Cyclone preheater

Wolter (W14) determined the phase compositions of kiln inlet meals from about 20 plants using cyclone preheaters, usually with precalciners. XRD showed that the decomposition products of the clay minerals were especially reactive, though some reaction of quartz also occurred. Phases detected, and some notable absences, were as follows ... 

Heat recovery in rotary kilns with grate preheaters or cyclone preheaters... 

In rotary kilns the raw powder is preheated by the hot flue gases. The use of special preheaters (grate preheaters, cyclone preheaters) can make heat recovery even more efficient. 

The largest rotary kilns with grate preheaters (Lepol furnaces) have a capacity of up to 3()()() t clinker per day and those with cyclone preheaters up to 5000 t per day. In the latest developments the calcining is partly carried out in a preliminary step, in so-called precalciners, which are between the preheater and the rotary tube, to reduce the thermal loading of the rotary tube. The required clinker throughput can then be achieved with smaller rotary tubes or existing plants can increase their capacities. The largest plants with cyclone preheaters produce up to 8000 t/day. 

Wachtler and Jannsen [90] studied the material phase composition in the kiln with partial precalcinahon, and without its shut down. In the material from cyclone preheater spurrite, C2F and aluminates CA and and free CaO were found. In the samples also these phases and additionally C AF were present Then the gehlenite appears, C2S and C3A. Appearing of CjA is accompanied with the disappearing of C2F and CA. When C3S appears the samples have already the composition of clinker. 

Table 2.16 Absorption degree of heavy metals in rotary kilns with cyclones preheaters...

Short dry kilns are usually accompanied by an external preheater or pre-calciner (Figure 1.7) in which the feed is dried, preheated, or even partially calcined prior to entering the main reactor (kiln). As a result the thermal load on the kiln proper is reduced. Hence kilns equipped with preheaters or precalciners tend to be short, on the order of 15-75 m (about 50-250 ft) depending on the process. The shorter kilns are those in which the entering feed material is almost calcined. Applications include cement and some lime kilns. Because of the large feed particle size encountered in limestone calcination, modern lime kilns are equipped with preheaters which function as a packed bed of stone with a countercurrent flow of kiln exhaust gas rather than the typical cyclone preheaters in cement kiln systems. 

Figure 1.7 Cement kiln equipped with cyclone preheaters.

Whereas in 1953 most cement kilns had clinker outputs of between 300 and 500t/day, nowadays kiln plants producing around 5000 t/day are not uncommon and are likewise equipped with cyclone preheaters embodying the same basic idea of heat transfer from the hot kiln exit gas to the raw meal in suspension in the gas stream. For further information the reader is referred to the article by Bomke (1978), which moreover contains a comprehensive list of literature references. 

By conversion to precalcining, the clinker output of existing rotary kilns with cyclone preheater equipment can be substantially increased - by up to about 100% in certain cases. In new kiln plants equipped with (pre)calciners it is possible to increase clinker output up to threefold, as compared with a conventional kiln-cum-preheater plant, while the kiln dimensions (diameter and length) can moreover be reduced. 

Rotary kiln with cyclone preheater and exit gas utilization in a... 

Rotary kilns with cyclone preheater equipment generally discharge exit gas at a temperature of between 320° and 360° C. This gas contains only about 12% moisture and is therefore very suitable for the drying of raw materials. In the following example a rotary kiln with cyclone preheater operating in combination with grinding/drying mill, in which the exit gas heat is utilized, will be considered. 

The operational behaviour of cement kilns is also determined by the cyclone preheater, in which part of the waste gas enthalpy is transferred to the kiln feed and thus recovered for the process. In addition to the gas mass flow, which in turn is governed by the fuel energy demand as well as by the air rate, the efficiency of the preheater is mainly dependent on the dust cycles in the preheater. It is normally between 50 to 65 % and may be markedly increased by the installation of additional dip tubes. Fig. 2 (2) indicates that by increasing the separation efficiency of both lower cyclone stages from 60 to 80 % each, the preheater energy loss may be cut by about 0.15 MJ/kg of clinker. In the past the installation of dip tobes was successfully effected in numerous plants. 

Fig. 2 Energy loss of a cyclone preheater with four cyclone stages in dependence on the separation efficiency of both lower cyclones.

Between 1960 and 1970 the capacity of numerous kilns was greatly enhanced without adjusting the cyclone preheater to the higher waste gas volume flows. This caused the pressure losses in the cyclone preheater and thus the specific electrical energy requirements of the ID-fan in some cases to double or to increase even more. In redevelopment work the gas cross sections were sometimes enlarged by 100 % and in addition the pressure losses could be reduced by aerodynamically adjusted geometries. In some cases the specific ener requirements of the entire kiln could be reduced by up to 15 %. 

From the heat balance of a cement kiln with cyclone preheater and 30 % bypass in table 1 it appears that about 20-30 percent of the heat losses are due to the pre-heater waste gas and the exhaust air from the clinker cooler (1). 

Different amonnts of exit gas or waste heat may arise from modem cement kilns eqnipped with cyclone preheaters. This will depend on the methods and machineiy employed. Fie. 1 schematically indicates the principal points of gas discharge from the rotary kiln process with precalcination. The waste gases differ with regard to qnantity, composition, temperature, pressure and dnst content. It is presnpposed that these gases cannot be directly ntilized in the bnming process. 

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