Numerical Simulation of NOx Formation in Non-Premixed Methane Combustion Using Internal Flue Gas Recirculation

Internal flue gas recirculation (IFGR) is one of the most effective techniques for NOx reduction in boilers. Previous studies mainly focused on premixed combustion; however, the NOx formation principles in non-premixed methane/air flames are still unclear. This work aims to investigate the NOx forma...

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Bibliographic Details
Published in:Combustion science and technology 2024-12, Vol.196 (16), p.3777-3801
Main Authors: Cheng, Jiaying, Zong, Chao, Ji, Chenzhen, Bao, Jinrong, Zhu, Tong
Format: Article
Language:eng ; jpn
Subjects:
Combustion
Computational fluid dynamics
Flue gas
Fluid dynamics
Hydrodynamics
Methane
Nitrogen oxides
Nitrous oxide
Oxidizing agents
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Summary:Internal flue gas recirculation (IFGR) is one of the most effective techniques for NOx reduction in boilers. Previous studies mainly focused on premixed combustion; however, the NOx formation principles in non-premixed methane/air flames are still unclear. This work aims to investigate the NOx 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 NOx 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 NOx reduction performance. Influenced by the decreased temperature by IFGR, the generation of thermal NOx is largely eliminated from 66 to 1 mg/m3. Prompt NOx contributes more than thermal NOx for the IFGR rates above 12.2%. The reaction range of thermal NOx is found moving upstream toward the nozzle with the axial distance of 0.1 m at most. IFGR gradually reduces prompt NOx 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 NOx formation pathways in non-premixed methane combustion, and highlights the importance of eliminating prompt NOx to achieve low-NOx combustion rather than sole NOx suppression method, which can provide foundation for designing low-NOx techniques.
ISSN:0010-2202
1563-521X
DOI:10.1080/00102202.2023.2195557