Variation of chemical composition of thermal bitumen during Huadian oil shale pyrolysis

The variation of chemical composition of thermal bitumen during Huadian oil shale pyrolysis was studied. Spent shale samples obtained by retorting oil shale at 300-550℃ were subjected to sequential Soxhlet extraction-acid pickling-Soxhlet extraction procedures to obtain free bitumen (FB), bitumen bound with carbonates (BB-1) and bitumen bound with silicates (BB-2). The bitumen samples were characterized by liquid chromatography fractionation, FT-IR and GC-MS. The results show that the total bitumen yield first increases and then decreases with increasing temperature from 300 to 550℃, and reached the maximum value of 4.63% at 400℃. Especially, the intense vaporization and decomposition of bitumen occurring at 400-450℃ causes a dramatic decrease in bitumen yield from 4.63% to 0.98%. Decarboxylation of aliphatic acids, decomposition of esters and cracking of long-chain alkanes take place at 350-450℃, which decreases the contents of acids and esters in FB and shortens the chain length of alkanes. The carboxylic acids derived from kerogen pyrolysis can react with carbonates to form carboxylates, leading to a high amount of aliphatic acids in 400℃ BB-1 (78.82%). The contents of oxygenated compounds (acids, esters and phenols) and alkanes of 400℃ BB-2 are 80.79% and 19.21%, respectively, due to the combination between oxygenated compounds and clay minerals, and the insertion of alkanes into the interlayer space of montmorillonite.