Editor’s note: Naturalist Don Hendershot is writing a series of articles exploring alternative fuels.

Natural gas is the fossil alternative to fossil fuels. It is a fossil fuel composed of the remains of eons-old plants and animals. And like oil it is found in underground reservoirs. Natural gas proponents note that reserves of natural gas are greater than those of oil. But critics are quick to point out that those reserves are based on today’s usage and those reserves will begin to dwindle more quickly as natural gas becomes a bigger part of the energy picture. The scenario would likely be similar to the current oil situation with “cheap” natural gas being replaced by “expensive” natural gas as demand and usage increase.

Natural gas is used in three forms, LNG (liquefied natural gas), CNG (compressed natural gas) and LPG (liquefied petroleum gas). Propane is probably the most common LPG and the most common “alternative” fuel. Propane has been used to fuel vehicles in this country since 1912. Many of the farmers where I grew up, in the Mississippi Delta, had a propane tank in the back of their pickup in the 1960s and 1970s. Today there are well over 300,000 vehicles in the U.S. (most are fleet vehicles) that run on propane.

Propane is a cleaner fuel than gasoline or diesel and could provide a 60 percent reduction in greenhouse emissions. Because of its higher octane (105-110) and because it enters the engine as a vapor, propane doesn’t gunk the engine as much as petroleum-based fuels, thus most propane engines are longer lived and require less maintenance.

However, there is a reason those fuel tanks were in the beds of those farmers’ pickups. Propane has less energy output than gasoline or diesel, so you get fewer miles per gallon. The tank is also bulkier than gasoline tanks because the propane is pressurized.

Automakers aren’t keen on LPG vehicles. Most LPG vehicles are converted “after market” at a cost of $2,000 to $4,000.

If you want to order a LPG vehicle many automakers will customize one for you at the factory. Most of these are “bi-fuel” vehicles, which store gasoline and LPG in separate tanks.

Propane is probably the most available alternative fuel with about 3,000 fueling stations across the U.S. But that is still small-scale compared to the nearly 200,000 gasoline and/or diesel stations.

And because propane has a number of other uses — such as heating — it can undergo wide price fluctuations just like gasoline. In fact, in January 2007 the average price of regular unleaded gasoline was $2.22 per gallon and propane was $2.33 per GGE (gallon-equivalent).

The other natural gases, CNG and LNG have even more limited availability. There are about 1,600 CNG fueling stations and fewer than 100 public LNG stations. However, if you have natural gas at home you will soon be able to purchase an appliance to fill your auto at home. The appliance is expected to list for about $4,000 but incentives and/or tax credits should greatly reduce that price.

While natural gas is considerably cleaner than conventional gasoline engines, one study reported that Honda’s CNG Civic GX emitted about 30 percent more greenhouse gas than their hybrid Civic over a year of driving.

So the pros of natural gas include:

• Primarily a domestic product

• Cleaner than gasoline/diesel

• Slightly cheaper at this time

Some of the cons:

• Fuel tank space requirements

• Lack of fueling infrastructure

• Lack of range compared to conventional engines

• Initial and/or conversion costs

• Natural gas is still a fossil fuel

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