[Objective] To address the problems of excessively high exhaust gas temperature, exceeding the design value by more than 40 ℃, and low -temperature corrosion of the air preheater in a 1000 MW ultra -supercritical 2 once-reheat coal-fired unit, an optimization strategy for cascade recovery and utilization of flue gas waste heat was proposed to improve unit thermal economy and operational reliability. 

[Method] Based on the Flownex simulation platform and field test data obtained from actual unit operation, a thermodynamic model of the flue gas waste heat recovery system was establish ed and validated. According to the principle of energy cascade utilization, the flue gas temperature was divided into high -, medium-, and low -temperature regions. Four retrofit schemes were developed and comparatively analyzed, including a combined low -temperature economizer and air heater system, a flue gas –air cascade waste heat recovery system, an air preheater bypass cascade flue gas waste heat recovery system, and a multi -stage coupled cascade utilization system for flue gas waste heat. The thermal performance and energy -saving characteristics of different schemes under variable operating conditions were evaluated in terms of exhaust gas temperature reduction, equivalent enthalpy drop of extraction steam, coal-saving effect, and turbine heat consumption rate. 

[Results] The simulation results show that the deviation between the calculated and measured exhaust gas temperatures was within 1%, indicating good model accuracy and reliability. With the decrease in exhaust gas temperature, the additional power g eneration from extraction steam replacement and the equivalent enthalpy drop per unit extraction steam increased significantly, resulting in enhanced coal-saving performance and reduced turbine heat consumption rate, while the energy -saving benefit in the low-temperature region gradually diminished. The application of the secondary air duct heater effectively mitigated the low -temperature corrosion problem of the air preheater and simultaneously recovered part of the flue gas waste heat, thereby improving b oiler efficiency. Under 100% turbine heat acceptance (THA) condition, the optimal bypass ratios of Scheme 3 and Scheme 4 were 0.12 and 0.18, respectively. The corresponding coal-saving effects reached 5.464 g/(kW· h) and 6.868 g/(kW· h), while the turbine he at consumption rate decreased by 54.110 kJ/(kW· h) and 71.919 kJ/(kW· h), respectively. Among the four retrofit schemes, Scheme 3 and Scheme 4 exhibited superior comprehensive energy -saving performance, especially under high -load conditions. 

[Conclusion] Scheme 1 and Scheme 2 are more suitable for low -load operating conditions and exhibit stronger adaptability to coal quality fluctuations, with relatively low investment cost but limited energy -saving potential. In contrast, Scheme 3 and Scheme 4 demonstrate r emarkable advantages in reducing coal consumption and turbine heat consumption rate. Scheme 4 is more applicable to units requiring higher energy -saving performance, whereas Scheme 3 achieves a better balance between technical feasibility and economic effi ciency. The proposed Flownex -based cascade flue gas waste heat recovery optimization strategy provides an effective technical pathway for energy-saving retrofit and efficiency improvement of large-scale coal-fired power units.