4.9.3.1Atmospheric pollutants from fuels
The combustion of fuels is a major source of atmospheric pollutants.
Most fuels, including coal, contain carbon and/or hydrogen, and may also contain some sulfur.
The gases released into the atmosphere when a fuel is burned may include carbon dioxide, water vapour, carbon monoxide, sulfur dioxide and oxides of nitrogen. Solid particles and unburned hydrocarbons may also be released, that form particulates in the atmosphere.
You should be able to describe how carbon monoxide, soot (carbon particles), sulfur dioxide and oxides of nitrogen are produced by burning fuels. Each has a different cause, and the mark is for the cause, not the name:
The oxides of nitrogen are the odd ones out, and worth pausing on. The nitrogen does not come from the fuel at all — it is the nitrogen in the air drawn into the engine, which is normally unreactive but reacts with oxygen at the very high temperatures inside a running engine.
You should be able to predict the products of combustion of a fuel given appropriate information about the composition of the fuel and the conditions in which it is used.
Worked example
A fuel contains carbon, hydrogen and a small amount of sulfur. It is burned in an engine with a limited supply of air. Predict the products.
- Every element in the fuel will be oxidised as far as the oxygen allows.
- Hydrogen gives water vapour.
- Carbon with a limited supply of oxygen gives carbon dioxide, but also carbon monoxide and soot, because there is not enough oxygen for complete combustion.
- The sulfur in the fuel is oxidised to sulfur dioxide.
- The engine is hot, so nitrogen and oxygen from the air react to form oxides of nitrogen.
Answer: Water vapour, carbon dioxide, carbon monoxide, soot, sulfur dioxide and oxides of nitrogen — plus some unburned hydrocarbons.
Common mistake
Read the conditions before answering. With plenty of air the products are only carbon dioxide and water, plus sulfur dioxide if the fuel contains sulfur. Carbon monoxide and soot appear only when the oxygen supply is limited.
4.9.3.2Properties and effects of atmospheric pollutants
Carbon monoxide is a toxic gas. It is colourless and odourless, and so is not easily detected.
Sulfur dioxide and oxides of nitrogen cause respiratory problems in humans and cause acid rain.
Particulates cause global dimming and health problems for humans.
You should be able to describe and explain the problems caused by increased amounts of these pollutants in the air.
Common mistake
Acid rain and global dimming are separate problems with separate causes: acid rain comes from sulfur dioxide and oxides of nitrogen dissolving in water, while global dimming comes from solid particulates reflecting sunlight. Neither is the greenhouse effect.
- Carbon monoxide
- a toxic gas produced by incomplete combustion, which is colourless and odourless and so not easily detected
- Acid rain
- rain made acidic by sulfur dioxide and oxides of nitrogen released when fuels are burned
- Global dimming
- the reduction in sunlight reaching the Earth's surface caused by particulates in the atmosphere
Before you practise
- Most fuels contain carbon and/or hydrogen, and may contain some sulfur.
- Complete combustion gives carbon dioxide and water vapour.
- Incomplete combustion, from a limited supply of oxygen, gives carbon monoxide and soot, and unburned hydrocarbons may be released.
- Sulfur dioxide comes from sulfur impurities in the fuel being oxidised.
- Oxides of nitrogen come from nitrogen and oxygen in the air reacting at the high temperature inside an engine.
- Carbon monoxide is toxic, and colourless and odourless so not easily detected.
- Sulfur dioxide and oxides of nitrogen cause respiratory problems and acid rain.
- Particulates cause global dimming and health problems.