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 sources 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 control 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/utility/mats_rtc_chapters_f

oreword‐1‐2‐3‐4_121611.pdf.
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/utility/mats_rtc_chapters_5

‐6‐7‐8‐9‐10‐app_a‐b_v2_121611.pdf.

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