There are various options, but if you cannot avoid the problem by better control of the kiln feed chemistry or burning conditions in the kiln then there are two possibilities:
(i) install the equipment to fire Cardox charges in the region where you have persistent problems, and
(ii) by an industrial gun and shoot the rings out.

It is no surprise that you have problems. Your instrumentation and control technology is inadequate. I am surprised that you can operate a large modern kiln without coal flow metering equipment to the main burner. I think your problems are caused by an excess of sulphate over alkalis. This results in the very hard coating in the riser duct. The best solution is to increase the alkli content and pass the alkali sulphate out in the clinker. However, with your inadequate coal flow control there is likely to be reductive burning and then there will be a heavy sulphate cycle in the kiln. This is also likely to be contributing to your current problems. You could try installing acoustic cleaners to try to prevent the sulphate build-up in the riser. The high temperature excursions in the precalciner and stage four cyclone are caused by interruption in feed down the preheater tower caused by the build-up. Solve the build-up problem and you will solve the high temperature problem. For the oil compensation you need expert control, however it is doubtful if your general control can be the basis for expert control.

We are facing serious problem of build-ups at stage four riser duct starting from kiln housing and at start of mix chamber it becomes to thick and remains thick through out the riser duct so that opening remains only 30-40 per cent. Some coating has also observed at cylindrical part of cyclone and at its roof, but coating at riser duct becomes very tough and hard and similar to sintered material. My first question: is there any special method to get rid off it during operation as normal pocking do not results any way. Is there any chemical treatment, operational technique or physical methods. Secondly, what could we do to prevent such things.

I am not aware of any commercial operations using fluidised bed for industrial cement clinker production. There are pilot and research installations but not of commercial capacity. There are number of kilns with six-stage preheaters. This brings the exhaust gas temperature down to less than 300 degrees Celcius. However, the most economic number of preheater stages is usually five as increased capital costs for six stages cannot be recovered by the energy savings. A company might choose a six-stage preheater if there is a shortage of water for exhaust gas cooling and conditioning. Therefore most of the existing six-stage preheater kilns are in desert environments.