A thermodynamic study of dolomite as hydrogen sulphide adsorbent when pyrolysing or partially gasifying coal

Abstract

Hydrogen sulphide absorption on dolomite when pyrolysing or partially gasifying coal is discussed with respect to thermodynamics. It is assumed that the absorption equilibrium over half-calcined dolomite sets the hydrogen sulphide concentration and that the gas phase composition is determined by the equilibria of the water-gas shift and the Boudouard reactions. When partially gasifying coal, temperature, pressure, air potential, moisture in and ultimate composition of the coal are the only parameters to vary in practice. Given the assumptions, theoretical minimum equilibrium hydrogen sulphide concentrations are derived as functions of these parameters. In the computations, the carbon content in the gas phase is accounted for indirectly, as it cannot be directly controlled. Results indicate that theoretical minimum hydrogen sulphide (i.e. sulphur) concentrations are influenced mainly by temperature, pressure and moisture content in the coal. An increase in temperature will enhance the absorption potential, whilst a pressure increase has the opposite effect. The theoretical minimum hydrogen sulphide concentrations were found to be 10–20 mg S MJ −1, depending on the parameter values chosen. The reason for the existence of a minimum is that above temperatures of around 800 °C, an increase in temperature has to be coupled to an increase in total pressure to prevent the decomposition of the half-calcined dolomite.