Effect of spent liquor recycle during hydrothermal carbonization on the properties of hydrochar

Hydrothermal carbonization is a feasible way to convert biomass into valuable hydrochar, the recycle of spent liquor during HTC is beneficial to reduce the output of spent liquor. The effects of spent liquor recycle on the properties of hydrochar and the biogas potential of spent liquor are investigated in this study. Part of the spent liquor (50% volume fraction) was recycled four times at 220 and 240 °C HTC, respectively. The results showed that the surface area of hydrochar was in the range of 7.2-8.6 m²/g after spent liquor recycle, and the peak of -OH, C = O and C-O became more intense. The Cd²⁺ adsorption capacity of hydrochar was around 1.3 mg/g and it decreased by 0.02-0.15 mg/g after spent liquor recycle. The biogas yield of spent liquor kept declining from 263 to 29 mL/g-COD with each cycle of spent liquor. Our results suggest that spent liquor recycle is a promising method to improve hydrothermal carbonization process.