Hydrothermal liquefaction of Scenedesmus obliquus using a novel catalyst derived from clam shells: Solid residue as catalyst for hydrogen production

[Display omitted] •Microalgae biomass was cultivated from wastewater in a mixotrophic mode.•Novel catalyst for liquefaction was derived from clamshell waste.•Hydrothermal liquefaction resulted bio-oil yield (39.6 wt%) for catalyst (0.6 wt%).•Solid residue from liquefaction was used as catalyst for biomass gasification. This study explores the catalytic application of waste clam shell in hydrothermal liquefaction (HTL) of microalgae (Scenedesmus obliquus) for liquid hydrocarbons production. Novel catalyst (calcium hydroxide) was derived from clam shells. Catalytic HTL was performed at varying temperature of 240–320 °C for catalyst load (0.2–1 wt%) at a reaction time of 60 min. Bio-oil yield was maximum (39.6 wt%) at a temperature of 300 °C for catalyst load of 0.6 wt% at a reaction time of 60 min with calorific value of 35.01 MJ/kg. Compounds like phenols, aromatic hydrocarbons, acids and aldehydes were detected in bio-oil through Gas Chromatography Mass Spectrophotometry (GC–MS). Gasification of microalgae with waste solid residue obtained from HTL was carried out for hydrogen production. Valuable hydrogen gas production was maximum (37 wt%) at a temperature of 400 °C for 3 wt% of solid residue. Water-gas shift, methanation and steam reforming reactions favoured the hydrogen gas production.