TECHNOLOGIES TO REDUCE NOX
“Mild” combustion: the staged combustion
This is a well-known technique for limiting nitrogen oxide (NOx) emissions in preheated air burners. In fact, the nitrogen and oxygen atoms bind more easily as combustion temperatures increase, however, if the oxidizer and fuel are mixed at different points/times in the burner (“Stages”), a colder and more homogeneous flame is generated and the NOx emissions are substantially reduced.
The design of a stage burner therefore includes one or more air (or gas) sources, each of which is managed with its own combustion ratio (“lambda”). In the first stage there is a tendency to supply excess fuel (or oxidizer), while in the subsequent stages the ratio returns to stoichiometric, obtaining both complete combustion and lower temperatures.
Schematic representation for a refractory burner
In this case it is air to be supplied in stages, initially producing a rich, reducing flame which progressively becomes oxidizing. Note how the flue gas recirculation also contributes to the reduction in NOx.
Burner with refractory head
As in the previous diagram, the secondary air circulation channels are created inside the block, and the air outlet slots can be seen externally.
Burner with SIC combustion head
In the case of high-speed recuperative burners with SIC combustion head we use a particular design of the SIC head to distribute the air.
Self-recuperative burner 3 stages
In this case, given the high recovery temperatures and the high speed, we even use three different air stages.
EFFICIENCY OR NOX?
Differentiated “staging” for each preheated air burner model.
We use 5 different staging modes depending on the heat recovery system (with external or plug in or recuperator, self recuperative, regenerative, self-regenerative), the type of flame (long or rapid and tense) and the material (SIC, refractory, metal) effectively solving the trade off between energy efficiency and NOx.
FAQ
All “mild” combustion techniques to reduce NOx are based on imperfect combustion, also called “diluted” precisely because it is not stoichiometric, and capable of reducing temperature peaks, thus causing a loss of radiation. But the loss of efficiency is largely compensated by the energy savings resulting from the use of pre-heated air!
Yes, although it is less effective than flameless if used alone.
Similarly to flameless combustion, combustion increases the homogeneity of temperatures, thus avoiding the formation of hot spots, a highly appreciated condition in heat treatment furnaces as well as in melting/holding furnaces. The same applies to radiant tubes, where the homogeneity of temperatures reduces the creation of soot, the main cause of break downs of the radiant tubes themselves.
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