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Natural gas consists to 85-99% of methane (w1). It is primarily extracted from gas wells or in conjunction with crude oil production, but it can also be produced as a "by-product" of landfill operations (w2). "World reserves of gas are more widely distributed than those of liquid fuels so that developing countries like Argentina, Bangladesh and Pakistan may see a strategic advantage in its development" (1).

Because natural gas is mostly methane, NGVs have much lower non-methane hydrocarbon emissions than gasoline vehicles, but higher emissions of methane. Since the fuel system is sealed, there should be no evaporative emissions and refueling emissions are negligible. Cold-start emissions from NGVs are also low, since cold-start enrichment is not required. In addition, this reduces both VOC and CO emissions. NOx emissions from uncontrolled NGVs may be higher or lower than comparable gasoline vehicles, depending on the engine technology, but are typically slightly lower. Light-duty NGVs equipped with modern electronic fuel control systems and three-way catalytic converters have achieved NOx emissions more than 75 percent below the stringent California Ultra Low-Emission Vehicle (ULEV) standards. However, methane is the hydrocarbon that is most difficult to oxidize in a catalyst.

As a substitute for diesel, NGVs should have somewhat lower NOx and substantially lower PM emissions. Catalysts in methane fuelled vehicles have shown considerably reduced durability compared to gasoline fuelled vehicles.

Given equal energy efficiency, greenhouse gas (GHG) emissions from NGVs will be approximately 15 percent to 20 percent lower than from gasoline vehicles, since natural gas has lower carbon content per unit of energy than gasoline. Lean burning NGVs have about the same GHGs as diesel fuel vehicles. Due to slippage of methane from the exhausts and the much lower energy efficiency of the Otto engine, GHG emissions can be higher or substantially higher.

Natural gas must be stored onboard a vehicle either in a compressed gaseous state (CNG) or in a liquefied state (LNG) (w3). CNG is less costly and a far more common option for vehicles, which is the reason that LNG is not highlighted in the following discussion.

Natural gas is lighter than air. A spill would disperse into the air as opposed to pooling on the ground or entering sewer or water systems. However, in an enclosed space, natural gas will rise to the ceiling, which could be a potential fire and ignition risk (w2).

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Regarding the infrastructure necessary for the establishment of a CNG-based transportation system, "a local reserve of gas is clearly the first requirement, both because the expense of transport internationally is likely to make it uneconomic and because that increases the probability of securing a substantial balance of payments advantage by its exploitation. More important than large gas reserves is the availability of a city gas distribution network" (1).

"A network of low-cost refueling stations is needed to provide a service equivalent to that expected with other fuels. Other needs include education and training programs, quality control and industry support systems, regulations and equipment standards" (w4). The varying proportions of methane in natural gas must be taken into account with regard to the processing of the gas, in order to ensure a standard product.

According to (2), safety modifications to fuel depots are required. For storage purposes the gas is kept under a pressure of 3,000-4,000 pounds per square inch (3). CNG fuel tanks add more weight to the vehicle compared to diesel buses (4) and the maximum driving distance per tank fill is considerably lower.

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CNG vehicles cost $3500 to $6000 more than their gasoline powered counterparts. This is primarily due to the higher costs of the fuel cylinders. The cost of CNG fuel depends on the location and availability of the gas as well as local tax policies. In the United States and Argentina for example, where natural gas has a reasonable market share, it tends to cost less than diesel fuel (4) (w4). In December 1999, the price of natural gas in Argentina was US$ 0.331 (incl. US$ 0.082 tax) per cubic meter, approximately equivalent in energy content to 1 liter of diesel fuel, which cost US$ 0.499 (incl. US$ 0.211 tax) (4).

In addition to the fuel itself, for the use of natural gas as a transport fuel the infrastructure outlined above must be financed. Natural gas has not been as widely used as a transport fuel for as long a period of time as diesel has, which is why large parts of the infrastructure for its distribution have yet to be created. In the program descriptions Cleaner Bus Fleets in New York City and CNG Buses in the United States, some cost experiences with CNG in the United States are provided (see also documents (5), (6), (7), (8) and (9)). For more in-depth information on cost implications of CNG use, see also (3), (w1), (4), (2), (10), (11) and (12).

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"Natural gas is widely used for space heating in both homes and industries, as well as for electricity generation. Networks of gas pipelines easily serve these stationary uses of natural gas. Natural gas has not been widely used for transportation, primarily due to its lower energy density and distribution difficulties" (w4).

"Argentina has the greatest penetration of gas powered vehicles, based on rich supplies of natural gas, which accounts for nearly 50 percent of the country's primary energy. There is a network of 11,000 miles of trunk pipelines and 93,000 miles of distribution lines. Filling stations costing US$1 million pay back capital in less than three years, and there are now 774 public filling stations through the country" (1). "Many large cities in Bangladesh, Brazil, Colombia, Indonesia, Pakistan and eastern Europe and the Former Soviet Union already have extensive gas networks" (1).

The International Association for Natural Gas Vehicles (IANGV) (w5) provides statistics on the distribution of natural gas vehicles in countries throughout the world.

Within the Info Pool, the following projects are described which feature CNG usage: CNG Buses in the United States, CNG Buses in Delhi and Cleaner Bus Fleets in New York City. Case studies of Argentina and New Zealand can be found in (4).

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Obstacles to the widespread use of NGVs include the absence of transportation and storage infrastructure, cost, loss of cargo space, increased refueling time, and lower driving range.

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Compressed Natural Gas (CNG)

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