The flame of a fireplace or the gas flame in the kitchen are common phenomena in our life; we all think that we know exactly what happens, but if we examine the flame carefully, we notice the extreme variability of behaviour; the flame becomes longer, then shorter, then changes colour in a portion, sometimes there is smoke, sometimes it is clean.
Flames are per se determined by many catastrophic (non-linear) parameters, and, are therefore intrinsically chaotic, each elemental domain being a separate entity and averaging the mean flame forefront only statistically. The reduction of nasty by-products (soot, dioxins, furans), as well as NOx and CO/ CO2, seems bound to the fate of each and every single element combustion domain.
It’s very difficult to modify the behaviour of each domain (it’s where contaminants like soot are created and reactions leading to the creation of NOx and concentrations of CO and CO2 take place); even if it was possible, any intervention would cause a statistical effect, as it is not possible to intervene on each special domain; in addition, specific variations in the composition or form of the fuel may cause effects which can be evaluated only statistically; in conclusion, scientific and technological domain of phenomena occurring in the traditional flame is still a distant goal to be achieved.
An important step outside traditional flames was marked by Wuenning et al. in 1992, who demonstrated that by decoupling some steep flame parameter gradients e.g. oxygen concentration and temperature, combustion could take place in a loose and “mild” way throughout the entire “volume” of the combustor. Thus, “flameless” combustion was born. Conceptually, flameless combustion is completely opposite that of flames. It represents a quantum leap in emission reduction: no soot, 10.000 times less PAH (polyaromatics), ten times less NOx, lower CO/ CO2.
