NETL: Mercury-Control Program Achieves Success
June 23, 2008 // Published as a news service by IHS
According to the National Energy Technology Laboratory (NETL), coal-fired power plants, municipal waste combustors and medical-waste incinerators are just some of the sources of mercury emissions that have raised environmental concern for decades.
Scientists have known for some time that the mercury these sources emit can eventually be converted into methyl mercury, posing a threat to people and animals, said NETL.
The threat arises when mercury accumulates in the food chain, moving from plants to small animals to large animals and to humans.
Human exposure largely derives from the consumption of fish with high levels of mercury; at least 43 states have issued food advisories regarding high levels of mercury in fish.
Recognizing the impact that controlling emissions could have on U.S. coal-fired power plants, the U.S. Department of Energy (DOE) embarked on a research program more than 15 years ago to develop and demonstrate cost-effective mercury-control technologies, said NETL.
This program was carried out as part of the NETL Innovations for Existing Plants research area, which is directed at improving the overall environmental performance of the current fleet of coal-fired power plants.
Over the 15-year period that NETL managed the DOE mercury-control R&D program, NETL worked with the U.S. Environmental Protection Agency (EPA), the Electric Power Research Institute (EPRI), power plant operators, state and local agencies and a host of research organizations, including academic institutions.
NETL and its research partners identified factors associated with mercury speciation and capture in the coal-combustion process, then developed a suite of technologies to reduce mercury emissions.
The collaborative effort has met the program's goal: to bring technology to commercial readiness by 2007 that can achieve at least 50-70% mercury removal at costs considerably less than 1999 baseline estimates. Achieving this goal culminated with the success of field testing that was initiated in 2000, said NETL. Coal type and other plant-specific factors determine what percentage of each form of mercury - elemental, oxidized and particulate - will be released and how it will be controlled.
According to NETL, it was critical that field testing encompass a broad suite of power plant configurations and coal types, especially plants that burn lower-rank coals, which produce the more difficult to capture elemental form of mercury. NETL conducted large-scale field testing of the most promising mercury-control technologies on more than 40 coal-fired boilers burning a range of coal types, including bituminous, subbituminous and lignite, and equipped with variety of air-pollution control devices.
A major focus of the field testing under the NETL program was the development of activated carbon injection (ACI) technology. ACI involves injecting powdered carbon into a power plant's flue gas to grab hold of the mercury. The activated carbon is then captured and removed from the flue gas using the plant's particulate control device. The field program also tested technologies that would enhance the capture of mercury in a plant's sulfur dioxide control system.
According to NETL, the success of the mercury-control program may be best demonstrated by the fact that more than 100 full-scale ACI systems have been ordered by U.S. coal-fired power generators as of April 2008. These contracts include both new and retrofit installations and represent more than 44 gigawatts (GW) of coal-based electric generating capacity.
Approximately 33 GW of existing electric generating capacity (about 10% of total U.S. coal-based capacity) will be retrofitted with ACI to control mercury emissions. This includes NETL-developed halogen-treated carbon systems that can capture elemental mercury.
The ACI systems have the potential to remove 70% or more of the mercury, and in some cases, 90% or greater mercury capture, at a cost that can dip below $10,000 per pound of mercury removed. By comparison, the cost of technology to capture mercury available in 1999 was estimated to be about $60,000 per pound, said NETL.
The mercury-control program also resulted in two NETL patents for new methods to control mercury emissions. In the first, researchers extracted partially combusted coal from a boiler and used the unburned carbon as a mercury sorbent. The second patent uses ultraviolet light to convert elemental mercury into an oxidized form that can be more readily captured by existing air pollution control devices.
Source: National Energy Technology Laboratory (NETL).