Hydrogen transfer and reaction mechanism during in-situ pyrolysis of Fushun oil shale with steam injection

The upgrading of pyrolysis oil and gas during the in-situ mining of oil shale is the key focus point to realize its large-scale clean and efficient utilization. Steam injection significantly improved the quality of shale oil and gas during in-situ pyrolysis of oil shale. However, the oil and gas upgrading mechanism of oil shale during steam pyrolysis is still unclear, which is closely related to the migration behavior of hydrogen in steam and oil shale. Thus, in this work, isotope tracing method have been adopted to investigate the hydrogen migration and reaction mechanism during the steam pyrolysis of oil shale. The results show that the presence of steam promotes the pyrolysis behavior of oil shale, and the yield of shale oil is 1.5 times of anhydrous pyrolysis. The H2 concentration in the pyrolysis gas during steam pyrolysis of oil shale is up to 65.4%. The hydrogen in steam can migrate to all pyrolysis products, with migration amounts of 89.1% for pyrolysis water, 1.5% for pyrolysis gas, 3.5% for shale oil, and 5.9% for solid residue. The H center dot free radicals are inclined to attack the O site of cracked oil fragments during pyrolysis, followed by the alpha site. However, the O content of shale oil decrease during steam pyrolysis of oil shale, and the hydrogen in steam mostly enteres into shale oil in the form of H center dot, combining with cracked oil fragments to form short-chain aliphatic hydrocarbons and inhibiting the polymerization of cracked oil fragments into shale char. Meanwhile, steam also promote the migration of inherent hydrogen from oil shale to shale oil during pyrolysis, resulting from the cracking of kerogen induced by H center dot and displacement effect of steam. The steam pyrolysis of oil shale is controlled by multiple reaction mechanisms in both primary and secondary reactions, including induced cracking, hydrogenation, and steam flooding. This work provides theoretical support and technical guidance for the in-situ clean and efficient development of oil shale.