High pressure CO2 gasification of Morupule coal: Kinetics and morphological development of chars

Abstract

A direct gasification approach, coupling pyrolysis and gasification, was employed to study the kinetics of Morupule coal gasification in CO2 using a wire-mesh reactor (WMR) at elevated pressures (up to 20 bara) and temperatures of 900 °C and 1000 °C. A novel non-destructive analytical technique based on X-Ray computed tomography (CT) was used, for the first time, in conjunction with scanning electron microscopy (SEM) to unravel the morphological evolution of pyrolysis and gasification chars produced from the WMR. Lower total volatile yields were obtained at high pressures compared to atmospheric pressure for 0 s (quenching the reaction upon reaching temperature) holding time experiments. A temperature dependence of the gasification reaction rate at elevated pressures was observed. At 900 °C, increasing the pressure beyond 10 bara did not yield a commensurate increase in the reaction rate. Further increases in pressure to 20 bara at 1000 °C led to a pronounced increase in the reaction rate. The applicability of the Langmuir – Hinshelwood (L – H) rate model was investigated and showed a suitable characterisation of the gasification kinetics driven by a CO2 chemisorption-limited reaction mechanism. X-Ray CT imagery reveal the development of an internal macroporosity produced by the explosive release of volatiles during pyrolysis. In contrast, SEM images indicate that high pressure coal gasification is characterised by the development of micropores, with a rapid consumption of the outer char surface.