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CNG

Natural gas is an organic compound that is found deep within the earth. It is a fossil fuel formed over millions of years of geological pressures and changes. Natural gas is primarily methane comprised of hydrogen and carbon. It is a safe fuel source that is commonly used in homes and businesses for heating, lighting and cooking. It can also be compressed into CNG (compressed natural gas) so high quantities of energy can be stored in relatively small containers. Why is CNG safer than gasoline or diesel?

Compressed natural gas (CNG) is an alternative fuel for vehicles. Unlike gasoline and diesel, CNG is economical, clean burning, and available from domestic sources. CNG is non-toxic. It does not contain poisonous ingredients. It cannot contaminate the earth and water. Compressed natural gas burns cleanly. In fact, it is the cleanest burning fossil fuel today. Natural gas burns with a clean blue flame.

CNG is lighter than air; so if a CNG system leaks, the gas will quickly disperse into the atmosphere and form a diluted mixture that is outside its range of flammability. By comparison, if a gasoline or diesel system leaks, the fuel will puddle on the floor because they are dense liquids. The liquid puddle of gasoline or diesel will have a significantly higher the risk of ignition. CNG has a narrow range of flammability compared to gasoline and diesel fuel. It needs 5% to 15% of fuel to air mixture to burn. Natural gas also has an ignition temperature of 1100 degrees Fahrenheit. This means that it takes a higher temperature to ignite natural gas compared to gasoline, which can ignite at approximately 600 degrees.

CNG storage tanks are designed to hold gas between 3,000 to 3,600 psi. They are stringent tested to meet federal safety standards that include impact testing to ensure integrity even in vehicle collisions. They have release valves that open automatically if excessive heat or pressure are detected.

How is CNG used in a vehicle?

CNG vehicles are filled in a manner very similar to the way a car is filled with gasoline. A hose carries CNG to a nozzle that is inserted in the vehicle. Some CNG fueling systems fill vehicles in a few minutes (similar a gasoline station) and some fill large fleets of vehicles slowly overnight. When a driver steps on the vehicle’s accelerator, CNG leaves the vehicles storage cylinder, passes through a high-pressure fuel line and enters the engine compartment. Then it passes through a regulator that reduces pressure back to atmospheric conditions. The natural gas is then mixed with air. It flows through a carburetor or fuel injection system and enters the engine's combustion chambers.

CNG station criteria

The first step in designing a CNG station is to identify the type of vehicles will be fueled (light-duty vehicles, forklifts, buses, solid waste, etc.). The type of vehicles often determines the best station design a specific fleet. For example, a school bus fleet can best be served by a slow-fill system, while a fast-fill system would be better for a light-duty vehicle fleet. The number of vehicles and size of their tanks must be identified so the station can be correctly sized. If vehicles will be purchased over a period of time, decisions should be made about whether to phase the installation of compressors over time or whether to build out the station in one step. The fleet manager is likely to have preferences about the fuel management system. Some require detailed records showing the quantity of fuel placed in each vehicle so it can be compared to mileage and other factors. Others are interested only in the gross amount of fuel consumed by the fleet in a period of time. Public access stations need credit card payment or tracking systems.

Will CNG demand grow?

Many fleet owners are prepared to shift to alternative fuels, but they want to start with a portion of their fleet so they can verify the benefits. Public access CNG stations provide fueling locations for small or partial fleets. Owners often build their own CNG stations when the vast benefits of CNG have been validated. Progressive municipalities encourage fleet users to shift to safer, cleaner fuels by seeding the marketplace with public access stations.