I want to share a typical question that when the hot meal sample is checked @ the kiln feed end its calcination will be 97%.In a hypotetical condition,if the calcination @ the kiln inlet is 50%,what happens to the hot meal?How far it affects the kiln?If kiln Becomes Dusty.Why?
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If only 50% of the kiln feed is calcined by the time it enters a precalciner kiln you will have no chance of controlling the free lime in the burning zone. Calcination requires a lot of heat input, which is why approx. 60% of the total coal used in a modern kiln is burned in the precalciner.
The temperature of the burning zone would fall as a portion of the heat from the flame is absorbed by the calcination reaction. This would result in very high free limes and underburnt clinker.
This underburnt clinker would be inherently dusty and break apart more easily in the cooler causing very dusty conditions which would further lower the temperature of the burning zone by obscuring the radiant heat of the flame and transporting and diffusing this heat up the kiln toward the back end.
In the worst case scenario there might even be a meal slide through the kiln as the calcining meal becomes fluidised, through rapid loss of CO2, and flushes through the kiln, bogging the cooler.
Because of all these problems, in a modern precalciner kiln it is essential to keep the calcination degree of hot meal entering the kiln inlet to between 90 and 95%
If the calcination degree gets too high, it is an indication that the temperature in the precalciner/lower cyclones is too high. This promotes buildup formation in the smoke box and lower cyclones causing blockages. In extreme cases, clinkerisation can actually occur in the lower cyclones and meal pipes causing catastrophic plugging.
Even if buildup and plugging does not occur, high calcination degree could also mean that the burning zone is probably too long, so clinker formation will occur earlier than intended in the kiln which may adversely affect the clinker mineralisation and ultimately cement performance and possibly even clinker grindability.