Energy recovery from garden and park waste by hydrothermal carbonisation and anaerobic digestion

Abstract

Hydrothermal carbonisation (HTC) can transform wet lignocellulosic biomass, which is not considered an effective biofuel for energy production at the industrial level, into a carbonaceous product called hydrochar (HC) that is suitable for combustion and a process water (PW). PW is an interesting by-product that can be valorised for biogas production via anaerobic digestion (AD). This study presents a new approach for the valorisation of garden and park wastes (GPW) by integrating HTC to generate HC for energy production, while PW is subjected to AD for biogas production. The hydrothermal treatment was performed at 180, 210, and 230 °C, yielding HC with improved physicochemical properties, such as an elevated higher heating value (21–25 MJ kg−1); low ash (<5 wt.%), nitrogen (1.3 wt.%), and sulphur (0.2 wt.%) contents; better fuel ratio (0.4–0.6); and a broad comprehensive combustibility index (8.0×10−7 to 9.6×10−7 min−2 °C−3). AD of the generated PW was conducted under mesophilic conditions (35 °C), resulting in a methane production in the range of 253–326 mL g−1 CODadded and COD removal of up to 65%. The combination of HTC and AD allowed the recovery of 91% and 94% of the energy content feedstock, as calculated from the combustion of HC and methane, respectively.