Power Sector Mercury Emissions

Many factors, including the Hg content (by speciation—elemental Hg versus various Hg-containing compounds), chlorine content, and other chemical constituents of the coal used; the rank of the coal (i.e., bituminous or subbituminous); the boiler temperature and firing type and the flue gas temperature; and the types of existing control equipment for NO_x, SO₂, and particulates affect the level of Hg emissions from a particular power plant. In recent years data collection and analysis efforts have focused on these factors so that better estimates of current power sector Hg emissions could be developed; however, substantial uncertainty remains. As additional tests are performed, factors currently unaccounted for may turn out to be important.

Section 112(n)(1)(A) of the Clean Air Act Amendments of 1990 required the U.S. Environmental Protection Agency (EPA) to perform a study of possible public health problems associated with hazardous air pollutants from steam-electric power plants. That study was completed in December 1997 and transmitted to the Congress.^a One of its key findings was that Hg emissions from coal-fired power plants posed the greatest public health concern among the hazardous air pollutants identified; however, the EPA determined that more data were needed before regulatory decisions could be made.

Using its authority under section 114 of the Clean Air Act, in November 1998 the EPA issued an information collection request (ICR) requiring coal-fired power plants to provide data associated with Hg emissions. The ICR data were collected in three phases. The first phase involved the collection of basic information—boiler type, size, existing emissions control equipment, etc.—for every coal-fired generator with 25 megawatts or greater capacity. The second phase was the collection of fuel shipment information for each of the electric power plants identified in the first phase. Each of the electric power plants was required to report the quantity and source of each coal shipment received for the calendar year 1999. For every sixth shipment (a minimum of 3 analyses per month) the plants also had to report the Hg and chlorine content of the coal received. In the third phase of the ICR, 75 plants were selected to test the Hg emissions at the inlet and outlet of the last pollution control device on one or more units. The plants used were chosen to be representative of the different types of existing coal plants.

The ICR data are the primary information used in this report to assign Hg content to the coal supply curves in the NEMS Coal Market Module and the Hg emissions modification factors for each coal plant type represented in the Electricity Market Module. On average the sample data show that the Hg content of coal shipped in 1999 was 7.3 pounds per trillion Btu (approximately 0.2 pounds of Hg per thousand short tons of coal); however, there was considerable variation among coals from different seams, even within a given coal supply region. For example, the 1999 ICR data indicated that coal shipments from the Pittsburgh seam in Northern Appalachia had an average Hg content of 8.2 pounds per trillion Btu, whereas shipments from the Upper Freeport seam averaged 16.4 pounds Hg per trillion Btu. Even within the same coal seam the tested shipment data show considerable variation in Hg content. For example, although the average Hg content for the Pittsburgh seam was 8.2 pounds per trillion Btu, the minimum for shipments from that seam was 0.1 pounds per trillion Btu and the maximum was 73.1 pounds per trillion Btu. In statistical terms, the standard deviation for Hg content at the Pittsburgh seam is 4.04, indicating that most samples should have Hg contents between 0.1 and 16.3 pounds of Hg per trillion Btu.

The Hg removal rates for the various coal plant configurations also showed significant variation. Data from the third phase of the ICR show that on average a cold-side electrostatic precipitator (CSE)—a particulate removal device—removes 31 percent of the Hg that passes through it. However, the variation among plants with CSEs was large, ranging between 0 percent and 87 percent removal. The situation was similar for facilities with fabric filters—another type of particulate removal device. On average they removed 69 percent of the Hg passing through them, but, after excluding plants that actually reported increases in Hg after passing flue gas through the fabric filter, the removal rate ranged between 54 percent and nearly 100 percent. In addition, there is very little information on the impact of new NO_x control devices—selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR) equipment—on Hg emissions because, while many plants plan to add them in the near future, only a few are using them now. This study assumes that, when combined with an SO₂ scrubber, an SCR enhances Hg removal with an emissions modification factor of 0.65; however, no additional removal is assumed for plant configurations that have an SCR but do not have an SO₂ scrubber.

Additional research is needed on the variations seen in the available data. Over the next several years the National Energy Technology Laboratory (NETL), the EPA, and others plan to conduct full-scale tests of various Hg removal technologies on several coal plants. This analysis assumes the use of activated carbon injection technologies to remove Hg, because they have been tested at pilot scale; however, there are other technologies in development, including advanced coal cleaning techniques, alternative absorbents, and more efficient use of absorbents (recycling absorbents rather than once-through systems) to remove Hg from flue gas.

In addition, efforts to understand the role of chlorine and other chemicals in coal on the amount of Hg removed are underway. Data from those tests and from other ongoing research should allow a better understanding of the factors influencing Hg emissions and improve analyses of options for reducing them. Although this report uses the best data available, considerable uncertainty exists about the measurement of and options for reducing Hg emissions from coal-fired power plants.

aU.S. Environmental Protection Agency, Mercury Study Report to Congress, EPA-452/R-97-003 (Washington, DC, December 1997).