Biowaste hydrothermal carbonization for hydrochar valorization: Skeleton structure, conversion pathways and clean biofuel applications.

Hydrothermal carbonization (HTC), as one of thermal conversion techniques, shows promising commercial potential for hydrochar production from wet biowaste. This technique was re-discovered and regraded as artificial coalification to mimic natural process. In recent years, researchers concern more about hydrochar obtained from HTC, since large amount of organic waste including sludge, algae, food waste, manure etc. are generated with high moisture, which can be directly used as reaction medium, and hydrochar has high carbon density and energy retention. With this regard, application of hydrochar as biofuel is a renewable and sustainable way for biowaste recycling. In this review, HTC process and pathways about hydrochar formation from (N-free/N-rich biowaste), carbon-skeleton structure, critical elements on clean properties, and hydrochar pelletization for biofuel production were presented. Potential applications and challenges for HTC as green and sustainable way were presented, which will provide prospect for hydrochar as clean and renewable biofuel.