Study of Calcium-Based Sorbents for High-Temperature H2S Removal. 3. Comparison of Calcium-Based Sorbents for Coal Gas Desulfurization

Abstract

H{sub 2}S sorption by 18--35 mesh particles (average mass radius of 0.40 mm) of three different calcium-based sorbents (limestone, CaCO{sub 3}; dolomitic limestone, [MgCO{sub 3}-CaCO{sub 3}]{sub 1}[CaCO{sub 3}]{sub 3}; dolomite, MgCO{sub 3}-CaCO{sub 3}) was tested under simulated coal gas in a differential tube reactor. Two fundamentally different behaviors were observed. Above the calcination temperature of CaCO{sub 3}, complete conversion of CaCO{sub 3} to CaS can be achieved with all three sorbents; the reaction rate increases as the magnesium-to-calcium ratio increases in the sorbent and the reaction rate is controlled by the diffusion of H{sub 2}S through the CaS product layer and by the kinetics of the calcination of CaCO{sub 3} to CaO. However, below the calcination temperature of CaCO{sub 3} (about 900 C under 1 bar of CO{sub 2}), less than 20% of the CaCO{sub 3} in limestone can be converted to CaS compared to 100% in dolomite. For the dolomitic limestone, all the calcium atoms associated with the dolomite regions can be converted to CaS whereas only 20% of those associated with the limestone regions can be converted. Above 710 C, the sulfidation rate of dolomite and dolomitic limestone is controlled by the diffusion of H{sub 2}S through themore » product layer. Below 710 C, the kinetics of calcination of MgCO{sub 3} as well as the rate of the chemical reaction between CaCO{sub 3} and H{sub 2}S become the limiting steps in the overall reaction kinetics.« less