Statistical Clarification of the Hydrothermal Co-Liquefaction Effect and Investigation on the Influence of Process Variables on the Co-Liquefaction Effect

Hydrothermal co-liquefaction of different types of biomass has recently attracted great interest as it has the potential to reduce logistics costs and increase the biocrude yield/quality. Although a positive co-liquefaction effect (CE) has been reported in previous studies, the statistical significance of CE is uncertain, and the effects of process variables on the CE remain unexplored. In this study, the carbohydrate-rich feedstock (sawdust and spent coffee grounds) was hydrothermally co-liquefied with algal biomass (Chlorella sp. and seaweed) at 270 and 320 °C with varying mixing ratios of 25:75, 50:50, and 75:25. A statistically sound method, one-sample t-test, was, for the first time, applied to evaluate if the positive or negative CE is significantly greater or less than zero. A significantly positive CE of 22.4% (synergistic effect) on the biocrude yield was obtained in the co-liquefaction of spent coffee grounds/Chlorella sp. Co-liquefying sawdust/Chlorella sp. and spent coffee grounds/seaweed showed negative and positive values of CE, respectively, but these numbers were not statistically significant, taking the experimental error into consideration, and thus should not be considered as an antagonistic or synergetic effect for the two types of mixtures. Co-liquefaction of sawdust/seaweed exhibited a significantly negative CE (antagonistic effect) on the biocrude yield (−14.8%). The feedstock mixing ratio (varying biochemical composition of mixture) did affect the CE, which was reasoned using the knowledge of biomass model components’ interactions under liquefaction conditions. As observed, sufficient and comparable contents of protein and carbohydrate in the feedstock blends led to the synergistic effect on the biocrude yield. Temperature was also influential for the CE; for instance, increasing temperature diminished the synergistic effect in co-processing of spent coffee grounds and seaweed. Within the experimental scope, spent coffee grounds/Chlorella sp. were identified to be the most favorable feedstock combination, giving a biocrude yield of 37.2 wt %, dry ash-free basis and a synergistic effect of 22.4%, when co-liquefying at 320 °C and a mixing ratio of 50:50.