Hydrothermal liquefaction of blackcurrant pomace and model molecules: understanding of reaction mechanisms

Abstract

Hydrothermal liquefaction (HTL) refers to the conversion of carbonaceous resources into oily substances in hot pressurized liquid water. During this process, constitutive biomass molecules decompose into thousands of organic compounds, following complex reaction mechanisms. The chemistry behind HTL processes is highly complex and still poorly understood to date, in spite of many research efforts. After a detailed analysis of a wet bioresource, blackcurrant pomace, a selection of representative model compounds was subjected to hydrothermal liquefaction conditions (300 °C, 60 min), either alone or as binary, ternary and quaternary mixtures: glucose, xylose, and microcrystalline cellulose were chosen to represent carbohydrates; guaiacol and alkali lignin for native lignin; glutamic acid for proteins; and linoleic acid for lipids. The results show that the reaction products mainly arise from degradation of individual compounds. The main reactions that can be identified are decarboxylation, dehydration, and condensation reactions producing heavy compounds found in the bio-oil and the char. Some binary interactions have been identified such as the Maillard reaction between carbohydrates and proteins, and also a strong interaction between carbohydrates and lipids for bio-oil formation. Comparative experiments showed that HTL of the real resource (blackcurrant pomace) could be qualitatively represented by model mixtures, in terms of the molecular composition of the products, especially when model fibres were used. The quantitative representativeness of the simulating monomers is lower than that obtained by using model polymers.