ICSE Class 8 Chemistry Chapter 03 Carbon based Fuels

Carbon-based Fuels

We need energy to meet all our needs. We get this energy from various sources, such as coal, petroleum and water. When we use a source of energy, we say we have harnessed it.

Thermal Energy and Related Terms

Thermal energy, i.e., heat, is the most common form of energy that mankind has been able to use. Though thermal energy is obtained from nuclear reactions also (which you will read about in a later chapter), it is generally obtained from combustion.

When a substance combines with oxygen giving out heat and light, the phenomenon is called combustion.

Thus, combustion involves the burning of a substance. But remember that every substance does not burn. Wood or coal burns, but glass or gold does not.

A substance that undergoes combustion is said to be combustible. For example, wood, coal, petrol, diesel, natural gas and petroleum gas are combustible substances.

A substance that is burnt with a view to obtaining heat (or light) from it is called a fuel.

Thus, wood, coal, petrol, diesel, kerosene, etc., are all fuels. As all these contain carbon in the free or combined state, they are carbon-based fuels.

Supporter of Combustion

A supporter of combustion is a substance whose presence is essential for the combustion of another substance.

For all those substances that burn in air, oxygen is the supporter of combustion. Carbon dioxide generally behaves as a nonsupporter of combustion, though it supports the combustion of magnesium. It is possible that carbon dioxide decomposes into carbon and oxygen first and then the oxygen supports the combustion of magnesium. Magnesium burns in steam also.

Ignition Temperature

Wood or coal is a fuel, but does it start burning on its own? No, it has to be heated with a flame and only then does it begin to burn. But once it starts burning, the reaction is so highly exothermic (i.e., so much heat is produced) that the whole piece of wood or coal burns. So, a fuel starts burning only when it is heated to a temperature called its ignition temperature.

The temperature to which a substance must be heated before combustion takes place is known as its ignition temperature.

You will understand the significance of ignition temperature upon doing the following activity.

Activity Place a paper cup containing water on a flame. The water will become hot, but the cup will not burn. This is because the water takes away the heat from the cup and does not allow it to reach its ignition temperature.

Now, we can easily understand why (i) fire is extinguished by water, and (ii) a log of wood takes a longer time to start burning than wood shavings, when heated in a flame.

When water is poured over a burning substance, it absorbs heat from the substance. As a result, the temperature of the substance falls below the ignition temperature, and it stops burning.

A log of wood has a huge mass. So, when we heat it with a flame, the heat the log receives is dissipated through its bulk. And the log takes a long time to attain the ignition temperature. On the other hand, wood shavings, having a much smaller mass, attain the ignition temperature more readily. So, a log or a large piece of wood takes a longer time than wood shavings to start burning.

Calorific Value of a Fuel

The amount of heat given out by 1 g of a fuel on complete combustion in air or oxygen is known as the calorific value of the fuel.

The term is derived from calorie, which is a unit of heat. In the SI system, however, joule is used as a unit of energy. In SI, the calorific value of a fuel is expressed in kJ/g.

The calorific values of some fuels are given in Table 3.1.

Hydrogen has the highest calorific value. However, it is difficult to handle in the gaseous state as it forms an explosive mixture with air. In the liquid state, it is used as a fuel in spacecraft.

What Makes a Good Fuel?

Several factors are considered while choosing a fuel. Apart from its easy availability at a reasonable price, a good fuel should have the following characteristics.

1. It should have a high calorific value.

2. It should have a low ignition point, but not lower than room temperature.

3. It should not burn too fast or too slowly.

4. The combustion of the substance should not produce harmful substances like soot and poisonous gases.

5. No residues should be left on combustion.

6. It should be safe to store, handle and transport.

Hydrocarbons

Hydrocarbons constitute a big family of fuels.

Hydrocarbons are organic compounds that contain only carbon and hydrogen.

Hydrocarbons may contain small to large numbers of carbon atoms. They are classified into several types, and each type may be represented by a general formula. For example, a class of hydrocarbons called alkanes have the general formula \(C_nH_{(2n+2)}\). Its first member (\(n = 1\)) is methane, whose formula is \(CH_4\). Some other common examples of alkanes are ethane (\(C_2H_6\)), propane (\(C_3H_8\)) and butane (\(C_4H_{10}\)). Acetylene (\(C_2H_2\)) belongs to a different class of hydrocarbons.

Physical State

In nature, hydrocarbons are generally found as mixtures. For example, natural gas and petroleum are mixtures of hydrocarbons.

Under ordinary conditions, among alkanes, the lower members (containing 1-4 carbon atoms) are gases, the higher ones (containing 5-17 carbon atoms) are liquids, and the still higher ones (containing more than 17 carbon atoms) are solids.

From Table 3.2, we find that all the hydrocarbon fuels, except biogas, are associated with petroleum. So, we will discuss biogas here, and the other fuels in a later section along with petroleum.

Biogas

Biogas is obtained by the anaerobic fermentation of cattle dung and domestic sewage. Anaerobic fermentation is fermentation in the absence of air. A mixture of methane (\(CH_4\)), carbon dioxide (\(CO_2\)), hydrogen (\(H_2\)) and hydrogen sulphide (\(H_2S\)) is thus obtained, methane being the main constituent.

The fermentation takes place in an underground tank (made of bricks), called the digester. Slurry of cattle dung and water is fed into the tank. On fermentation of the dung, biogas is evolved and collected in the gas holder (Figure 3.3) or the fixed dome (Figure 3.4).

The gas holder, made of steel, floats over the slurry. The holder moves up when gas collects in it and moves down when gas is drawn from it. (There is an outlet pipe for the gas on top of the gas holder.)

In the fixed-dome type, the gas collects under the fixed dome. When sufficient gas collects, it exerts pressure over the slurry and the slurry is forced into the outflow tank. The spent slurry obtained from the outflow tank is rich in nitrogen, phosphorus and potassium, and is used as a manure.

Larger plants can be established for a community where the cattle population is large. Such plants can be put up easily in rural India. The government subsidises and promotes the establishment of biogas plants through various agencies.

Apart from animal dung, biogas is obtained by a similar process from municipal sewage.

Combustion of biogas

Methane, the main constituent of biogas, burns to give carbon dioxide and water vapour. As methane contains only one carbon atom, it burns out completely. Thus no soot (unburnt carbon particles) or carbon monoxide is formed.

\[CH_4 + 2O_2 \rightarrow CO_2 + H_2O + \text{heat}\]

Fossil Fuels

You know that coal and petroleum are fossil fuels, and natural gas collects over petroleum in a petroleum well. Though their stocks are limited, they are the most widely used fuels of today.

Coal

Coal is the most widely found fossil fuel. It has been used from ancient times. It is obtained from mines by opencast mining, when the coal deposit is near the surface of the earth, and underground mining, when the deposit lies deep under the surface.

Of the total world reserves, more than half lie in North America and South America, one-third in Asia and the rest in other parts of the world. In India, coal is found mainly in West Bengal, Jharkhand, Orissa, Chhattisgarh and Madhya Pradesh.

Formation of coal

Millions of years ago, plants thrived in large, shallow swamps. They were buried under the earth owing to some natural phenomena, and fossilised in course of time. A combination of heat, pressure and bacterial action gradually converted these buried remains into coal. Plants contain carbon compounds. So, coal is mainly carbon.

Peat is the first stage of formation of coal. When the plants died, they fell into the swamps, and were covered by sand and silt. Below the water, they were decomposed by anaerobic bacteria, which do not require the presence of oxygen. This led to the formation of peat. (In contrast, when dead plants fall on the ground, they are decomposed by aerobic bacteria. The products then are liquid or gaseous.)

Because of earthquakes and volcanic upheavals, the remains of these plants and peat sank below the ground. Several such events occurred, and many layers of plant remains were deposited beneath the ground over millions of years. These layers, especially the lower ones, experienced tremendous pressure and temperature. Because of the high temperature and pressure, certain gaseous products including water vapour were driven out, and the carbon content in the plant remains increased. This process, as you know, is called carbonisation. This is how vast deposits of rocky carbon, called coal, were formed.

The type of coal formed depends upon the pressure and temperature to which the layer of plant remains has been subjected, and the time for which the plant remains have been buried.

All these factors, in turn, depend upon the depth of the layer. Thus, four stages of the formation of coal have been identified - peat, lignite, bituminous coal and anthracite (Figure 3.5). It is natural that the extent of carbonisation, and therefore, the carbon content increases with depth. Anthracite is the best quality coal and then come bituminous coal, lignite and peat.

Apart from carbon, coal contains nitrogen and sulphur as impurities.

The destructive distillation of coal

The destructive distillation of coal can be carried out just as that of wood (Figure 3.6). Some vapours are produced. A part of these vapours is condensed into coal tar and ammoniaca liquor on being cooled. The rest escapes as coal gas. A solid residue (coke) is left behind.

Coal gas

Coal gas is a mixture of hydrogen, carbon monoxide and methane, all of which are combustible. So, coal gas is an important fuel. Previously, it was used for street lighting. It is still used in industries for several purposes.

\[2H_2 + O_2 \xrightarrow{\text{burn}} 2H_2O\]

\[2CO + O_2 \xrightarrow{\text{burn}} 2CO_2\]

\[CH_4 + 2O_2 \xrightarrow{\text{burn}} CO_2 + 2H_2O\]

Coal tar

It is a thick, black liquid collected on the condensation of the vapours. It contains many organic compounds. The pitch obtained from coal tar is used in making roads.

Ammoniaca liquor

It is a solution that collects over coal tar, and contains ammonium hydroxide. It is used for making fertilisers.

Coke

Coke is the solid residue left behind in the process. It contains more than 98% carbon. It is used (i) as a reducing agent in metallurgy, and (ii) for producing fuel gases like water gas and producer gas.

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