1 MEMORANDUM

From: Gail Tonnesen, EPA Region 8 Air Quality Mode ler

To: Fort Berthold FIP File

Date: June 15, 2012

Re: Impacts of Well Pad VOC Enclosed Combustors and Utility Flares on the Ozone and NO 2 NAAQS

Summary

EPA evaluated the impacts of changes in Volatile Or ganic Compounds (VOC) and Nitrogen Oxides (NO X ) emissions from enclosed combustors and utility fl ares used for control of VOC emissions at oil and natural gas production facilities on the Fort B erthold Indian Reservation (FBIR) as part of the technical analysis for the FBIR Federal Implementat ion Plan (FIP). Emissions categories that are affected by this FBIR FIP include VOC and NO X . Both VOC and NO X are precursors to ozone, a criteria pollutant, and NO X is a precursor to Nitrogen Dioxide (NO 2 ), also a criteria pollutant. As discussed in the TSD, EPA reviewed data from synthe tic minor permit applications 1 and determined that a typical uncontrolled well pad 2 with two wells would have VOC emissions of about 4 ,330 tpy. 3 After installing VOC control equipment, including a combi nation of enclosed combustors, pit flares, and utility flares with destruction efficiencies rangin g from 90-98%, potential controlled VOC emissions from a typical well pad with two wells and no pipel ine access were estimated to be 150 tpy. Additionally, NO X emissions resulting from the associated combustion of VOCs were estimated to be 8.8 tpy 4 for a two-well facility with no pipeline access. A ctual emissions are expected to be much lower due to the anticipated development of pipeline infr astructure in future years.

EPA performed a qualitative assessment of ozone imp acts and current ozone air quality in the region an d determined that the proposed action is not expected to cause violations of the 8-hour ozone NAAQS. As discussed below, EPA also evaluated two scenarios o f NO X emissions from enclosed combustors and utility flares using the AERMOD nearfield dispersio n model to evaluate impacts on the 1-hour average
NO 2 NAAQS. Input data to the AERMOD model for VOC encl osed combustors includes emission rates
of NO X , stack height and diameter, stack gas exit velocit y and temperature. Input data for utility flares includes flare height and calories per second gener ated by VOC combustion. The highest modeled impacts on NO 2 were 27 ppb, or 27% of the 1-hour NO 2 NAAQS. When combined with background concentrations of 11 ppb, the highest modeled conce ntrations were 38 ppb or 38% of the 1-hour NO 2 NAAQS. Thus, the AERMOD modeling results do not ind icate exceedances of the NO 2 NAAQS for
NO X emissions from enclosed combustors and utility fla res.

1 The applications can be found in the docket for th e interim final rule, Docket ID: EPA-R08-OAR-2012-0 479, which can be accessed at: http://www.regulations.gov.

2 A well pad is an oil and natural gas production fa cility that consists of one or more wells and produ ction and storage operations.

3 A summary of VOC emissions from application data t itled “VOC Emissions Analysis from Syn Minor Applic ation Data”
can be found in the docket for the interim final ru le, Docket ID: EPA-R08-OAR-2012-0479, which can be accessed at: http://www.regulations.gov.

4 A summary of NO X emissions from application data titled “NO X Emissions Analysis from Syn Minor Application Data ”
can be found in the docket for the interim final ru le, Docket ID: EPA-R08-OAR-2012-0479, which can be accessed at: http://www.regulations.gov.2

Background Air Quality Data

Impacts of the proposed action on ambient concentra tions of NO 2 and ozone must be evaluated relative to current background concentrations of NO 2 and ozone in the region. The State of North Dakota (ND) operates 3 ambient monitoring sites in the western part of the state that are designed to characterize regional background concentrations of criteria poll utants including ozone, NO 2 , SO 2 , and particulate matter. These sites are located at Dunn Center (Dun n), Theodore Roosevelt National Park-Northern Unit (TNRP), and Lostwood National Wildlife Refuge (LNWR ). The State also operates 13 additional sites that are designed to characterize source specific i mpacts or significant source and population centers

, and these sites are not appropriate for evaluating regional background air quality.

Figure 1. Locations of ambient air quality monitor ing sites in ND.

Background Ozone Levels The NAAQS for ozone is 75 parts per billion (ppb), and compliance with the NAAQS is determined by comparison to a “design value” that is calculated a s the 3-year average of the fourth highest daily 8- hour
ozone concentration at each monitor site. Based on the most recent ambient monitoring data in ND, the 2011 ozone design values at the Dunn, TRNP and LNWR sites are 55, 59, and 60 ppb, respectively. 5 Thus, current ozone air quality for western ND is s ubstantially lower than the ozone NAAQS.

Background NO 2 Levels Compliance with the NAAQS for this pollutant is det ermined by calculating a “design value” which is the eighth highest daily 1-hour average concentrati on averaged over a 3-year period at each monitor si te. Source specific model impacts must be combined with background NO 2 concentrations to evaluate potential exceedances of the NO 2 NAAQS. Current background concentrations of NO 2 in the region are quite low. Air Quality System (AQS) data from 2009 through 2011 shows 1-hour NO 2 design values for

5 The Air Quality System (AQS) – the EPA’s Repositor y of Air Quality Data, available online at: http://www.epa.gov/ttn/airs/airsaqs/. The AQS moni toring data for the ND sites can be found in the do cket for the interim final rule, Docket ID: EPA-R08-OAR-2012-0479, which can be accessed at: http://www.regulations.gov.3

the Dunn, LWNR and TNRP monitors at 11 ppb, 17 ppb, and 9 ppb, respectively, compared to the standard of 100 ppb. The Dunn monitor site is locat ed about midway between the oil development along
the North Dakota – Montana border and the seven coa l conversion facilities to the east. EPA selected t he Dunn monitor data as the most appropriate backgroun d site for the FBIR because it is a rural background monitor that is influenced by regional e nergy development activity.

Expected Future Development in the FBIR

In an effort to determine future development likely to occur on the FBIR, the EPA evaluated the Octobe r 2, 2009 Bureau of Land Management (BLM) report titl ed “Reasonable Foreseeable Development Scenario for Oil and Gas Activities on Bureau manag ed Lands in the North Dakota Study Area”. This report was supplemented on February 25, 2011 with t he document titled “Revised Activity and Surface Disturbance Projections for the Reasonable Foreseea ble Development Scenario for Oil and Gas Activities on Bureau Managed Lands in the North Dak ota Study Area”. Both documents are included in the docket for this rule and are publicly available at the following website: http://www.blm.gov/mt/st/en/fo/north_dakota_field/r mp/RFD.html.

The FBIR is located entirely within the North Dakot a study area that was evaluated by BLM in the report. Therefore, it was possible to obtain the oi l and gas development potential for projected pad
densities and associated wells for the FBIR from in formation in the report. Based on the projected pa d densities in the 2011 supplemental report, EPA dete rmined the potential for oil and gas development on the FBIR was approximately 990 well pads during the report planning period of 2010 to 2029. Although the BLM report estimated an average well density of 1.5 wells per pad, EPA used a value of two wells per pad to provide a more conservative estimate of well pad density in future development. EPA believes the average of two wells per pad to be rep resentative of typical operations on FBIR, based on information from synthetic minor permit application s and environmental assessments conducted by the
Bureau of Indian Affairs. 6 Thus, EPA concluded that reasonably foreseeable de velopment within the exterior boundaries of the FBIR would consist of ap proximately 2,000 wells or 1,000 well pads between
2010 and 2029.

Qualitative Assessment of Ozone Impacts

Ambient ozone is a secondary pollutant that is form ed from precursor emissions of VOC and NO X . High ambient ozone levels are typically observed in urba n areas during stagnant meteorological conditions i n the summer, and more recently, in areas of intensiv e oil and gas production in mountain basins that experience strong inversion conditions and snow cov er during winter. The primary air pollutants that a re affected by this action are VOC and NO X , both of which are precursors to ozone formation. For a typical well pad consisting of two wells, EPA estimated tha t the use of enclosed combustors and/or utility fla res would result in VOC reductions of about 4,180 tpy. Additionally, EPA projected an increase in NO X emissions of 8.8 tpy due to the combustion of natur al gas emissions at the well pad. EPA estimated the reasonably foreseeable development of 1,000 well pa ds with an enclosed combustor and/or utility flare
controls would result in a cumulative reduction of 4,180,000 tons of VOC and an increase of 8,800 tpy
of NO X distributed over the FBIR region of 1,544 square m iles. Because the proposed action would result in large VOC reductions and comparatively sm all NO X increases, the action could result in either
net increases or net decreases in ambient ozone lev els depending on the sensitivity of ozone formation to

6 A document containing our analysis of well pad den sities from multiple environmental assessments can be found in the docket for the interim final rule, Docket ID: EPA-R 08-OAR-2012-0479, which can be accessed at: http://www.regulations.gov.4

changes in VOC and NO X . Generally, a photochemical modeling analysis is n eeded to determine the
extent to which VOC and NO X contribute to ozone formation, and photochemical m odel simulations would be required to predict whether the impacts of this action would increase or reduce ambient ozone levels. However, significant resources and time are required to develop a photochemical modeling analysis. Because ND and the FBIR generally have me teorological conditions with high winds and good ventilation during the summer ozone season, the emi ssions changes from the proposed action are not expected to significantly increase or reduce ambien t ozone levels. Elevated winter ozone concentration s have not been observed in ND. However, preliminary analysis of winter ozone in Wyoming indicates
that winter ozone formation is more sensitive to VO C emissions than to NO X emissions. Thus, if high
winter ozone conditions are observed in ND in the f uture, the large VOC reductions from the proposed action would be expected to result in net ozone red uctions in the FBIR. Given that current ambient ozone levels in ND are substantially lower than the ozone NAAQS, small changes in ambient ozone levels that result from the proposed action are not expected to cause exceedances of the ozone NAAQS. Therefore, EPA does not propose to perform photoche mical modeling for this analysis. Instead, EPA proposes to continue to monitor ambient ozone level s in the region, and if increasing levels of ozone are observed in the future, additional analysis of VOC and NO X impacts on ozone will be performed at that
time.

AERMOD Scenario Descriptions for NO 2 Impacts

EPA used the AERMOD nearfield dispersion model to e valuate several scenarios of NO X emissions from enclosed combustors and utility flares to eval uate nearfield impacts on the 1-hour average ambien t NO 2 NAAQS. The scenarios are outlined in Table 1 and d iscussed in more detail below.

EPA staff analyzed permit application data for exis ting and proposed well pads on the FBIR and determined that VOC control at the well pad is impl emented using either VOC enclosed combustors, a utility flare, or a combination of VOC enclosed com bustors and utility flares. Enclosed combustors are utilized to control low pressure VOC emissions from storage tanks. The utility flares are used to con trol the high pressure produced natural gas emissions fr om the heater treaters. At larger facilities with multiple wells and storage tanks, storage tank natu ral gas emissions may be routed to a dual-tip utili ty flare that is capable of handling higher volumes of storage tank natural gas emissions as opposed to having multiple enclosed combustors. Using data obt ained from the permit applications and conversations with various operators 7 , a typical VOC enclosed combustor was determined t o have a stack height 25 feet, stack diameter of 48 inches, exit velocity of 1 meter per second (m/s) and an ex it temperature between 400 to 800 F. A typical utility flare was determined to have a height of 25 ft and a heat release rate of 796,261 calories per second (c al/s). The heat release rate of 796,261 calories pe r second was determined by averaging the btu/scf valu es for heater-treater produced natural gas from six permit applications, each from a different operator 8 . A corresponding calories per second value was al so obtained for storage tank waste gas. However, sinc e the heater-treater produced natural gas comprises

7 Enclosed combustor specifications were included in the synthetic permit applications for some facilit ies. EPA obtained utility flare and enclosed combustor specifications during discussions with various operators. Copies of the specifications
can be found in the docket for the interim final ru le, Docket ID: EPA-R08-OAR-2012-0479, which can be accessed at: http://www.regulations.gov. EPA also discussed faci lity operations with various operators to further d evelop modeling
scenarios.

8 A document containing the calories per second calc ulations can be found in the docket for the interim final rule, Docket ID: EPA-R08-OAR-2012-0479, which can be accessed at: ht tp://www.regulations.gov.5

90% of the volume of total waste gas at the facilit y, the difference resulting from the lower calories/second value for storage tank natural gas emissions was assumed to be negligible 9

.

Based on a subset of 64 permit applications, EPA al so determined that on average, each well would have maximum allowable emissions of 4.4 tpy of NO X . The subset of 64 applications was chosen from thr ee operators that did not have full access to pipeline infrastructure, thus creating a worst-case scenari o in which all of the waste natural gas would have to be flared. Since EPA’s reasonable foreseeable development (RFD) scenario proposed an average dens ity of two wells per well pad, the average well pad will have a total of 8.8 tpy of allowable NO X emissions. Among well pads in the 64 permit applications surveyed, the largest number of wells per pad was 10 wells. EPA determined that well pads larger than 10 wells would likely need to obtain a permit, and would therefore be subject to an independent air quality impact analysis (AQIA). Thu s, using emissions data for a 10-well facility was appropriate for a maximum emissions scenario.

Using data from the 64 permit applications and info rmation provided by various operators, EPA developed three scenarios for modeling analyses. Ta ble 1 below outlines the details of each scenario.
Scenario 1 was assumed to be the worst-case conditi on and modeled as a utility flare with no enclosed combustor. Given that the largest number of wells p er pad was 10, the NO X emission rate was assumed to be 44 tpy or the sum of 4.4 tpy from each of the 10 wells. Based on data provided to EPA staff, the stack height of the utility flare was assumed to be 25 feet. The stack diameter for the utility flare was not specified for this scenario because the models use a parameterization to calculate the flare diameter. Scenario 2 was assumed to be the average condition and modeled as the combination of a smaller utility flare and an enclosed combustor. The stack height f or the utility flare was assumed to be 9 feet. The stack height of the enclosed combustor was also ass umed to be 25 feet with a stack diameter of 48 inches. Based on the information provided to EPA st aff, the average NO X emission rate for the average utility flare was determined to be 15 tpy, while 3 tpy, or the average per well of four wells was assu med
for the enclosed combustor. Scenario 3 was assumed to have the lowest impacts and had NO X emissions
less than the de minimis level of 10 tpy. As a result, no additional analys is was performed for Scenario 3.

AERMOD Input Parameters for NO 2 Impacts

The AERMOD dispersion model (version 12060) was use d to predict the maximum 1-hour average
ambient NO 2 concentration from utility flares and enclosed com bustors at well pads on the FBIR in North Dakota. The enclosed combustors and utility f lares were represented as point sources in AERMOD. Table 2 outlines the input parameters for t he AERMOD model for each of the modeled
scenarios of NO X emissions from enclosed combustors and utility fla res. Scenario 1 included a single AERMOD simulation performed for a 25 foot flare wit h 44 tpy NO X emissions. North Dakota’s flare modeling procedure 10 was used to determine the remaining model input pa rameters for Scenario 1. For a typical flare, North Dakota’s flare modeling proced ure assumes a stack exit velocity of 40 m/s, stack gas temperature of 1000 K, and 55 percent heat loss is due to entrainment of ambient air. The total flare heat release was also assumed to be 796,261 calories per second. These assumptions were then used to calculate the effective height and diameter of a ut ility flare. Scenario 2 included two AERMOD simulations, one for a 9 foot flare with 15 tpy NO X emissions and one for enclosed combustor with 3 tp y

9 A summary of flare gas volumes from application da ta titled “Flare Gas Volumes from Syn Minor Applica tion Data” can be found in the docket for the interim final rule, Doc ket ID: EPA-R08-OAR-2012-0479, which can be accesse d at: http://www.regulations.gov

10 North Dakota - Department of Health Memorandum to Air Quality Staff Regarding Model Input Parameters for Flares
issued on 11/10/2010.6

year NO X . Similar to Scenario 1, North Dakota’s flare model ing procedure was used to determine the model input parameters for the utility flare. The i nput parameters for the AERMOD model for simulating a typical enclosed combustor were based on data obtained from various operators.

Table 1. Modeled scenarios of NO X emissions from enclosed combustors and utility fla res.

Scenario Emission Rate [tpy] Stack Height

[ft] Stack Diameter

[inches] 1 – Worst-Case Utility Flare 44 25 NS a

2 – Average Case Utility Flare 15 9 NS a
Enclosed Combustor 3 25 48

3 – Lowest Impacts Utility Flare 7.4 25 NS a
Enclosed Combustor 1.4 12 48

a Parameterizations were used to calculate this vari able for AERMOD.

Table 2. Model input parameters assumed for each mo deled scenario.

Scenario Emission Rate [tpy] Stack

Height [ft] Stack/Flare

Diameter [inches] Exit

Velocity [m/s] Stack G as

Temperature [K] 1 – Worst-Case Utility Flare 44 25 17.3 40 1000

2 – Average Case Utility Flare 15 9 17.3 40 1000

Enclosed Combustor 3 25 48 1 700

To more realistically model NO 2 , the Plume Volume Molar Ratio Method (PVMRM) was a lso used for the AERMOD analyses. This method accounts for the a mbient conversion of nitrogen oxide (NO) to
NO 2 in the presence of ozone. To apply the PVMRM optio n in AERMOD, an in-stack ratio of NO 2 /NO X emissions of 0.5 was assumed for the utility flare simulation and enclosed combustor simulation. Typical values for the NO 2 /NO X ratio in combustion emissions are about 0.1, but b ecause source test data for NO 2 /NO X ratios are not available for flares and enclosed c ombustors, we conservatively
assumed an NO 2 /NO X ratio of 0.5.

The AERMOD analyses also used 5 years of National W eather Service (NWS) meteorological data collected at the Bismarck, North Dakota station fro m 2004 to 2008. This meteorology station provided both surface and upper-air meteorological data 11 . Background ozone concentrations were based on hourly ozone data collected at the Hannover, North Dakota station between 2004 and 2008 12 , and missing ozone background concentrations were assume d to be 55 ppb. The modeled scenarios also
assumed flat terrain.

AERMOD Results for NO 2 Impacts

Table 3 lists the AERMOD modeled direct impacts for each scenario. Table 3 also lists the background
NO 2 design value for the Dunn monitor site and the sum of the modeled impacts plus the background concentration. The 5 year average of the 8 th highest daily modeled NO 2 concentration was combined with the background 1-hr NO 2 design value to evaluate the source impacts. The l ast row of Table 3 shows the combined maximum impacts of the flare and the enclosed combustor for Scenario 2. For Scenario 1, the combined modeled and background NO 2 design value was 28% of the NAAQS. For Scenario 2, the combined modeled and background NO 2 design value was 38% of the NAAQS.

11 Documents containing the surface and upper-air met eorological data obtained from ND can be found in t he docket for the interim final rule, Docket ID: EPA-R08-OAR-2012-047 9, which can be accessed at: http://www.regulations .gov.
12 The document containing background ozone data obta ined from ND can be found in the docket for the int erim final rule, Docket ID: EPA-R08-OAR-2012-0479, which can be acce ssed at: http://www.regulations.gov7

Although Scenario 1 had larger NO X emissions than did Scenario 2, with 44 tpy in Scen ario 1 compared to 18 tpy in Scenario 2, Scenario 2 had a greater f lare height and increased buoyancy of the plume because of the greater VOC mass combusted. As a res ult, the AERMOD model predicted lower surface
level NO 2 concentrations compared to Scenario 2. As describe d above, Scenario 3 had less than the 10 tpy de minimis threshold, so modeling was not performed for Scena rio 3. As shown in the last column of Table 3, the AERMOD NO 2 concentrations were significantly lower than the 1 -hour NO 2 NAAQS.

Table 3. AERMOD predicted impacts on NO 2 from utility flare and enclosed combustor.

Scenario Five year average of 8 th highest model impact (ppb) Background

1-hour average NO 2 design value (ppb) Combined

Modeled and Background NO 2 (ppb) NO 2

NAAQS 1-
hr Average (ppb) Percent of

NO 2 NAAQS

1 – Worst - Case Utility Flare 17 11 28 100 28%

2 – Average Case Utility Flare 14

Enclosed Combustor 13

Sum of Flare and Combustor 27 11 38 100 38%