Although you are recording excess oxygen ex-kiln, with a low-volatile fuel such as pet-coke, it is quite likely that - without careful adjustment of burner settings - there may be sufficiently reducing conditions in the burning zone, al least locally, to decompose calcium sulfate, so leading to increased levels of clinker free-lime. A check on volatile cycles in the kiln system may illuminate the situation, perhaps comparing your current percentage sulphur retention against that seen during a trial with a different fuel mix that is easier to combust.

Dear Auther

Clinker is not dusty but brown core is always seen inside the big sizes. Well i agree that brown core could be the basic reason of high free lime . To take care for high free lime raw meal is finer as seen , coal mix residue is also finer. The same mix is fired in calcinator along with kiln. No separate grinding of coal mix. Same VRM with same fine fine coal bin .Our kiln system is ILC.Further finer grinding of Raw Meal is not possible due to Raw Mill capacity shortage.

I agree that increase in petcoke usage will lengthen the BZ & will make the burning harder but ay the same time silica from fuel mix will come down & will result in more lime than the previous one for the same LSF of Raw Mix.

One of the observation is that if free lime increases to more that 2% the calcination degree of hot meal also increases to more than 98%.I do not understand this phenomenon.

Also the comments of 123Red are not clear at all.

Please do comment

The comments of 123red are valid.  The petcoke contains sulphur which mixes with the feed and probably enters the kiln as calcium sulphate (or alkali sulphate if there are sufficient alkalis).  Calcium sulphate can pass through the kiln and leave with the clinker if there is sufficient oxyen at the inlet (typically 5% for high clinker sulphate levels).  If there is not sufficient oxygen for combustion (and the brown cores suggest that this is the case) then the flame will take oxygen from the calcium sulphate to form calcium sulphite.  This volatilises at about 1200 degrees giving sulphur dioxide which volatilises and lime, which is freed too late to be combined with silica and therefore is present as free lime.

The free lime combined with high calcination could be due to a few things.  Most commonly are either false calcination due to high levels of recirculating dust, or early formation of belite which forms clusters which are difficult to combine to make alite.  Its worth measuring the free lime of the hotmeal to see whether this is occurring.  If you have access to microscopy then this will show you what is happening.

Arthur

Dear Arthur

How do I know that the problem is due to oversulfurization.

What are the parameters to be checked in hot meal to conclude.

I normally check SO3,LOI, Calcination in the hot meal.

SO3 in clinker is around 0.8% 

Also after monitoring what is th ebst way to reduce free lime without loosing production.

Please do reply