Co-hydrothermal liquefaction (co-HTL) of microalgae and spent coffee grounds (SCGs) was systematically investigated under milder operating conditions for the production of high-quality biocrude oil and hydrochar. The study focused on evaluating product yield, physiochemical characteristics, and fuel properties to assess the feasibility of these feedstocks as sustainable bioenergy resources. The present study aims to model and optimize the co-HTL of SCGs and microalgae, with the objective of systematically evaluating the effects of key process parameters such as reaction temperature, residence time, and feed-to-solvent ratio using response surface methodology (RSM). The use of a methanol–water cosolvent system promoted high biocrude yield while simultaneously reducing the hydrochar yield compared to pure water. The optimal operating conditions were identified at a reaction temperature of 271.1 °C, a residence time of 39.5 min, and a feed concentration of 5 wt %. The yield of biocrude obtained at the optimized condition was 72.6 wt %. The synergistic interactions between microalgae and SCGs were found to enhance the conversion efficiency. GC–MS analysis of biocrude demonstrated a high content of fatty acid methyl esters (FAMEs). The heating value of the hydrochar obtained at the optimized condition was 27 MJ/kg. Life cycle assessment of the co-HTL process revealed that the global warming potential (GWP) attributable to electricity consumption was 0.27 kg CO2 eq. Future research should prioritize scaling the co-HTL process from batch operations to continuous reactor configurations in order to assess process stability, heat integration performance, and overall energy recovery efficiency.
Bhushan et al. (Mon,) studied this question.