It produces harmful products like sulphur and nitrogen oxides in very small amounts.
Explanation
Compressed Natural Gas (CNG) is a fossil fuel substitute for gasoline (petrol), diesel fuel, and propane/LPG. It is primarily composed of methane ($CH_4$) compressed to less than 1% of the volume it occupies at standard atmospheric pressure. Its classification as a "cleaner" fuel is derived from its chemical composition and combustion properties.
Option Analysis
- It has a higher calorific value than petrol. is incorrect: While CNG does have a high calorific value (approximately 50,000 kJ/kg) compared to petrol (approximately 45,000 kJ/kg), a higher calorific value indicates energy efficiency rather than environmental cleanliness. The "clean" nature of a fuel is determined by its emission profile, not just its energy density.
- It produces harmful products like sulphur and nitrogen oxides in very small amounts. is correct: CNG is considered a cleaner fuel primarily because it burns more completely than petrol or diesel. Upon combustion, it produces negligible amounts of harmful pollutants such as sulphur dioxide ($SO_2$) and significantly lower levels of nitrogen oxides ($NO_x$) and particulate matter (soot). Unlike petrol and diesel, which often contain sulphur impurities, natural gas is processed to remove impurities before compression.
- It does not produce any carbon dioxide upon combustion. is incorrect: CNG is a hydrocarbon. The combustion of any hydrocarbon produces carbon dioxide ($CO_2$) and water ($H_2O$). While CNG produces less $CO_2$ per unit of energy compared to coal or oil due to its high hydrogen-to-carbon ratio, it does not produce zero carbon dioxide.
- It is a liquid fuel and thus easier to transport. is incorrect: As the name suggests, Compressed Natural Gas is stored in a gaseous state under high pressure, not as a liquid. Liquefied Natural Gas (LNG) is the liquid form. Furthermore, the physical state of the fuel (liquid vs. gas) does not inherently determine its cleanliness.
Key Takeaway: CNG is preferred as a green fuel because its combustion leaves no unburnt residue (particulate matter) and emits significantly lower quantities of toxic gases like sulphur dioxide and nitrogen oxides compared to liquid fossil fuels.