Natural gas is one of several fuels that can help ease our dependence on imported petroleum for transportation. Extracted from underground reservoirs, natural gas is a fossil fuel composed primarily of methane, along with other hydrocarbons such as ethane, propane, butane and inert gases such as carbon dioxide, nitrogen and helium.
Its composition varies and depends on its source. Natural gas is distributed in an extensive pipeline system after it is cleaned and purified and an odorant is added.
The United States is one of the world's largest producers and consumers of natural gas. The fuel accounts for about one quarter of the energy consumed in the United States, primarily in the industrial, residential, commercial and electric utility sectors. The transportation sector consumes only three percent of our natural gas, primarily to power compressors on natural gas pipelines.
Interest in using natural gas as a transportation fuel has increased in recent years, particularly in urban areas, for two main reasons. First, natural gas burns cleanly, emitting little carbon monoxide or reactive gases that pollute the air. Second, our nation has very large resources of natural gas, so this fuel can help us reduce our dependence on foreign oil at a competitive cost. Converting as little as ten percent of America's 200 million cars and trucks to natural gas can reduce oil imports by 570 million barrels each year or about 20 percent of our imports.
Most gasoline-powered cars and trucks can be converted to operate on compressed natural gas or liquefied natural gas in either a single-fuel natural gas only or a dual-fuel either gasoline or natural gas configuration. Modifications include changes to the fuel delivery and carburetion systems. Diesel vehicles can also be converted to use either diesel fuel or natural gas in a dual-fuel configuration.
The physical and chemical properties of natural gas provide good performance in these modified vehicles. Natural gas has a higher octane number than gasoline. In
addition, because it is introduced into the engine as a gas rather than as a liquid that must first be vaporized natural gas can provide quicker cold starts with lower emissions.Natural gas vehicles are refueled at stations specially designed to deliver compressed or liquefied natural gas. Compressed natural gas stations use either the "slow-fill" or "fast-fill" methods. Slow-fill stations are simpler in design and less costly than fast-fill stations, but they take several hours to refuel a vehicle in comparison to the two-five minutes associated with fast-fill stations. Liquefied natural gas stations require cryogenic storage vessels to maintain the natural gas in a liquid state and refueling times are comparable to those of conventional gasoline stations. Liquefied natural gas vehicles generally provide a longer driving range between refuelings than compressed natural gas vehicles; however, the driving range of both is shorter than that of gasoline-powered vehicles because of the lower energy density of natural gas fuels.
Today, more than 30,000 natural gas vehicles are in use in the United States and nearly a million operate worldwide. Most of these are compressed natural gas dual-fuel vehicles. Public and private fleets are especially good candidates for natural gas conversion because of their relatively short driving ranges and centrally located refueling facilities. Transit authorities are also introducing hundreds of natural gas buses into their urban transportation systems.
In response to federal and state legislation, 38,500 natural gas vehicle fleets will be mandated for conversion by 2000. A total of 861,000 natural gas vehicles are projected to be purchased between 1995 and 2000 to meet these mandates. The American Gas Association projects that there will be 11 million natural gas vehicles on the road by 2010.
Advantages of natural gas include that it can be used directly as it is taken from the ground, while other fuels must be refined. It has an octane rating of about 130 as compared to 90 for gasoline and costs less on a per-gallon equivalent basis than gasoline or diesel. Natural gas leads to reduced engine maintenance and to longer engine life. In general, natural gas vehicles burn 80 percent cleaner than conventional vehicles.
However, there are several disadvantages to using natural gas as well. Natural gas occupies about four times the space of an energy equivalent of gasoline resulting in added weight and space of fuel storage tanks. New fuel tanks and some fuel system modifications are required on retrofits. At the present time, there is limited availability of refueling stations and natural gas vehicles experience a shorter driving range between fill-ups.