4. Air/flue gas circuits

4.1 Nature of circuits

Atmospheric air, used for combustion in the furnace, has the disadvantage of containing only 20% oxygen, forcing four times more nitrogen into the system as ballast than the oxygen required for combustion. In addition, this atmospheric air contains water vapour. The heat contained in the fumes produced by combustion is partially captured in the furnace by the screen tubes and, downstream of the furnace, in exchangers with a variety of functions: superheaters, re-superheaters or evaporators and economizers (figure