Usage
The Wobbe index is commonly expressed in either British Thermal Units (BTU) per standard cubic foot, or else megajoules per standard cubic meter (1,000 BTU/scf = 37.3 MJ/Sm3). In the case of natural gas (molar mass 17 g/mol), the typical heating value is around 1,050 BTU per cubic foot and the specific gravity is approximately 0.59, giving a typical Wobbe index of 1,367 BTU/scf (51 MJ/m3).
There are three ranges or "families" of fuel gases that have been internationally agreed based on Wobbe index. Family 1 covers manufactured gases, family 2 covers natural gases (with high and low ranges) and family 3 covers liquefied petroleum gas (LPG). Combustion equipment is typically designed to burn a fuel gas within a particular family: hydrogen-rich town gas, natural gas or LPG.
| Family | Type of gas | Wobbe Index range (MJ/Nm3) | Wobbe number range from |
| 1 | Town gas / Syngas | 22.5 – 30 | 24 – 29 |
| 2 L | Natural | 39 – 45 | |
| 2 H | 45.5 – 55 | 48 – 53 | |
| 3 | LPG | 73.5 – 87.5 | 72 – 87 |
| 4 | DME | 51.3 - 54 | 52 – 53 |
Other flame characteristics and composition limits may determine the acceptability of the replacement gas, e.g. flame speed, "yellow tipping" due to incomplete combustion, sulfur content, oxygen content, etc.
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