Pulverized coal combustion: Fuel nitrogen mechanisms in the rich post-flame.

Abstract

Chemical kinetic mechanisms governing the fate of coal nitrogen in the fuel-rich stage of a pulverized-coal staged combustion process were investigated. Emphasis was on determination of the effects of coal rank, temperature and stoichiometric ratios on the speciation and rates of destruction of nitrogenous species and correlation of coal data by a unif1ed mechanism. The relative importance of homogeneous and heterogeneous mechanisms during post-flame interconversion reactions of the fuel nitrogen pool was quantified. Experiments with doped propane gas and a high- and low-grade coals, burned under a variety of conditions in a 2 Kg/h downflow combustor, yielded timeresolved profiles of temperature, major (H₂, CO, CO₂, O₂ and N₂), nitrogenous (NO, HeN and NH₃) and hydrocarbon (CH₄ and C₂H₂) species. These profiles allowed global mechanisms describing the speciation and destruction of fuel nitrogen species to be explored, using predictive models of increasing levels of sophistication. Fuel nitrogen speciation varied significantly from coal to coal and depended on stoichiometric ratio and temperature, which were varied independently. A general correlation describing the destruction rate of NO was derived from data. This rate, which was first-order in both NO and NH₃, was generally valid for all coals and all conditions examined. Fuel nitrogen interconversion reactions, especially destruction of NO and HeN, was predominantly homogeneous, but no single elementary reaction was controlling. Temperature quench down the combustor is the origin of OH equilibrium overshoot. Expressions for estimating the OH equilibrium overshoot as a function of the axial temperature decay along the combustor were derived both empirically and kinetically from fundamental considerations using data from doped propane gas runs. These expressions, together with available literature values of gas phase rate coefficients, could adequately describe the post-flame NO and HeN profiles of coal and gas runs. HeN profiles in the far postflame zone of the coal flames are strongly influenced by the slow release of nitrogen from the coal residue. This devolatilization plays a critical role in supplying the HeN that drives the multistep process converting fuel N into molecular nitrogen.