CURRENT RESEARCH Evaluation of Candidate Solids for High-Temperature Desulfurization of Low-Btu Gases Phillip R. Westmoreland‘ and Douglas P. Harrison* Departmentof Chemical Engineering,Louisiana State University, Baton Rouge, La. 70803
High-temperature processes for desulfurization of low-Btu gases are receiving increased attention. In this study, results of thermodynamic screening of the high-temperature desulfurization potential of 28 solids, primarily metal oxides, are reported. By use of the free energy minimization method, equilibrium sulfur removal and solid compound stability were determined a t temperatures to 1500 O C. Eleven candidate solids based upon the metals Fe, Zn, Mo, Mn, V, Ca, Si-, Ba, Co, Cu, and W show thermodynamic feasibility for hightemperature desulfurization of low-Btu gas. Commercial processes for the removal of H2S from gases ( I ) involve wet scrubbing and, consequently, operate near 250 O F . High-temperature H2S removal capability is needed if low-Btu coal gasification is to be a viable alternative to oil imports. The reactivity of certain metal oxides with H2S has long been known. The Appleby-Frodingham process ( 2 ) utilized ferric oxide at 400 OC for coke oven gas desulfurization. ZnO at 400 OC is used to desulfurize hydrocarbon feedstock for ammonia synthesis ( 3 ) .Currently, the use of limestone and dolomite for high-temperature desulfurization is being studied (4,5). The literature contains these and numerous other reports of reaction between H2S and metal oxides. In this study, a systematic thermodynamic screening, using the method of free energy minimization, was performed to determine which inorganic solids have desulfurization potential and to define the limits of the potential application.
High-Temperature Desulfurization Criteria Generically, the process of low-Btu coal gasification can be represented by the combination of coal, air, and steam t o produce gaseous products rich in CO, H2, and N2 with smaller quantities of steam and CO2. Sulfur in the coal will be converted predominantly to H2S. In a high-temperature desulfurization process the gasifier product will contact the solid reactant with which the sulfur species will react to form solid sulfur compounds. Thermodynamic criteria have been established with respect to fractional desulfurization and solid stability. High fractional desulfurization is an obvious requirement. For this general study, we have taken 95% equilibrium desulfurization to be the minimum acceptable although the exact requirement will depend upon turbine specifications, coal composition, and local environmental regulations. The stability criterion ensures that unreacted solid is stable a t the conditions of interest. For example, in a low-Btu gas atmosphere, lead oxide will be reduced to metallic lead, whose melting point (327 “C) is too low to be of interest. ConsePresent address, Oak Ridge National Laboratory, Oak Ridge, Tenn.
quently, all oxides subject to reduction to low-melting metals were deemed unsatisfactory. Finally, the temperature range to be considered must be specified. While desulfurization at the gasifier exit temperature is preferable, properly designed heat recovery systems will allow certain temperature adjustments without appreciable efficiency losses. After studying the processes described by Robson et al. (6), we have concluded that desulfurization from 400-1200 “C is of potential interest. In this study, a single coal composition and gasifier feed mixture are reported. The elemental analysis of each is found in Table I. The fuel is typical of much of the U S . high-sulfur coal. Gasifier feed analysis corresponds to 59% of the oxygen required for total combustion and a 0.36 molar ratio of steam to air, conditions which are representative of the “third generation” processes described by Robson et al. (6). Only elemental analyses are tabulated as this information alone is necessary for free energy minimization. Chemical composition of the gasifier effluent may be read from Figure 2 of Reference 7. Characteristics of other fuels and gasification mixtures reported by Stinnett et al. (7) were also examined (8).Differences were found to be minor so that the results reported are applicable to a broad range of low-Btu gas compositions.
Thermodynamic Analysis Oxides of the 28 elements listed in Table I1 were subjected to detailed thermodynamic analysis. These elements were ~~~
Table 1. Elemental Analysis of Fossil Fuel and Gasifier Feed Mixture Element
Fuel analysis, mass %
Gaslfler leed analysisa, atomic yo
5.3 68.5 8.5 1.4 4.1 12.2
30.1 15.8 18.6 35.1 0.4
Hydrogen Carbon Oxygen Nitrogen Sulfur Ash a
-
Ash-free basis.
Table 11. Elements Whose Oxides Were Considered in Detailed Thermodynamic Analysis 1. 2. 3. 4.
5. 6. 7. 8. 9. 10.
Lithium Sodium Magnesium Aluminum Potassium Calcium Titanium Vanadium Chromium Manganese
11. 12. 13. 14.
15. 16. 17. 18. 19.
Iron Cobalt Nickel Copper Zinc Strontium Zirconium Molybdenum Silver
20. 21. 22. 23. 24. 25. 26. 27. 28.
Cadmium Tin Antimony Barium Tungsten Lead Bismuth Lanthanum Cerium
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Table 111. Manganese Compounds Considered in System Mn-C-H-N-0-S 1. Mn(s) 7. Mnp03(s) 13. MnS04(s) 2. Mn(l) 8. a-Mn304(s) 14. Mn4N(s)a 3. Mn(g) 4. MnO(s) 5. MnO(g)a
9. /?-Mn304(s)
15. MnC03(s)
MnS(s) 16. a-Mn3C(s) MnS(1) 17. @-Mn3C(s) 6. MnOp(s) 12. MnS&Ja a Estimated thermodynamic data were employed for these compounds. 10. 11.
selected after a preliminary screening of the periodic table eliminated nonmetals, radioactive and extremely toxic elements, prohibitively expensive elements, and elements for which sufficient thermodynamic data were unavailable. The general principles of free energy minimization are discussed by van Zeggeren and Story ( 9 ) ,while the computational procedure used in this study is described by Westmoreland (8).All gases are assumed ideal while condensed species are assumed to form pure phases. The analysis requires free energy-temperature data for all possible equilibrium species. Possible gaseous species were previously defined by Stinnett et al. (7). In this study, extension of the free energy data base to include solid, liquid, and vapor species associated with the desulfurizing metals was necessary. Altogether, the free energy-temperature functions of 434 species were utilized. As an example, the possible equilibrium compounds considered in the manganese system are listed in Table 111. Similar data sets were prepared for the other 27 elements. Thermodynamic results should always be qualified to reflect the availability and reliability of thermodynamic data. Data for the 434 compounds were compiled from a number of sources (10-14). In certain cases, a portion of the data was estimated; manganese compounds which utilized estimated data are indicated in Table 111. An additional qualification regarding the accuracy of the equilibrium treatment, particularly at low temperature, is in order. Meeting the established thermodynamic criteria should be considered a necessary but not sufficient condition for establishing desulfurization feasibility. Kinetic experiments should follow the thermodynamic screening.
Fractional desulfurization results are summarized in Figures 1 and 2. From Figure 1, it is obvious that manganese, for example, satisfies the 95% desulfurization criterion at all temperatures below 1060 "C. On the other hand, calcium satisfies this requirement only a t temperatures above 770 "C. Figure 3 summarizes solid stability results. As an example, consider barium. At low temperature, BaC03 is the stable solid form. Sulfiding of the carbonate begins a t approximately 800 "C, and, from that point to the 1200 "C melting temperature of Bas, simultaneous existence of solids BaS and BaC03 is predicted. By properly combining information from Figures 1-3, the desulfurization potential of each candidate can be established. These results are summarized in the following paragraphs. Barium: Sulfiding of BaC03 begins near 800 "C and reaches the required 95%sulfur removal near 900 OC. From 900 to 1200 "C, the melting point of Bas, all thermodynamic criteria are satisfied. This temperature range is higher than needed using current gasification technology. The behavior of barium and
\ 400
T E M P E R A T U R E , 'C Figure 1. Desulfurization potential of candidate solids
680
Environmental Science & Technology
z 0 !a N
I C
a
3 LL _I
3
ul
;98 v)
a
0 J
a
z
Gc
0
a
n 96 LL
I 3 -
a
m zi
3 O
w
94
TEMPERATURE,
Figure 2.
I
1200
Thermodynamic Results Desulfurization potential was analyzed by minimizing the 'free energy of a C-H-N-0-S elemental system defined by Table I in the presence of an excess of the metal oxide being evaluated. In this manner, fractional sulfur removal could be determined simultaneously with the stable state of the excess metal. Pressure was fixed a t 20 atm while temperature was varied between 360 and 1560 "C. Of the 17 of 28 elements judged unsatisfactory, 13 were rejected because of inadequate desulfurization. Of these, six-Al, Ce, Cr, Mg, Ti, and Zr-formed stable, unreactive oxides (or carbonates) throughout the temperature range. Similarly, the alkali metals were rejected because the stable solids-LizCO3, NazC03, and KzCO3-were unreactive. It should be noted that initial sulfiding of the alkali metals was predicted near the melting temperature of the carbonate. These cases were rejected under the solid stability criterion. Four additional elements-Ag, La, Ni, and Sb-were rejected because of inadequate desulfurization, although sulfiding may occur below the minimum temperature of interest. Finally, four elements-Bi, Cd, Pb, and Sn-were rejected because excess metal oxide could be reduced to a low-melting metal. The remaining 11 candidates satisfied thermodynamic criteria over at least a portion of the temperature range.
I
800
"C
Desulfurization potential of candidate solids
BARIUM
CALCIUM
COBALT
COPPER
IRON
MANGANESE
MOLYBDENUM
STRONTIUM
TUNGSTEN
VAN A Dl UM
ZINC 400
600
800
1000
1200
I400
TEMPERATURE, 'C
Figure 3.
Stable solid phases
calcium are similar but calcium provides a wider applicable temperature range and is less expensive. Calcium: Sulfiding of CaC03 begins near 600 "C and reaches the required 95% desulfurization just below 800 "C. Maximum desulfurization capacity occurs near 880 "C, the temperature a t which CaC03 decomposes to CaO. Note that the CONSOL process ( 5 ) being developed reacts half-calcined dolomite, CaCOs-MgO,at 900 "C and 15 atm to form Cas-MgO. Perhaps the major drawback to calcium is that use is restricted to temperatures above 800 OC. Lower temperature desulfurization is suitable for many applications without loss of efficiency. Further, materials of construction problems would be reduced a t lower temperatures. Cobalt: Cobalt satisfies desulfurization criterion to a maximum temperature of 600 "C with COS the sulfided product. In the reducing atmosphere of coal gas, excess cobalt would be present as the metal at temperatures in excess of 300 "C. Copper: The behavior of copper and cobalt is similar, although copper maintains 95% desulfurization capability to a temperature in excess of 900 "C. In the reducing atmosphere, excess copper would be present in metallic form over the entire temperature range. Iron: Iron is a suitable desulfurizing material a t temperatures up to 700 "C. At these temperatures, Fe304 is the stable form of excess iron. The rapid decrease in fractional desulfurization near 700 "C corresponds to Fe304 reduction to FeO. Manganese: Oxide stability and high fractional desulfurization are predicted to temperatures in excess of 1000 "C. Below 400 "C, MnC03 is the stable solid, while above 400 "C, MnO is stable. Importantly, manganese shows desulfurization
potential in the temperature range of 600-700 "C where metal oxides currently known to be reactive with H2S are unsatisfactory. Molybdenum: Molybdenum exhibits satisfactory desulfurization to a temperature in excess of 800 OC. The temperature at which fractional desulfurization drops below 95% corresponds closely to the temperature at which MoOz is reduced to the metal. Strontium: The behavior of strontium, barium, and calcium is similar. At low temperature, SrC03 is stable and sulfiding does not begin until 800 "C. By 900 "C desulfurization has reached the 959.0 level and maximum desulfurization occurs near 1100 "C, where SrC03 decomposes into the oxide. Calcium would be preferred to strontium because of the wider range of operating temperature. Tungsten: The behavior is similar to molybdenum with desulfurization dropping below 95% near 1000 "C. Excess tungsten may exist in several oxidation states. Wo3 is stable to 550 "C where reduction to W02 occurs. Near 600 "C the WOz is converted to WC. Vanadium: In the reducing atmosphere, V2O3 is the stable form of the excess metal. Essentially 100% desulfurization, with VzS3 as the sulfided product, is predicted UQ to the melting temperature of V2S3 near 650 "C. Zinc: On the basis of fractional desulfurization, zinc is acceptable to 1150 "C with ZnS as the sulfided form and ZnO as the stable form of excess zinc. However, zinc is limited to a maximum temperature of approximately 700 "C because of the formation of zinc vapor. Experimental observations in this laboratory have confirmed the formation of zinc vapor in similar atmospheres at temperatures in excess of 700 OC. L i t e r a t u r e Cited (1) Hydrocarbon Process., NG/LNG/SNG Handbook, April 1973.
(21 Bureau, A. C., Olden, M. J. F., T h e Chem. Eng., 49,55 (1967). (3) Phillipson, J. J., "Desulfurization", in Catalyst Handbook, Wolf Scientific Book, London, 1970. (4) Ruth, L. A,, Squires, A. M., Graff, R. A., Enuiron. Sci. Technol., 6, 1009 (1972). (5) Curran, G. P., Clancey, J . T.,Pasek, B., Pell, M., Rutledge, G. D., Gorin, E. P., Consolidation Coal Co. Final Report, EPA Contract NO.EHSD 71-15.1973. (6) Robson, F. L., Giramonti, A. J., Lewis, G. P., Gruber, G., Final Report CPA-22-69-114, National Air Pollution Control Administration, 1970. (7) Stinnett, S. J., Harrison, D. P., Pike, R. W., Enuiron. Sci. Technol., 8,441 (1974). (8) Westmoreland, P. R., "Desulfurization of Gasified Coal: Evaluation of Candidate High-Temperature Sulfur Acceptors", M.S. Thesis, Louisiana State University, Baton Rouge, La., 1974. (9) Van Zeggeren, F., Storey, S.H., "The Computation of Chemical Equilibria", Cambridge Press, London, 1970. (10) Kelley, K. K., U.S. Bur. Mines, Bull. 584 (1960). (11) Stull, D., et al., "JANAF Thermochemical Tables", PB-168-370, 1965; PB-168-370-1,1966;NSRDS-NBS-27,1971 (2d ed.). (12) Mah, A. D., U.S. Bur. Mines, Rep. Znuest., 5600 (1960). (13) Rau, H., et al., J . Chem. Thermodyn., 5,833 (1973). (14) Parsons, T., et al., NAPCA Contract PH-86-68-68, Final Rep., July 1969. Receiued for reuieu: May 19,1975. Accepted January 21,1976. T h e high-temperature gas desulfurization analysis was supported by the Enuironmental Protection Agency (Grant R802036). Original development of free energy minimization capability was supported by the National Aeronautics and Space Administration (Grant N G R 19-001-059).
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7, July 1976
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