In case the combustion air or gas are progressively mixed in different stages of the burner, it is possible to obtain modulation and control of the flame temperature and structure, strongly reducing the formation of NOx.
The burner that takes advantage of this effect is provided of more than one air (or gas) source, that correspond to the different combustion stages; each source is controlled with its own air/gas ratio. In the first stage ratio tends to be in excess of gas (or of air), while during the secondary stage, the ratio tends to return stoichiometric.
In this way it is possible to modulate the process by ensuring complete combustion as well as a lower temperature level reducing therefore NOx emissions.
In case of burners with refractory block, the circulation channels of secondary air are integrated inside the block , while externally it will be noticed just the air slots as for the below ENM NXT burner.