Comparison Of 1000MW Single-reheating And Double-reheating S-CO₂ Coal-fired Power Generation System

China is a big consumer of coal,and thermal power generation plays a dominant role.At the same time,China faces the challenge of energy conservation and emission reduction.The advanced power cycle of supercritical carbon dioxide（S-CO₂）has attracted extensive attention in recent years.The coal-burning S-CO₂ cycle is expected to replace the Rankine cycle due to its advantages of high cycle efficiency and compact system.When S-CO₂ is coupled with coal fired boiler,there are problems such as large mass flow of working medium in the furnace and absorption of flue gas heat in full temperature zone.Studies have shown that the above problems can be solved by adopting modular boiler and connected-top-bottom-cycle method.At present,S-CO₂ coal-fired power generation system is in the research process of further improving cycle efficiency.Introduced secondary reheat is one of the important methods to improve the cycle efficiency of water generating units.In recent years,the number of double-reheating units put into operation is increasing.However,there is still a lack of detailed and systematic comparison between the S-CO₂ single-reheating and double-reheating coal-fired power generation systems.In this paper,a comparative study of the generalized S-CO₂ cycle in which single-reheating and double-reheating cycle are not coupled with specific heat sources is carried out to analyze the efficiency gain mechanism of the reheating system and the penalty effect of heater pressure drop.Then,the S-CO₂ coal-fired power generation systems with 1000MW single-reheating and double-reheating were taken as the research object.The cyclic structure adopted the connected-top-bottom-cycle based on the repeated utilization of energy.The system performance of the conventional heating surface layout of S-CO₂ boiler was compared and the three factors affecting the pressure drop of the boiler cooling wall were revealed:heat transfer amount on heating surface,uneven distribution of heat load in furnace and pipe diameter of heating surface are analyzed.Finally,according to the influence mechanism,the conventional heating surface of the boiler is optimized,and the system performance after optimization is compared and analyzed.The results show that the double-reheating cycle has a thermal efficiency advantage of 0.68%when the heater pressure drop is zero.The pressure drop of the heater leads to efficiency penalty.The pressure drop of the reheat heater has a significant influence on the pressure drop of the mainstream heater,and the pressure drop of the secondary reheat heater has a significant influence on the pressure drop of the primary reheat heater.Since the pressure drop of the secondary reheat heater has the most significant influence on the cycle efficiency,the critical secondary pressure drop is defined（the pressure drop of the mainstream heater,the pressure drop of the primary reheat heater and the cycle thermal efficiency of single-reheating and double-reheating cycle are corresponding to each other respectively,and the pressure drop of the secondary reheat heater is obtained）.The critical secondary pressure drop is sensitive to the pressure drop of the primary reheat heater and is not affected by the pressure drop of the mainstream heater.When the cycle is coupled with the boiler,the pressure drop of the cooling wall is the leading factor of the pressure drop in the furnace,and the pressure drop of the secondary reheat heater cooling wall module（Part4）is the leading factor of the pressure drop of the cooling wall.According to the influencing factors of cooling wall pressure drop,the double-reheating boiler can be optimized by adjusting the position of Part4 and increasing pipe diameter.After the position adjustment,when the inner diameter of Part4 is 30mm,the cycle thermal efficiency of the double-reheating system is increased by 0.66%,and the cycle thermal efficiency of the double-reheating system is 0.22%higher than that of the single-reheating system.As the diameter of Part4 varies between 30mm and 40mm,the thermal efficiency of the double-reheating cycle ranges from 51.57%to 51.85%,which can be 0.50%higher than the thermal efficiency of the single-reheating cycle.In addition,the spatial distribution of heat in the single-reheating and double-reheating boilers is different.The influence of the spatial distribution of heat in the furnace on the critical secondary pressure drop is analyzed.The results show that the change of pressure drop caused by the different distribution of heat on the heating surface can be ignored.At the same time,the sensitivity analysis of the turbine inlet parameters is carried out.Increasing the temperature of the working medium at the turbine inlet will make the double-reheating cycle more efficient.Reduce the pressure of working medium at turbine inlet,and the single-reheating cycle has more advantages in efficiency.This study provides theoretical support for whether double-reheating is used in S-CO₂ coal-fired power generation system.Its significance for industrial application is as follows:when the inlet parameter of the turbine is 620℃/30MPa,the inner diameter of Part4 is 26.6mm and the critical secondary pressure drop is 0.380MPa,the cycle thermal efficiency of the double-reheating system is equal to the cycle thermal efficiency of the single-reheating system;When the inner diameter of Part4 is 40mm and the pressure drop of the secondary heater is 0.152MPa,the cycle thermal efficiency of the double-reheating system is 0.50%higher than that of the single-reheating system.