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What are the factors affecting the number of cyclone stages in preheater kiln system?

By: Nael Shabana qatar.cement@yahoo.com

The number of cyclone stages in a preheater system largely determines the system heat efficiency. It also depends on the intended use of the heat of the exit gas leaving the system. In modern practice, most of the kiln gases from a preheater or precalciner system are used for drying in both the raw mill and the coal mill. As a rule, the throughput and moisture content of the raw materials and fuels, which have to be dried by the remaining exhaust gas heat, decide whether 2, 3, 4, 5 or 6 cyclone stages are used. There are criteria for selecting the number of preheater stages other than drying the materials. Typical determining factors include construction cost, electricity and fuel price, gas conditioning system, heat exchange efficiency, radiation losses, and pressure drop (figure-8) which is one of the most important factors affecting the number of cyclone stages.

Initially number of stages used to be 4 only. Pressure drop across a 4 stage preheater used to be 500 to 550 mmwg. Any increase in number of stages resulted in additional pressure drop which unfavorably offset gain in fuel efficiency. Therefore cement industry continued for quite some time with 4 stage preheaters till cyclones of low pressure design were developed. Attention was paid by designers of preheaters to the geometry of cyclones and their internals to reduce pressure drop. As a result designs have become similar and pressure drops comparable As a result, fuel consumption has steadily gone down and specific gas volume expressed in Nm3/kg clinker has also come down. Presently pressure drops in 6 stages preheater are comparable to or even less than that in a 4 stage preheater of old designs (Table-2).

With reduced pressure drop, temperature and reduced gas volume specific power for preheater fan expressed in kwh/ton of clinker has also come down. Table-3 & figure-9 shows typical values for specific fuel consumption, specific gas volume and specific power for preheater fan. It would be seen that development of low pressure drop cyclones has resulted in bringing close specific power consumption of preheaters of 4 to 6 stages. Thus with this development, it has been possible to increase number of stages from 4 to 6 without paying penalty in terms of power consumption.