Hydrothermal Co-Liquefaction of Lignite and Oak Biomass in the Presence of Formic Acid as a Hydrogen Precursor

Lignocellulosic biomass and low-grade coals are potential sources of industrial chemicals, such as single-ring aromatics. This work investigated the hydrothermal co-liquefaction (HTL) of oak sawdust and a Polish lignite under subcritical water conditions and in the presence of formic acid as an in situ hydrogen source. Oak sawdust and lignite exhibited opposite reactivities, with high conversion of the biomass (1.8 wt %, daf solids) while 57.9 wt %, daf of solid product was recovered from the HTL of lignite. Co-liquefaction of the two feedstocks led to a synergistic performance, enhancing the total oil yield to 35.3 wt %, daf. The presence of formic acid significantly improved the HTL oil yield, reducing the recovered solid yield and enhancing oil quality through demethoxylation reactions involving the emergence of guaiacol and reduction of syringol. Methoxyphenols and ketones dominated the light oil fraction product from oak/lignite co-liquefaction in the presence of formic acid. CO₂ was the main product gas from HTL experiments using both oak sawdust and/or lignite, accounting for over 70 vol % of the gas yield. The study showcased the significant synergistic benefits of biomass and coal co-liquefaction as well as the positive influence of an in situ hydrogen-producing precursor. The process conditions used in this work led to an enhanced oil yield over what has hitherto been reported in the literature.