Calciners, indirect heat

Indirect-Heat Calciners Indirect-heat rotary calciners, either batch or continuous, are employed for heat treating and drying at higher... 

Equipment commonly employed for the diying of sohds is described both in this subsection in Sec. 12, where indirect heat transfer devices are discussed, and in Sec. 17 where fluidized beds are covered. Diyer control is discussed in Sec. 8. Excluding fluid beds this subsection contains mainly descriptions of direct-heat-transfer equipment. It also includes some indirect units e.g., vacuum diyers, furnaces, steam-tube diyers, and rotaiy calciners. 

The thermal efficiency of steam-tube units will range from 70 to 90 percent, if a well-insiilated cylinder is assumed. This does not allow For boiler efficiency, however, and is therefore not direc tly comparable with direct-heat units such as the direct-heat rotaiy diyer or indirect-heat calciner. 

Because indirect-heat calciners frequently require close-fitting gas seals, it is customaiy to support aU parts on a selFcontained frame, for sizes up to approximately 2 m in diameter. The furnace can employ elec tric heating elements or oil and/or gas burners as the heat source for the process. The hardware would be zoned down the length of the furnace to match the heat requirements of the process. Process control is normaUy by shell temperature, measured by thermocouples or radiation pyrometers. When a special gas atmosphere must be maintained inside the cyhnder, positive rotaiy gas se s, with one or more pressurized and purged annular chambers, are employed. The diaphragm-type seal ABB Raymond (Bartlett-Snow TM) is suitable for pressures up to 5 cm of water, with no detectable leakage. 

Design Methods for Calciners In indirect-heated calciners, heat transfer is primarily by radiation from the cyhnder wall to the solids bed. The thermal efficiency ranges from 30 to 65 percent. By utilization of the furnace exhaust gases for preheated combustion air, steam produc tion, or heat for other process steps, the thermal efficiency can be increased considerably. The limiting factors in heat transmission he in the conductivity and radiation constants of the shell metal and solids bed. If the characteristics of these are known, equipment may be accurately sized by employing the Stefan-Boltzmann radiation equation. Apparent heat-transfer coefficients will range from 17 J/(m s K) in low-temperature operations to 8.5 J/(m s K) in high-temperature processes. 

PSYCHROMETRY, EVAPORATIVE COOLING, AND SOLIDS DRYING TABLE 12-24 Indirect-Heat Rotary Calciners Sizes and Purchase Costs ... 

FIG. 12-72 Gas-fired indirect-heat rotary calciner with a water-spray extended cooler and feeder assembly (ABB Raymond/ Bartlett-Snow. )... 

Calcining the Bicarbonate to Soda Ash. Crude filtered bicarbonate is continuously calcined by indirect heating. Various techniques are used to heat, 2430 kj/kg (581 kcal/kg) this material to 175—225°C in the calciners. Carbon dioxide, produced at 95% or higher purity, is compressed and recycled to the carbonating tower in order to enrich the makeup kiln-gas feed. The hot soda ash discharged from the calciner is cooled, screened, and packaged, or shipped in bulk. This product, called light ash has a bulk density of around 590 kg/m3. A certain amount is sold in this form the majority, however is converted to dense ash. 

Typically current performance of the individual units in the WCF off-gas cleaning system is summarized in Figure 4 (9). Five campaigns have been completed in the WCF. Overall off-gas decontamination factors achieved, when indirect heating was used for the calciner, varied from 0.8 X 10 to 0.9 X 10 for particulates. In campaign 4, in-bed combustion was used and the overall decontamination factor for particulates was 1.4 X 10, and for ruthenium-106 it was 0.8 X 10 . 

Table VI summarizes typical operating conditions for the WCF as applied to the two major liquid wastes at the ICPP. The conditions for the calciner vessel are based on indirect heating with NaK in-bed heating gives similar results except that the bed might be subject to short periods of exposure to a high temperature in the immediate vicinity of the burning kerosene. Similarly, the off-gas composition leaving the calciner will depend on the method of heating.

In general, the temperature range of operation for indirect heat calciners can vary over a wide range, from 4 K at the low end to approximately 1475 K at the high end All types of carbon steel, stainless, and... 

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