Direct numerical simulations of polypropylene gasification in supercritical water

In order to reduce environmental pollution by plastic wastes, supercritical water gasification (SCWG) appears as a promising technology. The present study investigates the SCWG process of polypropylene (PP) plastic waste using particle-resolved direct numerical simulations (PR-DNS). A directional ghost-cell immersed boundary method has been used to solve the reacting boundary condition, including detailed molecular diffusion models. To validate the procedure, SCWG of a coal particle has been first investigated as a benchmark, analyzing in detail interphase momentum and heat and mass transfer, and chemical reactions are analyzed. Surface reactions and the resulting Stefan flow expand the boundary layer around the particle, impacting the efficiency of heat and mass transfer. Adding then a suitable reaction mechanism, SCWG of PP plastic wastes leading to combustible gases is analyzed by PR-DNS and found to be very efficient. The gasification temperature is an important parameter to control SCWG efficiency. To the authors' knowledge, this is the first PR-DNS study investigating the SCWG process for plastic wastes, and it provides interesting information regarding transfer processes and their limitations.