CNG is natural gas that has been compressed to a pressure of 3,000 to 3,600 pounds per square inch (psi) so that the gas takes up less than 1% of the volume it occupies at standard atmospheric pressure. CNG is stored and transported in hard containers made of steel, aluminum with a fiber-reinforced polymer, aluminum with a carbon fiber composite overlap, or all composite material from a carbon fiber or carbon/glass fiber. The containers must be strong enough to contain the high-pressure gas and to also resist any external penetration.
CNG is used as a fuel for vehicles and as a means of limited storage. When larger amounts are required, it is more common to utilize liquified natural gas (LNG) storage instead since the equivalent amount of LNG takes up significantly less volume than CNG. CNG is often used on mobile storage units such as trucks or trailers. The stored CNG can then be hooked up directly to customers or to sections of the gas utility system to maintain service while the utility works on upstream lines. A large CNG truck/trailer may be sufficient to keep a small town in service during a pipeline replacement or other upstream maintenance.
CNG vehicles include passenger cars, light commercial vehicles, buses, trucks, and rail locomotives. Penetration has been limited though due to lack of CNG fueling stations and concerns that while CNG transport is significantly cleaner than gasoline or diesel vehicles, CNG vehicles do still emit greenhouse gases and other pollutants.