ReaxFF MD simulations of petroleum coke CO2 gasification examining the S/N removal mechanisms and CO/CO2 reactivity

Abstract

High S content (>4 wt%) petroleum coke (petcoke) are used as fuel but are also potential gasification feedstocks (commonly achieved with CO2). Petcoke S and N heteroatoms are in thermally stable structures and thus will contribute to the gasification process during consumption. Here, the removal and transformation of S and N from a petcoke atomistic representation during CO2 gasification were explored via the reactive force field (ReaxFF) NVT simulations. Pyrolysis, reducing, and oxidizing conditions were examined at 3000 K for 250 ps. Under these conditions the slow pyrolysis and slower gasification processes overlap, thus both contribute to the overall gasification behavior. The S/N transformations were: thiophenic-S → COS, C1-2S → CnOnS, COnS → H1-2S, SO2; with pyrrolic and pyridinic N → CON, CN, NO1-2 → C1-2O1-2N, COnN, HNO1-2 → HCN, HNO1-2, NO1-3 (0 < n < 5). The ReaxFF simulations captured the Boudouard reaction with one O atom in CO2 bonding with a C atom then the O-C bond in CO2 breaks after producing CO. The petcoke C atom that bonded with the O atom is then removed producing the second CO molecule. Reactive molecular dynamic simulations such as these are helpful in capturing both the structural diversity (scale challenge) and the complexity of the interactions to elucidate contributions to coke consumption.