Gasification is a versatile technology used to convert coal into synthesis gas for use in cleaner power generation and production of high-value products. This work investigates the pyrolysis and gasification behaviour of a relatively unknown Morupule coal, from Botswana, using a wire-mesh reactor. Morupule coal was pyrolysed in a helium atmosphere and gasified in CO2 at temperatures of 400 – 1050 ºC and heating rates of 1 – 1000 ºC s-1 under pressures of up to 30 bara and holding times of up to 400 s at peak temperature.
Elevated pressures induced a suppression of the volatile release during the temperature ramp up. However, ultimate yields at 1000 ºC prove insensitive to pressure. A novel direct gasification approach with in-situ coal pyrolysis, as opposed to decoupling the pyrolysis and gasification experiments, was used to study the intrinsic CO2 gasification kinetics as would be observed in a gasifier. An activation energy of 320 kJ mol-1, higher than published data, is reported. Enhanced gasification rates were obtained at pressures of up to 20 bara, with reduced gasification lag periods previously observed under atmospheric pressure conditions. Morupule coal gasification propagated through the consumption of C-C/C=C bonds, without preference for smaller aromatics. Char morphology was characterised by a developing external surface porosity as gasification proceeded.
A distributed activation energy model, assuming a Gaussian distribution, accurately represented the pyrolysis behaviour of Morupule coal. The shrinking core, volumetric and random pore models adequately represented the early-stage atmospheric pressure gasification kinetics. The Langmuir – Hinshelwood rate expression described the high-pressure gasification kinetics fairly well but could not account for the initial gasification lag. In studying the effect of particle size on pyrolysis, larger particles exhibited lower product yields during the heating up period. However, identical yields were obtained under prolonged holding at 1000 ºC at both atmospheric and elevated pressures.