UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
 

REGION 8
 

1595 Wynkoop Street 

DENVER, CO 80202-1129 

Phone 800-227-8917 

http://www.epa.gov/region08 

Ref: 8P-AR 

Bruce Brodie 

Refinery Manager 

ExxonMobil Refining & Supply 

P.O. Box 1163 

Billings, MT 59103-1163 

Re: Results of "Subject to" Visibility Modeling for Best Available Retrofit Technology (BART) 

Dear Mr. Brodie: 

On June 19, 2006, EPA received a letter from the Montana Department of Environmental 

Quality that stated it was withdrawing its efforts to meet the requirements of the Regional Haze 

Rule (RHR) specified at 40 CFR 51.308. On September 27, 2006, EPA Region 8 sent a letter 

notifying you that we would be conducting modeling to determine which sources in Montana are 

"subject to" the Best Available Retrofit Technology (BART) requirements in the RHR as part of 

our effort to address the requirements of that rule. Since it was determined that ExxonMobil 

Refining and Supply Company (Exxon) located in Billings, Montana was BART eligible, we 

informed you that Exxon would be included in the modeling and asked you to certify the data 

that would be used for this modeling. 

In discussions with EPA Region 8, Exxon raised objections to the use of the 2001-2003 

actual emissions data for the subject to BART modeling and requested that EPA use values for 

the BART modeling based on past operational practices and limits in the consent decree. The 

consent decree, United States, et at. v. ExxonMobil Corporation, et al (consent decree, CD), 

applies to certain BART eligible units at Exxon, including the Fluid Catalytic Cracking Unit 

Carbon Monoxide Boiler (FCCUCOB) and the Fluid Coker Carbon Monoxide Boiler (FCCOB). 

In the consent decree, Exxon agreed to comply with specific provisions of New Source 

Performance Standards (NSPS) 40 CFR part 60 subpart J, "Standards of Performance for 

Petroleum Refineries". In response to EPA's September 27, 2006 letter, Exxon calculated what 

it considers to be the potential to emit (PTE) ofSOz from the combustion of Sour Water Stripper 

Overhead (SWSOH) gases in the FCCUCOB based on the limits in the consent decree and past 

operational practices (maximum daily flow rate during the three-year BART baseline period) as 

a substitute for the historical actual emissions. 

EPA believes that the use of Exxon's estimate of its future 24-hour SOz emissions from 

combusting SWSOH gases in the FCCUCOB is a reasonable one, but given the magnitude of the 

facility's impacts and the potential for the source to increase its short term emissions through 

changes at the facility, Region 8 needs to ensure that the estimate ofthe facility's impacts is 
backed up with a short-term emission limit on SOz emissions from the combustion of SWSOH 

gases in the FCCUCOB. So, Region 8 intends to include in the FIP a short-term limit on SOz 

emissions from the combustion of SWSOH gases in tile FCCUCOB consistent with what was 

used in CALPUFF modeling, which is described below. 

EPA Region 8 asked the Western Regional Air Partnership (WRAP) to conduct visibility 

modeling using Exxon's projected PTE for SOz emissions from combustion ofSWSOH gases in 

the FCCUCOB and actual emissions for other units. Based on the results of this modeling, EPA 

Region 8 believes that your facility will not be subject to BART, if other requirements are met. 

The results of the modeling and the additional requirements are described below. 

. States (or EPA) have several options for determining whether a source is subject to 

BART, including the use of dispersion modeling to determine whether an individual source may 

be reasonably anticipated to cause or contribute to any visibility impairment in any qass I area. 

When dispersion modeling is used, part of the process of determining whether a source is subject 

to BART is the selection of a deciview threshold against which to measure the visibility impact 

of a source. For its analysis of Montana sources, EPA Region 8 has established a threshold of . 

0.5 deciviews as the appropriate threshold based on the number and distribution of sources' 

impacting Class I areas in and around Montana. If dispersion modeling predicts that a source 

will have visibility impacts of 0.5 deciviews or more at a Class I area on the 98th percentile of 

values, EPA Region 8 considers that source to be subject to BART. Further information 

regarding this approach to determining whether a source is subject to BART can be found in 

section lILA of EPA's BART Guidelines. 

For Exxon, the modeling results using Exxon's suggested mix of actual emissions and 

projected PTE show a maximum 98th percentile impact within a Class I area of 0.10 deciviews, 

which does not exceed the threshold of 0.5 deciviews. A summary of the BART modeling 

results is enclosed. The enclosure provides a summary ofthe 98th percentile predictions for the 

three years modeled for Exxon at all federal Class I areas in the vicinity of the plant. More 

detailed modeling results, including the electronic CALPUFFICALMET files, are available on 

the WRAP website at http://pah.cert.ucr.edu/agm/308/bart.shtml. 

A source can request a "synthetic minor" emission limit to avoid BART. In order to rely 

on the calculated PTE used in the modeling for SOz emissions from combustion of SWSOH 

gases in the FCCUCOB, we need to create such a synthetic minor limit. To do so, we intend to 

include a 24-hour limit in the FIP for SOz emissions from the combustion of SWSOH gases in 

the FCCUCOB. This limit would be 163.3 lbs SOzlday, which is the value Exxon suggested and 

is similar to the value that WRAP modeled. This limit would not be subject to exceptions during 

startup, shutdown, and malfunction periods. In order for the synthetic minor emissions 

limitations to be enforceable from a practical standpoint, they must be short term and specific so 

as to enable the Agency to determine compliance at any time. The daily limit would be 

consistentwith the 24-hour averaging time used in the CALPUFF modeling to determine that the 

Exxon facility's visibility impact would be less than the BART threshold of 0.5dv. Additionally, 

monitoring to meet a pound per day emission limit is feasible for Exxon given that Exxon must 

comply with NSPS J for the SWSOH gases. As we understand it, there will be either a CEM or 

an alternative monitoring plan for the SWSOH gas H2S concentration, and there is an existing 

flow meter on the SWSOH gas stream. Concentration and flow values could be used to 
w"ith a limit of 163.3 lbs S02/day. Along with the limit, the FIP would also include the relevant 

compliance determining methods and recordkeeping and reporting requirements. to ensure the 

limit would be enforceable. 

Please let us know within 14 days whether this approach is acceptable to you. If this 

approach is not acceptable to you, we anticipate that a BART analysis will be necessary. 

Please note that regardless of outcome on the BART issue, EPA may have to evaluate t~e

need for further controls on Exxon to ensure that Montana meets the requirements for reasonable 

progress. We will be considering this in the near future. 

If you have any questions or would like to discuss this matter further, please call Laurel 

Dygowski at (303) 312-6144 for questions on the RHR requirements and BART guidelines and 

Kevin Golden at (303) 312-6442 for questions on the modeling. 

Sincerely, . ,...... ? 

.. ~L/~W;;iL .V· 

['-'Callie A. Videtich, Director v 

Air and Radiation Program 

(1)Printed on Recycled Paper 
Source = Exxon Billings Refinery: SOx =70 TPY; NOx =998 TPY 

99tn 98tn 98 Minimum Max
 th

Class I Area
 Distance Delta-dv Delta-dv Delta-dv # Days 98th Percentile by Year 3-Year 

(km) (dv) (dv) (dv) > 0.5 dv 2001 I 2002 I 2003 Average 

noab I North Absaroka Wilderness I 143 I 0.30 1 0.12 1 0.09 1 a 1 0.09 I 0.10 I 0.07 1 0.09 

yell Yellowstone NP I 150 I 0.16 I 0.07 I 0.06 I a I 0.07 I 0.05 I 0.05 I 0.06 

wash Washakie Wilderness 176 0.18 0.07 0.05 a 0.06 0.07 0.03 0.05 

ulbe I UL Bend 1 187 I 0.17 I 0.09 1 0.07 1 a 1 0.08 I 0.03 1 0.07 I 0.06 

teto I Teton Wilderness 1 205 I 0.08 I· 0.03 0.02 0'1 0.02 1 0.02 I 0.01 I 0.02 

gamo Gates of the Mountains WA a 0.01 

redr Red Rock Lakes a 0.01
 

scap I Scapegoat \. 348' "r>~o,o4··I·~·0:00 I o:oO~·Sl O---C:I--O;OO» I,Q;OO·"I··O;oO I 0.00
 

anac 1 Anaconda - Pintler I 368·1<0:04 L'~o.oo~lo.oo'-<I<:'o:·'lo:oo'pl:\o:ob:,';li~o~oO I 0.00
 

boma;. IBob Marshall> '1384: ·"~l[·":01b2'·.jo.ob 10.b.O~;:I;1 'O.bo;::\I;\'Q~db'~.d.Qb 10.00
 

thro • 1Theodore RoosevelHJP " 1393;J:;;;o~·bO·.1 '-·0;00' "'0.00' I /';;0 I ..o.bo . n.:6b.!.?'o.:OO I 0.00
 

medi . I' MediGine Lake I 419>0:QOi\!0:60',;:f io.oo'·I,,··61 0.00 >1 0: bO?rio;00 1 0.00 

mimo .' .' I' Mission MountainsWA I 438.' 1·;·fi'(r;b1;·\il(~2.,o:do>Lr·6.oGI.;"~'Q " "0.00;' ',~o"h';,o-'-oO' 1 0.00 

selw· ISelway-BitlerrootWilderness I 451' 1.:.,o'.()1.:·~f7:0;OO'·' .·1 .. cLoo:·1 " a-.. '-.1 ..... 0.00 ," r:;6;·do.;:::I~·:;·:C:}:·Ob:: I :' 0.00 

glaG. I Glacier NP 1464·T:·\;o::oo-~'I~:,>,o::OOJ· 10.00" I a ,I, 0.00 .J!.:O~do.,,:R7q;,OO ", 0.00 -.--,,'..... 

lost Lostwood " 0.00 

saWt Sawtooth Wilderness' "; .0.00; 

'c.amo Cabinet MountainsWA 0.00 

heca 1 HelisCany6nWilde'rness.' . >1 623·J';~o;(j(L!'·TTIo:Ob··1 .6.00:', ·0.'" 1 ~'O.OO li,o~bo::·~;:P/!;o;o61/0.ob

eaca I Eagle Cap Wilderness '1 '674:' .• J::... e.oo':o.OO '1'0.00'··, .a . I .0.00 . ·1;'o~Oo·'J'::d;O() 1',,0:00