Hot desulfurization of coal gas with copper
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I.
INTRODUCTION
C O A L cannot be used universally as a source of energy or as a raw material in chemistry. But this is the case if the coal is gasified. The resulting gas can be employed as a heating gas, as an energy bearer in power plants, as a synthesis gas in the chemical industry, or as a reducing gas in the metallurgical industry. Disturbing, however, is the sulfur content of the gas which is about 0.3 vol pct at a sulfur level of 1 wt pct in the coal. The gas may readily be desulfurized at low temperature with available chemical processes. However, if the gas is to be used at a high temperature directly after its generation, a hot desulfurization process may be appropriate because the energy loss occurring in cooling and reheating, and precipitation of carbon (in strongly reducing gases) can be avoided. A hot desulfurization process is particularly favorable in the new combined cycle power plants. 1.2 Another advantage of desulfurization prior to combustion is in the fact that the gas volume to be desulfurized is much smaller than in flue gas desulfurization. The main sulfur-containing molecule in coal gas is H2S. Other sulfur-bearing species are present in much smaller concentration. In a hot desulfurization process the removal of H2S from the coal gas can be performed by an oxide, a carbonate, or a metal with suitable thermodynamic properties. The reaction product is a sulfide. Since in a modern process the spent absorbent, containing the sulfide, may not be dumped, a regeneration process has to follow for converting the absorbent to its original form. Usually, the regeneration is by oxidation treatment yielding an off-gas containing SO2. The volume of the off-gas must be small and the SO2 content high so that the SO2 can be processed into sulfuric acid or elemental sulfur. Several known hot desulfurization processes are based on reactions with calcium oxide or calcium carbonate. 3.4,5In the desulfurization phase the calcium oxide or calcium carbonate react to form calcium sulfide. The chemical equilibria of these reactions are very favorable and desulfurization of coal gas is possible thermodynamically to very low H2S contents. But the problem is that the regeneration process is very complicated and the reactivity of the absorbent decreases if it is used repeatedly. 6'7's Another material employed in hot desulfurization processes is iron oxide 9'1~ or metallic iron. 12 In the desulfurization phase the iron oxide reacts to FeS. The regeneration is by roasting with air. GEORG SICK is with Leybold Heraeus, Wilhelm Rohn Str. 25, D-6450 Hanau, Germany. KLAUS SCHWERDTFEGER is with lnstitut ffir Allgemeine Metallurgie, Technische Universit~it Clausthal, Robert-Koch-Str. 42, D-3392 Clausthal-Zellerfeld, Germany. Manuscript submitted May 27, 1986. METALLURGICALTRANSACTIONS B
Several hot desulfurization processes based on copper have been proposed, t3-t6 But none of these processes is currently being developed further. The most extensive testing in large scale was performed with the Johns Hopkins process in the
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