Atmospheric Pollutants from Fuels

The combustion of fuels is a major source of atmospheric pollutants (substances released into the air that harm health or the environment). Most fuels, including coal, contain carbon and/or hydrogen, and some also contain sulfur as an impurity.

When a fuel burns, the atoms in it combine with oxygen. What comes out depends on which elements the fuel contains and how much oxygen is available. The gases and solids released may include:

Product	Where it comes from
Carbon dioxide (CO₂)	Carbon burning fully in plenty of oxygen
Water vapour (H₂O)	Hydrogen in the fuel burning
Carbon monoxide (CO)	Carbon burning in too little oxygen
Sulfur dioxide (SO₂)	Sulfur impurities in the fuel burning
Oxides of nitrogen (NOₓ)	Nitrogen and oxygen from the air reacting at high temperature
Particulates	Soot (solid carbon) and unburned hydrocarbons

Carbon dioxide and water vapour are the products of clean, complete burning. The remaining products are unwanted pollutants, and the next slides explain how each one forms.

Whether a fuel burns cleanly depends on the oxygen supply. This is the key idea behind carbon monoxide and soot.

Complete combustion happens when there is plenty of oxygen. A hydrocarbon (a compound of only hydrogen and carbon) then burns to give only carbon dioxide and water. For example, methane:

$C{H}_4+2O_2\to C{O}_2+2H_{2}O$

Incomplete combustion happens when there is not enough oxygen. The carbon in the fuel is not fully oxidised, so as well as carbon dioxide and water you also get carbon monoxide (CO) and carbon particulates (soot). For example:

$4C{H}_4+5O_2\to 2CO+2C+8H_{2}O$

Both equations balance: in the second one there are 4 carbon, 16 hydrogen and 10 oxygen atoms on each side. Incomplete combustion also releases less energy than complete combustion.

Complete combustion (plenty of oxygen): carbon dioxide and water only. Incomplete combustion (limited oxygen): also carbon monoxide and soot (carbon particulates).

Two pollutants come from sources other than the carbon and hydrogen in the fuel.

Sulfur dioxide (SO₂) forms when sulfur impurities in the fuel burn. Fuels such as coal and some oils contain small amounts of sulfur, and this reacts with oxygen:

$S+O_2\to S{O}_2$

Oxides of nitrogen (NOₓ) are different: they do not come from the fuel at all. They form when the high temperatures inside an engine or furnace make the nitrogen and oxygen from the air react together. A worked example for nitrogen monoxide:

$N_2+O_2\to 2NO$

This is why oxides of nitrogen are strongly linked to vehicle engines, where combustion temperatures are very high.

Sulfur dioxide comes from sulfur in the fuel. Oxides of nitrogen come from nitrogen and oxygen in the air reacting at the high temperatures inside engines.

For the exam you must describe and explain the specific problems caused by each pollutant. Match the pollutant to its effect precisely.

Pollutant	Properties and problems caused
Carbon monoxide (CO)	Toxic, colourless and odourless, so it is not easily detected
Sulfur dioxide (SO₂)	Respiratory (breathing) problems; acid rain
Oxides of nitrogen (NOₓ)	Respiratory (breathing) problems; acid rain
Particulates (soot)	Global dimming; health problems

Carbon monoxide is dangerous because it is toxic and cannot be seen or smelled. It combines with the blood and reduces the amount of oxygen the blood can carry, and because there is no warning smell or colour people can be poisoned without noticing.

Sulfur dioxide and oxides of nitrogen cause respiratory problems when breathed in, and they dissolve in water in the atmosphere to form acid rain, which damages buildings, plants and aquatic life.

Particulates cause global dimming by reflecting sunlight back into space, reducing the light reaching the ground, and they also cause health problems such as lung damage when breathed in.

A common exam task gives you information about a fuel and the conditions, and asks you to predict the products. Work through it with two questions.

Step 1: Which elements are in the fuel?

• Carbon and hydrogen present → the products can include carbon dioxide and water.
• Sulfur present as an impurity → sulfur dioxide can also form.

Step 2: Is there enough oxygen?

• Plenty of oxygen (complete combustion) → carbon dioxide and water only.
• Limited oxygen (incomplete combustion) → also carbon monoxide and carbon particulates.

Worked example. A fuel is a hydrocarbon containing a small amount of sulfur, burned in a limited supply of oxygen. Predict the products.

• It contains carbon and hydrogen, so carbon dioxide and water form.
• Oxygen is limited (incomplete combustion), so carbon monoxide and particulates (soot) also form.
• It contains sulfur, so sulfur dioxide forms too.

Predicted products: carbon dioxide, water, carbon monoxide, particulates and sulfur dioxide. (Remember oxides of nitrogen come from the air at high temperature, so include them only if the question points to high-temperature combustion such as in an engine.)

1. Saying oxides of nitrogen come from the fuel

Oxides of nitrogen form when nitrogen and oxygen from the air react at high temperatures inside engines. They are not an impurity in the fuel, unlike sulfur.

2. Mixing up which pollutant causes which problem

Carbon monoxide is toxic (colourless and odourless); sulfur dioxide and oxides of nitrogen cause respiratory problems and acid rain; particulates cause global dimming. Keep these effects matched to the correct pollutant.

3. Confusing global dimming with the greenhouse effect

Global dimming is caused by particulates blocking sunlight, reducing light reaching the surface. It is a separate idea from the greenhouse effect, which is about greenhouse gases trapping heat.

4. Forgetting why carbon monoxide is hard to detect

The mark is for stating that it is colourless and odourless (as well as toxic), so it cannot be seen or smelled. Do not just write "poisonous".

5. Leaving carbon monoxide or soot out of incomplete combustion

Incomplete combustion produces carbon monoxide and carbon particulates as well as carbon dioxide and water. Do not describe it as producing only carbon monoxide.