SIMULATION OF CEMENT ROTARY KILN

Abstract

Rotary kilns have numerous industrial applications including cement production. Frequent operational problems such as low thermal efficiency, refractory failure, and poor product quality have prompted extensive efforts to improve and optimize their design. Mathematical modeling and Computational Fluid Dynamics constitute effective tools recently used for the process. A cement kiln consists of three major parts: the hot flow, the bed, and the wall. Gas flow is typically counter-current to the bed movement, and the gas is heated to supply the requisite energy for processing the material. The gas can be heated by way of a direct flame or by passing through an external furnace. Both physical processes and chemical reactions can. occur inside a kiln. Both physical processes and chemical reactions can occur inside a kiln. For example in a lime kiln, granular material proceeds through a drying section, then a pre-heating section, and finally the calcining and burning sections where chemical reaction occurs. Within a 'rotary kiln heat can be transferred by conduction, convection and radiation, from gas to wall, gas to bed and wall to bed. The mechanism whereby the gas heats, by direct contact, the exposed wall of the kiln, which then rotates to be under the bed and thereby heats it, is sometimes referred to as regenerative heat transfer. The simplest models of rotary kiln operation treat temperatures within the kiln as depending only on axial position, so the gas and the bed are well-mixed over a cross-section, and lumped heat transfer coefficients have been used to describe each heat transfer mechanism. Different forms of solid motions, such as sliding, slumping, rolling, and the transition behavior between these motions are well described. The solid transverse motions are found to influence the solid axial motion in the kiln. Furthermore, it is also found that the change of the bed depth in axial direction inversely influences the solid transverse motion, as well as the residence time of the particles in the active layer and the hold-up of solids in kilns. For the axial bed depth some models exist. These models require the bed depth at the discharge end as initial 'condition to solve the differential equations. However, this actual depth is still unknown for different operational conditions. In most rotary kiln operations the chemical reactions in the bed require high temperature, for example cement kilns will require temperatures of approximately 15000C. The energy to raise the temperature and drive endothermic reactions is from the combustion of a range of fuels such as natural gas, coal and more and more alternative fuels. Heat transfer from the gas to the bed is complex and occurs from the gas to the bed surface and kiln wall to bed surface via conduction, convection and radiation. A number of rotary kiln models have been proposed over the years and recent computational fluid dynamic models can be developed but all have their limitations. Most assume isothermal conditions through the bed at any axial position. However, practically it is known that such conditions do not exist as product variations are known to occur, for example in lime kilns where fine particles are calcined and larger particles are only partly calcined. The bed 'motion regime, either cascading, rolling or slumping depends on the rotational speed of the kiln, the percentage fill and the feed physical properties.