Kilns zones

The pyrolysis of tires based on rotary kiln technology started quite early in the 1970s. A field-scale rotary kiln at Rocky Flats is documented in [5]. Very new developments can be found in [6]. The rotary kiln developed by Faulkner has several distinct heating zones independent from each other. The system consists of a rotary feed cylinder that includes a screw-Uke flight extending from the inner wall of the feed cylinder. As the feed cylinder rotates, the flight directs the supply of vehicle tire pieces into the infeed end of the pyrolysis section. The temperature levels of the kiln zones decrease from a maximum of 800°C to 500°C at the end of the kiln. A separation of char and scrap steel... 

The internal kiln surface can contain up to six empty defect zones or zones with thermal physical characteristics different from those of material layers. The initial data are taken from a text file, prepared by any editor in the format MS-DOS. The obtained values of temperatures are written into an output file in the format MS-DOS for subsequent processing with visualization programs. 

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. 

Thermal Reduction. Thermal reduction is usually accompHshed ki a high temperature countercurrent rotary kiln. "Hot zone," a region near the kiln spik, temperature is usually controlled at 1100—1200°C. The reaction rate has been shown to be only slighdy lower at 1050°C than at 1130°C (9). About 6% of the feed BaSO remains unreacted after 30 min at 1050°C. Reaction completion is approached ki less than 10 min at 1100°C (10). 

Air Drying Equipment. Tunnel kiln dryers (70) are long furnaces comprised of several zones of different temperature, humidity, and air flow through which the ware travels on a moving car or belt. These kilns afford continuous processing. Periodic kiln cross-circulation dryers (70) are box furnaces in which ware is stacked on permanent racks or on a car that can be shuttled in and out of the furnace. Fans or jets are used to circulate heat uniformly through the ware. The process is not continuous, but production rates can be enhanced by shuttling multiple cars. 

Scoop systems are provided for introducing collected dust or, in some cases, a feed component through the shell at some intermediate point or points. Ports are installed in the shell for admitting combustion air at points beyond the hot zone these are used in reducing kilns for burning carbon monoxide and volatiles from materials oeing processed. 

Generally, a diy-feed kiln will have three zones of heating, and a wet-feed kiln will have four ... 

Equation (14) is the equation for the straight lines in Fig. 10 used to demonstrate the approximate constancy of a over each zone of the kiln. Similar consideration of the heat balance shows that... 

There are 21 parameters in Eq. (25) (Table I). These provide the five dimensionless groups given in Table II. A 10-dimensional process variation space is needed to characterize the slow coke preformance of the two-zone air lift TCC kilns, even when a is constant for each zone. It is not surprising that engineers and operators had problems in understanding the responses observed in the In. 

Parameters in the model are listed in Table I. The flow, structural, and boundary conditions are known quantities. The frequency factor and activation energy for coke burning were the values determined by Weisz and Goodwin (1966) from the experiments discussed earlier, and the catalyst diffusivity D was measured directly in the laboratory. The value of a was determined from direct observations of the CO/CO2 ratio in each zone of the operating kiln. The remaining parameters are known quantities. Thus, there are no adjustable parameters available to tune the fitting of predicted values to observed data, for the fraction of coke remaining and for the vertical temperature versus distance from the top of the kiln. 

Flow of air up Flow of air down Air temperature at inlet Oxygen in air to kiln a (upper zone) a (lower zone)... 

A major part of the fast coke is probably desorbed from the catalyst bed and burned in the gas phase. Even if none of the fast coke was desorbed, a calculation of the Thiele modulus tj for conditions in the plume burner and the top of the first zone of the kiln shows that rj is in the range 0.92-0.99. Thus, the fast coke can be assumed to bum without significant diffusion limitation. 

Figure 19 shows the slow coke profiles for the top and bottom zones obtained when 0.0, 0.1, and 0,2% fast coke is present. When no fast coke is present, the catalyst does not bum clean ( 0.2% residue carbon at kiln exit). This is consistent with the rapid rise in bum-off distance at 875 F (469°C) seen in Fig. 17. However, the presence of 0.1% fast coke gives an essentially clean catalyst, using only 0.6 of the bottom zone. Only a quarter of the bottom zone is needed when 0.2% fast coke is present. This improvement in bum-oflf distance is caused by the temperature boost obtained from the rapidly burning fast coke. This temperature boost is shown by the temperature curves in Fig. 20b. The temperature of the catalyst at the top of the upper bed has increased from 875°F (741 K) to 900°F (755 K). This is sufficient to make a large improvement in bum-off distance as shown by Fig. 17.

Figure 21 shows the response of the temperature above the air inlets and above the cooling coils at the bottom of the bottom zone of the kiln, due to a step change in catalyst temperature when no fast coke is present (Wcf = 0). Figure 22 shows that there is a large improvement in stability of the kiln temperature when fast coke [as per Eq. (49)] is present. An unexpected response to a temperature rise occurs when the fast coke is absent the temperature above the cooling coils falls before it finally rises, and residual coke is present until 60 min after the tempera-... 

The feedstock to the TCC reactor varies in its coke-forming properties. This is an important source of disturbance in the kiln operation. Figure 23 shows the effect of a 20% change in coke. The temperature above the air inlet responds with a damped oscillation of the temperature before reaching a steady state 40°F (22°C) above the previous steady state. The temperature at the bottom of the lower zone rises monotonically 75°F (42°C) to the new steady-state value. This rise in catalyst temperature will influence the performance of the reactor, since the catalyst is returned to the top of the reactor after some heat losses from cooling coils in the bottom of the kiln and in the air lift and the separator. The change of temperature... 

Fig. 23. Response of kiln to coke increase (a) coke (b) bottom of lower zone (c) above air inlets. 

Ko Mole fraction of oxygen in air, moles 02/mole air / Location of air inlet L Length of kiln, ft Li Length of ith zone of kiln m [kcolDcoy ... 

Solid materials are often dried or heated using combustion gas exhaust from a fired heater as the material is conveyed through a hot combustion gas zone. Solids handling dryers may take a number of forms, e.g., a rotary kiln. Losses involving dryers usually involve internal fires or explosions. 

Under USEPA s BIF mle, manufacturers are required to closely monitor numerous conditions in the kiln and to observe limits on the following aspects of the process (a) the maximum feed rate of hazardous waste fuel (b) the maximum feed rate of metals from both raw materials and fuels (c) the maximum feed rate of chlorine from raw materials and fuels (d) the maximum feed rate of raw materials (e) the maximum temperature at the inlet to the air pollution control devices (f) the maximum concentration of carbon monoxide and total hydrocarbons in the flue gas (g) the maximum temperature in the combustion zone or minimum temperature at the kiln inlet and (h) any decrease of pressure at the baghouses or any decline in the strength of the electric field of electrostatic precipitators (both are types of air pollution control devices). 

The kilns built for Houdriflow units, the first employing the gas lift principle, are cylindrical vessels 12 to 20 feet in diameter and contain one or two split-flow or counterflow burning zones. These regenerators are operated at pressures of from 3 to 10 pounds per square inch gage. The increase in pressure effects a further saving in burning volume, as... 

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