Molecular dynamics investigation on the co-gasification of various components of sewage sludge in supercritical water

The supercritical water gasification (SCWG) technology is a promising method for the disposal and utilization of sewage sludge. As a mixture with various components, detailed studies on the co-gasification mechanism of sewage sludge in supercritical water (SCW) are needed. In this paper, three typical components, namely, alpha-O-4 structured lignin (C20H24O6), triglyceride (C9H14O6), and alanine (C3H7O2N) are selected to present the sludge mixture. The detailed SCWG processes of these components under different conditions are simulated by using reactive force field (ReaxFF) molecular dynamics method. The three components first undergo their own cleavage in SCW, and the cleavage order is C-O-C bond breakage of triglyceride, ether bond detachment of lignin and deamination of alanine. The order of further gasification is triglyceride, alanine, and then benzene ring segments of lignin. The presence of alanine improves the gasification efficiency of lignin, and thus increases the overall gasification of the system. The SCW mass fraction increases the production of H2 but decreases that of CO. SCW mass fraction of 66 %-81 % is suggested for the simulation cases. In addition, the SCWG process is highly temperature-dependent because of the high energy needed for opening the benzene rings of lignin, but a much higher temperature above ring-opening temperature cannot further increase the gasification efficiency.

Automated summary

In this study, the co-gasification mechanism of sewage sludge in supercritical water was simulated using the reactive force field molecular dynamics method. The results showed that the components of sewage sludge underwent their own cleavage before further gasification. The presence of alanine improved the gasification efficiency of lignin. The mass fraction of supercritical water had a significant effect on the gas production.