This article is the third in a series examining alternative fuels production and source materials. Previously we have taken a look at biodiesel and natural gas. In the next couple of months we will cover ethanol, hydrogen, and electricity, as a vehicle fuel. This month, we will focus on propane autogas.

Propane, called propane autogas when used as a vehicles fuel, is a colorless, odorless heavier-than-air gas that, when compressed, can be stored as a liquid. Propane autogas has been in use since the 1920sas fuel for light-duty and heavy-duty vehicles. Today, 3% of propane produced is used as transportation fuel.

Fuel Sources
Nearly all of the United States’ propane autogas is produced domestically. Propane is a byproduct of the process of refining oil (petroleum) or natural gas (methane). More than half (55%) of the propane autogas produced is a byproduct from natural gas purification from wet gas wells and the rest (45%) comes from crude oil refining. These two sources are very different. But once refined, there is little difference between the propane autogas processed from these sources. As nearly all natural gas is produced domestically, the production of propane autogas is an important advantage for the U.S.

Manufacturing and Production
Natural Gas Refining
The processing of natural gas involves removal of propane, butane, and large amounts of ethane from the raw gas, which prevents condensation of the volatile elements in natural gas pipelines.

Raw natural gas consists mostly of methane, but not at the desired level of purity. A variety of substances must be removed, some of which are hydrocarbon gases like propane. Propane, along with butane and ethane, are made of heavier hydrocarbon chains. These heavier gases are removed from natural gas in the final stage of processing.

The still-raw natural gas is cryogenically distilled. Normal distillation works by heating a compound to a set temperature where the desired elements will vaporize and boil off for later collection. Cryogenic distillation, however, uses low temperatures to cause the gas to condense into a liquid.

The raw natural gas is cooled to the point at which the heavier hydrocarbons liquefy, while the lighter methane remains gaseous. Finally, the butane, ethane, and propane are separated from one another in a similar manner using a distillation device called a fractionator.

Crude Oil Refining
Oil refineries produce some propane as a byproduct of cracking petroleum into gasoline or heating oil. Cracking is the process of breaking down hydrocarbon gases into simpler ones with lower boiling points. During the oil refining process, the propane and other impurities are pulled out through a boiling process. In modern fluid catalytic cracking, the residue from previous refining is pumped into a chamber with heated catalyst, which vaporizes the petroleum residue and breaks apart certain complex carbon chains. The result is a variety of lighter hydrocarbons, including propane. Many fuels and other products are created from oil refining and a small percentage is propane. The volume of total refined products may be larger than 42 gallons. Typically, the propane volume produced from a 42-gallon barrel of crude would be just over 4.5 gallons.

The propane is then separated using a fractionator or distillation column. Different gases have different boiling points, and as they rise in the distillation column, they cool. Once they cool enough to return to a liquid state, they are trapped at that level in compartments.

Once refined and processed, propane is stored in large pressurized tanks and transported to retailers all over the U.S. From there, it is distributed to consumers at retail stores, campgrounds, and fuel stations across the country.

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Schematic of a typical propane distribution route. Source: U.S. Energy Information Administration.

Propane that is used as a vehicle fuel is commonly referred to a propane autogas. Propane autogas is one of the world’s most common engine fuels. It burns cleanly and can run at higher compression ratios than conventional vehicles. This results in a high energy output, while producing lower emissions levels than conventional vehicles. While the cost of a gallon of propane autogas fuel varies, it is typically less expensive than a gallon of gasoline.

This article is based on the Clean Cities Learning Program Petroleum Reduction Technologies and Propane Autogas Technician Training curriculum. For more information, visit the Clean Cities Learning Program, Propane Autogas Gateway, or contact the NAFTC.




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