In this paper, the Thermogravimetric Analysis-Fourier Transform Infrared Spectroscopy (TG-FTIR) technique is used to analyze the pyrolysis behavior of kerogen of two different oil shales at different heating rates. The pyrolysis reaction mechanism of kerogen and the regularity of change in the composition of its pyrolysis products are discussed. Furthermore, the apparent activation energy (E) and the frequency factor (k0) are determined through the distributed activation energy model (DAEM), and the relationships between E and the kerogen chemical structure, conversion rate, frequency factor, and the amount of kerogen pyrolysis products generated are established. The results show that the kerogen structure is similar to that of aliphatic chains, its pyrolysis takes place mostly in the range of 350–520 °C, and the post-pyrolysis semicoke residue accounts for less than 32.5%. In the kerogen pyrolysis process, first the precipitation of free water takes place, followed by depolymerization and decarboxylation, so that the main alkyl side chains are constantly parting and cycling, and the oxygen-containing group gradually breaks up and produces substances such as alkanes, carboxylic acids, alcohols, and aldehydes until a more stable graphite-like structure of kerogen is formed. In the products of kerogen pyrolysis, the concentrations of released lightweight noncondensable volatiles (CH4, CO, CO2) are lower than those of liberated condensable volatiles containing macromolecules (e.g., CHx, C=O groups) that show the Gaussian-like distribution. The apparent activation energy in the two kinds of kerogen varies in the range of 100–495 kJ·mol–1. At the same time, during the entire pyrolysis system, the apparent activation energy and logarithm values of the frequency factor (lnk0) exhibit a good linear relationship. The study reveals the pyrolysis reaction mechanism of oil shale in terms of the relationship between the chemical structure of kerogen macromolecules and the degree of oil shale pyrolysis.
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