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 slow decomposition of plant material under pressure and without air.

Behavioural objectives

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

1. describe how coal was formed and name the stages of carbonisation;
2. state the four types of coal and their approximate carbon content;
3. describe the destructive distillation of coal, name its products and state one use of each;
4. state the uses of coal and coke;
5. describe how producer gas, water gas and synthetic gas are prepared;
6. state the differences between producer gas and water gas.

What is coal?

Coal is an impure form of carbon made up of a complex mixture of compounds.

• Main elements: carbon, hydrogen and oxygen.
• Impurities: small amounts of nitrogen, sulphur and phosphorus.
• Origin: plant vegetation buried for a very long time.

How was coal formed?

Coal was formed by the gradual decomposition of plant vegetation under pressure and in the absence of air.

• Gases given off: carbon(IV) oxide, methane and steam were liberated.
• Residue: a material with a very high percentage of carbon was left behind.
• Carbonisation: this slow conversion of plant material into coal in stages.

What are the stages of carbonisation?

Plant material changes into coal in four stages, from peat to anthracite.

1. Peat: the first stage, formed directly from the plant material.
2. Lignite: brown coal, formed from peat.
3. Bituminous coal: soft coal, formed from lignite.
4. Anthracite: hard coal, the last stage.

Peat → lignite → bituminous → anthracite

What are the types of coal?

There are four types of coal: peat-like coal, lignite, bituminous coal and anthracite.

What is the destructive distillation of coal?

Destructive distillation of coal is the heating of coal strongly in the absence of air to obtain useful substances.

What are the steps in the destructive distillation of coal?

1. Coal is placed in a closed container so that no air enters.
2. It is heated to a very high temperature.
3. The volatile components distil off as vapours and gases.
4. The vapours are cooled, so coal tar and ammoniacal liquor condense.
5. Coal gas is collected as the uncondensed gas.
6. Coke remains as the solid residue.

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

What are the products of destructive distillation of coal and their uses?

Destructive distillation of coal gives four products: coke, ammoniacal liquor, coal tar and coal gas.

What are the uses of coke?

Coke is the solid, non-volatile residue left after the destructive distillation of coal.

1. Fuel: coke is mainly used as a fuel.
2. Reducing agent: it is an important industrial reducing agent, used to extract metals, especially iron, from their ores.
3. Fuel gases: it is used to produce gaseous fuels such as water gas and producer gas.
4. Manufacture: it is used to make graphite, calcium carbide, silicon carbide and carbon(IV) sulphide.

What are the uses of coal?

Coal is used mainly as a fuel and as a raw material for chemicals.

1. Fuel: it generates power for steam engines, factories and electrical plants.
2. Chemicals: it is used for making various chemicals.

What are fuel gases?

Fuel gases are gases that burn to give heat; the three studied here are producer gas, water gas and synthetic gas.

• Producer gas: made from coke and air.
• Water gas: made from coke and steam.
• Synthetic gas: made from methane (natural gas) and steam.

What is producer gas?

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

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

• Cost: it is inexpensive.
• Heating: used in furnaces and limekilns.
• Retorts: used for firing retorts in the manufacture of zinc and coal gas.
• Nitrogen source: supplies nitrogen for making NH₃ (Haber process).

What is water gas?

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

H₂O(g) + C(s) → CO(g) + H₂(g)

Steam + white-hot coke → water gas

• Fuel: it is an important industrial fuel.
• Hydrogen: it is used in the manufacture of hydrogen.
• Organic compounds: it is used to make compounds such as methanol and butanol.

How is hydrogen obtained from water gas?

1. Mix water gas with excess steam.
2. Pass the mixture over an iron(III) oxide catalyst at 450°C.
3. The carbon(II) oxide reacts with the steam to give hydrogen and carbon(IV) oxide.
4. Absorb the carbon(IV) oxide with caustic soda or water.
5. Remove unreacted carbon(II) oxide with ammoniacal copper(I) chloride.
6. Hydrogen is left as the final product.

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

What is synthetic gas?

Synthetic gas is a mixture of hydrogen and carbon(II) oxide, prepared from methane and steam over a nickel catalyst at about 800°C.

CH₄(g) + H₂O(g) → CO(g) + 3H₂(g)

• Source of methane: natural gas.
• Pollution: it is not a major source of air pollution.
• Reason: sulphur is removed in the gasification process, so it contains no sulphur or sulphur compounds.

What are the differences between producer gas and water gas?

• Producer gas: a mixture of carbon(II) oxide and nitrogen, made by passing air over red-hot coke.
• Water gas: a mixture of carbon(II) oxide and hydrogen, made by passing steam over white-hot coke.

Note: Industrially, both gases can be made in the same plant by passing air and steam through heated coke above 1000°C.

Evaluation

1. Name, in order, the four stages of carbonisation from peat to 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 the four products of the destructive distillation of coal and state one use of each.
2. List two differences between producer gas and water gas.