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STATE OF NEW MEXICO

BEFORE THE ENVIRONMENTAL IMPROVEMENT BOARD

IN THE MATTER OF PROPOSED REVISIONS

TO THE STATE IMPLEMENTATION PLAN

FOR THE SAN JUAN GENERATING STATION

BEST AVAILABLE RETROFIT TECHNOLOGY

DETERMINATION 

No. EIB 13- 02(R)

_____________________________________________________________________________

NMED EXHIBIT 14 – NMED RESPONSES

TO COMMENTS OF THE 

NATIONAL PARK SERVICE

______________________________________________________________________________

This exhibit presents the Department’s responses to comments received from the National Park 

Service (NPS), dated July 24, 2013, on the proposed Nitrogen Oxide (NOx) best available 

retrofit technology (BART) determination for the San Juan Generating Station (SJGS). 

(Document N3615 (2350)) (“NPS comments”).

As a threshold matter, the Department notes that the NPS comments appear to be based on an 

incorrect assumption that the Department is proposing a “better-than-BART” alternative, 

pursuant to 40 C.F.R. § 51.308(e)(2). The comments further assume that “BART” for the 

purpose of a “better-than-BART” determination is as defined by the U.S. EPA in the federal

implementation plan (FIP) promulgated on August 22, 2011. (See, e.g., comments at p. 1, “The 

State Alternative needs to demonstrate greater visibility impairment than the EPA FIP 

proposal.”) This is not the case. 

The Department proposed SIP revision comprises a revised BART determination pursuant to 40 

C.F.R § 51.308(e)(1), based on new information. Specifically, the revised determination 

considers the voluntary shut-down of two of the four units at San Juan, as envisioned in the 

“State Alternative” negotiated between NMED, PNM, and EPA. Therefore, while the 

Department’s BART analysis of the “State Alternative” is informed by information contained in 

the FIP, as well as in NMED’s own prior NOx BART determination, the analysis does not 

purport to rely on the option allowed in § 308(e)(2) of demonstrating “greater reasonable 

progress” than would the application of BART to all BART eligible sources. In other words, the 

State Alternative is not “better than BART,” it is BART.

Response to Comment Regarding “Step 3: Evaluate Control Effectiveness of Remaining 

Control Technologies

 
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The NPS commented that it believes that it is feasible for the SJGS to achieve a NOx emission 

rate of 0.04 lb/MMBtu on an annual basis, citing the fact that three units with retrofit SCRs 

operate at or below 0.04 lb/MMBtu, according to the 2012 CAMD database.

The NMED concurred with EPA’s determination that a NOx emission limit of 0.05 lb/MMbtu 

was an appropriate emission limit for SCR based on source specific factors and commonly 

accepted removal efficiencies of a retrofit SCR installation. In order for PNM to meet an 

enforceable emission limit of 0.05 lb/MMbtu based on a 30-day rolling average, the SCR system 

would need to be designed to achieve greater removal efficiency for most periods of operation. 

This enforceable emission limit also includes consideration of emissions during startup, 

shutdown, malfunction events that typically result in emissions in excess of those generated 

during normal operations. See 76 Fed. Reg. 52,388, 52,405 (“The CEMS data indicate that our

proposed NOX BART limit can be achieved without separately limiting startups, shutdowns, and 

malfunctions.”) Air permitting authorities typically take into consideration these types of 

emissions in addition to normal operating emissions to establish federally enforceable emission 

limits. This margin ensures that the emission limit is achievable and appropriate for all operating 

situations. 

The NPS comments do not specify what enforceable emission limits apply to the three SCRretrofit units that NPS cites as operating at or below 0.04 lbs NOx/MMBtu, or whether such 

limits are greater than the actual emission rates cited. As mentioned above, permitting 

authorities do not establish enforceable emission limits based on actual emission rates, but 

normally include emissions during startup, shutdown, and malfunction as a small safety factor 

into the calculated emission limit. In any case, the mere existence of three units with lower 

actual emission rates, out of hundreds of units employing SCR nationally, does not outweigh the 

site-specific determination by the EPA that 0.05 lbs/MMBtu is an appropriate SCR-based limit 

for the SJGS.

Response to Comment Regarding “Step 4: Perform Impacts Analysis of Remaining 

Control Technologies”

The NPS presented an estimate of SCR costs based on the EPA’s Control Cost Manual and 

partly based on the EPA’s Integrated Planning Model (IPM), concluding that the dollar per ton 

annual costs of SCR are less than half of what PNM estimated.

The cost estimates provided by the NPS were partly based on a methodology that is more 

appropriate for regulatory planning purposes and less effective at estimating costs for a source 

specific SCR installation. This economic model is the Integrated Planning Model (IPM) and is 

used by EPA to analyze projected impacts from proposed environmental regulations. 
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Even if the NMED were to accept the NPS cost estimates for SCR installation, this information 

would not affect the Department’s final determination of BART at the SJGS. The NPS estimates 

indicate the total capital investment for SCR technology would be around $374,152,055 for a 

four unit installation. In contrast the total capital investment estimated for the 2 SNCR/2 Unit 

Shutdown scenario is only $34,556,000, making this proposal the most cost effective proposal, 

especially considering the emissions reduction and visibility improvement that would be 

achieved by this proposed alternative. 

Use of actual maximum 24-hour emission rate

The NPS asserts that PNM should have modeled 24-hour maximum emission rates rather than 

30-day rolling average emission rates, because the latter are higher than the former, and should 

have used actual 24-hour emission rates modified by a percentage reduction to represent control 

technology. 

In fact, PNM did not model the 30 day rolling average, but rather added a 10% safety factor to 

the 30-day rolling emission limit of 0.30 lb/MMBtu, and therefore assumed a NOx emission rate 

of 0.33 lb/MMbtu to establish the baseline visibility impact. PNM used this methodology in the 

original 2007 BART analysis, and all subsequent updated visibility modeling submittals. NMED 

accepted this methodology because it is actually more conservative (ie, a higher emission rate) 

than use of the 24 hour maximum emission rates. Specifically, as shown below, use of the 0.33 

lb/MMBtu rate results in higher pound-per-hour NOx rates than use of 24-hour maximum rates 

for three of the four units. For the fourth unit, Unit 4, use of the 0.33 lb/MMBtu rate results in an 

emission rate that is 29 lb/hr less than the 24 hour maximum rate, an insignificant difference.

Modeled Emission Rates to Establish Baseline Visibility Impacts (lb/hr NOx)

Unit 1 Unit 2 Unit 3 Unit 4

CAMD 1203.8 1105.2 1795.5 1893.1

PNM Baseline 1223.3 1217.0 1900.1 1864.2

The Department notes that the use of the maximum 24-hour actual emission rate inherently overestimates the emission rate for this facility for all other 24-hour periods of the review and does 

not represent a realistic depiction of actual emissions over the review period. This emission rate 

represents the highest emission level and we appropriately note that the actual emissions are 

below this level. 

The Department also notes that in other BART evaluations, EPA has accepted the use of average 

use of engineering estimates of post-control emission rates rather than percentage reductions 

from maximum 24 hour rates (as suggested by the NPS). Specifically, in its evaluation of 

Nevada’s NOx BART determination for the Reid-Gardner Generating Station, EPA explained:

“Regarding the use of control scenario emission rates based upon annual average 

emission factors (in lb/MMBtu) instead of 24-hour average emission factors (lb/MMBtu), 

we disagree with the commenter that these emission rates do not provide acceptable 
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estimates of visibility benefits. The methodology for calculating control scenario model 

emission rates described by the commenter involves applying the estimated control 

efficiencies of particular technology to the baseline (pre-control) model emission rate. 

While this methodology has been used by EPA, it does not preclude the use of other 

methodologies for calculating control scenario emissions. In the case of control 

technology performance, engineering estimates of a particular technology’s post-control 

level of performance will often be expressed in terms of lb/MMBtu, either on a 30-dayor 

annual average basis. To the extent that the engineering estimate represents more 

accurate depiction of future anticipated emissions at a particular facility, it may be 

appropriate to rely on the specified post-control level of performance rather than on a 

control efficiency applied to a pre-control emission rate. In fact, using model emission 

rates based on an annual average, instead of a 24-hour average, results in more stringent 

emission rates. 77 Fed. Reg. 50,936, 50,944 (Aug. 23, 2012)

The Department’s approach is therefore similar to approaches previously approved by EPA.

The Department does not agree with the NPS assertion that the decrease in SO2 emissions is 

only a “paper” reduction in SO2 emissions. The shutdown of Units 2 and 3 results in a 

significant decrease in SO2 emissions from the facility and a real reduction in visibility impacts. 

In addition, a lower federally enforceable emission limit for remaining Units 1 and 4 will reduce 

the variability in emissions from these units. The only legal mechanism to ensure that those 

reductions are realized at these units is by establishing a lower federally enforceable emission 

limit of 0.10 lb/MMBtu. Without this emission limitation, the facility could have operated at the 

higher emission rate. Simply because the data reviewed by NPS indicated the facility’s actual 

SO2 emissions did not approach the upper limit of their SO2 emission limit does not preclude the 

facility from being able to do so.

Response to Comments Regarding “Step 5: Visibility Impacts Analysis of Remaining 

Control Technologies

The NPS commented that the Department’s reference to EPA’s conclusion that ammonia slip 

would not significantly affect visibility was misleading, because EPA never addressed ammonia 

slip from SNCR, which can be much higher than from SCR.

The Department agrees that EPA’s reference to ammonia slip was with respect to SCR, not 

SNCR. Nonetheless, for several reasons the Department believes that ammonia slip from SNCR 

need not be addressed in visibility modeling. There are a couple of reasons that the impact of 

ammonia slip from SNCR is expected to be negligible in visibility modeling. First, the constant

1-ppb ammonia background concentration used in the PNM BART visibility modeling is 

representative of actual ammonia concentrations directly around the San Juan Generating Station 

(SJGS) emission sources, according to the EPA. See 76 Fed. Reg. 52,388, 52,434 (Aug. 22, 

2011). Second, the ammonia removal efficiency from the wet scrubber for SNCR is 90%, the 

same value that was used in EPA Region 9’s BART determination for Navajo Generating 

Station. See Docket Number: EPA-R09-OAR-2013-0009 – Technical Support Document for 

Proposed Rule, at p. 122. EPA was also concerned with the impact of the remaining ammonia

after the 90% control for the Navajo project, and performed modeling analysis to take into 
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account the impact of the net ammonia slip. EPA determined that the net ammonia slip effect 

would be negligible. The PNM modeling used the same ammonia background value and the 

same wet-scrubber removal efficiency as the EPA modeling for the Navajo Generating Station. 

As with that analysis, the projected increase in ammonia concentrations due to ammonia slip is 

expected to be a small fraction of ammonia background levels. Therefore, the impact of the 

ammonia slip from SNCR should be negligible.

Response to Comments on “2013 BART Determination”

The NPS commented that it was not clear on the bases for this statement in the BART 

determination:

“The retirement of Units 2 and 3 will reduce the facility annual NOx emissions by an 

additional 10,550 tons. When added to the controlled emission rate of Units 1 and 4, 

total annual NOx emission will be reduced by 12,989 tons.”

The bases for this statement can be found in Table 10 on p. 18 of Appendix D. Current (i.e., 

“Consent Decree”) NOx emissions are 4119 tons per year (tpy) for Unit 2 and 6431 tpy for Unit 

3. The sum of these is 10,550 tpy, which is therefore the reduction that would be achieved by 

shutting down these units.

The reductions from controlling Units 1 and 4 with SNCR are, respectively, 966 tpy (4,140 tpy 

under consent decree minus 3,174 tpy under SNCR) and 1,472 tpy (6,309 tpy under consent 

decree minus 4,837 tpy under SNCR).

Total annual NOx reductions are therefore 10,550 + 966 + 1,472 = 12,988 tons. 

The NPS pointed to a condition requiring optimization of NOx reduction contained in a prior 

consent decree between the Department, PNM, and the Grand Canyon Trust, and requested that 

the Department describe how that condition has been implemented. See comments at p. 6, 

footnote 8 (setting forth language that is contained in Consent Decree CV 02-552 BB/ACT 

(ACE), Paragraph V.9 (b).

In 2009 and 2010, PNM submitted the required performance reports for each unit, covering the 

first 12 months of operation after installation of NOx controls at that unit. Based on the results 

of the tests and the provision in the referenced paragraph calling for the addition of a 10% 

margin of safety, PNM recommended that the NOx emission limit remain at 0.30 

lb/MMBtu. The Department, however, disapproved PNM’s test reports as containing 

insufficient information to allow the Department to determine whether an emission rate of less 

than 0.30 lb/MMBtu was feasible. This disapproval triggered a dispute resolution procedure 

under the consent decree. In May of 2011, the parties (including Grand Canyon Trust) agreed to 

terminate the dispute resolution process if either the Environmental Improvement Board (EIB) or 

the EPA adopted a NOx limit for SJGS of 0.28 lb/MMBtu or lower, unless PNM challenged such 

limits. In June 2011, the EIB adopted the Department’s proposed NOx BART limit of 0.23 

lbs/MMBtu, based on installation of SNCR, and PNM did not challenge this standard. In 
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accordance with the parties’ agreement, no further efforts were made to determine whether the 

consent-decree technology could achieve a NOx rate lower than 0.30 lbs/MMBtu.