Wobbe index
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TheWobbe index(WI) orWobbe number[1]is an indicator of the interchangeability offuel gasessuch asnatural gas,liquefied petroleum gas(LPG), andtown gasand is frequently defined in the specifications of gas supply and transport utilities.
Ifis thehigher heating value,or highercalorific value,andis thespecific gravity,the Wobbe index,,is defined as:
is the density of the gas atstandard conditions,thedefinitionof which changed in 1982. Published Wobbe data may be using 0 °C, 15 °C, 15.56 °C, 20 °C or 25 °C. EU directives on gas quality use 15 °C in accordance withISO 13443andISO 6976.
is the density of air at standard conditions,is themolar massof the gas andis the molar mass of air which is about 28.96 kg/kmol. . The Wobbe index is used to compare the combustion energy output of different composition fuel gases in an appliance (fire, cooker etc.). If two fuels have identical Wobbe indices then for given pressure and valve settings the energy output will also be identical. Typically variations of up to 5% are allowed as these would not be noticeable to the consumer.
The Wobbe index is a critical factor to minimise the impact of the changeover when analyzing the use ofsubstitute natural gas(SNG) fuels such aspropane-airmixtures. The Wobbe index also requires the addition of propane to some upgraded biomethane products,[2]particularly in regions where natural gas has a high calorific value such as Sweden.[3]
The Wobbe index has its origins in the 1920's withItalianphysicist and engineer Goffredo Wobbe.[4]
Note that these Wobbe numbers below are not calculated at 15 °C and so are not correct according toISO 13443,seeStandard temperature and pressure.
Fuel gas | Upper index kcal/Nm³ |
Lower index kcal/Nm³ |
Upper index MJ/m³ |
Lower index MJ/m³ |
---|---|---|---|---|
Hydrogen | 11,528 | 9,715 | 48.23 | 40.65 |
Methane | 12,735 | 11,452 | 53.28 | 47.91 |
Ethane | 16,298 | 14,931 | 68.19 | 62.47 |
Ethylene | 15,253 | 14,344 | 63.82 | 60.01 |
Natural gas | 12,837 | 11,597 | 53.71 | 48.52 |
Propane | 19,376 | 17,817 | 81.07 | 74.54 |
Propylene | 18,413 | 17,180 | 77.04 | 71.88 |
n-butane | 22,066 | 20,336 | 92.32 | 85.08 |
Iso-butane | 21,980 | 20,247 | 91.96 | 84.71 |
Butylene-1 | 21,142 | 19,728 | 88.46 | 82.54 |
LPG | 20,755 | 19,106 | 86.84 | 79.94 |
Acetylene | 14,655 | 14,141 | 61.32 | 59.16 |
Carbon monoxide | 3,060 | 3,060 | 12.80 | 12.80 |
Usage
[edit]The Wobbe index is expressed inMJ/Nm³(where 'Nm³' indicates'm³ in Normal conditions), or sometimes inBTU/scf.In the case of natural gas (molar mass17 g/mol), the typical heating value is around 39 MJ/Nm³ (1,050 BTU/scf) and the specific gravity is approximately 0.59, giving a typical Wobbe index of 51 MJ/Nm³ (1,367 BTU/scf).
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[1] |
---|---|---|---|
1 | Town gas/Syngas | 22.5 – 30.0 | 24.0 – 29.0 |
2 L | Natural | 39.0 – 45.0 | |
2 H | 45.5 – 55.0 | 48.0 – 53.0 | |
3 | LPG | 73.5 – 87.5 | 72.0 – 87.0 |
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.
Limitations
[edit]In spite of its usefulness, Wobbe index alone is not a good indicator of the interchangeability of two or more gases, or mixtures of them. It is necessary to bear in mind other criteria while determining the plenty substitution of a fuel by other, different of the one used to adjust the burning system.[6]
References
[edit]- "The Wobbe Index and Natural Gas Interchangeability"(PDF).Emerson Process Management. 2007-07-30.
- ^abTreloar, R.D. (2005).Gas Installation Technology.Blackwell. p.24.ISBN978-1-4051-1880-4.
- ^"Bio Gas max"(PDF).Archived fromthe original(PDF)on 2015-09-23.Retrieved2012-09-07.
- ^Ingemar Gunnarsson
- ^"American Gas Association"(PDF).Archived fromthe original(PDF)on 2021-12-29.Retrieved2022-03-01.
- ^Wobbe Index and Fuel Gases
- ^Flórez-Orrego, Daniel, Escola Politécnica da USP."Fuel Gas Interchangeability".Retrieved8 Sep2012.
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:CS1 maint: multiple names: authors list (link)