Research On Boiler And Recuperator In Supercritical Carbon Dioxide Coal-fired Power Generation System

The goals of carbon peak and carbon neutralization indicate the development direction of energy structure transformation for China.Coal plays a role as fundamental energy source to improve the development of large-scale renewable energy.The traditional coal-fired water-steam power plant has insufficient flexibility due to huge components size,which cannot meet the demand of energy development.Supercritical carbon dioxide cycle（sCO₂ cycle）coal-fired power generation systems is regarded as a revolutionary technology with benefits of higher efficiency,compact size and fast response to external load variation.Lots of researches have been conducted focusing on cycle optimization and system conceptual design.It is urgent to carry out researches on components.Focusing on sCO₂ boiler and recuperator,this paper establishes detailed numerical models on flow and heat transfer in component level and comprehensive models coupling sCO₂ cycle with component models to study on three topics:scale law of sCO₂ boiler dependent on power capacities,strategy to decrease cooling wall temperatures in sCO₂ boiler,and design of parallel network for large capacity recuperators.The effects of capacity on power generation system and boiler is studied,and the scale law of sCO₂ boiler dependent on power capacities is revealed.Once sCO₂ cycle is integrated in coal-fired power plant with conventional boiler design（total flow mode,TFM）,ultra-large pressure drop of sCO₂ boiler occurs to suppress system efficiency,namely pressure drop penalty effect.Partial flow mode（PFM）is proposed to deal with this problem for 1000 MW power plant in reference,but it is not clear whether PFM is applicable for small scale power plant.The effects of PFM and TFM on system performance are examined at various power capacities covering 50-1000 MW.A numerical model coupling thermodynamic cycle and thermal-hydraulic characteristics of boiler is established.Scale laws of boiler size,mass flux and pressure drop in boiler tubes are developed.With power capacities decrcase,the surface to volume ratio in sCO₂ boiler increases,reducing the mass fluxes in cooling wall tubes.Hence,the pressure drop penalty effect is weakened,increasing the thermal efficiency.It is concluded that PFM is necessary for power plants with power capacities larger than 100 MW,however,TFM can be used for power plants with power capacities smaller than 100 MW.A comprehensive strategy to decrease cooling wall temperatures is proposed to avoid overheating of sCO₂ boiler cooling wall.Due to the deep recuperation of sCO₂ cycle and special physical properties of CO₂,sCO₂ boiler introduces challenges compared with water-steam boiler:the cooling wall is more prone to overheating and bursting due to higher wall temperatures because of the higher working fluid temperature and lower convective heat transfer coefficient in tubes.The boiler in a 1000 MW power plant with double reheating is selected as the research object.The work is based on fundamental consideration of the thermal coupling between radiation heat flux of flue gas side and heat transfer and flow characteristics of sCO₂ in heat exchanger tubes.Numerical model is established by coupling thermodynamic cycle of system,boiler design,and thermal-hydraulic and heating transfer characteristics of cooling wall.The research is conducted in two stages:（1）Considering the non-uniform distribution of heat flux along the flue gas flow direction only,the matching strategy between flue gas side and sCO₂ side is proposed.Over entire flow path of flue gas in boiler,the heat flux decreases with the decrease of flue gas temperature.Thus the traditional matching method of low-temperature working medium with high heat flux can be adopted,by which the low-temperature CO₂ enters the cooling wall first,and then enters the convective heat exchange surface with low heat flux.In furnace,the cooling wall consists of several modules,each having different CO₂ temperature in tubes and different thermal resistances.The conventional method fails to obtain the optimal arrangement for sCO₂ modular cooling wall.Allowable-heat-flux is proposed,which integrates the sCO₂ temperature and the comprehensive thermal resistance,representing the maximum heat flux the tube can bear.Based on allowable-heat-flux,a new method is proposed to match higher heat flux with higher allowable-heat-flux,which decreases the cooling wall temperatures further.（2）Considering the 3D non-uniform distribution of radiation heat flux in furnace,a comprehensive solution to decrease cooling wall temperatures is proposed.The solution includes four consecutive techniques:improved coupling in furnace width direction（CWD）,flue gas recirculation for heat flux reduction（FGR）,improved coupling in furnace height direction（CHD）,and enhanced heat transfer in cooling wall tubes（EHT）.A comprehensive thermal-hydraulic model is developed for a 1000 MW power plant.The new solution can reduce the cooling wall temperatures from 670.5℃ to 635.0℃,among which CWD.FGR.CHD and EHT contribute to the decrement of cooling wall temperatures by 13.3℃.4.4℃,6.8℃and 11.0℃,respectively,concluding that CWD and EHT are more effective than other techniques.The flow characteristics of parallel network for large capacity recuperators are analyzed,and the effects parallel network parameters on recuperator performance are studied and the suggestions for the design of parallel network of recuperators are given.Due to large thermal load,it is necessary for sCO₂ recuperator to adopt efficient and compact heat exchanger,among which printed circuit heat exchanger（PCHE）has attracted much attention.However,the capacity of a single PCHE is limited,thus a large capacity recuperator shall be composed of multiple PCHEs in parallel to form a parallel network.The component of high temperature recuperator（HTR）in a 1000 MW power plant with recompression cycle is selected as the research object.Calculation model coupling thermodynamic cycle of system and flow characteristics of parallel network in recuperator is established.The parallel network is assumed to consist of N1 main pipes,and each main pipe has N2 branch pipes with PCHEs connected in parallel.The results show that,the pressure drop of the parallel network for large capacity recuperator is equivalent to that of PCHE,and the flow rates of branch pipes increase along the flow direction of the main pipe.Effects of N1 and N2 on the performance of recuperator are explored,taking thermal efficiency,weight of PCHEs and pipes,footprint area and flow maldistribution as the evaluation indexes of the recuperator.In theory,larger N1 yields better recuperator performance.However in practice,it is necessary to do comprehensive optimization considering design and layout of all components in the power plant.The lower limits of N1 for different power capacities are recommended as follows:1 for power plants below 350 MW,2 for 350～700 MW and 3 for 700～1000 MW.On the premise of meeting the size restrictions,the PCHE size is recommended to be increased,and N2 to be decreased.