Experimental assessment of velocity and narrow-band measurements in oxy-fuel combustion of pulverized walnut shells

Self-sustained pulverized biomass swirl flames are investigated experimentally under oxy-fuel atmosphere concerning the effect of thermal load, and oxygen content of the oxidizer stream on flame characteristics. Experiments are conducted employing a swirl burner in a down-fired cylindrical chamber. Operating conditions are selected in ranges of thermal loads between 60 k W th to 100 k W th and oxidizer oxygen contents between 25 and 30 vol.-% while keeping the flame stoichiometry constant. Particle velocity profiles are obtained employing non-intrusive 2D laser Doppler velocimetry. Spectrally narrow-band imaging of the near burner region enables the identification of reaction zones via OH* radical emission imaging as well as regions of high particle temperature by imaging their thermal radiation. The results show that the occurrence of an fuel particle recirculation is related to the ratio of the momentum flow rate of swirled secondary stream over the momentum flow rate of the fuel with its carrying stream. By either reinforcing the flame thermal power or lowering the oxidizer oxygen content, the momentum ratio increases, progressively leading to the fuel particle recirculation at the central axis of the flame. In the flames with a recirculation of hot fuel particles towards the burner, homogeneous devolatilization and char combustion regions are observed in the radical emission of OH* and thermal radiation images, respectively. • Particle recirculation zone (PRZ) depends on swirl-to-fuel momentum. • Hot particle recirculation promotes uniform volatile release and char burning. • Raising O 2 content in oxidizer stream shifts the reaction zone into the diffusor.