A ReaxFF study of the effect of pressure on the contribution of char-CO2 gasification to char conversion during pressurized oxy-fuel combustion

Abstract

The effect of pressure on the contribution of char-CO2 gasification to char conversion during pressurized oxy-fuel combustion (POFC) at 10–25 bar was investigated using reactive force field molecular dynamics (ReaxFF MD) simulation. The results showed that char conversion rates under air (O2/N2), CO2/N2, and oxy-fuel (O2/CO2) atmospheres all increased with the increase of total pressure. Both the char-O2 oxidation under air atmosphere and char-CO2 gasification under CO2/N2 atmosphere followed the Langmuir-Hinshelwood (L-H) mechanism. The char reaction rates calculated by the L-H model based on the assumption of separate active sites and common active sites were compared with the ReaxFF MD simulation results under O2/CO2 atmosphere at different pressures. It was found that char-O2 oxidation competed with char-CO2 gasification under pressure of 10–15 bar, while the competition did not occur under pressure of 20–25 bar. The contribution of char-O2 oxidation and char-CO2 gasification to char conversion during char combustion in O2/CO2 atmosphere were quantitatively determined from the consumption of O2 and CO2. The char-O2 oxidation and char-CO2 gasification rates in O2/CO2 atmosphere at different char conversions were also examined. It indicated that the char-CO2 gasification rate was mainly controlled by the competition between char-CO2 gasification and char-O2 oxidation under low pressures, but was controlled by the partial pressure of CO2 under high pressures.