Study on the gasification mechanisms of key intermediates of oily sludge in supercritical water by ReaxFF-MD method

Supercritical water gasification has been confirmed as an advanced treatment method for organic waste including oily sludge. As a supplement to our previous studies, molecular dynamics (MD) simulations must be implemented to understand the gasification mechanism thoroughly. Five representative model compounds were selected to investigate the gasification mechanism using ReaxFF-MD method. The molecular evolution of five key intermediates in SCWG was simulated and compared at 3500 K. To overcome the gasification challenge of oily sludge, toluene was selected for further investigation due to its prominent proportion and relatively low reactivity. Simulations were systematically discussed under different temperatures (2500–3500 K) to study the product distribution. It was found that the yields of the main products (H2, CO and CO2) continuously increased with reaction time and temperature. The yields of C1-C3, C4-C7 and C7+ increased more rapidly at higher temperature, with the yield of short-chain carbonaceous organics (C1-C3) being significantly higher than that of other carbonaceous organics. The decomposition kinetics of toluene in supercritical water were also analyzed, and the apparent activation energy was 235.22 ± 22.66 kJ⋅mol−1. Furthermore, four main gasification pathways were identified by tracking molecular animations. This study would provide valuable insights into the transformation characteristics of oily sludge.