 0.846 0.269 1.15 0.261 1.41 0.32 1.85 0.399

46.6 53.7 64.1 60.0 77.2 68.5 87.5 73.5 86.9 78.0 76.9 84.1 71.3 88.6

99.5 99.1 99.7 99.2 94.7 99.1 93.2 98.7 88.2 98.7 77.5 98.5 71.5 98.5

0.433 0.548 0.587 0.638 0.623 0.581 0.555

a F CH4 ) 1333 mL/h; FSO2 ) 2667 mL/h. Catalyst weight used in kinetic domain: 1 g (30-50 mesh). Catalyst weight used in internal diffusion domain: 2 g (d ) 5 mm, h ) 3 mm).

of hydrogen sulfide byproducts nor did it change the performance of the catalyst in terms of the sulfur and COS yields to any appreciable amounts. 5. Influence of Internal Diffusion. The aforementioned experiments were performed with catalysts of fine grain sizes (30-50 mesh), where the internal surfaces of the catalysts were exposed and were readily accessible to the reactants. Consequently, the catalytic reactions occurred in the kinetic domain such that the mass-transfer contribution on the reaction rates was small. In a commercial practice, however, catalysts in granular forms with diameters larger than 5 mm are often employed in order to reduce aerodynamic resistance (pressure drop). In such cases, the internal diffusion is expected to influence the kinetics of the catalytic reactions. To determine the extent of the contribution from the diffusion, the performance of a catalyst with a grain size (d ) 5 mm, h ) 3 mm) which is larger than the one used previously (30-50 mesh) was studied (Table 1). The effectiveness factor of the catalyst (η) is used as a measure of the internal diffusion and is defined as follows:

η) reaction rate measured in the internal diffusion domain ) reaction rate measured in the kinetic domain

γint (2) γkin where γint and γkin are the reaction rates measured in the internal diffusion domain and in the kinetic domain, respectively. If we assume that the catalytic reduction of SO2 by methane over Co3O4/γ-Al2O3 catalysts is first order with respect to the concentration of SO2, then the empirical reaction rate equation can be expressed as

γkin ) kkinCSO2

(3)

where kkin is the rate constant in the kinetic domain, and CSO2 is the concentration of sulfur dioxide. If the reaction order of SO2 in the internal diffusion domain remains first order, then the reaction rate equation in the internal diffusion domain can be expressed as

γint ) kintCSO2

(4)

where kint is the rate constant measured in the internal

Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2133

diffusion domain. If the reaction rate is expressed as an average rate, then

γ ) ∆C/∆τ

(5)

where τ is the contact time. When τ ) 0 and C ) 0,

γ ) C/τ ) FSO2C/Wcat

(6)

where FSO2 and C are the flow rate of sulfur dioxide feed and the sulfur dioxide conversion efficiency, respectively. Wcat is the weight of the catalyst. Assuming that C remains unchanged in both the internal diffusion domain and the kinetic domain, we can substitute eq 6 into eq 2 and obtain

η ) Wkin/Wint

(7)

where Wkin and Wint are the catalyst weights used in the kinetic domain and in the internal diffusion domain, respectively. If C is directly proportional to the catalyst weight, then the C values obtained in the internal diffusion domain can be corrected to meet the C values in the kinetic domain according to the catalyst weights employed. The η values calculated according to eq 7 are listed in Table 1, and its average value is 0.566. From eq 7 we can estimate

Wint ) 1/η ) 1/0.566 ) 1.8 g Therefore, the C values obtained from 1.8 g of coarsegrain (d ) 5 mm, h ) 3 mm) catalyst (in the internal diffusion domain) should be nearly the same as those from 1 g of fine-grain (30-50 mesh) catalyst (in the kinetic domain). The result of 1.8 g obtained based on equations derived from the first-order assumption is close to the actual amount of catalyst used, 2 g (Table 1). This implies that the first order with respect to the concentration of SO2 under both kinetic and internal diffusion domains is a reasonable assumption (eqs 3 and 4). In addition, YH2S increased more rapidly with temperature (Table 1) using coarse-grain catalysts than using fine-grain catalysts. The increasing YH2S can be attributed to the secondary reaction between the sulfur product and hydrogen which results from the increase of reaction time in the case of coarse-grain catalysts. YH2S can be decreased by increasing the space velocity or decreasing the reaction temperature. Conclusions Cobalt oxides supported on several types of carriers (silica, molecular sieves 5A and 13X, and γ-Al2O3) were evaluated for the reduction of sulfur dioxide by methane to elemental sulfur. Results showed that the yield of elemental sulfur in the temperature range 740-820 °C exhibited the following order: Co3O4/γ-Al2O3 > Co3O4/ SiO2 > Co3O4/13X > Co3O4/5A. With a stoichiometric molar ratio of sulfur dioxide to methane of 2 to 1 in a feed gas, a 87.9% sulfur dioxide conversion efficiency with a 98.5% selectivity to elemental sulfur and a 80% conversion efficiency with a 98.7% selectivity could be achieved at 820 and 780 °C, respectively. The sulfur

yield reached a maximum value of 87.5% at a space velocity of 5000 h-1 and a temperature of 840 °C. X-ray diffraction results revealed that the catalyst was composed of Co3O4 and γ-Al2O3 before the reaction and changed to structures of β-COS1.035, COS2, Co3O4, and γ-Al2O3 after the reaction. The addition of 10% hydrogen sulfide to the feed gas did not result in the accumulation of hydrogen sulfide byproduct; the added hydrogen sulfide was converted to elemental sulfur by reaction with sulfur dioxide. A catalyst size-dependent study was performed to determine the influence of internal diffusion. The reaction occurred in the kinetic domain with a catalyst having fine grain sizes (30-50 mesh), whereas the reaction performed in the internal diffusion regime has a granular catalyst of d ) 5 mm h ) 3 mm. The effectiveness factor of the granular catalyst studied was 0.566. Acknowledgment This work was supported by the Assistant Secretary for Fossil Energy, U.S. Department of Energy, under Contract DE-AC03-76SF00098 through the Pittsburgh Energy Technology Center, Pittsburgh, PA. Literature Cited Akhmedov, M. M.; Shakhtatinskii, G. B.; Agaev, A. I.; Gezalov, S. S. Study of the Process for Sulfur Dioxide Reduction with Methane over Alumina Chromium Catalyst. Zh. Prikl. Khim. (Leningrad) 1986, 59, 504-508. A. S. T. M. Powder Diffraction File No. 9-418; 10-425. A. S. T. M. Powder Diffraction File No. 25-1081; 41-1471; 9-418; 10-425. Bierbower, B. G.; Vansciver, J. H. Allied’s SO2 Reduction System. Chem. Eng. Prog. 1974. Bobrin, A. S.; Anikeev, V. I.; Yermakova, A.; Kirillov, V. A. High Temperature Reduction of SO2 by Methane at Various CH4/SO2 Ratios. React. Kinet. Catal. Lett. 1989a, 40, 363-367. Bobrin, A. S.; Anikeev, V. I.; Yermakova, A.; Zheivot, V. I.; Kirillov, V. A. Kinetic Studies of High-Temperautre Reduction of Sulfur Dioxide by Methane. React. Kinet. Catal. Lett. 1989b, 40, 357362. Fleming, E. P.; Fitt, T. C. High Purity Sulfur from Simelter GassReduction with Natural Gas. Ind. Eng. Chem. 1950, 42, 2249-2250. Helstrom, J. J.; Atwood, G. A. The Kinetics of the Reaction of Sulfur Dioxide with Methane. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 114-117. Mulligan, D. J.; Berk, D. Reduction of Sulfur Dioxide with Methane over Selected Transition Metal Sulfide. Ind. Eng. Chem. Res. 1989, 28, 926-931. Mulligan, D. J.; Berk, D. Reduction of Sulfur Dioxide over Alumina Supported Molybdenum Sulfide Catalyst. Ind. Eng. Chem. Res. 1992, 31, 119-125. Nekrich, E. M.; Kontsevaya, A. N.; Ganzha, G. F. Study of the Process for Sulfur Dioxide Reduction with Methane over Supported Oxide Catalyst. Zh. Prikl. Khim. (Leningrad) 1978, 51, 526-529. Sarlis, J.; Berk, D. Reduction of Sulfur Dioxide with Methane over Activated Alumina. Ind. Eng. Chem. Res. 1988, 27, 1951-1954. Young, S. W. The Thiogen Process for Removing Sulfur Fumes. Trans. Am. Inst. Chem. Eng. 1915, 8, 81.

Resubmitted for review February 20, 1997 Accepted February 27, 1997X IE950575I X Abstract published in Advance ACS Abstracts, April 15, 1997.