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 degrees 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 degrees C) and burnout (581-619 degrees 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 degrees 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 degrees 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 degrees C) accounted around 85% of the total energy content of feedstock, allowing a potential valorization route for dairy manure. (C) 2020 Elsevier Ltd. All rights reserved.