Gas and oil fired boilers require only bi-sector design air preheaters. Coal fired boilers also require an air-stream to dry the coal and to transfer the pulverised coal from the coal pulveriser to the furnace. This air is called primary air and it has to be at a higher pressure than the secondary air to overcome the pressure drop through the coal mill, the pulverised fuel pipework and the coal burner.
In coal fired boilers there are two air supplies :-
Secondary Air (SA)
which is supplied direct to the burners and is the bulk of the combustion air supply.
Primary Air (PA)
which is used to dry the coal and transport the pulverized coal from the coal mills to the burners.