Wobbe number

The Wobbe number, or Wobbe index, is a description of a fuel gas used in deciding whether that gas can be substituted for another in a particular burner, for example, can propane be used in a kitchen range made to burn natural gas? The Wobbe Index is sometimes called the Interchangability Factor.¹ Symbol, WN or WI.²

The Wobbe number is named for the Italian engineer Goffredo Wobbe, who in 1926 published a paper³ defining a measure of fuel gas "quality", with the idea that fuels of the same quality could be interchangeable. . energy obtained by burning the gas, which is obviously directly proprotional to the gas's heating value. Then the question becomes, how mcuh of the gas will be burned. inversely proportional to the square root of the gases specific gravity, it's density relative to air. pointing out that in a burner fed with fuel gas at a constant pressure flowing through a hole with a fixed size, the heat output is proportional to the flow volume per time.

 through a fixed hole size at constant pressure, the flow velocity is proportional to the specific gravity of the gas


The heating value of a gas is proportional to its specific gravity, its density compared to that of air.

 

In concept, the Wobbe index takes into account not only the volumetric energy value of the gas, but also the rate at which molecules of that gas can be delivered through the burner. The inverse of the root of the relative density, which is a good approximation of the relative mass flow rate of a gas through a fixed orifice at a fixed pressure.

 

 The usefulness of the Wobbe number is that in a burner where gases pass through hole of a fixed size, gas mixtures that have the same Wobbe number will deliver roughly the same amount of heat.

The Wobbe number is calculated by dividing the heating value by the square root of the gas's specific gravity (its density relative to air).

heating value over square root of specific gravity

In the United States, the heating value is measured in Btu per standard cubic foot (scf).

heating value over square root of specific gravity

Differing definitions of an scf exist, but for fuel gases in the U.S., a standard cubic foot is 1 cubic foot of gas at 60°F (15.6°C) and 14.73 pounds per square inch absolute.  In Canada, a pressure of 101.6 kilopascals is specified. It is important to list the temperature and pressure being used as standard, since many standards are used worldwide.

Notice that the scf (and the normal cubic meter Nm³ and standard cubic meter Sm3  described below) are not units of volume. Because they specify temperature and pressure, for any particular gas they are a measure of quantity of matter. As a result, converting standard cubic feet to normal cubic meters is not the same as converting cubic feet to cubic metres (0.0283168) since the standard temperature and pressure used can be different.

In the rest of the world, the heating value is measured in

heating value over square root of specific gravity

The expression Nm³ or Nm3 stands for Normal cubic meter (nothing to do with newtons) A Normal cubic meter is a cubic meter of gas at a temperature of 0°C at 101.325 kilopascals absolute (DIN 1343). A standard cubic meter has the same pressure, but a temperaure of  15deg;C (ISO 13443:1996). (In Canada, for example, a normal cubic meter is a cubic meter at at 15°C at 101.325 kPaA.)

Which set of units is being used is usually obvious from the great difference in the values. For example, using SI units the Wobbe number for methane is 50.7. Pure methane has a Wobbe number of 1363; natural gas as piped to homes in the United States typically has a Wobbe number between 1310 and 1390.

Fuel High heating value
MJ/Sm3
MJ/Nm3 btu/scf  
hydrogen 45.5   1220  
methane 49.9 53.28 1339  
ethane 63.6 68.19 1708  
ethylene   63.82    
natural gas 46-52 53.72 1235-1396  
propane 74.8 81.07 2008  
propylene   77.04    
n-butane 84.4 92.32 2266  
iso-butane   91.96    
propylene   77.04    
pentane 92.5   2482  
butylene-1   88.46    
LPG   86.64    
acetylene 57.0   1531  
carbon monoxide        
LNG 52.8   1417  

The Wobbe number is sometimes said to be dimensionless, but it is not, as a glance at the fraction shows. The denominator is dimensionless. The numerator, however, has the same dimensions as heating value, and divided by one, retains those dimensions. Nonetheless a Wobbe number is often given without units, because:
a) People tend to confuse the Wobbe number with heating value. Showing the units encourages such confusion.
b) The only use of the Wobbe number of a gas is to see if it is bigger or smaller than the Wobbe numbers of other gases, calculated in the same way. In this comparison, the units used in the original measurements are irrelevant.

The Wobbe number is sometimes given as a pair of numbers, the first based on the high heating value of the gas, and the second on its low heating value. The high heating value includes the heat recovered by condensing the products of combustion, for example, in a condensing home furnace. The low heating value does not include the energy lost with the products of combustion, for example, in an oven in a kitchen range.

1. NGC+ Interchangeability Work Group.
White Paper on Natural Gas Interchangeability and Non-Combustion End Use. February 28, 2005, page 7.

2. International Gas Union and BP.
Guidebook to Gas Interchangeability and Gas Quality. 2011.
symbol, page 146.

3. Goffredo Wobbe.
La definizione della qualità del gas.
L'industria del gas e degli acquedotti, vol 15, no. 11, Rome, 30 November 1926.
Pages 165-172.

ASTM D 1945 (American Gas Association Bulletin No. 36)

Alternatives to the Wobbe number include the Weaver [email protected] and the AGA Bulletin 36 index.

@. Elmer R. Weaver.
Research Paper 2193
Formulas and Graphs for Representing the Interchangeability of Fuel Gases.
Journal of Research of the National Bureau of Standards, Vol. 46, No. 3, March 1951.

Examples of use

Sweden gets much of its fuel gas from the Netherlands. It is also very interested in encouraging local production of biogas. The Swedish biogas has a much lower Wobbe number than the Dutch natural gas, and so propane is added to the biogas to increase its Wobbe number to approximate that of the Dutch gas.

T. Q. Frandsen, L. Rodhe, A. Baky, M. Edström, I. Sipilä, ??, K., S.L. Petersen, K. Tybirk.
Best Available Technologies for Pig Manure Biogas Plants in the Baltic Sea Region.
Stockholm: Baltic Sea 2020, 2011.

Modified Wobbe Number

General Electric, a major manufacturer of gas turbines, used a modified Wobbe number in recommending fuels for its turbines. in addition to heating value and density, took into account temperature. the specific gravity is multppiled by the temperature, before taking the sqaure root. and lower heaing value.is the square root of the specific garvity  tenperature in kelvin

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