Product 3

Liquefied Petroleum Gas

LPG is a mixture of light hydrocarbons which are gaseous at normal temperatures and pressures, and which liquefy readily at moderate pressures or reduced temperature. It is odourless and so, for safety reasons, a pungent compound, ethyl mercaptan, is added to make any leaks easily detectable.

The main component gases of LPG are:

  • Propane (C3H8)
  • Butane (C4H10)

These gases undergo the following reactions during combustion:

Propane: C3H8 + 5O2 -> 3CO2 + 4H2O,
Butane: 2C4H10 + 13O2 -> 8CO2 + 10H2O,

LPG occurs naturally in crude oil and natural gas production fields and is also produced in the oil refining process.


Main users are residential (cooking and heating), commercial/industrial (as fuel), autogas (instead of petrol) and petrochemical (as raw material).


Propane Butane
Molecular weight 44.1 58.12
Explosive range in air 2.1 – 9.5 % vol 1.6 – 8.5 % vol
Boiling point – 42.1oC – 0.5 oC
Freezing point – 187.8 oC – 138 oC
Gross energy per unit volume MJ/L 25.5 28.7
Density @ 15 oC kg/L 0.510 0.580
Litres per tonne 1960 1720
Motor octane number (MON) 95.4 89
Research octane number (RON) 100 92

The Mix

Motor vehicles run on propane or a mixture of propane and butane. Propane and butane are gases at atmospheric pressure and temperature but can easily be liquefied for storage by an increase in pressure. The most common blend by volume is 60 per cent propane to 40 per cent butane. Industry uses propane or butane and the petrochemical industry may use both as a feedstock.


Transport Fuel

The motor-vehicle sector continues to be the fastest-growing market for LPG in the world. LPG consumption and automotive use has been growing at a rate of about 17 per cent per year. This growth has been largely due to the tax-free status of LPG in the world, which results in a significant pump price differential compared to petrol. LPG vehicle conversion kits are also exempt from sales-tax.


The Environment

There have been several studies of the effects of LPG on the environment, particularly automotive LPG. Recent studies of car emissions have shown that when compared to unleaded petrol, LPG emissions for oxides of nitrogen and carbon monoxide were slightly higher in well tuned vehicles. Evaporative emissions of hydrocarbons are virtually zero. It does not need lead or other additives to boost its otane rating.

It is therefore fair to say that LPG has the potential to be environmentally friendly provided equipment and engines are installed and maintained correctly. This is particularly true of vehicles powered by LPG only rather than being set up for dual fuel operation.



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