STUDY OF CALCIUM-BASED SORBENTS FOR HIGH-TEMPERATURE H2S REMOVAL .3. COMPARISON OF CALCIUM-BASED SORBENTS FOR COAL-GAS DESULFURIZATION

H2S sorption by 18-35 mesh particles (average mass radius of 0.40 mm) of three different calcium-based sorbents (limestone, CaCO3; dolomitic limestone, [MgCO3-CaCO3](1)[CaCO3](3); dolomite, MgCO3-CaCO3) was tested under simulated coal gas in a differential tube reactor. Two fundamentally different behaviors were observed. Above the calcination temperature of CaCO3, complete conversion of CaCO3 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 H2S through the CaS product layer and by the kinetics of the calcination of CaCO3 to CaO. However, below the calcination temperature of CaCO3 (about 900 degrees C under 1 bar of CO2), less than 20% of the CaCO3 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 degrees C, the sulfidation rate of dolomite and dolomitic limestone is controlled by the diffusion of H2S through the product layer. Below 710 degrees C, the kinetics of calcination of MgCO3 as well as the rate of the chemical reaction between CaCO3 and H2S become the limiting steps in the overall reaction kinetics.