Research and Application of Low-NOx Combustion Technologies for Natural-Gas-Fired Boilers: A Comprehensive Review

Natural-gas-fired boilers remain widely used for building and industrial heat, making NOx control relevant even as energy systems decarbonize. This comprehensive review synthesizes the published literature on staged combustion, flue-gas recirculation (FGR), premixed combustion, oxy-fuel combustion, humidified combustion, catalytic combustion, flameless/MILD combustion, and integrated systems. The evidence indicates that staged burners and moderate external FGR are the most mature retrofit options, whereas lean premixed combustion is generally better suited to new or deeply retrofitted small and medium boilers. Humidification coupled with waste-heat recovery can reduce NOx while increasing total heat recovery, but water management, corrosion, fouling, and auxiliary demand must be considered. Oxy-fuel/FGR systems facilitate CO2 capture but impose substantial oxygen-production, recycle, and CO2-conditioning requirements. Hydrogen blending widens lean operability while increasing flashback sensitivity and altering thermal-NO and NNH chemistry. Technology selection should therefore balance NOx, CO, efficiency, stability, auxiliary resources, retrofit constraints, and long-term reliability.