coal firing involves a single pipe burner through which the mill carrying air together with entrained coal are injected at a

tip velocity of approximately 80M/sec. Tip velocity must always be substantially greater than the flame propagation velocity

which may be up to 25M/sec. Flames produced by coal nozzle velocities in excess of 80M/sec are susceptible to severe

instabilities. The pipe is usually narrowed near the tip to minimize parasitic pipe losses and to convert the flow into the desired

static pressure. For indirect firing, multi-channel burners of various designs are employed. One annulus is used for conveying

pulverized fuel from the mill and one or more separate streams are used to supply primary air for controlling the flame

Kiln rings are sections of heavy coating, usually in the burning zone, though sometimes also near the back of the kiln, which can grow to restrict both gas and material flow and eventually force shut-down. Conversely, ring collapse causes a flush of unburned material. Ring formation in the burning zone is commonly attributed to operational fluctuations though a low coal ash-fusion temperature or high mix liquid phase will increase the risk 

 Early detection is possible with a shell scanner and rapid reaction is essential. Such ring growth may be countered by varying kiln speed or by small movements (10cm) of the burner in and out. Rings at the back of the kiln are usually associated with volatiles cycles, particularly excessive sulphur at the kiln inlet. It is evident, though of little help, that rings are structurally more stable in small diameter kilns.

the most concentrated the heat transfer the highrer the thirmal load on refractories the shell temp. should be observed  the short flame have to be used IF sufficintely thick coat has been formed this will protect the the sintering zone , depending upon raw material proprties could give your sintering zone thick or thin coat and they are often reduced reduced by very high heat radition So you can used the short flame just when you be sure that your material tendes to coat in sintering zone even under intens heating .

If material dosen`t show tedency to coat with long slim flame must be chosen .the airia ratio of flame surface and inner kiln shell must be reduced to lower the amount of heat transfer to the shell . You can use the long flame to sinter large lumps of material and low flux containt in material for example make sintering zone difficult .

I hope that`s could help you and others friends

Best regards

ILA