Figure 1: Chloride-removing efficiency of bypass systems in various works
The most efficient point for removing chloride from the system is the kiln inlet chamber or the lower part of the riser duct, where the chloride concentration in the kiln flue gas is highest. To minimise the loss of kiln feed, the takeoff of the bypass gas should be located at a point where the flue gas carries a minimum of kiln dust. CFD modelling helps to describe the flow pattern of the flue gas, and to evaluate the point where the bypass gas takeout should be located.
Figure 2: Sulphate-removing efficiency of bypass systems
Gas bypass systems are highly efficient in removing chloride from the system, but merely moderately efficient for sulphate and alkalis. A hot meal bypass is capable of taking off sulphate or alkalis due to the discharge point (Figure 3).
Figure 3: Modern cyclone preheater with a gas bypass
The removal of bypass gas is linked to the precipitation of considerable masses of bypass dust. Besides partly carbonated and partly decarbonated hot meal, the dust contains alkali chlorides and sulphur compounds as well as intermediate products such as C2S and CA. The utilisation of bypass dust depends on the composition of the dust. The biggest portion of the bypass dust is used as a minor additional constituent in cement, other possibilities are its use as construction material in different applications.