Coal pyrolysis in a rotary kiln: Part II. Overall model of the furnace
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. INTRODUCTION
ROTARY kilns are widely employed in the cement, metallurgical, and chemical industries due to their simplicity of use and their ability to continuously treat granular or powder solids with excellent heat and matter transfer between the solid and gas phases. The present article considers the use of the rotary kiln for the pyrolysis of coal to produce high-reactivity coke for electrometallurgical applications. The coal is introduced at one end of the furnace in the form of 1- to 2-cm-diameter grains and flows parallel to the kiln axis, which is slightly inclined to the horizontal, due to the rotation of the tubular body. The coke produced is discharged at the other end. A counterflow of air is introduced at the coke end and becomes laden with volatile species evolved by the pyrolysis, while at the same time being deplenished of oxygen due to the various combustion reactions, which provide the energy necessary for the process. The goal of the present article is to describe a complete mathematical model of the kiln intended for process optimization. The model describes all the physicochemical and thermal phenomena of importance for the process, including drying of the coal, the removal of volatiles, cracking of tars, the combustion of the volatile species, the partial combustion and extinction of the coke, the solid and gas flows, and heat transfer between the charge, the kiln walls, and the gas. Although several rotary kiln simulation models have been FABRICE PATISSON, Research Scientist, is with the Centre National ´ de la Recherche Scientifique (CNRS), ´ Laboratoire de Science et Genie des ´ ´ Materiaux Me talliques (LSG2M), E cole des Mines, 54042 Nancy, Cedex, ´ France. ETIENNE LEBAS, Research Scientist, formerly with LSG2M, ´ Ecole des Mines, is with the Institut Franc¸ais du Petrole, 69390 Vernaison, France. FRANC¸OIS HANROT, Research Scientist, formerly with the Centre de Pyrolyse de Marienau, is with IRSID, Usinor Research Center, 57283 ` ` Maizieres-le ABLITZER, Professor, is with ´ s-Metz, Cedex, France. DENIS ´ LSG2M, Ecole des Mines. JEAN-LEON HOUZELOT, Professor, is with ´ ´ the Laboratoire des Sciences du Genie Chimique, Ecole Nationale ´ Superieure des Industries Chimiques, 54001 Nancy, Cedex, France. Manuscript submitted January 18, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
published in the literature,[1–5] none applies specifically to the pyrolysis of coal. Another point which distinguishes the present study from previous work is that the description of the conversion of an individual coal grain to coke led us to develop a specific kinetic and thermal model, which has been termed the grain model. The latter has been described in detail in Part I of this article[6] and is incorporated in the overall model of the kiln presented in this Part II of the article. After considering the movement of the charge, heattransfer, and combustion phenomena, the article goes on to describe the model itself and the results obtained. II. MOVEMENT OF THE CHARGE In a rotary coal pyrolysis kiln, the c
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