An innovative ‘sea-thermal’ synergetic biorefinery for biofuel production: Co-valorization of lignocellulosic and algal biomasses using seawater under hydrothermal conditions

This study explores the co-hydrothermal treatment (co-HTT) of almond hulls and Chlorella Vulgaris using seawater as an alternative HTT medium. The influence of the feedstock (individual biomass and all the possible binary mixtures) was systematically evaluated under different conditions (reaction temperatures and times). The feedstock mixture and hydrothermal conditions significantly influenced the overall product distribution: gas (1-5%), hydrochar (6-56%), biocrude (6-55%), and aqueous fraction (33-52%), along with the most representative physicochemical and fuel properties of these products. Notably, the biocrude had a calorific value of 24-31 MJ/kg, whereas the hydrochar shifted between 3 and 26 MJ/kg. The degradation of abundant polysaccharides in almond hulls produced acidic species, promoting the degradation of proteins to N-containing species in biocrude. The synergies between microalgae and almond hulls favored the deamination of amino acid and repolymerization of formed monomers. Process optimization revealed that the best biocrude production (59% yield and HHV=28 MJ/kg) was obtained by treating C. Vulgaris at 268 °C for 180 min. Contrarily, the HTT of almond hulls under optimum processing conditions (300 ºC and 112 min) also produced an energy-dense biocrude (29 MJ/kg) but with a much lesser yield (16%). However, such a low biocrude production can be synergistically increased up to 33 %, maintaining the HHV (31 MJ/kg), including up to 61 wt% of C.Vulgaris into the feedstock, with a feedstock energy recovery of 75%. Holistically, the co-HTT of 40 wt% C. Vulgaris and 60 wt% almond hulls at 300 °C for 180 min produced energy-dense liquid (23% yield and HHV=32 MJ/kg) and solid (29% yield and HHV=25 MJ/kg) biofuels simultaneously, with a feedstock energy recovery of 80%. Given these excellent prospects, this strategy provides timely and new insights into developing synergetic strategies to utilize microalgal and lignocellulosic biomasses more efficiently, paving the way toward developing holistic and unseasonal biorefinery processes.