Kinetic Modeling of Fuel-Nitrogen Conversion in One-Dimensional, Pulverized-Coal Flames

Abstract

A detailed reaction mechanism for converting fuel nitrogen to nitric oxide and molecular nitrogen in gas flames is used to model nitrogen chemistry in fuel-rich coal-dust/oxidizer flat flames. In the devolatilization zone a simplified pyrolysis model is used and a hydrocarbon oxidation scheme supplement the nitrogen reactions. The predicted distributions of nitrogenous species (HCN, NH₃, NO and N₂) are compared to time-resolved experimental data obtained for two stoichiometrics and coal-types. The model accounts for the radical removal on particle surfaces, and various heterogeneous reactions for reduction of NO are considered.

In the devolatilization flame zone the coal-N conversion mechanisms appear to be augmented by physical and chemical processes occurring in the vicmity of the devolatilizing particles. The current analysis supported by other investigations indicate that processes such as formation of fuel-rich volatile clouds and heterogeneous reduction of NO on surfaces of coal-particles and soot is responsible for the significant conversion of fuel-N to N₂ early in these flames. These processes, however, are at present not very well understood, and more work is needed to quantify their effect.

The post-flame conversion of fuel-N was found to be controlled by heterogeneous processes, since heterogeneous removal of radicals practically quenches the gas-phase chemistry in this region. In the bituminous coal flames the experimental data can be explained in terms of char-N release as HCN and perhaps, at higher temperatures, N₂. In the subbituminous flames there is considerable heterogeneous removal of NO, apparently with surface-catalyzed reduction of NO by NH₃ being the most important process under these conditions. For the subbituminous coal char-N release seems to occur primarily as NH₃.