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"Subjectto" Visibility Modeling for Best Available Retrofit Technology (BART)

Dear Mr. Brodie:
OnJune 19, 2006, EPA received a letter from the Montana Department ofEnvironmental Quality that stated it was withdrawing its efforts to meet the requirements ofthe 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 "subjectto" the BestAvailable Retrofit Technology (BART) requirements in the RHR as part of
oureffort to address the requirements ofthat 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 ofthe 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, UnitedStates, et at. v. ExxonMobil Corporation, etal (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 CarbonMonoxide Boiler (FCCOB). In the consent decree, Exxon agreed to comply with specific provisions ofNew Source Performance Standards (NSPS) 40 CFRpart 60 subpart J, "Standards ofPerformance for Petroleum Refineries". In response to EPA's September 27, 2006 letter, Exxon calculated what it considers to be the potential to emit (PTE) ofSO

### zfrom the combustion ofSour 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 BARTbaseline period) as a substitute for the historical actual emissions.

EPA believes that the use ofExxon's estimate ofits future 24-hour 

### SOz emissions from combusting SWSOH gases in the FCCUCOB is a reasonable one, but given the magnitude ofthe facility's impacts and the potential for the source to increase its short term emissionsthrough changes at the facility, Region 8 needs to ensure that the estimate ofthe facility's impacts isbacked up with a short-term emission limit on 

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

### SOz emissions from the combustion ofSWSOH 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 ofSWSOHgases in the FCCUCOB and actual emissions for other units. Based on the results ofthis modeling, EPA Region 8 believes that your facility will not be subjectto BART, ifother requirements are met. The results ofthe 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 ofdispersion modeling to determine whether an individual source may be reasonably anticipated to cause orcontribute to any visibility impairment in any qass I area. When dispersion modeling is used, part ofthe process ofdetermining whether a source is subject to BART is the selection ofa deciview threshold against which to measure the visibility impact ofa source. Forits analysis ofMontana sources, EPA Region 8 has established a threshold of

. 0.5 deciviews as the appropriate thresholdbased onthe number and distribution ofsources' impacting Class I areas in and around Montana. Ifdispersion modeling predicts that a source will have visibility impacts of0.5 deciviews or more at a Class I area on the 98th percentile of values, EPA Region 8 considers that source to be subjectto BART. Further information regarding this approach to determining whether a source is subject to BART can be found in section lILA ofEPA's BART Guidelines.

For Exxon, the modeling results using Exxon's suggested mix ofactual emissions and projected PTE show a maximum 98th percentile impact within a Class I area of0.10 deciviews, which does not exceed the threshold of0.5 deciviews. A summary ofthe 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 ofthe plant. More detailed modeling results, including the electronic CALPUFFICALMETfiles, 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 orderto rely
onthe calculated PTE used in the modeling for 

### SOz emissions from combustion ofSWSOH gases in the FCCUCOB, we need to create such a synthetic minor limit. To do so, we intend to include a 24-hourlimit in the FIP for 

### SOz emissions from the combustionofSWSOH gases in the FCCUCOB. This limit would be 163.3 lbs 

### SOzlday, which is the value Exxon suggested and is similarto the value that WRAP modeled. This limit would not be subjectto 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. Thedaily 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 BARTthreshold of0.5dv. 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 tow"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. Ifthis approach is not acceptable to you, we anticipate that a BART analysis will be necessary.
Please note that regardless ofoutcome on the BART issue, EPA may have to evaluate**t~e** need for further controls on Exxon to ensure that Montanameets the requirements for reasonable progress. We will be considering this in the near future.

Ifyou have any questions or would like to discuss this matter further, please call Laurel Dygowski at (303) 312-6144 for questions onthe RHR requirements and BART guidelines and Kevin Golden at (303) 312-6442 for questions on the modeling.

Sincerely,

. ,...... ?



# ..**~L/~W;;iL** .V· ['-'CallieA. Videtich, Director 

## v Air and Radiation Program



# (1)Printed on RecycledPaperSource = Exxon Billings Refinery: SOx

## = 70 TPY; NOx

## =998 TPY 99tn 98tn 98th

Minimum Max 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 1HelisCany6nWilde'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