Abstract
The semi-dry Lepol Process of cement manufacture consists of drying and calcining a shale and limestone nodule mix on a travelling "Lepol" grate. The resulting semi-calcined mixture then flows down a shute into a shortened version of a conventional rotary kiln. A better understanding of the fluid flow and heat transfer taking place around the nodules on such a Lepol Grate Calciner will unquestionably lead to the more efficient operation and design of such installations. The work carried out for this thesis involved first establishing the fundamental aerodynamics of a Lepol Grate -Rotary Kiln system and then using this knowledge to develop a realistic mathematical model to predict the bed temperature along the calciner section of the furnace; neither of these two experiments having been attempted before. The aerodynamics of the system was established using a 1:24 scale, perspex, water model with the flow suitably adjusted to represent plant flow conditions and obtaining typical flow diagrams to present the results. The salient features observed in these studies, using polystyrene beads as tracers, were:- (a) The flow patterns obtained in the Lepol grate model were essentially independent of both the Reynolds numbers range and the bed voidages investigated. (b) The initial assumption of plug flow, through the drier and calciner was erroneous. Indeed, for the calcining section, the flow was found to be restricted to 4/5ths of the grate area, due to the geometrical constraints on the system. (c) The fluid flowing above the calcining section formed a double vortex. (d) The fluid issuing into the drier chamber behaved as a confined jet. A numerical mathematical model to describe the processes occurring on the Lepol grate calciner at steady state was derived, and tested against experimental results of Weber (1). This analysis involved dividing the bed of the calciner into 4 equal strata and then subdividing each stratum into increments. This model yielded the following equation for the top stratum of the bed: T1= a - y + s + E -n + m. c. To m. c. where To = Temperature at beginning of increment. T1= Temperature at end of increment, m = Mass flowrate of solids, c = Specific he at of solids, a = Heat transfer by convection, y = Energy used for calcining. s = Energy radiated from the furnace walls. E = Energy radiated from the furnace gases, p = Energy re-radiated from the increment. For the other 3 strata, T1 = a + m. c. To - y + V/m. c. where V= Energy transferred by radiation between the increment being considered and the increment above. In this way, the heating curves of the bed for the four strata at steady state were obtained using an iterative procedure and compared with the results of Weber. Several correlations to characterise the heat transfer from the gas to the bed of particles in the calciner were tested and finally that of Lof and Hawley (2) was found to give the best fit to Weber's results.