Coal is a solid particle. In thermal power plants the coal is ground to a fine powder and then burned. This is to ensure that each coal particle comes into to contact with air so that the chemical reaction can take place. If the furnace is only provided with the theoretical air there may be a possibility that some coal particles do not get sufficient air. If this happens that part of the coal will not burn and will be an energy loss. To avoid this air is provided in excess of the theoretically calculated value. This is called the Excess Air factor. In large coal fired power plants the Excess Air is around 20 %.
Problems of Less Air.
If the air is less than required
• part of the Coal will not burn and goes out through the ash as un-burnt coal particles. This is equivalent to losing a part of the potential energy of the coal. This is known as Unburnt Carbon Loss.
• in the normal combustion reaction Carbon and Oxygen combine to form Carbon dioxide giving out heat. Actually this takes place in two stages , first is the formation of Carbon monoxide and then the Carbon monoxide reacts to form Carbon-dioxide. If the air is less the second stage does not take place, and Carbon monoxide is formed. This means a part of the potential energy of the coal is not released. This is an energy loss.
Also Carbon monoxide is a toxic pollutant.
Problems of More Air.
If the air supplied for combustion is more than required, the air that is in excess of the combustion requirement does not contribute to the combustion process , but takes away a part of the combustion heat then goes up the stack as waste heat. This is an energy loss. This is called the Dry Gas loss.
Modern combustion control systems in power plants optimise the air requirements to minimize the effects of both these conditions.