A Reactive Molecular Dynamics Study of Thermal Pyrolysis Behavior and Mechanisms of Lignin During the Hydrothermal Process: The Function of the Water Molecules

Reactive molecular dynamics simulations were conducted on the hydrothermal process of lignin in the temperature range of 1200-3000 K. The role of water and the evolution of molecular numbers, chemical bonds and products in the hydrothermal process were deeply analyzed. Lignin was significantly inhibited from further decomposition into smaller molecules due to the influence of water at 2100-2400K. Based on the role of water, the lignin hydrothermal process can be divided into three stages: promotion (1200-1800K)-inhibition (2100-2400K)-promotion (2700-3000K). In the first two stages, water molecules were not consumed. Water molecules will promote or inhibit the pyrolysis process of lignin depending on the temperature. While in the third stage, water molecules participated in the pyrolysis reaction. The breaking of ether, cleavage of the ring, and the participation of water molecules are the main factors affecting the difference between the three stages. This paper supplied a deep understanding of lignin hydrothermal process.