Energy valorization of cow manure by hydrothermal carbonization and anaerobic digestion

The work evaluates the energy recovery from hydrothermal carbonization (HTC) of cow manure through the thermal analysis of hydrochar and the anaerobic digestion (AD) of the process water (PW). The increase of the HTC temperature in the range of 170–230 °C improved solid-fuel properties (higher heating value (16.4–20.1 MJ kg−1), fuel ratio (0.19–0.33)) of dairy manure, but less energy can be recovered by its combustion ascribing to reduction of hydrochar yield (65.0–54.0%). Fixed carbon content (12.5–18.7%), and ignition (263–278 °C) and burnout (581–619 °C) temperatures increased with carbonization temperature, thereby reducing risks of fire and explosion. However, the highest value of the comprehensive combustion index, related with good fuel characteristics, was obtained for the hydrochar carbonized at 170 °C. The high organic matter content of the PW allows energy recovery by AD, obtaining the highest methane yield for the PW generated at 170 °C (294 ± 2 mL STP CH4 g−1 VSadded). HTC led to higher energy recovery than conventional AD of dairy manure (4.1 MJ kg−1). The energy recovery by AD combined with the energy content of the hydrochar (13.7 MJ kg−1 feedstock for HTC conditions below 200 °C) accounted around 85% of the total energy content of feedstock, allowing a potential valorization route for dairy manureAuthors greatly appreciate funding from Spain’s MINECO (project PID2019-108445RB-I00; project RYC-2013-12549), Madrid Regional Government (project S2018/EMT-4344), and National Science Foundation (NSF#CBET-1856009