Abstract
Flameless or ‘mild’ combustion is a well known measure to reduce NO x -emissions. This work aims at the application of the technique to coal combustors. Experiments have been carried out with lignite as well as bituminous coals showing an overall NO x -reduction capability with the current burner design of about 20–50% depending on fuel type and the stoichiometry at the burner. The most important design feature is a high momentum of the combustion air inducing a strong recirculation which reduces the temperature variations and temperature peaks in the combustion chamber and thus prevents thermal NO formation. An investigation of NO x -emissions under Ar/O 2 as well as CO 2/O 2 atmospheres was carried out in order to quantify the ratio of fuel- to thermal-NO. This investigation showed a high reduction of thermal NO in the flameless combustion mode. It also showed an increase of fuel-NO which was primarily related to the decrease of the peak flame temperature in flameless combustion. The intensive mixing in flameless combustion also allows a reduction of the oxidiser oxygen concentration under oxyfuel conditions to 15 vol % as compared to 17 vol % with a standard swirl burner.
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