Thermochemical valorization of manure digestate via hydrothermal liquefaction and thermal treatment of hydrochar to obtain soil improver

Second-generation biomasses like sewage sludge, digestate, animal/agricultural waste, are renewable energy sources of particular interest as alternatives to fossil fuels to reduce the energy sector’s environmental footprint. These biomasses rich in organics can be valorized to obtain liquid, gaseous, or solid energy carriers, depending on the thermochemical/biological process they undergo. However, for the abovementioned biomasses it is important to consider their high moisture content in the valorization process. Currently, anaerobic digestion is the most established process for converting wet waste into energy, as biogas, a mixture mainly containing CH4 and CO2. The co-product of the process is a digestate rich in carbon and nutrients, but it still has a high moisture content, making it unattractive in energy recovery processes (i.e. combustion, pyrolysis, and gasification). Following the concept of circular economy and biorefinery, the possibility of valorizing the digestate through hydrothermal liquefaction (HTL) can be evaluated. HTL is a thermochemical route to obtain a liquid energy vector (biocrude) from high-water-content biomasses. In fact, in HTL conditions (250–374°C and 40–200 bar), water acts as a catalyst, reactant, and thermal flywheel, and thus the biomass organic components can be directly converted without a drying step. At the end of the HTL process, in addition to the target biocrude, a gas phase, an hydrochar, and an aqueous phase are also produced. The aim of this work will be focused on the exploitation of digestate through the HTL process in a 500 mL batch autoclave to study the yield and properties of the biocrude produced at different temperatures (300–350°C) and isothermal reaction times (10–60 min). The hydrochar (HC), is not under regulation for its use as amendment, but its conversion into biochar could be a valuable route. Co-pyrolysis of HC with lignocellulosic biomass representative of agricultural waste like straw (S) will be tested to obtain a biochar and evaluate its performance as a soil improver. Different blends and ratios of S and HC will be tested for biochar production, and the volatile phase will be explored as energy vector to sustain the whole process or as a source of chemicals. Preliminary results on biochar produced at 450°C showed that the blends S90%-HC10% up to S50%-HC50% have respectively 28.39–51.66% ash content, values lower the threshold limits reported in Regulation (EU) 2019/1009 for fertilizers.