Technology▼Solutions Capturing mercury with ultraviolet light
CARLA SANTEE, DEPARTMENT OF ENERGY
removal technologies being developed to help meet whatever standards are agreed upon (Environ. Sci. A new technology based on ultraviothropogenic mercury emissions, acTechnol. 2003, 37, 283A–284A). let (UV) light could provide an attraccording to DOE. In December 2000, The concept of using UV light to tive alternative to current methods the U.S. EPA determined a need to oxidize mercury is not new, but the for capturing mercury from coal-fired regulate mercury because of a link NETL researchers are the first to depower plants. The process, which was between emissions and the bioaccuvelop the technology to where it developed by scientists at the U.S. mulation of mercury in fish. EPA has might be commercially viable in a Department of Energy’s (DOE’s) Nasince begun developing Maximum power plant environment. A typical tional Energy Technology Laboratory Achievable Control Technology untreated flue gas stream from an (NETL), was recently licensed by (MACT) standards for power plants electric power plant contains many Powerspan Corp., a cleancomponents that can oxienergy company based in dize mercury when irradiNew Hampshire. ated, including oxygen, Mercury exists in two water, hydrogen chloride, forms in flue gas streams: carbon dioxide, sulfur elemental and oxidized. dioxide, and nitrogen oxElemental mercury is less ides. The researchers have reactive and therefore patented the process and more difficult to capture. described it at length in a In the new photochemical paper from Industrial & oxidation system, or PCO, Engineering Chemistry a flue gas stream is irradiResearch (Ind. Eng. Chem. ated with UV light at 253.7 Res. 2002, 41, 5470–5476). nanometers to oxidize eleActivated carbon injecmental mercury to mertion is currently the most curic oxide, mercurous common technique for resulfate, and mercurous moving mercury from flue chloride. gas streams, but Henry When coal is burned, it liberates elemental mercury, which is diffiPreliminary tests conPennline, a chemical engicult to capture with conventional technologies for cleaning power ducted in Powerspan’s labneer at NETL and co-inplant emissions. Laboratory-scale tests of the photochemical oxidaoratories have shown that ventor of the PCO process, tion system developed by Evan Granite of DOE’s National Energy Technology Laboratory show that the technology has promise for the PCO technology can sees some advantages to remove 90% of the elemen- converting 90% of that elemental mercury into oxidized forms that using UV light. “The main tal mercury from simulated can be captured with existing equipment. advantage is that you don’t flue gas streams, converting have to inject a sorbent, it to the oxidized form that can be reunder the Clean Air Act (CAA), which which can be quite costly,” he says. moved by existing equipment. This should be issued by December 2004. A typical 500-megawatt coal-burning makes the technology capable of meetAt the same time, several new plant can run up a bill of nearly $5 ing the most stringent standards that multi-pollutant control strategies— million a year for activated carbon. could be proposed for the toxic metal, including the Bush Administration’s While definitive numbers aren’t yet observers agree. Just as importantly, Clear Skies Initiative—have been available to compare with activated the technique should be inexpensive proposed that would supplant the carbon, “initial cost estimates indito implement and run, according to its MACT standards (Environ. Sci. cate that PCO would cost less than developers. It requires no additives, Technol. 2002, 36, 181A–182A). Each other mercury control methods,” says accommodates existing ductwork, of these, if adopted, would regulate Stephanie Procopis, director of marand requires only simple equipment— mercury with a different level of keting for Powerspan. They have calsimilar to irradiation systems in wasteseverity, adding a note of confusion culated that the lamps used to irradiate water treatment plants. to the researchers developing new the flue gas streams would require Coal-fired power plants in the technologies. Despite the inherent less than half of 1% of a plant’s elecUnited States emit about 48 tons of difficulties of designing a product in tric output, she says. mercury per year, generating almost the absence of definitive standards, Both NETL and Powerspan have one-third of the country’s total anPCO is only one of several mercury run bench-scale tests in their labs, 158A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / MAY 1, 2004
© 2004 American Chemical Society
and the results have been extremely promising. “We’re very excited about the technology based upon the smallscale tests,” says NETL’s Evan Granite, who shares a patent on the PCO process with Pennline. In NETL’s best run at the laboratory scale, the technology cut mercury levels from 300 parts per billion (ppb) to 90 ppb, but Granite’s paper describing the tests acknowledges that the results contain a significant level of uncertainty. The PCO technology can also be used in other systems that require some type of mercury control, such as municipal solid waste incinerators, natural gas refining, metal recovery plants, industrial boilers, and engine exhaust treatments. Granite and Pennline have received inquiries from companies interested in developing some of these other applications. Procopis expects Powerspan’s technology to be commercially available in the next two or three years. “We plan to test the PCO process at one or more power plants in the next year with a focus on oxidation and removal of elemental mercury from the flue gas of low-rank coals,” she says. Coal comes in many varieties. Anthracite has the highest carbon content and heat value, followed by bituminous coal, which is the most plentiful type in the United States. Bituminous is mainly found in the eastern and middle parts of the North American continent. Subbituminous coal is the next lowest rank, followed by lignite, both of which are found chiefly in the western United States. These low-rank coals are not as efficient, but they have a lower sulfur content and thus burn more cleanly. Many power producers across the country have switched to the lowerrank coals to avoid installing flue gas desulfurization units, but there is a catch, according to Pennline. Plants burning subbituminous and lignite coals generally emit more mercury than those using bituminous coal, which some experts attribute to higher chlorine levels in bituminous coal. At the moment, there is no cost-effective, commercially available technology for power plants that burn these lower-rank coals, and owners of these plants are eager for a technique like PCO that might reduce mercury emissions while minimizing the impact on plant operation costs. The PCO process still needs to prove itself in the real world. “I think
there are some real challenges with the technology,” says John Pavlish, a senior research advisor with the Energy & Environmental Research Center at the University of North Dakota, which studies mercury measurement and control technologies. PCO has only been tested on simulated flue gases, and Pavlish is concerned about how UV light will behave in a real flue gas environment with sulfur dioxide, acid gases, and most importantly, particulates. Lab results don’t always translate to a more complex system, he says. Pavlish advocates continued work with activated carbon to improve its removal efficiency as the best solution for removing mercury from power plant emissions. He says that his group is currently testing a couple of promising technologies that use additives to reduce the amount of carbon needed to capture mercury, which could translate into major cost savings. Granite and Pennline are also working on a method to improve activated carbon. Their “thief process” is a radical variation on activated carbon injection, in which partially combusted coal from the furnace is extracted by a lance—or “thief”—and then re-injected downstream to adsorb mercury. Tests at NETL’s pilotscale combustion facility and a slipstream from a utility in Wisconsin demonstrated mercury removal at similar levels to those obtained with activated carbon, but using sorbents that are much cheaper. Powerspan has also been developing a multi-pollutant approach to controlling power plant pollution, which it calls ECO because of its use of electro-catalytic oxidation. The technique removes sulfur dioxide, nitrogen oxides, fine particulate matter, and mercury from coal-fired power plants, while producing a commercially valuable fertilizer as a byproduct (Environ. Sci. Technol. 2000, 34, 379A–379A). If PCO is successful, it could significantly enhance Powerspan’s ability to capture mercury with this process. With the current uncertainty about the future of mercury regulation, developers and producers can find it difficult to determine their goals. “It presents a lot of challenges for everybody,” Pavlish says. “The uncertainty continues to add uncertainty to everything.” —JASON GORSS MAY 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 159A
