Reaction kinetics of pulverized coal-chars derived from inertinite-rich coal discards: Characterisation and combustion

Abstract

An investigation was undertaken to determine the chemical, structural, mineralogical and petrographic (maceral groups) properties of coal-chars derived from typical South African discards, in order to evaluate a suitable combustion reaction rate model. Detailed characterisation results of typical coal-chars involving conventional and advanced measurements (such as CCSEM) are presented and the following unique properties were found (1) the chars have very low porosities (<5%), which can be attributed to the presence of a high content of inertinite (>75%) in the parent coals and the minerals with low porosities. (2) The coal-chars consists of large amounts of kaolinite and quartz and to a lesser extent of pyrite and calcite, very different to conventionally used bituminous coals, and (3) a microscopic inspection of the coal-chars (CCSEM) showed that the pulverised form of the coal-char samples, used for the reaction rates studies, were a collection of mainly carbon rich particles (<10% minerals) and mineral rich particles (<10%) carbon) which had been liberated in the milling process to particles with diameters of 20 and 70 μm. Combustion experiments were carried out in a TGA at atmospheric pressure with varying mixtures of oxygen and nitrogen and consisted essentially of generating results for the validation of a suitable overall reaction rate model and associated kinetic constants. Using isothermal and non-isothermal methods it was found that the shrinking core model with a rate controlling surface reaction (no internal effects) was applicable for temperatures between 643 and 823 K. A description of a non-isothermal method using linear heating rates and analytical expressions with application to combustion is given and sets of intrinsic reaction rate constants for the coal-chars examined are reported. It was found that the intrinsic reaction rates were determined essentially by the activity of the carbon-rich particles with a low concentration of minerals (ash).