Hot Gas Clean-Up with Dolomites: Effect of Gas Composition on Sulfur Removal Activity

In this study high temperature desulfurization performance of dolomite was investigated for coal-derived gases. Experimental results indicated that dolomite was active and could be effectively used for gas desulfurization at temperatures greater than 1023K. However, due to thermodynamic equilibrium restrictions, in the presence of water vapour the H₂S concentration could not be reduced below 150-200ppmv by dolomite. Thermodynamics predicts that water vapour can adversely affect the chemical equilibrium between H₂S and calcium oxide. The presence of CO₂ in the gas stream did not have a suppressing effect on the sulfidation reactions, whereas it tended to depress the calcination of CaCO₃ to CaO. During the dolomite-based sulfur removal process, both the water-gas shift and reverse water-gas shift reactions could also occur depending on the reacting gas compositions. The Boudouard reaction could take place and proceed at 1023K depending on the CO/CO₂ ratio in the feed stream. This was dictated by the thermodynamic characteristics of the reaction. COS formation was observed due to the reactions taking place between H₂S and CO/CO₂ during H₂S removal by dolomite. Trace amounts of methyl mercaptan were also detected in the reactor effluent gases during H₂S removal. Methyl mercaptan formation might be attributed to the reactions taking place between H₂S, H₂, and CO and/or CO₂.