Abstract
At present, the main cost items associated with cement manufacture are due to the fuel and the electric energy consumed. Reducing the fuel cost can be achieved either by reducing the specific consumption or by using lower grade cheap fuels in the process. One of the simplest and most important methods of reducing the specific consumption is the flame control, which, in addition to saving energy also results in better quality product and steadier kiln operation. By changing the process from wet to suspension preheater dry system, the industry can reduce its fuel consumption up to 50%. Application of precalcining system to these dry process suspension kilns can even further improve the heat transfer conditions in the kiln and the preheater. Precalcining can be achieved either by introducing some proportion of the total heat input into the riser duct connecting the kiln to the preheater or by adding another stage of separate calciner chamber. The advantages obtained by precalcining are several, some of the important ones being increase in production capacity for a given kiln unit, improved kiln lining life, steadier operation, better heat transfer conditions in suspension state for the decarbonation of the material, and the possibility of utilizing lower grade fuels in the secondary firing unit in the calciners. In order to achieve the aim of energy saving, it is essential to have a very good understanding and investigation of the aerodynamics of the furnace, and the effects of the modifications introduced to the system for energy saving purposes, like auxiliary burners. Modelling techniques prove to be very useful in such cases. In this present work, the technique of physical modelling has been chosen and the acid/alkali mixing method has been applied for flow visualization. By comparing the model results concerning the length and behaviour of the flame under different kiln operating conditions with the results of the experiments carried out on the prototype itself by earlier workers, the reliability of the technique and its usefulness in flame studies have been proved. By applying the same technique and flow visualization methods to a 1 :40th scale down geometric model of a suspension preheater kiln with auxiliary firing arrangement, the optimum operating conditions for such systems in case of secondary firing have been determined. It has been established that for the auxiliary firing systems in four-stage cyclone preheaters with combustion air being supplied through the kiln, the proportion of the fuel supplied at the back-end of the kiln in the riser duct should not exceed 30% of the total heat input, the optimum value being 27%. In designing auxiliary burners for such systems, it has also been found that the stream to jet velocity ratios (p) should be in the range 0.62 < p < 0.83 for burners 30° inclined to the horizontal, and 0.41 < p < 0.49 for the burners perpendicular to the stream flow in the riser duct for purpose of having an axisymmetric flame in the riser duct.