necessary at these high DSI performance levels and so our cost
estimates do not include these potential additional costs. For
Harrington Unit 061B (the only unit with an existing ESP), our cost
estimate for DSI at 80 percent control efficiency does not include the
cost of a new ESP or fabric filter even though we do not know with
certainty whether the existing ESP would be able to handle the high
sorbent injection rates needed at high SO2 removal
efficiency. Therefore, without additional site-specific information
regarding the range of maximum control efficiency achievable and
associated costs needed to consider DSI at higher control levels, we
are not further considering DSI at 80/90 percent control efficiency in
our weighing of the factors. We welcome site-specific information and
comments on the potential for these units to consistently achieve DSI
SO2 control efficiencies much higher than 50 percent (which
may be as high as 80 to 90 percent).
---------------------------------------------------------------------------

    \326\ Harrington Unit 062B and Welsh Unit 1 show small
improvement in cost effectiveness at the higher level of DSI
control.
---------------------------------------------------------------------------

    In comparing DSI at 50 percent control level with SDA and wet FGD,
we found that DSI at the 50 percent control level was more cost-
effective than either SDA or wet FGD. In general, DSI systems have low
capital costs in comparison to SDA or wet FGD. At 50 percent control
level, the ongoing annual operation and maintenance costs of DSI are
comparable to those of SDA and wet FGD. Given the relatively low
initial capital costs of DSI as compared to the installation of SDA or
wet FGD, DSI may be a more favorable control option from a cost
perspective for a coal-fired EGU that may have plans to retire in the
next several years. However, we are not aware of any federally
enforceable and permanent commitment to cease operations for these
sources that would impact the remaining useful life of controls.

Therefore, we do not place extra weight on the capital cost benefit of
DSI at 50 percent control over the visibility benefit gained by SDA. In
considering CAMx modeled visibility benefits, wet FGD and SDA provide
approximately twice the amount of visibility benefits as DSI at 50
percent control level. Additionally, for all units, with the exception
of Harrington Unit 061B, we conclude that scrubbers are approximately
$4,900/ton or less, and thus within the range we regularly find to be
cost-effective. We are proposing to find that, with the possible
exception of Harrington Unit 061B, the resulting visibility benefit
offered by scrubbers outweighs any possible advantage DSI at 50 percent
control may hold in terms of cost-effectiveness. At higher control
efficiencies, DSI may become more favorable as the difference in
visibility benefits between DSI and SDA or wet FGD decreases and
estimated cost-effectiveness for DSI even at higher control is
estimated to be less than that for SDA or wet FGD, resulting in
increasing incremental costs between DSI and scrubbers. However, as
noted elsewhere, there is uncertainty as to what DSI control
efficiencies are achievable for these particular units and the
associated costs at these higher control efficiencies. We will further
consider site-specific information provided to us during the public
comment period in making our final decision on SO2 BART and
potentially re-evaluate DSI for one or more particular units.
    As we indicate elsewhere in our proposal, both SDA and wet FGD are
mature technologies that are in wide use throughout the United States.
In comparing wet FGD versus SDA, wet FGD is slightly less cost-
effective than SDA in all cases evaluated for this proposed action. Wet
FGD has slightly higher SO2 removal efficiency than SDA and
generally requires lower reagent usage and has lower associated reagent
costs than a comparable dry scrubber. However, as the Control Cost
Manual explains, ``In general, dry scrubbers have lower capital and
operating costs than wet scrubbers because dry scrubbers are generally
simpler, consume less water and require less waste processing.'' \327\
The Control Cost Manual also notes that SDA has lower auxiliary power
usage and lower water usage than wet FGD and does not require any
wastewater treatment, unlike a wet FGD.\328\ These factors all
contribute to the generally lower capital and operating costs of SDA
compared to wet FGD. Further, the wet FGD cost algorithms were updated
in version 6 of our IPM model to incorporate the capital and operating
costs of a wastewater treatment facility for all wet FGDs. The IPM wet
FGD Documentation states:
---------------------------------------------------------------------------

    \327\ EPA Air Pollution Control Cost Manual, Seventh Edition,
April 2021, Section 5, Chapter 1, titled ``Wet and Dry Scrubbers for
Acid Gas Control,'' page 1-11. The EPA Air Pollution Control Cost
Manual is available at https://www.epa.gov/economic-and-cost-analysis-air-pollution-regulations/cost-reports-and-guidance-air-pollution#cost%20manual.
    \328\ Id. At 1-3 and 1-4.

    Industry data from ``Current Capital Cost and Cost-effectiveness
of Power Plant Emissions Control Technologies'' prepared by J. E.
Cichanowicz for the Utility Air Regulatory Group (UARG) in 2012 to
2014 were used by Sargent & Lundy LLC (S&L) to update the wet FGD
cost algorithms from 2013. The published data were significantly
augmented by the S&L in-house database of recent wet FGD and wet FGD
wastewater treatment system projects. Due to recently published
Effluent Limitation Guidelines (ELG), it is expected that all future
wet FGDs will have to incorporate a wastewater treatment
facility.\329\
---------------------------------------------------------------------------

    \329\ IPM Model--Updates to Cost and Performance for APC
Technologies, Wet FGD Cost Development Methodology, Final January
2017, Project 13527-001, Eastern Research Group, Inc., Prepared by
Sargent & Lundy, p. 1.

    The anticipated need for a wastewater treatment facility for all
future wet FGDs also contributes to the higher capital and operating
costs of wet FGD compared to SDA. We discuss the cost differences and
the factors that result in wet FGD being slightly less cost-effective
than SDA for the evaluated units in greater detail in our 2023 BART FIP
TSD. We solicit comment on any additional factors or information that
may affect the costs of wet FGD and/or SDA for the evaluated units and
weigh in favor of one control option or the other. Although wet FGD
would offer slightly greater SO2 emission reductions
compared to SDA, that the estimated visibility benefits of the two
control options are very similar in all cases. In consideration of the
additional costs and non-air environmental impacts associated with wet
FGD, we propose to conclude that, based on a weighing of these factors,
the selection of SDA is appropriate for Coleto Creek Unit 1, W. A.
Parish Units WAP5 and WAP6, Welsh Unit 1, and Harrington Unit 062B. We
propose that SO2 BART should be based on the emission limit
associated with SDA control levels. For those units with existing
fabric filters, DSI could potentially meet the same emission
limitations as SDA but this would need to be confirmed with site-
specific performance testing. For Harrington Unit 061B, as discussed in
Section VIII.A.2., there are unique circumstances that impact the
evaluation of controls. For this unit, we propose that SO2
BART should be an emission limit based on SDA and we propose in the
alternative an emission limit based on DSI at 50 percent control level.
    We discuss in further detail our consideration of the cost-
effectiveness and anticipated visibility benefits of controls for each
of the facilities. Tables 15 thru 17 and 19 thru 26 provide summary
CAMx and CALPUFF model results of the benefits from the recommended
BART controls. The CAMx model results shown in the following tables for
each evaluated BART source summarize the benefits from the recommended
controls at the three Class I areas most impacted by the source or unit
in the baseline modeling. The benefit is calculated as the difference
between the maximum impact modeled for the baseline and the maximum
impact level modeled under the control scenario. Also summarized are
the cumulative benefit and the number of days impacted over 0.5 and 1.0
dv. Cumulative benefit is calculated as the difference in the maximum
visibility impacts from the baseline and control scenario summed across
the 15 Class I areas included in the CAMx modeling. The baseline total
cumulative number of days over 0.5 (1.0) dv is calculated as the sum of
the number of modeled days at each of the 15 Class I area impacted over
the threshold in the baseline modeling. The reduction in number of days
is calculated as the sum of the number of days over the chosen
threshold across the 15 Class I areas included in the CAMx modeling for
the baseline scenario subtracted by the number of days over the
threshold for the control scenario.
    In addition to these metrics, to further inform the impacts and
potential benefits of emission reductions, we also provide the average
of modeled potential impacts from CAMx on a broader set of high impact
days. The CAMx model results tables include the average impact across
the top ten highest impacted days at the most impacted class I areas
(and cumulative across all Class I areas) for the baseline and the
recommended control scenario, as well as the calculated visibility
benefits, to assess the potential visibility benefits that could be
anticipated due to

controls during the ten days with meteorological/transport conditions
that result in the largest visibility impacts. These varying conditions
affect the reaction rates and transport of pollutants which can be
simulated within the photochemical grid model. While the BART analysis
is focused on examination of the maximum potential visibility
impairment and benefits, these additional metrics provide a sense for
the potential benefit across days other than just the maximum impact
day.
    For Coleto Creek, Parish and Welsh units, we also present the
benefits of SDA control levels for comparison with wet FGD, though
these SDA control levels were not directly modeled in CAMx. To evaluate
SDA control levels using the available CAMx model results, we
calculated an estimate of the visibility benefits using a mathematical
extrapolation method, which is further discussed in the 2023 BART
Modeling TSD.
    The CALPUFF model results in the following tables for the evaluated
BART sources include the 98th percentile modeled impact and the number
of days impacted over 0.5 and 1.0 dv for those Class I areas within the
range of CALPUFF typically used for BART. See the 2023 BART Modeling
TSD for a complete summary of our visibility benefit analysis of
controls, including modeled benefits and impacts at all Class I areas
included in the modeling analyses, plus additional metrics considered
in the assessment of visibility benefits.
1. Coleto Creek Unit 1
    In reviewing Coleto Creek Unit 1, we conclude that the installation
of SDA or wet FGD results in significant visibility benefits. We
summarize some of these visibility benefits in Table 15 and discuss
them after the table.

                Table 15--CAMx-Predicted Wet FGD (SDA) Visibility Benefits at Coleto Creek Unit 1
----------------------------------------------------------------------------------------------------------------
     Coleto Creek Unit 1                     Baseline                                 Controlled
----------------------------------------------------------------------------------------------------------------
                                                                       Visibility
                              Impact (dv)   Avg impact   Number of     improvement   Avg visibility    Impacted
        Class I area             on the      (dv) for   days >=0.5/    (dv) on the     improvement    number of
                                maximum     the top 10    >=1.0 dv       maximum      (dv) for the   days >=0.5/
                               impact day      days                   impact day *    top 10 days *    >=1.0 dv
----------------------------------------------------------------------------------------------------------------
Caney Creek.................         1.55         0.89         18/2     1.38 (1.34)     0.80 (0.78)          0/0
Breton......................         1.19         0.47          4/1     1.08 (1.05)     0.43 (0.42)          0/0
Wichita Mountains...........         1.13         0.86         23/3     1.00 (0.98)     0.79 (0.76)          0/0
Cumulative (all Class I              8.54         5.14         69/6            7.75            4.71          0/0
 areas).....................
----------------------------------------------------------------------------------------------------------------
* Secondary values in parentheses indicate estimated visibility benefits for SDA.

    The visibility benefits predicted by CAMx with wet FGD control
levels applied to Coleto Creek Unit 1 are summarized in Table 15. We
also present the estimated benefits of SDA (shown in parentheses) for
the visibility improvement at the top three impacted Class I areas. The
small difference in visibility benefits between SDA and wet FGD is
consistent with the relatively small difference in control efficacy,
with an estimated difference between wet FGD and SDA on the maximum
impacted day of 0.04 dv at Caney Creek and an average top 10 days
difference of 0.02 dv at Caney Creek and Wichita Mountains.
    CAMx modeling results indicate that wet FGD will
eliminate all 69 days impacted over 0.5 dv across all Class I areas. At
each of the three most impacted Class I areas (Caney Creek, Breton, and
Wichita Mountains), wet FGD will result in visibility improvements of
more than 1.0 dv on the maximum impacted days at each Class I area, and
for the average of the top 10 most impacted days, CAMx
predicts an average improvement of 0.43 to 0.80 dv at those same three
Class I areas. Overall, there is a cumulative improvement to the
average of the top 10 impacted days of approximately 4.7 dv with wet
FGD across all impacted Class I areas and 7.7 dv cumulative improvement
on the maximum impacted day. When compared to wet FGD, we estimate that
SDA will result in very similar visibility benefits, ranging from 0.98
to 1.34 dv at the three most impacted Class I areas on the maximum
impacted days and an average improvement of 0.42 to 0.78 dv at those
same three Class I areas for the average of the top 10 most impacted
days. See the 2023 BART Modeling TSD for more information on our
estimation of the visibility benefits of SDA. Additional evaluation of
the visibility benefits of DSI are presented in the 2023 BART Modeling
TSD, but in summary, we find that DSI averaged 46 percent reduction in
cumulative visibility impacts at the Class I areas, while wet FGD
averaged 91 percent reduction in cumulative visibility impacts overall
on the most impacted days. At Caney Creek (highest baseline maximum
impact of 1.55 dv), DSI results in improvement on the maximum impacted
day of 0.66 dv compared to 1.38 dv for wet FGD and 1.34 dv for SDA.
Thus, we conclude that the resulting visibility benefit offered by
scrubbers outweighs the possible advantage DSI at 50 percent control
may hold in cost-effectiveness.
    We also conclude that both SDA and wet FGD are cost-effective at
$2,692/ton and $2,911/ton (respectively) and, as discussed in Section
VIII, well within a range that we have previously found to be
acceptable. Wet FGD is less cost-effective than SDA and we estimate
that it would have only a slight additional visibility benefit over
SDA. As discussed earlier, in weighing the factors between SDA and wet
FGD, we determined the additional visibility benefits did not outweigh
the additional cost, water requirements, and wastewater treatment
requirements associated with wet FGD. We consider the significant
visibility benefits that will result as justification for the cost of
SDA at the Coleto Creek Unit 1. We therefore propose that
SO2 BART for Coleto Creek Unit 1 is an emission limit of
0.06 lbs/MMBtu on a 30 BOD rolling average based on the installation of
SDA.
2. Harrington Units 061B & 062B
    From our identification of available controls, we conclude that
both DSI and SDA are technically feasible on both Harrington units.
Harrington Unit 061B is distinct from the other coal-fired units we
evaluated in that it has an existing ESP rather than a fabric filter.
Additionally, this unit had relatively low utilization at times during
the 2016-2020 baseline we used in our BART analysis, which has resulted
in a cost per SO2 tons removed for SDA that is relatively
high compared to the other units evaluated for SDA. Based on these
facts, we are proposing and taking comment on two alternative BART

determinations. We are proposing BART is an emission limit reflective
of the installation and operation of SDA on both Unit 061B and 062B. In
the alternative, we are proposing BART to be an emission limit
reflective of the installation and operation of DSI at 50 percent
control for Unit 061B and SDA on 062B. We provide the reasoning for
each determination in detail in the following paragraphs and solicit
comment on both approaches.
    In order to evaluate visibility benefits of control options for the
Harrington units, we performed modeling using both CALPUFF and
CAMx. As discussed in Section VII, and in more detail in our
2023 BART Modeling TSD, there are a number of differences between CAMx
and CALPUFF with one of the concerns being CALPUFF's simpler chemistry
mechanism that may underestimate the benefit of SO2
reductions versus CAMx generated values using more state of
the science chemistry.
a. Control Scenario 1: SDA on Unit 061B and Unit 062B

                                    Table 16--CALPUFF Predicted Visibility Benefits of SDA on Both Harrington Units.*
--------------------------------------------------------------------------------------------------------------------------------------------------------
                      Harrington                                   2016-2018 baseline impact              Modeled Benefit of SDA on both
--------------------------------------------------------------------------------------------------------              units                 Cumulative
                                                                                Cumulative   --------------------------------- 2016-2018 # of
                                                                                2016-2018 # of                                      days with
                     Class I Area                        2016 dv    2017 dv    2018 dv      days with                                      impacts >=0.5
                                                                                impacts >=0.5   2016 dv    2017 dv    2018 dv     dv/>=1.0 dv
                                                                                dv/>=1.0 dv
--------------------------------------------------------------------------------------------------------------------------------------------------------
Carlsbad Caverns......................................       0.39       0.41       0.56            16/5       0.24       0.27       0.31             1/1
Bandelier.............................................       0.17       0.12       0.14             2/0       0.12       0.09       0.11             0/0
Pecos.................................................       0.22       0.28       0.24             9/0       0.15       0.17       0.16             0/0
Salt Creek............................................       0.49       0.59       0.54            27/3       0.23       0.39       0.32             2/0
Wheeler Peak..........................................       0.12       0.15       0.16             2/0       0.07       0.10       0.11             0/0
White Mountain........................................       0.26       0.43       0.33             7/0       0.17       0.26       0.24             0/0
Wichita Mountains.....................................       0.54       0.45       0.58            24/8       0.35       0.23       0.33             3/0
--------------------------------------------------------------------------------------------------------------------------------------------------------
* Benefit of control values are the decrease in deciview between baseline and the control scenario. Number of days is the number of days that are equal
  or greater than 0.5 and 1.0 dv after controls.

    As in Section VII, we compared the visibility benefits (as
predicted by CALPUFF) of the SDA control levels on both units to the
baseline impacts in terms of percent reduction in visibility impacts.
To make this comparison, we first calculated the average of the 98th
percentile (8th highest value) for the three years modeled for each
Class I area and the average for the seven Class I areas. For
Harrington, Salt Creek was the highest impacted of the seven Class I
areas and SDA control on both units compared to baseline resulted in a
reduction of visibility impacts by 58 percent, from 0.54 dv to 0.23 dv.
At the second highest impacted Class I area, Wichita Mountains, SDA on
both units result in a reduction of visibility impacts by 58 percent,
from 0.52 dv to 0.22 dv. SDA on both units also resulted in an average
reduction of visibility impacts across the seven Class I areas combined
of 61 percent. Using the CALPUFF modeling results from the baseline, we
determined the total number of days when facility impacts were greater
than 0.5 dv and 1.0 dv. Harrington had a total of 87 days with
visibility impacts above 0.5 dv and 16 days above 1.0 dv at the seven
Class I areas modeled with CALPUFF. In comparison, SDA on both units
results in a large reduction in impacted days with only six days still
above 0.5 dv and one day above 1.0 dv at the same seven Class I areas.
In conclusion, the CALPUFF modeling results show that SDA on both units
would provide notable visibility improvements.

                                       Table 17--CAMx-Predicted Visibility Impact and Benefit of Controls for SDA
--------------------------------------------------------------------------------------------------------------------------------------------------------
                       Harrington                                            Baseline                                       Controlled
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Visibility
                                                          Impact (dv) on    Avg impact    Number of days    improvement   Avg visibility     Impacted
                      Class I area                          the maximum    (dv) for the    >=0.5/ >=1.0     (dv) on the     improvement   number of days
                                                            impact day      top 10 days         dv        maximum impact   (dv) for the   >=0.5/>=1.0 dv
                                                                                day         top 10 days
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                  Harrington Unit 061B
--------------------------------------------------------------------------------------------------------------------------------------------------------
White Mountain..........................................            1.43            0.48             3/1            0.96            0.35             0/0
Bandelier...............................................            0.83            0.28             1/0            0.64            0.23             0/0
Salt Creek..............................................            0.79            0.55             6/0            0.50            0.43             0/0
Cumulative (all Class I areas)..........................            6.59            3.15            10/1            4.61            2.48             0/0
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                  Harrington Unit 062B
--------------------------------------------------------------------------------------------------------------------------------------------------------
White Mountain..........................................            1.36            0.48             3/1            0.95            0.36             0/0
Bandelier...............................................            0.82            0.29             1/0            0.65            0.23             0/0
Salt Creek..............................................            0.79            0.56             6/0            0.52            0.45             0/0
Cumulative (all Class I areas)..........................            6.55            3.17            10/1            4.79            2.56             0/0
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                             Harrington Units 061B and 062B
--------------------------------------------------------------------------------------------------------------------------------------------------------
White Mountain..........................................            2.64            0.93             8/3            1.78            0.70             1/0
Bandelier...............................................            1.60            0.56             4/1            1.24            0.45             0/0
Salt Creek..............................................            1.52            1.08            13/6            0.97            0.86             1/0

Cumulative (all Class I areas)..........................           12.77            6.23           44/10            9.08            5.00             2/0
--------------------------------------------------------------------------------------------------------------------------------------------------------

    The CAMx results reinforce that installation of SDA at the
Harrington units would provide significant visibility benefits. CAMx
modeling results indicate SDA on the individual Harrington units will
eliminate all days impacted over 0.5 dv at all Class I areas. When
considering the combined impacts of the two units, visibility benefits
from SDA installed on both units predicts only one day to exceed the
0.5 dv threshold at each of the White Mountain and Salt Creek Class I
areas. This is an overall (cumulative Class I areas) reduction from 44
days over 0.5 dv in the baseline to a total of only two days with SDA.
The overall cumulative visibility improvement is 9.08 dv on the maximum
impacted days and 5.0 dv improvement when considering the average of
the top ten days across all 15 Class I areas.
    For Harrington Unit 061B, the CAMx results show that SDA would
eliminate all days impacted over 0.5 dv for that unit. On the maximum
impacted day at White Mountain, SDA results in 0.96 dv improvement over
baseline (1.43 dv), an additional 0.44 dv improvement over DSI at 50
percent control (from Table 12). On the maximum impacted day at
Bandelier, SDA results in 0.64 dv improvement over the baseline (0.83
dv), an additional 0.3 dv improvement over DSI at 50 percent control.
Furthermore, the CAMx results predict that the cumulative visibility
benefit provided by SDA on just Unit 061B is 4.6 dv, with eight Class I
areas seeing improvements of 0.25 dv or more.\330\ SDA control on both
units resulted in a reduction of maximum visibility impacts by 67
percent at White Mountain and an average reduction of maximum
visibility impacts across all 15 Class I areas of 71 percent. This
highlights that emissions and reductions from Harrington impact
visibility conditions at several Class I areas. Visibility benefits for
SDA on Unit 062B are very similar to Unit 061B.
---------------------------------------------------------------------------

    \330\ Bandelier, Guadalupe Mountains, Carlsbad Caverns, Salt
Creek, Upper Buffalo, White Mountain, Wheeler Peak, and Pecos
visibility improvements with SDA on Harrington Unit 061B ranging
from 0.25 dv to 0.96 dv.

                                                 Table 18--Cost Analysis Summary for Units 061B and 062B
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                2020
                                                                                SO2 reduction                       2020 Cost-      Incremental
                   Facility                                    Control                      (tpy)       2020 Annualized    effectiveness       cost-
                                                                                cost            ($/ton)      effectiveness
                                                                                ($/ton)
--------------------------------------------------------------------------------------------------------------------------------------------------------
Harrington 061B...............................  DSI w/ESP--50% control efficiency....           1,892         $7,075,817          $3,740  ..............
Harrington 061B...............................  SDA..................................           3,327        $21,967,236          $6,603         $10,377
Harrington 062B...............................  DSI w/BGH--50% control efficiency....           2,703         $7,408,200          $2,742  ..............
Harrington 062B...............................  SDA..................................           4,812        $23,369,564          $4,857          $7,568
--------------------------------------------------------------------------------------------------------------------------------------------------------

    A summary of our cost analyses from Section VII.B.3. are presented
in Table 18. In our analysis, we find SDA to have a cost of $6,603/ton
for Harrington Unit 061B, which is above the range for controls that we
have previously found to be cost-effective. It is reasonable to expect
that similar controls installed on units that are designed for similar
capacity would result in similar tons reduced and cost effectiveness.
Units 061B and 062B are designed to produce 360 MW of electricity but
based on a review of heat input data from 2010 to 2021, differences in
utilization or heat input have resulted in different estimates of tons
reduced and cost effectiveness.\331\ The resulting control cost
effectiveness for Harrington Unit 061B ($6,603/ton) is higher than at
the similarly designed and sized Unit 062B ($4,857/ton) because of a
lower utilization rate.
---------------------------------------------------------------------------

    \331\ See ``CAMD Heat Input Data for Harrington Station.xlsx''
available in the docket for this action.

---------------------------------------------------------------------------

[GRAPHIC] [TIFF OMITTED] TP04MY23.115

    As shown in Figure 1, the utilization rate of Unit 061B was much
lower than Unit 062B during the 2016-2020 baseline period we evaluated
for this proposed action. However, utilization rates both before and
after the baseline period have been more consistent between the two
units, and the utilization rate at Unit 061B has at times exceeded the
annual utilization at Unit 062B. The difference in utilization during
the baseline period used for the BART analysis results in a relatively
smaller estimated reduction of SO2 emissions (3,327 tons per
year with SDA for Unit 061B compared to 4,812 tons per year reduced
with SDA for Unit 062B) used to calculate the cost-effectiveness in $/
ton removed.
    Further examination of the historical heat input for these units
shows that Unit 061B annual heat input for 2015 and for 2021 are higher
than during the 2016-2020 period, and for both 2015 and 2021, heat
input for Units 061B and 062B are similar. During Fall of 2016 through
spring of 2017, Unit 061B was utilized less than the other two units at
the facility.\332\ This pattern continued for 2017/2018 and 2018/2019,
resulting in lower overall heat input for the unit during those years.
Starting in Fall of 2019, utilization of the BART units at the facility
became roughly similar again, except during periods where a unit at the
facility was down. We also note that July 2022 heat input for Unit 061B
is higher than in any other single month from 2015-2022. These changes
in utilization in the more recent period may suggest that the
historical pattern of lower utilization of Unit 061B compared to Unit
062B that was observed in the majority of the 2016-2020 period may not
continue in the future, which could result in more favorable (lower $/
ton) cost-effectiveness for SDA and other controls at Harrington Unit
061B. Furthermore, because there are no enforceable limitations on
utilization for these units, there is no assurance that Unit 061B will
operate in the future at the lower utilization rates seen between 2016
and 2020.
---------------------------------------------------------------------------

    \332\ The Harrington facility has three EGUs. The third unit,
Unit 063B, is not BART-eligible.
---------------------------------------------------------------------------

    We find that SDA on Units 061B and 062B provides significant
visibility benefits. For Unit 062B we find SDA at $4,857/ton within the
range we have previously found to be cost effective for BART. While
above the range we have previously found to be cost effective, we still
find SDA at $6,603/ton for Unit 061B to be reasonable based on the
visibility benefits. Additionally, the estimated higher cost-
effectiveness associated with SDA is driven by past lower utilization
of Unit 061B during the baseline period. We propose and are taking
comment on our determination that BART for Units 061B and 062B is an
emission limit of 0.06 lb/MMBtu consistent with the installation and
operation of SDA.
b. Control Scenario 2: DSI on Unit 061B and SDA on Unit 062B
    Because we recognize the cost effectiveness of SDA at Harrington
Unit 061B is above a range of costs we have previously required for
BART, we are proposing in the alternative to determine that BART is DSI
at a control level of 50 percent, with a requirement to conduct a DSI
performance evaluation.

                     Table 19--CALPUFF Predicted Visibility Benefit of DSI (50 Percent) on Harrington Unit 061B and SDA on Unit 062B
--------------------------------------------------------------------------------------------------------------------------------------------------------
                   Harrington                                     2016-2018 Baseline                   Benefit of DSI--50% at Unit 061B and
-----------------------------------------------------------------------------------------------------            SDA at Unit 062B             Cumulative
                                                                                Cumulative --------------------------------------- 2016-2018 #
                                                                                # of days                                            of days
                                                                                with                                                with
                  Class I area                      2016 dv      2017 dv      2018 dv      impacts      2016 dv      2017 dv      2018 dv      impacts
                                                                                >=0.5 dv/                                           >=0.5 dv/
                                                                                >=1.0 dv                                            >=1.0 dv
--------------------------------------------------------------------------------------------------------------------------------------------------------
Carlsbad Caverns................................         0.39         0.41         0.56         16/5         0.18         0.21         0.23          5/1
Bandelier.......................................         0.17         0.12         0.14          2/0         0.09         0.06         0.08          0/0
Pecos...........................................         0.22         0.28         0.24          9/0         0.11         0.13         0.12          0/0
Salt Creek......................................         0.49         0.59         0.54         27/3         0.16         0.30         0.25         11/1
Wheeler Peak....................................         0.12         0.15         0.16          2/0         0.05         0.08         0.08          0/0
White Mountain..................................         0.26         0.43         0.33          7/0         0.14         0.20         0.19          0/0
Wichita Mountains...............................         0.54         0.45         0.58         24/8         0.27         0.20         0.25          8/0
--------------------------------------------------------------------------------------------------------------------------------------------------------
* Benefit of control values are the decrease in deciview between baseline and the control scenario. Number of days is the number of days that are equal
  or greater than 0.5 and 1.0 dv after controls.

    For Harrington, CALPUFF results show installation of DSI at a 50
percent control level on Unit 061B and SDA on Unit 062B resulted in a
reduction of visibility impacts by 44 percent from the baseline at the
highest impacted Class I area (Salt Creek) from 0.54 dv to 0.31 dv, and
an average reduction of visibility impacts across seven Class I areas
of 47 percent. For the 2016-2018 modeled years (baseline period),
Harrington baseline had a total of 87 days with visibility impacts
above 0.5 dv and 16 days above 1.0 dv at the seven Class I areas
modeled with CALPUFF. DSI at 50 percent on Unit 061B and SDA on Unit
062B resulted in 24 days above 0.5 dv and two days above 1.0 dv. The
incremental visibility benefit between DSI and SDA is larger with the
CAMx modeling than with the CALPUFF modeling.\333\
---------------------------------------------------------------------------

    \333\ See the 2023 BART Modeling TSD for detailed discussion of
differences between CAMx and CALPUFF models and modeling results.

                            Table 20--CAMx Predicted Visibility Benefit of DSI (50 Percent) on Unit 061B and SDA on Unit 062B
--------------------------------------------------------------------------------------------------------------------------------------------------------
                       Harrington                                            Baseline                                       Controlled
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Visibility
                                                          Impact (dv) on    Avg impact    Number of days    improvement   Avg visibility     Impacted
                      Class I area                          the maximum    (dv) for the    >=0.5/ >=1.0     (dv) on the     improvement   number of days
                                                            impact day      top 10 days         dv        maximum impact   (dv) for the    >=0.5/ >=1.0
                                                                                day         top 10 days         dv
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                   Harrington Unit 061B with DSI (50 percent) control
--------------------------------------------------------------------------------------------------------------------------------------------------------
White Mountain..........................................            1.43            0.48             3/1            0.52            0.19             1/0
Bandelier...............................................            0.83            0.28             1/0            0.34            0.12             0/0
Salt Creek..............................................            0.79            0.55             6/0            0.26            0.23             1/0
Cumulative (all Class I areas)..........................            6.59            3.15            10/1            2.56            1.34             2/0
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                          Harrington Unit 062B with SDA control
--------------------------------------------------------------------------------------------------------------------------------------------------------
White Mountain..........................................            1.36            0.48             3/1            0.95            0.36             0/0
Bandelier...............................................            0.82            0.29             1/0            0.65            0.23             0/0
Salt Creek..............................................            0.79            0.56             6/0            0.52            0.45             0/0
Cumulative (all Class I areas)..........................            6.55            3.17            10/1            4.79            2.56             0/0
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                          Harrington Unit 061B with DSI (50 percent) and 062B with SDA controls
--------------------------------------------------------------------------------------------------------------------------------------------------------
White Mountain..........................................            2.64            0.93             8/3          * 1.34          * 0.54          ** 1/1
Bandelier...............................................            1.60            0.56             4/1          * 0.94          * 0.34          ** 1/0
Salt Creek..............................................            1.52            1.08            13/6          * 0.73          * 0.66          ** 3/0
Cumulative (all Class I areas)..........................           12.77            6.23           44/10          * 7.03          * 3.86          ** 5/1
--------------------------------------------------------------------------------------------------------------------------------------------------------
* We did not model this combination (50 percent DSI on 061B and SDA on 062B) directly, so we estimated these values by subtracting the difference
  between the 50 percent DSI (Low Control) and SDA for 061B improvement values from the combined units SDA-only values in the previous table.
** Again, we did not model this combination directly, so we estimated the number of days based on the High (SDA) and Low (50 percent DSI) control number
  of days.

    The CAMx results for Harrington for this second control scenario
show that White Mountain was the most impacted of the 15 Class I areas,
the same as in the first control scenario, which had SDA on both units.
From Table 17 of the first control scenario, we calculate that SDA
control on both units compared to baseline resulted in a reduction of
visibility impacts at White Mountain by 67 percent and an average
reduction of visibility impacts across the 15 Class I areas of 71
percent; whereas, from Table 20 we calculate that the 50% DSI on Unit
061B and SDA on Unit 062B

compared to the baseline resulted in a reduction of visibility impacts
at White Mountain by 51 percent and an average reduction of visibility
impacts across the 15 Class I areas of 55 percent.
    For Unit 061B, by itself, DSI at 50 percent control results in
visibility benefits approximately one half of those achieved through
SDA. On the maximum impacted day at White Mountain, DSI at 50 percent
on Unit 061B results in 0.52 dv improvement compared to 0.96 dv with
SDA on that unit; at Bandelier, DSI at 50 percent results in 0.34 dv
improvement compared to 0.64 dv with SDA on that unit. The cumulative
visibility benefit across all Class I areas on the maximum impacted
days for Unit 61B with DSI at 50 percent is 2.56 dv compared to 4.61 dv
with SDA. For the average of the top 10 most impacted days, SDA
provides for a 0.43 dv benefit at Salt Creek compared to 0.23 dv for
DSI at 50 percent control, and SDA provides for 0.35 dv benefit at
White Mountain compared to 0.19 dv for DSI at 50 percent control--
almost twice the improvement with SDA over DSI at 50% on Unit 061B.
    When considering the combined benefits of DSI for Unit 061B and SDA
for Unit 062B, the visibility improvement at White Mountain Class I
area is estimated to be more than 1.3 (1.78 minus 0.44) dv on the
highest impact day, while the average of the top 10 most impacted days
visibility improvement is approximately 0.6 (0.86 minus 0.20) dv at
Salt Creek. Overall, for the visibility improvement at the cumulative
Class I areas from the Harrington facility, CAMx predicts an average
improvement of almost 4.0 (5.00 minus 1.14) dv across all the Class I
areas evaluated on the top 10 days and an improvement on the maximum
impacted days of approximately 7.0 (9.08 minus 2.05) dv with SDA
controls on Unit 062B and DSI at 50 percent on Unit 061B. Thus, we find
that SDA on Unit 062B and DSI at 50 percent control on Unit 061B
results in a significant reduction in visibility impacts from these
units and that the benefits are spread across a number of Class I areas
in New Mexico, Texas, and Oklahoma. As previously discussed, SDA on
both units provides an additional cumulative visibility benefit (the
difference between DSI at 50 percent control and SDA on Unit 061B) on
the average of the top 10 days from the Harrington facility of 1.14 dv
across all the Class I areas evaluated and an additional improvement on
the maximum impacted days of 2.05 dv. However, DSI at 50 percent
control for Harrington is more cost-effective ($2,742/ton for Unit 062B
and $3,740/ton for Unit 061B) than SDA ($4,857/ton for Unit 062B and
$6,603/ton for Unit 061B) and is well within the range of what we have
previously found to be acceptable in other BART actions. For Harrington
Unit 062B, we consider SDA to also be cost-effective and within the
range of what we have previously found to be acceptable in other BART
actions. As discussed earlier, the cost of SDA at Unit 061B is above
the range we have previously found to be cost-effective, and the
incremental cost-effectiveness of SDA (going from DSI at 50 percent
control efficiency to SDA) is $10,377, which we consider to be
relatively high. The cost of SDA at Unit 061B is relatively high, but
we still find SDA to be reasonable based on the important visibility
benefits of SDA on this unit. However, given the relatively high cost
of SDA at Unit 061B, we propose in the alternative that BART for this
unit is based on DSI. While the visibility benefits of DSI are
approximately half those from SDA on Unit 061B using the CAMx results,
installation of DSI is significantly less costly than SDA. Therefore,
we are proposing in the alternative that BART for Unit 061B is 0.27 lb/
MMBtu based on DSI at 50 percent, with a compliance period of no later
than two (2) years from the effective date of the final rule.\334\
---------------------------------------------------------------------------

    \334\ The proposed regulatory language for this rulemaking only
covers our first proposed approach (SDA on Harrington Units 061B and
062B). If the EPA finalizes an action consistent with our
alternative proposed approach (DSI at 50% control on Unit 061B and
SDA on Unit 062B), we will revise the regulatory language
accordingly.
---------------------------------------------------------------------------

    We believe Unit 061B is likely capable of achieving an
SO2 emission limit of 0.27 lb/MMBtu with DSI but are not
certain whether the unit could achieve a lower emission limit on a 30
BOD or what the potential impacts to PM emissions could be at higher
injections rates necessary for higher control efficiencies using the
existing ESP. We evaluated DSI at a 50 percent control level as a
conservative representative of what DSI can achieve on average. Because
the control efficiency of DSI is dependent on several operational
variables, we also propose to require a performance evaluation (as
provided for in Section IX.A.3) to determine the maximum control
efficiency of DSI for Harrington Unit 061B specifically along with an
estimate of the cost to operate DSI at this control level.\335\ Based
on available information, on a unit-specific basis, using sodium-based
sorbents, we believe DSI could potentially achieve up to 80 percent or
higher SO2 control, even with an ESP. However, as noted
earlier, because of unit-specific uncertainty we are proposing an
emissions limit of 0.27 lb/MMBtu based on DSI at 50 percent. If a DSI
performance evaluation finds that Unit 061B can meet a lower rate, we
will propose to adjust this limit in a future notice to reflect the
maximum control efficiency that the unit can consistently meet. As
discussed in Sections VII.B.2.a and VII.B.3.a, we are also soliciting
comments on the range and maximum control efficiency that can be
achieved with DSI at the evaluated units, including Harrington Unit
061B, and estimates of the range of associated costs. We are especially
interested in comments on any site-specific DSI testing for Unit 061B
to determine the range and maximum control efficiency that can be
achieved with DSI at the unit. Any data to support the control
efficiency range, maximum control efficiency, and cost of DSI for the
unit should be submitted along with those comments. We will further
consider DSI site-specific information provided to us during the public
comment period in our final decision and potentially re-evaluate DSI
for this particular unit.
---------------------------------------------------------------------------

    \335\ The purpose of the DSI performance evaluation is to
determine the lowest SO2 emission rate Unit 061B would be
able to sustainably achieve on a 30 BOD with DSI under three
different scenarios for particulate removal ((1) using the existing
ESP; (2) with a new ESP installation; and (3) with a new fabric
filter installation) and to determine how compliance with such an
emission rate would impact our cost estimates for DSI. The proposed
DSI performance evaluation requirements are discussed in greater
detail in Section IX.A.3.
---------------------------------------------------------------------------

c. Option To Convert to Natural Gas
    Additionally, we recognize that Xcel Energy has announced its
intent to convert Harrington Station to natural gas by January 1, 2025.
We understand this has been formalized further in an Agreed Order with
TCEQ,\336\ a PSD permit revision,\337\ and approval from the Texas
Public Utility Commission (PUC).\338\ The BART Guidelines state in
situations where a future operating parameter will differ from past or
current practices, and if such future operating parameters will have a
deciding effect in the BART determination, then the future operating
parameters need to be made federally enforceable and permanent in order
to consider them in the BART

determination.\339\ Thus, we are providing Xcel Energy the option to
make this conversion to natural gas a permanent and federally
enforceable commitment by incorporating it into this FIP. We are
proposing that should Xcel Energy agree to these future operating
parameters (i.e., operating as a natural gas source no later than
January 1, 2025), then for purposes of this analysis we will consider
Harrington to be a natural gas source. We noted earlier that for
natural gas units, there are no practical add-on controls to consider
for setting a more stringent SO2 BART emission limit.
Therefore, under this option, we propose that BART for both Harrington
units is the burning of pipeline natural gas, as defined at 40 CFR
72.2.\340\ Because the conversion to natural gas no later than January
1, 2025, would occur before the deadline to comply with a BART emission
limit reflective of the installation of DSI or scrubbers, there is no
need to evaluate whether an interim SO2 emission limit is
necessary prior to the conversion to natural gas. Additionally, the
visibility benefits of a conversion to natural gas would be greater
than with the limits we are proposing based on either SDA or DSI. We
are interested in comments on this option and specifically invite
Harrington to provide comments as to their interest in this option.
---------------------------------------------------------------------------

    \336\ In the Matter of an Agreed Order Concerning Southwestern
Public Service Company, dba cel Energy, Harrington Station Power
Plant, TCEQ Docket No. 2020-0982-MIS (Adopted Oct. 21, 2020). A copy
of the Order is available in the docket for this action.
    \337\ See Harrington's revised PSD permits (NSR1529 and NSR1388)
located in the docket for this action.
    \338\ See the Texas PUC Order, Docket No. 52485-201, located in
the docket for this action.
    \339\ 70 FR at 39167.
    \340\ ``Pipeline natural gas'' means a naturally occurring fluid
mixture of hydrocarbons (e.g., methane, ethane, or propane) produced
in geological formations beneath the Earth's surface that maintains
a gaseous state at standard atmospheric temperature and pressure
under ordinary conditions, and which is provided by a supplier
through a pipeline. Pipeline natural gas contains 0.5 grains or less
of total sulfur per 100 standard cubic feet. This is equivalent to
an SO2 emission rate of 0.0006 lb/MMBtu. Additionally,
pipeline natural gas must either be composed of at least 70 percent
methane by volume or have a gross calorific value between 950 and
1100 Btu per standard cubic foot. 40 CFR 72.2.
---------------------------------------------------------------------------

3. Welsh Unit 1
    In reviewing the modeling results for Welsh Unit 1, we conclude
that the installation of a wet FGD or SDA will provide significant
visibility benefits. As discussed in Section VII.A.1, we modeled Welsh
Unit 1 with both CALPUFF and CAMx. The visibility benefits for Welsh
are summarized in Tables 21 and 22.

                                    Table 21--CALPUFF-Predicted Wet FGD and SDA Visibility Benefits at Welsh Unit 1 *
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                        2016-18 baseline                                High control scenarios (WFGD/SDA)
                                       -----------------------------------------------------------------------------------------------------------------
                                                                           Cumulative      Visibility benefit at Class I area    Cumulative 2016-2018 #
             Class I area                                                 2016-18 # of       (dv) from baseline (WFGD/SDA)        of days with  impacts
                                         2016 dv    2017 dv    2018 dv      days with   ---------------------------------------      >=0.5 />=1.0 dv
                                                                          impacts >=0.5                                        -------------------------
                                                                           dv/>=1.0 dv     2016 dv      2017 dv      2018 dv        WFGD         SDA
--------------------------------------------------------------------------------------------------------------------------------------------------------
Caney Creek...........................       0.70       0.94       0.96           77/13    0.28/0.27    0.37/0.35    0.53/0.53         18/1         18/1
Upper Buffalo.........................       0.36       0.49       0.60            16/0    0.25/0.24    0.33/0.32    0.42/0.40          0/0          1/0
Wichita Mountains.....................       0.25       0.35       0.24             3/0    0.17/0.16    0.28/0.26    0.16/0.16          0/0          1/0
--------------------------------------------------------------------------------------------------------------------------------------------------------
* Benefit of control values are the decrease in deciview between baseline and the control scenario. Number of days is the number of days that are equal
  or greater than 0.5 and 1.0 dv after controls.

    The Welsh facility is within 450 km of three Class I areas (Caney
Creek, Wichita Mountains, and Upper Buffalo), and therefore, within the
range that the CALPUFF model has been used for assessing visibility
impacts in BART analyses. CALPUFF results for Welsh indicate that
installation of wet FGD or SDA resulted in a reduction of visibility
impacts by 45 percent (0.39 dv average visibility benefit) and 44
percent (0.38 dv average visibility benefit), respectively from the
baseline (0.86 dv) at the highest impacted Class I area (Caney Creek),
and an average reduction of visibility impacts across the three Class I
areas of 57 percent and 55 percent respectively.
    Using three years (2016-2018) CALPUFF modeling results, we assessed
the annual number of days when the facility impacts were greater than
the 0.5 dv and 1.0 dv threshold at each of the Class I areas and then
summed this value for all Class I areas to determine the total number
of days in the 2016-2018 modeled period where visibility impacts were
above 0.5 dv and 1.0 dv. These results indicate that the installation
of wet FGD or SDA will eliminate 78 days (81 percent decrease) and 76
days (79 percent decrease) respectively where visibility is greater
than 0.5 dv and 12 days (92 percent decrease) where visibility is
greater than 1.0 dv over the three modeled years for these three Class
I areas. Comparing the CALPUFF modeled improvement with the
installation of wet FGD versus SDA on Unit 1 indicates the visibility
benefits are very similar (within 1.3-5.4 percent of each other).

                                       Table 22--CAMx-Predicted Wet FGD (SDA) Visibility Benefits at Welsh Unit 1
--------------------------------------------------------------------------------------------------------------------------------------------------------
                      Welsh Unit 1                                           Baseline                                       Controlled
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Visibility
                                                          Impact (dv) on    Avg impact    Number of days    improvement   Avg visibility     Impacted
                      Class I area                          the maximum    (dv) for the    >=0.5/ >=1.0     (dv) on the     improvement   number of days
                                                            impact day      top 10 days         dv        maximum impact   (dv) for the   >=0.5/>=1.0 dv
                                                                                day *       top 10 days *
--------------------------------------------------------------------------------------------------------------------------------------------------------
Caney Creek.............................................            1.58            1.11            27/6     1.08 (1.02)     0.83 (0.79)             0/0
Wichita Mountains.......................................            1.54            0.71             6/2     1.34 (1.29)     0.60 (0.57)             0/0
Upper Buffalo...........................................            1.12            0.68             8/1     0.83 (0.79)     0.53 (0.50)             0/0
Cumulative (all Class I areas)..........................            6.67            3.97            46/9            5.27            3.21             0/0
--------------------------------------------------------------------------------------------------------------------------------------------------------
* Secondary values in parentheses indicate estimated visibility benefits for SDA.

    Table 22 displays the visibility benefits predicted by CAMx with
wet FGD control levels applied to Welsh Unit 1. We also present the
estimated benefits of SDA (shown in parentheses). Since SDA is slightly
less effective at reducing SO2 emissions than wet FGD, the
comparative results between SDA and wet FGD are consistent with the
difference in control efficacy, with a difference between wet FGD and
SDA on the maximum impacted day of 0.06 dv at Caney Creek and 0.05 dv
at Wichita Mountains and an average top 10 days difference of 0.03-0.04
dv at each of the top three Class I areas.
    CAMx modeling results indicate that wet FGD on Welsh Unit 1 will
eliminate all days impacted by the unit over 0.5 dv at all Class I
areas, from 46 days in the baseline to zero with wet FGD, and SDA
controls eliminate all but one day with impacts over 0.5 dv. At the
most impacted Class I areas, wet FGD control results in visibility
improvements of up to 1.35 dv on the maximum impacted day at Wichita
Mountains and 1.29 dv with SDA control compared to the baseline maximum
impact of 1.54 dv. Similarly, wet FGD control results in visibility
improvements of up to 1.08 dv on the maximum impacted day at Caney
Creek and 1.02 dv with SDA control compared to the baseline maximum
impact of 1.58 dv. For the average of the top 10 most impacted days,
wet FGD control results in 0.82 dv, while SDA results in 0.79 dv
visibility improvements at Caney Creek (baseline impact 1.11 dv). For
the average of the top 10 most impacted days, wet FGD control results
in 0.60 dv, while SDA results in 0.57 dv visibility improvements at
Wichita Mountains (baseline impact 0.71 dv).
    Overall, there is a cumulative improvement to the average of the
top 10 days of approximately 3.2 dv with wet FGD across all impacted
Class I areas and approximately 5.3 dv cumulative improvement on the
maximum impacted day. The 2023 BART Modeling TSD shows that DSI control
achieved approximately 39 percent average improvement in visibility,
while wet FGD averaged 79 percent overall visibility improvement. At
Caney Creek, DSI results in improvement on the maximum impacted day of
0.48 dv compared to 1.08 dv for wet FGD and 1.02 dv for SDA. At Wichita
Mountains, DSI results in improvement on the maximum impacted day of
0.69 dv compared to 1.35 dv for wet FGD and 1.29 dv for SDA. At Caney
Creek, the baseline had 27 days over 0.5 dv and 6 days over 1.0 dv, but
with DSI these number of days were reduced to 8 and 1, respectively,
and further reduced with wet FGD to zero days over 0.5 dv and zero days
over 1.0 dv. At Wichita Mountains, the baseline had 6 days over 0.5 dv
and 2 days over 1.0 dv, but with DSI these number of days were reduced
to 2 and zero, respectively, and further reduced with wet FGD to zero
days over 0.5 dv and zero days over 1.0 dv.
    We conclude that both SDA and wet FGD are cost-effective at $4,370/
ton and $4,497/ton (respectively) and remain within a range that we
have previously found to be acceptable. Wet FGD is less cost-effective
than SDA and as discussed in the preceding paragraphs, it would have
only a slight additional visibility benefit over SDA. As discussed
earlier, in weighing the factors between SDA and wet FGD, we determined
the additional visibility benefits did not outweigh the additional
cost, water requirements, and wastewater treatment requirements
associated with wet FGD. DSI at 50 percent control is more cost-
effective but results in much less visibility benefit. We consider the
significant visibility benefits that will result from the installation
of SDA at Welsh Unit 1 to justify the cost, and therefore, we propose
that SO2 BART for Welsh Unit 1 should be based on the
installation of SDA at an emission limit of 0.06 lb/MMBtu based on a 30
BOD.
    We recognize that at $4,370/ton, the cost of SDA for Welsh Unit 1
is in the upper range of cost-effectiveness of controls found to be
acceptable in other BART actions nationwide. Nevertheless, we consider
it to be cost-effective and provides for significant visibility
benefit. Since BART is defined as an emission limitation,\341\ sources
have the flexibility to decide what controls to install and implement
so long as they comply with the BART emission limitations and
associated requirements that are promulgated. As discussed in Section
VIII.A, based on available DSI cost information, some EGUs with an
installed baghouse may be able to achieve 90+ percent SO2
control efficiency using DSI with sodium-based sorbents. Therefore,
Welsh Unit 1 could potentially comply with our proposed SO2
emission limit of 0.06 lb/MMBtu with DSI operated at a high
SO2 control level, but this would need to be confirmed with
site-specific performance testing. If the unit is capable of meeting
this SO2 emission limit with DSI, this control technology is
likely to be even more cost-effective than SDA.
---------------------------------------------------------------------------

    \341\ See 40 CFR part 51, Appendix Y--Guidelines For BART
Determinations Under the Regional Haze Rule, section IV.A.
---------------------------------------------------------------------------

    As discussed in Sections VII.B.2.a and VII.B.3.a, we also invite
comments on the range and maximum control efficiency that can be
achieved with DSI at Welsh Unit 1 and estimates of the range of
associated costs. We are especially interested in any site-specific DSI
testing for Welsh Unit 1 to determine the range and maximum control
efficiency that can be achieved with DSI at this unit. Any data to
support the control efficiency range, maximum control efficiency, and
cost of DSI for the unit should be submitted along with those comments.
We will further consider site-specific information provided to us
during the public comment period in making our final decision on
SO2 BART and potentially re-evaluate DSI for this particular
unit.
4. W. A. Parish Units WAP4, WAP5 & WAP6
    W. A. Parish Unit WAP4 is the only gas-fired unit we determined to
be subject to BART. Gas-fired EGUs have inherently low SO2
emissions and there are no known SO2 controls that can be
evaluated. While we must assign SO2 BART determinations to
the gas-fired unit, there are no practical add-on controls to consider
for setting a more stringent BART emission limit. As explained earlier
in Section VII.B.1.c, the BART Guidelines state that if the most
stringent controls are made federally enforceable for BART, then the
otherwise required analyses leading up to the BART determination can be
skipped. As there are no appropriate add-on controls and the status quo
reflects the most stringent control level, we are proposing that
SO2 BART for W. A. Parish Unit WAP4 is to limit fuel to
pipeline natural gas, as defined at 40 CFR 72.2.\342\
---------------------------------------------------------------------------

    \342\ As provided for in 40 CFR 72.2, pipeline natural gas
contains 0.5 grains or less of total sulfur per 100 standard cubic
feet. This is equivalent to an SO2 emission rate of
0.0006 lb/MMBtu.
---------------------------------------------------------------------------

    In evaluating W. A. Parish Units WAP5 and WAP6, we conclude that
the installation of wet FGD or SDA will result in significant
visibility benefits. We summarize some of these visibility benefits in
Table 23.

                                      Table 23--CAMx Predicted Visibility Benefit of Wet FGD (SDA) at W. A. Parish
--------------------------------------------------------------------------------------------------------------------------------------------------------
                      W. A. Parish                                           Baseline                                       Controlled
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Visibility
                                                          Impact (dv) on    Avg impact                      improvement   Avg visibility     Impacted
                      Class I area                          the maximum    (dv) for the   Number of days    (dv) on the     improvement   number of days
                                                            impact day      top 10 days   >=0.5/>=1.0 dv  maximum impact   (dv) for the   >=0.5/>=1.0 dv
                                                                                day *       top 10 days *
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                    W. A. Parish WAP5
--------------------------------------------------------------------------------------------------------------------------------------------------------
Wichita Mountains.......................................            2.01            0.83            12/1     1.86 (1.80)     0.77 (0.75)             0/0
Caney Creek.............................................            1.57            1.09            36/6     1.38 (1.36)     0.97 (0.94)             0/0
Breton..................................................            1.08            0.52             4/1     0.94 (0.92)     0.47 (0.45)             0/0
Cumulative (all Class I areas)..........................            8.82            5.18           86/10            7.93            4.71             0/0
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                    W. A. Parish WAP6
--------------------------------------------------------------------------------------------------------------------------------------------------------
Wichita Mountains.......................................            2.24            0.93            15/1     2.07 (2.01)     0.86 (0.84)             0/0
Caney Creek.............................................            1.75            1.22            47/9     1.52 (1.50)     1.08 (1.05)             0/0
Breton..................................................            1.21            0.58             4/2     1.05 (1.02)     0.52 (0.50)             0/0
Cumulative (all Class I areas)..........................            9.86            5.80          119/15            8.81            5.27             0/0
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                               W. A. Parish WAP5 and WAP6
--------------------------------------------------------------------------------------------------------------------------------------------------------
Wichita Mountains.......................................            3.97            1.71           35/12            3.61            1.56             0/0
Caney Creek.............................................            3.13            2.22           86/38            2.59            1.91             1/0
Breton..................................................            2.21            1.08            12/4            1.89            0.96             0/0
Cumulative (all Class I areas)..........................           17.96           10.72          269/91           15.66            9.56             1/0
--------------------------------------------------------------------------------------------------------------------------------------------------------
* Secondary values in parentheses indicate estimated visibility benefits for SDA

    Table 23 displays the visibility benefits predicted by CAMx
modeling with wet FGD control levels applied to Units WAP5 and WAP6. We
also present the estimated benefits of SDA (shown in parentheses) for
each unit individually. Since SDA is slightly less effective at
reducing SO2 emissions than wet FGD, the comparative results
between SDA and wet FGD are consistent with the difference in control
efficacy, with a maximum difference between wet FGD and SDA on the
maximum impacted day of 0.06 dv at Wichita Mountains for each unit
(0.02-0.03 dv for Caney Creek and Breton) and an average top 10 days
difference of 0.03 dv at Caney Creek (0.02 dv at Wichita Mountains and
Breton) for each unit, with SDA always showing marginally less
improvement from the baseline. These values indicate that SDA per unit
results in approximately 2-4 percent less benefit than wet FGD on a per
unit basis.
    CAMx modeling results indicate that wet FGD installed on each of
Units WAP5 and WAP6 will eliminate all days impacted by each unit over
0.5 dv at all Class I areas, and our estimates for SDA control also
show no days over 0.5 dv at any Class I areas. When considering the
combined impacts from all three units taken together with wet FGD on
WAP5 and WAP6, the CAMx results predict one day to exceed the 0.5 dv
threshold (at Caney Creek).\343\ We would expect similar results in
looking at SDA for Units WAP5 and WAP6 as the visibility differences
for SDA and wet FGD are small. Overall, there is a cumulative reduction
from 269 days over 0.5 dv in the baseline to a total of just one day
over the threshold with wet FGD across all impacted Class I areas.
---------------------------------------------------------------------------

    \343\ W. A. Parish Unit WAP4 is a gas-fired unit for which we
are locking in the requirement to burn pipeline quality natural gas.
---------------------------------------------------------------------------

    Installation of wet FGD on both units results in 3.61 dv
improvement (91 percent reduction of 3.97 dv baseline) on the maximum
impact day at Wichita Mountains and a 1.56 dv improvement (91 percent
reduction of 1.71 dv baseline) on the top 10 average days at Wichita
Mountains. Installation of wet FGD on both units results in 2.59 dv
improvement (83 percent reduction of 3.13 dv baseline) on the maximum
impact day at Caney Creek and a 1.91 dv improvement (86 percent
reduction of 2.22 dv baseline) on the top 10 average days at Caney
Creek. SDA visibility benefits on a unit basis result in 95 percent or
more of the visibility benefit of wet FGD on a unit basis. At the most
impacted Class I areas, either wet FGD or SDA on each unit will each
result in visibility improvements of more than 1.8 dv per unit at
Wichita Mountains, and the top 10 days average visibility improvement
for the individual units are more than 0.9 dv at Caney Creek for each
unit with wet FGD or SDA. Across all impacted Class I areas, the top 10
days average improvement from all three units combined is predicted to
be approximately 9.5 dv, or approximately 89 percent reduction in
visibility impairment due to wet FGD controls or SDA. As provided in
Section VII.B.4, DSI operated at 50 percent control (``low control
scenario'') results in 43 percent visibility improvement for the
overall three units, whereas wet FGD visibility benefits result in 87
percent improvement at the most impacted Class I areas for the three
units and the cumulative 15 Class I areas included in the modeling.
    We conclude that both SDA and wet FGD are cost-effective at $3,044/
ton and $3,074/ton (respectively) for Unit WAP5 and $2,651/ton and
$2,717/ton (respectively) for Unit WAP6 and remain well within a range
that we have previously found to be acceptable. While DSI at 50 percent
control is more cost-effective at $2,262/ton for Unit WAP5 and $2,244/
ton for Unit WAP6, it results in less visibility benefit. The
incremental cost-effectiveness of SDA (going from DSI at 50 percent
control efficiency to SDA) is $4,006/ton for Unit WAP5 and $3,155/ton
for Unit WAP6, which we consider to be reasonable. Thus, we conclude
that the resulting visibility benefit offered by scrubbers outweighs
the possible advantage DSI at 50 percent control may hold in cost-
effectiveness.

    Wet FGD is slightly less cost-effective than SDA and we estimate
based on scaling of our CAMx modeling results that it would have only a
slight additional visibility benefit over SDA. As discussed earlier, in
weighing the factors between SDA and wet FGD, we determined the
additional visibility benefits did not outweigh the additional cost,
water requirements and wastewater treatment requirements associated
with wet FGD. We consider the cost of SDA at the two W. A. Parish units
to be justified by the significant visibility benefits that will
result. We therefore propose that SO2 BART for W. A. Parish
Units WAP5 and WAP6 should be based on the installation of SDA at an
emission limit of 0.06 lb/MMBtu based on a 30 BOD.

B. SO2 BART for Coal-Fired Units With Existing Scrubbers

1. Martin Lake Units 1, 2, and 3
    The BART Guidelines state that underperforming scrubber systems
should be evaluated for upgrades.\344\ Other than upgrading the
existing scrubbers, all of which are wet FGDs, there are no competing
control technologies that could be considered for these units at Martin
Lake. These units were modeled with both CALPUFF and CAMx. We summarize
some of these visibility benefits from upgrading Martin Lake's existing
scrubbers in Tables 24 and 25.
---------------------------------------------------------------------------

    \344\ 70 FR 39171 (July 6, 2005).

                                     Table 24--CALPUFF-Predicted Scrubber Upgrade Visibility Benefits at Martin Lake
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                        2016-18 Baseline impacts                          Scrubber upgrades
                                                             -------------------------------------------------------------------------------------------
                                                                                Cumulative   Visibility benefit at class I    Cumulative
                                                                                2016-2018 #      area (dv) from baseline      2016-2018 #
                        Class I area                                                             of days   ---------------------------------   of days
                                                               2016 dv    2017 dv    2018 dv       with                                          with
                                                                                impacts                                       impacts
                                                                                >=0.5 dv/    2016 dv    2017 dv    2018 dv    >=0.5 dv/
                                                                                >=1.0 dv                                      >=1.0 dv
--------------------------------------------------------------------------------------------------------------------------------------------------------
Caney Creek.................................................       3.28       3.60       3.35      338/215       2.12       2.36       2.16       133/44
Upper Buffalo...............................................       2.12       2.54       2.27      212/115       1.58       1.90       1.72         33/8
Wichita Mountains...........................................       1.45       1.07       1.15        79/36       1.21       0.89       0.91          5/2
Cumulative..................................................       6.84       7.21       6.78      629/366       4.90       5.15       4.79       171/54
--------------------------------------------------------------------------------------------------------------------------------------------------------

    In evaluating Martin Lake, there are three Class I areas (Caney
Creek, Upper Buffalo, and Wichita Mountains) within the typical 450 km
range that CALPUFF has been used for assessing visibility impacts. The
modeled scrubber upgrades result in large visibility improvements of
over 2.2 dv at Caney Creek and 1.7 dv at Upper Buffalo. Visibility
benefits at Wichita Mountains also exceed 1.0 dv. CALPUFF results for
Martin Lake indicate that upgrading the scrubbers resulted in a
reduction of visibility impacts by 65 percent from the baseline at the
highest impacted Class I area (Caney Creek), and an average reduction
of visibility impacts at the three Class I areas of 71 percent. Using
the three years (2016-2018) of CALPUFF modeling results, we assessed
the annual average number of days, averaged across the three years,
when the facility impacts were greater than 0.5 dv at each Class I
area; we also looked at the cumulative number of days summed across the
three years at all the Class I areas (three in this case). The
reduction in the number of days (annual average) was calculated using
the cumulative value of the number of days (three-year total) over the
0.5 dv threshold across the three Class I areas for the baseline
scenario minus the cumulative number of days (three-year total) over
the threshold for the control scenario. For the three Class I areas,
2016-2018 CALPUFF modeling results indicate that upgraded scrubbers on
the three units will eliminate 152 days annually (3-year average), or
458 days cumulatively across the 3 years, when the facility has impacts
greater than 0.5 dv in the baseline. The same analysis for the 1.0 dv
threshold, as reported in Table 24, has 104 days (312 days total)
reduced on annual average. CALPUFF modeling results indicate large
improvements at the individual Class I areas and the cumulative
improvement of almost 5 dv; these scrubber upgrades markedly improve
the overall cumulative predicted visibility by approximately 71 percent
from the baseline.
    Table 25 includes each affected Martin Lake unit and the combined
facility along with the resulting CAMx-modeled visibility benefits from
upgrading Martin Lake's existing scrubbers.

                                    Table 25--CAMx Predicted Visibility Benefit of Scrubber Upgrades for Martin Lake
--------------------------------------------------------------------------------------------------------------------------------------------------------
                       Martin Lake                                           Baseline                                       Controlled
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Visibility
                                                          Impact (dv) on    Avg impact                      improvement   Avg visibility     Impacted
                      Class I area                          the maximum    (dv) for the   Number of days    (dv) on the     improvement   number of days
                                                            impact day      top 10 days   >=0.5/>=1.0 dv      maximum      (dv) for the   >=0.5/>=1.0 dv
                                                                                impact day      top 10 days
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                   Martin Lake Unit 1
--------------------------------------------------------------------------------------------------------------------------------------------------------
Caney Creek.............................................            2.60            1.98           74/22            2.00            1.56             2/0
Wichita Mountains.......................................            2.08            1.01            17/3            1.76            0.85             0/0
Upper Buffalo...........................................            1.93            1.39            48/8            1.66            1.18             0/0
Cumulative (all Class I areas)..........................           12.39            7.90          197/38           10.36            6.64             2/0
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                   Martin Lake Unit 2
--------------------------------------------------------------------------------------------------------------------------------------------------------
Caney Creek.............................................            2.54            1.94           72/22            1.94            1.52             2/0

Wichita Mountains.......................................            2.03            0.99            17/3            1.71            0.82             0/0
Upper Buffalo...........................................            1.89            1.36            44/8            1.62            1.14             0/0
Cumulative (all Class I areas)..........................           12.09            7.71          188/38           10.06            6.44             2/0
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                   Martin Lake Unit 3
--------------------------------------------------------------------------------------------------------------------------------------------------------
Caney Creek.............................................            2.81            2.14           85/24            2.23            1.73             2/0
Wichita Mountains.......................................            2.24            1.09            18/3            1.93            0.93             0/0
Upper Buffalo...........................................            2.09            1.51           51/12            1.84            1.30             0/0
Cumulative (all Class I areas)..........................           13.44            8.59          223/48           11.45            7.34             2/0
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                              Martin Lake Units 1, 2, and 3
--------------------------------------------------------------------------------------------------------------------------------------------------------
Caney Creek.............................................            6.69            5.27         150/101            5.00            4.07            32/7
Wichita Mountains.......................................            5.49            2.83           51/27            4.57            2.35             3/0
Upper Buffalo...........................................            5.16            3.83          111/70            4.39            3.21             7/0
Cumulative (all Class I areas)..........................           33.79           22.16         521/301           27.91           18.44            47/7
--------------------------------------------------------------------------------------------------------------------------------------------------------

    Table 25 shows that the Martin Lake units individually cause or
contribute to visibility impairment at Wichita Mountains, Caney Creek,
and Upper Buffalo on a large number of days. CAMx predicts baseline
impacts for these combined three units to be more than the 0.5 dv
visibility threshold 150 days of the year at Caney Creek, 111 days of
the year at Upper Buffalo, 51 days of the year at Wichita Mountains,
and in total for 209 days per year for the other 12 Class I areas
modeled. The average visibility impact across the top 10 days for the
combined units is more than 5.2 dv at Caney Creek and more than 3.8 dv
at Upper Buffalo. CAMx modeling results indicate that upgrades to
Martin Lake's wet FGD scrubbers to 95 percent control efficiency
installed on each of the units will eliminate all but two days impacted
by each individual unit over 0.5 dv at all Class I areas. When
considering the combined impacts from all three units, the modeling
results show an overall (across all impacted Class I areas) reduction
from 521 days over 0.5 dv in the baseline to a total of 47 days over
the threshold after the scrubber upgrades are installed, for an overall
reduction of more than 90 percent in the number of days over the
threshold. With the modeled scrubber upgrades, the number of days
impacted over 1.0 dv are reduced from 101 days to 7 days at Caney
Creek. Days over the 1.0 dv threshold at all other Class I areas are
eliminated, decreasing from 200 in the baseline to zero with the
scrubber upgrades. At the most impacted Class I Areas, the scrubber
upgrades on each unit will each result in visibility improvements of
approximately 2.0 dv on the most impacted days at Caney Creek, and the
top 10 days average visibility improvement for the individual units is
more than 1.5 dv at Caney Creek. Across all 15 Class I areas, the top
10 days average impact from all three units combined dropped from
baseline of 22.2 dv to 3.7 dv after control upgrades, for an overall
cumulative improvement of approximately 83 percent reduction due to
improved scrubber efficiency. Similarly, across all 15 Class I areas,
the maximum daily impact from scrubber upgrades results in a visibility
improvement of 27.91 dv compared to the 33.79 dv baseline total, which
is a reduction of 83 percent.
    As we state elsewhere in this proposal, we estimate scrubber
upgrades at the Martin Lake units to be very cost-effective and less
than $1,200/ton. We conclude that these scrubber upgrades are very
cost-effective and result in very significant visibility benefits,
significantly reducing the impacts from these units and reducing the
number of days that Class I areas are impacted over 1.0 dv and 0.5 dv.
We propose SO2 BART for each Martin Lake unit should be to
upgrade the wet FGD scrubbers to a control efficiency of 95 percent,
with an emission limit of 0.08 lb/MMBtu on a 30 BOD basis. This cost
analysis, the reasons set forth in previous sections regarding the
overall SO2 emissions impact of these units, and the modeled
benefits, support this proposed BART determination.
2. Fayette Units 1 and 2
    Fayette Units 1 and 2 are currently equipped with high performing
wet FGDs. Both units have demonstrated the ability to maintain a
SO2 30 Boiler Operating Day (BOD) average below 0.04 lb/
MMBtu for years at a time.\345\ As discussed in Section VII.B.2.a,
retrofit wet FGDs should be evaluated at 98 percent control or no less
than 0.04 lb/MMBtu. Table 26 shows the visibility impacts for the
baseline emissions, the current permitted emission limit (which is
greater than the baseline emission rate), and an emission limit of 0.04
lb/MMBtu (which is representative of controlled emissions with wet
FGD).
---------------------------------------------------------------------------

    \345\ See our 2023 BART FIP TSD for additional information and
graphs of this data.

          Table 26--CAMx-Predicted Visibility Impacts of Baseline, Permit Limits, and Wet FGD Limit of 0.04 lb/MMBtu for Fayette Units 1 and 2
--------------------------------------------------------------------------------------------------------------------------------------------------------
                  Fayette Units 1 and 2                        2016 Baseline impacts      Permitted limit (0.2 lb/MMBtu)      Wet FGD (0.04 lb/MMBtu)
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Number of days
                                                             Impact at    Number of days     Impact at    >0.5 dv/number     Impact at    Number of days
                      Class I area                         Class I area   >=0.5 dv/>=1.0   Class I area    of days >1.0    Class I area   >=0.5 dv/>=1.0
                                                               (dv)             dv             (dv)             dv             (dv)             dv
--------------------------------------------------------------------------------------------------------------------------------------------------------
Caney Creek.............................................            0.52             1/0            1.04            11/1            0.52             1/0
Wichita Mountains.......................................            0.34             0/0            1.02             3/1            0.31             0/0
Upper Buffalo...........................................            0.33             0/0            0.73             5/0            0.34             0/0
Cumulative (all 15 Class I areas).......................            2.24             1/0            5.31            21/2            2.12             1/0
--------------------------------------------------------------------------------------------------------------------------------------------------------

    Fayette modeling shows increased visibility impacts when modeling
the existing permit limit (Title V permit level of 0.2 lb/MMBtu to meet
NSPS UUUUU). At this higher permitted rate, the Fayette source would
have visibility impacts greater than 1 dv at Caney Creek and Wichita
Mountains. However, Fayette routinely emits at rates less than this
permit limit. We also modeled wet FGD at 0.04 lb/MMBtu, which these
units already consistently meet on a 30-day BOD basis. The results are
very similar to baseline modeling results reflecting the maximum 24-hr
emissions from 2016-2020, but did result in a slight overall benefit
from baseline conditions. Therefore, we propose that additional
scrubber upgrades for Fayette are not necessary and that Fayette Units
1 and 2 maintain a 30 BOD rolling average SO2 emission rate
of 0.04 lb/MMBtu. We believe that based on their demonstrated ability
to maintain an emission rate below this value on a 30 BOD basis, these
units can consistently achieve this emission level.

C. PM BART

    As discussed in Section VI.B, we propose to disapprove the portion
of the Texas Regional Haze SIP that sought to address the BART
requirement for EGUs for PM. We present our analysis of the BART
factors and the potential costs and visibility benefits of PM controls
in Section VII.B.5. All the coal-fired units are either currently
fitted with a baghouse, an ESP and a polishing baghouse, or an ESP. As
part of our BART determination, we propose to conclude that the cost of
retrofitting the subject units (Harrington Unit 061B, Martin Lake
Units, and Fayette Units) with a baghouse would be extremely high
compared to the visibility benefit for any of the units currently
fitted with an ESP. The BART Guidelines state it is permissible to rely
on MACT standards for purposes of BART unless there are new
technologies subsequent to the MACT standards which would lead to cost-
effective increases in the level of control. Because the costs of
installing a baghouse would be extremely high, we propose that PM BART
for the coal-fired units is an emission limit of 0.030 lb/MMBtu along
with work practice standards. This limit is consistent with the MATS
Rule, which establishes an emission standard of 0.030 lb/MMBtu
filterable PM (as a surrogate for toxic non-mercury metals) as
representing MACT for coal-fired EGUs.
    For the gas-fired BART unit, W. A. Parish Unit WAP4, there are no
appropriate add-on controls and the status quo reflects the most
stringent controls. We are proposing to make the requirement to burn
pipeline natural gas federally enforceable. We are proposing that PM
BART for W. A. Parish Unit WAP4 is to limit fuel to pipeline natural
gas, as defined at 40 CFR 72.2.

IX. Proposed Action

A. Regional Haze

    We are proposing to withdraw the Texas SO2 Trading
Program set forth in 40 CFR part 97 Subpart FFFFF, which constitutes
the FIP provisions the EPA previously promulgated to address
SO2 BART obligations for EGUs in Texas. In its place, we are
proposing to promulgate a FIP as described in this notice and
summarized in this section to address the SO2 BART
requirements for those BART-eligible sources participating in the Texas
SO2 Trading Program. Additionally, as described in Section
VI, we are proposing that our prior approval of the portion of the
Texas Regional Haze SIP related to PM BART for EGUs was in error and
are correcting that through disapproving that portion of the SIP and
promulgating source specific BART requirements to address the
deficiency. Our proposed FIP includes SO2 and PM BART
emission limits for 12 EGUs located at 6 different facilities.
1. SO2 BART
    We propose that SO2 BART for the subject-to-BART units
is the following SO2 emission limits to be met on a 30 BOD
period:

               Table 27--Proposed SO2 BART Emission Limits
------------------------------------------------------------------------
                                                           Proposed SO2
                          Unit                            emission limit
                                                            (lb/MMBtu)
------------------------------------------------------------------------
                    Scrubber Upgrades
Martin Lake Unit 1......................................            0.08
Martin Lake Unit 2......................................            0.08
Martin Lake Unit 3......................................            0.08
                 Emission Limit as BART
Fayette Unit 1..........................................            0.04
Fayette Unit 2..........................................            0.04
W A. Parish Unit WAP4 *.................................  ..............
                   Scrubber Retrofits
Harrington 061B.........................................            0.06
Harrington 062B.........................................            0.06
Coleto Creek Unit 1.....................................            0.06
W. A. Parish WAP5.......................................            0.06
W. A. Parish WAP6.......................................            0.06
Welsh Unit 1............................................            0.06
                           DSI
Harrington 061B.........................................    0.27 (in the
                                                            alternative)
------------------------------------------------------------------------
* For Unit WAP4, BART is to limit fuel use to pipeline natural gas, as
  defined at 40 CFR 72.2. As provided for in 40 CFR 72.2, pipeline
  natural gas contains 0.5 grains or less of total sulfur per 100
  standard cubic feet. This is equivalent to an SO2 emission rate of
  0.0006 lb/MMBtu.

    We propose that the following sources comply with these limits
within five years of the effective date of our final rule: Coleto Creek
Unit 1; Harrington Units 061B (for a limit consistent with scrubber
retrofit) and 062B; W. A. Parish Units WAP5 and WAP6; and Welsh Unit 1.
This is the maximum amount of time allowed under the Regional Haze Rule
for BART compliance. We based our cost analysis on the installation of
wet FGD and SDA scrubbers for these units, and in past actions we have
typically required that scrubber retrofits under BART be operational
within five years.\346\
---------------------------------------------------------------------------

    \346\ See 76 FR 81729, 81758 (December 28, 2011) and 81 FR
66332, 66416 (September 27, 2016), where we promulgated regional
haze FIPs for Oklahoma and Arkansas, respectively. These FIPs
required BART SO2 emission limits on coal-fired EGUs
based on new scrubber retrofits with a compliance date of no later
than five years from the effective date of the final rule.

---------------------------------------------------------------------------

    We are proposing an alternative BART limit based on DSI at 50
percent for Harrington Unit 061B with a proposed compliance date within
two years of the effective date of our final rule. We believe that two
years is appropriate as the installation of DSI systems is less complex
and time consuming than the construction of a scrubber. We also propose
to require a DSI performance evaluation, as more fully described in
Section IX.A.3, within one year of the effective date of our final
rule. In Section VIII.A.2 we also provide an option for Harrington to
agree as part of this FIP to convert to natural gas by no later than
January 1, 2025.
    For Martin Lake Units 1, 2, and 3, we propose that compliance with
these limits be within three years of the effective date of our final
rule. We believe that three years is appropriate for these units, as we
based our cost analysis on upgrading the existing wet FGD scrubbers of
these units, which we believe to be less complex and time consuming
than the construction of a new scrubber.
    For Fayette Units 1 and 2, we propose that compliance with these
limits be within one year. We believe that one year is appropriate for
these units because the Fayette units have already demonstrated their
ability to meet these emission limits.
2. Potential Process for Alternative Scrubber Upgrade Emission Limits
    In our 2023 BART FIP TSD, we discuss how we calculated the
SO2 removal efficiency of the units we analyzed for scrubber
upgrades. Since we do not have CEMS data for the inlet of the scrubbers
(we only have CEMS data for the outlet of the scrubbers) and we do not
have recent site-specific testing from the facility to more accurately
determine the current control efficiency of the scrubbers, we estimated
the current removal efficiency of each scrubber using formulas. These
formulas utilize the reported sulfur content and tonnages of the fuels
burned at each unit to calculate the theoretical uncontrolled
SO2 emissions. The calculated theoretical uncontrolled
SO2 emissions and CEMS data for the scrubber outlet
SO2 emissions are then used to calculate scrubber
efficiency. Given a lack of updated source-specific information
resulting in an estimated control efficiency based on available fuel
usage and SO2 emissions data, we cannot assure accuracy in
our quantification of scrubber efficiency. However, despite the
potential for inaccurate information regarding scrubber efficiency,
based on the results of our scrubber upgrade cost analysis, we do not
believe that any such error in calculating the true tons of
SO2 removed affects our proposed determination that scrubber
upgrades are cost-effective. Even if we were to make reasonable
adjustments in the tons removed to account for any potential error in
our scrubber efficiency calculation, we would still propose to upgrade
these SO2 scrubbers. We believe we have demonstrated that
upgrading an underperforming SO2 scrubber is one of the most
cost-effective pollution control upgrades a coal-fired power plant can
implement to improve the visibility at Class I areas. However, our
proposed FIP does specify an SO2 emission limit that is
based on 95 percent removal. This is below the upper end of what an
upgraded wet SO2 scrubber can achieve, which is 98-99
percent, as we have noted in our 2023 BART FIP TSD. We believe that a
95 percent control assumption provides an adequate margin of error for
the units for which we have proposed scrubber upgrades, such that they
should be able to comfortably attain the emission limits we have
proposed. However, for the owner of any unit that disagrees with us on
this point, we propose the following:

    (1) The affected unit should comment why it believes it cannot
attain the SO2 emission limit we have proposed, based on
a scrubber upgrade that includes the kinds of improvements (e.g.,
elimination of bypass, wet stack conversion, installation of trays
or rings, upgraded spray headers, upgraded ID fans, using all
recycle pumps, etc.) typically included in a scrubber upgrade.
    (2) After considering those comments, and responding to all
relevant comments in a final rulemaking action, should we still
require a scrubber upgrade in our final FIP we will provide the
company the following option in the FIP to seek a revised emission
limit after taking the following steps:
    (a) Install a CEMS at the inlet to the scrubber.
    (b) Pre-approval of a scrubber upgrade plan conducted by a third
party engineering firm that considers the kinds of improvements
(e.g., elimination of bypass, wet stack conversion, installation of
trays or rings, upgraded spray headers, upgraded ID fans, using all
recycle pumps, etc.) typically performed during a scrubber upgrade.
The goal of this plan will be to maximize the unit's overall
SO2 removal efficiency.
    (c) Installation of the scrubber upgrades.
    (d) Pre-approval of a performance testing plan, followed by the
performance testing itself.
    (e) A pre-approved schedule for 2.a through 2.d.
    (f) Should we determine that a revision of the SO2
emission limit is appropriate, we will have to propose a
modification to the BART FIP after it has been promulgated. It
should be noted that any proposal to modify the SO2
emission limit will be based largely on the performance testing and
may result in a proposed increase or decrease of that value.
3. DSI Performance Evaluation for Harrington Unit 061B
    We are proposing that SO2 BART for Harrington Unit 061B
should be based on the installation of SDA at an emission limit of 0.06
lb/MMBtu based on a 30 BOD and in the alternative, we are proposing
that SO2 BART should be based on DSI at 50 percent control
efficiency at an emission limit of 0.27 lb/MMBtu based on a 30 BOD with
the requirement to conduct a DSI performance evaluation and submit to
the EPA no later than one (1) year from the effective date of our final
rule. We believe Unit 061B is likely capable of achieving an
SO2 emission limit of 0.27 lb/MMBtu with DSI, but are not
certain whether the unit could achieve a lower emission limit on a 30
BOD or what the potential impacts to PM emissions could be at higher
injections rates necessary for higher control efficiencies using the
existing ESP. The purpose of the DSI performance evaluation is to
determine the lowest SO2 emission rate Unit 061B would be
able to sustainably achieve on a 30 BOD with DSI as well as the
potential control efficiencies achievable with upgraded particulate
removal and to determine how compliance with such an emission rate
would impact our cost estimates for DSI. Therefore, as part of the
performance evaluation, we are also proposing to require an estimate of
the costs of DSI for each of the three control scenarios specified in
1.a through 1.c.
    Should we require an SO2 emission limit based on DSI for
Harrington Unit 061B in our final FIP, we are proposing the following
requirements for a DSI performance evaluation:
    (1) The performance evaluation must be conducted by a third-party
engineering firm and must determine the potential lowest sustainable
SO2 emission rate on a 30 BOD with DSI for each of the
following control scenarios:
    (a) DSI with the existing ESP for particulate removal;
    (b) DSI with a new ESP installation for particulate removal;
    (c) DSI with a new fabric filter installation for particulate
removal.
    (2) The performance evaluation must include an estimate of the
costs for each of the three control scenarios specified in 1.a through
1.c. The cost estimates must include a detailed breakdown of the
capital costs and annual operation and maintenance costs for each
control scenario as well as an estimate of the annual SO2
emissions reductions under each control scenario. The cost estimates
should adhere to the costing methodologies recommended in the

EPA Air Pollution Control Cost Manual.\347\
---------------------------------------------------------------------------

    \347\ EPA Air Pollution Control Cost Manual, Seventh Edition,
April 2021 available at https://www.epa.gov/economic-and-cost-analysis-air-pollution-regulations/cost-reports-and-guidance-air-pollution#cost%20manual.
---------------------------------------------------------------------------

    (3) The facility must submit a detailed report of the performance
evaluation and all supporting documentation to the EPA no later than
one year from the effective date of our final BART FIP.
    Based on the DSI performance evaluation, we will determine whether
a revision of the SO2 emission limit for Harrington Unit
061B is appropriate. Should we determine that a revision of the
SO2 emission limit is appropriate, we will propose a
modification to the BART FIP after it has been promulgated.
4. PM BART
    We propose that PM BART limits for the coal-fired units, Martin
Lake Units 1, 2, and 3; Coleto Creek Unit 1; W. A. Parish Units WAP5
and WAP6; Welsh Unit 1; Harrington Units 061B and 062B; and Fayette
Units 1 and 2 are 0.030 lb/MMBtu and work practice standards, shown in
Table 28.

    Table 28--PM BART Emissions Standards and Work Practice Standards
------------------------------------------------------------------------
                 Unit type                        PM BART proposal
------------------------------------------------------------------------
Coal-Fired BART Units.....................  0.030 lb/MMBtu filterable PM
                                            Table 3 to Subpart UUUUU
Gas-Fired Only BART Units.................  Pipeline quality natural gas
------------------------------------------------------------------------

    We propose that compliance with these emissions standards and work
practice standards be the effective date of our final rule, as the
affected facilities should already be meeting them.
    We propose that PM BART for W. A. Parish WAP4 is to limit fuel to
pipeline natural gas, as defined at 40 CFR 72.2.

B. CSAPR Better-Than-BART

    We propose that, if this proposal to implement source-specific BART
requirements at certain EGUs in Texas is finalized, the EPA's
analytical basis for our 2017 CSAPR Better-than-BART determination will
be restored,\348\ which concluded that implementation of CSAPR in the
remaining covered States will continue to meet the criteria for a BART
alternative. This will also resolve the claims in the 2017 and 2020
petitions for consideration. We are therefore proposing to deny the
2020 petition for partial reconsideration of our September 2017 Final
Rule affirming 40 CFR 51.308(e)(4) and our subsequent 2020 denial of a
2017 petition for reconsideration of that rule. This proposed
reaffirmation will allow the continued reliance on CSAPR participation
as a BART alternative for BART-eligible EGUs for a given pollutant in
States whose EGUs continue to participate in a CSAPR trading program
for that pollutant.
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    \348\ 82 FR 45481.
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X. Environmental Justice Considerations

    The EPA defines environmental justice (EJ) as ``the fair treatment
and meaningful involvement of all people regardless of race, color,
national origin, or income with respect to the development,
implementation, and enforcement of environmental laws, regulations, and
policies.'' The EPA further defines the term fair treatment to mean
that ``no group of people should bear a disproportionate burden of
environmental harms and risks, including those resulting from the
negative environmental consequences of industrial, governmental, and
commercial operations or programs and policies.'' \349\ Recognizing the
importance of these considerations to local communities, the EPA
conducted an environmental justice screening analysis around the
location of the facilities associated with this action to identify
potential environmental stressors on these communities and the
potential impacts of this action. However, the EPA is providing the
information associated with this analysis for informational purposes
only. The information provided herein is not a basis of the proposed
action.
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    \349\ See https://www.epa.gov/environmentaljustice/learn-about-environmental-justice.
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    The EPA conducted the screening analyses using EJScreen, an EJ
mapping and screening tool that provides the EPA with a nationally
consistent dataset and approach for combining various environmental and
demographic indicators.\350\ The EJScreen tool presents these
indicators at a Census block group (CBG) level or a larger user-
specified ``buffer'' area that covers multiple CBGs.\351\ An individual
CBG is a cluster of contiguous blocks within the same census tract and
generally contains between 600 and 3,000 people. EJScreen is not a tool
for performing in-depth risk analysis, but is instead a screening tool
that provides an initial representation of indicators related to EJ and
is subject to uncertainty in some underlying data (e.g., some
environmental indicators are based on monitoring data which are not
uniformly available; others are based on self-reported data).\352\ For
informational purposes, we have summarized EJScreen data within larger
``buffer'' areas covering multiple block groups and representing the
average resident within the buffer areas surrounding the BART
facilities. EJScreen environmental indicators help screen for locations
where residents may experience a higher overall pollution burden than
would be expected for a block group with the same total population in
the U.S. These indicators of overall pollution burden include estimates
of ambient particulate matter (PM2.5) and ozone
concentration, a score for traffic proximity and volume, percentage of
pre-1960 housing units (lead paint indicator), and scores for proximity
to Superfund sites, risk management plan (RMP) sites, and hazardous
waste facilities.\353\ EJScreen also provides information on
demographic indicators, including percent low-income, communities of
color, linguistic isolation, and less than high school education.
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    \350\ The EJSCREEN tool is available at https://www.epa.gov/ejscreen.
    \351\ See https://www.census.gov/programs-surveys/geography/about/glossary.html.
    \352\ In addition, EJSCREEN relies on the five-year block group
estimates from the U.S. Census American Community Survey. The
advantage of using five-year over single-year estimates is increased
statistical reliability of the data (i.e., lower sampling error),
particularly for small geographic areas and population groups. For
more information, see https://www.census.gov/content/dam/Census/library/publications/2020/acs/acs_general_handbook_2020.pdf.
    \353\ For additional information on environmental indicators and
proximity scores in EJSCREEN, see ``EJSCREEN Environmental Justice
Mapping and Screening Tool: EJSCREEN Technical Documentation,''
Chapter 3 and Appendix C (September 2019) at https://www.epa.gov/sites/default/files/2021-04/documents/ejscreen_technical_document.pdf.
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    The EPA prepared EJScreen reports covering buffer areas of
approximately 6-mile radii around the BART facilities. From those
reports, one BART facility, Harrington Station, showed EJ indices
greater than the 80th national percentiles,\354\ which were for ozone,
lead paint, and RMP facility proximity, none of which are regulated by
this proposed action. No BART facility showed an EJ index greater than
80th national percentile for PM2.5, diesel particulate
matter, air toxics cancer risk, air toxics respiratory hazard index,
traffic proximity, hazardous waste site proximity, underground storage
tanks,

or wastewater discharge. The full, detailed EJScreen reports are
provided in the docket for this rulemaking.
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    \354\ For a place at the 80th percentile nationwide, that means
20% of the U.S. population has a higher value. EPA identified the
80th percentile filter as an initial starting point for interpreting
EJScreen results. The use of an initial filter promotes consistency
for EPA programs and regions when interpreting screening results.
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    This action is proposing to promulgate a FIP to address BART
requirements that are not adequately satisfied by the Texas Regional
Haze SIP. The proposed rule is proposing SO2 and PM BART
limits on EGUs in Texas to fulfill regional haze program requirements
and additionally disapproving portions of the Texas Regional Haze SIP
related to PM BART. Exposure to PM and SO2 is associated
with significant public health effects. Short-term exposures to
SO2 can harm the human respiratory system and make breathing
difficult. People with asthma, particularly children, are sensitive to
these effects of SO2.\355\ Exposure to PM can affect both
the lungs and heart and is associated with: premature death in people
with heart or lung disease, nonfatal heart attacks, irregular
heartbeat, aggravated asthma, decreased lung function, and increased
respiratory symptoms, such as irritation of the airways, coughing or
difficulty breathing. People with heart or lung diseases or conditions,
children, and older adults are the most likely to be affected by PM
exposure.\356\ Therefore, we expect that these requirements for EGUs in
Texas, if finalized, and resulting emissions reductions will contribute
to reduced environmental and health impacts on all populations impacted
by emissions from these sources, including populations experiencing a
higher overall pollution burden, people of color and low-income
populations. There is nothing in the record which indicates that this
proposed action, if finalized, would have disproportionately high or
adverse human health or environmental effects on communities with
environmental justice concerns.
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    \355\ See https://www.epa.gov/so2-pollution/sulfur-dioxide-basics#effects.
    \356\ See https://www.epa.gov/pm-pollution/health-and-environmental-effects-particulate-matter-pm.
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XI. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Overview

    This action is exempt from review by the Office of Management and
Budget (OMB) because the proposed FIP, if finalized, would not
constitute a rule of general applicability, as it proposes source
specific requirements for electric generating units at six different
facilities located in Texas.

B. Paperwork Reduction Act

    This action does not impose any new information collection burden
under the PRA. OMB has previously approved the information collection
activities contained in the existing regulations and has assigned OMB
control number 2060-0667. Because the proposed source specific BART
emission limits apply to only six different facilities, the Paperwork
Reduction Act does not apply. See 5 CFR 1320.3(c).
    Additionally, the proposed withdrawal of the Texas SO2
Trading Program does not impose any new or revised information
collection burden under the provisions of the Paperwork Reduction Act
(PRA), 44 U.S.C. 3501 et seq. OMB has previously approved the
information collection activities for the Texas SO2 Trading
Program as part of the most recent information collection request
renewal for the CSAPR trading programs, which was assigned OMB control
number 2060-0667. The withdrawal of the Texas SO2 Trading
Program does not change any collection requests required as part of the
CSAPR trading programs. Furthermore, the withdrawal of the Texas
SO2 Trading Program will cause no change in information
collection burden related to SO2 requirements because the
sources that are currently participating in the Texas SO2
Trading Program have the same SO2 monitoring and reporting
requirements under the Acid Rain Program. Thus, the withdrawal of the
Texas SO2 Trading Program proposed in this action will not
change any collection burden that these sources are subject to under
either the CSAPR trading programs or the Acid Rain Program.

C. Regulatory Flexibility Act

    I certify that this action will not have a significant impact on a
substantial number of small entities under the RFA. This action will
not impose any requirements on small entities. The proposed FIP action,
if finalized, will apply to EGUs at six facilities, none of which are
small entities as defined by the RFA.

D. Unfunded Mandates Reform Act

    The EPA has determined that Title II of UMRA does not apply to this
proposed rule. In 2 U.S.C. 1502(1) all terms in Title II of UMRA have
the meanings set forth in 2 U.S.C. 658, which further provides that the
terms ``regulation'' and ``rule'' have the meanings set forth in 5
U.S.C. 601(2). Under 5 U.S.C. 601(2), ``the term `rule' does not
include a rule of particular applicability relating to . . .
facilities.'' Because this proposed rule is a rule of particular
applicability relating to specific EGUs located at six named
facilities, the EPA has determined that it is not a ``rule'' for the
purposes of Title II of UMRA.

E. Executive Order 13132: Federalism

    This proposed action does not have federalism implications. It will
not have substantial direct effects on the States, on the relationship
between the national government and the States, or on the distribution
of power and responsibilities among the various levels of government.

F. Executive Order 13175: Consultation and Coordination With Indian
Tribal Governments

    This proposed rule does not have tribal implications, as specified
in Executive Order 13175. It will not have substantial direct effects
on tribal governments. Thus, Executive Order 13175 does not apply to
this rule.

G. Executive Order 13045: Protection of Children From Environmental
Health Risks and Safety Risks

    EPA interprets Executive Order 13045 as applying only to those
regulatory actions that concern environmental health or safety risks
that EPA has reason to believe may disproportionately affect children,
per the definition of ``covered regulatory action'' in section 2-202 of
the Executive Order. Therefore, this action is not subject to Executive
Order 13045 because it does not concern an environmental health risk or
safety risk. Since this action does not concern human health, EPA's
Policy on Children's Health also does not apply.

H. Executive Order 13211: Actions That Significantly Affect Energy
Supply, Distribution, or Use

    This proposed action is not subject to Executive Order 13211 (66 FR
28355 (May 22, 2001)), because it is not a significant regulatory
action under Executive Order 12866.

I. National Technology Transfer and Advancement Act

    Section 12 of the National Technology Transfer and Advancement Act
(NTTAA) of 1995 requires Federal agencies to evaluate existing
technical standards when developing a new regulation. To comply with
NTTAA, the EPA must consider and use ``voluntary consensus standards''
(VCS) if available and applicable when developing programs and policies
unless doing so would be inconsistent with applicable law or otherwise
impractical. The EPA

believes that VCS are inapplicable to this action. This action does not
require the public to perform activities conducive to the use of VCS.

J. Executive Order 12898: Federal Actions To Address Environmental
Justice in Minority Populations and Low-Income Populations

    Executive Order 12898 (59 FR 7629, February 16, 1994) directs
Federal agencies, to the greatest extent practicable and permitted by
law, to make environmental justice part of their mission by identifying
and addressing, as appropriate, disproportionately high and adverse
human health or environmental effects of their programs, policies, and
activities on minority populations (people of color and/or Indigenous