Hydrothermal liquefaction of residues of Cocos nucifera (coir and pith) using subcritical water: Process optimization and product characterization

Highlights: • Coconut coir and pith are important waste products of the coconut processing industry. • Hydrothermal liquefaction (HTL) transforms the wastes in bio-oil, bio-gas and char. • The HTL process was optimized through a Response Surface Methodology (RSM). • Time and temperature were optimized for biomass conversion and low char yield. • A bio-oil yield of 34.6% and 32.72% for coir and pith were obtained, respectively. The effects of time (10–60 min) and temperature (250–350°C) on TOC_crude and biochar yields from coir and pith through hydrothermal liquefaction (HTL) process was investigated. The parameters were optimized for minimum biochar and maximum total organic carbon (TOC) in aqueous crude using response surface methodology (RSM). The optimal time and temperature for HTL of coir and pith were 35 min, 302°C, and 35.2 min, 300°C, respectively. Higher biomass conversion and bio-oil yield of 87.34%, 83.76% and 34.6%, 32.72%, was observed for coir and pith, respectively. The biochar yield for coir and pith was reduced from 40 to 12.66% and 34–16.24%. The oxygen and carbon content in the HTL products such as heavy bio-oil (HBO), light bio-oil (LBO) were lower and higher increasing the High Heating Value (HHV), respectively. The HHV of HBO, and LBO for coir and pith were 31 MJ/kg, 22 MJ/kg and 28 MJ/kg, 19 MJ/kg, respectively. The GC-MS/MS analysis revealed that the oil was a mixture of compounds such as alcohols, aldehydes, ketones, amines, amides, esters, ethers, phenols, and their derivatives. Therefore, the conversion of coir and pith to bio-oil can be effectively achieved through the HTL process.