Preheaters and Precalciners

Dry process cement production facilities often have several other types of manufacturing equipment designed to increase fuel efficiency. First, many dry process kilns add a preheater to the feed end of the kiln to begin heating of the feed prior to its entrance to the kiln. Two main types of preheaters exist, the suspension preheater and the traveling grate preheater both use hot, exiting kiln air to facilitate a more efficient heat transfer to the feed than could occur in the feed end of the kiln itself.1 This 

The second development to increase fuel efficiency in a dry process kiln is a precalciner. For this system, a vessel called a flash precalciner is located between the preheater and the kiln, and is fueled by a separate burner. A discussion of tire use to supplement precalciner fuel is discussed in section 4.2.6 below. 

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Improvement of the energy efficiency in the pyroprocessing is reached by increasing the fuel efficiency of the kiln for example by replacing the wet kilns with preheater and precalciner systems, resulting in significant reduction of the CO2 emissions. 

DeHayes, S.M. Grady, J.M. and Vidergar, D.M., "Clinker Microstructure—Comparison of Dry Process, Preheater, and Precalciner Kilns," Proceedings ofthe 8th International Conference on Cement Microscopy, International Cement Microscopy Association, Orlando, Florida, 1986, pp. 1-12. 

Late in the nineteenth century the rotary kiln was developed in Britain, introduced into the United States and, from that country, adopted in Continental Europe. With this kiln it had become possible to use any type of fuel solid, liquid or gaseous (coal, oil, gas). The raw materials were introduced into the rotating tube in the form of "slurry" (wet process) or "raw meal" (dry process). In comparison with the shaft kiln, the capacity of the rotary kiln was soon greatly increased, especially after very effective homogenization methods, preheating and precalcining systems had been developed, and efficient measuring and control instrumentation had been introduced. 

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. 

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. 

This chapter is devoted to the actual process of making cement. The various stages are described. The wet process and the shaft kiln are only briefly considered. On the other hand, the dry process with raw meal preheating and the precalcination principle are treated in some detail, as are the preparation of the raw materials, the storage and homogenization of the raw meal, and the cooling of the cement clinker. 

Fig. 3 NSP Cement Kiln (Raw meal, Precalciner, Preheater and Kiln)...

SAC High (precalciner, preheater and long kilns 20-50% 120-160 180-220 250- 00 660-940 None CO, THC may increase... 

Seven cement kilns in the United States utilize about 6 million scrap tires per year to replace conventional fuels. Cement kilns appear to be ideal for scrap tires because of their high operating temperatures (2,600 F) and good conditions for complete combustion, which minimize air pollution problems. Also, there is no residue, since the ash is incorporated into the cement product. Of the 240 cement kilns in the United States, about 50 are equipped with precalciner/preheaters, making them most suitable for tire combustion. 

In the U.S. there are approximately 240 active cement kilns (52). Of these, there are 50 precalciner/preheater kilns built since 1971, which would be the kilns most likely to burn tdf. However, about 20 percent of these kilns are at locations, such as the southeast Gulf coast, where they can probably obtain petroleum coke at a lower price than tdf, and hence they would not be likely tdf buyers. The remainder could become tdf users if the economics and the environmental permitting procedures were favorable. If 40 cement plants each used the equivalent of 2 million tires per year, there could conceivably be a national usage of 80 million tires per year, or one third of the annual number of scrap tires generated. 

When whole tires are used as supplemental fuel in cement manufacture, they generally enter the process at the upper feed end of the kiln. Depending on the specific process flow at a facility, TDF can be added at the feed end, at the lower (firing) end, or in a raw feed preheater/precalciner that is located before the raw feed entrance. These options are described in more detail in Chapter 4, Tire and TDF Use in Portland Cement Kilns. 

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 ... 

The burning temperature for production of Portland cement clinker can be decreased by about 150°C through the use of fluxes, but opinions have differed as to the energy saving thereby obtainable, Klemm and Skalny (K52), who reviewed the subject, estimated it at 630kJkg" . Christensen and Johansen (C56) considered that this figure, while possibly realistic for an inefficient, wet process kiln, was unlikely to be so for a modern, precalciner-preheater kiln, in which heat recovery is efficient. They considered a value of lOSkJkg" more realistic. 

In the early 1980s the preheater rotary kiln size reached a typical capacity of about 3000 (m)tpd. Further increasing of fhe oufpufs were limifed by a span life of fhe brick refractory lining of a rofary kiln shell wifh a diameter more of 6 m. The precalciner/preheater kilns were developed to reach production up to 10,000 (m)tpd and more, by splitting the total fuel input between the main burner and the auxiliary burners of an additional vertical vessel, called precalciner, located in between the rotary kiln section and the suspension preheater. 

By providing air directly to the calciner and not as excess air in the kiln, 60% of the total heat can be burnt in the calciner, allowing more than 90% calcination of the raw meal before entering the kiln. The relatively low temperatures in the calciner (800°C-900°C) indicate that a conventional flame with a defined shape and geometry is not possible. Several factors affect the calciner efficiency, especially the uniformity of air flow, raw meal, and fuel dispersion. With a preheater/ precalciner kiln system, a nominal production of more than 10,000 tpd clinker can be reached with as little as 3 MJ/kg (700 kcal/kg). 

Figure 10.6 Typical schematic of a cement kiln with preheater, precalciner, and grate cooler.

With these arrangements, in cases where the raw material has a low moisture content and requires little exhaust air from the cooler for drying, additional expenditure on dust collection equipment for cleaning the exhaust air can be cut down. At the other extreme, raw material with as much as 14% moisture content can be dried without any extra heat input by utilizing the exhaust gas from the clinker cooler in combination with exit gas from a preheater kiln with precalcining. 

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