Combustion and Acid Rain

When things burn they undergo the chemical reaction of combustion during which the fuel combines with from the air. This forms of the elements they contain. A fuel is anything that will release energy when burnt. Most fuels contain carbon and/or hydrogen.

When pure carbon burns in plenty of oxygen we achieve 'complete combustion':

Carbon	+		→
C	+	O2	→	CO2

When pure carbon burns in limited oxygen we only achieve 'incomplete combustion':

	+	Oxygen	→	Carbon Monoxide	+	Carbon
3C	+	O2	→	2CO	+	C

When Hydrogen burns:

Hydrogen	+	Oxygen	→	Water
2H2	+	O2	→	2H2O

When a (eg Methane) burns in plenty of oxygen:

Hydrocarbon	+	Oxygen	→	Water	+	Carbon Dioxide
CH4	+	2O2	→	2H2O	+	CO2

However some hydrocarbon fuels may also contain some sulfur and when that burns:

Hydrocarbon	+	Sulfur	+	Oxygen	→	Water	+	Carbon	+	Dioxide
CH4	+	S	+	3O2	→	2H2O	+	CO2	+	SO2

As we have seen the gases released into the atmosphere when a fuel burns may include:

• carbon dioxide
• carbon monoxide
• water (vapour)
• sulfur dioxide.

When carbon dioxide dissolves in clouds it forms carbonic acid, this is why normal rain is slightly acidic. There are some scientists that believe that carbon dioxide in the atmosphere traps heat from the sun causing an increase in global temperature. This is known as the effect. The amount of atmospheric Carbon Dioxide has been increasing since the revolution and as our energy demands continue to increase. Some scientists believe that the man-made carbon dioxide is what is causing climate change. Not all scientists agree with this, and offer contradictory evidence such as:

Carbon Monoxide is toxic, but is colourless and odourless, so is known as a 'silent killer'. It is essential that things like gas fires and boilers are adaquately.

The water vapour in the air, and the increased evapouration due to the increased global temperature may form more clouds which may reflect back more of the Sun's heat.

When sulfur dioxide dissolves in clouds it forms sulfuric acid which falls as acid rain. This can kill off vegetation and aquatic life. Sulfur can be removed from fuels they are burned, for example in vehicles - city diesel is low sulfur diesel as the sulfur has been removed during refining. Sulfur dioxide can be removed from the waste gases combustion, for example in power stations - this is known as Flue Gas Desulfurisation (FGD) and is achieved through the use of scrubbers.

Burning hydrocarbons with a large number of carbon atoms often results in 'incomplete combustion'. The increase of particles in the atmosphere caused by incomplete combustion can cause global . This is when the Sun's light is blocked from reaching the Earth's surface.

Pollutant	Effect
Carbon Dioxide	Global
Carbon Monoxide
Unburnt Carbon (soot)	Global
Sulfur Dioxide
Methane	Global
CFCs	Hole in layer