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ExxonMobil DEC	 - 3 2007 

Refining & Supply Company 

700 ExxonMobil Road 

PO Box 1163 

Billings, Montana 59103-1163 

406 657 5380 Telephone 

4066575374 Facsimile 

EJf(onMobil 

Refining & Supply 

November 29,2007 

Callie A. Videtich, Director 

Air and Radiation Program 

USEPA Region VIII 

1595 Wynkoop Street 

Denver, CO 80202-1129 

Re:	 Exxon Mobil Corporation Billings Refinery Response to EPA's "Results of "Subject to 

Visibility Modeling for BART" 

Dear Ms. Videtich: 

This letter serves as Exxon Mobil Corporation Billings Refinery's (ExxonMobil) response to EPA 

Region 8's letter referenced above, received by ExxonMobil on October 29, 2007. The letter 

was addressed to Bruce Brodie, Refinery Manager, Exxon Mobil Refining and Supply Company 

(ExxonMobil) in Billings, MT. The EPA letter addresses dispersion modeling results that were 

performed for the Best Available Retrofit Technology (BART) requirements to meet the Regional 

Haze Rule (RHR) specified in 40 CFR 51.308(e). ExxonMobil received verbal confirmation to 

extend the date of our response to November 29,2007. 

Briefly, EPA's letter suggested that a short-term mass emission limit from the combustion of 

sour water stripper overhead (SWSOH) in the Fluid Catalytic Cracking Unit Carbon Monoxide 

Boiler (FCCUCOB) should be established (through the proposed Federal Implementation Plan 

[see 71 FR 39259, et al.l) in order to create a "synthetic minor" status. Such a designation 

would, according to the letter, avoid BART. You requested whether such an approach was 

acceptable to ExxonMobil. 

ExxonMobil does not agree with this approach for several reasons outlined in Attachment 2 to 

this letter. However, ExxonMobil has determined that EPA's proposed approach is unnecessary 

in the base case. We are submitting new modeling results performed by our modeling 

contractor. The results of additional dispersion modeling based on actual emissions from 2001, 

2002, and 2003, consistent with the BART Guideline, indicates ExxonMobil is not subject to 

BART, having a modeled deciview impact below the 0.5 threshold. Those results, by 

themselves, negate the need for any emission limitation on the FCCUCOB as the model shows 

visibility impact less than the threshold values contained in the BART protocols. Attachment 1 

to this letter explains this direct resolution to the issue you have raised. 
We would be pleased to discuss this matter further with you at your convenience. We believe 

that the attachment is self-explanatory and resolves the concerns raised by your letter. In 

support of ExxonMobil's conclusions, enclosed are three CD-ROMs with copies of the 

CALPUFF input files and CALPOST output files. 

Sincerely, 

Ron 

~/fL/ J. Kuhler 

Environmental and Safety Section Supervisor 

Enclosures (2 Attachments and 3 CDs) 

cc:	 Kevin Golden, EPA Region VIII 

Laurel Dygowski, EPA Region VIII 

CERTIFIED MAIL 
ATTACHMENT 1 

Response to EPA Region 8 Letter
 

Regarding
 

"Subject to" BART and Emission Limits
 

November 29, 2007 

Background 

In general terms, a facility (or a portion of a facility) is BART-eligible if it meets certain 

emissions and construction or re-construction criteria. 1 The facility (or portion of a 

facility) is "subject to"2 a BART analysis if the constructed or reconstructed units "cause 

or contribute,,3 to visibility impairment at any of the 156 "mandatory Class I Federal 

4 Areas.. . 

On September 27, 2006, EPA sent a letter to ExxonMobil seeking to certify the emitting 

units and emissions data that would be used in the modeling. In a letter dated October 

17,2006, we provided a list of emitting units that we believe are "BART-eligible" sources 

in accordance with Section II of the "Guideline for BART Determinations Under the 

Regional Haze Rule" (herein referred to as the "Guideline") found in 40 CFR 51, 

Appendix Y. 

The October 17, 2006 response departed slightly from the Guideline and from the 

Montana "Modeling Protocol For Federal Mandatory Class I Areas"s. Those two 

documents require that the emission rate used in the dispersion modeling be the "24_ 

hour average actual emission rate from the highest emitting day of the meteorological 

period, unless this rate reflects periods of start-up, shutdown, or malfunction."6 

Issue 

Although the Regional Haze Rules (RHRf and Guideline suggest the use of "actual" 

emission rates, ExxonMobil found it inconsistent to use "actuals" data in the face of 

requirements that have since prohibited or will prohibit the emission rates observed 

during the modeling-year periods (years: 2001, 2002 and 2003). ExxonMobil believed 

1 Further description is available in 40 CFR 51, Appendix Y, Section II. 

2 As described in 40 CFR 51, Appendix Y, Section 111.1.
 

3 Ibid.
 

4 Defined in 40 CFR 301.
 

5 http://www.epa.gov/Region8/air/pdf/MontanaBARTProtocoI0906.pdf.
 

640 CFR 51, Appendix Y, Section 111.A.3.
 

7 40 CFR 51.308.
 
that the new potential emission rates that would be imposed by United States, et al. v. 

ExxonMobil Corporation, et al. (Consent Decree) would be a more realistic estimate of 

emissions than the 2001-2003 actual emissions. The result would be a higher quality of 

modeling with more realistic results reflecting the Consent Decree requirements. 

As a result, ExxonMobil requested and EPA agreed to model the lower emission rate 

constrained by the Consent Decree. 8 The modeling results demonstrated ExxonMobil's 

impact on all Class I areas to be less than the 98th percentile threshold value (0.5 

deciviews). Since the values are less than 0.5 deciviews, EPA concludes that 

ExxonMobil must not be "subject to" further BART review. 

It appears from the EPA's letter, however, that EPA has made an assumption that the 

0.5 deciview modeling results were achieved only because a lower emission rate 

constrained by the Consent Decree was used in the model. Expressed another way, 

EPA seems to have assumed that ExxonMobil would be "subject to" BART had the 

higher "actual" emission rates been used in the model (as outlined by the modeling 

protocol EPA used in this exercise9

). As a result of this assumption, EPA then argues 

that it is necessary to establish a "synthetic minor" emission limit to avoid BART. 1O 

We have tested this assumption using refined estimates of past actual emissions with 

new modeling. This new dispersion modeling, discussed below, has demonstrated that 

regardless of the emission rates used in the model, the conclusion remains the same. 

The results indicate modeled values below 0.5 deciviews regardless of Consent Decree 

or "actual" emission rates. 

Resolution 

The dispersion modeling was rerun using the "actual" emission rates as outlined in the 

Montana modeling protocol. 11 If the results of dispersion modeling indicate deciview 

impacts less than 0.5, a facility is not "subject to" BART since it would fail to meet the 

98th percentile threshold requirements found in the modeling protocols. 12 This 

methodology is identical to emission rates and results of all of the BART-eligible 

sources in Montana and Region 8. 

For reference, Table 1 provides a summary of the emission rates at issue. The table 

shows the emission rates that were used in previous (EPA) modeling demonstrations as 

well as the emission rates used for this (ExxonMobil) analysis. 

8 See 3'd paragraph of October 29,2007 letter from EPA to ExxonMobil.
 

9 http://www.epa.gov/Region8/air/pdf/MontanaBARTProtocoI0906.pdf.
 

10 See last paragraph of page 2 of October 29, 2006 letter from EPA to ExxonMobil.
 

11 EPA agreed that it would use the Montana modeling protocol as its methodology for conducting the
 

BART modeling "subject to" analysis. This is further confirmed via the modeling results presented by the
 

University of California Riverside (on behalf of EPA) and the accompanying summary document:
 

~2ttP://pah .cert.ucr.edu/aqm/308/bartlcalpuff/summary/MT_summary_v5_May30_2007.doc.
 

http://pah.cert.ucr.edu/aqm/308/bartIWRAP_RMC_BART_Protocol_Aug15_2006.pdf and 

http://www.epa.gov/Region8/air/pdf/MontanaBARTProtocoI0906.pdf. 
Bison Engineering, Inc., on behalf of ExxonMobil, has revisited the dispersion modeling. 

The model input files used by EPA (via University of California Riverside) were 

retrieved. The emission rates in the original modeling were replaced with the 2001, 

2002,2003 "actual" emission rate13 for the facility (see Table 1 above). The model was 

then re-run using the "actual" emission rates, while holding constant all other input data. 

The results of the modeling are summarized in Table 2. The table demonstrates that 

even with the use of these higher emission rates, the results at the 98th percentile are 

well below the 0.5 threshold value. 

Conclusion 

Given that the model has now been executed using actual emiSSion rates that are 

consistent with all other BART-eligible sources throughout the region, we conclude that 

ExxonMobil is not "subject to" a BART analysis since the modeling results are 

consistent regardless of the emission rate inputs. Also, consistent with other U.S. 

facilities, there is no justification or regulatory basis for a "synthetic minor" limit. 

13 Emission rates are contained in the 02/03/06 letter from ExxonMobil to Montana Department of 

Environmental Quality. 
Table 1: Summary of Emission Rates 

Emission Rates Used Submitted on October 17, 

2006 to EPA1 

Actual Maximum Emissions During 2001, 

2002, & 2003 (on date shown)2 

NOx S02 PM10 8/16/01 2/27/01 10/5/02 

Emission Unit 

tpy 

(Ibs/dav) 

tpy 

(Ibs/day) 

tpy 

(Ibs/day) 

NOx 

Ibs/dav 

S02 

Ibs/day 

PM10 

Ibs/day 

Fluid Cat Cracking CO Boiler 113 

(619) 

46 

(252) 

6 

(32.9) 1,780 10,291 379 

Fluid Coker CO Boiler 184 

(1008) 

14 

(76.7) 

5 

(27.4) 1,030 7 1 

Crude/Vacuum/F-2 236 

(1293) 

31 

(170) 

10 

(54.8) 1,076 11 37 

HF #2/3 - F-3X 25 

(137) 

3 

(16.4) 

1 

(5.5) 72 2 4 

HF #2/3 - F-5 20 

(110) 

3 

(16.4) 

1 

(5.5) 103 4 2 

Hydrogen Plant/F-551 76 

(416) 

12 

(65.8) 

5 

(27.4) 403 5 17 

Hydrocracker/F651 25 

(137) 

3 

(16.4) 

1 

(5.5) 146 2 6 

Powerformer/F-700 251 

(1375) 

10 

(54.8) 

4 

(21.9) 845 3 12 

Flare 23 

(126) 

219 

(1200) 

7 

(38.4) 19 0.1 4 

Tank Heater/F-10 2 

(11.0) 

0 

(0) 

0 

(0) 2 0.01 0.1 

Boiler House/B-8 145 

(795) 

13 

(71.2) 

5 

(27.4) 0 0 25 

Notes: 

1. Emission rates submitted to EPA on October 17, 2006, were provided in tons/year. 

2. Emission rates submitted to MDEQ on February 3, 2006, were provided in Ibs/day. 
Table 2: CALPUFF Modeling Results 

Source =ExxonMobii Billings Refinery 

99th 98th Minimum Max 98th Percentile by 98th 3-yr Class I Area1 Distance Delta-dv Delta-dv Delta-dv vear(dv Average 

(km) 

# Days 

(dv) 

noab 

(dv) (dv) (dv) > 0.5 dv 2001 2002 2003

North Absaroka Wilderness 0.27 0.19 0.22 

yell 

143 0.58 0.32 0.24 2 0.19

Yellowstone NP 0.17 0.14 0.15 

wash 

150 0.43 0.19 0.15 0 0.14

Washakie Wilderness 0.16 

ulbe 

176 0.32 0.22 0.16 0 0.14 0.22 0.12

0.31 

teto 

UL Bend 187 0.10 0.44 0.30 10 0.38 0.23 0.31

0.07 

gamo 

Teton Wilderness 205 0.17 0.11 0.08 0 0.06 0.10 0.05

Gates of the Mountains WA 0.07 0.13 

redr 

277 0.74 0.22 0.14 2 0.11 0.22

0.05 

Notes: 

Red Rock Lakes 282 0.24 0.09 0.05 0 0.03 0.09 0.04

1. Class I areas are those only within 300 km of the Billings Refinery. 
ATTACHMENT 2 

Response to EPA Region 8 Letter
 

Regarding
 

"Subject to" BART and Emission Limits
 

November 29,2007 

Objections to EPA Proposal 

The analysis in Attachment 1 responds to EPA's request and negates a need for a 

"synthetic minor" emission rate. This resolution notwithstanding, ExxonMobil would like 

to take this opportunity to outline a few of its additional objections regarding EPA's 

proposal. If you find it necessary, we would be pleased to discuss these concerns in 

more detail in the future. 

No Regulatory Basis for USynthetic Minor Status" under RHR 

A BART-eligible source would be required to conduct a BART control technology 

analysis only if it is determined to "cause or contribute" to visibility impairment at one of 

the 156 Federal Class I areas. The Guideline provides a methodology regarding how 

that determination is to be made. In summary, the BART-eligible source is subject to 

BART if dispersion modeling demonstrates that "actual" emission rates in the model 

yield an impact greater than 0.5 deciviews. 14 The key point is that the "actual" emission 

rate, not the potential to emit (PTE) rate, is the defining model input value that 

determines "cause or contribute." The "actual" rate, therefore, is not a regulatoryimposed "not-to-exceed" limit. The triggering event is based on historical past actuals. 

ExxonMobil originally proposed the use of a PTE rate for the BART modeling as a 

conservative estimate of emissions. EPA agreed that this approach was a reasonable 

one. This conservative approach, however, does not justify the imposition of a future 

limit on PTE based on a concern that ExxonMobil might increase emissions in the 

future. A source determined not to be subject to BART based on actual emissions 

would not be required to take a limit under the BART applicability requirements. EPA 

cannot create and apply a new requirement without a rulemaking. 

ExxonMobil also has some elemental concerns with EPA's use of this term in the 

context of "subject to" BART analyses. While the term has a clear meaning in the 

permitting program, it has no meaning in the Regional Haze Rule (RHR). The term 

synthetic minor really refers to limiting one's emissions in order to avoid an applicability 

determination. That is not the case here. 

14 It is understood that this is a guideline and that details in Appendix Y provide various options. However, 

EPA has taken a position that the 0.5 deciview value is appropriate in Montana for this and other BARTeligible facilities. 
For BART, there are (in simplistic terms) two tests that would cause a facility to be 

"subject to,,15 a BART review. The first is that the facility must meet an emissions 

threshold, date requirement and source category. [This first test is usually known as a 

test for "BART-eligible."] The second is that the facility (or portion of a facility) must 

"cause or contribute" to a visibility impairment. [This test is usually referred to as a 

"subject to" BART analysis.] For ExxonMobil's case, it is clear that the refinery meets 

the BART-eligible first test. It is only the second "subject to" test that could have been in 

question. 

For synthetic minor purposes, as that term is widely used in the air pollution field, it only 

has meaning for the first test. One could become a "synthetic minor" source within the 

meaning of BART if and only if the facility were to agree to an emission rate less than 

250 tons per year (the BART threshold for "BART-eligible" sources). Since that is clearly 

not the case here, the term has no meaning. 

Start-up, Shutdown and Malfunction Emission Limits 

EPA proposed that an emission limit be established that "would not be subject to 

exceptions during startup, showdown, and malfunction periods.,,16 This is wholly 

inconsistent with the BART guidelines. 

As noted earlier, the issue for ExxonMobil is one of "subject to" BART, rather than 

BART-eligible. The instructions provided in the Guideline for conducting the "subject to" 

analysis states that those emission rates do not include periods of startup, shutdown 

and malfunctions.17 

EPA has, on the other hand, proposed a limit that includes those periods of startup, 

shutdown and malfunction emissions. This is at direct odds with the instructions in the 

Guideline. The EPA letter states that this limit (including startup shutdown and 

malfunctions) is necessary for enforcement purposes for synthetic minor emissions. 

This is not the case. 

The issue here does not relate to limiting emissions for regulatory applicability purposes 

(BART-eligible). The issue is whether the facility, excluding startup, shutdown and 

malfunctions, causes or contributes to visibility impairment. The Guideline clearly 

instructs the states (and EPA) to use this methodology. To require that these emissions 

be included in the "subject to" analysis is inconsistent with the Guideline instructions. 

As a general matter, ExxonMobil objects to EPA's proposal to apply limits during 

startup, shutdown, and malfunction, as set forth in EPA's proposed Federal 

15 The term "subject to" is in reference to the Subject Line in the 10/29/97 letter as well as how that term is
 

discussed, and found in quotes, in 40 CFR 51, Appendix Y, Section III.
 

16 Page 2, last paragraph, EPA 10/29/07 letter to ExxonMobil.
 

17 40 CFR 51, Appendix Y, Section III.A.3.
 
Implementation Plan (FIP) for S02 in the Laurel/Billings area. ExxonMobil hereby 

incorporates its November 2,2006 comments on the proposed FIP by reference. 

CEM and Flow Monitor 

Because the above analysis confirms ExxonMobil is not "subject to" BART, the need for 

a CEMS is no longer applicable. As EPA correctly noted, under the Consent Decree, 

either a CEM or alternative monitoring plan is required for the SWSOH gas consistent 

with the NSPS J requirements.