Carbon and It’S Compounds IV: Coal & Fuel Gases – SS1 Chemistry Lesson Note (Third Term)

Coal is an impure form of carbon formed by the gradual decomposition of plant material under pressure and without air. Heating coal in the absence of air (destructive distillation) gives coke, ammoniacal liquor, coal tar and coal gas. Coke is then used to make fuel gases such as producer gas and water gas.

Behavioural objectives

By the end of the lesson, students should be able to:

• describe how coal was formed and name the stages of carbonisation;
• state the four types of coal and their approximate carbon content;
• describe the destructive distillation of coal and name its products;
• state the uses of coal and coke;
• describe how producer gas, water gas and synthetic gas are prepared, and state the differences between producer gas and water gas.

What is coal?

Coal is an impure form of carbon. It is a complex mixture of compounds composed mainly of carbon, hydrogen and oxygen, with small amounts of nitrogen, sulphur and phosphorus as impurities.

How was coal formed?

Coal was formed by the gradual decomposition of plant vegetation under pressure and in the absence of air. Carbon(IV) oxide, methane and steam were liberated, leaving behind a material that contained a very high percentage of carbon.

During this process of carbonisation, the vegetable material was converted in stages, first into peat. It then passed gradually through several stages:

Peat → lignite → bituminous → anthracite

What are the types of coal?

There are four types of coal.

What is the destructive distillation of coal?

The process by which a wide variety of substances can be obtained from coal is known as the destructive distillation of coal.

During the process, coal is heated to a very high temperature in the absence of air so that all the volatile components are distilled.

Coal → Coke + Ammoniacal liquor + Coal tar + Coal gas

Coal decomposes to yield several products. The solid product is coke, which is the non-volatile residue left behind after destructive distillation.

What are the uses of coke?

1. Coke is mainly used as a fuel.
2. It is a very important industrial reducing agent and is used in the extraction of metals, especially iron, from their ores.
3. It is used in the production of gaseous fuels, like water gas and producer gas.
4. It is used for the manufacture of graphite, calcium carbide, silicon carbide and carbon(IV) sulphide.

What are the uses of coal?

1. Coal is used mainly as fuel to generate power for steam engines, factories and electrical plants.
2. It is also used for making various chemicals.

What are fuel gases?

Fuel gases can be made by the gasification of coke. There are three types: producer gas, water gas and synthetic gas.

Producer gas

Producer gas is a mixture of nitrogen and carbon(II) oxide. It is prepared by passing a stream of air through red-hot coke.

2C(s) + O₂(g) + N₂(g) → 2CO(g) + N₂(g) + heat (producer gas)

Producer gas is inexpensive and is widely used in heating furnaces, in firing retorts (in the manufacture of Zn and coal gas) and in limekilns. It is also a source of nitrogen for the manufacture of NH₃ (Haber process).

Water gas

Water gas is a mixture of hydrogen and carbon(II) oxide. It is prepared by passing steam over white-hot coke.

H₂O(g) + C(s) → CO(g) + H₂(g)

Steam + white-hot coke → water gas

Water gas is an important industrial fuel and is used in the manufacture of hydrogen and other organic compounds, e.g. methanol and butanol.

To obtain hydrogen, water gas is mixed with excess steam, and the mixture is passed over an iron(III) oxide catalyst at 450°C. The carbon(II) oxide decomposes the steam, and the products are hydrogen and carbon(IV) oxide.

CO(g) + H₂(g) + H₂O(g) → CO₂(g) + 2H₂(g)

Caustic soda or water is used to absorb carbon(IV) oxide from the mixture. Ammoniacal copper(I) chloride can be used to remove unreacted carbon(II) oxide. The final product is hydrogen.

Synthetic gas

Synthetic gas is a mixture of hydrogen and carbon(II) oxide. It is prepared by mixing steam with methane (obtained as natural gas) and passing it over a nickel catalyst at about 800°C.

CH₄(g) + H₂O(g) → CO(g) + 3H₂(g)

Synthetic gas is not a major source of air pollution because sulphur is removed in the gasification process, so it does not contain sulphur or sulphur compounds.

What are the differences between producer gas and water gas?

Note: Industrially, producer gas and water gas can be made in the same plant, by passing air and steam through heated coke at a temperature above 1000°C.

Evaluation

1. Name the four stages through which plant material passes to form anthracite.
2. What is the destructive distillation of coal?
3. State three uses of coke.
4. How is water gas prepared? Write the equation for the reaction.
5. Why is synthetic gas not a major source of air pollution?

Assignment

1. Name four products of the destructive distillation of coal and state one use of each.
2. List two differences between producer gas and water gas.