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Show kiln, thus achieve both a reduction in energy consumption and emissions simultaneously. FCT was retained to advise on the aerodynamics of the new cooler and kiln hood, the effect of the changes on the performance of the main kiln burner, the effect on the existing electrostatic precipitator, as well as design of the flash calciner. All these tasks involved modelling, but it is only the design of the flash calciner which is described here. 3.3.1 Description of the Flash Calciner A flash calciner is a relatively simple system and consists essentially of a refractory lined vessel, vertically mounted, figure 19. Air which is normally preheated to some l300-2000 oF enters through an inlet at the base. Fuel is supplied via burners arranged around the wall near the base. The position of the burners varies with different designs and may be in the inlet duct, the conical section or the main body. Finally, the feed material to be calcined enters as a powder through the wall of the vessel, usually through a single inlet, and the product leaves with the exhaust gases through the top. Flash calciners are different from most furnaces in that the combustion and heat transfer takes place under near isothermal conditions, with the heat liberated by combustion being virtually equal to the heat absorbed by calcination, and little or no temperature rise occurring. Flash calciners from different suppliers vary considerably in detail design, for example one supplier has a cyclonic design, but most have the essential features described above. 3.3.2 Common Problems EXperienced with Flash Calciners Combustion in a flash calciner takes place under extremely adverse conditions owing to the high dust loading and the endothermic calcining reaction. In fact, the materials commonly calcined, limestone and alumina, are commonly used as fire extinguishing agents and explosion suppresants! Flash calciners are usually fired by oil, pulverized coal or natural gas and, as for most industrial systems, the fuel/air mixing largely controls the combustion. Natural gas gives special problems because of its low density and high ignition tempera ture. The former causes problems with stratification and can result in unburnt gas leaving with the exhaust gases whilst the latter can result in flame out in the event of a relatively small temperature drop. The distribution of the feed powder within the flash calciner is clearly important to ensure that all the material is processed, but it is very difficult or virtually impossible to make anyon-line measurements. Furthermore, it has been very difficult to model this dense, two phase flow system. As a result of these unknowns, flash calciners have been designed empirically and many problems have been experienced including:- 1. Incomplete combustion resulting in unburnt products exhausted to atmosphere and (occasionally) explosions in downstream plant, such as electrostatic precipitators. 2. Overheating of the product causing sintering and blockages. |