Carbon And It’S Compounds IV – Coal & Fuel Gases SS1 Chemistry Lesson Note

COAL AND FUEL GASES

1. 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.

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:

(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. 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 used 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 are distilled.

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

Coal decomposes to yield several products such as:

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.

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.

FUEL GASES/GASIFICATION OF COKE

There are 3 types of fuel gases.

1. 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

2. 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) + H2(g)

Steam > white-hot coke > Water gas

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

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.

3. 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 it over a Nickel catalyst at about 8000C.

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/it does not contain sulphur or sulphur compounds.

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.