Early evolution of coal nitrogen in opposed flow combustion configurations

Abstract

A laminar opposed flow, pulverized coal combustion configuration was used to explore the early evolution of light gaseous nitrogenous species into the bulk gas phase. Two coals of different ranks were considered. Concentration profiles of HCN, NH3 and NO were measured, under both oxidizing and reducing environments and at high heating rates provided by the hot flue gases of a CO/O2/Ar flame. Net rates of formation into the bulk gas phase were calculated from the experimental data after correcting for diffusion and convection effects. These rates were then related to particle time-temperature histories. Experimental data show that HCN precedes NH3 and NO for both coals. It is the first light gaseous product of coal nitrogen evolution entering into the bulk gas phase. For low rank coals, either only a small amount of tar nitrogen is released or its subsequent oxidation to light gaseous products is slow. For high rank coals secondary reactions of tar are rapid and lead to substantial levels of nitrogenous species. A simple kinetic model is proposed to relate gas phase measurements to coal devolatilization kinetics. The latter are similar for three experiments involving two coals and can be described by a single rate constant given by 63.8 exp(−5220/RT). Bituminous coal, under oxidizing conditions, shows substantially higher rates, possibly due to energy feedback mechanisms in the vicinity of the particles. Our value, which was derived from gas phase species measurements, compares reasonably well with literature values, which originated from solid phase measurements.