CNG stands for compressed natural gas. It consists mainly of methane (CH4) and is much less energy dense than LPG, which makes it less suitable for mobile applications unless they have very large tanks.
CNG tends to come from fossil sources, although these can include gases which would otherwise have been released into the environment, such as coal-bed methane and wellhead gas. Since methane is a very potent greenhouse gas, there are major benefits in making use of these gases.
Engines running on CNG may be either lean-burn or stoichiometric. Where the engine is lean-burn a two-way catalytic converter can be fitted to oxidise emissions of carbon monoxide and methane. If the engine runs stoichiometric this enables the use of a three-way catalytic converter.
Unfortunately catalytic oxidation of methane can only be achieved at relatively high temperatures i.e. 430 ℃ and over.