Ramsay Chang, EPRI
Activated carbon injection module used at Laskin Energy Center and Stanton Station
By 2007, coal-fired power producers will have to comply with regulations on mercury emissions currently being developed by the Environmental Protection Agency (EPA)—unless this timetable is extended by the EPA or unless legislation superceding this process is passed by Congress. Over the past decade, the Electric Power Research Institute (EPRI), the U.S. Department of Energy, and the EPA have undertaken extensive research to develop cost-effective methods for reducing these emissions, especially from coal-burning facilities.
Most recently, EPRI has led a collaborative research project with Great River Energy, Minnesota Power, and Xcel Energy to conduct pilot- and full-scale tests of three potential methods for curbing mercury emissions from low-sulfur, low-chloride western coals, such as lignite and Powder River Basin coal. The goal is to define the cost and performance of new technologies to reduce mercury emissions, to determine the impact on plant operations, and to provide information to EPA as it proceeds in its rulemaking process.
Three control technologies
The first full-scale demonstrations of two of the technologies were performed at Great River Energy’s Stanton Station and at
Minnesota Power’s Laskin Energy Center. One technology involved injecting activated carbon into flue gas entering Stanton’s spray dryer/baghouse system and Laskin’s wet particulate scrubber: The activated carbon adsorbs the mercury so it can be collected by existing (or new) air pollution control equipment.
The other technology involved impregnating the activated carbon with chemical additives or adding chemicals (halides) to the plants’ boilers to change the chemical form of mercury in the flue gas to a form that is more easily controlled with existing pollution control equipment.
In addition to the two full-scale tests, small-scale slipstream tests were also conducted with a third mercury removal process that is still in the early stages of development—an EPRI-patented method called Mercury Capture by Adsorption Process (MerCAP). In the MerCAP process, mercury is adsorbed onto plates or banks of tubes formed of (or coated with) materials that adsorb mercury and placed into the flue gas stream. Gold is the first method being investigated as an adsorbent because it forms an amalgam with mercury. When the gold surface becomes saturated with mercury, the plates can be regenerated by heating, and the desorbed mercury captured in a secondary recovery system.
Findings to date
Based on past tests and these new full-scale results, the first technology, carbon injection, appears to be capable of removing all species of mercury and therefore could be used with all types of coal with varying degrees of success. For example, for plants burning western coals with low-chloride content, there may be limitations on the removal efficiency when activated carbon is used ahead of electrostatic precipitators (ESPs) or wet particulate scrubbers. Similarly, the effectiveness of activated carbon was significantly reduced when used with spray dryer/baghouse systems as configured at Stanton.
Results showed that the second technology—chemical impregnation of the activated carbon and chemical additives—enhanced mercury removal at Stanton and Laskin by a factor of 2 to 4 times in short-duration tests. The potential impact of impregnated carbons and halide addition on long-term boiler operation, performance, and waste disposal will be determined in extended testing.
The small-scale test results indicate that MerCAP can remove at least 80 percent of mercury downstream of the spray-dryer baghouse at Stanton, and may be a cost-effective option to activated carbon injection. MerCAP with gold as the sorbent surface did not perform well in the non-scrubbed flue gas tested at Stanton and Laskin, and alternate sorbent surfaces are being developed for these conditions.
Speaking of the tests to date, Tim Hagley, Minnesota Power’s supervisor of air quality has noted, “It will take time and tenacity to achieve the technological breakthroughs needed. Our collaboration with EPRI, Great River Energy, and Xcel Energy provides an excellent opportunity to conduct cutting edge, full-scale testing of promising technologies. Our goal is to find cost effective, sustainable technologies for reducing mercury emissions from our facilities.”
Future research will proceed on several paths. To date, every new field site has been a “paradigm buster,” so there is a critical need for more long-term (3 months to 1 year) full-scale field tests with activated carbon injection. Utility host sites are being sought for these tests. In addition, short-term, pilot and full-scale tests need to be conducted with plant configurations and fuels other than those used in the Stanton and Laskin demonstrations. These tests might be conducted with EPRI’s TOXECON technology, which injects activated carbon between an ESP and a baghouse. This configuration offers two main advantages: (1) greater mercury reduction with less carbon addition than injection ahead of an ESP; and (2) retention of ash sales by keeping the fly ash and activated carbon separate.
MerCAP testing will continue with alternate coatings to gold and alternate geometries, such as carbon honeycombs, which can be incorporated into a rotating “wheel” structure with adsorption and regeneration sections.
Other tests may explore the use of novel sorbents. To date, the activated carbon has been Norit’s FGD. However, other less costly sorbents might be made from different carbonaceous sources such as other coals (ND lignite, eastern bituminous), biomass, waste tires, and soot. Besides activated carbon, other sorbents—such as those based on zeolites, clay, fly ash and activated limes—are also potential candidates for mercury adsorption.
Another control technology that will continue to be tested involves oxidation catalysts, which convert elemental mercury to an oxidized form in the gas phase so that it can be removed in an FGD (flue gas desulfurization) system. This technology is still in an early stage of development, but results to date show that some of the catalysts can provide a high level of mercury oxidation (>90 percent) in the short term.
Overall, mercury control research is still in its early stages, and many engineering questions remain to be answered—all of which affect technical feasibility and cost. One central challenge is the considerable variation in power plant configurations and fuel. Nevertheless, EPRI’s research program intends to explore all the options so that each power company finds the method best suited for its plant and coal type.
Chang is a project manager at EPRI, who has more than 25 years experience managing research in air emission control technology. He can be reached at 650-855-2535, firstname.lastname@example.org.