Hydrogen-free Conversion of Lipids into Fuel Range Hydrocarbons over Three Reaction Stages

Abstract

The chemical composition of lipids is much closer to hydrocarbons than any other type of biomass-derived feedstocks and could contribute to the source of feedstocks for large-scale production of fuels and chemicals. In this present work, a three-step process involving hydrothermal hydrolysis, catalytic decarboxylation and catalytic cracking has been investigated for hydrogen-free conversion of lipid feedstocks into liquid hydrocarbon fuels with high yields of aliphatics and aromatics using batch reactors. The hydrothermal hydrolysis stage investigated in the absence of catalysts was optimised and results show that the best conditions for the hydrolysis of rapeseed oil were 1 hat 300 °C, and oil-to-water mass ratio of 1:2. This led to a high conversion of rapeseed oil to give 88 wt.% yield of semi-solid product, dominated by C18 fatty acids, with oleic acid accounting for over 62 wt.% yield. Over 97% deoxygenation was achieved at 400 °C for 2 h, with heptadecane as the dominant compound. Further experiments at temperatures of 420 °C and 450 °C led to simultaneous decarboxylation and cracking of the hydrolysed oil to obtain organic liquid products with high proportions of gasoline and biojet fuel range hydrocarbons at the optimal conditions of 450 °C for 1 h. Finally, catalytic cracking of the decarboxylated oil was carried out and the optimal oil product found with 10:1 oil-to-ZSM-5 mass ratio, 400 °C for 1 h contained C7- C16 hydrocarbons with a dominance of single ring compounds. Significant shifts of aromatic contents towards undesirable soot-forming polycyclic aromatic hydrocarbons (PAHs) occurred with increased reaction severity. Overall, the three-stage process produced around 50 wt. % - 65 wt.% yield of liquid hydrocarbons, without the use of expensive hydrogen gas. The innovative combination of reaction steps, catalysts and reaction conditions presented a potentially low-cost for obtaining fuel range liquid hydrocarbons.