O Level Revision : Integrated Science - Science in Energy Uses - Fuels

Fuels are materials that are burnt to provide heat or chemical energy. Fuels can be gases, liquids or  solids. There are natural and man-made fuels. Some fuels are more efficient than others. Engines are machines designed to convert chemical energy into other forms of energy and thus do work. The use of fuels has social and economic implications on society. Fuels are either renewable or non-renewable. Non-renewable fuels should be used sustainably.

Types of fuels

Definition: A fuel is a material that can be burned to give out heat or provide chemical energy.

  • Solid fuels include wood, coal, coke, charcoal and cowdung.
  • Liquid fuels include paraffin, ethanol, petrol, diesel and methylated spirit.
  • Gaseous fuel includes methane, butane, hydrogen and ethane.
  • Burning a fuel is a chemical reaction. Chemical energy is converted to heat energy giving off carbon dioxide and water.

Fuel + oxygen  → carbon dioxide + water + energy


  • Coal is found as bituminous coal at Hwange Colliery in Zimbabwe.
  • It is a fossil fuel and is non-renewable.

Uses of coal

  1. Thermal power generation (Hwange Thermal Power Station).
  2. Domestic cooking and heating.
  3. Industry (heating and power generation).
  4. Agriculture (tobacco curing).
  5. Production of coke (coking).
  • Coal can be processed to produce a more efficient fuel known as coke.
  • The process is called the destructive or dry distillation of coal.
  • Coke is used for iron smelting in the blast furnace.

Destructive distillation of coal




(i)   crushed coal

(v)  2 beakers

(ii)  3 test tubes        (iii) class tubing

(vi) 3 retort stands   (vii)burner

(iv)   rubber tubing

(viii) water


Heat the crushed coal

Destructive distillation of coal


  1. Solid residue remain in the test tube 1.
  2. A gas is collected in test tube 3.
  3. An aqueous solution forms in test tube 2 and dark liquid settles at the bottom.


  1. The residue in test tube 1 is coke.
  2. The gas collected is coal gas.
  3. The aqueous solution is ammoniacal liquor (contains ammonia).
  4. The dark liquid is coal tar (contains benzol).

 Products of the destructive distillation of coal and their uses





Coal gas



Manufacture of fertilisers


Drugs, disinfectant, dyes, plastics, perfumes, solvent, insecticides


Road making, preserving wood, used in the chemical industry

Energy from biological waste (biogas production)

  • Recovery of energy from biological waste is achieved by fermenting the waste in the biogas digester.
  • Waste fermentation produces bio-gas which can be used as a fuel.
  • Fermentation refers to the decomposition of organic matter without the use of oxygen.

bio-gas production

Biogas collection

The biogas digester

Factors affecting biogas production

  1. Anaerobic bacteria ferments waste.
  2. An optimum temperature (35-55°C).
  3. Suitable pH of 6-7 for anaerobic bacteria.
  4. Moisture / water.



  1. saves wood
  2. is clean

The digester provides improved fertiliser.




Energy from fermentation of sugar or maize (ethanol production)

Ethanol is a liquid fuel produced by fermenting sugar or maize i.e.


Yeast (zymase)


Factors affecting ethanol production are:

  1. Yeast (enzyme).
  2. Optimum temperature (about 37°C).
  3. Suitable pH (about 7).
  • The ethanol produced in the ethanol plant is dilute.
  • It is concentrated by fractional distillation.

Investigating favourable conditions for ethanol production

Investigating conditions of ethanol production

Test tube



left at room temperature


left in a water bath maintained between 35 and 40oC


left in a refrigerator


contents of test tube boiled and test tube left at room temperature

The most ideal conditions are determined by noting the amount of carbon dioxide collected in the syringe.


  • The most carbon dioxide was produced in test tube 2.
  • Very little to almost no carbon dioxide was produced in test tube 4 where yeast had been denatured (killed) by boiling.


The ideal conditions for ethanol production are:

  • Optimum temperature of between 35oC and 40oC.
  • Presence of a catalyst, yeast.

Fuel efficiency

  • Same amounts of different fuels give different amounts of heat energy when burned.
  • The amount of energy produced by burning a specific amount of fuel is called its efficiency.
  • The efficiencies of fuels can be calculated after measuring the rise of temperature of the substance being heated by the burning fuel.
  • Simple laboratory experiments can be conducted, but care must be taken to minimize the loss of heat to the environment.



  • A fuel gives heat energy when it reacts with oxygen.
  • Carbon dioxide and water are produced during the reaction.
  • Complete combustion happens when there is sufficient oxygen supply.
  • When the supply of oxygen is insufficient, incomplete combustion results.
  • Complete combustion releases more energy than incomplete combustion.
  • A Bunsen burner flame is efficient when there is complete combustion i.e. when the air hole is open ensuring sufficient supply of oxygen. The flame will be blue.
  • When the air hole is closed, incomplete combustion occurs producing a yellow sooty flame which is less efficient.

Complete combustion equation

Fuel  +  oxygen → carbon dioxide  +  water  +  heat


Incomplete combustion equation

Fuel  +  oxygen  → carbon monoxide  +  carbon  +  water  vapour  +  heat

Products of combustion

  • Combustion produces noxious (irritating or harmful) gases.
  • The products of complete combustion are carbon dioxide and water has adverse effects when it accumulates in the atmosphere.
  • The products of incomplete combustion include carbon monoxide, carbon and water. Carbon monoxide is a poisonous gas.

Fuel engines

  • Fuel engines convert the chemical energy stored in fuels to kinetic energy.
  1. Intake
  • piston moves down
  • volume of cylinder increase thus pressure decrease
  • inlet valve open
  • petrol-air mixture drawn into cylinder
  • exhaust valve closed
  1. Compression
  • both valves closed
  • piston moves up
  • compresses petrol-air mixture
  1. Power
  • both valves closed
  • spark, produced by spark plug ignites petrol-air mixture
  • mixture explodes causing a rise in pressure
  • piston moves down
  1. Exhaust stroke
  • piston moves up
  • exhaust valve opens
  • exhaust gas move out
  • inlet valve is closed

Operational principles of a four stroke engine

Note: The engine has multiple cylinders to ensure even (smooth) firing and even power


The carburetor

The carburetor helps to regulate the correct and clean petrol-air mixture to the engine.

Parts of the carburetor and their functions


Jets               :

petrol is supplied into carburetor through jets.


Air filters     :

clean (filter) the air before it gets into the carburetor.


Petrol filter  :

cleans (filters) the petrol before it gets into the carburetor (it is important to supply



clean fuel to avoid blocking the jets).


Choke          :

controls the amount of air entering the carburetor (closing the choke gives a mixture



rich in petrol usually done to make starting the car easier).


Throttle       :

is controlled by the accelerator and allows more or less of the petrol air mixture into



the engine.

Note: The efficiency of an engine can be lowered by worn out carburetor jets and blocked filters leading to an incorrect petrol-air mixture thus incomplete combustion.

Comparing the operation of a diesel and a petrol engine



Has spark plug to ignite petrol-air mixture

No spark plug. Compression causes air to become very hot and when diesel is sprayed into cylinder it ignites because of the high temperatures

Has carburetor

Has fuel injectors

Less efficient i.e. produces more carbon


More efficient (efficiency is measured by fuel

economy - kilometres per litre)

Causes more air pollution

Causes less air pollution



Renewable and non-rewable fuels

  • Non-renewable fuels get used up and cannot be regenerated.
  • Fossil fuels like oil (petrol, diesel) and coal are non-renewable fuels. They cannot be regenerated once they are used up.
  • It is therefore important to conserve fuels.
  • Fuels that can be re-formed or replenished or are continually available are renewable e.g. wood, biogas, ethanol, wind and the sun.

Social and economic implications of using fuels

  1. Use of fire wood as a fuel may lead to uncontrolled cutting down of trees, resulting in deforestation and land degradation.
  2. Burning fuels produces:
  3. a) carbon dioxide which may cause global warming,
  4. b) carbon monoxide which is a poisonous gas and is a result of incomplete combustion of fuels, c)    unburnt carbon (soot which results in ‘smog’), and
  5. d) oxides of sulphur which may dissolve in rain water forming acid rain. Acid rain damages plants and acidic conditions are not suitable for aquatic life. It also affects limestone buildings and lead based paints and corrodes super structures made from metals such as bridges.

Safe handling of fuels

Fuels need careful handling:

  • During transportation, they should be transported in metallic containers.
  • Spillage in water bodies may lead to destruction of ecology of the area since the fuel form a layer above the water level and prevents oxygen from entering water. Birds’ feathers stick also be affected by the oil together and birds fail to fly and their bodies are exposed to the cold.
  • Fuels are inflammable thus care must be taken to avoid their catching fire and thus causing accidents.