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
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

Partial combustion of large size (< 50 mm) coke
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)

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