[Objective] Under the ambitious dual -carbon goals, the new generation of coal -fired power units is undergoing a critical transition towards functioning as fundamental guarantee and system regulation power sources. Consequently, these units are required to frequently operat e at ultra -low loads (e.g., 20%–30% power load ratio conditions) for extended periods to provide deep peak -shaving services. Such severe deviation from the design conditions inevitably leads to operational bottlenecks. It is highly urgent to clarify the en ergy consumption characteristics under low-load conditions and achieve optimized energy savings. 

[Methods] Taking a typical 660 MW supercritical coal-fired power unit as the targeted research object, a comprehensive system off-design analysis model was established to simulate the thermal performance across the entire load spectrum. Through this rigorous mathematical model, the decay laws of the extraction steam pressure differential were quantitatively obtained, revealing the underlying thermodynamic mechanism of drainage blockage induced by the insufficient driving force. Furthermore, based on the principle of energy cascade utilization, three targeted drainage retrofit configurations were proposed and systematically evaluated: Scheme A (rerouting the drains of the high-pressure heaters directly to the deaerator); Scheme B (rerouting the drains of the low -pressure heaters to the condenser); and Scheme C (installing variable-frequency low-pressure drain pumps). A comprehensive techno-economic evaluation model was also formulated to assess the life -cycle viability. 

[Results] The comparative simulation results effectively demonstrate the varying thermodynamic benefits of the proposed configurations. For the high-pressure regenerative system, Scheme A effectively rebuilds the necessary drainage driving pressure differential, guaranteeing the stable 2 directional flow of the drains. More importantly, it successfully recovers the high -grade thermal energy, subsequently reducing the net coal consumption rate by a subst antial 3.10 g/(kW·h) under the extreme 20% THA (Turbine Heat Acceptance) condition. Regarding the low -pressure regenerative system, although Scheme C introduces a minor amount of mechanical pump work consumption, the exergy analysis reveals that it fundamentally eliminates a significant portion of the irreversible mixing exergy loss at the cold end. Consequently, compared to the conventional bypass method in Scheme B, Scheme C can further decrease the net coal consumption rate by an additional 0.20 g/(kW·h) . 

[Conclusion] Comprehensive techno -economic evaluation indicates that the initial investment and operational costs of the active pumping strategy are well justified by the long-term fuel savings. It is concluded that Scheme A and Scheme C serve as the opt imal retrofit configurations for resolving the drainage blockage problems in the high -pressure and low -pressure regenerative systems, respectively. These optimized solutions excellently balance the unit's operational stability with life-cycle thermal economy.