Experimental study on ignition and combustion of pulverized coal particles clouds under laminar and turbulent conditions

The ignition and combustion behaviors of pulverized coal particles clouds in a jet with different levels of turbulence and primary O₂ mole fractions were evaluated in a lab-scale optical entrained flow reactor. Bituminous particles were injected into the hot flue gas environment produced by a Hencken burner, with environment temperatures varying from 1200 K to 1700 K and secondary O₂ mole fractions from 0.1 to 0.3. Digital photography and OH-Planar Laser-Induced Fluorescence (OH-PLIF) techniques were employed to record the coal jet flame behaviors and capture the transient structure of the flame. The coal jet flame was narrow and smooth under laminar conditions, and become wider as the turbulent intensity of the primary flow increased. The OH-PLIF data reveals that upstream of the turbulent coal jet flame, reactions occur only at the periphery of the clusters of the pulverized coal particles where the high-temperature environment provides sufficient heat and oxygen. Downstream of the turbulent coal jet flame, reactions could occur also within the clusters of the pulverized coal particles, because of the continuous coal devolatilization process along the streams, and the mixing process between the volatiles and the oxygen entrained by the secondary flow. The ignition delay time is reduced and the combustion intensity is enhanced with the increase of the environment temperature and O₂ mole fraction in the primary or secondary flow. The frontal OH edge expands inward to the centerline of the clusters of the pulverized coal particles with the increase of the primary O₂ mole fraction. In addition, the effects of the environmental temperature and O₂ mole fraction on the ignition of pulverized coal particles clouds exhibit a threshold phenomenon. Once the environment temperature or the O₂ mole fraction exceeds a certain threshold, its influence on the ignition delay of the coal jet becomes weaker and the leading factors controlling the ignition behavior of the pulverized coal particle clouds change accordingly.