Thermal decomposition of kerogens: Mechanism and analytical application

Abstract

Specific features of the chemical structures of organic matter (Lerogen) in oil shales caused by the nature of the starting materials for the formation of shales and the routes of their subsequent alteration are reflected in the composition of shale semicoking (retorting) oil. In order to establish the analytical possibilities of the thermal decomposition method in elucidating the kerogen structure and to obtain more data on the mechanism of the pyrolysis of kerogens, the effect of a series of factors (rate of heating, pressure, presence of carrier gases, water and mineral matter of shale, treatment with reagents) on the yield and composition of the pyrolysis products of oil shales was investigated. The yield of shale oil and the phenol content in the latter increase when semicoking is performed in a stream of hydrogen at atmospheric pressure. In shale pyrolysis in the presence of water under pressure, the yield of oil and, in particular, water-soluble organic compounds also increase, as well as the content of neutral heteroatomic compounds in oil. With increasing content of mineral substances in shale, the yield of the semicoking oil (kerogen basis) and the content of polar compounds in it diminish owing to an increasing influence of oil adsorption on the mineral matter and its additional decomposition as a result. In the initial stage of thermal decomposition, both longer aliphatic substituents and side-chains of iso-structures split off and alkenes with a double bond in the middle of the chain (probably the products of elimination and dehydration of the aliphatic substituents with a hydroxyl group) are formed. The formation of n-1-alkenes, particularly those of even carbon number, which originate from the side-chains of odd carbon number by the cleavage of carbon–carbon bonds in the β-position to the cyclic nucleus of kerogen, becomes more pronounced in the final stage of pyrolysis when, owing to significant aromatization of the cyclic part of kerogen, the selectivity of the β-cleavage increases.