Valorization of red macroalgae biomass via hydrothermal liquefaction using homogeneous catalysts

Hydrothermal liquefaction of red macroalgae species, Kappaphycus alvarezii (KA) and Eucheuma denticulatum (ED), was performed at 350 °C in the presence of 5 wt% neutral and alkali catalysts like Na₂CO₃, K₂CO₃, CaCO₃, Na₂SO₄, NaOH, and KOH. The maximum bio-crude yield of 26.7 wt% and 18.5 wt%, on a dry ash-free basis, was obtained from Na₂CO₃ treatment of KA and KOH treatment of ED, respectively. The bio-crude from both feedstocks mainly consisted of cyclic oxygenates, whose selectivities were maximum in K₂CO₃ and CaCO₃ treatments. The calorific value of the bio-crude was 38.5 MJ/kg from KA and 30.8 MJ/kg from ED, while that of biochars was 20-24 MJ/kg. A high degree of deoxygenation (64.2%) was observed in bio-crude produced from Na₂SO₄ treatment of KA biomass. Salts of Cl^-, SO₄^2- and K⁺ constituted the major inorganic portion of the aqueous phase. Maximum energy recovery (99%) was observed from the Na₂CO₃ treatment of ED.