Air emission inventories gather data from facilities so that air emissions can be quantified. Oil and gas (O&G) operators prepare air emission inventories for their facilities for a variety of reasons. These include the following:

  • Air permitting for new facilities and modifications to existing facilities
  • Annual criteria air pollutants and hazardous air pollutants (HAP) inventories for major sources
  • Greenhouse gas (GHG) inventories


Air Permit Applications Emission Inventories

Air permit applications/registrations require some estimation of emissions for the covered activities. The scope of these inventories includes the following:

  • For oil and gas production facility permitting, the emissions inventory is for permanent, stationary emission sources that burn natural gas or diesel fuel and for emission sources that vent natural gas or crude/condensate vapors. Permits may be required for temporary sources.
  • Uses potential to emit (PTE) emissions based on maximum expected hourly and annual emissions. Many States allow an O&G facility to limit their PTE if the emission control method, work practice or production limit is in an enforceable air permit.
  • Mass emission rate (e.g., average and maximum lbs/hour) and annual mass amount (e.g., tons/year) are calculated for each air pollutant.
  • Calculations based on emission factors (e.g., manufacturer, EPA’s AP-42, stack testing, industry/agency accepted) and process simulation models (e.g., GLYCalc, AMINECalc, HYSYS, ProMax).
  • Facility wide and equipment specific emissions data is used to determine the applicability and compliance with regulations (e.g., Title V air permitting, NSPS OOOO/OOOOa)
  • Pollutants included in air permit inventories include criteria pollutants, HAPs and GHGs.
  • A copy of the emission calculations used are included in the air permit application.


Annual Emission Inventories (Criteria, HAPs)

State agencies require annual emission inventories for major source facilities with a Title V permit, PSD permit or located in a  nonattainment area or areas adjoining a nonattainment area. These emission inventories estimate “actual” emissions for the reporting period.

The reasons for conduct an air emission inventory include the following:

  • Equipment included are those that burn a fuel (e.g., natural gas, diesel) and those that vent natural gas or crude oil/condensate vapors.
  • Inventories can include criteria air pollutants and/or hazardous air pollutants (HAP) depending on the applicable regulations.
  • Facilities track “activity data” during the calendar year covered. Activity data includes hours operated, product throughput, gas flared or vented, volume of fuel used and operating parameters.
  • Calculations are based on the activity data for the calendar year covered.
  • Calculations based on emission factors (e.g., manufacturer, EPA’s AP-42, stack testing, industry/agency accepted) and process simulation models (e.g., GLYCalc, AMINECalc, HYSYS, ProMax).
  • Mass emission rate (e.g., average and maximum lbs/hour) and annual mass amount (e.g., tons/year) are calculated for each air pollutant.
  • Many State regulatory agencies require data be submitted in an agency set electronic format.
  • State regulatory agencies vary on whether sample emission calculations used are required to be submitted with the supplied as a part of the report.
  • Many States assess fees on reported air emissions based dollars per ton of actual emissions.


Greenhouse Gas Inventories

Greenhouse gas (GHG) emission inventories may be prepared to comply with EPA regulations in 40 CFR 98, in-house corporate requirements and voluntary GHG reporting actions.

  • Greenhouse gas reporting is required by the U.S. EPA under 40 CFR 98 Subpart W for onshore oil and gas facilities and Subpart C and W for offshore (Gulf of Mexico) production platforms.
  • Equipment included are those that burn a fuel (e.g., natural gas, diesel) and those that vent natural gas (e.g., methane).
  • The rules in Subparts C and W are very prescriptive and specify the emission factors, equations and data to be used for the emission calculations. Metric units are used to report the mass amounts of GHGs.
  • Emission calculations are based on actual activity data (e.g., hours operated, product throughput, gas vented and flared, volume of fuel used and operating parameters)
  • Onshore production and gathering facilities report under 40 CFR 98 Subpart W based on aggregated emissions for the reporting “Basin.” If Basin totals for the aggregated facilities equal or exceed 25,000 metric tons CO2e per year, then an annual report is required using the e-GGRT
  • Offshore (Gulf of Mexico) platforms must report emissions based on 40 CFR 98 Subparts C and W requirements. Annual reporting required if have 25,000 metric tons CO2e or more per year from a platform (or group of platforms connected by a bridge). Annual reporting also used the e-GGRT system.


Offshore Gulf of Mexico Criteria Air Pollutant Emission Inventories

  • The Bureau of Ocean Energy Management (BOEM) requires their Air Quality Reporting (AQR) spreadsheet be used for calculating emissions. These calculations are used to permit drilling and production platform emissions operating in the Central and Western Gulf of Mexico.
  • On a 3-year cycle, the BOEM requires the annual emission inventory for Gulf of Mexico operations. For the 2021 reporting year, the new online OCS AQS system will be used. .
  • Equipment included are those that burn a fuel (e.g., natural gas, diesel) and those that vent natural gas or crude oil/condensate vapors.
  • Similar to other emission inventories, GOADS uses activity data summarized for each month of the reporting year.
  • The operator submits to the BOEM the emission inventory descriptive data and activity data in the GOADS formatted database and the BOEM calculates emissions based on this data.


Emission Inventory Data Needs

Typical descriptive/static data needed include:

  • Latitude and longitude of facility and exhaust stack or vent
  • Emission source name and Identification number
  • Manufacturer, make, model
  • Emission factor data
  • Rated capacity (e.g., horsepower for engines, MMBTU/hr for heaters, SCF/day for flares/enclosed combustors)
  • Stack exhaust/vent parameters – height, inner diameter, temperature, exit velocity
  • Storage tank product stored, throughput, color, dimensions, volume, operating pressure/temperature, etc.
  • Fuel type (natural gas, diesel)
  • Rated amount of fuel usage
  • Operating pressure and temperature
  • Emission controls used
  • Representative samples/chemical analyses for fuel gas, vent gas, flare gas, and flash gas

Typical activity data needed include:

  • Hours operated
  • Throughput of natural gas or crude oil/condensate
  • Volume of natural gas and crude oil/condensate vapors vented to the atmosphere or flared
  • Volume of fuel used
  • Actual operating parameters (e.g., pressure, temperature, circulation rate)


Emission Inventory Steps

  1. Specify type of emissions inventory.
  2. Gather descriptive/static data for each of the equipment that have the potential for emitting air pollutants. Use standardized forms to ensure all of the needed data is gathered.
  3. Determine expected activity data for each emission source.
  4. Determine appropriate emission factors or process simulation models to use for calculations.
  5. Calculate emissions and summarize emissions for equipment and facility.


O&G Air Pollutants of Concern

  • Criteria pollutants: NOx, CO, VOCs, SO2, PM2.5/PM10
  • Hazardous air pollutants: benzene; toluene; ethylbenzene; xylenes; 2,2,4-trimethylpentane; n-hexane; acetaldehyde; formaldehyde
  • Greenhouse gases: CO2, CH4, N2O


Cimarron Services and Products

Cimarron can assess your existing vapor control systems for compliance with Federal (NSPS OOOO/OOOOa) and State air quality rules using our IQR Emissions Services. Our IQR team members are ready for the following:

  • Monitor for leaks from vapor (vent gas) control systems (tank hatch, closed vent systems).
  • Determine adequacy of existing vent gas systems including vapor recovery units and enclosed combustors.
  • Based on site visits and measurements, supply data that can be used to prepare a Modeling Guideline according to the requirements of Appendix B of the EPA New Owner Clean Air Act Audit Program for Oil and Gas

As a part of our IQR/LDAR services we can conduct the required leak monitoring for NSPS OOOOa.

Our best in class Vapor Recovery Units, Vapor Recovery Towers, and Vapor Combustion Units can minimize emissions so production is optimized with minimal regulatory burdens.

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