LPG (liquefied petroleum gas)
by Chris Woodford. Last updated: March 29, 2011.
Natural gas is one of the world's favorite fuels, but—in its usual piped
form—you can't use it just anywhere. You can't pipe gas into a car to power the engine or take it with you when you go hiking in the mountains. Nor can you
can run a gas pipe out to a boat in the middle of the ocean. And if
you live somewhere reasonably rural, you probably can't even have gas at home.
But there's a very simple way to enjoy the benefits of gas even when normal piped supplies are out of reach: you can convert
to LPG (liquefied petroleum gas)—a really convenient, super-pressurized
gas stored in liquid form in a tank, canister, or bottle. Let's find out more about how it works!
Photo: A typical large LPG tank outside a building. Tanks like this are easy to move from place to place. Conversions to LPG are relatively simple, though bring extra safety rules and regulations. Photo by courtesy of Clean Fueling Technologies and US Department of Energy (DOE) Digital Photo Archive.
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 two thirds of the LPG people use is extracted directly
from the Earth in the same way as ordinary natural gas. The rest is
manufactured indirectly from petroleum (crude oil) drilled from the
Earth in wells in the usual way.
Photo: Petroleum can be extracted from the ground by "nodding donkey" pumps like this one. Some of it is then turned into LPG. Picture courtesy of US Department of Energy (DOE) Digital Photo Archive.
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. Exactly which of
the gases is present in LPG depends on where it
comes from, how it is supplied, and what it is being used for; often
LPG contains a mixture of both gases. Butane tends to be used more
for small, portable LPG supplies in such things as boats and
gas-powered barbecue stoves. Larger household tanks are more likely
to contain propane. Tiny quantities of other gases are also naturally
present in LPG. Since LPG is normally odorless, small amounts of a pungent gas
such as ethanethiol (also known as ethyl mercaptan) are added to help people smell potentially dangerous gas leaks, which might otherwise go undetected.
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.
Artwork: LPG is compressed into 274 times less space than its gases would normally occupy, forcing it to turn into a liquid.
Using LPG
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.
Photo: LPG-powered buses like this are helping to cut emissions at Zion National Park, Utah. Photo by courtesy of US National Park Service and US Department of Energy (DOE) Digital Photo Archive.
The rest of the LPG people use (the other 50 percent) is split roughly equally
between cars and industrial 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. Something like 6 million European vehicles (mostly
low-emission buses and cars) run on LPG and there are about 17,500
gas stations in Europe supplying the fuel.
What are the pros and cons?
The main reason for converting to LPG is its superb portability and
convenience: it can be used in remote places where ordinary gas supplies are unavailable. That
doesn't just mean rural homes: large LPG canisters are extremely
useful in disasters and emergencies where supplies of electricity and
gas have either been interrupted or never existed in the first place.
Although LPG is a fossil fuel, it's relatively clean compared to such
things as coal and oil (it makes less air pollution—fewer soot
particulates, nitrogen oxides, and sulfur) and produces fewer
emissions of carbon dioxide (which cause global warming).
Photo: Portable cooking: This cylinder of LPG propane was used as
a temporary cooker by Georgian refugees living in emergency housing in August 2008. Note
the cooking ring mounted just on top. Picture by Jason T. Bailey courtesy of US Navy.
The two big disadvantages of LPG are safety and cost. Keeping a gas
pressurized at 274 times less space than it would normally occupy requires
extremely sturdy metal tanks and it's hardly surprising that LPG
containers do occasionally explode. Explosions of gas canisters on
building sites are far from uncommon, for example. In theory, since
LPG is highly flammable, transporting it by road and storing it in
large tanks in populated areas should be much more dangerous than
piping natural gas underground. Having said that, LPG suppliers go to
great lengths to ensure safety and LPG is generally recognized as
being just as safe, all-round, as an ordinary natural gas supply.
Cost is another big drawback: LPG is typically several times more expensive than ordinary natural gas.
But if you can't get piped gas, and LPG is your only option, maybe that's not really such an issue.
Further reading
On this website
Other helpful websites
- USA: Use Propane: Clear, informative, and reliable consumer advice site operated by the Propane Education and Research Council (PERC).
- UK: UKLPG: The British trade association for the LPG industry has lots of reliable, detailed information about switching to LPG for different applications (at home, at work, and in your vehicle).
- USA: Fuel economy information: Propane (liquefied petroleum gas): The US Government's Fuel Economy website explains the pros and cons of running your car on LPG.
- UK: Drive LPG: A British site designed for motorists who have LPG or want to convert. Includes a calculator to help you figure out potential savings and a list of official LPG gas (filling) stations.
Books
- Alternative Energy for Dummies by Rik DeGunther. Dummies, 2009. A very broad look at all kinds of alternative and renewable energy, including chapters on electric cars, LPG vehicles, and so on. In other words, not just a book about LPG!
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