Hydrothermal Liquefaction of Several Food Residues

International audienceAgricultural and food processing wastes are one of the most abundant resources of biomass worldwide. For instance in 2010, it was estimated that 89 Mt of food waste were generated in the EU-27. Unlike other traditional biomasses, such as wood, these wastes usually contain more than 50 wt. % water. In comparison with thermochemical processes such as incineration or pyrolysis, Hydrothermal Liquefaction (HTL) does not require the costly preliminary step of drying the biomass: water is a solvent and a reactant for this process. It is therefore possible to economically treat wet wastes. HTL typically generates 4 phases: an oily product known as 'bio-oil', an aqueous phase containing dissolved organics, a solid residue and a gaseous phase. In this study, several agricultural products were tested, including fruit residues and potato starch. All of the feedstocks were subjected to a detailed chemical analysis. These residues were then exposed to subcritical water in a batch autoclave. Operating conditions were 280-350°C, 8.5-20 MPa, and a reaction time between 15 and 60 min. For each experiment, the aqueous, solid and oily fractions were analysed. This paper focuses on the carbon distribution in the different phases recovered, and the organic compounds contained in the aqueous phase, as regards to the operating conditions. Raw resources are mainly acidic (3 < pH < 5), and initial pH values were modified by addition of NaOH or Na 2 CO 3. The results show that increasing the initial pH of the feed has a significant influence on the fraction of organic carbon remaining in the aqueous phase: it increases from 10% at pH = 4, to 35% at pH = 12.5. The nature of the biomass is also important: for example, more organic carbon is transferred from grape to the aqueous phase, compared to starch. This paper mainly consists in exploratory runs: further studies should focus on model compounds, in order to unveil the mechanisms leading to the products. Especially, the knowledge on the dependency to acidity of the aqueous phase will allow us to optimize the liquefaction process, and the role of alkali catalysts would be clarified