On December 20, 2000, the U.S. Environmental Protection Agency (EPA) announced it intends to regulate mercury emissions and discharges from coal-fired power plants. Proposed regulations will be made available for public comment by 2003, with the final rule published in 2004.
While good news for the public and environment, the regulatory drivers will inevitably force utilities to reexamine their existing control technologies, many of which seem inadequate considering statistics. In 1999, EPA estimates the electric utility industry emitted 43 tons of mercury from 1,149 units at 464 coal-fired power plants. Projections for 2010 without regulations include approximately 60 tons of emissions from 1,026 units at 426 coal-fired plants.
Mercury can be emitted into the atmosphere and water through hazardous and municipal waste incinerators and medical waste incinerators, but the fuel used by industry, namely coal, contain varying concentrations of mercury and other pollutants. According to a report by the Minnesota Department of Transportation, more than 800 million metric tons of coal is burned annually in the United States, making this fossil fuel the largest source of mercury emissions.
The reduction of mercury emissions can be achieved by several routes, including utilization of coal cleaning, substituting low mercury coal, using another fossil fuel such as oil, injection of mercury absorbents into the flue gas followed by electrostatic precipitators (ESP), employing wet scrubbing flue gas desulphurization and more.
Innovative Approaches to Hg Control
Hot on the heels of new regulations are several companies developing innovative technologies, which address mercury extraction and removal from coal-fired flue gas. The technologies listed below received federal funding through the EPA's Small Business Innovation Research (SBIR) Program. This and other federal SBIR programs support higher-risk research and development activities at companies with 500 or fewer employees.
Purity Systems Inc. has developed a silica-polyamine composite material to extract oxidized mercury from coal-fired flue gas desulfurization water to levels less than two ppb (parts per billion). Used in an efficient, environmentally benign system, the materials have proven highly effective at reducing mercury concentrations from relatively concentrated aqueous solutions.
More than 200 of the largest coal-fired utility boilers in the United States utilize a flue gas desulfurization (FGD or wet scrubbers) process. Although these processes were designed primarily to remove sulfur dioxide from the plant emissions, the wet scrubbers also entrain oxidized mercury in the scrubber water, and mercury levels in the scrubber water can reach 600 ppb. The process has been demonstrated to reduce the mercury concentration from 100 ppb to less than two ppb at treatment rates of 10 column volumes / hr. During Phase I testing, approximately 55 g of the silica-polyamine composite material was used to treat 15,500 grams (g) of water containing 100 ppb of mercury to below two ppb. This technology will find particular application in treating scrubber water and remediating sites with heavy mercury concentrations.
ADA Technologies Inc. has developed a series of disposable sorbents for mercury in flue gas streams. The sorbents consist of a natural silicate treated with chemicals to capture and immobilize vapor-phase mercury and mercury compounds. Laboratory testing in a packed bed configuration under typical flue gas conditions demonstrated that, in a nitrogen gas stream, the new materials have at least three times the capacity for mercury, as compared to activated carbon, under identical conditions. More significantly, in an acid gas stream with sulfur dioxide (SO2) and nitrogen oxides (NOx), mercury broke through the activated carbon bed in less than 15 minutes, but failed to break through the new sorbents, even after 24 hours. Although they are designed to be disposable, the sorbents are also regenerable, if necessary. The silicate base ensures that these sorbents will not degrade the quality of fly ash.
Further testing will characterize the relative mercury removal performances of the several candidate chemicals. Once the optimum formulation has been identified, a test will be run in which sorbent is injected into the exhaust gas from a pilot coal combustor. Additional applications for the new sorbent include a less expensive replacement for activated carbon in municipal waste and medical waste incinerators and in industrial processes where mercury control is an issue, such as base and precious metal production and refining.
TDA Research Inc. is developing a low-cost sorbent for mercury abatement in coal-fired electric utility flue gases. An ideal mercury abatement system could be retrofitted into most or all existing coal-fired electric utilities. An attractive approach that meets this requirement is dry sorbent injection. The dry sorbent will be injected into the flue gas and removed in the particulate control module (PCM). The sorbent should be readily removed with the fly ash by either the fabric filter (baghouse) or the ESP. Through its current work in the field, the company identified a new approach to sorbents for mercury abatement that offer cost and performance advantages over previous materials. The approach is to use a clay substrate, which is modified to improve mercury uptake. Tests in a packed bed apparatus have confirmed that the new sorbents are effective, and an engineering analysis indicates that they can lower the cost per pound of mercury removed.
Bright Future for Hg Abatement
EPA and state regulations are driving the development of new technologies. Numerous methods are beginning to address the issue of mercury abatement, and finding an efficacious, low-cost solution will benefit both the environment and industry as well.
This article originally appeared in the August 2001 issue of Environmental Protection, Vol. 12, No. 8, p. 34.
This article originally appeared in the 08/01/2001 issue of Environmental Protection.
Brennan Van Dyke is director of the Regional Office for the North America for the United Nations Environment Programme.