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Flameless oxidation
Flameless oxidation is a particular case of industrial combustion technology that deliberately avoids a burner stabilized flame front, which is the conventional design constraint. Flameless oxidation is an example of flameless combustion and can be considered a major design progress in high temperature combustion processes.

In Figure 1a, the burner is firing in flame mode: a stable, pale blue, noisy flame makes the high temperature at the burner nozzle visible because of the bright luminosity;  in  Figure1b  the same burner is firing in flameless oxidation mode (shortly FLOX  or flox, while FLOX® is a registered trademark of “WS GmbH”)  and the hot spots at the burner nozzle have disappeared: no flame front is visible or audible any more in spite of complete combustion within the furnace.

Fundamental features of flameless oxidation have been thoroughly investigated and practical solutions have been developed for industrial applications. Flameless oxidation can be defined as “stable combustion without a flame front with a defined recirculation rate of hot combustion products”. Flameless oxidation requires a hot combustion chamber above a threshold temperature, higher than self-ignition temperature of the fuel: in industrial furnaces this  threshold is fixed to be ≈ 850 °C for safety purposes. Recirculation of chemically inert flue gases from the furnace chamber into the reactants (fresh combustion air and fuel), reduces quite effectively the combustion temperature because of the large heat capacity involved (dilution). Figure 3 schematically shows three combustion regimes as a function of furnace temperature and of the “internal recirculation” rate Kv, which is defined as the flow of entrained (recirculated) combustion products with respect to the combustion products generated by the burner itself :

The influence of the recirculation ratio Kv can be approximately estimated as follows :
 * 1) a conventional, stable flame front can only be established  within limited recirculation ratios (Kv ≈< 0,3-0,5) because dilution of reactive species extinguishes the flame
 * 2) a stable flameless oxidation regime (shortly FLOX ) can be established  above the temperature threshold  850 °C for high recirculation ratios Kv
 * 3) an intermediate unstable combustion regime (lifted flames) is not suitable for steady operation of industrial processes

ɘ̃max ≈ ɘ̃0 + ɘ̃ad /(Kv +1)

where ɘ̃0  is the temperature of the combustion products within the furnace. This estimate shows than the maximum local flame temperature ɘ̃max can be effectively mitigated  from the adiabatic flame temperature ɘ̃ad (order of 2000 °C for natural gas or other fossil fuels) down to few hundred degrees in excess of the process temperature ɘ̃0. This is the main reason for curbing down thermal NO formation, that is very sensitive to local high temperature. By adopting flameless oxidation, NOx emissions can be abated by at least an order of magnitude.

Flameless oxidation has made use of high air temperature compatible with mandatory NOx emissions limits: very hot combustion air (say at about 1000 °C) could produce local hot spots well in excess of 2000 °C and consequently intolerable NOx, excessive thermal stress and undesired non uniformities in heat transfer. Flameless oxidation solves these problems and suits very well high air preheating: the importance of high air temperature techniques stems from the fact that air preheating by heat recovery from flue gases is the most efficient fuel saving strategy for high temperature furnaces.

In the last decades recuperative burners and regenerative burners have been developed for fuel saving purposes, whereby air is effectively preheated in a counter-current heat exchanger integrated into the burner itself. Flameless oxidation is the best technology that makes this design possible by avoiding too high NOx emissions. The basic principles can be embodied into burner design in a straightforward way. Thousands of industrial furnaces for heat treatment of steel, for aluminium, glass, ceramics etc have successfully applied these technology. Flameless oxidation also features merits as fuel flexibility, temperature uniformity and simpler burner design. Application to lean fuel gases with very low calorific value and to liquid or solid fuels has been tested with success. Also design of dry low-NOx gas turbine combustors has profited of the flameless oxidation principle.