Regenerative heat transfer in rotary kilns
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I.
INTRODUCTION
IN a rotary kiln, heat is transferred to the solids burden by two paths, across the exposed upper surface and the covered lower surface of the bed, as shown in Figure 1. The mechanisms of heat flow to the two surfaces are very different. At the upper surface the solids receive heat directly by radiation and convection from the freeboard gas whereas at the lower surface, heat flows by a combination of radiation and conduction from wall to solids. The latter heat-transfer path is part of the regenerative cycle of the kiln wall which, as it rotates through the freeboard, also receives thermal energy via radiation and convection from the hot combustion gases; of course, a portion of this heat is lost to the surroundings, as depicted in Figure 1. Radiative and convective heat flow to the upper surface of the bed have been investigated recently. 1,2.3The present work was initiated to examine regenerative heat transfer in a rotary kiln, therefore, as a complement to these studies. The regenerative action of the kiln wall has been investigated previously but some basic questions remain concerning its importance relative to the other heat-transfer steps, the effect of different kiln variables on regenerative as well as overall heat flow to the bed, and the possibility of employing a simplified model to predict the inside wall temperature, heat loss through the refractory wall, and overall heat transfer to the solids. In addition, as will be seen in the next section, there are questions regarding the adequacy of earlier calculations as a result of the paucity of data for convective heat-transfer coefficients and simplifications made in the mathematical formulation of radiation in the freeboard. The approach taken in this work is theoretical. The study may be divided into four sections: (a) The development of a mathematical model to predict the temperature field in a kiln wall; (b) The determination of heat-transfer coefficients for use in the model; (c) Application of the model to predict the regenerative J.P. GOROG and J.K. BRIMACOMBE are with the Department of Metallurgical Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1W5. T.N. ADAMS is with the Recovery Technology Section, Weyerhaeuser Technology Center, Tacoma, WA 98422. Manuscript submitted June 8, 1981. METALLURGICALTRANSACTIONS B
Gas
Fig. 1 - - Schematic diagram of rotary kiln showing major heat flow paths to the burden.
action of the wall and overall heat transfer to the solids as a function of kiln variables; (d) Development of a simplified model to predict average inside wall temperature, heat transfer to the solids, and heat loss through the wall. The calculations do not include the effect of a flame in the freeboard; this subject will be addressed in the next paper in this series.
II.
PREVIOUS WORK
The earliest attempts to predict the regenerative action of the kiln wall were relatively crude. Heilegenstaedt4 calculated the circumferential temperature in the wall by assuming it to be a slab of infinite thermal con
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