Effect of a water-tolerant Lewis acid catalyst on the yields and properties of hydrochars from hydrothermal carbonization of walnut wood

The hydrothermal carbonization of walnut wood chips was conducted at 200–250°C for 1–8 h. Increasing the hydrothermal carbonization temperature or the residence time decreased the volatile products and increased the fixed carbon content of the hydrochars. The hydrochars produced from the non-catalytic experiments at 250 °C for 6 and 8 h were in the lignite class. The lowest O/C and H/C atomic ratios were obtained after carbonization at 250 °C for 8 h. The catalytic hydrothermal carbonization experiments were carried out in the absence and presence of InCl₃ using 1, 2, and 4 mmol of InCl₃ at 200, 225, and 250 °C for 4 h. The highest heating value of hydrochar from the catalytic experiment was 24.73 MJ/kg and was obtained at 250 °C for 4 h using 1 mmol InCl₃. Process water reuse resulted in increased heating values of the hydrochars in both the non-catalytic and catalytic experiments. The use of InCl₃ promoted the coalification degree of the hydrochars. These results demonstrate that InCl₃ is a suitable catalyst for producing hydrochars via the hydrothermal carbonization of walnut wood chips, which can be used as a solid biofuel.