Valorization of kitchen waste hydrolysis residue for green solid fuel application: Conversion mechanism and fuel characteristics

[Display omitted] •Kitchen waste hydrolysis residue is upgraded into hydrochar microspheres via HTC.•Higher heating value increases significantly with a reduction of harmful elements.•Excellent combustion performance shows a promising prospect as green solid fuel.•Conversion mechanism of carbohydrate/protein hybrid in subcritical water is proposed.•Promotion of kitchen waste recycling is achieved. Hydrolysis pretreatment is widely adopted for the liquid fermentation with biomass waste (e.g., kitchen waste). However, a considerable proportion of organics would remain as the solid residues due to the incomplete hydrolysis, which would make energy loss. Thus, the recycling of such solid residues to fuels is significative for waste reduction, resource recovery and green energy development. Herein, the energy valorization of kitchen waste hydrolysis residue (KWHR) by hydrothermal carbonization (HTC) of the carbohydrate and protein components and the conversion mechanism in subcritical water were investigated. Hydrothermal treatment was effective for harmful element removal. HTC temperature showed a significant influence on the physicochemical properties of hydrochars. Higher temperature could accelerate the hydrolysis of organic matters and the subsequent polymerization and aromatization for microsphere formation. Due to the dehydration and decarboxylation, the carbon content of hydrochars increased. The increasing HTC temperature could improve the fuel properties of carbon microspheres except for the excessive temperature, which would degrade the combustion performance. The obtained carbon microspheres (KWHR-240) exhibited a higher heating value of 30.56 MJ kg−1, carbon recovery of 50.81 % and energy yield 55.14 % with applicable ignition temperature. Thus, KWHR derived hydrochar microspheres showed promising prospect in the application as green solid fuel and would promote kitchen waste recycling coupled with the fermentation of hydrolysate.