external fans are provided to give sufficient air for combustion. the primary air fan takes air from the atmosphere and, first warms the air in the air preheater for better economy. primary air then passes through the coal pulverizers, and carries the coal dust to the burners for injection into the furnace. the secondary air fan takes air from the atmosphere and, first warms the air in the air preheater for better economy. secondary air is mixed with the coal/primary air flow in the burners.