Liquefied petroleum gas or liquid petroleum gas, also referred to as simply propane or
butane, is a flammable mixture of hydrocarbon gases used as a fuel in heating appliances,
cooking equipment, and vehicles. It is increasingly used as an aerosol propellant
and a refrigerant, replacing chlorofluorocarbons in an effort to reduce damage to the ozone
layer. When specifically used as a vehicle fuel it is often referred to as autogas.
Varieties of LPG bought and sold include mixes that are primarily propane (C
3H 8), primarily butane (C
4H 10) and, most commonly, mixes including both
propane and butane. In winter, the mixes contain more propane, while in summer, they contain
more butane. In the United States, primarily two grades of LPG are sold: commercial propane
and HD-5. These specifications are published by the Gas Processors Association and the
American Society of Testing and Materials. Propane/butane blends are also listed in these
specifications. Propylene, butylenes and various other hydrocarbons
are usually also present in small concentrations. HD-5 limits the amount of propylene that can
be placed in LPG to 5%, and is utilized as an autogas specification. A powerful odorant,
ethanethiol, is added so that leaks can be detected easily. The international standard
is EN 589. In the United States, tetrahydrothiophene or amyl mercaptan are also approved odorants,
although neither is currently being utilized. LPG is prepared by refining petroleum or "wet"
natural gas, and is almost entirely derived from fossil fuel sources, being manufactured
during the refining of petroleum, or extracted from petroleum or natural gas streams as they
emerge from the ground. It was first produced in 1910 by Dr. Walter Snelling, and the first
commercial products appeared in 1912. It currently provides about 3% of all energy consumed,
and burns relatively cleanly with no soot and very few sulfur emissions. As it is a
gas, it does not pose ground or water pollution hazards, but it can cause air pollution. LPG
has a typical specific calorific value of 46.1 MJ/kg compared with 42.5 MJ/kg for
fuel oil and 43.5 MJ/kg for premium grade petrol. However, its energy density per volume
unit of 26 MJ/L is lower than either that of petrol or fuel oil, as its relative density
is lower. As its boiling point is below room temperature,
LPG will evaporate quickly at normal temperatures and pressures and is usually supplied in pressurised
steel vessels. They are typically filled to 80–85% of their capacity to allow for thermal
expansion of the contained liquid. The ratio between the volumes of the vaporized gas and
the liquefied gas varies depending on composition, pressure, and temperature, but is typically
around 250:1. The pressure at which LPG becomes liquid, called its vapour pressure, likewise
varies depending on composition and temperature; for example, it is approximately 220 kilopascals
for pure butane at 20 °C, and approximately 2,200 kilopascals for pure propane at 55 °C.
LPG is heavier than air, unlike natural gas, and thus will flow along floors and tend to
settle in low spots, such as basements. There are two main dangers from this. The first
is a possible explosion if the mixture of LPG and air is within the explosive limits
and there is an ignition source. The second is suffocation due to LPG displacing air,
causing a decrease in oxygen concentration. In addition, an odorant is mixed with LPG
used for fuel purposes so that leaks can be detected more easily.
Large amounts of LPG can be stored in bulk cylinders and can be buried underground.
Uses Rural heating
Predominantly in Europe and rural parts of many countries, LPG can provide an alternative
to electricity and heating oil. LPG is most often used where there is no access to piped
natural gas. LPG can be used as a power source for combined
heat and power technologies. CHP is the process of generating both electrical power and useful
heat from a single fuel source. This technology has allowed LPG to be used not just as fuel
for heating and cooking, but also for de-centralised generation of electricity.
LPG can be stored in a variety of ways. LPG, as with other fossil fuels, can be combined
with renewable power sources to provide greater reliability while still achieving some reduction
in CO2 emissions. Motor fuel
When LPG is used to fuel internal combustion engines, it is often referred to as autogas
or auto propane. In some countries, it has been used since the 1940s as a petrol alternative
for spark ignition engines. In some countries, there are additives in the liquid that extend
engine life and the ratio of butane to propane is kept quite precise in fuel LPG. Two recent
studies have examined LPG-fuel-oil fuel mixes and found that smoke emissions and fuel consumption
are reduced but hydrocarbon emissions are increased. The studies were split on CO emissions,
with one finding significant increases, and the other finding slight increases at low
engine load but a considerable decrease at high engine load. Its advantage is that it
is non-toxic, non-corrosive and free of tetraethyllead or any additives, and has a high octane rating.
It burns more cleanly than petrol or fuel-oil and is especially free of the particulates
present in the latter. LPG has a lower energy density than either
petrol or fuel-oil, so the equivalent fuel consumption is higher. Many governments impose
less tax on LPG than on petrol or fuel-oil, which helps offset the greater consumption
of LPG than of petrol or fuel-oil. However, in many European countries this tax break
is often compensated by a much higher annual road tax on cars using LPG than on cars using
petrol or fuel-oil. Propane is the third most widely used motor fuel in the world. 2008
estimates are that over 13 million vehicles are fueled by propane gas worldwide. Over
20 million tonnes are used annually as a vehicle fuel.
Not all automobile engines are suitable for use with LPG as a fuel. LPG provides less
upper cylinder lubrication than petrol or diesel, so LPG-fueled engines are more prone
to valve wear if they are not suitably modified. Many modern common rail diesel engines respond
well to LPG use as a supplementary fuel. This is where LPG is used as fuel as well as diesel.
Systems are now available that integrate with OEM engine management systems.
Refrigeration LPG is instrumental in providing off-the-grid
refrigeration, usually by means of a gas absorption refrigerator.
Blended of pure, dry propane and isobutane the blend "R-290a" has negligible ozone depletion
potential and very low global warming potential and can serve as a functional replacement
for R-12, R-22, R-134a and other chlorofluorocarbon or hydrofluorocarbon refrigerants in conventional
stationary refrigeration and air conditioning systems.
Such substitution is widely prohibited or discouraged in motor vehicle air conditioning
systems, on the grounds that using flammable hydrocarbons in systems originally designed
to carry non-flammable refrigerant presents a significant risk of fire or explosion.
Vendors and advocates of hydrocarbon refrigerants argue against such bans on the grounds that
there have been very few such incidents relative to the number of vehicle air conditioning
systems filled with hydrocarbons. One particular test, conducted by a professor at the University
of New South Wales, unintentionally tested the worst-case scenario of a sudden and complete
refrigerant expulsion into the passenger compartment followed by subsequent ignition. He and several
others in the car sustained minor burns to their face, ears, and hands, and several observers
received lacerations from the burst glass of the front passenger window. No one was
seriously injured. Cooking
LPG is used for cooking in many countries for economic reasons, for convenience or because
it is the preferred fuel source. According to the 2011 census of India, 33.6
million Indian households used LPG as cooking fuel in 2011, which is supplied to their homes
either in pressurised cylinders or through pipes. LPG is subsidised by the government
in India. Increase in LPG prices has been a politically sensitive matter in India as
it potentially affects the urban middle class voting pattern.
LPG was once a popular cooking fuel in Hong Kong; however, the continued expansion of
town gas to buildings has reduced LPG usage to less than 24% of residential units.
LPG is the most common cooking fuel in Brazilian urban areas, being used in virtually all households,
with the exception of the cities of Rio de Janeiro and São Paulo, which have a natural
gas pipeline infrastructure. Poor families receive a government grant used exclusively
for the acquisition of LPG. LPG is commonly used in North America for
domestic cooking and outdoor grilling.
Security of supply Because of the natural gas and the oil-refining
industry, Europe is almost self-sufficient in LPG. Europe's security of supply is further
safeguarded by: a wide range of sources, both inside and outside
Europe; a flexible supply chain via water, rail and
road with numerous routes and entry points into Europe;
According to 2010–12 estimates, proven world reserves of natural gas, from which most LPG
is derived, stand at 300 trillion cubic meters. Added to the LPG derived from cracking crude
oil, this amounts to a major energy source that is virtually untapped and has massive
potential. Production continues to grow at an average annual rate of 2.2%, virtually
assuring that there is no risk of demand outstripping supply in the foreseeable future.
Comparison with natural gas LPG is composed primarily of propane and butane,
while natural gas is composed of the lighter methane and ethane. LPG, vaporised and at
atmospheric pressure, has a higher calorific value than natural gas, which means that LPG
cannot simply be substituted for natural gas. In order to allow the use of the same burner
controls and to provide for similar combustion characteristics, LPG can be mixed with air
to produce a synthetic natural gas that can be easily substituted. LPG/air mixing ratios
average 60/40, though this is widely variable based on the gases making up the LPG. The
method for determining the mixing ratios is by calculating the Wobbe index of the mix.
Gases having the same Wobbe index are held to be interchangeable.
LPG-based SNG is used in emergency backup systems for many public, industrial and military
installations, and many utilities use LPG peak shaving plants in times of high demand
to make up shortages in natural gas supplied to their distributions systems. LPG-SNG installations
are also used during initial gas system introductions, when the distribution infrastructure is in
place before gas supplies can be connected. Developing markets in India and China use
LPG-SNG systems to build up customer bases prior to expanding existing natural gas systems.
LPG-based SNG or natural gas with localised storage and piping distribution network to
the house holds for catering to each cluster of 5000 domestic consumers can be planned
under initial phase of city gas network system. This would eliminate the last mile LPG cylinders
road transport which is a cause of traffic and safety hurdles in Indian cities. These
localised natural gas networks are successfully operating in Japan with feasibility to get
connected to wider networks in both villages and cities.
Environmental effects Commercially available LPG is currently derived
from fossil fuels. Burning LPG releases carbon dioxide, a greenhouse gas. The reaction also
produces some carbon monoxide. LPG does, however, release less CO
2 per unit of energy than does coal or oil. It emits 81% of the CO
2 per kWh produced by oil, 70% of that of coal, and less than 50% of that emitted by
coal-generated electricity distributed via the grid. Being a mix of propane and butane,
LPG emits less carbon per joule than butane but more carbon per joule than propane.
LPG can be considered to burn more cleanly than heavier molecule hydrocarbons, in that
it releases very few particulates. Fire/explosion risk and mitigation
In a refinery or gas plant, LPG must be stored in pressure vessels. These containers are
either cylindrical and horizontal or spherical. Typically, these vessels are designed and
manufactured according to some code. In the United States, this code is governed by the
American Society of Mechanical Engineers. LPG containers have pressure relief valves,
such that when subjected to exterior heating sources, they will vent LPGs to the atmosphere.
If a tank is subjected to a fire of sufficient duration and intensity, it can undergo a boiling
liquid expanding vapor explosion. This is typically a concern for large refineries and
petrochemical plants that maintain very large containers. In general, tanks are designed
that the product will vent faster than pressure can build to dangerous levels.
One remedy, that is utilized in industrial settings, is to equip such containers with
a measure to provide a fire-resistance rating. Large, spherical LPG containers may have up
to a 15 cm steel wall thickness. They are equipped with an approved pressure relief
valve. A large fire in the vicinity of the vessel will increase its temperature and pressure,
following the basic gas laws. The relief valve on the top is designed to vent off excess
pressure in order to prevent the rupture of the container itself. Given a fire of sufficient
duration and intensity, the pressure being generated by the boiling and expanding gas
can exceed the ability of the valve to vent the excess. If that occurs, an overexposed
container may rupture violently, launching pieces at high velocity, while the released
products can ignite as well, potentially causing catastrophic damage to anything nearby, including
other containers. See also
Compressed natural gas Filling carousel
Gasoline gallon equivalent Industrial gas
Intumescent World LP Gas Association
References
External links WLPGA World LP Gas Association
PERC Propane Education & Research Council NPGA National Propane Gas Association, USA
Propane 101 Explaining propane and LP Gas fundamentals
NIOSH Pocket Guide to Chemical Hazards Centers for Disease Control and Prevention