Fuels and Combustion
The main function of primary air in pulverised coal fired burner is to

Preheat the tertiary air used for complete combustion of CO to CO₂
Dry and transport the coal
Have proper combustion by supplying it around the burner
Burn CO to CO₂

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Fuels and Combustion
A carbonaceous fuel (containing no H₂ or hydrocarbons) is burnt and the resulting flue gas contains 21% CO₂. It means that

Complete combustion of fuel has taken place
No excess air has been used for combustion
21% excess oxygen has been used for combustion
21% excess air has been used for combustion

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