Hydrothermal liquefaction for biochar production from finger millet waste: its valorisation, process optimization, and characterization

Abstract

In this study, the potential of finger millet waste biomass (FMWB) as a source of biochar production through hydrothermal liquefaction (HTL) was investigated. The HTL process was designed using Box–Behnken design (BBD) and carried out with process variables, i.e., temperature (250 °C, 350 °C, and 450 °C), time (30 min, 45 min, and 60 min), and solid-to-water ratio (1 : 6, 1 : 8, and 1 : 10). The responses, i.e., biochar yield (%), bulk density (g cm⁻³), pH, and high heating value (HHV), were analysed. Optimisation was done using design expert software (version 13.0.1). The optimized finger millet waste biochar (O-F_MW_BC) was produced at optimum values (450 °C, 1 : 10, and 33.5 min). The results of proximate and elemental analysis revealed that moisture, ash, and volatile content, H, and O of O-F_MW_BC decreased while fixed carbon, thermal stability, and C content increased compared to FMWB. FT-IR, SEM-EDX, and XRD analyses were performed for O-F_MW_BC. The results of FT-IR showed the presence of O–H, C–H, CO, and CC functional groups. The SEM image revealed the rough surface of O-F_MW_BC, and XRD confirmed the production of a broad range of inorganic compounds and minerals. This study provides the full exploitation of F_MW_BC as a source of solid fuel.