Donnelly Team Designs Cryogenic Remediation of Power Pollutants

by Professor John M. Pfotenhauer, University of Wisconsin – Madison, pfot@engr.wisc.edu

The fields of low temperature physics and energy and environment do not frequently interact. Imagine my astonishment to find in the 2010 spring edition of a newsletter from the Department of Physics at my graduate school, the University of Oregon, that my thesis advisor from more than 28 years ago (wow!), Professor Russell Donnelly, who is well known for his many contributions to the field of low temperature physics (think quantized vortices in superfluid helium), was involved in a project to clean up the stack gases from coal-fired power plants! Promptly calling him and asking whether he was now making excursions into my field of mechanical engineering, I received a warm welcome to “come out and help.” The reunion grew even better when I arrived in Eugene that summer and learned that one of my former grad student classmates, Charlie Swanson, was also centrally involved in the same project.

The group at the University of Oregon, which also included scientists John Elzey and Robert Hershberger, was building on an analysis by Donnelly from 1979 that proposed using a cryogenic refrigeration system to remove odiferous pollutants from the smokestacks of a local paper mill. The group conceived applying the same idea to extract not only carbon dioxide, but a variety of other pollutants such as sulfur dioxide and mercury from the effluent of coal-burning power plants. Cooling the gas flow to the point where a given component condenses from the gas phase into either the liquid or solid phase allows that component to be extracted from the gas flow. The remaining gas flow, upon re-warming to room temperature, presents a environmentally friendly stream composed primarily of nitrogen, oxygen and argon.

Although a number of very serious efforts have been initiated by various groups around the world during the past decade to explore the idea of using cryogenics to capture carbon dioxide from coal-burning power plants, including those by Shell Global Solutions, Exxon-Mobil, Air Liquide and Alstom Technology Ltd, a simple analysis based on fundamental thermodynamics, missing from the open literature, was needed in order to identify and quantify the associated energy penalty of such an idea, in an ideal or best case scenario. The energy penalty is defined as the fraction of energy needed to run the cryogenic system, which then will not be available to sell. One distinct advantage of the cryogenic system is that the carbon dioxide and sulphur dioxide will exit the plant at pipeline pressure and can be sent hundreds of miles for sequestration, if needed.

Using an EXCEL-based program devised by Swanson, our team’s analysis considered an incremental cooling process, for which at each step-wise reduction in temperature, the energy extracted in order to cool, condense, or de-sublimate each component in the mixture was determined according to its thermodynamic location relative to its respective phase diagram, as well as the associated work required to extract that energy. As various fractional components such as H2O, SO₂ and CO₂ leave the gas phase, the composition and partial pressures of the remaining gas components are adjusted.

Such an analysis should determine, for example, if the effluent from a coal plant were cooled sufficiently to extract a significant fraction, say 90 percent, of the carbon dioxide by cooling it to 153K and causing it to de-sublimate, or freeze and fall out of the gas stream, how much of the energy production from the power plant that is also producing the exhaust would be consumed by such a cooling system? Pollutants in the effluent such as sulfur dioxide and mercury would also be extracted because they would freeze out at higher temperatures. The ideal cycle analysis determined a 9.1 percent energy penalty.

The energy penalty calculated from the ideal-cycle analysis was then adjusted by using a suitable second law efficiency, or “percent of Carnot,” or figure of merit, for a large-scale refrigeration system operating near 150K. Our analysis revealed that the energy penalty for cleaning up the effluent is approximately 25 percent of the energy generated by the same power plant. The amount is surprisingly competitive with all other approaches being considered for producing cleanly burning coal, and a cryogenic method has the additional advantage of capturing essentially all of the mercury. The details of the approach and analyses are published in Physical Review E 86, 016103 (2012).

The significance of these results are well summarized by the introductory paragraph of this article: “The generation of electricity through combustion of coal is the largest component of greenhouse gas emissions in the United States, and globally coal combustion constitutes 40% of carbon dioxide (CO₂) emitted to the atmosphere due to energy consumption. There are two systems for capturing carbon dioxide from coal-fired power plants that have been under consideration for some time. Oxy-fuel combustion burns coal in the presence of pure oxygen and recirculated CO₂ (instead of air) in order to produce a concentrated CO₂ stream that is subsequently purified and compressed. The fraction of electric power loss owing to this technology is about 26%. The monoethanolamine (MEA) process passes the exhaust gases through an absorber where the MEA absorbs the CO₂. The CO₂ is stripped from the MEA using steam. The CO₂ is then dried and compressed and the MEA is purged of contaminants and recirculated. The energy penalty for MEA is about 28%.”

Many further challenges remain before the cryogenic method for capturing carbon from the effluent of a coal-fired power plant can be put into reality. For example, reliable methods must be developed to separate solid particulates from the gas stream as the gas mixture is cooled, and to translate those solid phases into a storable liquid. Nevertheless, the scoping calculations presented in the Physical Review E article suggest an exciting opportunity for the field of cryogenics to enable a means of producing clean energy from a not-so-clean fuel.