What is LPG?

The basic idea of LPG is simple. If you’re far from a gas main (the ordinary system of natural gas supplied to buildings through a network of pipes), simply get your gas from a large fixed tank or a portable canister or bottle. As its name suggests, LPG is a fossil fuel closely linked to oil. About 60 percent of the LPG people use is extracted directly from the Earth as a byproduct of drilling for ordinary natural gas and oil. The rest is manufactured indirectly from petroleum (crude oil) during refining, and a tiny but growing amount (known as BioLPG or renewable LPG) is also produced from biomass or waste. [1]

Two side-by-side green LPG propane tanks.

Photo: These two large propane tanks supply LPG to a remote cliffside building in Dorset, England, far from the main gas supply network.

Propane and Butane

Chemically, LPG is a mixture of two flammable, but nontoxic gases called propane and butane. Both of these are hydrocarbons (their molecules are made from different combinations of hydrogen and carbon atoms): propane molecules (C3H8) have eight hydrogen atoms attached to three carbon atoms, while butane molecules (C4H10) have ten hydrogen atoms bonded to four carbon atoms. LPG sometimes contains a variation of butane called isobutane, which has the same component atoms (four carbons and ten hydrogens) connected together in a slightly different way.

Molecular structure of propane, C3H8, and butane,C4H10.

How is LPG different from LNG?
LPG is not the same thing as LNG (liquified natural gas). Natural gas is mostly methane (with small amounts of various other gases mixed in), and LNG is essentially just a compressed, liquified version of that. As we’ve just seen, LPG is mostly propane and butane.

From Liquid to Gas… and back again

If you could see inside an LPG tank or bottle, you’d see a liquid not a gas. That’s because the propane and butane have been compressed so they take up something like 274 times less space than normal. (By comparison, the air in a typical car tire is pressurized to roughly 2–3 times normal air pressure—so the gas in an LPG tank is squeezed about 100 times more!) Like lowering its temperature, compressing a gas (increasing its pressure) eventually turns it into a liquid. Compressed in this way, LPG takes up relatively little space, so those big LPG tanks you see next to people’s homes actually contain far more “gas” (in liquid form) than you might suppose. In the same way, even a tiny canister of camping gas (slightly bigger than a jam jar) contains a surprising amount of energy for your cooking. When you use LPG, it’s released slowly and safely from the container through a valve and, at that point, turns back into a gas. In that form, it’s just like natural gas: it’s a fuel rich in energy that you can burn to release heat for cooking, heating, or powering something like a car engine.

Using LPG

According to the US Energy Information Administration, about 5 percent of US homes use propane as their main fuel source for home heating, and (for obvious reasons) mainly during winter. [5] In wintertime, roughly half the LPG people use is consumed in homes for heating and cooking (in other words, instead of piped natural gas). Typically, this kind of LPG is delivered by road to a large tank placed safely outside a home or other building (for safety reasons). It’s also possible to buy reusable gas canisters (from such companies as Calor in the UK) for powering standalone stoves, heaters, barbecues, and outdoor patio heaters (now frowned upon because of the energy they waste to the open air). Tiny LPG canisters are also widely used in portable hair-care appliances, such as hair-curling tongs.

The rest of the LPG people use (the other 50 percent) is split between cars, industrial, agricultural, and commercial uses (such as gas burners on construction sites). Although LPG provides less than 2 percent of the total energy people use, it’s still one of the most important alternatives to gasoline. (Used this way, it’s sometimes called autogas.) [6] There are roughly 27 million vehicles worldwide running on LPG (including 15 million in Europe and 6 million in Asia), with about 76,000 gas filling stations worldwide (including 46,000 in Europe and 10,000 in Asia) supplying the fuel. [7]