(146f) Sulphur Dioxide Capture with Dolomites and Coal Ash in Fluidised Bed Coal Combustion

The capture of sulphur dioxide with dolomites and calcium-containing coal ashes in the temperatures range 750 C to 950 C and at atmospheric and elevated pressures with varying concentrations of carbon dioxide was investigated. This study was undertaken to get a better understanding of reaction kinetics of the simultaneous removal of sulphur dioxide during the combustion of poor quality coal discards in fluidized beds (FBC). Experimentation was carried out with a thermogravimetric analyser with typical gas mixtures consisting of 2500 ppm sulphur dioxide and carbon dioxide concentrations varying between 8% and 25% (mole). At atmospheric pressure with low partial pressures of carbon dioxide and temperatures above 850 C, calcium oxide is the active sorbent with kinetics described adequately with the shrinking core model incorporating a varying effective diffusion coefficient accounting for the structural changes. At high partial pressures of carbon dioxide and at low temperatures calcium carbonate is the active sorbent and in general has a lower overall capture capacity than the calcium oxide. The conversion of the calcium carbonate present in the coal ashes (approx. 10%) was found to be very similar to that of the dolomites with a high concentration of calcium carbonate. The shrinking core model with a varying effective diffusion co-efficient was found to be valid, but with parameters much different to results with calcium oxide.