RESOLUTE NATURAL RESOURCES

 for Resolute Natural Resources Company 

 1675 Broadway, Suite 1950 

 Denver, CO 80202

McElmo Creek Unit Oil Production Facility - 

Main Flare, Gas Compression Facility 

1-Hour SO2 Modeling with AERMOD

Resolute Natural Resources Company 

April 2011 

MMA Project Number 2440-10 

 

by McVehil-Monnett Associates, Inc. 

 44 Inverness Drive East, Building C 

 Englewood, CO 80112 
i 

TABLE OF CONTENTS

Page No.

1.0 INTRODUCTION.............................................................................................. 1-1 

2.0 PROJECT DESCRIPTION............................................................2-1 

3.0 MAIN FLARE EMISSIONS.............................................................................. 3-1 

3.1 CRITERIA POLLUTANTS............................................................................... 3-2 

4.0 EXISTING ENVIRONMENT ........................................................................... 4-1 

4.1 TOPOGRAPHY ................................................................................................. 4-1 

4.2 AIR QUALITY .................................................................................................. 4-1 

4.3 AREA CLIMATE AND METEOROLOGY ..................................................... 4-2 

5.0 IMPACT ASSESSMENT APPROACH ............................................................ 5-1 

5.1 AIR QUALITY MODEL DESCRIPTION ........................................................ 5-2 

5.2 MODEL INPUT ................................................................................................. 5-2 

5.3 METEOROLOGICAL DATA ........................................................................... 5-3 

5.4 SOURCE CHARACTERISTICS AND EMISSION RATES............................ 5-5 

5.4.1 Fuel Rate and Heat Release ................................................................................. 5-5 

5.4.2 Flare Height ......................................................................................................... 5-6 

5.4.3 Modeled Emission Rates......................................................................................5-6 

5.5 RECEPTOR SELECTION................................................................................. 5-7 

5.6 BACKGROUND MODELED SOURCES ........................................................ 5-8 

6.0 AIR QUALITY IMPACTS ................................................................................ 6-1 

6.1 SULFUR DIOXIDE (SO2)................................................................................. 6-2 

7.0 REFERENCES ............................................................................7-1 

LIST OF TABLES

TABLE 3-1 PRE-PROJECT & POST-PROJECT ACTUAL & PTE EMISSIONS 

TABLE 3-2 MAIN FLARE CRITERIA POLLUTANT EMISSION LIMITS 

TABLE 4-1 SO2 1-HOUR MONITORING IN THE McELMO CREEK AREA 

TABLE 4-2 JOINT FREQUENCY DISTRIBUTION OF WIND SPEED AND DIRECTION 

 CANYONLANDS AIRPORT, UTAH 

TABLE 4-3 ANNUAL STABILITY CLASS DISTRIBUTION AT CANYONLANDS 

 AIRPORT,UT 

TABLE 4-4 MONTHLY AVERAGE TEMPERATURE & TOTAL PRECIPITATION AT 

BLANDING, UT 

TABLE 5-1 SUMMARY OF NATIONAL AMBIENT AIR QUALITY STANDARDS AND 

 PSD INCREMENTS 

TABLE 5-2 ESTIMATED FLARE STACK PARAMETERS FOR AERMOD INPUT 

TABLE 5-3 MODELED STACK PARAMETERS AND EMISSIONS FOR THE MAIN 

FLARE 
ii 

TABLE 5-4 MODELED BACKGROUND SOURCE INPUT 

TABLE 6-1 COMPARISON OF MODELED CUMULATIVE SO2 IMPACTS TO THE 1-

HOUR SO2 NAAQS - FLAT 

TABLE 6-2 COMPARISON OF MODELED CUMULATIVE SO2 IMPACTS TO THE 1-

HOUR SO2 NAAQS - ELEV 

LIST OF FIGURES

FIGURE 2-1 LOCATION MAP OF McELMO CREEK OIL PRODUCTION FACILITY 

FIGURE 2-2 FLOW DIAGRAM OF EXISTING GATHERING SYSTEM OPERATIONS 

FIGURE 4-1 WIND ROSE OF CANYONLANDS AIRPORT, UTAH METEOROLOGICAL 

DATA 

FIGURE 5-1 1-HOUR SO2 MODELED RECEPTOR GRID INCLUDING FINE GRIDS 

FIGURE 6-1 MAXIMUM MODELED 1-HOUR SO2 IMPACTS WITH THE FLARE AND 

BACKGROUND SOURCES - 300 BTU/SCF CASE (ELEV) 

FIGURE 6-2 MAXIMUM MODELED 1-HOUR SO2 IMPACTS WITH THE FLARE AND 

BACKGROUND SOURCES - 682.4 BTU/SCF CASE (ELEV) 

LIST OF APPENDICES 

APPENDIX A DVD WITH AERMOD MODEL INPUT & OUTPUT FILES, 

METEOROLOGICAL DATA AND AERMAP FILES 
1-1 

1.0 INTRODUCTION 

Resolute Natural Resources Company (Resolute) is submitting this supplemental air modeling 

study for 1-hour SO2 impacts at the request of EPA Region IX during a meeting held on 

November 30, 2010. A Prevention of Significant Deterioration (PSD) permit application for its 

Main Flare at the McElmo Creek Oil Production Facility, located on the Navajo Nation near 

Aneth, Utah were submitted on March 22, 2007 to meet the terms of a Consent Decree (DOJ 

Case No. 90-5-2-1-2237, filed June 8, 2005). This Consent Decree was issued to the previous 

owner/operator of the facility, ExxonMobil Corporation (ExxonMobil). 

The PSD permit application document presented a facility process description for existing 

operations, criteria pollutant permit emission limits for the Main Flare, a Best Available Control 

Technology (BACT) analysis for the Main Flare, air modeling methodology, and air modeling 

results. Impacts were evaluated in both Class I and II areas. A revised permit application was 

submitted in April 2008 that responded to EPA Region IX comments concerning increased 

emissions due to processing flare gas with a higher BTU content than previously cited. This air 

modeling study is intended as a supplement to the already submitted permit application because 

none of the other non-air modeling information has changed. 

This air modeling study for 1-hour SO2 impacts is submitted to demonstrate modeled compliance 

to the 1-hour SO2 ambient air quality standard of 196 μg/m3, effective on August 23, 2010 with a 

detailed description of the final rule published in the Federal Register dated June 22, 2010. An 

ambient air quality standard for 1-hour NO2 of 188 μg/m3

 also became effective on April 12, 

2010 with a detailed description of the final rule published in the Federal Register dated 

February 9, 2010. Both of these standards were promulgated after the most recent revision of the 

PSD permit application submitted in April 2008. 

An additional modeling study for the 1-hour NO2 standard is not necessary because the 

SCREEN3 modeling presented in Section 7.1 of the PSD permit application document indicated 

a maximum 1-hour NO2 impact of 2.5 μg/m3

, well below the significant impact level (SIL) of 7.5 

μg/m3 (4 ppb). However, the maximum 1-hour SO2 impact presented in Section 7.2 of the 

application document was 11.6 μg/m3

, above the SIL of 7.9 μg/m3 (3 ppb). 
2-1 

2.0 PROJECT DESCRIPTION 

This section presents an overview of the McElmo Creek Oil Production Facility operations, 

including the gas compression plant. 

Currently, produced fluids at the oil production facility are processed, sold, and re-injected 

through a series of satellite facilities, main tank batteries, and injection plants. Produced fluids 

are gathered and separated at one of eleven satellite facilities located throughout the field. Gases 

from the satellite facilities are directed to the gas compression plant, and liquids are pumped to 

one of four central tank batteries. Gases sent to the gas compression plant are compressed, dried 

through a glycol dehydrator, blended with carbon dioxide (CO2), and re-injected into the field. 

Liquids are further separated at the central tank batteries, with oil being sold and transported 

through a pipeline system. Water from the central tank batteries is pumped to the water flood 

injection plant. 

The Main Flare at the gas compression plant combusts gases that have been bypassed around the 

plant’s electric-driven compressors or that come through various emergency relief valves in the 

compression plant. Additionally, the vent stream from the plant’s glycol dehydrator is directed 

to the Main Flare. 

A location map of the McElmo Oil Production Facility is presented in Figure 2-1. A flow 

diagram of the facility’s gathering system operations is presented in Figure 2-2, illustrating the 

flow through the satellite facilities, central tank batteries, water flood injection plant, and gas 

compression plant. 

Fig

Block Flow Di

Gathering Sy

McElmo Creek O
3-1 

3.0 MAIN FLARE EMISSIONS 

As requested in the EPA Region IX comment letter of May 22, 2006, this section presents preand post-project actual emissions, as well as pre-project estimates of the potential to emit (PTE) 

for all criteria pollutants, including particulate matter (PM10 and PM2.5) and lead. Pre- and postproject gas flow rates to the flare are also included. Per EPA’s comment letter, pre-project is 

defined as prior to the gas compression facility being dismantled in 1997, and post-project is 

defined as subsequent to the gas plant being dismantled. Thus, pre-project emission estimates 

provided in Table 3-1 are based on 1996 gas flare rates and a conservative gas analysis from that 

year and post-project emission estimates are based on the 2005 emission inventory, updated by 

gas analysis data from 2005-08. The post-project emissions presented in Table 3-1 reflect the 

updated estimates from 2005-08 gas analyses data. 

As clarified in the EPA Region IX comment letter of November 8, 2006, post-project PTE is for 

the period following the dismantling of the gas plant, but prior to the imposition of the Consent 

Decree limits. Therefore, the post-project PTE emissions provided are based on 2003 flare 

emissions data multiplied by a PTE factor of 1.2 since the annual flare rate in 2003 was the 

highest of the five year period 2001-2005. Updated post-project PTE emissions based on more 

recent gas analyses from late 2005 through 2008 and the natural gas flow rate from 2003 

multiplied by 1.2 are also provided. These post-project PTE determinations were made in 

accordance with the methodology outlined in 40 CFR §63.760(a)(1)(i)(B) (E&P MACT) and 

past Texas Commission on Environmental Quality (TCEQ) guidance on PTE calculations for oil 

& gas operations. Applicable calculation methodology with input data used to estimate 

emissions are presented in Appendix B of the PSD permit application document. 

The gas flow rates to the flare used for the emission estimates are 1,145.96 MMscf in 1996 (for 

both actual and PTE emissions), 727.704 in 2005 (rounded off to 730.0 MMSCF) for postproject actual emissions, and 1,070.366 MMscf for post-project PTE emissions. Because the 

flare is only used for emergency relief and to control dehydrator still vent emissions, the daily 

rates vary. 
3-2 

Table 3-1 

Pre-Project and Post-Project Actual and PTE Emissions

Criteria 

Pollutant

Pre-Project 

Actual Emissions 

(tpy) 

Pre-Project 

PTE1

(tpy) 

Post-Project 

Actual Emissions 

(tpy) 

Post-Project 

PTE2

(tpy) 

CO 159.2 159.2 82.5 135.1 

NOx 29.3 29.3 15.2 24.8 

VOC 183.6 183.6 91.7 151.8 

SO2 138.1 138.1 30.1 44.2 

Lead 0.00021 0.00021 0.000109 0.000179 

PM10 3.2 3.2 1.7 2.7 

PM2.5 3.2 3.2 1.7 2.7 

1

 Pre-project PTE is equivalent to 1996 actual emission estimates. 

2

 Post-project PTE is estimated by identifying the maximum annual gas flow rate from the most recent five-year 

period with available data (i.e., 2001 – 2005) and multiplying by a contingency factor of 1.2 to provide a worstcase post-project PTE estimate. The maximum gas flow year of 2003 was selected as the basis for this estimate. 

The maximum BTU content from 2005-08 gas analyses data (682.4 BTU/SCF) was assumed for emission 

estimates.

3.1 CRITERIA POLLUTANTS

Table 3-2 presents a summary of criteria pollutant emission limits for the Main Flare as listed in 

Paragraph 9.a of Exxon Mobil’s Consent Decree. Due to the higher BTU content occurring 

since issuance of the Consent Order, Resolute is requesting revised emission limits 

corresponding to the last column of Table 3-1. 

Table 3-2 

Main Flare Criteria Pollutant Emission Limits1

Pollutant tons/year2

SO2 60 

CO 73 

NOx 13.4 

VOC 95.4 

1. As listed in Paragraph 9.a of Consent Decree (DOJ Case No. 90-5-

2-1-2237) 

2. Cumulative 365-day rolling average. 
4-1 

4.0 EXISTING ENVIRONMENT 

4.1 TOPOGRAPHY

The McElmo Creek Oil Production Facility is located on Navajo Nation land in southeastern 

Utah, in San Juan County, near the town of Aneth. The gas compression facility is located 

approximately 23 km (14 miles) northwest of the Utah / Colorado border and 4.8 km (3 miles) 

north of State Highway 262. Arches National Park and Canyonlands National Park are located 

to the northwest, approximately 160 km (100 miles) and 96 km (60 miles), respectively. Mesa 

Verde National Park is located approximately 60 km (38 miles) east of the gas compression 

facility site. The western boundary of the Canyon of the Ancients National Monument is 

approximately 16 km (10 miles) northwest of the plant site. 

Elevations around the gas compression plant range from 5,500 feet to 5,900 feet. Terrain 

generally increases in both the north and south directions from the east-west valley formed by 

the San Juan River. Ground cover is relatively sparse because of the semiarid climatic 

conditions surrounding the gas compression facility site. 

4.2 AIR QUALITY

Background SO2 air quality monitoring data for 2008-2010 are presented in Table 4-1. No site 

or area-specific data are available for southeastern Utah; therefore, air quality data gathered from 

the monitoring station located at Salt Lake City International Airport, and the monitoring station 

located at USBR – Shiprock (Farmington, NM) are used as conservative indicators of 

background air quality. However, the Salt Lake City data are from an urban location and not as 

representative of the rural conditions surrounding the McElmo Creek Gas Plant as those 

surrounding the USBR Shiprock site. Therefore, the background concentrations from the USBR 

Shiprock site are used to represent conditions existing near the McElmo Creek Gas Plant. Per 

EPA’s guidance memo of August 23, 2010, the first tier approach for selection of monitored 

background SO2 concentration to add to modeled impacts is to use the highest daily maximum 

hourly concentration. As a result, the highest daily maximum hourly concentration of 79 μg/m3

from the USBR Shiprock site is the appropriate background concentration for the 1-hour SO2

modeling.
4-2 

Table 4-1. 

SO2 1-Hour Monitoring in the McElmo Creek Area. 

Daily 

Maximum 

Concentration 

99th

Percentile 

NAAQS2

Pollutant 

Monitor 

Location/County1

Averaging 

Period Year (ppm / μg/m3

) (ppm / μg/m3

) (ppm / μg/m3) 

2008 0.131 / 343 0.057 / 149

2009 0.069 / 181 0.038 / 100

Salt Lake City 

International 

AP/Salt Lake 

1-hour 

2010 0.031 / 81 0.029 / 76

0.075 / 196.5

2008 0.028 / 73 0.020 / 52

2009 0.030 / 79 0.025 / 65

Sulfur 

Dioxide 

(SO2) USBR Shiprock - 

Farmington, NM / 

San Juan 

1-hour 

2010 0.016 / 42 0.011 / 29

0.075 / 196.5

1

 No site-specific data is available. These monitoring locations provide the closest available current data. 2

 To attain this standard, the 3-year average of the 99th percentile of the daily maximum 1-hour average at each monitor within an area over each 

year must not exceed 75 ppb. 

Table 4-1 shows that monitored concentrations at both stations are well below the SO2 National 

Ambient Air Quality Standards (NAAQS). Monitored compliance with the 1-hour SO2 NAAQS 

is based on the 3-year average of the 99th percentile of the daily maximum 1-hour values. 

4.3 AREA CLIMATE AND METEOROLOGY

The southeastern area of the state, where the McElmo Creek facility is located, can be 

characterized as an area of low precipitation, high summer temperatures, and moderate winter 

temperatures. Meteorological monitoring has not been conducted onsite at the McElmo Creek 

facility. UDAQ supplied the pre-processed AERMET meteorological data for this study, which 

originates from the Canyonlands Airport, northwest of the facility. These data cover the years 

2005-2009 and is considered as representative of the southeastern Utah area. Table 4-2 presents 

a summary wind speed and direction distribution for the five year period. Figure 4-1 presents a 

graphical depiction of wind speed and direction distribution as a wind rose. The average wind 

speed was 2.6 meters/second (m/s), or 5.8 miles per hour (mph). The predominant wind 

directions were from the west-southwest (11.0%), from the west (10.3%), and from the northnortheast (10.8%). 

An atmospheric stability distribution for this period is in Table 4-3. This table shows that 

neutral (D) and stable (F) conditions are dominant. 

 
4-3 

 Table 4-2 

 10 METER WIND DATA ANALYSIS 

 FROM 01/01/2005 THROUGH 12/31/2009 

 CANYONLANDS AIRPORT METEOROLOGICAL STATION 

 FREQUENCY OF OCCURRENCE OF WIND SPEED BY DIRECTION 

 +----------WIND SPEED CLASSES (M/S)----------+ 

 AVERAGE 

 0.0- 1.8- 3.3- 5.4 8.4 > WIND 

 DIR 1.8 3.3 5.4 8.4 11.0 11.0 TOTAL SPEED 

------------------------------------------------------------------------- 

 N .0248 .0152 .0037 .0014 .0003 .0000 0.045 2.0 

 NNE .0461 .0224 .0142 .0080 .0020 .0004 0.093 2.5 

 NE .0164 .0071 .0037 .0035 .0010 .0001 0.032 2.6 

 ENE .0199 .0075 .0052 .0042 .0010 .0000 0.038 2.6 

 E .0219 .0123 .0084 .0020 .0001 .0000 0.045 2.2 

 ESE .0238 .0208 .0093 .0005 .0000 .0000 0.054 2.1 

 SE .0267 .0260 .0163 .0030 .0002 .0000 0.072 2.5 

 SSE .0204 .0131 .0083 .0035 .0005 .0000 0.046 2.5 

 S .0153 .0057 .0049 .0046 .0007 .0001 0.031 2.8 

 SSW .0160 .0048 .0063 .0092 .0045 .0007 0.041 4.0 

 SW .0300 .0088 .0126 .0120 .0048 .0007 0.069 3.5 

 WSW .0764 .0148 .0098 .0065 .0019 .0004 0.110 2.0 

 W .0571 .0183 .0173 .0079 .0019 .0006 0.103 2.4 

 WNW .0291 .0158 .0194 .0132 .0039 .0008 0.082 3.5 

 NW .0300 .0281 .0171 .0065 .0028 .0005 0.085 2.9 

 NNW .0210 .0213 .0087 .0016 .0004 .0001 0.053 2.4 

------------------------------------------------------------------------- 

 TOT: 0.475 0.242 0.165 0.087 0.026 0.004 1.000 

 AVG: 0.9 2.6 4.2 6.7 9.5 13.6 2.6 

 TOTAL NUMBER OF VALID READINGS FOR THIS TABLE => 43456 

 OUT OF 43456 TOTAL VALID HOURS 

 TOTAL NUMBER OF MISSING HOURS => 368 

 TOTAL NUMBER OF CALM HOURS => 4806 
4-4 

Table 4-3. Annual Stability Class Distribution at Canyonlands Airport, Utah. 

Stability Class Percent of Occurrence (%) 

A 4.2 

B 8.5 

C 8.4 

D 25.0 

E 4.1 

F 49.7 

Temperature and precipitation conditions in vicinity of the McElmo Gas Compression Facility 

are depicted by a long-term climatic monthly summary (12/8/1904 – 3/31/2005) of data from 

Blanding, Utah in Table 4-4. 

Table 4-4. Monthly Average Temperature and Total Precipitation at Blanding, Utah.

Month Avg. Temp. 

Max. (o

F) 

Avg. Temp. 

Min. (oF) 

Avg. Temp. 

Mean (oF) 

Precipitation Total 

(inches) 

January 39.1 17.1 28.1 1.39 

February 44.8 22.3 33.6 1.23 

March 52.7 27.8 40.3 1.04 

April 62.2 34.3 49.3 0.87 

May 72.2 42.0 57.1 0.71 

June 83.3 50.7 67.0 0.45 

July 88.6 57.8 72.2 1.15 

August 86.2 56.1 71.1 1.39 

September 78.2 48.3 63.3 1.28 

October 66.0 37.9 51.9 1.45 

November 51.4 26.6 41.0 1.06 

December 41.1 19.1 30.1 1.35 

Annual Avg. Temp./Annual Total Precip. 63.8 36.7 50.3 13.35 
WRPLOT View - Lakes Environmental Software

WIND ROSE PLOT

Canyonlands Airport Station

Meteorological Data

DATE:

4/22/2011

PROJECT NUMBER:

2440-10

Figure 4-1

NORTH

SOUTH

WEST EAST

2%

4%

6%

8%

10%

WIND SPEED

(m/s)

>= 11.0

8.4 - 11.0

5.4 - 8.4

3.3 - 5.4

1.8 - 3.3

0.4 - 1.8

Calms: 11.12%

TOTAL COUNT:

43205 hrs.

CALM WINDS:

11.12%

DATA PERIOD:

2005-2009 

Jan 1 - Dec 31

00:00 - 23:00

AVG. WIND SPEED:

2.64 m/s

DISPLAY:

 Wind Speed

Direction (blowing from)
5-1 

5.0 IMPACT ASSESSMENT APPROACH

This section presents the methodology used to evaluate the potential ambient air quality impacts 

resulting from the operation of the Main Flare. These analyses are compared against the 

National Ambient Air Quality Standards (NAAQS) and PSD increments presented in Table 5-1. 

TABLE 5-1 

SUMMARY OF NATIONAL AMBIENT AIR QUALITY 

STANDARDS AND PSD INCREMENTS 

 

 NAAQS Class II PSD 

 Averaging Primary Increment 

Pollutant Time μg/m3

 (ppm) μg/m3 (ppm)

1-houra Carbon Monoxide 40,000 (35) -- 

8-houra

 10,000 (9) -- 

Annual (arith. 

Mean) 

Nitrogen Dioxide 100 (0.05) 25 (0.013) 

1-hourb

 188 (0.10) -- 

Ozone 8-Hourc 147(0.08) -- 

24-houra Particulates (PM10) 150 (--) 30 (--) 

Annual (arith. 

Mean) 

50 (--) 17 (--) 

24-houra Particulates 35 (--) 9(--) 

(PM2.5) Annual (arith. 

Mean) 

15 (--) 4 (--) 

Sulfur Dioxide 1-hour 196 (0.075) -- 

 3-houra 1,300 (0.50) 512 (0.20) 

 24-houra 365 (0.14) 91 (0.035) 

 Annual (arith. 

Mean) 

80 (0.03) 20 (0.008) 

Rolling 3-month 

Average 

Lead 0.15 (--) -- 

Calendar Quarter 

(arith. mean) 

1.5 (--) -- 

a. Not to be exceeded more than once per year. 

b. The 3-year average of the 98th percentile of the daily maximum 1-hour average at each monitor within an area 

must not exceed 100 ppb. 

c. The 3-year average of the 4th highest daily maximum concentrations must not exceed this standard. 
5-2 

5.1 AIR QUALITY MODEL DESCRIPTION

AERMOD (AMS/EPA Regulatory Model), Version 09292, was used to assess potential nearfield air quality impacts of the facility. AERMOD has the capability of modeling for both shortterm and long-term averaging periods. The model was run in regulatory default mode with rural 

dispersion coefficients. 

AERMOD is a modeling system consisting of three separate modules: AERMAP, AERMET and 

AERMOD. 

AERMAP is a terrain preprocessor that uses Digital Elevation Model (DEM) or National 

Elevation Dataset (NED) electronic files, which are readily available from the Internet, for use 

with user-generated receptor grids. The output file consists of the x, y locations of each receptor, 

mean sea level (MSL) elevation and hill profile parameter. The hill profile parameter is used in 

determining plume flow around elevated terrain. Source elevation and receptor elevations and 

hill heights were determined through AERMAP using NED files. 

The meteorological preprocessor AERMET uses hourly surface observations, cloud cover/solar 

radiation-delta T and upper air parameters from twice-daily vertical sampling of the atmosphere 

to create two output files consisting of surface and vertical profile data, respectively. Preprocessed AERMET files were obtained from UDAQ for use in this study. 

AERMOD directly reads the three output files created by the pre-processing programs, and along 

with user-entered source information, predicts ambient air concentrations for a variety of 

pollutants and averaging periods ranging from 1-hour to annual. 

5.2 MODEL INPUT 

The Main Flare is a point (stack) source that routinely operates in the stand-by mode as reflected 

in the annual emissions presented as limits in Section 3. For modeling purposes, particularly for 

comparison to short-term standards and PSD increments, the worst-case impacts from the Main 

Flare would be during an upset episode when all process gas is sent to the flare, rather than reinjected into the ground. Thus, modeled flare stack parameters were adjusted to reflect the 
5-3 

instantaneous nature of heat release characteristics of a flare when combusting all process gas 

rather than re-injecting for input to AERMOD. The algorithm for this characterization is based 

on the Texas Commission on Environmental Quality’s “Technical Basis for Flare Parameters” 

(September 10, 2004). An effective stack diameter at an assumed stack velocity of 20 m/s at the 

estimated heat release (in calories per second) of the flare is estimated. An effective stack height 

is estimated based on heat release and the physical stack height of the flare. Table 5-2 presents a 

summary of these estimates at the four heat content conditions of the flared gas described in 

Section 5.4. A radiative loss of zero is assumed for consistency with buoyancy flux estimations 

from the algorithm used in SCREEN3. 

Supplemental modeling was performed to estimate impacts of 1-hour SO2 only, since PM10

emissions are insignificant and previous SCREEN3 modeling of NOx, CO and the other 

averaging periods for SO2 demonstrated modeled compliance to the applicable standards. VOCs 

are not modeled directly except as a precursor to ozone. The annual emission limit for VOCs 

(95.4 tons per year) is below the 100 ton per year criterion typically cited as that level requiring 

ozone modeling. Previous SCREEN3 modeling assumed flat terrain. AERMOD was run in both 

the FLAT and ELEV modes to assess the effects of terrain on modeled 1-hour SO2 impacts and 

to compare modeled AERMOD impacts to previously modeled SCREEN3 impacts. 

5.3 METEOROLOGICAL DATA

The selection of an appropriate meteorological data set for air quality modeling is based on 

proximity to the project site, similarity of topography and conformity of the data collection and 

reporting procedures to PSD monitoring requirements. Another consideration is that the 

meteorological data can be readily processed into a format compatible for use in air dispersion 

modeling. 

For this study, modeling was conducted using a Division-furnished five year (2005-2009) of preprocessed 10-meter tower data from the Canyonlands Airport (KCNY) station near Moab, UT 

for surface data, and concurrent upper air data from Grand Junction, CO. The base elevation of 

the KCNY meteorological monitoring site is 1387.8 meters, which was used as input for 

AERMOD. 
5-4 

Table 5-2. 

Estimated Flare Stack Parameters for AERMOD Input 

 

Assumptions: 

Actual Stack Height (m.), Hs: 59.45 

Stack Temperature (o

K), Ts: 1273 

Stack Velocity (m/s), Vs: 20.0 

Flare Gas Flow Rate (scf/hr): 618104.0 

Ambient Temperature (o

K), T: 294.3 

(Assumed as 70.0 o

F.) 

 

1. Hsl, Effective Stack Height (m.) = Hs + (4.56 x 10-3) * (Hr/4.1868)0.478

 where, 

 Hr = Net Heat Release (Joules/sec) = (1 - F) H 

 F = Radiative Loss Factor, 0 (2008 SCREEN3 modeling): 0.0 % 

 H = Heat release at from flared gas at a particular heat content, in joules/sec 

1 BTU = 1055.0 Joules = 252 Calories 

Heat Content Heat Release, H Net Heat 

Release, Hr

Effective 

Stack 

Height, Hsl

(BTU/SCF) (BTU/sec) (Joules/sec) (Joules/sec) (m.) 

300 51508.7 54341643.3 54341643.33 70.91 

443 76061.1 80244493.3 80244493.32 73.25 

538.8 92509.6 97597591.4 97597591.43 74.61 

682.4 117165.0 123609124.7 123609124.7 76.42 

2. ds, Effective Stack Diameter (m.) = 0.03249 * ((Hr/Vs) * (Ts/(T * (Ts - T))))0.5

 where Hr, Vs, Ts and T are input variables with assigned values described above 

Heat Content Net Heat 

Release, Hr

Stack 

Velocity, Vs

Stack 

Temp., Ts 

Ambient Air 

Temp., T 

Effective 

Stack 

Diameter, ds

(BTU/SCF) (Joules/sec) (m/sec) (oK) (oK) (m.) 

300 54341643.33 20.0 1273.0 294.3 3.561 

443 80244493.3 20.0 1273.0 294.3 4.327 

538.8 97597591.4 20.0 1273.0 294.3 4.772 

682.4 123609124.7 20.0 1273.0 294.3 5.370 

3. Flare Stack Parameters Used for Each Flare Gas Heat Content Scenario in AERMOD 

Heat Content 

Effective 

Stack 

Height, Hsl

Effective 

Stack 

Diameter, ds

Stack 

Temp., Ts 

Stack 

Velocity, Vs

SO2

Emission 

Rate 

(BTU/SCF) (m.) (m.) (oK) (m/sec) (g/s) 

300 70.908 3.561 1273.0 20.00 9.21 

443 73.254 4.327 1273.0 20.00 9.21 

538.8 74.609 4.772 1273.0 20.00 9.21 

682.4 76.421 5.370 1273.0 20.00 9.21 
5-5 

5.4 SOURCE CHARACTERISTICS AND EMISSION RATES

5.4.1 Flare Rate and Heat Release 

Short-term pollutant emission rates were estimated by assuming that the maximum volume of 

process gas that could be generated from the well field would be sent to the flare at four different 

heat content values (in BTU/scf). These four heat content values represent a range of estimates 

including the lowest allowable LHV, the lowest LHV from 2004 gas analyses, the lowest LHV 

from 2005-08 gas analyses and the highest LHV from 2005-08 gas analyses data. The lowest 

value input was 300 BTU/scf, which is the lowest allowable LHV as stated in 40 CFR 

60.18(c)(3)(ii). The next lowest value utilized was 443 BTU/scf, which is the lowest of four 

2004 actual plant gas samples from McElmo. (The four analyses are dated 4/28/04, 5/27/04, 

8/29/04, and 9/29/04; and their LHV values were 443, 505, 512, and 453 BTU/scf, respectively.) 

The LHV of 443 BTU/scf was utilized due to the assumption that it would provide the least 

amount of heat rise, thus producing the lowest effective release height. The lowest and highest 

LHVs from 2005-08 gas analyses data of 538.8 and 682.4 BTU/scf, respectively, were also used 

for emission estimation. The resultant range of BTU contents and effective release heights were 

modeled to evaluate the sensitivity of impacts to each emission rate/effective release height and 

diameter combination. 

The maximum flare rate utilized was 689,423 acf/hr (618,104 scf/hr), which assumes that the 

flare is operating at 14.83 MCFD for the entire year. The flare rate value of 618,104 scf/hr was 

based on the assumption that the new flare tip will be 24 inches in diameter and will have a flare 

tip exit velocity of 64.74 ft/s. This exit velocity is below the maximum exit velocity as specified 

by 40 CFR 60.18, which is 66.98 ft/s. 

The heat released from the flare tip was then calculated and converted to a rate of calories per 

second based on the conversion of 1 BTU = 252.1644 calories. The equation used for this 

calculation is as follows: 

s

LHV V cal heat cal s

3600

( )( )(252.1644 ) ( / ) =
5-6 

 Where: 

 LHV = lower heating value of gas burned (BTU/scf) 

 V = 618,104 scf/hr. 

5.4.2 Flare Height 

The actual flare stack height of 195 feet was taken from drawing D-SA-107 as provided by 

Zeeco, Inc. from 1991. This measurement was field verified to ensure proper stack height. 

5.4.3 Modeled Emission Rates 

The emission factors for NOx and CO were derived by the Chemical Section of the Texas 

Commission on Environmental Quality NSR Division based on data published in the 1983 

Chemical Manufacturer's Association document entitled "A Report On A Flare Efficiency 

Study." The factors used were as follows: 

 NOx = 0.0641 lb/MMBTU 

 CO = 0.5496 lb/MMBTU. 

Therefore, the equation used to calculate the emission rate for NOx and CO contaminants at the 

specified LHV is as follows: 

s

hr

lb

g

MMBTU

lb EF

BTU

MMBTU

scf

BTU LHV

hr

scf rate g s V

3600

* 453.59 * *

10 ( / ) * *

6 =

 Where: 

 V = 618,104 scf/hr 

 LHV = lower heating value of gas burned (BTU/scf) 

 EF = emission factor (lb/MMBTU). 

Tests subsequent to the March 22, 2007 application submittal at the McElmo site indicated the 

plant inlet gas contains approximately 500 ppm H2S. However, it was assumed that the flared 

gas contains 700 ppm H2S to estimate SO2 emissions input to AERMOD. This H2S value was 
5-7 

used in order to provide a conservative estimate of SO2 emissions from the flare. The emission 

rate was calculated as follows: 

rate (g/s) = (618,104 scf/hr * 700 ppm * 64 lb/lb-mole SO2)/(380 scf/lb-mole * 1,000,000 ppm)= 72.87 lbSO2/hr 

In summary, the stack parameters and estimated emission rates for the Main Flare at the four

representative heat content values during upset conditions are summarized below in Table 5-3. 

Table 5-3 

Modeled Stack Parameters and Emissions for the Main Flare 

(Maximum Volumetric Process Gas Flow of 618,104 SCF/hr) 

Heat 

Content 

(BTU/SCF) 

Stack 

Height 

(m.) 

Effective 

Release 

Height (m.) 

Effective 

Stack Dia. 

(m.) 

Stack 

Velocity 

(m/s) 

Short-Term 

NOx Emission 

Rate (gm/sec) 

Short-Term 

SO2 Emission 

Rate (gm/sec) 

Short-Term 

CO Emission 

Rate (gm/sec) 

300 59.45 70.908 3.561 20.0 1.50 9.18 12.84 

443 59.45 73.254 4.327 20.0 2.21 9.18 18.96 

538.8 59.45 74.609 4.772 20.0 2.69 9.18 23.06 

682.4 59.45 76.421 5.370 20.0 3.41 9.18 29.21 

5.5 RECEPTOR SELECTION

Discrete receptors in UTM Zone 12 NAD83 coordinates were processed by AERMAP using 10-

meter NED data with the following grid resolutions: 

• A single ring of receptors 200 meters from the center point of the flare 

• 100 meter spaced grid out to 2,200 meters from the flare 

• 250 meter spaced grid from 2.2 to 10 kilometers from the flare 

• 500 meter spaced grid from 10 to 20 kilometers from the flare 

Additional blocks of 100-meter spaced grids were also used to determine maximum 

concentrations within the coarse grids. Figure 5-1 presents a plot of the modeled receptor grid 

used for this study. 
5-8 

5.6 BACKGROUND MODELED SOURCES 

Data on background SO2 emission sources in the region surrounding the McElmo Creek Gas 

Plant were acquired for modeling of cumulative impacts because maximum 1-hour SO2 impacts 

from the flare exceed the 1- hour SIL of 7.9 μg/m3

. The following agencies were contacted to 

acquire background SO2 emissions data: 

1. Utah Department of Air Quality (UDAQ) 

2. Colorado Air Pollution Control Division (CAPCD) 

3. Southern Ute Indian Tribe Air Quality Program 

4. Ute Mountain Indian Tribe 

The following responses were received from these agencies: 

1. The UDAQ provided background SO2 data from a facility located south of Blanding with 

a total of 0.7 tons per year (tpy) average for the last two years and is the only facility 

reporting emissions within 50 km. of the McElmo Creek Gas Plant, approximately 40 

km. away. 

2. The CAPCD provided background SO2 data from various facilities with total emissions 

of approximately 34 tpy, located approximately 15 to 50 km. from the McElmo Creek 

Gas Plant. 

3. Responses received from the Tribal agencies indicated that information on background 

SO2 emission source data from facilities on Tribal Land are not publicly available 

An Internet search for recent NEPA studies performed in the Four Corners Area found that the 

only available study was the Desert Rock Energy Project Draft EIS. SO2 emission source data in 

this document are from large power plant sources only, located at least 100 km. from the 

McElmo Creek Gas Plant. Therefore, it is not appropriate to include these data to model 

cumulative 1-hour SO2 impacts from the flare and background sources. 

. 
5-9 

The flare was modeled with all background SO2 data provided by the UDAQ and the CAPCD 

and added to the maximum daily monitored 1-hour SO2 concentration of 79 μg/m3

 for 

comparison to the 1-hour SO2 standard of 196 μg/m3

. The EPA guidance memo, dated March 1, 

2011, on modeling of 1-hour NO2 and SO2 suggests “that the emphasis on determining which 

nearby sources to include in the modeling analysis should focus on the area within about 10 

kilometers of the project location”. The basis of this statement is that modeled 1-hour NO2 and 

SO2 impacts decrease significantly more with distance than when the longer-term averaging 

periods are modeled. However, all background sources from the UDAQ and CAPCD were 

included in the modeling even though the closest emission source is located at least 15 

kilometers from the McElmo Creek Gas Plant. 

Modeled background source input for 1-hour SO2 is presented in Table 5-4. 
Table 5-4. Modeled Background Source Input 

Point Source Emission Rates and Stack Parameters 

Location SO2

Easting Northing Elevation 1-Hour 

Stack 

Height Tempe

Facility Name (m) (m) (m) (g/s) (m) (oMcElmo Creek McElmo Creek Plant 659259.7 4125275.6 1459.5 9.2100 70.91 127Utah Sources Denison Mines Corp.: White Mesa Mill 634400.0 4159100.0 1752.6 1.1075E-05 26.82 293Denison Mines Corp.: White Mesa Mill 634400.0 4159100.0 1752.6 3.2219E-06 10.67 394Denison Mines Corp.: White Mesa Mill 634400.0 4159100.0 1752.6 1.2255E-05 3.05 295Denison Mines Corp.: White Mesa Mill 634400.0 4159100.0 1752.6 3.6879E-05 26.82 293Denison Mines Corp.: White Mesa Mill 634400.0 4159100.0 1752.6 1.5672E-04 26.82 308Colorado Sources Questar E&P -Spargo #1-36 675928.3 4162567.7 1996.6 6.6107E-05 2.13 644Questar E&P - Island Butte A 677249.2 4159645.1 1986.0 2.8767E-05 3.66 838Questar E&P - Island Butte A 677249.2 4159645.1 1986.0 2.8767E-05 3.66 838Questar E&P - Island Butte #3 677724.6 4158971.0 1981.9 1.2658E-04 3.05 583DJ Simmons - Compressor #1 677622.4 4163863.7 2027.0 1.6078E-04 0.00 255Questar Exploration - Spargo No 2 675923.5 4161781.3 1988.7 8.4547E-05 3.35 830Kinder Morgan CO2 Co. -Yellow Jacket H10 695330.9 4149292.4 2024.9 2.4452E-01 6.10 644Kinder Morgan CO2 Co. -Yellow Jacket H10 695330.9 4149292.4 2024.9 2.4452E-01 6.10 644Kinder Morgan CO2 Co. -Hovenweep Central 687300.6 4151258.9 1965.2 2.4228E-01 6.10 644Kinder Morgan CO2 Co. -Hovenweep Central 687300.6 4151258.9 1965.2 2.4228E-01 6.10 644Questar E&P Cutthroat Battery A 683113.0 4149203.7 1871.6 1.4309E-04 3.35 752Questar E&P Cutthroat Battery A 683113.0 4149203.7 1871.6 8.2878E-05 2.44 872Questar E&P Cutthroat Battery A 683113.0 4149203.7 1871.6 4.0907E-05 5.79 807Questar E&P Cutthroat Battery B 684827.8 4145395.2 1904.0 9.8786E-05 3.35 752Questar E&P Cutthroat Battery B 684827.8 4145395.2 1904.0 6.9041E-05 0.61 501Questar E&P Cutthroat Battery B 684827.8 4145395.2 1904.0 2.1374E-05 3.66 866Questar Exploration - Mcclean Basin #2 683232.7 4148526.9 1846.2 4.7466E-05 2.44 841Questar E & P- Island Butte - B 681223.5 4156947.6 1982.5 7.7671E-05 5.18 522Smith Energy Lp 1988 Cache Oil Field 674969.6 4124174.4 1531.1 1.3808E-04 3.05 255Questar Exploration - Cutthroat #5 684827.4 4145394.4 1904.0 5.1666E-05 2.13 877Questar Exploration - Island Butte #7AH 680030.7 4157102.6 1978.4 1.6570E-05 3.05 699Questar Exploration - Island Butte #7AH 680030.7 4157102.6 1978.4 2.2151E-05 3.05 699Questar Expl And Prod - Cutthroat #9 682805.0 4151855.0 1838.8 3.0205E-05 4.57 569Questar Exploration - Cutthroat #14 684630.6 4143114.5 1898.3 2.4567E-05 4.57 568Questar Exploration - Cutthroat #8 684833.6 4144650.5 1898.9 1.9792E-05 2.13 588
Table 5-4 (Continued). Modeled Background Source Input 

Area Source Emission Rates and Source Parameters 

Location 

Easting Northing Elevation SO2 Emission Rate 

(m) (m) (m) (g/s) (g/s-m2) 

Utah Sources Denison Mines Corp.: White Mesa Mill 634350.0 4159050.0 1752.6 0.0197 1.9734E-06 
640000 645000 650000 655000 660000 665000 670000 675000

UTM Easting (m)

NAD83, Zone 12

4110000

4115000

4120000

4125000

4130000

4135000

4140000

4145000

UTM Northing (m)NAD83, Zone 12

Figure 5-1

McElmo Creek Gas Plant

1-Hour SO2 Modeling

Modeled Receptor Grid

Including Fine Grids
6-1 

6.0 AIR QUALITY IMPACTS

This section presents modeled air quality impacts of the Main Flare facility to demonstrate 

compliance with the 1-hour SO2 National Ambient Air Quality Standards (NAAQS), using 

AERMOD. A Prevention of Significant Deterioration Increment for Class II Areas for 1-hour 

SO2 has not been established. Air quality impacts are presented from the Main Flare alone and 

combined with modeled background sources with a representative ambient air background 

concentration to assess cumulative impacts. 

As discussed in the previous section, several assumptions were made to conservatively calculate 

emission rates from the Main Flare for use in this modeling analysis. These assumptions 

included: 

1. Consideration of a range of heat value ratings and corresponding effective release heights 

for the gas at McElmo to evaluate and identify maximum impacts produced by the 

McElmo Main Flare; 

2. The maximum design flare rate of 14.83 MCFD; and 

3. An H2S concentration of 700 ppm instead of previously measured values of 

approximately 450 to 500 ppm. 

Emission limitations established by the Consent Decree for the Main Flare are 60 tons per year 

of SO2, 73 tons per year of CO, 13.4 tons per year of NOx, and 94.5 tons per year of VOC. 

However, the worst-case conditions based on the modeled maximum flare rate of 14.83 MCFD 

and LHV of 682.4 BTU/scf result in emissions of 319.12 tons per year of SO2, 1,015.41 tons per 

year of CO, and 118.54 tons per year of NOx. These very conservative worst-case conditions 

result in emission rates in the analysis that are at least five times greater than current air permit 

limits. In addition, the actual field conditions are not capable of sustaining a flow rate of 14.83 

MCFD. The remainder of this section will show that even with these worst-case emission 

estimates that far exceed the current air permit limits and actual flow rate conditions, the 

modeling results show no exceedances of the 1-hour SO2 NAAQS. 

A DVD of AERMOD model run input and output files, meteorological data and AERMAP files 

is contained in Appendix A. Model runs include estimated SO2 impacts for process gas with a 

heat content of 300, 443, 538.8 and 682.4 BTU/SCF. 
6-2 

6.1 SULFUR DIOXIDE (SO2)

The Main Flare was modeled at the heat content scenarios of 300, 443, 538.8 and 682.4 

BTU/SCF. The maximum modeled impact concentrations from AERMOD occurred with a flare 

heat content of 300 BTU/SCF. However, the results shown in Tables 6-1 and 6-2 for the FLAT 

and ELEV runs, respectively reflect all modeled heat content scenarios as well as discrete 

regional sources. Additionally, the maximum 1-hour background SO2 concentration was 

obtained and added to the maximum modeled SO2 impacts from the Main Flare and regional 

sources to obtain an estimate of cumulative impacts for comparison to the SO2 NAAQS. 

Table 6-1. 

Comparison of Modeled Cumulative SO2 Impacts to the 1-Hour NAAQS - FLAT 

Flare 

Heat 

Content 

Maximum 

Modeled 

Year 

Maximum 

Modeled 

Concentration 

Monitored 

Background 

Concentration 

Maximum 

Cumulative 

Concentration 

National 

Ambient Air 

Quality Std. 

 

Percent of 

NAAQS 

 (ug/m3

) (ug/m3) (ug/m3) (ug/m3) (%) 

 

300 2009 8.57 79 87.57 196.5 44.6 

443 2009 5.91 79 84.91 196.5 43.2 

538.8 2009 4.81 79 83.81 196.5 42.7 

682.4 2009 4.24 79 83.24 196.5 42.4 

Table 6-2. 

Comparison of Modeled Cumulative SO2 Impacts to the 1-Hour NAAQS - ELEV 

Flare 

Heat 

Content 

Maximum 

Modeled 

Year 

Maximum 

Modeled 

Concentration 

Monitored 

Background 

Concentration 

Maximum 

Cumulative 

Concentration 

National 

Ambient Air 

Quality Std. 

 

Percent of 

NAAQS 

 (ug/m3

) (ug/m3) (ug/m3) (ug/m3) (%) 

 

300 2005 26.76 79 105.76 196.5 53.8 

443 2005 19.01 79 98.01 196.5 49.9 

538.8 2005 16.35 79 95.35 196.5 48.5 

682.4 2005 13.46 79 92.46 196.5 47.0 
6-3 

Figure 6-1 presents a concentration contour plot of maximum modeled 1-hour SO2 impacts from 

the 300 BTU/SCF heat content input scenario with AERMOD in the ELEV mode. To illustrate 

modeled impacts as a function of flare gas heat content input, Figure 6-2 presents a 

concentration contour plot of maximum modeled 1-hour SO2 impacts from the 682.4 BTU/SCF 

heat content input scenario with AERMOD in the ELEV mode. 

The predicted maximum 1-hour averaged concentration from the Main Flare and regional 

sources is 26.76 μg/m3

, respectively, located approximately 15.5 kilometers from the flare stack. 

This maximum concentration added to the background concentration of 79 μg/m3

 results in a 

cumulative concentration of 105.76 μg/m3

, 53.8% of the 1-hour averaged NAAQS of 196 μg/m3.
640000 645000 650000 655000 660000 665000 670000 675000

UTM Easting (m)

Zone 12, NAD83

4110000

4115000

4120000

4125000

4130000

4135000

4140000

4145000

UTM Northing (m)Zone 12, NAD83

Figure 6-1

McElmo Creek Gas Plant

Maximum 1-Hour SO2 Modeling Results

Flare at 300 btu/scf

Including Background Sources

Model Year 2005
640000 645000 650000 655000 660000 665000 670000 675000

UTM Easting (m)

Zone 12, NAD83

4110000

4115000

4120000

4125000

4130000

4135000

4140000

4145000

UTM Northing (m)Zone 12, NAD83

Figure 6-2

McElmo Creek Gas Plant

Maximum 1-Hour SO2 Modeling Results

Flare at 682.4 btu/scf

Including Background Sources

Model Year 2005
7-1 

 7.0 REFERENCES

Chemical Manufactturer’s Association, 1983. “A Report on a Flare Efficiency Study”, 

referenced by the Texas Commission on Environmental Quality NSR Division, Chemical 

Section 

Consent Decree, United States of America v. Mobil Exploration & Producing, U.S., Inc., DOJ 

Case No. 90-5-2-1-2237, filed June 8, 2005 in United States District Court, District of 

Utah. 

Texas Commission on Environmental Quality, 2004. “Technical Basis for Flare Parameters”, 

September 10, 2004, Austin, TX.
APPENDIX A 

DVD WITH AERMOD MODEL INPUT & OUTPUT FILES, 

METEOROLOGICAL DATA AND AERMAP FILES