Pyrolysis kinetics of oil shale from northeast China: Implications from thermogravimetric and Rock-Eval experiments

Thermogravimetric analysis (TGA) is used extensively to study oil shale pyrolysis and kinetics. TGA measures the total weight loss of the sample during pyrolysis, including the decomposition of kerogen and minerals, as well as the removal of water from clays. Rock-Eval (RE) characterizes the hydrocarbon released during pyrolysis. In order to identify and quantify the stages of kerogen decomposition and other reactions during pyrolysis, we performed TGA and RE pyrolysis experiments at four heating rates (10, 20, 30, and 40 °C/min) on two oil shales from Huadian and Fushun in northeast China. We determined the kinetic parameters of oil shale pyrolysis by applying parallel first-order reaction models to data from TGA and RE experiments. Variations in pyrolysis temperature and activation energy occur across the two experimental methods. Most weight loss and hydrocarbon generation occurred between 400 and 550 °C using TGA, and between 400 and 475 °C using RE. For the Huadian oil shale, the average amount of weight loss and hydrocarbon generation across all heating rates were 151.32 and 73.48 mg/g, respectively; the corresponding values for Fushun oil shale are 151.97 and 34.02 mg/g. For the Huadian oil shale, kerogen decomposition occurred mainly in the temperature range of 300-500 °C. The contribution to weight loss of kerogen decomposition was 14.7%, 48.6%, and 3.5% in the temperature ranges of 200-300 °C, 300-500 °C, and 500-600 °C, respectively. Additional weight loss was caused by the decomposition of minerals such as smectite, kaolinite, and pyrite. The activation energies obtained by TGA span a greater range than those obtained by RE, and the average activation energies obtained by TGA are lower by 10 kJ/mol (Huadian) and 1 kJ/mol (Fushun) than those obtained by RE. The discrepancy between TGA and RE results indicates that non-hydrocarbon reactions are significant at temperatures between 200 and 600 °C, and that RE is the more accurate approach for quantifying hydrocarbon generation during oil shale pyrolysis.