The oxidation of calcium sulphide at the temperatures of fluidised-bed combustors

Abstract

Calcium sulphide is a major product from coal gasification processes that use limestone as a desulphurisingagent; also, it may be formed in fuel-rich regions of fluidised-bed combustors. Complete conversion of CaS to CaSO 4 via the gas-solid reaction CaS+2O 2 →CaSO 4 is desirable to allow safe disposal of the bed ash material. This reaction has been investigated at temperatures from 650°C to 1050°C using thermogravimetric analysis. The results revealed, inter alia, a discontinuity in product formation with increasing temperature: Above 850°C, CaO is also formed with the evolution of SO 2 . This has been shown to be a result of the apparent solid-solid reaction CaS+3CaSO 4 →4CaO+4SO 2 , which occurs above about 900°C, when a liquid melt is probably produced. The observation of CaO production at 850°C during CaS oxidation, however, is a consequence of the exothermicity of the reaction forming CaSO 4 raising the particle temperature above 900°C, thereby initiating the solid-solid reaction between product CaSO 4 and reactant CaS. This rise in temperature, coupled to the change in porosity caused by both CaSO 4 and CaO formation, gives rise to slightly sigmoidal mass-time plots from the thermogravimetric experiments. A kinetic study of the oxidation of CaS to CaSO 4 has shown it to be first order in O 2 , with a rate constant of 4.5±2.0×0 −4 exp[−(5.7±1.5) 10 3 / T ] m/s. The overall conclusion is that, although CaS is converted to CaSO 4 at the operating temperatures of fluidised-bed combustors, reaction between these two solids starts for particle temperatures between 850°C and 900°C, probably via a melt, and results in the undesirable evolution of SO 2 .