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Federal Register / Vol. 86, No. 39 / Tuesday, March 2, 2021 / Rules and Regulations
analysis showed that the cross-line averages at the highest 720-hour average across all data and also at the 99th percentile is 1.6 lbs NOX/MMBtu, and at the 95th percentile is 1.5 lbs NOX/
MMBtu. In addition, review of the CEMS data shows that U.S. Steel has largely transitioned toward firing with natural gas and away from co-firing with coal and natural gas. U.S. Steel stated that it has been primarily combusting natural gas since December 2016. 6 As previously stated, only two of Minntacs five lines Lines 6 and 7 are capable of burning coal, and CEMS data show that U.S. Steel has largely shifted its operations on Lines 6 and 7 away from co-firing with coal and natural gas and toward firing exclusively natural gas.
While Lines 6 and 7 co-fired with coal and natural gas 85% of the time in 2012, these lines co-fired with coal and natural gas only 3% of the time in 2017.7
EPA has determined that the 1.6 lbs NOX/MMBtu cross-line emission limit constitutes the appropriate BART
emission limit for Minntac Lines 3
through 7, regardless of fuel type. As previously discussed, the BART
Guidelines provide that a source may be permitted to average emissions across a set of BART-eligible units within a fenceline, so long as the emission reductions from each pollutant controlled for BART would be equal to those reductions that would be obtained by separately controlling each of the BART-eligible units that constitute the BART-eligible source. 40 CFR part 51, appendix Y, at V. Minntac Lines 3, 4, 5, 6, and 7 are all BART-eligible units that constitute a BART-eligible source within a fenceline. When averaging the level of NOX emission reductions achievable on each of Minntac Lines 3
through 7 individually, the resulting limit is 1.6 lbs NOX/MMBtu when burning natural gas. Therefore, it is reasonable for EPA to establish a single cross-line average emission limit of 1.6
lbs NOX/MMBtu, to apply at all times, for Minntac Lines 3 through 7. 1.6 lbs NOX/MMBtu is the most stringent limit the facility can consistently meet while providing for operational flexibility with regard to fuel choice, including burning exclusively natural gas.
Comment 2: EPAs proposal lacks alternative BART emission limits based on the type of fuel each line will burn under the FIP. Although the BART
Guidelines are fuel-neutral, where a 6 See Redacted U. S. Steel Confidential Settlement CommunicationSubject to FRE 408, May 1, 2018, included in the docket.
7 See Minntac CEMS Data and Analysis, included in the docket.

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source wants to operate under different scenarios and burn different fuels that create different levels of BART pollutant emissions, EPA must first set alternative BART emission limits for each unit based on fuel use. EPAs 2013 FIP
promulgated two BART emission limits based on fuel use, which apply to all five BART units: A limit when burning natural gas, and second limit when cofiring coal and natural gas. The record indicates the BART units historically used a variety of fuels, which included:
Coal; wood; co-firing; biomass; and natural gas. EPAs proposed facilitywide BART limit relies on emission data collected when only one fuel was used, natural gas. EPA fails to analyze the range of fuels burned at Minntac and how the fuel burned impacts revising the prior BART determinations.
Response: EPA disagrees with the commenters contention that EPA must set alternative BART emission limits for each unit based on fuel use. Neither the CAA nor the regional haze rule requires EPA to establish separate BART limits based on fuel type. While the 1.6 lbs NOX/MMBtu limit for Minntac is reflective of natural gas emission data, EPA evaluated all available CEMS data for 20122017. These data are reflective of scenarios where lines were burning exclusively natural gas and scenarios when lines were co-firing with solid fuels.
We are under no obligation to set fuelspecific limits and are not doing so here.
EPA has determined that 1.6 lbs NOX/
MMBtu is the most stringent limit the facility can consistently meet while providing for operational flexibility with regard to fuel choice, including burning exclusively natural gas. As discussed previously in response to Comment 1, in response to comments received, EPA calculated 720-hour rolling averages for each line over the entire period without separating fuel types the All Fuels scenario. The data demonstrate that the cross-line averages at the highest 720-hour average across all data and also at the 99th percentile is 1.6 lbs NOX/MMBtu, and at the 95th percentile is 1.5 lbs NOX/
MMBtu. However, as previously explained, to allow for fuel choice and a scenario in which the facility burns only natural gas, 1.6 lbs NOX/MMBtu is the appropriate limit for the facility.
Comment 3: The agency suggests using the new data to revise the five BART determinations in its 2013 FIP.
EPA fails to provide a reasoned analysis for using the new data to revise its prior determination. EPAs prior determination found that once low NOX
burners were installed and burned natural gas, NOX emissions were lower
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than when co-firing coal and natural gas, and therefore, based the 2013 FIP
BART emission limits on its record and findings. EPAs 2020 proposal flips its prior determination, contending that NOX emissions are higher when burning only natural gas, as compared to cofiring coal and natural gas.
Response: EPAs August 15, 2012
proposed FIP approval includes an analysis and proposed determination that BART for Minntac is based upon the use of low NOX burners. In the 2013
FIP final rule, EPA finalized this determination. EPAs analysis concerning low NOX burners as representing BART for Minntac continues to remain valid and it is appropriate for EPA to rely on it in this action. As discussed above, at the time EPA established limits in the 2013 FIP, low NOX burners had only been in operation on Lines 6 and 7 since April 2011 and May 2010, respectively, and there were limited emission data available upon which to base a limit.
However, since that time, U.S. Steel has continued to operate low NOX burners on Lines 6 and 7 and has installed low NOX burners on Lines 4 and 5.
Therefore, as discussed in the response to Comment 1, there are significantly more data available from which to determine whether the BART FIP
emission limits are actually achievable through the utilization of low NOX
burners at Minntac.
Comment 4: EPAs approach is not permissible under the Act. Instead of proposing BART emission limits based on maximum controls, EPAs proposal uses the new data from the operating scenario that is the least effective at controlling NOX emissions to derive a BART emission limit, and then suggests applying the least effective control at all five BART units, regardless of what the unit burns.
Response: The control technology used as the basis for establishing BART
limits in the 2013 FIP has not changed.
Since promulgation of the 2013 FIP, however, our understanding of the emissions levels achievable through the use of this technology has changed. The emission limits initially promulgated under the 2013 FIP were based on the installation and optimization of a low NOX burner on Lines 6 and 7, and the limited CEMS data available at that time. Since promulgation of the 2013
FIP, U.S. Steel has continued to collect CEMS data from Lines 6 and 7. U.S.
Steel has also installed low NOX burners on Lines 4 and 5, has adjusted and optimized each of those burners to reduce NOX, and has collected CEMS
data for each of the lines. EPA based the 1.6 lbs NOX/MMBtu limit on the
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