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 °C and 250 °C) were tested and recycling of the liquid fraction was studied to improve the quality of the resulting hydrochar. The hydrochars obtained at 210 °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 °C and 250 °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 °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 °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. • Hydrothermal carbonization was performed in a continuous pilot plant. • The hydrochars obtained fulfilled the criteria to be used as solid biofuels. • Recycling process water improved hydrochar characteristics as solid biofuel. • Recycling process water notably improved the ash agglomeration indexes of hydrochars. • Furfural and urea were the most abundant compounds in process water analyzed by GC/MS.