Role of secondary air in pulverised-coal (PC) fired boilers In a PC-fired steam boiler, what is the principal purpose of the secondary air supplied at the burner?

Introduction / Context: Pulverised-coal combustion uses two air streams: primary air to convey and dry the fuel, and secondary (and sometimes tertiary) air to complete mixing and ensure complete combustion at the burner throat.

Given Data / Assumptions:

• Primary air already carries and partially dries the coal.
• Secondary air is injected around the burner zone for mixing and burnout.

Concept / Approach: The fuel–air mixture leaving the mill needs staged, well-distributed air to achieve flame stability and complete burnout. Secondary air forms the main oxidant envelope that mixes with coal jets, controls flame shape, and sets swirl/recirculation patterns for efficient combustion.

Step-by-Step Solution:

Verification / Alternative check: Burner manuals depict primary air nozzles carrying fuel and secondary registers shaping the flame with the bulk of oxidant.

Why Other Options Are Wrong:

Common Pitfalls: Overlooking tertiary/overfire air used for NOx control; the core purpose of secondary air remains combustion and mixing.

Final Answer: combustion of coal by supplying it around the burner.