Q828. Can facilities covered by the Subpart W Onshore Petroleum and Natural Gas Gathering and Boosting industry segment use Subpart C emission factors for calculating combustion emissions?

A828. Combustion emissions for the Onshore Petroleum and Natural Gas Gathering and Boosting segment are calculated using methods outlined in 40 CFR 98.233(z).

For years 2024 and prior, reporters have the flexibility to use any Tier of subpart C methodologies to calculate emissions for combustion of natural gas that is pipeline quality and has a minimum high heat value (HHV) of 950 Btu per standard cubic foot. If the fuel meets those criteria, then you may use the CO2 emission factor for natural gas as listed insubpart C, Table C-1 and the CH4 and N2O emission factors listed in subpart C, Table C-2.  See 40 CFR 98.233(z)(1). The methods in 40 CFR 98.233(z)(2) (i.e., Equation W-39A for CO2, either Equation W-39B or Equation W-40 for CH4, and Equation W-40 for N2O) must be used for all other natural gas fuels.

In 2024, flexibility to use Tier 2, 3, and 4 methodologies of subpart C was added to combusted fuels that are not pipeline quality but meet the definition of “natural gas” in 40 CFR 98.238, have a minimum HHV of at least 950 Btu/scf, a maximum HHV less than 1,100 Btu/scf, and a minimum CH4 content of 70 percent by volume. If the fuel meets those criteria, then you may use the CO2 emission factor for natural gas as listed in 40 CFR Part 98, Subpart C, Table C-1 and the CH4 and N2O emission factors listed in subpart C, Table C-2.  See 40 CFR 98.233(z)(1)(iii). The methods in 40 CFR 98.233(z)(2) must be used for all other natural gas fuels, including field gas and process vent gas.

For RY 2025, 40 CFR(z)(1)(iii) was moved into 40 CFR 98.233(z)(2), and the old 40 CFR 98.233(z)(2) was moved into a new 40 CFR 98.233(z)(3) category.  The procedures for calculating CO2 and N2O did not change; however, the CH4 calculation procedures changed to no longer allow the use of the subpart C, Table C-2 CH4 emission factor for reciprocating internal combustion engine (RICE) and gas turbine (GT) combustion units.

 For both 40 CFR 98.233(z)(1) and (z)(2) if the combustion unit is a RICE or GT, a CH4 emission factor must be determined using one of the methods outlined in CFR 98.233(z)(4).  The new emission factor may be determined using performance testing, OEM data, or a new emission factor from 40 CFR Part 98, Subpart W, Table W-7 which is based on the engine type (i.e. 2-stroke rich burn).  

For 40 CFR 98.233(z)(3) if the combustion unit is a RICE or GT, the CH4 emissions must be quantified by either Equation W-39B using the mole fraction of CH4 in the fuel, or, a by conducting a performance test to determine a CH4 emission factor that is subsequently used in Equation W-40.