Mechanism of kerogen pyrolysis in terms of chemical structure transformation

This article presents an overview on carbon chemical structure transformation to understand kerogen thermal decomposition based on the chemical structure of kerogen. Formation of kerogen is highlighted to distinguish the typical types of kerogen containing in oil shale and coal. The oil production potential for oil shale and coal is found to little correlate with their organic amounts but to depend on the quality or chemical structure of organic matters. Aliphatic and aromatic carbons in kerogen are correlative with the yield of oil and carbon residue from Fischer Assay retorting, respectively. The aliphatic carbon moieties largely produce oil and gas, while aromatic carbon portion is apt to be converted directly to carbon residue during kerogen pyrolysis process. On this basis, an updated lumped mechanism model is proposed for viewing kerogen pyrolysis and provides a basis for understanding the transformation of carbon chemical structures. Further quantization and analysis conclude that: 1) 10-20% aliphatic carbon leaves in carbon residue as methyl groups and methylene bridges attached to aromatic rings, 2) 45-80% aliphatic carbon is directly distillated into oil, and 3) 15-40% aliphatic carbon is aromatized into aromatic carbon. The aromatization degree of aliphatic carbon varies with secondary reactions and its intrinsic chemical structure (alkyl chains, naphthenic and hydroaromatic hydrocarbons). Thus, the article justifies that primary pyrolysis determines the potentially maximal oil yield according to original carbon chemical structure, while the subsequent secondary reactions should be selective and minimized to determine the final oil yield and quality.