Recycled Hydrothermal Carbonization of Bamboo Shoot Shells in a Catalyst System as a Sustainable Energy Conversion Method: Energy Utilization, Nutrient Recovery, and Reaction Mechanism

To improve the fuel properties of bamboo shoot shells, enhance the fertilizer properties of the process water, and reduce the environmental pollution caused by the discharge of hydrothermal carbonization (HTC) process water, the process water was recycled five times under a HCl-catalyzed system, a HAc-catalyzed system, a KCl-catalyzed system, and a catalyst-free system. The energy properties, combustion behaviors, and ash fusion characteristics of hydrochar were determined. Process water was evaluated for its potential in nutrient recovery. The results showed that hydrochar in all catalytic systems was suitable for industrial biofuels based on ISO 17225–8 (higher heating value >17 MJ/kg, N < 3 wt %, volatile matter < 75 wt %, ash <10 wt %). Recycling the process water enhanced the energy yield (EY) from 2.54% in the catalyst-free system to 3.92% in the HAc-catalyzed system, 3.05% in the KCl-catalyzed system, and 5.91% in the HCl-catalyzed system. The maximum EY of 67.00%–72.18% was obtained in the HAc-catalyzed system. The removal of metal ions from ash was the most efficient, and the risk of slagging was minimum in the HCl-catalyzed system. The use of catalysts increased the total organic carbon content, especially in the HAc-catalyzed system (2.92 g/L–3.71 g/L). In the HCl-catalyzed system, the recycled process water resulted in high contents of total nitrogen (3.86 g/L), P (0.372 g/L), and K (2.789 g/L). This technology could produce high calorific value fuels with liquid fertilizers. It also provides a solution for waste treatment in bamboo shoot processing, canned food, and other industries.