Combustion without flames has always been known at low temperatures, whilst the discovery of flameless combustion that is visible at high temperatures is recent.
The phenomenon is created when the fuel temperature is above the ignition temperature or in general above 750°C .
In these conditions, if the fuel is injected directly into the furnace, at given levels of turbulence and velocity, and bypassing the mixer with combustion air, this generate an oxidation reaction which does not occur in the visible range, but still produces thermal energy.
In these conditions it is compulsory to disconnect the flame detection device as the flame is not visible anymore, and / or disconnect the ignition system: combustion will take place all the same.

Combustion in flameless mode considerably reduces the formation of NOx as:

- Temperature peaks inside the flame are contained
- Erasing the flame, it is erased the preferred zone of NOx formation: the flame front, the separation surface between reactants and combustion products.
- Lowers the relative concentration of oxygen in the flame, reducing the reaction speed of nitrogen in the air.
- The previously formed nitrogen oxides are carried back inot the oxidation reaction,  in fact burning them again.