Scale law of sCO2 coal fired power plants regarding system performance dependent on power capacities

Abstract

Once supercritical carbon dioxide (sCO2) cycle is used for coal fired power plant with conventional boiler design (total flow mode TFM), ultra-large pressure drop of sCO2 boiler occurs to suppress system efficiency, thus we proposed partial flow mode (PFM). A CO2 stream is segmented into two parallel lines, each having half flow rate and half length. The effectiveness of PFM was examined at various power capacities (net power output Wnet) covering 50–1000 MWe. A numerical model is established by coupling thermodynamic cycle of system and thermal-hydraulic characteristics of boiler. Scale laws of boiler size, mass flux and pressure drop in boiler tubes are developed, agreeing with numerical simulation results. We show that with increase of Wnet, TFM increases pressure drops to generate additional pressure and load for compressors. This effect not only increases energy loads, but also raises exergy destructions for all components, deteriorating the first and second law efficiencies of the system. Compared with TFM, PFM reduces pressure drops according to the 1/8 criterion, the pressure drop penalty effect is eliminated to have much higher efficiencies than TFM. Because at smaller Wnet such as 100 MWe, sCO2 boiler has larger surface to volume ratio, flow passages of CO2 are sufficient to keep acceptable pressure drop, hence the system performance is similar when using TFM and PFM. We conclude that the flow splitting strategy is necessary for Wnet > 100 MWe, but not recommended only at very small power capacity such as Wnet < 100 MWe. Our work is important to understand the distinct feature of sCO2 cycle driven by different heat sources.