Estimating Air Emissions from Oil and Gas Operations
December 29, 2015

Everyday environmental professionals are estimating emissions from oil and gas production facilities and supplying data that will be used to make decisions that cost companies money. Typically, the estimation is done using a combination of methods as described in this blog. Notably, this data affects the following: 

  • Type of air pollution controls installed. 
  • Air permit type that applies based on emission source or facility total emissions such as a minor source or a major source.
  • Greenhouse gas (GHG) annual reporting requirements such as the mandatory EPA reporting and voluntary industry reporting.
  • Criteria and hazardous air pollutant (HAP) annual reporting requirements to state agencies.



 Why is it important to determine emissions as accurately as possible?

  • Overestimating facility emissions may trigger major source Part 70/Title V air permitting or require mandatory greenhouse gas (GHG) reporting under 40 CFR 98. Also, keeping emissions as low as possible can help a facility qualify for a “general” minor source air permit (e.g., TCEQ PBR). General permits have a much shorter wait time for approval than individual minor source air permits.
  • Underestimating emissions may cause compliance issues if a future required stack test (e.g., NSPS JJJJ for engines) yields a higher emission rate than used in the air permit or required by an applicable standard. Typically, a site specific source/stack test will be more reliable than a published emission factor.
  • Companies operating major sources pay air pollutant emission fees based on actual air  emissions of criteria pollutants and HAP emissions.  Many expect that in the near future fees will be assessed on GHG emissions – possibly for retroactive past GHG emissions. 

Methods Used

Typical methods used to estimate air emissions include:    

  1. Emission factors and equations supplied by a regulatory agency. The U.S. Environmental Protection Agency (EPA) publishes the most widely used source known as AP-42, Compilation of Air Pollutant Emission Factors.
  2. Emission factors and equations required by the USEPA for GHG reporting under 40 CFR Subpart 98.
  3. Emission factors generated by a manufacturer or supplier. Most engine and catalytic converter suppliers will have published emission data for their products.
  4. Source/emission testing. This includes stack testing engines and vent gas measurement and chemical analysis. Many operators prefer direct measurement of vent gas from storage tanks.
  5. Process simulation software from regulatory agencies, trade organizations and facility process design simulators.
  6. Material balance calculations. Use the difference between inlet mass and outlet mass of a product to estimate losses to the atmosphere for volatile chemicals and particulate matter.
  7. Stack testing of engines and enclosed combustors.
  8. Direct measurement by metering gas volumes and gas chemical analysis. The ideal gas law relationships can be used with the direct measurement data to calculate mass of natural gas components vented or combusted.

Emissions data from a source test, process simulator, material balance and metered volumes are usually expressed in units of lbs of pollutant per hour. This data can be converted to a site specific emission factor by expressing the emissions data as the mass of emissions per unit of activity.

For example, HY-BON/EDI’s IQR services measure the volume of gas vented from a storage tank during a 24-hour period, then sample and chemically analyze the vent gas to yield a lbs of VOC per hour rate. Combining this data with the oil production rate (average barrels per day) allows the user to calculate an emission factor based on the lbs of VOC vented per barrel of oil throughput in the storage tank.

Emission Factor Formulas

The general equation for emissions estimation using emission factors is:

E = (A)*(EF)*(1-ER/100)


E = emissions

A = activity rate (e.g., BOPD, amount of fuel burned per hour, hours operated, volume of gas vented)

EF = emission factor (e.g., lbs VOC per standard cubic feet natural gas vented)

ER = overall emission reduction efficiency, % - if emission controls are operating


AP-42 Emission Factor Ratings

AP-42 supplies “Ratings” for emission factors that range from A through E and Unrated, “U”. An “A” rating being “Excellent” considered the most favorable factor. The rating are based on source test data taken from facilities in the industry population. The larger number of samples across a specific source type, the higher the rating.

Due to the large variation in emissions from internal combustion engine and changing emissions standards, AP-42 emission factors may not be the best choice if a stack test will be required.

Potential Issues When Calculating Air Emissions

  • Using the wrong emission factor based on the process or emission source type. For example, using emission factors for a 2-cycle, lean burn spark ignition engine for a 4-cycle, lean burn spark ignition engine based upon a lack of enough data for the engine operating.
  • Changes in published EPA and state regulatory emission factors can require a facility to recalculate emissions and comply with applicable standards based on use of the new/updated emission factor.
  • Changes in facility crude oil, condensate and natural gas chemical makeup based on new well production.
  • For process simulations, use of inaccurate data inputs (e.g., operating pressures and temperatures) or use of data not representative (e.g., gas analyses) of the process or products produced.
  • Applying a site specific emission factor to another facility that is not representative of operating parameters, fuel used or chemical makeup of the crude oil, condensate or natural gas produced.
  • Underestimating or overestimating the emissions reduction from an emission control device.

Air Pollutants calculated 

  • Criteria pollutants: NOx, CO, VOC, SO2, PM2.5, PM10
  • Hazardous air pollutants (HAP): benzene, toluene, ethylbenzene, xylenes, n-hexane
  • Greenhouse gases (GHG): CO2, methane, N2O


 O&G Emission Sources and Emission Factors

Emission Source Type Typical Sources for Emission Factors and Estimation Methods
Amine gas sweetening units AmineCalc; process simulators; direct measurement
Crude oil, condensate, produced water, volatile organic liquids storage tanks AP-42 Chapter 7 and EPA TANKS4 program; process simulators; direct measurement
Crude oil and condensate loading - trucks, barges, ships Ap-42, Chapter 5
Flares and enclosed combustors  AP-42, Chapter 13; state regulatory agencies (e.g., TCEQ)
Fugitive sources from valves, flanges, connections, pump and compressor seals  AP-42, Chapter 5; API Documents 4638, 4589, 4615; 40 CFR 98 Subpart W for GHGs; state agencies
Fugitive dust/particulate matter AP-42, Chapter 13; state regulatory agencies
Glycol dehydrators GRI-GLYCalc; process simulators
Heater treaters, line heaters, reboilers AP-42, Chapter 1
Internal combustion engines - reciprocating, turbines Vendor data; AP-42 Chapter 3; stack testing
Pneumatic pumps, controllers that use natural gas  Vendor data; 40 CFR 98 Subpart W for GHGs
Venting of natural gas from wells and facility processes Metered or calculated volume, gas analysis and ideal gas law equations

Resources for Air Emission Estimation

Below is a list of commonly used resources for emission factors and estimation methods for oil and gas industry stationary sources.

Criteria and Hazardous Air Pollutants

  • USEPA Technology Transfer Network (TTN) includes the CHIEF Clearinghouse for Inventories & Emissions Factors. Under CHIEF can be found emission factors and estimation tools (software) to calculate air emissions.
  • AP-42, Compilation of Air Pollutant Emission Factors, Volume 1: Stationary Point and Area Sources, Fifth Edition
  • TANKS software – based on methods in AP-42 Chapter 7: Liquid Storage Tanks. TANKS only includes standing and working losses – no flashing losses calculated.
  • E&P TANKS (API Publication 4697) by American Petroleum Institute is a software that uses site-specific information to estimate emission from petroleum production storage tanks. Includes standing, working and flash losses.
  • WebFIRE – a database management system containing EPA's recommended emission estimation factors for criteria and hazardous air pollutants
  • GRI-GLYCalc software – program for estimating air emissions from glycol units using triethylene glycol (TEG), diethylene glycol (DEG) or ethylene glycol (EG).
  • Oil and gas process simulators such as Aspen HYSYS,   Bryan Research & Engineering, Inc. ProMAX, Virtual Materials Group VGM.
  • AMINECalc Version 1.0 – Amine Unit Emissions Model (API Publication 4679)

Greenhouse Gases

Concerned About Venting of Natural Gas?

HY-BON/EDI is your expert at vent gas management.  We use our IQR Survey to help O&G operators find leaks, quantify storage tank emissions and rectify the emissions.  Our IQR services and VRUs help a company stay in compliance with their air permits and air emission reporting requirements.

Our products and services can also make you money by recovering valuable vent gas that may be lost to the atmosphere or sent to flare to be burned.

Let HY-BON/EDI take away some of your air pollution concerns so your facilities stay in compliance.