Flamelesss combustion characteristics in a lab-scale furnace

Abstract

Flameless
combustion, named as Moderate or Intense Low-oxygen Dilution (MILD) combustion
or high-temperature air combustion (HiTAC), is a promising technology to
improve the thermal efficiency while suppressing NOx formation in combustion
systems. Flameless combustion can occur when fresh air (and/or fuel) streams
are sufficiently diluted by entrained combustion products before reactions take
place. It has recently been experimentally studied on laboratory-scale setups
because of scientific challenges, environmental concerns and its potential
industrial applications. Some burning features in flameless combustion have
been observed in jet-in-hot-coflow burners which use hot coflows generated by a
secondary burner or diluting air with N2 or/and CO2 to mimic the diluted air
which is actually diluted by burnt gases entrainment in furnaces. With the help
of highspeed cameras, the time-resolved studies on such burners have been done
experimentally. E. Oldenhof et al. [1] reported that the jet-in-hot-coflow
flame is stabilized by autoignition kernels and the entrainment of hot oxidizer
plays an important role in the formation of autoignition kernels[2]. As O2 level
in coflow is reduced, reaction zone becomes less intense leading to a greater
degree of partial premixing in these flames[3]. P. R. Medwell et al.[4] also
concluded that large-scale vortices can lead to a weakening of the flame front
or even local extinction leading to a form of partial premixing, and may
contribute to the stabilization of the flameless combustion reaction zone. With
low level (5% by volume) hydrogen addition in the fuel, the flame also exhibits
autoignition kernels, but this was not observed at higher level (10% and 25%)
hydrogen addition cases[5]. However, how can these findings be related to the
flames in a furnace is still unclear because of the lack of similar
experimental observations in furnace.