CNG is made by compressing natural gas which is mainly composed of methane [CH4]). It is stored and distributed in hard containers, at a normal pressure of 200–220 bar (2900–3200 psi), usually in cylindrical or spherical shapes.
CNG cylinders can be made of steel, aluminum, or plastic. Lightweight composite (fiber-wrapped thin metal "ISO 11439 CNG-3" / fibre-wrapped plastic "ISO 11439 CNG-4") cylinders are especially beneficial for vehicular use because they offer significant weight reductions when compared with earlier generation steel and aluminum cylinders, which leads to lower fuel consumption. The CNG cylinders bundled with safety-valve generally follow the ISO 11439 standard.
Scientists are developing methods of storing methane in a new form known as ANG (Absorbed Natural Gas) 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. The benefits are that vehicles can be refuelled from the natural gas network without extra gas compression, the fuel tanks can be made of lighter, less strong materials, and the tank designed to be much slimmer.
