The 2015 Paris Climate Talks (COP21) were successful in achieving consensus from 196 countries that climate change must be given significant attention. There was a particular focus on the release of carbon, in the form of carbon dioxide and carbon monoxide, as something that must be curtailed.
Scientists have shown that the carbon emissions from the combustion of fossil fuels for the generation of electric power can most significantly be controlled by conservation and innovation through three strategies:
- Conservation efforts, which curtail the amount of power required
- A shift in power generation from fossil fuels to renewable energy sources
- Efficient capture and sequestration of carbon dioxide, the byproduct gas resulting from fossil fuel combustion particularly coal combustion
The first two strategies are being implemented, but fossil fuels are readily available and will still be needed as a power source for many years to come. This leaves the third strategy to help resolve the CO2 dilemma while providing reliable power generation. The United States Department of Energy (DOE) has been at the forefront of supporting innovative approaches to use coal combustion at high cycle efficiencies while also capturing the carbon dioxide (CO2) generated by power plant. Once captured, these emissions can be sequestered, thus reducing the environmental impact of gases known to contribute to climate change. For example, the Allam Cycle is under development with support by the D.O.E. This cycle utilizes an oxygen separation process to remove nitrogen from the air and then to utilize the remaining oxygen to burn with coal. The oxygen is pressurized to 4,000 psia and then used to oxidize pulverize coal to temperatures of 700C or higher that and without the presence of nitrogen, the products of combustion is almost 100% CO2. This high-pressure CO2 fluid stream is expanded through a turbine before it is sequestered into the ground at pressures of 1,100 psia. This system is thus not truly a cycle but rather an open-flow, supercritical SCO2 power system. The Allam Cycle also assumes that a new power plant facility is built and will serve to utilize the USA’s abundant supply of coal as the primary fuel for the life time of the plant.
An alternative to the Allam Cycle is one that has been theoretically developed by Concepts NREC that combines a Supercritical CO2 Brayton cycle (SCO2) with a Pressure Swing Adsorption (PSA) or an equivalent process that can capture CO2 from the products of combustion of an existing power plant. Although the Concepts NREC system also utilizes the recovered CO2 as the working fluid in an open-flow SCO2 cycle, there is one important distinction between the Concepts NREC system and the Allam Cycle: the proposed cycle is intended to be added to existing coal fired power plants as a thermodynamic topping cycle. As such, the proposed system can be properly labeled a “cross-over” system in that it can be retrofitted onto existing coal fired, powered generation plants until renewable energy power generation systems become the dominate source of utility power. Concepts NREC has labeled this system (SCO2)^2 because it is intended to use the highest combustion temperatures that an existing power plant can generate, while minimally impacting the continuous power generation of the existing steam boilers-turbine power generating units while producing power at a cycle efficiency of 40% to 50%. The high temperature, heat energy into the (SCO2)^2 system only reduces the temperature of the combustion gas that are used in the existing steam boiler by 50 to 100°F. After the (SCO2)^2 removes some heat energy from the combustion gases, the hot exhaust then proceeds as before to provide heat energy to the steam boiler.
All the exhaust from the power plant is directed to the Pressure Swing Adsorption (PSA) system where the CO2 gas is removed from the products of combustion. The (SCO2)^2 system utilizes the recovered CO2 gas to serve as the working fluid in the (SCO2)^2 system. The CO2 capture is accomplished using a PSA process that has successfully removed gases (including hydrogen) from a variety of industrial gas processes. Current independent studies indicate that the new cycle could utilize K-promoted hydrotalcite in a high-pressure, high-temperature PSA system, to recover CO2 from utility power plant exhaust gas.
The CO2 stream that is recovered from the PSA process is then directed to the sequestration compressors that are intercooled, reciprocating compressors. The compression of the CO2 from atmosphere to 1,000 psia is most effectively accomplished using positive displacement (reciprocating) compressors, due to the relatively low volume flow rate and high pressure ratio requirements for compression.
The ultimate goal of this hybrid cycle is to significantly reduce the external power required for carbon sequestration, by utilizing the highly-efficient SCO2 cycle to provide approximately 30 to 40% of the power required by the sequestration compressor. The discharge of the (SCO2)^2 cycle is sequestered into an underground geophysical vault, but only after the SCO2 is first passed through a let-down turbine that recovers a significant portion of the energy of compression, until the ground vault is pressurized—thus providing additional power generation.
The unique characteristics of the SCO2/PSA cycle include:
- An SCO2 cycle that uses, as a working fluid, the same CO2 that is recovered from the utility waste heat source.
- A PSA system that removes CO2 gas from the exhaust gas waste stream. The best choice for CO2 adsorbents that can operate at the temperatures anticipated in the integrated supercritical cycle have been researched by several contemporary and independent chemical engineering researchers [references 1, 2, and 3]. There is consensus that K-promoted hydrotalcite in a high-pressure, high-temperature Pressure Swing Adsorption (PSA) system can effectively recover CO2 from utility power plant exhaust gas at higher-than-typical operating temperatures. In addition, a novel application of a pressure reduction turbine can be integrated into the proposed hybrid system. If a pressure reduction turbine is used as part of the PSA process, a savings of 10% of the power typically used in the vacuum pumping of the PSA sequence can be shown.
- A reheat turbine, which is added to the cycle to improve the SCO2 cycle efficiency to 57%
- Recovery of pressure energy from the local sequestration process, which occurs as the pressurized CO2 leaves the open SCO2 cycle and begins to fill the underground sequestration vaults. (A remote sequestration ground vault may require the same pressure let-down CO2 turbine for some power recovery, but it would not be integrated into the SCO2 turbine-compressor-generator proposed for facilities that can have local sequestration of recovered CO2)