Lignite and Biomass Waste Hydrothermal Liquefaction Crude Upgrading by Hydrotreatment

Hydrothermal liquefaction (HTL) is a promising processfor theenergetic valorization of low-value feedstocks. However, HTL crudeoils are incompatible with the existing fuel standards, making theirupgrade imperative. Nonetheless, the hydrotreatment (HDT) of HTL crudeoils is still a challenge due to the complex nature of the feedstock.Therefore, this work explores the relationship between the HTL crudeoil origin and the HDT conversion. Five HTL crude oils from differentfeedstocks, namely, oak sawdust, Brewer's Spent Grain (spentgrain), sewage sludge from two origins, and lignite, and their detailedcharacterization by elemental analysis, C-13 and P-31 NMR, molecular weight distribution, comprehensive 2D gas chromatography,and SimDis were compared to those of the HDT liquid product. Lignitedisplayed the highest potential for producing hydrocarbons, particularlymonoaromatics and naphthenes, among the feedstocks tested. The capacityof lignite HTL crude oil to be transformed into hydrocarbons was associatedwith the absence of compounds resistant to HDT in this feedstock.Similarly, oak sawdust also displayed higher selectivity toward aromaticand naphthene hydrocarbons due to the significant concentration ofaromatic compounds in the crude oil. In opposition, the sewage sludgeand spent-grain crude oils were particularly selective toward aliphatichydrocarbons, particularly paraffins, produced from aliphatic components,such as amides, in the crude oil. However, these feedstocks were richin nitrogenated aromatics, for example, carbazole, which are recalcitrantto hydrotreatment and were not fully converted during the reaction.The differences observed in the HDT liquid composition show that theHTL crude oil composition dictates the potential for producing hydrocarbonfuels. Indeed, HTL crude oils rich in aromatic compounds will yieldpreferentially naphthenes and aromatic hydrocarbons. In opposition,crude oils richer in aliphatic carbon will be more selective towardparaffins. The nature and concentration of heteroatom components mustalso be considered since these must be imperatively eliminated.