Catalytic and non-catalytic low-pressure hydrothermal liquefaction of pinewood sawdust, polyolefin plastics and their mixtures

Hydrothermal co-liquefaction of biomass and polyolefin plastic feedstocks offers the advantage of potential synergistic reaction environments for producing liquid products of high fuel quality. In this present study, hydrothermal liquefaction and co-liquefaction of sawdust, low-density polyethylene and high-density polyethylene were investigated in a batch reactor from 350 °C to 450 °C and autogenic pressures below 30 bar. The novel low-pressure hydrothermal processing method was carried out with and without low-cost Ni–Cu/Al2O3 bimetallic catalyst. Thermal degradation of the sawdust started at 350 °C, whereas the plastics could only completely degrade at 450 °C, which was then chosen as the optimum reaction temperature. The catalysed process led to an increase in oil yield from the sawdust, with carbon enrichment by 16.3% and 22% deoxygenation. Furthermore, the catalyst promoted the formation of ketones and aromatic hydrocarbons, while consuming phenols and furfural in the sawdust-derived bio-oils. For the plastics, the catalyst, gave slight decreases in oils yield in favour of gas and/or char formation, with the promotion of in situ hydrogenation to enhance the yields of alkanes over alkenes. Results from hydrothermal co-liquefaction tests showed that synergistic interactions occurred between the degradation products of sawdust and the plastics. The observed synergy was further promoted by the presence of the catalyst, leading to dramatic deoxygenation of the oil products to produce hydrocarbon-rich fuels with less than 4 wt% oxygen contents (≈90% deoxygenation). This low-pressure hydrothermal co-liquefaction process is an efficient and cost-effective pathway for single-loop conversion of widely available biomass and plastics feedstocks into highly deoxygenated oils for use as sustainable fuels.