Effect of Preoxidation O2 Concentration on the Reduction Reaction of NO by Char at High Temperature

Preoxidation experiments were carried out to determine the effect of temperature, ranging from 1000 to 1600 degrees C, on the O-2 enhancement of NO reduction by a bituminous coal char. The results showed that there is a critical O-2 concentration that gives a maximum amount of NO reduced by char. The effect of O-2 on the reduction of NO indicates a balance between the oxidation reaction and the formation of surface complexes on the char. A large number of C(O) surface complexes can form during the gasification process of char but cannot stably exist at high temperature. The presence of active vacant sites in the complexes is an important factor affecting the reduction of NO and char. This analysis provides a theoretical foundation for further reducing NO emissions during coal combustion. In recent work, we found that a four-level air-staged combustion technology realized ultralow NOx emissions of similar to 100 mg/Nm(3) at 6% O-2, which is much lower than that obtained by the current single-level air-staged technology with NOx emissions of about 200-300 mg/Nm(3) at 6% O-2. The difference is because, in the four-level air-staged case, the O-2 concentration along the reducing zone is maintained at a uniform low value that is beneficial to the enhancement of the reaction between NO and char.