Supercritical CO2 Power Cycles for Fossil Energy

Supercritical CO2 Power Cycles for Fossil Energy

The supercritical carbon dioxide-based power cycles can be implemented in indirectly and directly heated applications. The indirectly heated power cycle, shown in Figure 1, is applicable to boiler-type plants where the combustion gases and cycle working fluid are separated. There is essentially no loss or addition of CO2 during operation after the system is charged initially. A heat source (boiler) is used to indirectly heat the CO2 working fluid through a heat exchanger, similar to current supercritical steam cycles. This cycle is a non-condensing closed loop Brayton cycle with heat addition and rejection on either side of the expander. Energy is extracted from the sCO2 as it is expanded in the turbine. Remaining heat is recovered from the stream post-expansion via recuperators and used to preheat the compressed CO2 returning to the primary heat source. This recovery of waste heat in the recuperators limits the heat rejection from the cycle and improves efficiency. Additionally, the ability to modulate the temperature of the supercritical CO2 at the bottom of the cycle without condensing provides potential for dry cooling with either low or no water consumption.

Closed Loop SCO2 Recompression Brayton Cycle Flow Diagram
Figure 1. Indirect-Fired Supercritical CO2 Recompression Brayton Cycle
(click to enlarge)

In the directly-fired supercritical CO2 power cycles, the combustion of fuels, such as natural gas or coal syngas, and oxygen produce a steam and CO2 mixed stream that is integral to the process, used as the working fluid to drive the turbine and produce power. The turbine for the direct-fired cycle operates at a higher inlet temperature than that of the indirect-fired cycle. One potential configuration of the cycle is illustrated in Figure 2. The heat remaining in the CO2/steam mixture is recuperated to preheat the cooled, compressed CO2 that is recycled and used as the combustion diluent. The direct-fired supercritical CO2 power cycle, after further cooling to remove the water, can produce a high purity stream of carbon dioxide that is ready for use/reuse or storage, without expensive and energy intensive capture/separation technologies.

Oxygen Fueled Directly Heated SCO2 Cycle
Figure 2. Oxygen-Fueled Directly-Fired Supercritical CO2 Cycle
(click to enlarge)