Effects of process water recirculation on solid and liquid products from hydrothermal carbonization of Laminaria

Hydrothermal carbonization (HTC) is a promising thermo-chemical technology to treat wet biomasses for production of hydrochars but produces excessive process water. In this study, recirculation of process water from HTC of macroalgae Laminaria was investigated for 12 rounds. Recycling process water increased the hydrochar yield, carbon recovery rate and high heating value from 13.3% to 17.1%, from 22.9% to 32.6%, and from 18.4 MJ/kg to 20.5 MJ/kg after 12 rounds, respectively. The process water recirculation could partly alleviate the toxicity of process water through seed germination test. Volatile fatty acids (VFAs) predominantly accumulate with process water recirculation. The increased proportion of VFAs on chemical oxygen demand could promote methane production of diluted process waters, a 12.3% increase was observed in the round 10, compared with initial process water. These results showed that recycling the process water could reduce water consumption significantly and enhance energy recovery efficiency.