Performance evaluation and energy-saving mechanism analysis of a regenerative scheme based on heat capacity flow rate matching for coal-fired power plants under typical load conditions

To reduce the structural exergy loss caused by mismatch in heat capacity flow rates between extraction steam and water in the regenerative process, an “exit splitting and parallel heating” method is proposed and applied to a 1000 MW unit. The thermodynamic mechanism underlying the variations in energy-saving performance associated with the superheat utilization of extraction steam under typical load conditions is also investigated. In this configuration, a portion of water is diverted from the outlet of each corresponding regenerative heater (RH) and heated by the extraction steam, thereby reducing the superheat degree of extraction steam entering RHs. Comparative analyses are conducted among the conventional, outer steam cooler (OSC), master cycle (MC), and proposed units. Results show that the proposed scheme reduces exergy loss in both the regenerative process and the boiler, increasing the power generation efficiency by 0.50∼0.55 percentage points within 40%THA to 100%THA load conditions and outperforming both the OSC and MC units. As unit load decreases, the increasing superheat degree of extraction steam aggravates the mismatch in heat capacity flow rates between extraction steam and water in the RH, increasing the exergy loss per unit heat released by extraction steam in the superheated section. Meanwhile, the proportion of sensible heat released in the superheated section to the total heat released by extraction steam decreases. These combined effects limit the energy-saving potential associated with the superheat utilization of extraction steam and explain why the efficiency enhancement of the proposed unit does not continuously increase as unit load decreases.