Santa Barbara County Air Pollution Control District

Technical Information and References

Gaseous Fuel SOx Emission Factor (Version 1.0, 1/31/97)

Gaseous Fuel SOx Emission Factor:

Applicability: External Combustion units such as boilers and process heaters for gaseous fuels (e.g., natural gas, oil field produced gas and propane).

Equations: Two equations are presented. The first is the fundamental equation showing how the emission factor is generated. The second is a reduced form of the basic equation for streamlined use. Finally, a check on the units is shown.


  • EF = SOx emission factor in units of lb/MMBtu (HHV based, as SO2)
  • ppmvd S = total sulfur concentration in fuel (as S)
  • HHV = higher heating value of the fuel (Btu/scf)
  • MM = million
  • mol vol = molar volume of the fuel at standard conditions (1 atm & 60 ?F, equals 379 std ft3/lb-mole)
  • mol ratio = stoichiometric molar ratio for the combustion of sulfur (1 S + 1 O2 1 SO2)

Defaults: Default emission factors can be arrived at by using standard default values for the heating value and sulfur concentrations for each fuel.

ppmvd (as S)
ppmvd (as H2S)
HHV (Btu/scf)
SOx Emission Factor (lb/MMBtu)
PUC Natural Gas
GPA Commercial Propane
GPA HD-5 Propane
Produced Gas - South Zone
Produced Gas - North Zone


  1. PUC Natural Gas: Sulfur concentration based on maximum allowed total sulfur content of 5 gr/100 scf (as S) per General Order 58-A. The calculations below show how the equivalent concentrations are derived (depending on the "basis"):

    {ppmvd as S = (5 gr S/100 scf)*(106 scf fuel/MM scf fuel)*(lb S/7000 gr S)*(379 scf S/lb-mole S)/(32 lb S/lb-mole S) = 85 ppmvd as S}

    {ppmvd as H2S = (5 gr H2S/100 scf)*(106 scf fuel/MM scf fuel)*(lb H2S/7000 gr H2S)*(379 scf H2S/lb-mole H2S)/(34 lb H2S/lb-mole H2S) = 80 ppmvd as H2S}.

    Heating value based on USEPA AP-42, Appendix A (Thermal Equivalents of Various Fuels).


  2. Propane: Sulfur concentration based on Gas Processors Association Engineering Data Book (Ninth Edition, 1972), Figure 15-50 (GPA Liquefied Petroleum Gas Specifications, rev. 1979), Commercial Propane = 15 gr/100 scf, HD-5 Propane = 10 gr/100 scf (both as S). Same equation as listed in (a) above for the ppmvd "as S" calculation. Heating value based on Perry's Chemical Engineers Handbook, Chapter 9, 5th Edition, Table 9-16.


  3. Produced Gas: Sulfur concentration based on APCD Rule 311 - Southern Zone limit of 15 gr/100 scf (as H2S) and Northern Zone limit of 50 gr/100 scf (as H2S). To use in the calculations (which are based on an "as S" basis), these limits are adjusted to an as sulfur (as S) basis by use of the equation in note (a) above. This has the same affect as taking the ratio of the molecular weights (MWS/MWH2S) such that the respective Zone limits are 14.12 gr/100 scf and 47.06 gr/100 scf (as S). Heating value based on USEPA AP-42, Appendix A (Thermal Equivalents of Various Fuels).


  4. Reporting References: Reporting "as H2S" means the total sulfur values are converted to an H2S basis by taking the ratio of the molecular weights (MWS/MWH2S). This is needed to determine compliance with Rule 311 and permit conditions that require reporting "as H2S". For PUC and GPA standards and the emission calculations, sulfur content "as S" is used. "S" in this case is mono-atomic sulfur (MW = 32 lb/lb-mole). When reviewing fuel analyses with di-atomic sulfur species, such as CS2, the amount of sulfur from the compound in question must be doubled to account for the extra mole of sulfur.


  5. Permit Condition Limits and Reporting: Since permits require sulfur content of fuels to be reported "as H2S", the associated limits for non-Rule 311 sulfur concentrations need to be also stated in an "as H2S" basis so as to minimize the confusion of reporting in two ways. As such, for PUC natural gas the standard of 85 ppmvd "as S" is listed in the permit condition as 80 ppmv "as H2S". For GPA propane/LPG, the standard of 254 ppmvd "as S" is listed in the permit condition as 239 ppmvd "as H2S".

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