Management of off-specification compost by using co-hydrothermal carbonization with olive tree pruning. Assessing energy potential of hydrochar

[Display omitted] •Energy valorisation of the off-specification compost (OSC) waste was accomplished.•Hydrothermal carbonization (HTC) can convert OSC into a lignite-like coal product.•75% of OSC and 25% of biomass blend presented the highest energy recovery.•Higher heating values were upgraded from 17.0 to 26.2 MJ/kg in the selected blend. In this work the management of a waste called off-specification compost (OSC) was proposed via hydrothermal carbonization (HTC). The composition of this residue makes it not suitable for agronomic purposes because of the Spanish regulation requirements. Therefore, a way of management and/or valorisation needs to be found. The energy recovery through co-HTC with olive tree pruning (OTP) was evaluated. Blending of OSC with lignocellulosic biomass allows to obtain a coal-like product with physicochemical properties similar to those of a lignite, characterised by its high carbon content. Blends of 25, 50 and 75% of OSC with OTP were analysed. The individual OSC does not present good parameters for being used as solid fuel based on its chemical composition, however, the blend of 75% of biomass with 25% of OSC does. With a higher heating value of 26.19 MJ/kg, this blend shows the best energy yield and energy densification ratio. Thermogravimetric and kinetic analysis reveal that as biomass content in the blend increases, the more the hydrochar behaves as a solid fuel, therefore OSC can be used for energy purposes while its current use of landfill disposal can be reduced.