Fuel combustion, pollution and hydrogen alternatives

When there is not enough oxygen for complete combustion, incomplete combustion occurs. This can happen in engines and in fuel-burning appliances such as boilers and stoves.

The carbon in the fuel is not all converted into carbon dioxide. Instead, some can form carbon monoxide, CO, or solid carbon particles called soot. Water still forms from the hydrogen. The exhaust can contain a mixture of products rather than just one carbon-containing substance.

Two possible reactions for methane show the difference:

$$\mathrm{2CH_4 + 3O_2 \rightarrow 2CO + 4H_2O}$$ $$\mathrm{CH_4 + O_2 \rightarrow C + 2H_2O}$$

The first produces carbon monoxide; the second produces solid carbon. Soot adds carbon particulates to the air, so incomplete combustion causes particulate pollution as well as releasing gases.

How soot harms health and the environment

Soot particles can be breathed into the lungs, where they can cause respiratory problems. Unlike carbon monoxide, which harms people by reducing oxygen transport in the blood, soot causes harm as inhaled solid particles.

Soot can settle on buildings and statues, blackening their surfaces. Airborne particulates can also reduce the amount of sunlight reaching the Earth's surface, contributing to global dimming. In fuel-burning appliances, soot deposits can block pipes that carry away waste gases.

Why carbon monoxide is poisonous

Red blood cells contain haemoglobin, a protein that normally binds oxygen in the lungs and carries it to body tissues.

Carbon monoxide binds strongly to haemoglobin. This leaves less haemoglobin available to carry oxygen, so less oxygen reaches the body's cells. Cells need oxygen for aerobic respiration, which releases energy for their activities. A reduced oxygen supply can cause dizziness, loss of consciousness and eventually death.

Carbon monoxide is especially dangerous because it is colourless and odourless: people cannot rely on seeing or smelling it to recognise exposure.

Sulfur impurities lead to acid rain

A pure hydrocarbon contains only carbon and hydrogen, but some hydrocarbon fuels also contain sulfur impurities. When the fuel burns, the sulfur reacts with oxygen to produce sulfur dioxide:

$$\mathrm{S + O_2 \rightarrow SO_2}$$

Sulfur dioxide enters the atmosphere and dissolves in rainwater, making it more acidic. Further reactions in the atmosphere can form sulfuric acid. The resulting acid rain can damage environments far from where the fuel was burned.

Acid rain can damage crops and make water more acidic, harming aquatic organisms. It also attacks carbonate rocks such as limestone, damaging buildings and statues, and can corrode metal structures. Sulfur dioxide itself can irritate the lungs, throat and eyes.

Hot engines produce nitrogen oxides

Air entering an engine contains both nitrogen and oxygen. These gases do not normally react readily, but the high temperatures inside an operating engine allow them to react and form oxides of nitrogen.

Nitrogen monoxide can form first:

$$\mathrm{N_2 + O_2 \rightarrow 2NO}$$

It can then react with more oxygen to produce nitrogen dioxide, NO₂. These pollutants are therefore formed from gases in the air, rather than from sulfur impurities or the carbon in the fuel.

Nitrogen oxides contribute to acid rain and photochemical smog. They can also cause breathing difficulties, particularly for people with asthma. Their formation is linked to high engine temperatures, not specifically to incomplete combustion.

Is hydrogen a better car fuel than petrol?

Hydrogen reacts with oxygen and releases energy:

$$\mathrm{2H_2 + O_2 \rightarrow 2H_2O}$$

Unlike petrol, hydrogen contains no carbon. Its reaction with oxygen therefore produces water rather than carbon dioxide, carbon monoxide or soot. It also releases more energy per kilogram than petrol. Hydrogen can power vehicles through fuel cells, which use its reaction with oxygen to supply electrical energy.

However, the fuel's reaction in the vehicle is only part of the comparison. Hydrogen must be produced, and production requires energy and can be expensive. If its production involves fossil fuels, carbon dioxide can be released before the hydrogen even reaches the car. Hydrogen is also difficult and potentially dangerous to store and transport, requiring specialised storage arrangements.

The way hydrogen is used matters too. In a fuel cell, it reacts without the high-temperature combustion found in an engine. If hydrogen is instead burned in air in a hot engine, nitrogen and oxygen in the air can still react to form nitrogen oxides.

Hydrogen therefore offers clear advantages in reducing carbon-containing exhaust pollutants, but it is not automatically an environmentally harmless replacement for petrol. A balanced judgement depends on how the hydrogen is produced, how it is used, and the practical costs of safe storage and transport.

• Complete: sufficient oxygen → carbon dioxide + water; energy released.
• Incomplete: insufficient oxygen → carbon monoxide and/or carbon (soot), with water.
• CO binds strongly to haemoglobin → reduced oxygen transport → potentially fatal oxygen shortage.
• Soot causes respiratory problems, blackens buildings and contributes to global dimming by reducing sunlight reaching the surface. Deposits can block waste-gas pipes.

Other combustion pollutants

• Sulfur impurities burn: $\mathrm{S + O_2 \rightarrow SO_2}$.
• SO₂ dissolves in rainwater → acid rain: harms aquatic organisms and crops, attacks carbonate buildings and corrodes metals.
• High engine temperatures allow nitrogen and oxygen from air to react → nitrogen oxides: acid rain, smog and breathing difficulties.

Hydrogen versus petrol

$$\mathrm{2H_2 + O_2 \rightarrow 2H_2O}$$

Advantages: no carbon-containing products from hydrogen's reaction; more energy per kilogram than petrol.

Disadvantages: costly, energy-demanding production; production may release CO₂; difficult and potentially dangerous storage and transport.

Judgement: compare production as well as vehicle emissions. Hydrogen combustion in a hot engine can still form nitrogen oxides; a fuel cell avoids high-temperature combustion.