Effect of H2O on char-nitrogen conversion during char-O2/H2O combustion under high-temperature entrained flow conditions

Abstract

The char-O2/H2O combustion under high-temperature entrained flow conditions was investigated experimentally and numerically to study the effect of H2O on the char-nitrogen (char-N) conversion. A numerical model was established based on the unsteady convection–diffusion equations for mass and heat transfer. Local Random Pore Model (RPM) was adopted to consider the change of specific surface area during char combustion. The Langmuir-Hinshelwood mechanism was used in the rate equations of main heterogeneous reactions to consider the competitions on surface active sites. The numerical results of char conversion and fractional conversion of char-N to NO reproduce the experimental results well. Both of the numerical and experimental results shows that the added H2O promotes the fractional conversion of char-N to NO and the promotion effect becomes weaker with the increase of O2 concentration. Based on numerical results, four crucial factors that affect char-N conversion were decoupled and discussed quantitatively with addition of H2O. The added H2O promotes nitrogen release rate at lower oxygen concentration but inhibits nitrogen release rate at higher oxygen concentration. The added H2O lowers the heterogeneous char-NO reduction rate constant greatly but enlarges the pore surface area slightly, resulting in a lower overall heterogeneous char-NO reduction rate. Char-NO reaction is the main NO reduction pathway, and most of HCN formed in char-N–H2O reaction is more likely to release out of char particle and converts to NO through HCN→NH3→NO pathway in the ambient flow.