Rotary Kilns
Transport Phenomena and
Transport Processes
Akwasi A. Boateng
B H
Butterworth-Heinemann is an imprint of Elsevier
AMSTERDAM • BOSTON • HEIDELBERG • LONDON
NEW YORK • OXFORD • PARIS • SAN DIEGO
SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO
2005
Contents
Foreword xv
Preface xvii
1 The Rotary Kiln Evolution and Phenomenon 1
1.1 The Rotary Kiln Evolution 1
1.1.1 Comparison of the Rotary Kiln with Other Contactors 4
1.2 Types of Rotary Kilns 7
1.2.1 Wet Kilns 9
1.2.2 Long Dry Kilns 9
1.2.3 Short Dry Kilns 10
1.2.4 Coolers and Dryers 10
1.2.5 Indirect Fired Kilns 12
2 Basic Description of Rotary Kiln Operation 15
2.1 Bed Phenomenon 17
2.2 Geometrical Features and Their Transport
Effects 18
2.3 Transverse Bed Motion 19
2.4 Experimental Observations of Transverse Flow Behavior 24
2.5 Axial Motion 26
2.6 Dimensionless Residence Time 28
3 Freeboard Aerodynamic Phenomena 33
3.1 Fluid Flow in Pipes: General Background 35
3.2 Basic Equations of Multicomponent Reacting Flows 40
3.3 Development of a Turbulent Jet 43
3.4 Confined Jets 46
3.5 Swirling Jets 48
3.6 Precessing Jets 49
3.7 The Particle-laden Jet 52
3.8 Dust Entrainment 53
3.9 Induced Draft Fan 56
4 Granular Flows in Rotary Kilns 59
4.1 Flow of Granular Materials (Granular Flows) 60
4.2 The Equations of Motion for Granular Flows 64
4.3 Particulate Flow Behavior in Rotary Kilns 67
4.4 Overview of the Observed Flow Behavior
in a Rotary Drum 68
4.4.1 Modeling the Granular Flow in the
Transverse Plane 72
4.5 Particulate Flow Model in Rotary Kilns 72
4.5.1 Model Description 73
4.5.2 Simplifying Assumptions 74
4.5.3 Governing Equations for Momentum Conservation 75
4.5.4 Integral Equation for Momentum Conservation 79
4.5.5 Solution of the Momentum Equation in the Active Layer of the Bed 83
4.5.6 Velocity Profile in the Active Layer 85
4.5.7 Density and Granular Temperature Profiles 87
4.5.8 An Analytical Expression for the Thickness of the Active Layer 88
4.5.9 Numerical Solution Scheme for the Momentum Equation 90
4.6 Model Results and Validation 92
4.7 Application of the Flow Model 94
5 Mixing and Segregation 101
5.1 Modeling of Particle Mixing and Segregation in Rotary Kilns 105
5.2 Bed Segregation Model 106
5.3 The Governing Equations for Segregation 110
5.4 Boundary Conditions 113
5.5 Solution of the Segregation Equation 114
5.5.1 Strongly Segregating System (Case I) 114
5.5.2 Radial Mixing (Case II) 115
5.5.3 Mixing and Segregation (Case III) 116
5.6 Numerical Solution of the Governing Equations 117
5.7 Validation of the Segregation Model 119
5.8 Application of Segregation Model 120
6 Combustion and Flame 129
6.1 Combustion 129
6.2 Mole and Mass Fractions 131
6.3 Combustion Chemistry 133
6.4 Practical Stoichiometry 135
6.5 Adiabatic Flame Temperature 136
6.6 Types of Fuels Used in Rotary Kilns 138
6.7 Coal Types, Ranking, and Analysis 139
6.8 Petroleum Coke Combustion 140
6.9 Scrap Tire Combustion 141
6.10 Pulverized Fuel (Coal/Coke) Firing in Kilns 143
6.11 Pulverized Fuel Delivery and Firing Systems 145
6.12 Estimation of Combustion Air Requirement 147
6.13 Reaction Kinetics of Carbon Particles 147
6.14 Fuel Oil Firing 149
6.15 Combustion Modeling 152
6.16 Flow Visualization Modeling
(Acid-Alkali Modeling) 154
6.17 Mathematical Modeling Including CFD 156
6.18 Gas-Phase Conservation Equations Used in CFD Modeling 158
6.19 Particle-Phase Conservation Equations Used in CFD Modeling 160
6.20 Emissions Modeling 161
6.20.1 Modeling of Nitric Oxide (NOx) 161
6.20.2 Modeling of Carbon Monoxide (CO) 163
6.21 CFD Evaluation of a Rotary Kiln Pulverized Fuel Burner 163
7 Freeboard Heat Transfer 173
7.1 Overview of Heat Transfer Mechanisms 174
7.2 Conduction Heat Transfer 175
7.3 Convection Heat Transfer 180
7.4 Conduction-Convection Problems 182
7.5 Shell Losses 184
7.6 Refractory Lining Materials 184
7.7 Heat Conduction in Rotary Kiln Wall 187
7.8 Radiation Heat Transfer 189
7.8.1 The Concept of Blackbody 190
7.9 Radiation Shape Factors 192
7.10 Radiation Exchange Between Multiple Gray Surfaces 194
7.11 Radiative Effect of Combustion Gases 195
7.12 Heat Transfer Coefficients for Radiation in the Freeboard of a Rotary Kiln 196
7.13 Radiative Exchange from the Freeboard Gas to Exposed Bed and Wall Surfaces 198
7.14 Radiative Heat Transfer among Exposed
Freeboard Surfaces 199
8 Heat Transfer Processes in the Rotary Kiln Bed 205
8.1 Heat Transfer Between the Covered Wall and the Bed 207
8.2 Modified Penetration Model for Rotary Kiln Wall-to-Bed Heat Transfer 208
8.3 Effective Thermal Conductivity of Packed Beds 211
8.4 Effective Thermal Conductivity in Rotating
Bed Mode 214
8.5 Thermal Modeling of Rotary Kiln Processes 215
8.6 Description of the Thermal Model 216
8.7 One-Dimensional Thermal Model for Bed and Freeboard 218
8.8 Two-Dimensional Thermal Model for the Bed 221
8.9 The Combined Axial and Cross Sectional
Model—The Quasi-Three-Dimensional Model for the Bed 223
8.10 Solution Procedure 224
8.11 Model Results and Application 228
8.12 Single Particle Heat Transfer Modeling for Expanded Shale Processing 232
9 Mass and Energy Balance 239
9.1 Chemical Thermodynamics 239
9.2 Gibbs Free Energy and Entropy 240
9.3 Global Heat and Material Balance 243
9.4 Thermal Module for Chemically Reactive System 244
9.5 Mass Balance Inputs 245
9.6 Chemical Compositions 246
9.7 Energy Balance Inputs 246
9.8 Site Survey—Measured Variables 247
9.9 Shell Heat Loss Calculations 251
9.10 Calcination Module Calculation 251
9.11 Combustion 251
9.12 Energy Balance Module 258
9.13 Sensible Energy for Output Streams 259
10 Rotary Kiln Minerals Process Applications 265
10.1 Lime Making 265
10.2 Limestone Dissociation (Calcination) 266
10.3 The Rotary Lime Kiln 272
10.4 The Cement Making Process 275
10.5 The Cement Process Chemistry 275
10.5.1 Decomposition Zone 276
10.5.2 Transition Zone 277
10.5.3 Sintering Zone 277
10.6 Rotary Cement Kiln Energy Usage 278
10.7 Mineral Ore Reduction Processes in Rotary Kilns 280
10.7.1 The Rotary Kiln SL/RN Process 280
10.7.2 Roasting of Titaniferous Materials 282
10.8 The Rotary Kiln Lightweight Aggregate Making Process 285
10.8.1 LWA Raw Material Characterization 288
10.8.2 LWA Feedstock Mineralogy 289
10.8.3 LWA Thermal History 290
Appendix 297
Index 339
Preface
The author was born in Ghana in a little village called Kentikrono, now a suburb of Kumasi, the Ashanti capital. After a brief stint as an engineering cadet in the merchant marine corps of Ghana, he accepted a scholarship to study marine engineering in the former Soviet Union. Changing fields, he pursued studies in mechanical engineering in Moscow specializing in thermodynamics and heat engines where he redesigned a turbo-prop engine for hydrofoil application. He pursued graduate school in Canada, thereafter, and completed a MS in thermo-fluids mechanics at the University of New Brunswick with a thesis under J. E. S. Venart on energy conservation in greenhouses using thermal night curtains to prevent low-temperature nighttime infrared radiation heat losses through polyethylene roofs. He accepted a faculty position at the University of Guyana in South America, where his wife hails from, and taught undergraduate thermodynamics and heat transfer. His research in fluidized bed thermochemical conversion of rice hulls to provide bioenergy and utilization of the rice hull ash for cement applications earned him a faculty Fulbright award to the United States where he spent a year working in Dr. L. T. Fan’s laboratory at the department of chemical engineering at Kansas State University. Upon an invitation to Canada, he joined Dr. Brimacombe’s group at the Center for Metallurgical Process Engineering at the University of British Columbia, where he pursued a Ph.D. in rotary kilns under the sponsorship of Alcan Canada. His dissertation, Rotary Kiln Transport Phenomena—Study of the Bed Motion and Heat Transfer, supervised by P.V. Barr, presented some pioneering works on the application of granular flow theories for the modeling of particle velocity distribution in mineral processing kilns from which heat transfer within the kiln bed could be adequately and sufficiently solved.
After a brief stint as an Assistant Professor at Swarthmore College in Pennsylvania, he joined Solite Corporation, a rotary kiln lightweight aggregatemanufacturingcompanyinVirginiafoundedbyJaneandJohn Roberts (Swarthmore ’39) as a research and production engineer. At Solite, he developed a two-part training manual on rotary kiln transport phenomena for project 10-10-10, an operational campaign promoted by John Roberts to increase production and product quality by 10% and also reduce fuel consumption by 10%. After Solite restructured in 1997, the author returned to Pennsylvania and joined Fuel and Combustion Technology (FCT) founded by colleagues from the UK, Peter Mullinger and Barrie Jenkins, who, having also completed their Ph.D. works on kiln combustion had developed methods of optimizing turbulent diffusion flame burners to match cement and lime kiln processes. When FCT’s owner, Adelaide Brighton Cement, was acquired by Blue Circle Cement, the author joined the process group of Fuller-FL Schmidt (FFE) Minerals, now FLS Minerals in Bethlehem, PA, where he participated in works leading to the design of several large direct-fired mineral processing kilns including limestone calcination, vanadium extraction, soda ash production, and so on. He later worked for Harper International in Lancaster, NY, a lead provider of indirectly heated, high-temperature, rotary kilns employed for niche applications including inorganic materials. After Harper, he became a consultant to the industry providing process expertise including training to the rotary kiln community where he was dubbed “the kiln doctor.” He is now a senior Research Scientist with the Agricultural Research Service of the USDA pursuing research in biofuels and bioenergy.
Rotary Kilns: Transport Phenomena and Transport Processes, is a culmination of the author’s work in rotary kilns in both academic research and in industry. It captures the author’s experiences in production, process design, commissioning, and more importantly, attempts to bridge the classroom and the rotary kiln industry. The focus of Rotary Kilns: Transport Phenomena and Transport Processes is to provide the process engineer and the researcher in this field of work some of the quantitative descriptions of the rotary kiln transport phenomenon including freeboard and bed process interactions. The latter combines the transverse bed motion and segregation of granular materials and the resultant effect of these phenomena on the bed heat transfer. Although other bed phenomena, such as axial segregation (sequential banding of small and large particles along the kiln length) and accretion (deposition or growth of material onto the refractory wall forming unwanted dams) are also not well understood, these are only qualitatively described. However, these phenomena can be better explained after careful elucidation of the transverse bed motion, segregation, and heat transfer. The work has been divided into sequential topics beginning with the basic description of the rotary kiln operation followed by fluid flow in rotary kilns where the freeboard phenomenon is presented. Here the similarities of fluid flow in conduits are drawn to describe the characteristics of confined flows that manifest themselves in combustion and flames typical of the rotary kiln environment.
In Chapter 4 the granular flow phenomenon in rotary kilns is presented. In rotary kilns, often the material being processed is composed of granules, hence the underlying theories for such flows are important to the bed motion, gas-solids reactions, and solid-solid reactions that take place in the bed. With the knowledge of these flows, it is only prudent to cover mixing and segregation as they develop in rotary kilns. This is accomplished in Chapter 5. The severity of mixing phenomena impacts greatly on the quality of the product since it influences the thermal treatment of any granular material. Mixing and segregation determines the extent to which the rotary kiln can be classified as a continuous stirred reactor. The flame is the heart of direct-fired kilns, thus combustion and flame is treated in Chapter 6. The types of flames developed in rotary kilns depend on the flow distribution in the freeboard, which, in turn, determines the heat fluxes to the charged material and also emissions. Treatment of heat transfer in freeboard is therefore a logical sequence and this follows in Chapter 7 by a review of the fundamentals of process heat transfer. Many mathematical models have previously been applied to describe freeboard heat transfer in rotary kilns including one-dimensional zone models, and two- and three-dimensional computational fluid dynamics (CFD). Some of these are presented including recent developments. Freeboard treatment is followed by bed heat transfer in Chapter 8. Like fixed bed heat transfer, rotary kiln bed heat transfer is composed of particleto- particle conduction, convection, and radiation. However, superimposed on this phenomenon is an advective transport component that is generated due to granular flow that sets apart rotary kiln heat transfer from packed bed heat transfer. Some existing packed bed models and their extension to rotary kilns are presented here. Following the bed heat transfer, the mass and energy balance is established in Chapter 9 by considering the kiln operation as a thermodynamic system that interacts with the atmosphere. A simple mass and energy balance is presented for a lime kiln. Having established all the above, it is only prudent to present some specific mineral processing applications for which the rotary kiln has been the main workhorse in Chapter 10.
Some of the processes discussed include lime making, cement making, carbothermic reduction kilns, and lightweight aggregate kilns.
The author is indebted to the many students both in the colleges he has taught and in industry where he has lectured. He is grateful to Solite Corporation, which gave him an unprecedented opportunity to test his theories and mathematical models on large rotary kiln processes in the early years. He is also indebted to FCT, FLS Minerals, Harper, and all the many members of the family of rotary kiln operators particularly Utelite Corporation, Graymont, Inc., and others who gave him an unparalleled education beyond the classroom. Finally, the author is indebted to Dr. Gus Nathan, Dr. Phillip Shaw, and Dr. Peter Cooke for the critical feedback they provided on the manuscript for this book.
A. A. Boateng, Ph.D.
Royersford, PA
aboaten1@gmail.com
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