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A fuel is a substance that can be combusted or burnt to release energy as a byproduct. The energy can be in the form of heat, light, electricity, sound etc. This energy can be harnessed to power machines or used for other purposes such as heating or lighting. Combustion is the burning of fuel with energy released as a byproduct. Fuel is a very important substance for the existence of a modern man. Examples of fuels include petroleum products (petrol, diesel, fuel oil, kerosene, spirits, etc), natural gas, coal, wood, charcoal, producer gas, water gas, etc. The following are the three classification of fuels according to their States:-

1.Solid fuels: Solid fuels include wood, charcoal, peat, lignite, coal, coke, etc. The immediate use of all these fuels is for heating and lighting. However, these fuels have a long history of industrial use. Coal was the fuel for the industrial revolution, from firing furnaces to running steam locomotives and trains. Wood was extensively used to run locomotives. Coal is still used for generation of power until now. For example, in Tanzania the coal mined at Kiwira is used for generation of electricity. Also Tanga Cement Company uses coal as a source of power to run machines for production of cement.

Wood is used as a solid fuel for cooking, heating or, occasionally, as a source of power in steam engines. The use of wood as a fuel source for home heating is as old as civilization itself. Wood fuel is still common throughout much of the world. It is the main source of energy in rural areas. Wood charcoal yields a large amount of heat in proportion to its quantity than is obtained from a corresponding quantity of wood, and has a further advantage of being smokeless. Wood charcoal is often used for cooking and heating, in blacksmithing, etc.

Animal charcoal is used for sugar refining, water purification, purification of factory air and for removing colouring matter from solutions and from brown sugar. Animal charcoal is made by destructive distillation of animal bones.

Coke is a fuel of great industrial use. Coke is obtained by destructive distillation of coal. Most of the coke produced in industry is used as a reducing agent in the production of metals such as pig iron. A substantial amount of coke is also used for making industrial gases such as water gas and producer gas. Coke is a better fuel than coal because when it is burning, it produces a clean and smokeless flame. When coal is used as a fuel, it produces many toxic gases during burning. Coke has high heat content and leaves very little ash.

Coal is a complex mixture of substances, and its composition varies from one place to another. It depends on coal’s age and condition under which it was formed. Anthracite is a very hard black coal and it is the oldest of all types of coal.

When coal is heated in a limited supply of air, it decomposes. This thermal decomposition is called destructive distillation of coal. The products are coke, coal tar, ammoniacal liquor and coal gas.

2. Liquid fuels: Liquid fuels include petrol (gasoline) diesel, alcohol (spirit), kerosene (paraffin), liquid hydrogen, etc. Liquid fuels have advantage over solid fuels because they produce no solid ashes, and can be regulated by automatic devices. They are relatively more convenient to handle, store and transport than solid fuels.

Most liquid fuels in wide use are derived from fossils. Fossil fuels include coal, natural gas and petroleum. These fuels are formed from remains of sea plants and animals which lived millions of years ago. The remains became buried under layers of sediment. Immense heat and pressure resulted in the formation of coal gas and oil.

Energy produced when petroleum products (diesel, petrol, kerosene, natural gas etc) are burned, originated from the sun. This energy was transferred to animals through their consumption of plants or plant products. When the animals died, got buried, and compressed by heat and pressure, they produced oil which gives off that energy when burnt. Petroleum fuels are used in cars and in various other machines. Fuels used in cars and lories (petrol and diesel), kerosene (for jet aircraft) and fuel oil (for ships), all came from crude oil. Some oil fuel is also used for electricity generation.

Ethanol burns with a clean, non-smoky flame, giving out quite a lot of heat. On a small scale, ethanol can be used as methylated spirit (ethanol mixed with methanol or other compounds) in spirit lamps and stoves. However, ethanol is such a useful fuel that some countries have developed it as a fuel for cars. In countries where ethanol can be produced cheaply, cars have been adapted to use a mixture of petrol and ethanol as fuel.

The idea about the use of biofuel for fuelling automobiles and other machines has been borrowed by other countries including Tanzania. However, the programme has raised a bitter concern among different activists. Their doubt is that emphasis on growing crops for biofuel production may take up land that could otherwise be used for growing food crops. This, therefore, would mean that there would not be enough land to grow enough food to feed the ever-increasing human population. Hence, hunger will prevail. Notwithstanding all these shouting, biofuel crop production is there to stay.

3. Gaseous fuels: The use of gaseous fuels for domestic heating is common in urban areas. Compressed gas that is delivered to our homes in steel cylinders is liquefied propane, butane, or mixture of the two. When the valve is opened, the liquid gas vapourizes quickly into gas and passes through a pipe to the stove. Gaseous fuels are the most convenient fuels to handle, transport and store. The following is a list of types of gaseous fuels.

(a) Producer gas: Producer gas is produced by burning a solid carbonaceous fuel, such as coke, in a limited supply of air in a producer furnace. The reaction is exothermic and this makes coke to get hotter. Carbonaceous fuels are fuels that contain a high proportion of carbon. The producer gas is a mixture of carbon monoxide and nitrogen.

When air, mixed with a little steam, is passed through the inlet in the lower part of the furnace, the coke (carbon) combines with oxygen (from air) to form carbon dioxide,As the carbon dioxide formed rises up through the red-hot coke, it is reduced to carbon monoxide:
Since more heat (406 kJ) is produced in the lower part than is absorbed in the upper part of the furnace (163 kJ), some excess heat is obtained in the long run.This heat keeps the coke hot. The nitrogen gas in the air is not affected at all during the process.

As a fuel, producer gas burns to give out carbon dioxide. Because a good deal of producer gas contains nitrogen, a gas that does not support combustion, it has a lower calorific value compared to water gas.

(b) Water gas: Water gas is produced by passing steam over white-hot coke at 1000°C. The gas is a mixture of hydrogen and carbon monoxide. The reaction is endothermic, causing the coke to cool.

Water gas burns as a fuel to give carbon dioxide and steam. However, carbon monoxide is a very poisonous gas. The gas made from petroleum or coal contains some carbon monoxide, which makes it poisonous. Natural gas is safer and efficient, as it contains no carbon monoxide.

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