| It’s of great significance to study ignition of pulverized coal jet, including theignition mechanism and ignition delay, under various oxidizer and coal feedingconditions for power equipment operation and maintenance and burner design andretrofit. Ignition and combustion characteristics of coal particles show a significantdifference under different conditions. Because of the interaction of coal particles and thegas-solid turbulence characteristics, there exists a great difference between ignitioncharacteristics of pulverized coal jet and of a single coal particle.Based on a systematic review of reported typical research instruments andexperimental methods of pulverized coal jet, in this paper, a flat flame entrained flow(FFEF) reactor system composed by a Hencken burner and a syringe coal feeder, wasdesigned and built, which provides one-dimensional reaction condition that is close tothe practical furnace environment for coal particles. Due to the ability of quick andeffective adjustment of experimental conditions, the FFEF reactor system was proved tobe appropriate and convenient for experimental study on pulverized coal jet ignition andcombustion. An optical diagnostic system based on high speed CCD camera and otherimage acquisition devices as well as a simple and effective image processing methodwere introduced in this paper. Qualitative optical oberservation and quantitativenumerical analysis of jet ignition characteristics were realized. Effects of oxdizer andcoal feeding condition on pulverized coal jet ignition characteristics were investigatedon the constructed FFEF reactor system using the designed image processing method.Characteristics of the observed three typical “Group Combustion Flame (GCF)”and their jet conditions were summarized based on the ignition experiments undervarious conditions. The characteristics and formation mechanism of GCF were studiedby using the intensified charge-coupled device (ICCD) camera as well as thenarrow-band filter. Based on the jet ignition image sequences acquired by the highspeed CCD camera, the flame reunion, separation and flame front instability wereinvestigated in this paper. The result showed that the stability of GCF of pulverized coaljet increased with the increase of coal concentration.Ignition characteristics under different oxidizer and coal feeding conditions were investigated experimentally, the results show that:1) High volatile content andappropriate stoichiometric ratio of coal and entrained flow are two prerequisites ofoccurrence of GCF;2) Due to the difference of heat and mass transfer behavior andgas-solid turbulence characteristics between laminar and turbulent flow, there existssignificant difference on pulverized coal jet flame ignition behavior under variousprimary flow velocities;3) The increase of coal concentration is proved to beconductive to the formation of GCF, and the ignition delay decreases first then increaseswith the increase of the coal feed rate, due to the competition between the increase ofheat capacity and the increase of volatile releasing.4) It is easier to form GCF whenusing coal with higher volatile content and under smaller particle diameter. Because ofmore volatile releases at the same height of the reactor, the ignition delay of GCFdecreases as the volatile content increases and particle diameter decreases.5) Theinduction time controlled by O2concentration accounts for a smaller part of the wholeignition delay time, as a result of which, the ignition delay is not significantlyinfluenced by O2concentration of both primary and secondary flow, but the combustionprocess is enhanced when the O2concentration increases. |
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