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• S-CO 2 coal fired power plant was investigated. • The overlap energy utilization (OEU) principle was proposed. • OEU increases the average heat absorption temperature for bottom cycle. • An external air preheater (EAP) recycles extra heat to the system for bottom cycle. • OEU and EAP reaches identical efficiencies for top and bottom cycles. For a coal fired power plant, a combined supercritical CO 2 cycle (S-CO 2) absorbs flue gas energy over entire temperature range. Because top/bottom cycles absorb high and moderate temperature flue gas energy respectively, there is an efficiency gap between the two cycles. To fill the efficiency gap, the overlap absorption of flue gas energy is proposed, using top cycle to absorb high temperature flue gas heat, but bottom cycle to absorb not only moderate temperature flue gas heat, but also a part of high temperature flue gas heat. The overlap energy absorption in a high temperature regime increases CO 2 heat absorption temperature to improve bottom cycle efficiency. A four steps roadmap guides us to construct a combined cycle with RC + DRH as top cycle and RC + DRH + EAP as bottom cycle to reach the maximum efficiency limit, where RC, DRH and EAP represent the cycles of recompression, double-reheating and external air preheater, respectively. The components sharing among top/bottom cycles ensures simple system layout, and EAP recycles extra heat of bottom cycle to the system, keeping a smallest heat dissipation to environment. The net power efficiency reaches 47.99% at the main vapor parameters of 620 °C/30 MPa. The overlap energy utilization can be extended for other systems. [ABSTRACT FROM AUTHOR]

For coal fired power plant, the supercritical carbon dioxide Brayton cycle (S-CO 2 ) is difficult to absorb flue gas heat in a wide temperature range of 120–1500 °C. Here, novel methods are developed to cascade utilize flue gas heat, in which energies in high, moderate and low temperature levels are extracted by top cycle, bottom cycle or flue gas cooler (FGC), and air preheater, respectively. The cascade utilization shall satisfy the criterion that CO 2 temperature entering boiler for top cycle equals to CO 2 temperature leaving boiler for bottom cycle . The separate-top-bottom-cycle (STB) is proposed, in which no any component is shared by top and bottom cycles. Six possible bottom cycles are studied. The thermodynamics analysis is coupled with heat transfer and pressure drop analysis for whole power plant. It is found that the main vapor pressure of bottom cycle can be the “best” parameter to be adjusted over a wide range of 15–35 MPa to couple and optimize top and bottom cycles. Then, the parameter coordination principle is proposed to share specific components for top and bottom cycles. Thus, the separate cycles are converted into a connected cycle to simplify the whole system layout. The connected cycle has a power generation efficiency of 51.82% at main vapor parameters of 700 °C/35 MPa, significantly higher than available supercritical water-steam Rankine cycle power plant. The findings in this paper give a clue to further raise the power generation efficiency for large scale S-CO 2 coal fired power plant. [ABSTRACT FROM AUTHOR]

• Supercritical regeneration process is originally proposed. • Steam Rankine cycle based on supercritical regeneration process is constructed. • The current thermal efficiency bottleneck of the Rankine cycle is broken through. • Steam Rankine cycle and S-CO 2 Brayton cycle are compared based on average temperature. Compared with the steam Rankine cycle, high efficiency is one of the core advantages of the supercritical carbon dioxide (S-CO 2) Brayton cycle. However, the steam Rankine cycle still has the possibility of breaking its efficiency limit. This paper originally proposes the supercritical reheating regeneration process and constructed the supercritical regenerative Rankine cycle. It's found that the supercritical regeneration process can not only heat the feedwater, but also heat the reheated steam. then the thermal efficiency of the Rankine cycle can be further increased. Meanwhile, under the framework of the Carnot cycle, the average temperature at which heat is added (T ave,h) and the average temperature at which heat is rejected (T ave,c) are set as the optimization value, to perform a comparative study between the S-CO 2 Brayton cycle and the supercritical steam Rankine cycle. Through analysis, by applying the supercritical regeneration process, T ave,h of the Rankine cycle is significantly increased. It can reach or exceed T ave,h of the S-CO 2 cycle. Meanwhile, the heat-rejection process in the condenser of the Rankine cycle is the isothermal process, and its T ave,c is significantly lower than that of the Brayton cycle, which is a non-isothermal heat rejection process. Thus, the thermal efficiency of the steam Rankine cycle can surpass the S-CO 2 cycle. When the turbine inlet parameter is 30 MPa/620 °C, the thermal efficiency and average temperature at which heat is added are 55.65 % and 573.6 °C for RRRC + RH and 54.72 % and 568.8 °C for TC + RH, respectively. Based on the supercritical regeneration process, the Rankine cycle overcomes the existing efficiency bottleneck, and has the potential to surpass the thermal efficiency of the S-CO 2 cycle. [ABSTRACT FROM AUTHOR]

• TC is preferable than RC due to the elevated heat absorption temperature. • OEU is better than FGC to extract moderate temperature flue gas energy. • FGC cannot thoroughly extract moderate temperature flue gas energy in TC. • Double-channel-tail-flue enjoys higher thermal efficiency and boiler efficiency. Supercritical carbon dioxide (sCO 2) cycle is suitable for high temperature heat source, but introduces challenging in absorbing moderate/low temperature flue gas energy for coal fired power plant. Here, we explore the effect of flue gas cooler (FGC) and overlap energy utilization (OEU) on sCO 2 cycle. FGC and OEU extract moderate temperature flue gas energy by a splitting CO 2 flow rate from the cycle and a combined cycle, respectively. 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. The analysis is performed for a 100 MW rated power capacity. For RC + RH, we show that OEU yields an electric power efficiency increment of 0.13% compared to FGC, which is caused by smaller pressure drops in boiler components when using OEU. Even though TC + RH introduces difficulty in absorbing moderate/low temperature flue gas energy, OEU still can decrease outlet flue gas temperature (T fg,ex) to 126 °C, which is acceptable. However, FGC achieves a higher T fg,ex of 172.6 °C by a pinch temperature difference limit of 30 °C, which deteriorates boiler efficiency thus it is not acceptable. We conclude that OEU is better than FGC, no matter for RC + RH or TC + RH. The double-channel-tail-flue concept is proposed to not only increase thermal efficiency of the system, but also elevate boiler efficiency. This paper presents an important clue to design thermodynamics cycle for small capacity coal fired power plant, which is expected to have fast response with respect to load variations. [ABSTRACT FROM AUTHOR]

• Triple-compression sCO 2 cycle was adopted to evaluate the economic feature of coal fired power plant. • Attention was paid on the economic consideration of sCO 2 boiler and recuperator. • sCO 2 power cycle shows better efficiency than water-steam Rankine cycle. • Specific cost increases by 29% but levelized cost of electricity decreases by 1.32% compared to water-steam Rankine cycle. • Recuperator is a key component to control the cost of the whole system. The supercritical carbon dioxide (sCO 2) coal-fired power generation system has received great attention, but its economic characteristic is not well understood. Here, we present an economic comparison when using the sCO 2 power cycle and the water-steam Rankine cycle. The triple-compression sCO 2 cycle is adopted, incorporating overlap energy utilization over entire flue gas temperature range, intercooling and double reheating techniques. The heat-resistant steel materials of boiler are carefully selected to ensure its safe operation. The cost models of sCO 2 boiler and recuperator are paid more attention. We show that the sCO 2 power system attains the net power generation efficiency of 49.01%, which is higher than 48.12% for the advanced water-steam Rankine cycle system. Compared with water-steam system, the cost of sCO 2 turbine decreases by 30.0%, but the cost of sCO 2 recuperator seems to be one magnitude larger than that of USC heater, the cost of sCO 2 boiler increase by 36.3%. Hence, the whole sCO 2 power system increases the specific cost by 29.0%. Over an entire 30 years lifetime of the power plant, the levelized cost of electricity (LCOE) is 60.56 $/MWh for sCO 2 power system, which decreases by 1.32% compared to water-steam system. Therefore, we declare that even though the fabrication cost increases, the sCO 2 power system is preferable to the water-steam system. The specific cost of the sCO 2 power system can be further decreased by optimization of the recuperator, which is a key component in the system. [ABSTRACT FROM AUTHOR]

• A roadmap is proposed to reach the limit efficiency. • The roadmap reflected in general analysis and practical analysis of sCO 2 cycle. • TC achieves the second largest contribution for efficiency increment after RH. • The limited efficiency for sCO 2 coal fired power plant is over 51%. The Chinese Government has issued a series of documents to explain China's control over the carbon dioxide (CO 2) emission. One of the major tasks is to develop higher efficiency coal fired power plant, compared with current running power plant. In this paper, we aim to explore the roadmap to reach the efficiency limit for coal fired power plant using supercritical carbon dioxide cycle (sCO 2 cycle). Referenced to Carnot cycle, the proposed roadmap is to increase the cycle efficiency by elevating average heat absorption temperature (T ave, h) and lowering average heat release temperature (T ave, l). In contrast to recompression cycle (RC), tri-compression cycle (TC) is introduced. Due to the increased T ave,h , 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) °C. 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 sCO 2 cycle. At the main vapor parameters of 35 MPa/630 °C, the sCO 2 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 CO 2 emission for 1000 MW capacity in a fascial year. Our work provides the guideline for the design and operation of large scale sCO 2 coal fired power plant. [ABSTRACT FROM AUTHOR]