COAL AND FUEL GASES

COAL

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

Table of Contents

Carbonization of coal.

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 carbonization, the vegetable material was converted in stages into peat. They gradually passed through several stages: Peat lignite bituminous anthracite.

Types of Coal

There are 4 different types of coal namely:

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(1) Peat-like coal: It contains about 60% of carbon by mass.

(2) Lignite coal (brown coal): It contains about 67% of carbon by mass.

(3) Anthracite coal (or hard coal): It is tough and hard. It contains about 94% of carbon by mass. Impurities present may include nitrogen, sulphur and phosphorus. Anthracite is the last stage of coal.

(4) Bituminous (soft) coal: These are use every day at home. It contains about 88% by mass of carbon.

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 distil over.

Coal Coke + Ammoniacal liquor + Coal tar + Coal gas.

Coal decomposes to yield a number of products such as:

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Solid product e.g coke: It is the non-volatile residue left behind after destructive distillation.

Uses of coke

(i) Coke is mainly used as a fuel.

(ii) It is a very important industrial reducing agent and is used in the extraction of metals, especially iron, from their ores.

(iii) It is also used in the production of gaseous fuels, like water gas and producer gas.

(iv) It is used for the manufacture of graphite, calcium carbide, silicon carbide and carbon (iv) sulphide.

Liquid product: It include: Ammoniacal liquor and coal tar.

(a) Ammoniacal liquor: is a solution of NH₃ in water.

(b) Coal tar: This is the thick, viscous substance from which more than a hundred other products are derived. Products obtained by fractional distillation of coal tar include toluene, phenol, benzene, naphthalene and anthracene which are used in the synthesis of important commercial product like dyes, paints, insecticides, drugs, plastics and explosives.

Gaseous product e.g coal gas: It consists mainly of about 50% hydrogen, 30% methane, 10% carbon (ii) oxide with small amounts of gaseous impurities such as SO₂, CO₂, ethane, N₂, hydrogen sulphide, carbon (iv) sulphide and hydrogen cyanide.

Distillates of Coal	Uses
1.Ammoniacal liquor	To produce (NH₄)₂SO₄ for fertilizer.
2.Coal tar	To produce useful chemicals such as disinfectants and perfumes
3.Coal gas	Used as industrial fuel.

Uses of coal

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

EVALUATION

List the types of coal.
List the products of the destructive distillation of coal and state the use of each product

FUEL GASES/GASIFICATION OF COKE

There are 3 types of fuel gases.

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)+ O_2(g) + N_2(g) 2CO_(g)+ N_2(g) + Heat

Producer gas

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

H₂O_(g) + C_(s) CO_(g) + H_2(g)

Steam white hot coke Water gas

Water gas is then mixed with excess steam, and the mixture passed over iron (iii) oxide catalyst at 450⁰C.The carbon (ii) oxide decomposes the steam and the product are hydrogen and carbon (iv) oxide.

CO_(g) + H_2(g)+ H₂O_(g) CO_2(g) + 2H_2(g)

Caustic soda or water is used to absorbed carbon (iv) oxide from the mixture. Ammoniacal copper (i) chloride can be used to remove unreacted carbon (ii) oxide. The final product is hydrogen.

Differences between Producer Gas and Water Gas

(1) Producer gas has a low calorific value while water gas has a high calorific value (i.e producer gas has a lower heating ability than water gas.

(2) Water gas consists of equal volumes of hydrogen and carbon (ii) oxide both of which are combustible whereas producer gas consists of 33% combustible CO and 67% non-combustible N₂.

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 1000⁰C.

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

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

Synthetic gas: It is a mixture of hydrogen and carbon (ii) oxide gas. It is prepared by mixing steam with methane (obtained as natural gas) and passing them over Nickel catalyst at about 800⁰C.

CH_4(g) + H₂O_(g) CO_(g) + 3H_2(g)

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

EVALUATION

List the three types of fuel gases.
What can be used to absorb carbon (iv) oxide from a water gas mixture?
What can be used to remove unreacted carbon (ii) oxide from a water gas mixture?

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