PAPER: COOLING FLUE GAS COULD HELP REDUCE CO2, SO2, NOX EMISSIONS

Tamar Hallerman
GHG Monitor
08/31/12

The cryogenic cooling of flue gas could help aid in the capture of carbon dioxide and other harmful emissions from pulverized coal-fired power plants at a cost competitive rate, physicists argue in a paper published recently in an American Physical Society journal. In the most recent edition of the journal Physical Review E, physicists from the University of Oregon argue that refrigerating flue gas from a pulverized coal plant to 156 degrees Kelvin could help capture 90 percent of a plant’s CO2 emissions, as well as significant levels of other harmful pollutants like SO2, NOx and mercury. Using lower temperatures to treat emissions from electric generating units would allow project operators to capture CO2 while it is in a solid phase and compressing it into a gas, which could then be transported via pipeline at near ambient temperatures to sequestration sites, according to the study, which was funded by the Department of Energy through Pacific Northwest National Laboratory.

Head author Russell Donnelly and his team developed a mathematical formula for determining costs and found that the energy penalty associated with the process is comparable to that of post- and oxy-combustion capture, at between 22 and 26 percent. “Cryogenic treatment of flue gases is a competitive option for capturing carbon dioxide, sulfur dioxide and for trapping several undesirable pollutants as well,” the study says.

Donnelly Examined Cryogenics in 1960s

Cryogenically cooling emissions is not a new concept. Donnelly said he conducted research on the issue in the 1960s as part of a project he conducted at a paper mill in Oregon while looking to remove odor-causing gases from the plant using cryogenics. His work continued with a subsequent National Science Foundation grant, where he looked at capturing sulfur dioxide emissions in connection to acid rain. As part of that study, Bechtel Corp. completed a detailed engineering study on the issue and found that while the process worked well for separating SO2, the process also condensed large quantities at CO2, which at the time was not considered desirable. Donnelly said he began revisiting the issue several years ago when climate change and the role greenhouse gases became more of a national issue. “The reason this is coming out now is because we didn’t know the importance of the greenhouse gases 30 years ago and we do now,” Donnelly said in an interview. In addition to pulverized coal systems, companies have begun looking at cryogenic cooling in relation to oil refineries and natural gas.

Donnelly said, however, that many hurdles must be overcome before the technology will be able to be realized even on a pilot scale. “More experiments need to be done, and if we had money we could be making quite rapid progress. I think that we’re some distance from being able to build a power plant—there are all sorts of engineering problems that need to be worked out first,” Donnelly said. Even if the technology were to get off the ground, the size of the cooling machinery would likely be large. “In a 1,000 MW pulverized coal plant, you’d probably have ducts maybe 100 ft wide and 25 feet high and you’d probably need five or six of them. And you would be refrigerating all of that gas, so it might be the size of a football field,” Donnelly said. “It would be big stuff.” 

However, Donnelly said that if developed it could be preferable to more common methods of CO2 capture, particularly monoethanolamine (MEA)-based post-combustion capture, he said. “In my opinion, [cryogenic cooling] is vastly superior to any chemical method because you’re not messing around with a lot of chemicals. Instead, you’re doing it by physical processes and overall there’s nothing new there. Most refrigeration challenges were settled 60 years ago. We have no problem with getting those temperatures and getting them to work,” he said.