Gas Interchangeability Assessment
Enter mole percentages. Total must equal 100%.
Reference temperature (60°F) used if equal. Different values affect Modified Wobbe Index.
Understand gas interchangeability, Wobbe Index significance, AGA indices, and gas blending applications
The Wobbe Index (WI) is a measure of gas interchangeability defined as the higher heating value (HHV) divided by the square root of specific gravity: WI = HHV / sqrt(SG). Two gases with the same Wobbe Index will deliver the same thermal input to a burner at the same gas pressure, making it the primary criterion for fuel gas substitution.
Pipeline-quality natural gas typically has a Wobbe Index between 1,310 and 1,390 Btu/scf. The acceptable interchangeability band is usually plus or minus 5% of the target gas Wobbe Index. LNG regasification gas ranges from 1,350 to 1,420 Btu/scf depending on source.
The American Gas Association defines three interchangeability indices: IL (Lifting Index) for flame stability, IF (Flashback Index) for flashback tendency, and IY (Yellow Tip Index) for incomplete combustion. All three must fall within acceptable ranges (typically IL = 0.8-1.2, IF < 1.2, IY < 0.8) for safe gas substitution.
The Modified Wobbe Index accounts for gas temperature by including a temperature ratio: MWI = HHV / sqrt(SG x T_gas/T_ref). It is commonly used for gas turbine fuel specifications where fuel gas temperature varies. Typical gas turbine MWI spec is 40-55 MJ/m3 or equivalent in Btu/scf.