Experimental Investigation of NOx Reduction by Varying Internal Flue Gas Recirculation Structures in Non-Premixed Methane Combustion
THERMAL SCIENCE AND ENGINEERING PROGRESS(2024)
Tongji Univ
Abstract
Internal flue gas recirculation (IFGR) is one of the most effective techniques for eliminating nitric oxides (NOx) in boilers, however, the NOx reduction principles of varying IFGR structures in non-premixed methane flames have not been fully investigated. This work aims to experimentally investigate the NOx reduction characteristics in non-premixed methane combustion by varying IFGR structures. Five reductive air tubes with different nozzle contraction ratios (alpha) have been made to induce IFGR, while a control case was set for comparison. The results show that the NOx emissions are reduced from 158 to 39 mg/m3@3.5 %O2 by IFGR structural variations with inner thermal insulation. However, the most intensified IFGR structure cannot achieve the best performance of NOx reduction. Further analysis reveals that the intensified IFGR structures within proper ranges gradually shorten the local high-temperature zones, and enlarge the flame reaction ranges. When the IFGR structure causes overpowering recirculation and afterward excessive nozzle exit velocity, the uneven mixing and the local hightemperature zone will eliminate the performance of NOx reduction, and flame blowoff happens. In strongintensity IFGR cases, the flame temperature is hardly affected by varying global equivalence ratios (Phi), and the consequent NOx emissions. This work establishes a link between IFGR structural variations and NOx emissions in non-premixed methane combustion, and highlights the importance of enhancing mixing in non-premixed combustion rather than purely intensifying IFGR strengths, which can provide a foundation for designing lowemission nozzles.
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Key words
Internal flue gas recirculation (IFGR),Nozzle contraction ratios,Non-premixed methane combustion,Thermal NO x,Nitric oxides (NOx) emissions
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