Prior to 1990, little reliable data was available on mercury concentrations in coal plants’ stack gas or anywhere within the plant gas stream. The ERA attempted to change that by developing an emissions inventory known as the 1999 Information Collection Request (ICR). The ICR collected basic information on all 1,140 coal-fired utility boilers in the U.S., and more detailed information as well. The detailed data included analyses of coal’s calorific value and its mercury, chlorine, sulfur, moisture, and ash content. Also included were analyses of total and speciated (elemental and oxidized) mercury in the inlet and outlet flue gases of 81 units representing a cross-section of the various types of boilers and fuels used in the U.S.
This “statistical” cross-section of plants was required for any future mercury maximum achievable control technology (MACT) rule-making. For new sources, MACT cannot be less stringent than the emission control that is achieved in practice by the “best controlled similar source,” as determined by the ERA. Although the ERA has yet to make a mercury MACT determination, an evaluation of the ICR data that the ERA will use to set the standard tells the story. The ICR data seem to indicate that the maximum mercury reduction achieved in practice is well above 90%.
For existing sources, MACT must not be less stringent than the average emission limitation achieved by the best-performing 12% of the existing sources in the category for which the ERA has emissions information. Because the ICR test data represents a large portion of the available information about emissions of mercury from individual coal-fired boilers, the importance of the ICR data is very clear. Since the ICR tests cover about 80 different boilers, the 12% group could represent as few as 10 boilers. This depends upon what the ERA considers as the test for “having emissions information.” Based on evaluation of the entire ICR data set as a single source category, a reasonable conclusion would be that the “MACT floor” level of control for existing coal-fired boilers may also be near 90%.
This is where the plot thickens and your Washington lobbyists make their money. If the final rules have no sub-categorization of plants by at least fuel type, all those plants that switched to Powder River Basin coal to save money are up the proverbial creek without a paddle, because there are no technologies that will get you to 90% mercury removal. Meanwhile, other plants burning high-chlorine bituminous coals may already be operating at the 90% level.
Other ways of slicing and dicing the data are possible. For example, the ICR data for the bituminous and sub-bituminous coal groupings are separated into two subgroups based on whether the electrostatic precipitator is installed on the hot side or the cold side of the air heater. Some other categories, such as hot-side ESPs burning lignite, are not even represented. In general, the mercury removal capability of existing ESPs typically does not even reach 50%. On the other hand, ICR data shows that bituminous coal-fired plants with fabric baghouses consistently show 85% mercury removal; lignite-fired plants only achieve levels near 50%. Wet flue gas desulfurization systems can remove as much as 90% of ionic mercury when firing bituminous coal but typically only about 60% firing lignite.
Finally, there are the disappointing results of the Utility MACT Working Group, which labored for over a year to develop regulatory subcategories, minimum levels of emissions, the format of the standard, and compliance methods. The group’s original constituency comprised 6 representatives of state/local/tribal agencies, 8 representatives of environmental organizations, and 16 representatives of affected sources/fuel producers and suppliers/labor groups. It should surprise no one that such a diverse group could come to little agreement on the issues.
One of the few areas of agreement was the important issue of sub-categorization, perhaps the most significant issue facing the group apart from the question of how to set actual MACT floor levels. All supported the principle of creating only a few subcategories and agreed that pulverized coal boilers burning bituminous and/or sub-bituminous coal should be in one subcategory.