Hydrothermal co-carbonization of industrial biowastes with lignite toward modified hydrochar production: Synergistic effects and structural characteristics

Attempts to prepare high-grade and clean solid fuels from biowastes and low-rank coals are essential for protecting the environment and utilizing the energy from low-grade resources. The improvement of hydrothermal carbonization (HTC) and co-carbonization (co-HTC) on the upgrading and denitrogenation capabilities of industrial biowastes with low-rank coal as lignite (LC) was evaluated at 120–300 °C. The results demonstrated that coupled upgrading and denitrogenation occurred with a more intense effect on industrial biowastes than LC during the HTC process. For co-HTC in the prevailing hydrolysis (180 °C) or polymerization (240 °C) stage, an optimal mixing ratio of industrial biowaste/LC of 1:1–3:1 could result in an enhanced experimental calorific value, energy recovery efficiency, nitrogen removal efficiency, and weakened experimental nitrogen content, demonstrating the significant positive synergies on both upgrading and denitrogenation capabilities. The corresponding synergistic coefficients were maximized at 7% and − 23% for the calorific value and nitrogen content of hydrochars, respectively. The combined analyses of XPS, 13C NMR, and FTIR could provide evidence that these synergies of co-HTC were intrinsically associated with LC: (1) its more stable carbon and nitrogen functionalities; (2) promotion of its components or reaction sites on prevailing hydrolysis and polymerization reactions at corresponding temperature stages.