From beer to biocrude through hydrothermal liquefaction of brewers’ spent grain enhanced with acid hydrolysis pretreatment

• BSG hydrolysis transfers 95 wt% hemicellulose and 54 wt% proteins to hydrolysate. • HTL of pretreated BSG increased biocrude yield by 10 % versus non-pretreated. • Prior acid hydrolysis moderately reduced O and N contents in HTL biocrude. • Biocrude from BSG contains ca. 53 wt% of volatile fractions up to 300 °C. • Pretreatment cuts pyrazine content in biocrude by 29 %, reducing Maillard reactions. Brewer’s spent grains (BSG) constitute an abundantly available raw material, which is highly prospective to process through hydrothermal liquefaction (HTL) towards sustainable substitutes of fossil-based products. However, HTL biocrude obtained from BSG presents a significant challenge in the hydroprocessing to drop-in biofuels as the initial biomass contain substantial amount of oxygen and nitrogen. One of the efficient solutions to reduce the hetero-organics content in biocrude is feedstock pretreatment, aiming for reduction of hemicellulose and proteins content. In this study, we investigated the HTL process combined with acid hydrolysis (AH) of the BSG, where AH was conducted under specific conditions (150 °C, 10 wt%, pH 2, 180 min) optimised to reduce hemicellulose and protein content. Consecutively, HTL parameters (temperature, biomass concentration, and retention time) were optimised to maximise biocrude yield from raw BSG. Finally, the derived optimal conditions (335 °C, 10 wt%, 15 min) were applied for the BSG after hydrolysis. AH pretreatment noticeably altered BSG’s structure, leading to 95 wt% hemicellulose degradation and a 54 wt% reduction in protein content. Mass and elemental balances, including fibre composition, Kjeldahl method, were determined based on products analysis, including TOC and TN measurements of the liquid phase, to ensure a comprehensive assessment of carbon and nitrogen distribution. Under these optimal conditions, HTL of pretreated BSG led to a 9 wt% increase in yield and a 5 % improvement in the heating value of the resulting biocrude. The pretreatment also resulted in a 10 % reduction in oxygen content and a 33 % decrease in nitrogen content, improving the biocrude’s quality. Gas chromatography analysis showed a relative reduction by 29 % in complex heterocyclic compounds, such as pyrazine, which is beneficial for further upgrading. Distillation potential indicated by thermogravimetric analysis shows that approximately 53 wt% of biocrude mass is volatile fractions up to 300 °C, which contains nearly 80 wt% of the middle distillate fraction. Furthermore, HPLC and UV–VIS analysis of the hydrolysate indicated that it is a valuable product stream, containing 49 wt% saccharides and 31 wt% proteins. Future research should scale this approach for saccharide and protein stream valorization to improve efficiency and ensure economic viability.