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Federal Register / Vol. 86, No. 39 / Tuesday, March 2, 2021 / Rules and Regulations technology. However, since EPA
promulgated the BART limits for Minntac, 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 5.3 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. To reevaluate the emission limit achievable by use of low NOX burners, EPA analyzed available hourly CEMS data showing emissions in lbs NOX/MMBtu by fuel type. These data were available for the 20122017
time period. From this data set, EPA
then compiled the emission data available for each line after the installation of low NOX burners. For Line 4, this included data from December 15, 2016 through November 19, 2017. For Line 5, this included data from December 12, 2015 through November 11, 2017. For Lines 6 and 7, emission data were available from May 8, 2012 and April 27, 2012 through November 11, 2017, respectively. There
are necessarily differing amounts of CEMS data for each line since the low NOX burners were installed at different times.
To ensure that any revised emission limit would be based upon emission reduction capabilities during normal operations, EPA excluded hours when a line was idle, when a measurement error was recorded, or when process or CEMS codes indicated anything other than normal operation. For each line, EPA separated hours when only natural gas was burned from hours when the line was co-fired with coal Lines 6 and 7 or co-fired with biomass Lines 4 and 5. EPA then calculated 720-hour rolling averages based upon fuel type.4 To establish an achievable emission limit, EPA assessed the highest 720-hour average, the 99th percentile 720-hour average, and the 95th percentile 720hour average.5 The 99th percentile is the emission rate that the source would be predicted to be below during 99 out of 100 720-hour averages. The 95th percentile is the emission rate that the source would be predicted to be below
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during 95 out of 100 720-hour averages.
The highest 720-hour average is the emission rate at which the source would be predicted to be able maintain continual compliance.
Under the BART Guidelines, a source may be permitted to average emissions across a set of BART-eligible emission units within a fenceline, so long as the emission reductions from each pollutant being controlled for BART would be equal to those reductions that would be obtained by simply controlling each of the BART-eligible units that constitute the BART-eligible source. See 40 CFR
part 51, appendix Y, at V. U.S. Steel expressed interest in utilizing this option. As shown in Table 1 below, averaging the individual limits across Lines 4 through 7 for natural gas results in a combined emissions limit of 1.6 lbs NOX/MMBtu averaged over 720 hours, regardless of whether the single line emission limit basis for the cross-line average was the highest 720-hour average, the 99th percentile 720-hour average, or the 95th percentile 720-hour average.

TABLE 1INDIVIDUAL LINE AND CROSS-LINE AVERAGING EMISSION RATES FOR LINES 4 THROUGH 7
High 720-hr average lbs NOX/
MMBtu
Fuel
Line 4
Line 5
Line 6
Line 7

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Cross-line Average

99%
720-hr average lbs NOX/
MMBtu
95%
720-hr average lbs NOX/
MMBtu
Natural Gas
All Fuels
Natural Gas
All Fuels
Natural Gas
All Fuels
Natural Gas
All Fuels

1.5
1.5
1.4
1.4
1.7
1.7
1.9
1.9

1.5
1.4
1.4
1.4
1.6
1.6
1.8
1.8

1.5
1.4
1.4
1.3
1.6
1.4
1.8
1.7

Natural Gas
All Fuels

1.6
1.6

1.6
1.6

1.6
1.5

While Line 3 will not become subject to the FIP limits until July 2021, U.S.
Steel has indicated that, when compared to the other lines, the Line 3
burner is most similar to Line 4. Line 4
is more similar in age, size and design to Line 3 than the other lines. Line 3
utilizes the same fuels natural gas and biomass as Line 4, and both Lines are managed by the same control room operators. In addition, operating parameters on Line 3 are similar to Line 4 for such measured parameters as Kiln Exit Temperature, Preheat Zone
Temperature, Burner Temperature, and Pellet Residence time on the grate and in the kiln. Absent an engineering study for Line 3, using the emission rates for Line 4 as an estimate of the emission rates that would be expected after installation of a low NOX burner on Line 3 is reasonable. Therefore, EPA also calculated a cross-line average considering actual emissions from all four lines currently utilizing low NOX
burners Lines 4 through 7, as well as the expected emissions from Line 3. The resulting cross-line average is 1.6 lbs
NOX/MMBtu averaged over 720 hours, regardless of selection of statistical analyses at the 99th or 95th percentiles, or highest 720-hour average.
While the 1.6 lbs NOX/MMBtu limit for Minntac is reflective of natural gas emission data, in response to the comment received, EPA calculated 720hour rolling averages for each line over the entire period without separating fuel types. As provided in Table 1, the data
3 U.S. Steel installed low NO burners on Lines X
4 and 5 on December 15, 2016, and December 20, 2015, respectively.
4 Operations at Minntac in a given 30-day period, or even a single day, may in some cases involve
both operation with only natural gas and operation with at least some firing of solid fuels. To be able to evaluate emissions from all hours when different fuels were used within a 30-day period, rather than only the times when a line used solely natural gas or solely co-fired for 30 consecutive days, EPA

evaluated emissions based on 720-hour averages.
Note that operations are typically 24 hours per day and 720 is the number of hours in a 30-day period.
5 See Lines 3, 4, and 5 Data-L4_7 NO Data files X
combined, included in the docket.

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