Abstract

Novel flue-gas internal recirculation (FIR) is gaining increasing attention owing to its overwhelming stable combustion and nitrogen oxide (NOx) reduction advantages. This paper aims to reveal the nitrogen reduction mechanisms in the exciting new FIR methods and discuss the main FIR techniques in terms of combustion performance. In particular, the basic concept and the nitrogen reduction mechanism of FIR are first introduced. Besides, three main categories of FIRs are reviewed in accordance with the characteristics and operation status of different devices. The analysis of 16 FIR systems suggests that the recirculation space, effective recirculation rate, and combustion ignition limit must be considered during the design process, in order to balance NOx reduction and combustion efficiency. The integrated FIR and external FIR can achieve NOx emission reductions of at least 20% and 15.5%–20%, respectively. The embedded FIR exhibits more outstanding NOx emission reduction performance, with NOx reduction ratios of up to 26.09%–65%. In addition, the influence elements of FIR on combustion efficiency and nitrogen reduction are also discussed. In conclusion, strategies aimed at enhancing the combustion performance of novel FIR burners are thoroughly delineated, and the challenges that need to be addressed are succinctly highlighted in a forward-looking perspective. This review is expected to clarify the NOx reduction and combustion characteristics of current FIR techniques and suggest a development direction for FIR design.

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