Numerical Simulation of NO x Formation in Non-Premixed Methane Combustion Using Internal Flue Gas Recirculation

Abstract

ABSTRACT Internal flue gas recirculation (IFGR) is one of the most effective techniques for NO x reduction in boilers. Previous studies mainly focused on premixed combustion; however, the NO x formation principles in non-premixed methane/air flames are still unclear. This work aims to investigate the NO x formation mechanisms in non-premixed methane combustion at various IFGR rates. A series of three-dimensional computational fluid dynamics (CFD) simulations with detailed mechanisms was conducted. The results show that NO x is generated below 30 mg/m3 for IFGR rates higher than 17.7%, corresponding to the oxygen concentration in the oxidant mixture below 0.15. In high IFGR rates, the incompletely mixed oxidants and the presence of the central recirculation zones weakens the NO x reduction performance. Influenced by the decreased temperature by IFGR, the generation of thermal NO x is largely eliminated from 66 to 1 mg/m3. Prompt NO x contributes more than thermal NO x for the IFGR rates above 12.2%. The reaction range of thermal NO x is found moving upstream toward the nozzle with the axial distance of 0.1 m at most. IFGR gradually reduces prompt NO x but its spatial reaction ranges are hardly influenced. IFGR has no obvious effects both on the reaction rates and ranges on the minorities of the N2O and the NNH routes. This work connects the IFGR rates and the NO x formation pathways in non-premixed methane combustion, and highlights the importance of eliminating prompt NO x to achieve low-NO x combustion rather than sole NO x suppression method, which can provide foundation for designing low-NO x techniques.