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?
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Atransportation of coal.
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Bdrying of coal.
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Ccombustion of coal by supplying it around the burner.
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Dpreheating the primary air.
Answer
Correct Answer: combustion of coal by supplying it around the burner.
Explanation
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:
Assign duties: primary = transport + drying; secondary = combustion/mixing.Identify the key burner zone role of secondary air.Select the option emphasizing combustion around the burner.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:
Transportation/Drying: Primary air’s job, not secondary.Preheating primary air: Achieved by air preheaters, not via secondary air function.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.