Several rotary kiln designs have evolved, each specific to the process application it is intended for. They also come in several forms and shapes. Although the majority consist of straight, cylindrical vessels, dumbbell-shaped designs (Figure 1.) take advantage of the benefits that variable drum sizes can bring to process application. With regard to internal kiln fixtures, most direct fired kilns are lined with refractory materials for several reasons but the primary purposes are to insulate and protect the outer shell, in high temperature applications, from thermal damage and to save energy. Kilns may also be equipped with dams to increase the material dwell time or with lifters and tumblers (Figure 2.) to aid the materials to flow axially and in some cases to improve particle mixing achieved through surface renewal. Table 1. presents some of the energy-saving advantages of using lifters in various applications and processes. Some of these savings can be substantial.
Figure 1. Schematic diagram of a dumbbell-type rotary kiln.
Figure 2. Schematic diagram of kiln internal fixtures: trefoil (left) and
J-lifter (right).
Table 1. Advantages of Using Lifters (Data in Imperial Units)
Owing to the poor thermal efficiency of earlier long kilns and the need for fuel efficiency, most designs are aimed at maximizing mixing and heat transfer. To accomplish this, kilns are often equipped with heat recuperators, such as preheaters, in which part of the energy in the exhaust gas is recovered to preheat the feed before it enters the kiln. Although coolers are often used to cool the product for safe material handling, they are also used to recuperate the energy, which would otherwise go to waste, as in the earlier-day kilns, to preheat the combustion air and/or to provide other energy needs. Of the modern day rotary kilns the following can be distinguished: wet kilns, long dry kilns, short dry kilns, coolers and dryers, and indirect fired kilns. Some of these are discussed on the following lines.
1. Wet Kilns
Wet kilns are those that are usually fed with slurry materials. Wet kilns are usually long with kiln lengths on the order of 150–180m (about 500–600 ft). The feed end is usually equipped with chains that serve as a heat “flywheel” by recuperating the heat in the exhaust gas for use in preheating the feed to assist the drying. Chains are also used to break up any lumps that the material might form during the transition phase of changing from slurry to solids upon drying. In the cement industry these kilns are often not efficient and are becoming a thing of the past replaced by long dry kilns. Nevertheless, there are certain applications that are not amenable to the alternative use of long dry kilns, for example, lime mud kilns found in the pulp and paper industry and some food applications.
2. Long Dry Kilns
These are shorter than wet kilns with lengths on the order of 90–120m (about 300–400 ft). For long dry kilns, as with wet kilns, the drying, preheating, and calcination all occur in the one single vessel (Figure 3.).
Figure 3. Wet, long cement kiln. (Courtesy of FLS Minerals.)
However, they work well when the feed particles are large. The reason for the relatively shorter length is that the feed is dry with a moisture content the same as granular solids rather than slurry. Applications include lime kilns and lightweight aggregate kilns where the mined stones are crushed to about 1.3–5cm (0.5–1.5 in.) before feeding them into the kiln.
3. Short Dry Kilns
Short dry kilns are usually accompanied by an external preheater or pre-calciner (Figure 4) 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 15–75m (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 4. Cement kiln equipped with cyclone preheaters.
4. Coolers and Dryers
Some coolers and dryers can be in a form of contactors such as the rotary kiln itself, although some are packed-bed contactors such as grate coolers. Rotary coolers can be either in-line or attached (Figure 5.), the number of which is determined by a simple formula
where D and d are the respective diameters of the kiln and the cooler. However, attached coolers place extra mechanical load that must be accounted for in design calculations. They also present maintenance challenges. Rotary coolers and dryers would normally be equipped with tumblers and lifters, which cascade the material well above its angle of repose to take advantage of better solid-gas
contact.
Figure 5. Schematic diagram of an attached cooler arrangement.
5. Indirect Fired Kilns
Indirect fired kilns are those heated externally. They are usually designed for applications where direct contact between the material and the gas providing the heat source is undesirable. In this case, the heat source is external to the kiln (Figure 6.). Any internally flowing gas that is in the freeboard is used for purging any volatile or gas that arise from the bed as a result of chemical/physical reactions. Because of their low thermal efficiency, externally heated kilns are small, typically up to 1.3m (50 in.) diameter and are used for niche applications such as calcining of specialty materials. A unique feature of indirect-fired rotary kilns is multiple and compartmentalized temperature control zones, which can be electrically heated or gas fired individually. Therefore, they provide the capability of achieving high temperatures. In some cases, for example graphite furnaces, they can attain temperatures on the order of 2400 C. The zones can also facilitate tightly defined residence times and controlled atmosphere including flammables. Typical applications include calcination, reduction, controlled oxidation, carburization, solid-state reactions and purification, including waste remediation on a small scale, that require extremely high temperatures and tight control. Materials processed in indirectly fired rotary kilns include phosphors, titanates, zinc oxide, quartz ferrites, and so on. These are usually small in quantity but with a high margin of commercial materials that are economical to process in small quantities.
Figure 6 Indirect-fired small rotary kiln used for niche applications. (Courtesy of Harper International, Lancaster, NY.)
References
R. G. Blezard. Reflections of the History of the Chemistry of Cement. Society of Chemical Industry (SCI) Lecture Series, UK, 1998.
Expanded Shale, Clay, and Slate Institute, ESCSI, 2225 Murray Holladay Road, Salt Lake City, http://www.escsi.org/.
K. E. Peray. The Rotary Cement Kiln. Chemical Publishing Inc., New York, 1986.
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