When coal is carbonized coke is formed along with coal gas.

                            1300⁰C

         Coal                                 coke + coal gas

                     ^{In absence of air}

Coal is taken in a silica retort and heated to 1300⁰C in absence of air. The coal gas produced is passed through a hydraulic main to remove tar. Any tar or ammonia present in gas is removed by scrubbing with water. The colled gas is then scrubbed with creosote oil where, benzol, naphthalene etc. gets dissolves and removed. The sulphur compounds can be removed by passing the gas in a column of ferric oxide, which can be regenerated on exposure to air.

     Fe₂O₃ (S) = 3H₂S (g)       Fe₂S₃(S) + 3H₂O(l)

Fe2O3 is regenerated by heating Fe2S3 in presence of air.

     2Fe₂S₃ (S) + 3O₂ (g)       2Fe₂O₃(S) + 6H₂S(g)

The purified coal gas is finally stored over water in gas holder. The average composition of coal gas is

     CH₄ =32%            H₂ = 40%          CO = 7%          N₂ = 4%

     C₂H₄ = 3%            C₂H₂ = 2%        CO = 1%

Calorific value as illuminant as a fuel. It provides reducing atmosphere in metallurgy.