The rotary kiln

Nodules of clinker, typically 3-20 mm in diameter, are formed in a semisolid state in the burning zone, and solidify completely on cooling, which begins in a short cooling zone within the kiln, and continues in a cooler. In modern plants, when the nodules leave the kiln, their internal temperatures are around 1350°C, but their surface temperatures are considerably lower. 

Liquid or pulverized solid fuels are blown into the kiln through a nozzle with primary air. Additional secondary air is drawn into the kiln through the clinker cooler. The flame in the rotary kiln must meet several requirements. The clinker must be correctly burned, so as to minimize its content of free lime, with the least expenditure of fuel. The ash from a solid fuel must be uniformly absorbed by the clinker. For normal Portland cements, the conditions must be sufficiently oxidizing that the iron is present as Fe however, for white cements, mildly reducing conditions may be preferable. Proper flame control also extends the life of the refractory lining of the kiln. Computer-aided or fully automated control of kiln operating conditions is increasingly used. 

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Rotary Kiln Incinerators. The rotary kiln has been used to incinerate a large variety of Hquid and soHd industrial wastes. Any Hquid capable of being atomized by steam or air can be incinerated, as well as heavy tars, sludges, pallets, and filter cakes. This abiUty to accept diverse feeds is the outstanding feature of the rotary kiln and, therefore, this type of incinerator is often selected by the chemical and waste treatment industries. 

SL/RN Process. In the SL/RN process (Fig. 4), sized iron ore, coal, and dolomite are fed to the rotary kiln wherein the coal is gasified and the iron ore is reduced. The endothermic heat of reduction and the sensible energy that is required to heat the reactants is provided by combustion of volatiles and carbon monoxide leaving the bed with air introduced into the free space above the bed. The temperature profile in the kiln is controlled by radial air ports in the preheat zone and axial air ports in the reduction zone. Part of the coal is injected through the centerline of the kiln at the discharge end. The hot reduced iron and char is discharged into an indirect rotary dmm cooler. The cooled product is screened and magnetically separated to remove char and ash. 

The latest installations incorporate a waste heat boiler in the off-gas cleaning system to recover sensible heat from the rotary kiln off-gas. There is sufficient sensible heat in the off-gas from the SL/RN process to generate 500 to 700 kWh/t of DRJ, depending on the type of reductant used. 

Solid wastes arc disposed of by two basic methods. The first is by some type of dumping or landfill procedure the second is by incinerating (burning) the waste. This section focuses on incinerators, namely the rotary kiln, liquid injection, fuidized-bed, and multiple-hearth dc ices, which are the four types... 

The key to efficient destruction of liquid hazardous wastes lies in minimizing unevaporated droplets and unrcacted vapors. Just as for the rotary kiln, temperature, residence time, and turbulence may be optimized to increase destruction efficiencies. Typical combustion chamber residence time and temperature ranges arc 0.5-2 s and 1300-3000°F. Liquid injection incinerators vary in dimensions and have feed rates up to 1500 gal/h of organic wastes and 4000 gal/h of aqueous waste. 

The process is basically a rotary kiln design. Waste is first pretreated and then inserted in the rotary kiln, where it is incinerated with air. The chlorinated hydrocarbons are converted into H2O, CO2 and HCl. After that, in a wet scrubber the HCl is recovered as aqueous HCl. If needs be, natural gas or liquid energy carriers can be added in order to reach the necessary high temperatures in the afterburner. 

The rotary kiln design allows for accepting a mix of high-chlorinated wastes (solvents, chlorinated tars, plastics). Such kilns are usually designed in relation to a specific optimal calorific value in the input. The input mix should be set in such a way that this optimal composition is approached (e.g., PVC waste and other waste streams with a lower calorific value). It is likely that a 100% input of PVC would lead to all kind of problems of temperature control due to its relatively high calorific value. Chlorine contents of over 50% can easily be accepted. A final demand is that the particle size should be 10 x 10 x 10 cm at maximum. This implies that sometimes waste has to be shredded before it can be put into the kiln. Other acceptance criteria have not been published in literature. 

Three different types of furnaces are generally in use for calcination. The shaft furnace is considered to be the most suited for calcining coarse limestone. Furnaces of the rotary kiln type are used for handling materials of mixed particle sizes and lumps which disintegrate during the process. Calcination can be carried out in a fluidized bed-reactor for materials of small and uniform particle size. These furnaces are usually fired with gas, oil or coke in some cases electric heating is resorted to. 

The rotary kiln technology is quite common in the incineration business. The conversion technology in this case is in the form of a rotating tube, see Figure 22. 

The solid phase will be rotated or mixed in the rotary kiln. The air and the solid phase could be said to have a crosscurrent configuration. 

The system is mounted on two mobile trailers. The first trailer contains a rotary kiln in which soil is heated to 300 to 600°F. The off-gas is ducted from the rotary kiln to a cyclone separator where relatively coarse matter is removed and a heat exchanger cools the gas to about 400°F. The cooled off-gas goes to the baghouse, which filters relatively fine particles from the off-gas stream. The final unit is an afterburner or fume incinerator that heats the contaminated air to approximately 1800°F to destroy the contaminants. 

The Sonotech Cello pulse combustion system has the same limitations as a nonpulsating burner attached to a combustion device. Preliminary testing of the Sonotech system showed that in order to prevent slag formation, the temperature of the rotary kiln gas should not exceed 1700°F. The system produces considerable noise, which may be controlled by sound insulation. The Sonotech system uses resonant frequency of the incinerator to create pulsations. In an older incinerator, if the sound energy is not properly applied, the Sonotech system could cause structural problems. 

Data from Polasek Jervis (1994), for pure tyre ash. Major elemeni oxides add up to only 71 wt% no further details available. Data from Lemieux (1994), collected after co-firing natural gas and TDF crumbs. Major element oxides add up to 85 and 87 wt%, respectively. Ash represents fly ash from combustion of TDF crumbs, whose fuel characteristics are listed in Tables 4 and 5. Lemieux (1994) suspected that some elements (Al, Si, Zr) may have originated from the rotary kiln insulation. 

The rotary-kiln type incinerator mentioned by S. Slemrod of Picatinny Arsenal (Ref 24) for destruction of Mred-water syrup, obtained as waste in US plants manufacturing TNT by the batch method. One of such kilns was in operation at Keystone... 

The rotary kiln is 510 feet (153 meters) long and is fired by oil. Maximum daily output of the kiln is 1,760 tons (1.584 metric tons) of clinker, equivalent to 9,700 barrels of cemenl. Operations are automatically controlled, and two television cameras, one at each end of Ihe kiln, observe all material flow. 

There is a wide choice of contacting methods and equipment for gas-solid reactions. As with other solids-handling problems, the solution finally adopted may depend very much on the physical condition of the reactants and products, i.e. particle size, any tendency of the particles to fuse together, ease of gas-solid separation, etc. One type of equipment, the rotary kiln, has already been mentioned (Chapter 2, Fig. 2.4) and some further types of equipment suitable for continuous operation are shown in Fig. 3.37. The concepts of macromixing in the solid phase and dispersion in the gas phase as discussed in the previous section will be involved in the quantitative treatment of such equipment. 

The dynamic model of solid transport through the rotary kiln is based on the geometrical model derived by... 

Fig. 2. Schemes of the rotary kiln with the main variables used in the equations.

Note that data from a boiler burning TDF at a silicon manufacturing facility, Dow Coming in Midland, Michigan, are reported in Chapter 5 with waste wood boilers, because the primary fuel for this boiler is wood chips. Further, data on TDF use at Boise Cascade, an "other" manufacturing facility, are included in Chapter 4 with cement manufacturing, because the rotary kiln used to manufacture lime is similar to the rotary cement kilns. 

The rotary kiln is simple in concept, but difficult to operate in practice. The rotary kiln is a refractory lined, steel cylinder mounted horizontally on trunions and riding rings. It is pitched slightly toward the discharge end to facilitate material flow through the kiln. The kiln is fed from the high end and can be fed either whole tires or TDF chips. It can be fired internally or heated externally. 

The rotary kiln is a long tube that is positioned at an angle near horizontal and is rotated. The angle and the rotation allow solid reactants to work their way down the tube. 

The rotary kiln is a continuous countercurrent heterogeneous reactor. Solids traveling down the kiln are in plug flow, as are the gases passing upward. 

The most common reactor of this type is the lime kiln. This is a noncatalytic reaction where gas reacts with calcium carbonate moving down the kiln. Other reactions performed in the rotary kiln include calcination, oxidation, and chloridization. 

When petroleum coke is utilized for anode and electrode production and some specialty applications, it is necessary to calcine it to remove moisture and hydrocarbon VCM. Product qualities, along with production rate, are based on feedstock composition, kiln temperature profile, kiln residence time and cooling procedures. The two methods available for calcining coke commercially are the rotary kiln (5 ) shown in Figure 8 and the rotary hearth (6J shown in Figure 9. 

In the rotary kiln process, coke is fed to a rotating cylindrical furnace sloped slightly toward the discharge end. Coke flows down the kiln countercurrent to the hot gas flow. Moisture is liberated from the coke in the feed zone, then the coke passes through the combustion zone where VCM is liberated. As coke leaves the kiln, it is discharged to a cooler where it is quenched with water and then cooled with ambient air. Recent designs have incorporated energy efficient features such as air preheat and steam... 

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