How is ultimate analysis of coal carried out in the laboratory?

The ultimate analysis includes determination of carbon, sulfur, nitrogen, ash and oxygen. It is the elemental analysis of coal. It is used for calculation of calorific values.

Procedure

Carbon and hydrogen

Generally, both the elements are estimated simultaneously. Accurately weighed coal sample is heated in a tubular furnace in excess of oxygen. hydrogen gets converted to water and carbon to dioxide. These are absorbed in U-tube containing anhydrous calcium and U-tube containing potassium hydroxide respectively.

Nitrogen

Nitrogen present in coal sample is estimated by Kjeldahl’s method. The basic principle is to convert nitrogen to ammonium salt. The ammonium salt on treatment with NaOH liberates ammonia which is estimated by back titration, using standard acid solution.

Concentrated H2 for about 1-2 hours. Here nitrogen present in the coal gets converted ammonium salts. The sample I of cooled, diluted with distilled water. The contents are transferred to round bottom flask concentrated NaOH is added and the flask is heated. Ammonia is distilled into a measured amount of acid. The residual acid is back titrated with standard NaOH. %N is calculated from the amount Of acid consumed.

Sulfur

Sulfur is determined from the bomb washings obtained due to combustion of a known quantity of coal in bomb calorimeter experiment. The washings contain sulfur in form of sulfate which is precipitated as BaSO4 the weight of sulfur present in coal is calculated.

Ash

Ash is determined by the heating known quantity of coal in a furnace at 400 0C for 1half hours and finally at 750 0C for 1 hour to complete the combustion. Heating, cooling and weighing are done till constant weight is obtained.

Oxygen
Oxygen is determined by the difference as follows –
% Oxygen = 100 –( % Carbon + % Hydrogen + % Sulfur +% Nitrogen + % Ash)

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