Multiple drivers, interaction effects, and trade-offs of efficient and cleaner combustion of torrefied water hyacinth

Highlights: • The main reactions of the torrefaction process were deoxygenation and dehydration. • The control and torrefied water hyacinth combustions were quantified and optimized. • Biomass torrefied at 300 °C had the lowest E_a (184.94 kJ/mol) in CO₂/O₂ atmosphere. • 473.7 °C, 5 °C/min, and CO₂/O₂ atmosphere were the optimal combustion parameters. • Torrefaction reduced CO₂, NH₃, HCN, NO, and NO₂ emissions from the combustion. Developing cleaner and affordable alternatives to the sole reliance on fossil fuels has intensified efforts to improve the thermochemical conversion property of the second-generation lignocellulosic biomass. This study aimed to explore the effects of the two torrefaction temperatures (200 and 300 °C), the two reaction atmospheres (N₂/O₂ and CO₂/O₂), and the three heating rates (5, 10, and 15 °C/min) on the combustion regime of water hyacinth (WH). Decomposition behaviors, reaction kinetics, thermodynamics, and mechanisms, evolved emissions and functional groups, and fuel microstructure properties were quantified. The deoxygenation and dehydration reactions acted as the main drivers of the torrefaction process, with the peak degree of deoxygenation of 86.21% for WH torrefied at 300 °C (WH300). WH300 significantly reduced the quantity of oxygen-containing functional groups and altered the fuel microstructure properties. The order of the decomposition rates of the pseudo-components were hemicellulose > cellulose > lignin for both WH and WH torrefied at 200 °C (WH200) and cellulose > lignin > hemicellulose for WH300. The average activation energy fell from 197.71 to 195.71 kJ/mol for WH, 287.90 to 195.97 kJ/mol for WH200, and 226.92 to 184.94 kJ/mol for WH300 when the atmosphere changed from N₂/O₂ to CO₂/O₂. The heating rate exerted a stronger control on their combustion behaviors than did the reaction atmosphere. CO₂, NO, and NO₂ emissions dropped by 46.0, 53.1, and 65.9% for WH200 and 29.6, 42.8, and 62.5% for WH300, respectively, when compared to WH. 473.7 °C, 5 °C/min, and the CO₂/O₂ atmosphere were the optimal settings for the maximized combustion efficiency. 717.1 °C was determined as the optimal setting for the minimized combustion emissions. Our study can yield new insights into the large-scale and cleaner combustion of the torrefied water hyacinth.