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

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

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Fuels and Combustion
Producer gas containing least amount of tar is produced by the

High pressure gasification of coal (e.g. in Lurgi gasifier)
Partial combustion of coal
Atmospheric pressure gasification of coal (e.g., in Kopper-Totzek gasifier)
Partial combustion of large size (< 50 mm) coke

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