Abstract
The results of an extensive series of trials on a 100 tonnes per day cement kiln have provided a fundamental insight into the aerodynamics, burning mechanism and heat transfer in the sintering zone of such kilns. Accurate monitoring of the input and output variables has enabled mass and heat balances to be made on the system, and from these results it has been possible to isolate the areas where major fuel savings can be achieved. Slurry moisture, excess air and external heat losses are all variables where improvements and better control would reduce fuel consumption and increase the efficiency of rotary kilns. It has been shown that the external temperature profile of the kiln shell provides a useful indication of the various reaction regions that exist in the process cycle. From calculations of the heat lost from the shell, it has been shown that half the external heat losses occur from the sintering zone of the kiln. Specialized instrumentation has been developed to measure gas temperatures and extract combustion gas samples during the normal range of operation of the kiln. An analysis of these results has led to a formula to predict the length of the flame as influenced by the significant operating parameters. The measured gas concentrations have been used to predict the combustion rate within the flame, and a favourable comparison of this rate has been made with published data. The measurement of flame temperatures in the kiln has shown that the average flame temperature that may be encountered in a cement kiln is approximately 1800°C. Point temperatures of up to 2100°C were measured, and it was observed that increased excess air produced a shorter, hotter flame, but reduced the temperature of the combusted gases, resulting in a poorer quality product. A mathematical model has been developed to predict the gas and refractory temperature and heat flux profiles occurring in a rotary kiln sintering zone. The method is based on that of Hottel and Sarofim, but modified to account for the specialised firing conditions necessary for cement production. The model has been tested against the measured data obtained from the kiln trials, and the degree of agreement found to be encouraging. Use of this model should enable the cement, lime, and refractory industries to comprehend the effect of changes to operational variables, with a resulting improvement in heat utilisation, product quality and plant life.