Hydrothermal carbonization of biogas digestate: Effect of digestate origin and process conditions

Hydrothermal carbonization (HTC) solids were produced in a 250-ml batch reactor from three different digestates at varying temperatures (170, 190, 210, 230, and 250 °C) and 2 and 5 h reaction time. Three potential feedstocks of biogas plants were tested: organic household waste, cow manure, and energy crops. The proximate composition, elemental composition, heating values, and thermal stability of the HTC solids were characterized. The dry ash-free basis yields and carbon recovery of HTC solids decreased with increasing HTC temperature and time. The HTC solids from energy crop digestate had the highest yields (dry ash-free basis) except at the temperature of 250 °C. Increased HTC severity was found to have a positive effect on the dry basis carbon content and leads to a higher heating values (HHV) of cow manure digestate HTC solids. The preferred reaction condition for organic household waste digestate and energy crop digestate HTC solids was found to be 210 °C for 5 h, because of the highest HHV (dry basis) and the overall combustion performance was reached. Enhancing HTC temperature increased the peak intensity of functional groups of cow manure digestate HTC solids. Scanning electron microscopy imaging showed the fibrous structure of the plants from the digestates was mostly deconstructed at 250 °C-5 h. Experimental results indicated that both digestate origin and HTC condition influenced the properties of the HTC solids.