Kiln geometry

Nijdam, J.J. and Keey, R.B., 2000. The influence of kiln geometry on flow maldistribution across timber stacks in kilns. Drying Technol., 18 1865-1877. 

To speed up the process of attainment of the temperature steady value one can use special operations calculation without a kiln rotation, using large time intervals and calculation in two-dimensional R-tp geometry without regard for heat and mass transfer along an axis The program for realization of discussed simulation algorithms enables to calculate temperature in cells, a total number of which can not exceed 130 thousands A circular kiln structure can contain up to three layers. 

The zinc salt and BaS solutions are mixed thoroughly under controlled conditions (vessel geometry, temperature, pH, salt concentration, and stirring speed, see (a) in Fig. 20). The precipitated raw lithopone does not possess pigment properties. It is filtered off (b2) and dried (c) ca. 2 cm lumps of the material are calcined in a rotary kiln (d) directly heated with natural gas at 650-700 °C. Crystal growth is controlled by adding 1-2 wt% NaCl, 2 wt % Na2S04 and traces of Mg2 + (ca. 2000 ppm), and K+ (ca. 100-200 ppm). The temperature profile and residence time in the kiln are controlled to obtain ZnS with an optimum particle size of ca. 300 nm. 

Wifh this type of kiln sysfem, a higher quantity of excess air is required af fhe kiln exit in order to burn the additional fuel in fhe riser duct, but this then fosters the ideal conditions for alfernative and waste fuel use. The limit on heat in the riser duct is limited by several factors including duct geometry, system design, and fuel fype, and is generally limifed fo 25% of the total heat. This limit stems from the maximum quantity... 

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). 

Primary air is usually defined by a percentage of the stoichiometric air calculated for the total amount of fuel. In the case of PF, it provides a transport air stream that can or can not be considered in the total amount of primary air. Since the very hot secondary air has to be entrained into the fuel-primary air jet, it can have an important impact on the fuel-oxidant macro-mixing. The flow patterns of the secondary stream are mainly determined by the design of cooler uptake and the kiln hood itself. The relationship between primary air jet momentum and secondary air velocity has a significant impact on the flame geometry as well as on the heat transfer to the material and refractory lining. 

Fig. 5.15. Ceramic tunnel kiln (not to scale) with unfired preheat vestibule for heat recovery. Long, narrow kiln or furnace geometry minimizes the proportion of heat loss at the conveyor entrance and exit. Air-lock chambers are even better.

One can also determine the maximum capacity of the kiln by the same geometrical considerations if the flow area can be estimated. From the geometry (Figure 2.5) the chord length can be expressed as... 

Figure 6.12 Burner nozzle geometry for pulverized fuel combustion in a rotary kiln. Left swirl vanes Right center insert for flame stabilization.

For the process or kiln engineer perhaps the important application is to develop a catalog of radiative heat transfer coefficients, which can be combined with convection heat transfer coefficients to estimate the kiln heat balance. Cognizant of the fact that the radiative exchange between an area element and any zone in the freeboard is a function of temperature, pressure, and geometry, the radiative heat transfer coefficient can be cast into the form... 

Between 1960 and 1970 the capacity of numerous kilns was greatly enhanced without adjusting the cyclone preheater to the higher waste gas volume flows. This caused the pressure losses in the cyclone preheater and thus the specific electrical energy requirements of the ID-fan in some cases to double or to increase even more. In redevelopment work the gas cross sections were sometimes enlarged by 100 % and in addition the pressure losses could be reduced by aerodynamically adjusted geometries. In some cases the specific ener requirements of the entire kiln could be reduced by up to 15 %. 

The most used refractory lined vessel geometry in industry appears to be the cylindrical refractory lined vessel. In the steel industry, the blast furnace, blast furnace stoves, torpedo cars, steelmaking ladles (7), electric arc furnaces, degasser vessels, and cyclone dust collector are a few examples of cylindrical vessels. In the petrochemical industry, the petrochemical processing vessels are typically cylindrical. Fluid bed reactor vessel walls are typically cylindrical with a spherical vessel top. Many other basic industries use cylindrical refractory lined vessels such as rotary kilns and wood pulp process vessels. 

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