Reactions of Calcium and Magnesium Materials in High-Temperature Desulfurization of Flue and Fuel Gases

Abstract

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.