Research On The Effect Of Power Capacities On Supercritical Carbon Dioxide Coal-Fired Power Generation Systems

As an novel technology.the supercritical carbon dioxide cycle（sCO₂ cycle）coal-fired power generation system is still under development.Most of the research focused on several typical power capacities,such as 300 MWe or 1000 MWe.Considering the diversity of practical demands,it is necessary to explore the performance at different power capacities.In this paper,the performance of sCO₂ coal-fired power plants is investigated,with power capacities ranging from 100 MWe to 1000 MWe using simulation calculation methods.The research contents include the comparison of different flue gas absorption methods,the roadmap towards the efficiency limit for 1000 MWe sCO₂ coal-fired power plant,efficiency of sCO₂ coal-fired power plants at various power capacities,and the scaling law of sCO₂ boiler wall temperature dependent on power capacities.To absorb flue gas heat in a wide temperature range,the applicable range of different methods was determined.Both small and large sCO₂ coal-fired power generation system need to absorb flue gas heat in a wide temperature range.Therefore,we explore the effect of flue gas cooler（FGC）and overlap energy utilization（OEU）on sCO₂ cycle.Recompression cycle plus reheating（RC+RH）and tri-compression cycle plus reheating（TC+RH）are two basic cycle types for coal fired power plant.A thermodynamics model coupling with thermal-hydraulic characteristic is developed.It is shown that when using RC+RH,both OEU and FGC have the capability to extract moderate temperature flue gas energy.OEU has a 0.13%efficiency improvement compared with FGC.due to slightly decreased pressure drops in boiler components.Generally,both OEU and FGC can be adapted to RC+RH.and the system performance has weak difference between them.The situation is changed when using TC instead of RC.Even though TC+RH introduces difficulty in absorbing moderate temperature flue gas energy,OEU can decrease the outlet flue gas temperature to 126℃,which is acceptable practically.FGC has not sufficient capability to extract moderate temperature flue gas energy.The outlet flue gas temperature is 173℃.The double-channel-tail-flue concept is proposed to not only increase thermal efficiency of the system,but also elevate boiler efficiency.The study focused on 100 MWe rated power capacity,but the conclusions can be extended to other capacities.The roadmap to improve the efficiency of sCO₂ coal fired power plant is proposed and the limit efficiency is clarified.One of the objectives of developing coal-fired sCO₂ power generation systems is to improve the efficiency.The requirements for coal-fired power plant in China is that full-load power generation for large scale capacities,such as 1000 MWe,while emphasizing flexibility for the power capacities below the 600 MWe.Therefore,it is necessary to explore the limit efficiency of 1000 MWe coal-fired sCO₂ power generation system.This study consists of two parts.Referenced to Carnot cycle,the proposed roadmap is to increase the cycle efficiency by elevating average heat absorption temperature and lowering average heat release temperature.TC achieves the second largest contribution for efficiency increment,followed by the reheating technique.Then,TC,double reheating（DRH）and intercooling（IC）are integrated as TC+DRH+IC in the power plant.To completely absorb flue gas energy over entire temperature range of（1500～120）℃.A top cycle and a bottom cycle are connected for cascade utilization of flue gas energy.Overlap energy utilization is further utilized to fill the efficiency gap between top and bottom cycles.The proposed cycle also integrates the module boiler design to suppress the pressure drop penalty,and the flue gas recirculation to keep the heater surface temperature in an accepted level.A numerical model is developed for the comprehensive sCO₂ cycle.At the main vapor parameters of 35 MPa/630℃,the sCO₂ coal fired power plant reaches the net power generation efficiency of 51.03%,which is higher than 48.12%for a supercritical water-steam power plant at the same capacity.Such efficiency improvement saves 175.2 kilotons of coal and reduces 396.4 kilotons of CO₂ emission for 1000 MWe capacity in a fascial year.The efficiency of sCO₂ coal-fired power plants at various power capacities was studied and the capacity with optimal efficiency is determined.Recently,the thermal efficiency of the system was found to decrease with the increase in the power capacity.The research focused on the effect of the pressure drops in the sCO₂ boiler on the system performance by neglecting the pressure drops in the heat exchangers,except for the sCO₂ boiler,and by assuming a constant isentropic efficiency of the turbomachinery.A comprehensive model was established wherein the sCO₂ cycle was coupled with the models of various components.For the sCO₂ boiler,the total thermal load was assigned to various heaters,and the pressure drop in each heater was calculated.Owing to the strong penalty effect of the pressure drop,both total flow mode（TFM）and partial flow mode（PFM）were applied to the sCO₂ boiler.A fluid network integrating the recuperator units was established for heat recovery in the system.A thermal-hydraulic model was proposed for a single unit and an integration package.Various losses were considered in the prediction of the efficiencies of axial flow turbines and compressors.The thermal efficiency increases,attains a maximum,and then decreases with increase of W_net.This parabolic distribution results from the tradeoff between the decreased efficiency owing to pressure drops in the heat exchangers and the increased efficiency of the turbomachinery.The maximum thermal efficiency occurred at W_net of 300 MWe and 200 MWe when using the PFM and TFM,respectively.This work provides guidelines for selection of the optimal power capacity.The effect of power capacities on cooling wall temperature is studied,and the scale laws of cooling wall temperature dependent on power capacities is revealed.Heat transfer is one of the key issues when sCO₂ cycle coupled with boiler.Exploring the effect of power capacities on cooling wall temperature enables to recognize the coal-fired sCO₂ power plant at different power capacities comprehensively.The scaling law of cooling wall temperature was proposed and validated with power generation capacities ranging from 100 MWe to 1000 MWe.The convective heat transfer coefficient αf inside tubes satisfy α_f～W_net^0.40～0.53,decreasing with decreasing power capacities,which is unfavorable for controlling the wall temperature.The average thermal load qave in the furnace satisfy qave～W_net^0～1/3,decreasing with decreasing power capacities,which is beneficial for controlling the wall temperature.The cooling wall temperature is the tradeoff between αf and qave.With the power capacities decrease,the inner wall temperature(T_wi)of the cooling wall increase,while the outer wall temperature(T_wo)decrease,indicating that T_wi is sensitive to αf,while T_wo is sensitive to qave.The maximum wall temperature of the cooling wall is lower at small scale capacity.Finally,a control strategy was proposed to reduce the wall temperature of PFM boiler by changing the ratio of flow rate and ratio of thermal load of the cooling wall modules.