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Properties

Typical Fuel Gas Data

The data given below are typical values for the specified substances and, unless otherwise noted, are taken from "The Investigation and Control of Gas Explosions in Buildings and Heating Plant", 1983 (Harris, R). It is important to note that these are typical values and that Natural Gas properties are specified in the Gas Safety (Management) Regulations and Commercial Propane/Butane properties are defined by BS 4250. Wobbe Number is used to define whether a gas is 1st, 2nd or 3rd family and is not quoted here (It can be calculated by dividing the calorific value by the square root of the relative density).

Property

Methane

Propane

Butane

Chemical formula

CH4

C3H8

C4H10

Molecular weight

16

44

58

Relative density*1

0.6

1.5

2.0

Lower flammability limit (% gas in air)

5

2.2

1.9

Upper flammability limit (% gas in air)

15

9.5

8.5

Maximum laminar burning velocity*2 (m/s)

0.45

0.52

0.50

Minimum ignition energy (mJ)

0.29

0.25

0.25

Auto ignition temperature (K)

813

723

678

Net calorific value (MJ/m3)

34

86.4

112.4

Dynamic viscosity*3 (µP.s)

11

8

7

*1 Calculated as a ratio of Molecular weight compared to that of air (taken as 28.8).  

*2 Commonly quoted as flame speed which in practice is different to burning velocity.

*3 Data taken from http://webbook.nist.gov/chemistry/ at 288K and 1 atm.

Key Points

  • The trends below follow similar patterns as the number of carbons are increased, so a prediction could also be made for substances with higher number of carbons (Not for viscosity to due phase differences at atmospheric conditions)
  • Methane is lighter than air so will rise, Propane/Butane are heavier than air so will gather at lower level
  • Propane/Butane have lower flammability limit and so require a smaller proportion of gas to form a flammable mixture
  • Methane has a slower burning velocity. Burning velocity is important to ensure a stable flame is kept on an appliance - if supplied too fast the flame could lift off a burner or if supplied too slowly the flame could light back
  • less energy is required to light a mixture of Propane/Butane compared to Natural Gas
  • Propane/Butane produce more energy per unit volume of fuel burnt (Net values mean the water produced in combustion remains as vapour)
  • Natural gas has a greater resistance to shear or tensile stesses.

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