Reaction kinetics of pulverized coal-chars derived from inertinite-rich coal discards: Gasification with carbon dioxide and steam

An investigation was undertaken to determine the kinetics of gasification of coal-chars (pulverized) derived from typical South African inertinite-rich (high-ash) coals involving char reactions with carbon dioxide and steam and the effects of carbon monoxide and hydrogen. The chars used were characterized with respect to structural, chemical, mineralogical and petrographic (maceral content) properties and gasification experiments were conducted in a TGA at atmospheric pressure with different gas mixtures within a temperature range of 1073–1223 K. The shrinking core model with a controlling surface reaction was shown to be applicable for the gasification of pulverized coal-chars consisting of essentially of carbon-rich particles. The validity of this model can be attributed to the core having an exceptional low porosity (high inertinite parent coal) and consequently negligible penetration of the gases. It was found that the gasification intrinsic reaction rates could be adequately described by Langmuir–Hinshelwood type rate equations and that established equations have been validated with corresponding constants according to new data processing procedures. It was found that the reaction rate constants for coal-chars derived from inertinite-rich (76–80%) coal discards were different to results published in the literature and that the intrinsic reaction rates differed only slightly (order of magnitude) for coal-chars with similar maceral (inertinite) compositions and different total ash contents. The marked inhibiting effect of the carbon monoxide and hydrogen on the carbon dioxide/carbon monoxide and steam/hydrogen gasification reactions is shown and relevant constants are reported. Experiments were done and models evaluated for a multi-component gasification mixture consisting of a feed mixture of carbon dioxide, carbon monoxide, steam and hydrogen. Reaction constants determined with results from binary mixtures were used to predict results and it was found that the overall rate is best described with the assumption that the important reactions proceed on separate sites.