Reactions of Calcium and Magnesium Materials in High-Temperature Desulfurization of Flue and Fuel Gases
Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Prague
Basic aspects are investigated of the use of limestone and dolomite in removing SO2 from flue gas and H2S from coal gas at high temperatures. While CaO is capable of removing essentially all SO2 from the flue gas, this is not the case for removal of H2S with CaO from coal gas because of equilibrium constraints under practical conditions. MgO is effective in sorbing SO2 at much lower temperatures than CaO. Unlike CaS, CaSO4 has a large molar volume and, consequently, a strong interparticle diffusion resistance develops very quickly in the course of sorption. As a result, the initially fast reaction rapidly slows down and only incomplete conversions of CaO to CaSO4 are attained. The sulfidation reaction of CaO is also rapid but, in contrast to sulfation, high conversions to CaS can be obtained. In contrast to CaS, CaSO4 is a stable and quite inert compound. The solid formed in the sulfidation must be stabilized before disposing, usually by oxidizing to CaSO4, or regenerated. Neither of these processes is straightforward.
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