Anne Roubaud

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

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.

ECONOMIC EVALUATION OF A HYDROTHERMAL LIQUEFACTION PROCESS

Wet waste streams include a wide variety of products such as food processing residues, sewage sludge, but also the organic fraction of municipal solid waste. Hydrothermal liquefaction is a thermochemical conversion in hot compressed water that produces a hydrophobic product. This paper gives presents how hydrothermal liquefaction can produce a biocrude or a heavy fuel oil from blackcurrant pomace, grape marc and sewage sludge. The paper presents experimental results as well as a technical and economic evaluation of the process. The results from hydrothermal liquefaction depend on the resource. Typical biocrude yield is 50% of the dry resource while bio-oil yield can be up to 25%. High ash resources are however less interesting for this technology. The production costs are high compared to their fossil counterparts but gate fees in the order of 50 to 130 € tonne-1 could ensure economic competitiveness compared to fossil fuels.​