Effects of oil shale mineral composition on char combustion reactions

Abstract

Six powdered oil shales with differing mineral compositions were retorted and the resulting chars were subjected to isothermal combustion kinetic studies using TGA techniques. Base line combustion rates were first determined for each sample to evaluate mineral effects. Thermal decarbonation and water extraction of soluble minerals removed catalytically important species, and the subsequent combustion results are compared in terms of intrinsic first order kinetic expressions. In each case the char combustion rate was found to be first order with respect to both the char remaining and to the oxygen partial pressure. This was also true for a Devonian shale, Michigan Antrim, once the char combustion rate was successfully separated from the combustion rate of sulphur bearing minerals. Intrinsic western spent shale char combustion rate constants were found to vary from 1.2 × 10 −3 to 1.5 × 10 −4 kPa −1 s −1 at 700 K. The saline zone shales were the most reactive, and shales low in mineral carbonates the least reactive. Shales high in Group Ia and IIa metal carbonates combusted up to six times faster when these minerals were present as oxides. The combustion rate of Devonian shale char was slower by a factor of ten than that of the slowest western shale char. This was attributed to the virtual lack of catalytic minerals and is evidence that iron oxide is not a significant char oxidation catalyst.