Hydrothermal carbonization of swine manure in a continuous flow reactor pilot plant with process water recycling

This paper deals with hydrothermal carbonization (HTC) of swine manure in a pilot plant setup operating in continuous mode. Two temperatures (210 degrees C and 250 degrees C) were tested and recycling of the liquid fraction was studied to improve the quality of the resulting hydrochar. The hydrochars obtained at 210 degrees C fulfill the criteria to be used as solid biofuels (ISO 17225-8:2023). Increasing the reaction temperature led to a dramatic reduction of hydrochar yield (from 50 to 20 %) in conventional HTC (with tap water), accompanied by a moderate improvement of higher heating value (HHV, 18.2-20.4 MJ/kg), which decreased the energy yield (from 52.2 % to 23.8 %). Process water recycling significantly improved the hydrochar yield, reaching more than 80 % and 55 % at 210 degrees C and 250 degrees C, respectively, because of the formation of secondary hydrochar. C content and HHV also increased, giving rise to substantially higher energy recovery, which surpassed 93 % after four recycling tests at 210 degrees C. Fouling and alkali indexes of hydrochars showed much lower values than those of the feedstock mainly attributed to the solubilization of Na and K. At 210 degrees C, process water recycling favored a further reduction of those indexes. N and P were largely transferred to the liquid fraction, particularly the latter. Zn and Cu were, by far, the most abundant heavy metals in hydrochars, with a Zn content being slightly above the value established in Decision (EU) 2022/1244 for their application as a soil amendment.