Effect of metal elements in coal ash on NO release characteristics during oxy-fuel combustion at high temperature

Abstract

High temperature (up to 1873K) oxy-fuel combustion experiments were conducted on a fixed-bed reactor. Two combustion modes were designed: coupled combustion (volatile combustion and char combustion are simultaneous) and separated combustion (volatile combustion and char combustion are separated and individual). Bases on it, effect of alkali and alkali earth metals (AAEMs) and transition metals (TMs) in coal ash on the fuel-NO release characteristics was studied. Results show that in the coupled combustion, AAEMs accelerate the fuel-NO release rate. Besides, AAEMs decreased fuel-NO release amount by around 10% at different temperatures. This inhibition effect is related to activity of metal elements. The effect of TMs is sensitive to temperature. Mn promotes the oxidation of fuel-N intensively at lower temperature, while Fe and Cu can improve the fuel-NO reduction efficiency at higher temperature. Coal ash is beneficial to NO reduction and the reduction efficiency is higher than 50% at 1873K. In the separated combustion, Mg, Ca and Mn have almost no effect on volatile-NO and char-NO release. Na and K can decrease the volatile-NO and char-NO release amount below 1573K. Both volatile-NO and char-NO reduction efficiency of K and Na decreases from 20% to 0% as temperature rises. Fe and Cu can suppress the release of volatile-NO and char-NO. The volatile-NO reduction efficiency of Fe and Cu increases but the char-NO reduction efficiency decreases with temperature increasing. Coal ash can improve the proportion of volatile-NO in fuel-NO, about 25% at 1573K. At last, the potential catalytic mechanisms in coupled and separated combustion were proposed.