12 – Fossil energy

As a higher efficiency replacement to steam-Rankine cycles, indirectly heated sCO2 power cycles warrant in-depth consideration for use in fossil-fueled power plants. In this configuration, the heat of combustion from an external coal or natural gas combustion process is transferred into the supercritical CO2 (sCO2) cycle via a primary heat exchanger, in a similar manner to a boiler in a steam-Rankine power plant. In addition to its typically large-scale, fossil-fueled power plants offer additional challenges to the application of sCO2 power cycles, most notably in the range of temperatures required for making efficient use of the flue gas thermal energy for power generation. Options for sCO2 power cycles and efficiently coupling them to primary heat exchangers are considered in this chapter, including recovery of lower temperature flue gas thermal energy, which is critical for effective thermal resource utilization and minimizing the size and cost of the sCO2 cycle. Performance and cost of fossil-fueled indirect sCO2 cycles from the literature to date are compared, as well as particular sCO2 cycle component challenges that are unique to fossil-fueled power plants. In addition, open cycle, direct-fired sCO2 cycles have also been proposed that are more analogous to gas turbines than steam-Rankine cycles. In addition to the efficiency advantages of sCO2 power cycles, these cycles also allow for extraction of CO2 at inherently high purity and high pressure, making them very valuable for carbon capture and storage consideration. These cycles employ stoichiometric fuel combustion with high-purity oxygen in a heavily diluted sCO2 environment, such that the working fluid also contains water vapor, incomplete combustion products, and other impurities from the fuel and oxidizer streams. In addition to discussing these challenges, this chapter also discusses oxycombustion issues, turbine cooling needs, and other component-specific challenges. As with the indirect sCO2 cycles, the performance and costs of direct sCO2 cycles from the literature are also compared, as well as prospects for future utilization.