Among the following fuels of energy, which is the most environment friendly ?

Concept: How environment friendly a fuel is depends on what its combustion puts into the air. Every carbon-bearing fuel must release carbon dioxide, a greenhouse gas, and incomplete burning of it adds carbon monoxide, unburnt hydrocarbons and soot. A fuel whose molecule contains no carbon at all cannot release any carbon-based emission.

Application: Read the composition of each fuel offered here and work out what its combustion yields.

• Ethanol, C2H5OH — two carbon atoms per molecule, so burning it yields carbon dioxide and water.

• Biogas — mainly methane, CH4, mixed with carbon dioxide; one carbon atom per methane molecule, so burning it yields carbon dioxide and water.

• CNG — natural gas that is largely methane, CH4; one carbon atom per methane molecule, so burning it yields carbon dioxide and water.

• Hydrogen, H2 — no carbon atom at all. 2H2 + O2 → 2H2O, so the only product is water vapour.

Cross-check: Set the carbon content of each fuel against what it emits.

Hydrogen is the one carbon-free fuel in this set, so it is the only one that emits no carbon dioxide, no carbon monoxide, no unburnt hydrocarbon and no soot when it burns — which is why it is described as the cleanest fuel. Two honest qualifications: nitrogen oxides can still form from the nitrogen of the air at the high temperature of a hydrogen flame, and hydrogen's overall footprint depends on how it was made (electrolysis run on renewable electricity is clean, steam reforming of natural gas is not). At the point of use, among the fuels offered, hydrogen is the most environment friendly.

Answer: Hydrogen.