Multistep process kinetics of the non-isothermal pyrolysis of Moroccan Rif oil shale

Abstract

The kinetic analysis of the complex process of Moroccan Rif oil shale pyrolysis was studied in detail by thermogravimetric (TG) analysis and its derivative (DTG) under non-isothermal conditions. The investigated pyrolysis region, located between 325 and 600 °C, is formed by partially overlapping peaks and it was successfully separated into three discrete processes using the mathematical Fraser-Suzuki algorithm that accommodates asymmetric functions. For each separated process, the isoconversional differential and integral Model Free methods were used to evaluate activation energy and two Master plots fitting methods were explored to get reliable kinetic models. The main pyrolysis step of Moroccan Rif oil shale involves the devolatilization of organic matter including bitumen and kerogen respectively and subsequent decomposition of pyrite mineral FeS2. The first process represents the principal pyrolysis kinetic in accordance with dominance of bitumen products in the studied oil shale. By applying the nonlinear least square analysis to kinetic data, it was found that all processes were well described by n-th order kinetic equations, with n ≈ 1, for the first and third processes and n ≈ 1.67 for the second process. Besides, the thermodynamic functions of activated complexes (ΔS*, ΔH* and ΔG*) for the pyrolysis step of studied oil shale were calculated and discussed.