Fuel quality improvement and energy performance through co-hydrothermal carbonization of swine manure and coffee waste

This study evaluates the fuel properties and energy performance of hydrochar produced through co-hydrothermal carbonization (Co-HTC) of swine manure (SM) and coffee waste (CW). Reaction temperature (160 °C, 200 °C, 240 °C) and SM:CW mixing ratio (1:1, 3:1, 5:1) were varied to examine their effects on carbonization behavior, energy densification, and combustion characteristics. The hydrochar yield (HY), except at 160 °C (3:1), ranged from 53.4% at 240 °C (1:1) to 67.6% at 160 °C (5:1), with higher SM content and lower temperatures favoring yield due to ash and volatile retention. The highest energy densification ratio (EDR) of 1.38 was achieved at 240 °C and 5:1, whereas the energy yield (EY) increased from 69.3% at 160 °C (5:1) to 79.9% at 240 °C (5:1), reflecting the synergistic effects of the feedstock composition and temperature. The combustion reactivity, as indicated by the S index, did not show a consistent correlation with temperature, in contrast to the clear trends in EDR and EY. These findings highlight the complementary roles of SM as a nutrient-rich feedstock and CW as carbon-dense lignocellulosic biomass, enabling Co-HTC to produce customized solid fuels for diverse energy applications. Thus, Co-HTC offers a flexible and scalable approach for converting high-moisture organic waste into energy-dense solid fuels under moderate conditions, supporting circular bioeconomy strategies.