Conversion of high-ash coal under steam and CO2 gasification conditions

Fluidised-bed steam gasification tests have shown that approximately 23-27% of the carbon contained in the coal (~ 40-45% of the overall coal, considering also hydrogen and oxygen released in the gas) is quickly converted during the devolatilisation stage. Between 12 and 22% of the carbon contained in the remaining coke is converted to gas within the first 25 min of steam gasification. Finally, 55-80% of the carbon in the coke remaining after steam gasification is converted during the first 25 min of oxidation with air. Conversion of the high-ash coal is favoured at higher steam partial pressures and/or higher gasification temperatures. The conversion rate under fluidised-bed conditions is significantly lower than that obtained in TGA tests at similar temperature and steam partial pressure values. Differences in coal particle size, heating rate during devolatilisation, inhibition issues, and other fluid-dynamic effects influence the reactivity of the produced coke as well as the heat and mass transport rates. The overall effect of these phenomena is a shift in reaction regime. Operation under realistic FB conditions has an equivalent effect to a decrease in the gasification temperature under TGA conditions.