Co-hydrothermal carbonization of different feedstocks to hydrochar as potential energy for the future world: A review

Abstract Co-hydrothermal carbonization (co-HTC) has emerged recently as a promising thermochemical technique that can transfer organic solid residue (e.g., biomass, sewage sludge, plastic, livestock manure) into value-added products (i.e., hydrochar). This advanced mechanism conceals the weakness of hydrochars derived from the hydrothermal carbonization (HTC) of a single feedstock. The co-HTC of various feedstock blends used for hydrochar is reviewed in this article. Biomass/sludge, biomass/plastic, biomass/coal, biomass/manure were the most common feedstock blends. Many studies have found that co-HTC, compared to single feedstock, enhances the yield, ultimate and proximate properties, higher heating value (HHV), thermal behaviors, and overall quality of hydrochars. The synergistic effects of feedstock types, blending ratio, temperature, and residence time are also discussed. The feedstock mixing ratio emerges as the most important variable, followed by feedstock types, temperature, and reaction time. Thermogravimetric (TG) and derivative thermogravimetric (DTG) analyses have made it possible to better understand the thermal decomposition characteristics of hydrochars. In most papers on this subject, the range of combustion temperature and comprehensive combustion index (CCI) has increased. Overall, the physicochemical and thermal behaviors of hydrochar derived from co-HTC make it energy-efficient and environmentally sound by reducing the waste load and avoiding pollution issues. Additionally, several important suggestions have been recommended for future advances being made in co-HTC of feedstocks and its comprehensive application.