Pergamon Prog. Energy Combust. Sci. VoI. 20, pp. 487--618, 1994 Copyright OI995 Elsevier Sci©nc¢ Ltd Printedin GreatBritain.All rightsresave, d 0360--1285/94 $26.OO 0360-1285(94)0002ff4 INTERACTIVE PROCESSES IN GASIFICATION AND COMBUSTION-- PART III: COAL/CHAR PARTICLE ARRAYS, STREAMS AND CLOUDS KALYAN ANNAMALAI,* WILLIAM RYAN t and SENTH1LVELAN DHANAPALAN Department of Mechanical Engineering, Texas A & M University, College Station, TX 77843-3123, U.S.A. Received 30 August 1994 Alrstract--A comprehensive review is presented on the interactive transport processes in the gasification and combustion of a cloud of drops and solid particles. The review is divided into three parts. Part I is concerned with the interactive processes for arrays, streams and clouds of drops, Part II presents a review of isolated coal, carbon and porous char particles pertaining to interactive processes, and finally Part III deals with the interactive processes for solid particle arrays, streams and clouds. Isolated particle gasification (pyrolysis and heterogeneous) and combustion were briefly reviewed in Part II. Because of strong analogy of the group ignition and combustion, to porous char ignition and combustion, the literature on porous char combustion was also included in Part II and new results were presented on the internal ignition of porous char particle using Frank-Kamanetsky type of analysis. Part III presents an integrated approach starting from arrays to clouds and gasification to combustion. The interactions occur through processes ranging from pure diffusive to convective transport processes. Approximate criteria for interactive processes are given. As opposed to liquid drop arrays and clouds, there is no systematic study for arrays of char or coal particles. Due to the similarity between droplet evaporation and char combustion, new results are presented for the combustion of char arrays in quiescent atmosphere. Convective effects are also briefly discussed. Expanding the Frank-Kamenetsky analysis to ignition of clouds, simple solutions are presented for cloud ignition temperatures. A comparison of results between different techniques and between theory and experiment is given. Interesting results for the ignition characteristics of coal dispersions are obtained in that the particles with relatively small or low volatile matter which ignite heterogeneously when isolated are found to ignite homogeneously under cloud conditions. The minimum ignition temperature is found to increase with decrease in size under isolated mode while the opposite is true under interactions. The problems of the gasification, ignition, and combustion of clouds in confined and unconfined volumes are reviewed. Experiments conducted with streams (laminar flow reactors, LFR) and clouds (TGA, heated grids, shock tubes, batch of particles in LFR, Hertzberg Ignition apparatus) are reviewed. Following the drop literature, the relation between array and group combustion is presented. Finally, the relevance of the reviewed literature to pollutants' formation and destruction and spray combustion modeling is briefly discussed. CONTENTS Notation 1. Introduction and Objectives 2. Elementary Concepts on Interactive Processes 2.1.1. Double film model 2.1.2. Single film model (from gas phase) 2.2. Sublimation 2.3. Coal combustion 2.4. Coal pyrolysis 2.5. Modes and levels of interactions 2.6. Correction factor 2.6.1. Combustion 2.6.2. Pyrolysis 3. Gasification and Pyrolysis 3.1. Arrays 3.1.1. Two particles 3.1.2. Two or more particles 3.1.3. Point source method (PSM) 3.1.4. Coal 3.1.5. Relative velocity effects 3.1.6. Non-steady effects * Author to whom correspondence should be addressed. t Currently at Westinghouse Electric Corporation, Power Generation Business Unit, Orlando, FL, U.S.A. 488 490 492 493 494 495 495 495 496 496 496 497 497 497 497 500 502 502 502 507 487