Rotary kiln performance

Many kilns that formerly were direct coal-fired or producer-gas verticals were retrofitted to natural gas firing with center-burners and after World War II, dramatically improving lime quaUty, kiln capacity, and fuel efficiency. By the 1960s, this improved vertical kiln had lost favor to rotary and other special kilns because of the supply and cost problems of oil and gas in the United States and the spectacular improvement in rotary kiln performance. Many natural gas-fired center burners were permanently closed and dismanded because they could not be converted to coal. However, the reverse occurred in Europe where the extensive oil and gas discoveries heightened interest in the new, advanced vertical kilns. 

A furnace is a device (enclosure) for generating controlled heat with the objective of performing work. In fossil-fuel furnaces, the work appHcation may be direct (eg, rotary kilns) or indirect (eg, plants for electric power generation). The furnace chamber is either cooled (waterwaH enclosure) or not cooled (refractory lining). In this article, furnaces related to metallurgy such as blast furnaces ate excluded because they ate coveted under associated topics (see... 

Heat Transfer in Rotary Kilns. Heat transfer in rotary kilns occurs by conduction, convection, and radiation. In a highly simplified model, the treatment of radiation can be explained by applying a one-dimensional furnace approximation (19). The gas is assumed to be in plug flow the absorptivity, a, and emissivity, S, of the gas are assumed equal (a = e ) and the presence of water in the soHds is taken into account. Energy balances are performed on both the gas and soHd streams. Parallel or countercurrent kilns can be specified. 

A more simplified description is a unit that combusts materials in the presence of oxygen at temperatures normally ranging from 800 to 1650°C. A typical configuration of an incinerator is shown in Figure 9. Typical types of incineration units that are discussed herein are catalytic oxidation, fluidized beds, hquid injection, multiple hearth furnaces, and rotary kiln. Thermal desorption is also discussed. However, an overview of the main factors affecting incinerator performance is presented first, below. 

Studies of the incineration of liquid and solid wastes must determine the rates at which hazardous compounds are released into the vapor phase or are transformed in the condensed phase, particularly when the hazardous materials make up a small fraction of the liquid burned. We must be particularly concerned with understanding the effects of the major composition and property variations that might be encountered in waste incinerator operations—for example, fluctuations in heating value and water content, as well as phase separations. Evidence of the importance of variations in waste properties on incinerator performance has been demonstrated by the observation of major smges in emissions from rotary-kiln incinerators as a consequence of the rapid release of volatiles during the feeding of unstable materials into the incinerator. 

Particle Size and Desorption Rates. Bench-scale reactor studies of the desorption of toluene from single, 2- to 6-mm porous clay partides (14) showed desorption times that increased with the square of the particle radius, suggesting that diffusion controls the rate desorption. Parallel experiments performed in a small, pilot-scale rotary kiln at 300°C showed no effect of day partide size for diameters ranging from 0.4 to 7 mm. Additional single-partide studies with temperature profiles controlled to match those in the pilot-scale kiln had desorption times that were a factor of 2—3 shorter for the range of sizes studied (15). Hence, at the conditions examined, intrapartide mass transfer controlled the rate of desorption when single particles were involved and interpartide mass transfer controlled in a bed of particles in a rotary kiln. These results apply to full-scale kilns. As particle size is increased, intraparticle resistances to heat and mass transfer eventually begin to dominate. 

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. 

Batch Furnaces This type of furnace is employed mainly for the heat treatment of metals and for the drying and calcination of ceramic articles. In the chemical process industry, batch furnaces may be used for the same purposes as batch-tray and truck dryers when the drying or process temperature exceeds 600 K (620°F). They are employed also for small-batch calcinations, thermal decompositions, and other chemical reactions which, on a larger scale, are performed in rotary kilns, hearth furnaces, and shaft furnaces. 

The different technologies involve indirect heating and are characterized by the method of heat transfer to the material and by the method used for mixing the charge in the furnace. Slow pyrolysis can be performed in rotary kilns, or in static furnaces equipped with moving... 

Fusion or Roast Technique for Hard Scrap [5.27-5.29]. Fusion is performed at high temperature with sodium nitrite or sodium nitrate as oxidizing agent and sodium carbonate as dilutant and must be carefiilly controlled. Roasting is done in rotary kilns at 800 °C using a 1 3 mixture of oxidizer and diluent After cooling, the cake must be crushed prior to dissolution in water. 

In addition to the formation of alite, a-belite is transformed to P form and recrystalization and crystal growth proceeds for already formed components. If the clinker is underburned, it will contain an excessive quantity of CaO. The clinker, as a low alite content, is poor quality. If the clinker is overburned, the clinker results in large crystals. Table 31.5 shows the main characteristics and performances of the different classes of cement rotary kilns. 

There are typical parameters for the rotary kiln design and operation and they are considered critical in optimizing the performance of a kiln itself. When designing a cement rotary kiln for a certain nominal capacity, or when evaluating an existing one for potential output, there are a number of key parameters that must be evaluated. These... 

As a major step in the evaluation of the above mentioned high-throughput tools and techniques, a scale-down of different types of catalysts for several applications was performed. For that purpose, two well established commercial catalysts, one of the mixed metal oxide type for selective olefin oxidation and one impregnated catalyst for ethylene acetoxylation to vinyl acetate monomer (VAM), respectively, were prepared in the small-scale and their catalytic performance was compared. As shown in Fig. 1 with the selective oxidation catalyst, the scale-down of this catalyst was successful, since both, the commercial and the high-throughput prepared catalyst are showing identical performances. Regarding the calcination procedure one can point out, that only if this step is carried out in the 5-fold rotary kiln, equal catalysts were obtained. 

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