Valuable Crude Oil from Hydrothermal Liquefaction of an Aliphatic Coal

Artificial maturation techniques, such as hydrothermal liquefaction (HTL), have demonstrated the ability to generate petroleum-like products, termed bio-oils, from feedstocks including manure, algae and agricultural wastes; however, the oxygen, nitrogen, and sulfur content of bio-oils often requires upgrading. The objective of this study was to utilize HTL on a low rank coal to produce a high quality crude oil sample that can be used as a transportation fuel. Coal from the Wyodak-Anderson coal seam was selected for HTL due to its predominant aliphatic character and low heteroatom content. HTL was performed at 360 °C for 72 h. The untreated coal, spent solid, and expelled oil were analyzed using several advance analytical techniques including solid-state ¹³C NMR, two-dimensional gas chromatography–mass spectrometry and electrospray ionization-Fourier transform ion cyclotron resonance-mass spectrometry to achieve a comprehensive molecular characterization. Solid-state ¹³C NMR analyses of the untreated and spent coal estimate that 22% of the untreated coal, mostly the aliphatic portion, is lost as expelled hydrocarbon products. The expelled oil exhibits molecular characteristics similar to high quality crude oils, including low heteroatom content, n-alkanes ranging from C₉–C₃₂, and other desirable crude oil components such as cycloalkanes, alkyl benzenes and naphthalenes. It is suggested here that upon thermal maturation of the coal, preserved ester bonds in refractory biopolymers present in the coal are being cracked to yield fatty acids, which upon heating undergo decarboxylation, and finally random cleavage of C–C bonds to yield the lower molecular weight n-alkanes. Our results confirm that HTL is an effective technique for the production of a high quality crude oil from a low rank coal.