Flue Gas Recirculation in Steam Boilers: A Comprehensive Assessment Strategy for Energy Optimization and Efficiency Enhancement

In modern steam boilers, flue gas recirculation (FGR) is generally adopted as a temperature control method for reheated steam. Some research suggests that FGR does not affect the thermal performance of steam boilers, while other studies report enhanced thermal performance. This investigation aims to enhance energy efficiency by using an iterative calculation approach to provide a thorough energy evaluation of a steam boiler with an integrated FGR system. The research applies thermodynamic and heat transfer principles to model combustion characteristics, radiative heat transfer in the furnace waterwall, and convective and inter-tube radiative heat transfer in the economizer, superheaters, and reheater. The model is validated using specifications from an existing power plant boiler and a parametric analysis to examine the effects of varying FGR rates on heat distribution and thermal performance at full and partial loads. The results show that radiative heat accounts for an average of 45% of the total heat supplied, with up to 10% in the heat recovery section. As the FGR rate increases, radiative heat in the heat recovery section decreases by 50%, while the convective heat transfer increases then drops. The model shows that the ideal FGR is bounded between 0.3 at 50% boiler load and 0.4 at full load. An analysis of the impact of FGR on the various parts of the boiler reveals that the economizer experiences the most significant net change in heat gain, followed by the reheater. The effect of gas recirculation on the economizer can be nearly twice as great as on the reheater, indicating that FGR has substantial influence on components beyond the reheater. The findings indicate that reducing excessive heat in the economizer and reheater can be accomplished under different load conditions by regulating the fuel consumption rate according to the analysis of the effects of FGR on radiative and convective heat transfer across various components.