Some people think they’re the same thing, but they’re not.
For a start, you never see Natural Gas in cylinders in Australia. In other parts of the world you do – like NZ, where you can get cars that run on CNG (compressed natural gas), but I’ll say a bit more about this later.
The difference simply is that they are different chemicals. Natural gas, which is the gas sitting on top of crude oil deposits, is a mixture of methane and ethane, whereas LPG is a mixture of butane and propane.
They all belong to the same class of compound – hydrocarbons – the only difference is the size of the molecule.
Methane is CH4, ethane is C2H6, propane is C3H8 and butane is C4H10
The smaller molecules, methane and ethane, whiz around at a greater rate and therefore have a higher pressure, whereas the larger molecules, butane and propane, are slower moving and don’t have as much pressure.
Because the butane and propane don’t have as much pressure, they can be compressed into a liquid. That is, if you pressurise them enough, the molecules start p\bumping into each other, and eventually stick together form liquids – hence “liquefied” petroleum gas (LPG).
The smaller molecules, however, having a much higher pressure (about 10 times as much as the butane and propane), cannot be compressed into liquids unless extremely high pressures are applied, pressures too high to be practical industrially.
The implications of this are that you can get a lot more propane and butane into a cylinder – because it liquefies – then you can for the natural gas. And this is the biggest single complaint of CNG car owners in New Zealand – even with a large cylinder the cars can’t get very far, as you simply can’t get much gas into them. For LPG car owners, the fact that the gas is turned into a liquid means that the cars have a much greater range.
And this applies to home heaters to – whereas most home heaters are designed for natural gas, if you have LPG cylinders you must get a gas heater that is set up for LPG.