The mechanism of H2O in the superheated steam affecting pyrolysis of the kaolinite-associated kerogen

Abstract

In this work, ReaxFF molecular dynamics (MD) simulation was adopted to investigate the effect of superheated steam on the conversion of the kaolinite-associated Barkol kerogen (BLK) and reveal the corresponding mechanism. The ReaxFF simulated weight loss rate (DTG) and release tendency of H₂O, H₂ and CO₂ for BLK and the kaolinite-associated BLK agreed well with the results of Py-MS experiments. H-rich rate, double bond equivalents (DBEs), and hydrocarbon content were adopted to assess the quality of C₅-C₄₀, and configurations of C₄₀₊ were extracted to investigate the characteristics of residues. And the H₂O_steam- and kaolinite-involved (_steam refers to the superheated steam) reactions were analyzed. The results indicate that B- and L-acid sites of kaolinite co-catalyzed decomposition of BLK into heavy oil and shale gas in the kaolinite-pyrolysis system, and in steam/kaolinite-pyrolysis system, H₂O_steam further promoted decomposition of BLK into higher quality shale oil, especially for C₅-C₁₃ components, remaining higher aromatic and porous residues. And this enhanced effect of H₂O_steam is attributed to kaolinite and the induced decomposition of H₂O_steam molecules and their participation as reactants in reactions in two aspects: i) interaction between kaolinite and H₂O_steam, on one hand, inhibited formation of L-acids and facilitated generation of B-acids to catalyze carbocation ion reactions process, further weakened dehydrogenation of organics catalyzed by L-acid sites, and enhanced cracking of residues catalyzed by B-acaid sites, on the other hand, promoted decomposition of H₂O_steam molecules to form H-rich environment and further weakened dehydrogenation of organics; ii) attacking C_ar directly, H₂O_steam promoted shedding of alkyl side chains and ring-opening of aromatics to increase the –CH₂– content in shale oil. This paper provides theoretical guidance for further understanding mechanism of H₂O_steam on pyrolysis of kaolinite-associated kerogen and corresponding catalyst development and preparation.