## Comment 32: Commenter 16122 states that U.S. Government Accountability Office (GAO), states and even EGUs have made clear that because the EPA uses long-term standards (e.g., 30- day compliance period under the EGU MACT Rule), no variability analysis is needed (reference is October 2009 GAO report entitled “Mercury Control Technologies at Coal fired Power Plants...”). Commenter adds that EGUs concluded that this long-term averaging approach takes into account dramatic swings in Hg emissions from coal and allows them to meet stringent state Hg standards. The commenter further states that if the EPA is concerned that EGUs will have difficulty meeting the proposed 30-day standard then the EPA should lengthen the compliance time period (e.g., to one year).

Comment 33: Commenter 17845 states that several institutions (e.g., GAO) and states have concluded that no variability analysis is needed because when longer-term data are used (e.g., 30-day compliance period), the average of the data takes into account any variability in Hg emissions allowing units to meet stringent state Hg standards.

Comment 34: According to commenter 17846, the use of the UPL dramatically over-estimates the emissions achieved by the top 12 percent performers. The commenter adds that the proposed MACT floor standard allows Hg emissions that are 100 times the average of the top 12 percent performers for EGUs burning non-lignite coal and 11 times the average of the top 12 percent burning lignite coal. The commenter adds that the EPA must show that its variability analysis provides an accurate picture of the relevant s ources performance. The commenter adds that there is no reason for incorporating variability analyses into the EGU MACT Rule because the EPA is already using a longer-term compliance standard. The commenter adds that if the EPA is truly concerned that EGUs will have difficulty meeting the 30-day standard it is proposing, the EPA should lengthen the compliance time period (e.g., to one year).

Response to Comment 32-34: Emissions data have different sources of variability; therefore, incorporating uncertainty analysis into the MACT floor accounts for sources of variation present in the data. Because a single number (e.g., average) may not capture the variability observed under different designs, operations and control technologies, a confidence type statistic like the UPL, along with a 30-day average format, is used to ensure that different sources of variability are considered and that the proposed MACT floor remains valid under different scenarios. As mentioned elsewhere, we believe that a 30-day boiler operating day period provides flexibility sufficient for sources to operate processes and co ntrol devices to assure ongoing compliance and that the 30-day period provides for the level of environmental protection intended for this rule. The 30-day rolling average requires that the operator review and act on measurement data on at least a daily basis consistent with the enforcement and compliance provisions of the CAA (e.g., CAA section 113(d)). A rolling 12-month average would reduce that frequency to once per month. Variability – including that attributed to load/operational variability, fuel variability, and other sources of variability – has been accounted for in the setting of the proposed and final emission limits. The method used to account for this variability was described in the preamble for the proposed rule. The method is consistent with methods used in other NESHAP rules that the EPA has promulgated.

Comment 26: Commenter 18443 states that a major factor in Hg emissions concentration is the Hg fuel content. According to the commenter, the currently proposed Hg limit does not account Commented [PB1]: This is consistent to what I mentioned
during our phone call. For your sources, this means “720

hour” instead of “30

‐ day” and “hourly” instead of “daily.”

This is from pages 497 and 498 of Volume 1 of the MATS
Response to Comment Document, available at https://www3.epa.gov/ttn/atw/u tility/mats_rtc_chapters_f

oreword

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## for the variability of Hg in the fuel or varied plant operations and draws only on 40 hand-picked best performing units, and it is possible that the Hg content in a particular coal shipment could be high enough over a 30-day period to exceed the Hg limit despite the best efforts of a utility to limit Hg emissions.

Response to Comment 26: The agency disagrees with the commenter and notes that the Hg emissions limit was established using data from the best performing sources that were adjusted statistically using the UPL procedure to account for variability. Properly maintained and operated Hg control devices should enable source owners or operators to meet the rule’s Hg

emissions limit.

Comment 86: Commenter 18487 urges the agency to either discard its “Low Emitting EGU” provisions, or to substantially revise them; if the emissions variability asserted by the agency exists, the threshold conditions required for qualification as a “low emitting” unit provide no significant assurance that emissions will remain below the proposed standards.

Response to Comments 85 - 86: The agency disagrees with the comment to discard the incentive for an EGU owner or operator to seek LEE status. While emissions data collected as a result of the ICR and adjusted using the UPL calculation shows an EGU’s potential variability, well-operated EGUs – such as those qualifying for LEE status - are expected to have much less variable emissions. The Agency believes that the requirement to revert to the original monitoring frequency should subsequent emissions testing show the EGUs no longer meet LEE status will keep source owners or operators interested in maintaining LEE status. Commented [PB2]: From pages 38 and 39 of Volume 2 of

the MATS Response to Comment Document, available at https://www3.epa.gov/ttn/atw/u tility/mats_rtc_chapters_5

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Commented [PB3]: From page 243 of Volume 2.