Hydrothermal Liquefaction of Digestate and Copyrolysis of Hydrochar-Straw Blends: An Integrated Approach for Bio-crude and Biochar Production.

Anaerobic digestion converts wet biogenic wastes into biogas and a byproduct, the digestate, which is a wet material rich in carbon and nutrients widely used in agriculture. However, the large amount produced determines some limitations in this exploitation, thus different thermochemical processes are under investigation. The hydrothermal liquefaction (HTL) process is considered a viable thermochemical route to convert high-water-content biomasses into a liquid energy vector, the bio-crude. However, considerable uncertainties remain regarding the use of the HTL solid residue, the hydrochar (HC), which is not regulated as soil amendment. Thus, its transformation into biochar, already recognized by law as soil improver, could be a promising valorization route. In this context, copyrolysis of HC with abundant agricultural residues may represent an effective strategy to further improve resources' valorization. This study proposes an integrated approach that combines the conversion of digestate through HTL into bio-crude and copyrolysis of hydrochar and straw (S), an abundant agricultural waste to produce biochar. HTL of digestate has been performed in a 500 mL batch autoclave reactor and optimized, based on the biocrude yield and properties, investigating different temperatures (300 and 350 °C) and isothermal reaction times (0-30 min). HC produced under the optimal operating conditions for bio-crude production and S have been pyrolyzed individually and in blends with varying compositions (10-50 wt % S) under slow pyrolysis conditions at a final temperature of 450 °C. Pyrolysis products' yields and properties of biochars have been investigated to assess the compliance of biochar with EU Fertilizing Products Regulation (EU 2019/1009) and to identify possible nonlinear effects arising from the combination of the two feedstocks. Results show high biochar yields in the range of 42.8-62.9 wt % and high H/C ratios in the range of 0.68 to 0.61 which comply with the threshold limits imposed by the EU regulations.