Insights into gasification conversion mechanism of lignin in CO₂/H₂O environment: ReaxFF molecular dynamics...

To tackle global energy and environmental challenges, address the unclear gasification mechanism of lignin in CO₂/H₂O environments, this study uses reactive molecular dynamics simulations to investigate lignin gasification rules and mechanisms. A lignin system was simulated across a temperature gradient of 2000-3500 K via LAMMPS to elucidate molecular evolution, solid-liquid-gas phase transitions, and intrinsic reaction pathways. The results show that temperature is the core driving factor for lignin gasification. With the increase in temperature, C-C and C-H bonds continuously cleave; C-O bonds first decrease due to initial decomposition and then increase due to the enhanced formation of CO, which promotes the conversion of lignin from solid phase to liquid phase and further to gas phase. At high temperatures, the gas phase becomes the dominant product. Key findings reveal fundamentally different regulatory effects between the 2 atm: the CO₂ atmosphere exhibits higher reactivity and preferentially attacks C-C bonds, leading to thorough solid-liquid conversion at low temperatures, and generating a high yield of CO (approximately 1180 molecules) at 3500 K. Conversely, the H₂O atmosphere has low initial reactivity, preferentially attacks C-O bonds, and requires higher temperature (>2600 K) to accelerate reactions. Moreover, thermal decomposition dominates at high temperatures, resulting in the gas phase yield of the H₂O atmosphere surpassing that of the CO₂ atmosphere. Mechanistically, CO₂ promotes CO formation through C-C bond cleavage and water-gas shift reaction, whereas H₂O promotes H₂ formation through water-gas reaction and hydroxyl group interaction. By uncovering these distinct intrinsic mechanisms, this study highlights specific applications for biomass gasification technology. It provides theoretical guidelines for tailoring gasification parameters to selectively produce specific high-purity CO or H₂-rich syngas, thereby directly assisting in the design of high-efficiency biomass conversion reactors and facilitating the high-value utilization of lignin.