Hydrothermal carbonization of spent mushroom compost waste compared against torrefaction and pyrolysis

The effects of operating conditions (temperature, residence time, and water contents) of hydrothermal carbonization (HTC) of spent mushroom compost (SMC) waste on the hydrochars (HCs) and liquid effluent characteristics were experimentally revised and ranked in increasing order: residence time < dilution factor < temperature. HTC upgraded the energy capabilities by doubling their heating values and increasing their fixed carbon contents four times. HTC also enhanced the soil amendment characteristics of SMC feedstock in terms of increasing the adsorption polar heads concentration, enriching its calcium and heavy metals contents after a thorough inorganic contents evaluation, doubling the surface area and increasing the pore size by a factor of five. When compared against biocoal from torrefaction in another study, HCs contained less toxic oxygenated compounds and had an 11% higher HHV at lower temperature (i.e. lower energy cost). On the other hand, HCs showed higher surface area (25 m2/g at 250 °C in HTC compared to 16 m²/g at 550 °C in pyrolysis), close adsorption characteristic, and comparable energy capabilities (22.72 MJ/kg at 700 °Cs in pyrolysis compared to 20.7 MJ/kg at 250 °C in HTC) to pyrolysis at significantly lower temperature. GCMS along with UV were used to verify the reviewed degradation mechanism and evaluate the effect of process parameters on this mechanism and on the composition and toxicity of the HTC liquid effluent. They showed that acetic and formic acids, ethanol, phenol, and acetaldehyde were the major compounds that had resulted from the degradation of cellulose, hemicellulose, and lignin. Their concentrations increased with temperature and residence time, but was dependent on temperature in the case of increasing the dilution factor. Nevertheless, HTC degradation enhanced the total acids-phenols concentration in the liquid effluent by 700%.