Burning Coal : How much Air is required ?

Written by: johnzactruba • Edited by: Lamar Stonecypher
Updated Oct 20, 2009 • Related Guides: Carbon

The most important requirement to burn coal is the correct quantity of air. This article explains how to calculate the correct quantity of air .

Coal is the most widely used fuel source used to produce Electricity. Air provides the necessary Oxygen for burning coal. It is the chemical reaction between Carbon in coal and the Oxygen in the air that produces the heat energy.

Elements in Coal

There are 3 elements in coal that combine with Oxygen in the air duing the combustion process. The main and important element is Carbon which could be around 30% - 60 % .Then there is Hydrogen in the range of 1 % to 3 % and Sulphur in the range of 0.3 % to 3 % . Apart from this three there is Nitrogen 1% to 2 % and Oxygen from 5 % to 12 % . The Oxygen is also used up in the Combustion process. The exact amounts of each element depends on the type and rank of coal, how the coal was formed millions of years ago and the location or mines from which the coal is sourced.

Stoichiometric or theoretical air quantity.

This is calculated based on the chemical reaction between the elements and oxygen.

• Carbon combines with Oxygen to form Carbon-dioxide and heat.

C+ O₂ > CO₂

1 kg Carbon +2.67 kg Oxygen > 3.67 kg Carbon Dioxide1C+32/12 O > 44/12 CO₂

• Hydrogen Combines with Oxygen to form Water and heat

2 H₂+ O₂> 2H₂O

1 H +32/4 O >36/4 H₂O

1 kg Hydrogen +8 kg Oxygen >9 kg Water

• Sulphur Combines with Oxygen to form Suphur Dioxide

S + O₂ > SO₂

1 S +32 /32 O >64 /32 SO₂

1 kg Sulphur +1 kg Oxygen >2 kg Sulphur Dioxide.

For the purpose of calculation we consider a coal having 57.2 % Carbon, 2.2 % Hydrogen , 0.5 % Sulphur and 6.9 % Oxygen.

• The theoretical Oxygen required to burn this coal is then

2.67 x C % + 8 x H% + 1 x S % - O% = 1.64 kg of Oxygen for 1 kg of Coal.

Air contains 23.2 % by weight of Oxygen.

• The theoretical Air required to burn the coal is

= 1.64 / 23.2% = 7.1 kg of Air for 1 kg of Coal.

This is the theoretical Air required to burn the coal.

Quick Calculation of Theoretical Air.

The Heating value of Coal also depends on the elemental Carbon and Hydrogen. This means that the air required and the heating value have an almost fixed relationship. The theoretical air required for a unit heating value is practically a fixed value. This is around 0.332 kg of air for one MJ of heat input. This is true for a wide range of coals used in power plants.

By knowing the calorific value of coal , the theoretical air quantity can be directly calculated using this factor.