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?

Let's analyze the statements regarding hydrogen-enriched CNG (H-CNG) as a fuel:

1. The main advantage of the use of H-CNG is the elimination of carbon monoxide emissions. Incorrect. While H-CNG significantly reduces carbon monoxide (CO) emissions compared to pure CNG due to improved combustion, it does not eliminate them entirely. The term 'elimination' is generally too strong for most combustion technologies.

2. H-CNG as fuel reduces carbon dioxide and hydrocarbon emissions. Correct. The addition of hydrogen to CNG leads to more complete combustion of the fuel. This results in a reduction of unburnt hydrocarbons (HC) and also lower carbon dioxide (CO2) emissions per unit of energy, as hydrogen combustion produces only water, making the overall combustion cleaner.

3. Hydrogen up to one-fifth by volume can be blended with CNG as fuel for buses. Correct. Research and pilot projects (including those in India) have successfully demonstrated that blending hydrogen up to 20% by volume (one-fifth) with CNG is technically feasible and provides environmental benefits without requiring major modifications to existing CNG engines or infrastructure.

4. H-CNG makes the fuel less expensive than CNG. Incorrect. Currently, hydrogen production, storage, and distribution are generally more expensive than those for natural gas (CNG). Therefore, H-CNG fuel, despite its environmental advantages, is typically more expensive than pure CNG.

Thus, statements 2 and 3 are correct.