Sulphur dioxide capture under fluidized bed combustion conditions

Abstract

An investigation was undertaken to determine the desulphurization properties of industrially available dolomites for use in fluidized bed (coal) combustion (FBC). The performance and kinetics of sulphur dioxide capture were examined at atmospheric pressure under conditions favouring the presence of calcium oxide. Experimentation was carried out with a thermo gravimetric analyzer with typical gas mixtures occurring in FBC consisting of 2500ppm sulphur dioxide and carbon dioxide concentrations varying between 8% and 25% (mole). The structural properties that are important in desulphurization reactions were determined by BET and mercury porosimetry methods and it was found that the dolomite samples consisted of a nonuniform distribution of pore sizes with porosities (*25%) similar to dolomites used by other investigators. Experimentation consisted of calcination with pure nitrogen of the raw dolomite samples, followed by reaction with different gas mixtures to assess possible recarbonation (phase transition of calcium oxide) accompanying sulphation. It was found that phase transition temperatures and carbon dioxide partial pressures for the relevant calcium - based compounds were different to predictions from thermodynamic equilibrium calculations involving pure compounds. This effect is attributed to the presence of these compounds in a mineral complex structure and the impurities present, which was also observed by other investigators. Both calcium oxide and calcium carbonate are suitable for desulphurization and in this study attention was confined essentially to the calcium oxide phase. Sulphation with calcium oxide was found to occur above 850°C with low carbon dioxide concentrations, and results were obtained which did not show any blocking of pores as a result of molar density differences. Calcium oxide conversions of the order of 10% to 15% were obtained after 120 minutes (on-line), which compared well with some results in the literature. A shrinking core model incorporating an effective diffusion coefficient accounting for the structural changes was found to be valid for most experiments.