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Show Fuel Consumption (kcal/kg) 2 3 Oxygen In Kiln Exhaust Gases (%) Figure 4 Effect of Excess Air on Kiln Fuel Consumption for a typical semi dry process kiln (15) 3.2 Modeling of Combustion and Heat Transfer in Cement Kilns Effective modeling requires that the important parameters of the process being studied are identified and represented in the model. Since it is not possible to scale nature completely, physical modeling can only give part of the answer. Mathematical modeling is similarly limited both by computing power available and our ability to describe the combustion and heat transfer process mathematically. As a result each modeling technique represents a partial understanding of the process. The objective is to provide predictive techniques which work for real flames in real kilns and contribute to improved kiln performance. To achieve this objective normally requires simultaneous use of several modeling techniques. 3.3 Physical modeling of flames Despite the growth in computer modeling, physical modeling is still the most effective method for determining flame length and shape in rotary kilns. Acid/alkali modeling was developed by Sir William Hawthorn (8) at M I T as long ago as 1938 and is used to model the combustion process in rotary kilns where fuel/air mixing determines the flame characteristics. A physical model of the cooler, hood and kiln is constructed to an appropriate scale in clear acrylic plastic. The fuel is represented by dilute caustic soda solution containing phenolphthalein indicator, while the combustion air is represented by dilute hydrochloric acid. The concentration of the alkali and the stoichiometric ratio of alkali to acid is chosen to represent the correct air/fuel air requirement for the particular fuel. The flow of acid is adjusted to simulate different excess air levels, hence determining the relationship between flame length and excess air. The phenolphthalein becomes colorless at the boundary where the mixing is complete, thus the model flame envelope is defined by the colored region. The aerodynamics of the full size system are reproduced on the physical model thus allowing an accurate simulation of the fuel/air mixing characteristics and hence flame length under representative conditions. These model results should be corrected since the model is run under isothermal conditions, while in the kiln, considerable changes in temperature usually occur as combustion takes place. This results in a reduction in the gas density and an increase in volume giving a longer flame in the kiln than in the model. For most practical purposes the model flame length needs to be corrected for the density changes only. 3.4 Heat Transfer Modeling The combustion process and its integration into energy transfer equipment design, is the most complex of all process engineering problems, requiring the simultaneous solution of heat, mass and momentum transfer. For effective modeling of combustion and heat transfer in a rotary kiln many factors must be taken into account. Rotary kiln flames are turbulent jet diffusion flames which are fortunately relatively well understood owing to the work of Thring and Newby (9), Craya and Curtet (10), and Becker (11). Their analysis of momentum transfer in free and confined jets has yielded theories to predict the macro-turbulent 8 |
