Thermochemical characterisation of hydrochar from agricultural waste and its efficiency as a supplement with solid fuel

Circular approaches to revalorise waste biomass from agriculture and food production sectors are crucial for developing a sustainable bioenergy strategy. For instance, while the demand for edible mushroom cultivation has increased globally, the production generates a substantial amount of waste biomass, known as Spent Mushroom Substrate (MS). Thermochemical biomass conversion technologies such as hydrothermal carbonisation offers a robust strategy to produce “hydrochar” from the wet biomass and can be used downstream for various environmental applications. In this study, we assess the feasibility of MS-derived hydrochar for energy application, specifically as a blend with coal. The key parameters for the hydrochar production such as temperature, time and moisture content were optimised (205 °C, 3.65 h, and 73.18 %, respectively) using a statistical tool “Response Surface Methodology (RSM)” to obtain a carbon material with higher yield and calorific value. The hydrochar from MS exhibited an acidic pH (4.42), increased fixed carbon content (23.7 %), reduced sulphur content (0.26 %), coarser porous surface, enhanced oxygenated functional groups (hydroxyl, carboxyl and ketonic) and the formation of minerals like Sodium Carbonate (NaCO3), whewellite (CaC2O4·H2O) and gypsum (CaSO4). Combustion behaviour of hydrochar was also assessed using calorimetry and thermogravimetry, specifically to test different coal and hydrochar blends on the feasibility of using hydrochar as a supplement to conventional solid fuels. Our results suggest that a blend of 20 % hydrochar with 80 % coal as an ideal blending ratio (with a calorific value of 27.65 MJ kg−1) highlighting the use of hydrochar as supplement with conventional fuel like coal. •Optimised HTC temperature at 205 °C, time at 3.65 h and moisture content at 73.18 %.•Carbon microspheres and fluffy sponges were formed after hydrothermal carbonisation.•Hydrochar attained energy densification of 1.23 with an energy yield of 88.4 %.•Coal:Hydrochar blend of 80:20 showed efficient calorific value as 27.65 MJ kg−1•Hydrochar substitution reduced CO2 (5.3 %), NO2 (6.8 %) and SO2 (10.6 %) emissions.