CNG, or Compressed Natural Gas, is a gas which consists primarily of methane. Natural gas is found in association with fossil fuels, in coal beds, as methane hydrates and can be created by methanogenic organisms in marshes, bogs, swamps and landfills. The history of compressed natural gas (CNG) as a transportation fuel dates back to World War II. Natural gas vehicles are a proven technology that have been enhanced and refined over the years into a convenient and extremely safe method of transportation. Daily use of natural gas vehicles can be found throughout the country in a variety of applications.
CNG, has the unfortunate tendency to still inspire fear in those who are not familiar with it. The simple truth is that CNG is a safer fuel that either gasoline or diesel. To understand and adopt this truth, it simply requires letting go of the belief that gasoline is the "only" fuel just because it's what has always been utilized.
The physical properties of natural gas demonstrate that it has a limited range of flammability. This means that it requires a specific mixture of air and gas to burn. The range is in the 5 to 15% range and it requires an ignition temperature of approximately 1100 degrees F. Both gasoline and diesel fuel have lower concentrations of flammability and lower temperatures of ignition. It is far more likely that gasoline or diesel will ignite than natural gas.
In addition to the range and temperature requirements, natural gas is lighter than air. In the event that a leak were to occur, the gas would rise and disperse through the atmosphere, leaving very little change of ignition. Gasoline and diesel are both dense liquid fuels which pool during a leak and are easily ignitable.
CNG compared to LNG
CNG is often confused with Liquefied Natural Gas (LNG). While both are stored forms of natural gas, the key difference is that CNG is gas that is stored (as a gas) at high pressure, while LNG is in uncompressed liquid form.
CNG has a lower cost of production and storage compared to LNG as it does not require an expensive cooling process and cryogenic tanks.
CNG requires a much larger volume to store the same mass of gasoline or petrol and the use of very high pressures (3000 to 4000 psi, or 205 to 275 bar).
CNG can be stored at lower pressure in a form known as an ANG (Adsorbed Natural Gas) tank, at 35 bar (500 psi, the pressure of gas in natural gas pipelines) in various sponge like materials, such as activated carbon and metal-organic frameworks (MOFs). The fuel is stored at similar or greater energy density than CNG. This means that vehicles can be refueled from the natural gas network without extra gas compression, the fuel tanks can be slimmed down and made of lighter, less strong materials.