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
21% excess air has been used for combustion
21% excess oxygen has been used for combustion
No excess air has been used for combustion

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Fuels and Combustion
Fixed carbon in coal is defined as

Hundred minus the percentage of volatile matter, ash and moisture
The total quantity of carbon present in the coal
That present in volatile matters
The one which is present in the residue after combustion

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