Swapan Basu, Ajay Kumar Debnath, in Power Plant Instrumentation and Control Handbook (Second Edition), 2019. 12.5.2.1 Secondary Air Damper Control for Wall-Fired Boiler (With Flue Gas Recirculation System) For wall-fired boilers, secondary air (fuel air and tertiary air) combined together enters the individual burner and the proportioning of fuel air and tertiary air is done within the burner.