Analysis and optimisation of a novel “bio-brewery” approach: Production of bio-fuels and bio-chemicals by microwave-assisted, hydrothermal liquefaction of brewers’ spent grains

Abstract

This work firstly explores the feasibility of using a novel microwave-assisted, catalysed, hydrothermal process for the valorisation of brewers’ spent grains (BSGs), examining the effects of the temperature (180–250 °C), pressure (50–130 bar), reaction time (0–2 h) and catalyst amount (Ni-Co/Al-Mg, 0–0.25 g cat/g biomass). This route turned out to be a very promising approach for the production of bio-fuels (bio-oil and bio-char) and platform chemicals (sugar rich aqueous solutions) in a single unit, helping the development of an innovative bio-refinery around BSGs. The overall conversion and the yields to gas, aqueous fraction and bio-oil varied by 31–68%, 10–33%, 9–48% and 4–14%, respectively. The bio-oil was made up a complex mixture of phenols (0–26%), ketones (0–80%), aldehydes (0–57%), carboxylic acids (0–18%) and nitrogen compounds (0–76%). The proportions of C, H, N and O in the bio-oil varied as follow: 15–61 wt%, 5–10 wt%, 1–6 wt% and 26–77 wt%, respectively, which shifted its higher heating value (HHV) between 9 and 27 MJ/kg. The liquid fraction comprised a mixture of DP > 6 oligosaccharides (67–98 C-wt.%), DP2-6 oligosaccharides (0–10 C-wt.%), saccharides (0.2–7 C-wt.%), carboxylic acids (0–7 C-wt.%) and furans (0–27 C-wt.%). The spent solid after the experiments resembled an energetic like bio-char product, whose proportions of C, H, O and N varied by 35–72 wt%, 4–8 wt%, 1–4 wt% and 18–57 wt% and its HHV shifted between 9 and 32 MJ/kg. The optimisation of the process revealed that using a temperature of 250 °C and a pressure of 125 bar for 2 h, it is possible to convert the original material into high-energy biofuels: (8%) bio-oil (26 MJ/kg) and (35%) bio-char (32 MJ/kg); together with a (31%) saccharide-rich (>99 C-wt.%) aqueous solution, thus converting this process into a very promising approach to achieve an environmentally-friendly and integral valorisation of brewers’ spent grains.