Catalytic hydrothermal liquefaction of Scenedesmus sp. biomass integrated with dark-fermentation: biocrude and low-carbon fuel production in a biorefinery approach

Abstract

Herein, we report the catalytic hydrothermal treatment of dried microalgae biomass (DAB) of Scenedesmus sp. SVMIICT1 for biocrude production employing various alkali and metal-based catalysts at 200 °C and 100 bar pressure under N₂ and H₂ atmospheres with twelve designed experiments. The specific influence of inert and reducing conditions together with the catalyst on the yields of bio-oil as well as the composition, off-gases and energy recovery were explored. The results depicted that H₂ atmosphere and catalyst loading resulted in bio-oil with higher heating ratios (HHV: 29–37 MJ kg⁻¹) and yields by hydrodeoxygenation (H/C: 1.52–1.64; O/C: 0.51–0.08). Among the catalysts, Pt/C (H₂) afforded bio-oil with the highest calorific value (∼37 MJ kg⁻¹), which is compositionally equivalent to fossil-based hydrocarbon fuels. H₂–Pt/C condition increased the saturated hydrocarbon (biodiesel) content by 38.7–89.3% of the total bio-oil. Aliphatic/aromatic hydrocarbons were predominantly observed in the bio-oil followed by carboxylic acids, furan derivatives, indanone derivatives, ketones, and substituted benzenes. The by-products of HTL (aqueous fraction, off-gases, and solid residues) were analyzed for their specific characteristics. Further, the aqueous fraction was valorized through dark-fermentation to produce bio-hydrogen and bio-methane, wherein maximum biogas yield (H₂, 186 mL g⁻¹ TOC conversion and CH₄, 131 mL g⁻¹ TOC conversion) were observed under NaOH-catalyzed conditions. This study depicts the comprehensive utilization of algal biomass for energy and chemical intermediates in a circular chemistry context.