Evaluation on the enhanced solid biofuel from co-hydrothermal carbonization of pharmaceutical biowastes with lignite

Pharmaceutical biowastes are typical wet low-grade industrial biowastes, exhibiting a dual characteristic of hazardous waste and renewable resource. Their efficient and clean energy utilization is important and indispensable from the viewpoint of environmental protection and resource savings. However, low energy density and high nitrogen content are two typical defective characteristics to be resolved. In this study, targeting at solid biofuel product, co-hydrothermal carbonization of two pharmaceutical biowastes (Chinese herb residue and antibiotic fermentation residue) with a low-rank coal (ShenMu lignite coal, SLC) were performed at a fixed prevailing temperature (240 ?) and different mixing mass ratios. The compositional features, the structural properties, and the thermo-chemical behaviors of co-hydrochar products were evaluated to determine their fuel properties. The results indicated that co-hydrothermal carbonization of pharmaceutical biowastes with 25-50% SLC could achieve favorable energy recoveries and nitrogen removal for the reaction system. Under the circumstances, notable synergistic enhancements on both upgradation and denitrogenation capabilities were revealed, producing compatible co-hydrochar with high energy density (HHV up to 24-25 MJ/kg) and stable nitrogen structures (nitrogen content as low as 1.4-2.0 wt%). Furthermore, compared to feedstock or monohydrochar, co-hydrochar product exhibited better pyrolysis behavior with less N-containing and incombustible gaseous components, and longer combustion process with more stable flame. Thus, co-hydrothermal carbonization with lignite coal would be a potential strategy to transform pharmaceutical biowastes into clean and high-grade solid biofuel.