[Objective] Coal-fired power units are increasingly required to operate over a full load range and participate in deep peak regulation to accommodate renewable energy integration. However, under such variable conditions, issues such as efficiency deterioration and corrosion at the cold end of the air preheater become prominent. To address these challenges, this study investigates the off-design performance of the flue gas waste heat utilization system in a dual turbine regenerative coal-fired unit. The goal is to achieve efficient and safe operation across the entire load spectrum, from low to full load, by optimizing the allocation of waste heat recovery. 

[Methods] A detailed simulation model of a 1 000 MW dual turbine regenerative coal-fired power unit was developed using Ebsilon professional software. The model incorporates all major components of the power cycle, including the 2 http ://rlfd.cbpt.cnki.net boiler, turbine, regenerative heaters, air preheater, flue gas bypass duct, and air heater. The simulation covers a n entire range of loads, from 20% to 100% turbine heat acceptance (THA). Key parameters influencing the waste heat distribution and overall system efficiency were systematically evaluated. These parameters include: the bypass flue gas ratio (the fraction of flue gas diverted around the air preheater), the mixed flue gas temperature downstream of the air preheater (after mixing bypass and main streams), and the outlet air temperature from the air heater. A collaborative optimization approach was employed to identify the combination of these three parameters that minimizes the net coal consumption rate for power generation at each load level. 

[Results] The simulation results demonstrate that the three key parameters fundamentally determine how the recovered flue gas waste heat is partitioned between “air-side regenerative heating” and “water-side regenerative heating”. This partition strongly influences both the economic performance and operational safety. Specifically, the optimal bypass flue gas ratios were found to decrease as the unit load decreases: 18% at 100%THA, 16% at 80%THA, 15% at 75%THA, 12% at 60%THA, 12% at 50%THA, 11% at 40%THA, and 11% at 20%THA. The corresponding optimal values for the mixed flue gas temperature after the air preheater and the air heater outlet air temperature were de termined as: 132 ℃/85 ℃(100%THA), 134 ℃/87 ℃(80%THA), 135 ℃/90 ℃(75%THA), 137 ℃/92.5 ℃(60%THA), 138 ℃/95 ℃(50%THA), 140 ℃/97 ℃(40%THA), and 140 ℃/100 ℃(20%THA). Compared to a reference configuration without flue gas waste heat utilization, the proposed optimized system reduces the gross coal consumption rate for power generation by 2.88 , 2.66, 2.63, 2.36, 2.17, 2.04, and 1.93 g/(kW·h) at the respective load levels. 

[Conclusion] The off-design characteristics of the flue gas waste heat utilization system in a dual turbine regenerative coal-fired unit are highly dependent on the coordinated control of the air preheater bypass ratio, mixed flue gas temperature, and air heater outlet temperature. Properly matching these parameters to the unit load enables a balanced recovery of waste heat while maintaining the flue gas temperature above the acid dew point, thus mitigating low-temperature corrosion risks. The optimal parameter sets obtained in this s tudy provide a practical reference for the design and variable -load operation of such waste heat recovery systems in large -scale coal-fired power plants aiming for higher flexibility and efficiency.