Palo Alto, Calif., April 1, 2003 — In 2007, the Environmental Protection Agency (EPA) is slated to begin regulating mercury emissions from coal-fired power plants (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 teamed up with Great River Energy, Minnesota Power, and Xcel Energy to perform pilot tests and some of the first full-scale demonstrations 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 and to provide information to EPA as it proceeds in its rulemaking process.
Full-scale demonstrations of two technologies were performed at Great River Energy’s Stanton Station and at Minnesota Power’s Laskin Energy Center.
In the first process tested, activated carbon was injected 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.
In the second process tested, chemicals (halides) were added to the plants’ boilers to change the chemical form of mercury in the flue gas to one that is more easily controlled with existing pollution control equipment.
Another mercury removal process tested, but still in the early stages of development, is an EPRI-patented method called Mercury Capture by Adsorption Process (MerCAPTM). In the MerCAPTM 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.
Based on past tests and these new full-scale results, 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 or wet particulate scrubbers. Similarly, the effectiveness of activated carbon was significantly reduced when used with spray dryer/baghouse systems as configured at Stanton.
When the activated carbon was impregnated with chemical additives, mercury removal at Stanton and Laskin was enhanced by a factor of 2 to 4 times in short duration tests. The addition of halide compounds into the boiler also improved mercury removal at these plants. The potential impact of impregnated carbons and halide addition on long term boiler operation, performance, and waste disposal will be determined in extended testing.
Small scale test results to-date indicate that MerCAPTM can remove at least 80% of mercury downstream of the spray-dryer baghouse at Stanton, and may be a cost-effective option to activated carbon injection. MerCAPTM 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.
Ramsay Chang, program manager at EPRI says, “We are still at early stages with some of this research, and many engineering questions remain to be answered, all of which affect technical feasibility and cost. Our goals are to find lower cost options for meeting future regulations, determine the maximum mercury removal achievable, and assess the impact on plant operation, particulate emissions, and by-product utilization or disposal.
“There is so much variation in power plant configurations and fuel. We plan to explore all the options so that we can give each power company the method best suited for its plant and coal type,” Chang added.
Mark Strohfus, Great River Energy’s environmental policy analyst says, “These studies help Great River Energy understand how effectively and at what potential costs we may be able to control mercury emissions. While these tests increased our level of understanding of mercury emissions and control options, there are many questions remaining. Great River Energy plans to continue working with EPRI to answer some of these questions through further testing at our power plants.”
Tim Hagley, Minnesota Power’s supervisor of air quality said, “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.”
EPRI, headquartered in Palo Alto, Calif., was established in 1973 as a non-profit center for public interest energy and environmental research. EPRI’s collaborative science and technology development program now spans nearly every area of power generation, delivery and use. More than 1,000 energy organizations and public institutions in 40 countries draw on EPRI’s global network of technical and business expertise. Visit the EPRI website at http://www.epri.com.