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

Abstract

The use of domestic high-ash coal reserves contributes to the security of energy supply, and therefore high-ash coal is expected to remain as a key energy source in several countries (e.g., India, Turkey) for at least the next 30–40years. However, the use of high-ash coals for energy production (currently performed mainly via combustion processes) poses a number of technical and economic challenges, e.g., low efficiency and environmental issues. Gasification is an attractive option, since it allows a more efficient, more environmentally friendly conversion of the coal. In particular, integrated gasification combined cycle (IGCC) offers high efficiency, reduced emissions and potential for the implementation of CO2 capture.With the aim of optimising the design and operation of high-ash coal fluidised-bed gasification processes, this paper studies the effect of temperature and partial pressure on the conversion and reactivity of coke from an Indian high-ash coal under CO2 and steam gasification conditions using thermogravimetric analysis. Moreover, additional steam gasification tests have been carried out in order to determine the conversion rate under realistic fluidised-bed gasification conditions (e.g., coal ash as bed material, coal particle size, heating rate, bed hydrodynamics, gasification atmosphere), thus taking into account the effect of mass and heat transfer phenomena.Results of isothermal TGA tests have shown that coke reactivity increases at higher temperatures and/or partial pressures of gasifying agent. The experimental data have been fitted to two conversion models (shrinking core and volumetric). The determination of kinetic parameters (reaction order b, pre-exponential factor A and activation energy Ea) has been carried out at three conversion levels: X=0.2, X=0.5, and X=0.8. In the case of CO2 gasification, the reaction order b ranges between 0.2 and 0.8, although at temperatures of 850–900°C, the reaction order has a value around 0.6. In the case of steam gasification, the reaction order ranges between 0 and 1.1, and increases with reaction temperature.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 25min of steam gasification. Finally, 55–80% of the carbon in the coke remaining after steam gasification is converted during the first 25min 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.