Question

why is CNG called a clean fuel?

Answer 1

CNG is the compressed natural gas and cause less pollution as compared to other source of fuels like coal . The carbon dioxide  and nitrogen oxide as well as the sulphur oxide are produced less than the petroleum or coal etc . For example carbon dioxide produced by the coal burning release about 50% more as compared to the CNG  and CO2 is the  major pollutant in the air.

That's why CNG is called the Clean fuel.

Answer 2

Compressed natural gas is a fuel that can be used as a substitute for other fuels like petrol and diesel. When the gas is burnt, the quantity of carbon dioxide that is burnt is half of what is emitted when coal or other fossil fuels are burnt. Other pollutants like sulfur oxide, nitrogen oxide and particulate matter are also emitted in lesser quantities. Thus, the atmosphere is relatively cleaner when this gas is used. Hence, CNG is called clean fuel.