High-temperature reaction between sulphur dioxide and limestone-V. The effect of periodically changing oxidizing and reducing conditions

Sulphur capture by limestone has been studied in a laboratory reactor developed to simulate the periodically changing oxidizing and reducing conditions experienced by limestone particles in a fluidized-bed combustor. Under oxidizing conditions, sulphur is captures ad CaSO₄. Under reducing conditions, and in the presence of CO, sulphur is captured as CaS. Transformation of CaSO₄ to CaS and vice versa appears to proceed via CaO. Substituting CO with H₂ reducing agent causes an increase in the rate of reductive decomposition of CaSO₄, and no formation of CaS is observed. Using CH₄ neither reductive decomposition of CaSO₄ nor formation of CaS is observed. The sulphur capacity of 14 European limestones was studied under constant oxidizing as well as under alternating oxidizing and reducing conditions. The relative ranking of the limestones appears to be little influenced by the reaction conditions. Generally, a slight reduction in the sulphur capacity is observed under alternating conditions. The exceptions are limestones with a high content of Fe₂O₃, which lowers the sulphur capacity significantly, presumably due to reduced stability of the sulphated limestone under reducing conditions. Rates of CaS formation and of reductive decomposition of CaSO₄ differ greatly for different limestones. Reduction of particle size increases the SO₂ release due to CaS oxidation but decreases the release of SO₂ due to reductive decomposition. Both CaS oxidation and reductive decomposition of CaSO₄ may lead to a diminished degree of desulphurization in real combustors. A temperature optimum observed for desulphurization in fluidized-bed combustors appears to be caused primarily by the competition between sulphur capture and sulphur release, the latter of which becomes increasingly important at high temperatures.