Capture the Carbon: Decreasing CO2 Emissions from Coal-Fired Power Plants
By Virbin Nath A. Sapkota
Coal fired power plants produce more than a third of the nation’s electricity (1), but because they also produce 32% of America’s CO2 emissions (2), they contribute significantly to global climate change. In the short term, it would be impractical to shut these coal-fired power plants down, because they produce so much electricity. A practical way to reduce their emissions is to capture the CO2 from the plants after the coal is burned, using a technology called amine scrubbing. For the past two years, I have worked with Dr. Gary T. Rochelle’s group at the University of Texas at Austin to make amine scrubbing safer and more efficient.
In amine scrubbing, CO2 within the gas that is released from burning coal, known as flue gas, is absorbed into a solution composed of water and nitrogen-containing molecules called amines. This solution is heated in a unit called a “stripper column.” The heat separates the CO2 from the solution, and the purified CO2 is then compressed to high pressures and injected into rock layers deep underground.
Amine scrubbers require a substantial energy input to operate, and the choice of amine in the scrubber significantly affects the amount of energy required. Currently, the most commonly used amine is monoethanolamine (MEA). For scrubbers using MEA, 27-30% of the energy produced by the coal power plant would have to be used to power the amine scrubber.3 Our group is developing methods to use piperazine (PZ), an amine that can be heated to higher temperatures without degrading. When PZ is used, there is an energy tradeoff: more energy is required to heat the stripper to a higher temperature, but the CO2 also separates from the amine solution at a higher pressure. Because the CO2 is at a higher pressure, much less energy is required to further compress the CO2 for storage. The net effect of this tradeoff is that a hotter PZ system uses only 20% of the energy produced by the coal power plant, much less than the colder MEA system. (3)
However, piperazine has downsides. PZ can react with nitrogen dioxide (NO2), another component of the flue gas, to form carcinogens called nitrosamines. These carcinogens pose a safety risk if they leak out of the scrubber. The nitrosamine problem has been the primary obstacle to widespread use of PZ in amine scrubbers. Last year, I worked with a doctorate student, Nathan Fine, to solve the problem of nitrosamine formation by removing the problematic NO2 from the flue gas.
A promising technique for removing NO2 is limestone slurry scrubbing, which is already used by coal-fired power plants to remove another component of the flue gas: sulfur dioxide (SO2). In limestone slurry scrubbing, the flue gas is passed through a solution of limestone and water. (4) This causes a chemical reaction that absorbs SO2 and small quantities of NO2. We believe we can increase the absorption of NO2 by treating the limestone slurry scrubber with a chemical called thiosulfate (S2O32-). We studied the absorption of NO2 in a thiosulfate-treated limestone slurry scrubber and attempted to predict what conditions would allow us to remove 90% of NO2 in flue gas.5 If we can discover such conditions, then we can keep NO2 out of the amine scrubber, prevent toxic nitrosamines from forming in the amine scrubber, and safely use PZ to capture carbon dioxide.
In order to determine these optimal conditions, we passed mock flue gas composed of air, NO2, and CO2 through a solution which mimicked the one inside a limestone slurry scrubber. We varied the thiosulfate treatment and other variables and measured the NO2 absorption rate. The main findings were as expected: for untreated solutions, the NO2 absorption rate was too low, but treated solutions performed very well with high NO2 absorption rates. The data suggested that to remove 90% of the NO2 in the flue gas with a limestone slurry scrubber, the solution only needs to contain .1% thiosulfate. (5)
Because of amendments to the Clean Air Act in 1990, many coal-fired power plants are already required to use limestone slurry scrubbers to remove SO2 from their flue gas. Adding thiosulfate to these pre-existing scrubbers is simply a matter of mixing another chemical salt into the solution, so the cost of the thiosulfate is trivial in comparison to the risk of handling carcinogens. In the future, pilot tests using more realistic industrial equipment must be conducted to confirm these results. If these pilot tests confirm the laboratory results, the last major hurdle against using PZ instead of MEA within amine scrubbers will be eliminated, and carbon scrubbing will become much more economical. This development would allow us to significantly decrease CO2 emissions in a cost-effective manner, providing us with one more weapon in the fight against climate change.
- “Table 1.1. Net Generation by Energy Source: Total (All Sectors), 2005-October 2015.” U.S. Energy Information Administration Electricity Data. U.S. Energy Information Administration, 26 August 2015. Web. 5 January 2016.
- “Table 12.1 Carbon Dioxide Emissions From Energy Consumption by Source.” U.S. Energy Information Administration Environment Data. U.S. Energy Information Administration, 24 November 2015. Web. 5 January 2016.
- Rochelle, Gary T. “Amine Scrubbing for CO2 Capture.” Science 325.5948 (2009): 1652-1654.
- US Environmental Protection Agency. Air Pollution Control Technology Fact Sheet, n.d.: 3. Web. 24 January 2016. <http://www3.epa.gov/ttn/catc/dir1/ffdg.pdf>
- Sapkota, Virbin Nath A.; Fine, Nathan A.; Rochelle, Gary T. “NO2-Catalyzed Sulfite Oxidation.” Industrial & Engineering Chemistry Research. 54.17 (2015): 4815-4822.