TG-FTIR-MS Study on the Co-combustion of Digestate-Derived Hydrochar and Pyrochar with Anthracite: Combustion Characteristics, Emission Evolution and Ash Behavior

In line with China’s biomass valorization strategy, this study compares hydrothermal carbonization (HTC) and pyrolysis of digestate for co-combustion with anthracite using TG-FTIR-MS, with particular emphasis on three key aspects: (i) combustion performance, (ii) gaseous pollutant emissions, and (iii) ash-forming tendency. The results show that hydrochar enhances the reactivity of the initial combustion stage and reduces the ignition temperature of anthracite to approximately 300 ℃, with the comprehensive combustion characteristic index (S), combustion stability index (Fi), and combustibility index (Ci) increasing by 0.4%-64.1%, 89.2%-201.6%, and 91.0%-198.8%, respectively. In contrast, pyrochar decreases the burnout temperature by 12%-15%, indicating a distinct combustion enhancement pathway. Regarding gas-phase emissions, hydrochar exhibits 2.3-3.2 times greater CO2 suppression than pyrochar, while also reducing by-product emissions of SO2 by 6.9%-11.1% and NO by 4.4%-4.8%. These results suggest that hydrochar enables multi-pollutant control through the dual mechanisms of free-radical quenching and sulfur fixation. Mechanistic investigation based on two-dimensional correlation spectroscopy further clarified the sequential release behavior of gaseous products, while ANN modeling showed good predictive performance for thermogravimetric behavior within the experimental domain (R2= 0.99526). Solid-state 13C NMR further revealed the carbon structural basis underlying the distinct combustion reactivity and gaseous emission behavior of hydrochar and pyrochar. In addition, hydrochar improved ash fusion characteristics, leading to a 17.8%-35.7% reduction in the slagging propensity index compared with the raw blends, thereby mitigating ash-related operational risks in power plants. Overall, this comparative study demonstrates the superior performance of hydrochar over pyrochar and establishes a ”carbonization-combustion-emission-ash” framework for sustainable coal-biomass energy conversion.