Natural Gas and Propane Fuels

In the developed world, natural gas is used extensively as a fuel. It consists mainly of methane but contains small amounts of ethane and propane. Normally the gas is transported by pipelines from its source to domestic consumers, who use it for cooking and heating, and to some utilities, which burn it instead of coal or oil in power plants to produce electricity.

CH4(g) + 2 02(g)-> C02(g) + 2 H20(1) AH = -890 kj moP1

Unfortunately, where pipelines do not exist, the natural gas that is produced as a by-product of petroleum production at oil wells, etc. is simply wasted by venting or flaring it off, thereby adding to the atmospheric burden of greenhouse gases.

Highly compressed natural gas (CNG) is used to power some vehicles, especially in Canada, Italy, Argentina, the United States, New Zealand, and Russia. Due to the cost of converting a gasoline engine to accept natural gas as the fuel, the current use of CNG in vehicles is mainly restricted to taxis and commercial trucks that are in almost constant service. For such vehicles, the additional capital cost of converting the fuel system is much less in the long run than are the savings from the lower cost of the fuel. Because the compressed gas must be maintained at very high pressures to keep its storage volume reasonable, heavy fuel tanks with thick walls are required. In order to keep the weight and size of the tank to reasonable values, the driving range (before refilling) of CNG vehicles is usually considerably shorter than that of gasoline-powered vehicles.

Compressed natural gas has both environmental advantages and disadvantages as a vehicular fuel when compared to gasoline. Since methane molecules contain no carbon chains, neither organic particulates nor reactive hydrocarbons are formed or emitted into air as a result of its combustion; however, a small amount of each pollutant type is formed from the ethane and propane component of commercial natural gas. Overall, regional air quality is improved by the use of natural gas rather than gasoline or diesel oil. However, the release of methane gas from pipelines during its transmission or from tailpipes of vehicles due to its incomplete combustion could lead to increased global warming since methane is a potent greenhouse gas. A massive conversion in North America to CNG as a vehicular fuel would be limited by supply problems for the gas, which is now used extensively for domestic heating and cooking and increasingly as the fuel in new electric power plants.

Some interesting proposals have been made recently to improve the performance of natural gas as a vehicular fuel. More efficient burning of the methane results if a small amount—about 15% by volume—of hydrogen gas is added to it. Alternatively, a smaller storage volume for methane results if it is liquefied rather than simply compressed, but more energy is expended in the process.

Similar but somewhat less serious considerations apply to propane, CjHg, also a main component of liquified petroleum gas (LPG), in its use as a gasoline replacement in vehicles. The heat energy produced per gram of propane combusted, 50.3 kj, is not quite as high as that of 55.6 kj for methane. The heat released per gram by burning gasoline depends on the composition of the particular blend under consideration, but it is generally slightly less than that of propane. Both LPG and propane are readily liquefied under pressure, so they can be stored much more efficiently than can natural gas.

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