Chemistry 8.14 - 8.15 - Choosing fuels
Compare hydrogen and petrol by their sources, production, exhaust products, storage and supply. Use these comparisons to make a justified fuel choice for a particular situation.
Fossil fuels and renewability
A fuel is non-renewable when it is used much faster than natural processes replace it. Fossil fuels form over geological timescales, so extracting and burning them depletes a finite store on a human timescale.
| Fuel | Fossil source | Required classification |
|---|---|---|
| petrol | obtained from crude oil | non-renewable fossil fuel |
| kerosene | obtained from crude oil | non-renewable fossil fuel |
| diesel oil | obtained from crude oil | non-renewable fossil fuel |
| methane in natural gas | found as the main useful fuel in natural gas | non-renewable fossil fuel |
Petrol, kerosene and diesel oil are mixtures of hydrocarbons separated from crude oil; they are not single pure compounds. In this specification, the methane claim refers to fossil methane obtained from natural gas. The fact that methane can also be produced by biological processes does not make natural-gas methane renewable.
This source information matters when comparing fuels. Petrol is convenient to store and has an established distribution system, but using it consumes a non-renewable crude-oil fraction and its carbon can become carbon dioxide, carbon monoxide or soot depending on combustion conditions.
Choosing a car fuel
An evaluation must compare both fuels against the same criteria and then make a judgement. It is not enough to list an advantage of hydrogen and stop.
Hydrogen contains no carbon or sulfur. Its overall reaction with oxygen is:
The equation is balanced: both sides contain 4 H atoms and 2 O atoms. Water is the product from the hydrogen itself, so using hydrogen does not directly produce carbon dioxide, carbon monoxide, soot or sulfur dioxide from that fuel. Avoiding use-stage carbon dioxide is an environmental advantage because carbon dioxide contributes to climate change. However, a high-temperature hydrogen engine can still make oxides of nitrogen from nitrogen and oxygen in the air. The accurate claim is therefore no carbon-containing exhaust from the hydrogen, not "no pollution under every condition".
Hydrogen is an energy carrier and must first be manufactured. Splitting water using electricity from wind or solar power can give hydrogen a low fossil-carbon source. Producing it using fossil fuels or fossil-generated electricity can release carbon dioxide elsewhere, so hydrogen is not automatically renewable or low-carbon.
| Criterion | Hydrogen rather than petrol | Petrol rather than hydrogen |
|---|---|---|
| products during use | water from the hydrogen; no carbon-containing products from the fuel | complete combustion produces and water; incomplete combustion can produce and soot |
| resource | can be made using renewable electricity | non-renewable fossil fuel from crude oil |
| production | making, compressing and transporting hydrogen requires energy | extraction, refining and transport also require energy |
| storage and safety | low energy per unit volume, so compact storage often needs high-pressure tanks; leaks ignite easily and require engineered controls | liquid at everyday conditions, easy to store in compact tanks, but also flammable |
| infrastructure | fewer production and refuelling facilities may be available | established refuelling and distribution network |
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