Comparative Study between Flameless Combustion and Swirl Flame Combustion Using Low Preheating Temperature Air for Homogeneous Fuel NO Reduction

Abstract

The reduction of NOx emission in the combustion process is of vital importance to satisfy the increasingly strict NOx emission standards. Although substantial progress has been made for fuel NO reduction in conventional flame combustion, the homogeneous fuel NO mechanism in flameless combustion has not been systematically investigated. This paper performs a detailed comparative study between flameless combustion and conventional swirl flame combustion for homogeneous fuel NO reduction by both experiments and numerical simulations. The simulations are validated with experiments. It is found that the equivalence ratio (Φ) is significant for the stability and performance of flameless combustion. First, we found that the critical Φ to achieve flameless combustion is reduced with the increase of air preheating temperature. Second, importantly, with the increase of Φ from 0.68 to 0.82 a decreasing tendency of fuel NO emission is found in our experiments, which is the opposite of high temperature combustion. Considering that CO emissions are sharply increased when Φ > 0.8, there is a recommended Φ (i.e., Φ = 0.8 for the present study) to obtain the minimum emissions of fuel NO and CO in flameless combustion. Moreover, our experiments for the first time show that flameless combustion can reduce the homogeneous fuel NO emission by 60–85% relative to swirl flame combustion. Reaction pathway analysis demonstrates that in flameless combustion fuel NO is mostly formed via the NH3 → NH2 → HNO → NO path while NO is reduced through the pathway NO → HCN → NCO → N2. The present study reports the advantage of flameless combustion using low preheating temperature air for significant reduction of homogeneous fuel NO and provides new fundamental understandings of the fuel NO mechanism.

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