In the context of proposals to the use of hydrogen-enriched CNG (H-CNG) as fuel for buses in public transport, consider the following statements:

1. The main advantage of the use of H-CNG is the elimination of carbon monoxide emissions.

2. H-CNG as fuel reduces carbon dioxide and hydrocarbon emissions.

3. Hydrogen up to one-fifth by volume can be blended with CNG as fuel for buses.

4. H-CNG makes the fuel less expensive than CNG.

Which of the statements given above is/are correct?

The correct option is 2 and 3 only.

Let us analyze each statement regarding the use of Hydrogen-enriched Compressed Natural Gas (H-CNG) as a fuel for public transport buses:

1. Statement 1: "The main advantage of the use of H-CNG is the elimination of carbon monoxide emissions."
This statement is incorrect. While blending hydrogen with CNG significantly reduces carbon monoxide (CO) emissions (by up to 70%), it does not completely eliminate them. Since natural gas still contains carbon (methane), some carbon monoxide emissions remain.

2. Statement 2: "H-CNG as fuel reduces carbon dioxide and hydrocarbon emissions."
This statement is correct. H-CNG improves the combustion efficiency of fuel and leads to lower emissions of carbon dioxide (${\text{CO}}_2$) and unburnt hydrocarbons (HC), making it a cleaner alternative to standard CNG.

3. Statement 3: "Hydrogen up to one-fifth by volume can be blended with CNG as fuel for buses."
This statement is correct. Studies and trial runs (such as those conducted in Delhi) demonstrate that blending hydrogen up to 20% by volume (which is one-fifth) with CNG requires minimal modifications to existing CNG engines and infrastructure while delivering substantial emission reductions.

4. Statement 4: "H-CNG makes the fuel less expensive than CNG."
This statement is incorrect. Currently, H-CNG is more expensive than standard CNG due to the high cost of hydrogen production, storage, and blending technologies.

Therefore, only statements 2 and 3 are correct.