elsevier industrial and process furnaces principles design and operation apr 2008

Figure 1.1 The iron bridge at Coalbrookdale showing the detail adjacent Figure 1.4 Cross-section through a traditional downdraft pottery Figure 1.5 Schematic of a mixed feed vertical sha

Furnaces

Principles, Design and Operation

Peter Mullinger

Associate Professor, School of Chemical Engineering

University of Adelaide, South Australia

Barrie Jenkins

Consulting Engineer, High Wycombe, Bucks, UK

AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK • OXFORD PARIS • SAN DIEGO • SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO

Butterworth-Heinemann is an imprint of Elsevier

30 Corporate Drive, Suite 400, Burlington, MA 01803, USA

First edition 2008

Copyright © 2008 Elsevier Ltd All rights reserved

No part of this publication may be reproduced, stored in a retrieval system or transmitted

in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher

Permissions may be sought directly from Elsevier ’ s Science & Technology Rights

email: permissions@elsevier.com Alternatively you can submit your request online by visiting the Elsevier web site at http://elsevier.com/locate/permissions , and selecting

Obtaining permission to use Elsevier material

Notice

No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein

British Library Cataloguing in Publication Data

A catalogue record for this book is available from the British Library

Library of Congress Cataloguing in Publication Data

A catalogue record for this book is available from the Library of Congress

ISBN: 978-0-7506-8692-1

For information on all Butterworth-Heinemann publications

visit our web site at http://books.elsevier.com

Typeset by Charon Tec Ltd (A Macmillan Company), Chennai, India

www.charontec.com

Printed and bound in Hungary

08 09 10 11 12 10 9 8 7 6 5 4 3 2 1

directly heated by the flame.

Foreword xvii

1.1.3 Principle objectives of furnace designers and operators 5

1.2 Where are furnaces used? Brief review of current furnace applications

References 29

2.3.3 Reaction rate behaviour 40

Nomenclature 86

References 87

Chapter 4 An introduction to heat transfer in furnaces 89

4.2.3 Evaluating convective heat transfer coefficients 104

Atomisation of liquid fuels and pulverisation of coal 146

5.2.1 The essential importance of heat flux profiles 154

Effect of excess air (mixture ratio) on flame temperature 160

5.3.2 Turbulent jet diffusion burners 165

Nomenclature 204References 205

6.3 Application of modelling to furnace design 238Nomenclature 239References 241

7.4.4 Environmental benefits and health hazards of

Nomenclature 284References 284

Chapter 8 Furnace control and safety 287

8.3.1 Extractive gas sampling systems and analysers 302

Corrective action for unintentional sub-stoichiometric operation 318

8.6.1 Safety requirements for burner management systems 320

8.6.3 Achieving acceptable safety standards with programmable

8.6.4 Choosing an appropriate safety integrity level 3248.6.5 Determining the safety integrity level of the BMS system 326

Nomenclature 332References 332

9.2.1 On-site measurement 342

Nomenclature 372References 372

11.1 Basic performance requirements of the furnace structure 414

Traditional installation of castable refractory 420

11.3 Practical engineering considerations in the use of refractories 431

11.5.1 Effect of elevated temperature on metal properties 439

11.6 Practical engineering considerations in the use of high

Appendix 11A General properties of selected refractory materials 447

12.2.4 Reliability of available process knowledge 466

12.2.5 Effect of upstream and downstream processes 468

12.2.7 Potential for heat recovery and choice of equipment 474

Estimating the potential for heat recovery from hot

Estimating the potential for heat recovery from hot

Estimating the potential for heat recovery from shell

12.5 Detailed analysis and validation of the furnace design 500

Nomenclature 503 References 504

Author index 507

Subject index 511

Furnaces have been used by humans for thousands of years and yet, beyond the basic chemical reactions and heat release calculations, engineers rarely have any formal training in relation to furnace design, combustion and their integration into industrial processes It is therefore not surprising that the solution to issues of emis-sions, throughput and performance related problems have relied heavily on trial and error and experience Within industry in general equipment would be more success-ful designed using the principals outlined in this book rather than relying on correla-tions and scale up factors that have little, or no scientific basis to support them

In the early 1970s the authors set themselves the goal of applying more tific methods to burner design than were currently used This led to the realisation that heat release from flames needed to be closely matched to the process require-ments and that this was intimately related to the design of the furnace itself Now, more than ever before, the need to reduce ongoing energy costs and greenhouse gas emissions requires decisions to be made on the basis of knowledge rather than guesswork and past experience This book, being one of only a few ever published

scien-on the subject, highlights the applicable science which can be used to take much of the guesswork out of furnace design This book also emphasises the importance of ensuring that individual pieces of equipment are appropriate for the whole process and not simply selected on the basis of capacity or lowest capital cost

Alcoa ’ s alumina refineries operate a range of processes and equipment ing boilers, rotary kilns, gas suspension alumina calciners and regenerative ther-mal oxidisers and, like many other industries, have needed to address emissions, throughput, performance and safety issues without a clear understanding of the sci-ence and underlying design basis This makes it difficult to undertake reliable root cause analysis when problems occur

In the early 1980s I was a mechanical engineer in Alcoa ’ s Equipment Development Group I had insufficient knowledge of, and certainly little experience with, combustion processes and was faced with having to address throughput and emissions related issues with alumina calciners Fortunately I met the authors of this book, Peter Mullinger and Barrie Jenkins, and was delighted to discover that a scientific approach to furnace design is possible and methods are available to inves-tigate and optimise many aspects of the combustion and associated processes

It was highlighted through physical modelling of the flow patterns and acid alkali modelling of the combustion process mixing that both the throughput and emissions could be significantly improved by simply relocating fuel injection points These modifications proved to be effective and have now been employed on all applicable alumina calciners at Alcoa ’ s refineries around the world saving other-wise significant capital expenditure with potentially ineffective outcomes

Since those early days the science as described in this book has been employed over a wide range of process issues, from the design of new equipment and in the solution of problems with existing equipment, positively impacting on performance and reliability More recently there has been a major application in relation to the design of safety systems

In particular the application of CFD modelling has highlighted to me that CFD doesn ’ t replace the need for a deep understanding of the science of combustion and furnace heat transfer processes as there are many traps for the unwary and the uninformed

Whether you are engaged in modelling, design of original equipment or ment upgrades or operation of combustion and furnace heat transfer processes, this book provides much of the essential understanding required for success

Greg Mills Senior Consultant – Calcination Technology Delivery Group Alcoa World Alumina

This book has been more than 20 years in gestation; its lineage can be traced back

to Barrie ’ s lecturing at the University of Surrey in the late 1970s and early 1980s and Peter ’ s first combustion course, provided internally to Rugby Cement ’ s engi-neers in 1981

We are not attempting to explain how to design any particular furnace but are advocating a more scientific approach to furnace design than the traditional meth-ods of scaling from the last design New approaches are essential if we are to make the advances needed to develop new processes for the twenty-first century and to significantly reduce industrial energy consumption and emissions

We have worked together to improve the efficiency of furnaces since 1977, ing with rotary kilns in the cement industry when Roger Gates, Technical Director

start-of Rugby Cement, allowed Peter Mullinger to try the techniques developed by the Fuels and Energy Research Group at the University of Surrey (FERGUS) on Rugby ’ s South Ferriby No 3 Kiln This work was strongly encouraged by the plant manager, the late Jim Bowman The project was an immediate success and led to significantly increased production and reduced fuel consumption The success of that project encouraged Rugby to sign a research agreement with the late Frank Moles, founder

of FERGUS, which committed Barrie Jenkins and the rest of the FERGUS team to support Rugby Cement ’ s efforts to improve the production capacity, product quality and fuel economy of their 21 kilns

Following time in senior technical management roles with a company supplying combustion equipment to the petrochemical industry, we founded our own business where we applied more scientific methods to combustion and heat transfer problems

in all industries but principally those where the product was directly heated by the flame

We commercialised techniques that had been successfully developed and used in-house by organisations such as British Gas, CEGB, and British Steel ’ s Swinden Laboratories, Rotherham We added acid/alkali modelling as a means of determin-ing fuel/air mixing and flame shape, a technique that had seen little application outside of research institutions at that time

We built a successful business on this philosophy that continues today, managed

by our successors During the time we managed it we applied these techniques to over 250 projects in a wide range of furnace types in the alumina, cement, ceramic, chrome, copper, lead, lime, steel, mineral sands, nickel, petrochemical, pulp and paper and even the nuclear industry

The idea for this book arose from the short course we provided on behalf of the International Kiln Association to industry and to the Portland Cement Association where we were regularly asked to recommend a book We would have suggested

Professor Thring ’ s book The Science of Flames and Furnaces but it was long out of

print so all that we could offer were the course notes We hope that this book goes some way to filling the gap It is the culmination of 30 years of working together, albeit for the last few years from across the globe

Peter Mullinger Barrie Jenkins

Over the years it has been our privilege to work with many engineers, process operators and other people who have encouraged and cooperated with us Those who are mentioned below are but a small selection, whose influence has strongly encouraged the preparation of this book or who have directly contributed to it

Of special influence was the late Frank Moles, founder of the Fuels and Energy Research Group at the University of Surrey (FERGUS), who changed our think-ing about industrial combustion, and Roger Gates, Technical Director of Rugby Cement, who allowed us to implement Frank ’ s and our ideas on the company ’ s plants

We would like to thank many of the engineers at the former Midlands Research Station of British Gas, especially Neil Fricker, Malcolm Hogarth, Mike Page, Rachel Palmer, Jeff Rhine and Bob Tucker all of whom encouraged us to found Fuel and Combustion Technology Ltd (FCT) in 1984 and to apply modelling techniques to industrial combustion and heat transfer problems We believe that we were the first

to use these techniques commercially on a large scale

We owe a special debt of gratitude to those who were brave enough to give us our early work at FCT, including Len May, Terry Henshaw and John Salisbury

of ARC Ltd, Erik Morgensen and Lars Christiansen of Haldor Topsoe A/S, Greg Mills of Alcoa Australia, Ian Flower and Con Manias of Adelaide Brighton Cement, Philip Alsop of PT Semen Cibinong, Terry Adams and Peter Gorog of the Weyerhaeuser Company, all of whom were very influential in providing FCT with its early projects

Peter Mullinger would also particularly like to thank Emeritus Professor Sam Luxton, who strongly supported his change of direction to an academic career in

1999 Without that change, it is unlikely that time would have ever been available

to complete this task Peter would also like to thank his colleagues at the University

of Adelaide who either contributed directly to the book or who covered his ing duties during the first half of 2005 and first half of 2007, when the majority of this book was written, in particular Prof Keith King, Dr Peter Ashman, Prof Gus Nathan and Dr Yung Ngothai, A.Prof Dzuy Nguyen and A.Prof Brian O ’ Neill

We should also like to thank those commercial companies who provided data, photographs and drawings (who are acknowledged in the captions) but special thanks are due to Adam Langman, who tuned our woeful sketches into artistic mas-terpieces and Dave Crawley of DCDesign Services, who produced the process and instrument drawings and flow diagrams Grateful thanks are also due to Dr Christine Bertrand, Mr Dennis Butcher and Dr John Smart for their invaluable contribution to the sections on ‘ CFD modelling ’ , ‘ Furnace control and safety ’ and ‘ NOx formation and control ’ respectively

We hope that the errors are minimal, but there would be many more if it were not for the excellent proofreading of Victoria Jenkins and Sheila Kelly, to whom very special thanks are due We could not have managed without you Sheila, in particular, has read every word but maintains that it is not as much fun as Harry Potter!

Finally to all those who attended our industrial combustion short courses and asked, ‘ What book is available? ’ It is available at last; we hope that you won ’ t be disappointed

Figure 1.1 The iron bridge at Coalbrookdale showing the detail adjacent

Figure 1.4 Cross-section through a traditional downdraft pottery

Figure 1.5 Schematic of a mixed feed vertical shaft lime kiln and a

battery of six oil fired vertical shaft lime kilns 9

Figure 1.7 Modern cement kiln technology showing the cyclone preheater

tower on the left and the satellite cooler on the right 11

Figure 1.9 Schematic of a modern blast furnace with Abraham Darby’s

Figure 1.10 A reverbatory furnace for smelting copper sulphide ores 15

Figure 1.13 Cross-section through a copper anode furnace showing the

Figure 1.14 A hydrogen atmosphere furnace for de-sulphurisation of

Figure 1.16 Flash furnace for alumina, lime or cement raw material

Figure 1.18 Continuous rapid heating furnace for small billets 22

Figure 1.20 Two large gearbox cases entering a large annealing furnace 23Figure 1.21 A small incinerator designed by the authors to recover energy

Figure 1.22 Reducing kiln used in the mineral sands industry 25

Figure 1.24 Two types of refinery heater showing a cylindrical

Figure 1.25 Heat transfer coils for refinery heaters showing a coil

Figure 2.1 Effect of temperature on reaction rate in the extended

Figure 2.2 Consequence of extended Arrhenius equation on temporal

consumption of species A 40

Figure 2.3 Extracted reactions from the ‘chemical soup’ of fossil fuel

Figure 2.4 Relationship between reactedness and mass consumption of

Figure 2.5 Dependence of fuel reaction rate on the reactedness of

Figure 2.6 Equilibrium rate values for combustion reactions 44

Figure 2.8 Entrainment of secondary fluid into a free jet 48Figure 2.9 Entrainment of fluid into a high momentum confined jet

Figure 2.10 Entrainment of fluid into a swirling free jet with internal

Figure 2.11 Typical aerodynamics in a rotary kiln associated with a grate

cooler, obtained by water-bead and air modelling 60Figure 2.12 Typical aerodynamics in a flash calciner – obtained using

Figure 2.13 The effect of excess air on heat consumption (i.e fuel

Figure 2.14 The effect of excess air on flue gas heat losses 63

Figure 3.2 Viscosity temperature relationship for petroleum-based fuels 77Figure 3.3 Effect of carbon/hydrogen ratio on flame emissivity 82Figure 3.4 Effect of fuel type on heat transfer in a rotary kiln 82Figure 4.1 Representation of section through compound refractory

Figure 4.2 Representation of ingot shape and reheating furnace firing

Figure 4.4 Ingot heating predictions using Gurnie-Lurie chart 95

Figure 4.7 Representation of slab showing slice details with surface

Figure 4.8 Development of boundary layer over a flat plate 100

Figure 4.11 Diagrammatic representation variation of E with  for

Figure 4.12 Evaluation chart for approximate flame gas emissivity

Figure 4.15 Example of crossed string evaluation of exchange area

Figure 4.20 Mean beam length values for two geometric systems 128

Figure 5.1 Premixed and diffusion flames produced by a Bunsen burner 143Figure 5.2 Schematic diagram of the stability limits of a premixed flame 144Figure 5.3 Carbon monoxide concentrations in an 8 MW rotary

Figure 5.4 Effect of final droplet size on the additional surface area

Figure 5.5 Mechanism of spray formation from a typical atomiser 149Figure 5.6 Rosin-Rammler distribution for four different oil atomisers 151Figure 5.7 Optimum combustion intensities for selected rotary kiln

Figure 5.11 Flame stabilisation using flow deviation caused by bluff body 157Figure 5.12 Typical nozzle premix as applied to a tunnel burner

Figure 5.13 Effect of excess air on adiabatic flame temperature for natural

Figure 5.14 Partial premix radiant wall Walrad™ burner 161Figure 5.15 Principle of flame arrestor fuel gas nozzle 163

Figure 5.19 The main components of a typical furnace oil burner – water

Figure 5.22 Illustration of flame stabilisation by internal recirculation

zone caused by air entrainment into a hollow cone spray 171Figure 5.23 Classification of oil burners by atomiser type 172Figure 5.24 Schematic of simple and spill return pressure jet atomisers 173Figure 5.25 Calibration curve for wide turndown spill return pressure

Figure 5.26 Schematic and ‘exploded view’ of duplex wide

Figure 5.27 A low pressure air atomising burner 176Figure 5.28 Examples of medium and twin fluid atomisers 177Figure 5.29 Typical calibration curve for a high pressure twin fluid

Figure 5.34 Typical combustion air duct supplying multiple burners 188Figure 5.35 Poor combustion air distribution caused by inadequate

Figure 5.40 Typical register oil burner – note the radial air flow

Figure 5.41 Schematic and graphical representation of a sound wave 197Figure 5.42 The Clyde Refinery flares in normal mode and

Figure 5.43 Schematic of gas supply pipework for Clyde Refinery flares 203Figure 6.1 Physical modelling facility set-up for water modelling 211Figure 6.2 Flow visualisation in an arched furnace roof using water-bead

Figure 6.8 Construction of a one-dimensional cylindrical furnace model

Figure 6.9 Cement kiln flame temperature calculation using 3D zone

Figure 6.11 Comparison of measured and predicted flame temperatures 232Figure 6.12 3D, 1D and well-stirred furnace model predictions of wall

Figure 6.13 Coordinate grid systems used in CFD modelling 236Figure 6.14 Modelling inter-relationship for engineering design 239

Figure 7.1 Typical gas valve safety shutoff system – double block and

Figure 7.2 Oil system typical ring main with multiple furnace off-takes

Figure 7.3 Typical pumping and heating unit with shell and tube oil

Figure 7.8 Ball mill for grinding coal with integral drying chamber 256Figure 7.9 Two types of vertical spindle mill for grinding coal 258Figure 7.10 Sardon Saxifrage Mill showing a sectional view 259

Figure 7.12 Schematic of typical direct firing system applied to a

Figure 7.13 Schematic of typical indirect firing system 265

Figure 7.15 Coal mill inlet temperatures required to dry coal from

various raw coal moistures to a fine coal moisture of 2%

for various mill airflows showing the effect of false air 269

Figure 7.17 Schematic of a typical fine coal storage bin showing the

Figure 7.18 Examples of volumetric fine coal feeders showing screw

Figure 7.20 Schematic of a loss in weight fine coal feeder system 277

Figure 7.22 CERL riffle box for pulverised coal distribution 280Figure 8.1 Sources of error in temperature measurement using

Figure 8.4 Principle of remote temperature measurement using

Figure 8.5 Principle of ultrasonic temperature and flow measurement 295

Figure 8.7 Schematic of a conventional flue gas sampling system 303Figure 8.8 Typical flue gas sampling probe installation 304Figure 8.9 Water-cooled hot gas portable sampling probe designed by

Figure 8.12 Schematic of cross-duct optical obscuration dust monitor

Figure 8.13 A high temperature (700–1700°C) zirconia analyser for flue

gas oxygen measurement showing a cross-section of the cell 309Figure 8.14 Procal folded beam in-situ flue gas analyser 310Figure 8.15 An Opsis cross-duct multi-component optical gas analyser 311

Figure 8.17 Typical furnace start sequence – actual configuration will vary

with furnace application and compliance requirements 315Figure 8.18 Principle of fail safe relay logic employed in BMS systems 321

Figure 8.20 Example of two flow transmitters (4–20 mA) used to prove

Figure 8.21 Logic associated with three flow transmitters (4–20 mA) used

to prove combustion air flow and reduce spurious trips 329Figure 8.22 Ultraviolet and infrared wavelength compared with visible

Figure 8.23 Principle of flame ionisation showing half wave rectification

Figure 9.1 Schematic diagram of a generic furnace process showing

flows through the system 336

Figure 9.4 Plant trials using a suction pyrometer to measure combustion

air temperature and an optical pyrometer to measure product

Figure 9.7 Temperature/enthalpy data for cement, lime and alumina

Figure 9.8 Composite curves for hot and cold streams for pinch analysis 362Figure 9.9 Types of flow path configurations through recuperative heat

Figure 9.10 Relationship between heat transfer rate and surface area for

different recuperative heat exchanger flow regimes 365Figure 9.11 Types of flow path configurations through regenerative heat

Figure 9.13 Process flowsheet with air preheat and convection section

Figure 10.1 The structures in which nitrogen is commonly bound in

Figure 10.2 A simplified schematic representation of the paths of NOx

Figure 10.3 A schematic representation of the role of HCN in the NOx

Figure 10.4 The temperature dependence of the NOx formation process 383

Figure 10.6 Soot burning times for 500 Å particle at an oxygen partial

Figure 10.7 Tendency of fuels to smoke in relation to their composition 392Figure 10.8 Schematic mechanism for soot formation, showing the

Figure 10.11 An example of the trade-off between NO and CO emissions,

here from an internal combustion engine, which operates as

Figure 10.12 Effect of stoichiometry on NOx emissions for diffusion and

Figure 10.13 The effect of FGR on NOx emissions in an oil fired boiler 402

Figure 10.15 The principle of ‘reburn’ illustrated schematically 404Figure 10.16 The principle of ‘reburn’ applied to an entire boiler 405Figure 10.17 The principle of CO2 sequestration using oxy-fuel

Figure 10.18 Post-combustion gas composition from coal burned in a

Figure 11.1 Basic types of furnace showing direct fired and indirect fired 415Figure 11.2 Typical brick lining construction showing alternative layers

of bricks and expansion joints with casing reinforcing 416Figure 11.3 Typical monolithic lining held to the shell by anchors 416Figure 11.4 Typical ceramic fibre lining showing fixing details on

Figure 11.7 Typical anchors for installation of castable refractory as

Figure 11.8 Use of split mould formwork to cast refractory on a

Figure 11.9 Gunning refractory into place in a rotary kiln 423Figure 11.10 Strength development during curing of castable refractory 424

Figure 11.13 An example of an internally tiled furnace flat roof 428Figure 11.14 Simplified schematic of closed cooling system and open

Figure 11.15 Effect of raw materials on refractory properties 432Figure 11.16 Example of alloying element providing strength to a

metal by distortions of the crystal structure 439

Figure 11.17 Schematic illustration of how a passive oxide layer

Figure 12.2 Evolutionary development of the rotary kiln cement-making

Figure 12.4 Furnace process functions within the overall manufacturing

Figure 12.5 Schematics of cross-flow and counter-flow recuperators and

regenerative heat wheel for preheating combustion air 477Figure 12.6 Time/temperature/enthalpy diagram for metal slab heating 481Figure 12.7 Time/temperature/enthalpy diagram for multi-component oil

Figure 12.8 Time/temperature/enthalpy diagram for mineral aggregate

Figure 12.9 Temperature/enthalpy relationships for metal slab Oil and

Figure 12.10 Schematic diagram of a typical single zone reheating furnace 487Figure 12.11 Effect of slab length on principal parameters in the design of

Figure 12.12 Schematic diagram of a typical rectangular tube-still heater 490Figure 12.13 Effect of furnace height on principal parameters in the design

Figure 12.14 Effect of furnace height on principal parameters in the design

Figure 12.15 Effect of kiln diameter on principal parameters in the design

Table 1.1 Examples of furnaces meeting the classifications specified in

Table 3.3 Flammable limits for gases at room temperature 73Table 3.4 Typical properties of selected petroleum-based furnace fuels 76Table 3.5 Characteristic properties of selected solid fuels 78Table 4.1 Convective heat transfer correlations for a number of

Table 6.2 Typical diffusion coefficients and source terms 235

Table 7.1 General comparison between ball, vertical spindle and high

Table 8.4 Safety integrity level defined by IEC standard 61508 324

Table 9.8 Energy balance for the process with reduced in-leakage and

Table 9.9 Maximum energy conversion for various processes based on

Table 9.10 Common types of recuperative heat exchangers used in

Table 10.2 Occurrence of prompt NOx for some combustion processes 384Table 10.3 Chemical pathways to the formation of acid mist 385Table 10.4 World Health Organisation total equivalence factors for

human risk assessment of PBCs, dioxins and furans 388Table 10.5 Estimated dioxin and furan emissions in the UK 389Table 11.1 Characteristics of the main classifications of stainless steels 441Table A11.1 General properties of selected refractory materials 447Table A11.2 General properties of alumina and aluminous refractory

Table A11.3 General properties of chromite/magnesite refractory materials 449Table A11.4 General properties of magnesite/alumina refractory materials 450Table A11.5 General properties of dolomite, zirconia, and carbon refractory

Table A11.6 General properties of insulating refractory bricks 452Table A11.7 General properties of insulating refractory board,

Table 12.1 Ash analysis for several South Australian lignites 471Table 12.2 Furnace design data derived from Figures 12.6, 12.7 and 12.8 483Table 12.3 Calculation of heat transfer areas and average furnace heat

Table 12.4 Input data for well-stirred furnace model analysis 486Table 12.5 Well-stirred furnace analysis of slab heating furnace designs 488Table 12.6 Well-stirred furnace analysis of rectangular oil heating

Table 12.9 Analyses of the effect of kiln diameter on pressure drop

Table 12.10 Well-stirred furnace analysis of aggregate processing

shaft furnace designs including integral product cooling

Table 12.11 Manufacturer’s technical data on nozzle mixing gas burner

Chapter contents

1.2 Where are furnaces used? Brief review 7

of current furnace applications and

Combustion and furnaces are the very heart of our society Furnaces and kilns are used to produce virtually everything we use and many

of our food and drink items, either directly such as the metal ucts used in everyday life, the packaging for our food and drink, or indirectly such as the tools that are used to grow that food Even

prod-a nprod-aturprod-al product, such prod-as timber, requires drying to mprod-ake it able for most uses, and that drying occurs in a kiln From the earli-est days of human existence food was cooked over open fires and sticks were charred to harden them However, open fires provided little control over the heating process, and the birth of the Bronze Age some 5000–6000 years ago would have required the construc-tion of a forced draft furnace to achieve the temperature required

suit-to smelt the ore and produce liquid metal for casting Metal duction remained small scale for centuries owing to the scarcity

pro-of suitable fuel (charcoal) and the high cost pro-of its production The breakthrough came as a result of the determination and tenacity of Abraham Darby who worked much of his life to reduce the cost of cast iron by using coke in place of charcoal He finally succeeded and a lasting testament to his work is the world ’ s first iron bridge, completed in 1779 crossing the River Seven at Coalbrookdale in North West England, Figure 1.1

Darby ’ s pioneering work laid the foundations for the industrial revolution because iron production was no longer constrained by fuel supply and blast furnaces proliferated in the Severn Valley and even changed the way artists perceived the landscape, as exemplified in

Philip James de Loutherbourg ’ s famous landscape Coalbrookdale by

Night , now exhibited in the Science Museum, London In addition

to pioneering the use of coke, Darby also utilised steam engines for powering his furnace bellows and hence made the operation far less dependent on water power and less reliant on local rainfall, which

Figure 1.1 The iron bridge at Coalbrookdale showing the detail adjacent to an abutment

and ‘ Coalbrookdale by Night ’ – Philip James de Loutherbourg

was problematic and on many occasions drought had interrupted production for long periods By establishing his furnaces independ-ently of the vagaries of the charcoal burners and the weather, Darby triggered an industrial revolution that is continuing to this day The vast majority of the materials we use, and the food and drink

we consume, have been heated at some stage during the tion process Combustion processes are conveniently divided into high and low temperature process Although there is no strict divid-ing line between the two, processes with wall temperatures below 400–500 ° C are often considered low temperature, while those above 500–800°C are generally considered high temperature High temperature processes include cement and lime manufacture, brick and ceramic manufacture, most metal processing, glass making, etc., while low temperature processes include drying processes, food processing and sterilisation, steam raising, oil refining, etc

It is much more difficult to ensure efficient use of the fuel ’ s energy

in high temperature processes than their low temperature parts For low temperature processes, such as steam raising, effi-ciencies of over 80% are commonly obtained, whereas for high temperature processes, efficiencies exceeding 50% are rare Furnace design engineers of the future will be required to maximise overall process efficiency in a carbon constrained world This requirement will be driven both by the community need to achieve greenhouse gas reduction and by the process economics owing to high fossil fuel costs, especially for processes requiring premium fuels, such as oil and gas The aim of this book is to assist engineers to achieve higher per-formance both from existing and from new furnace designs, by pro-viding the reader with access to the tools that assist with gaining an in-depth understanding of the fundamentals of the individual proc-esses The book does not attempt to provide an in-depth understanding

counter-of any individual process because that would require a book in itself, for each industry, as can be seen from the wide variety of furnaces out-lined in this introduction In any case, we consider that the weakness of that approach is that the knowledge remains confined to that particular industry For example, the excellent kiln efficiencies achieved by the lime industry were almost unknown in the pulp and paper industry in the early 1980s, hence their lime kilns used some 30–50% more fuel than those in the mainstream industry at that time

1.1 What is a furnace?

The Oxford English Dictionary defines a furnace as ‘ an enclosed

structure for intense heating by fire, esp of metals, or water ’ , whereas a kiln is described as ‘ as furnace or oven for burning, baking

or drying, esp for calcining lime or firing pottery ’ In reality, there

is little difference between the two; kilns and furnaces both ing within a similar temperature range The two names owe more to tradition than to functional differences

Furnaces are the basic building block of our industrial society, indeed they are the foundation of our entire civilisation, as already discussed The principal objective of a furnace is to attain a higher processing temperature than can be achieved in the open air Although some processes could be carried out in the open air, to do so would

be far less efficient, the fuel consumption would be much higher and control of the process would be much more difficult

Furnaces can be used to facilitate a wide range of chemical tions, or in some cases simply for physical processes, such as anneal-ing or drying Design of the former is normally more complex than the latter but not exclusively so One of the challenges of furnace design is to determine the critical rate determining step(s) and to ensure that these are effectively addressed in the design By achieving this, smaller more efficient furnaces and more cost-effective designs can often be developed

In this book we shall be dealing with furnaces for processing materials at high temperatures, that is, above 400°C, especially those where the product is directly exposed to the flame We shall not dis-cuss steam boilers, the design and manufacture of which is a highly specialised subject In any case steam boilers have reached such high efficiencies that only marginal gains are possible Future gains in the efficiency of electricity generation, for example, depend on improv-ing the cycle efficiency; while this will impact on boiler design, this is not where the gains will originate Readers should consult Babcock and Wilcox (2005) for further information on boiler design

1.1.1 Furnace outline

The basic concept of a furnace is shown in Figure 1.2

Heat loss Fuel

Air

Flue gas

Flame Heat transfer Product

Figure 1.2 The basic elements of a furnace

Heat is liberated by burning fuel with air (or oxygen), or from electrical energy, and some of this heat is transferred to the product The remaining heat leaves in the flue gas and through openings such

as charging doors, or is lost from the external surface The efficiency

of the furnace can be defined as:

  Q p s

where:

  furnace efficiency

Q p  heat embedded in the final product

Q s  heat supplied by combustion

The heat embedded in the product is often quite small compared with the overall heat supplied, much heat being lost in the flue gases and by-products or waste materials such as slag While equation (1.1)

is a general expression for efficiency and is equally applicable to boilers, etc., it has many limitations with respect to determining the effectiveness of a furnace Later we shall examine other, more use-ful, measures of furnace performance

1.1.2 Furnace classification

There are an almost infinite number of ways of classifying furnaces, e.g by shape, industry, product, etc., but a very simple classifica-tion based on the heat transfer concepts of the type of heat source and the type of heat sink is shown in Figure 1.3 This classification system is highly simplified but it is useful because the nature of the product, the type of fuel, and the heat transfer mechanism all have

a major influence on the physical arrangement of the furnace It should be noted that many furnaces have multiple heat sinks and use several fuels, either concurrently or alternatively, which also affects the furnace design Examples of furnaces falling in the classifications shown in Figure 1.3 are provided in Table 1.1 Descriptions and illustrations of a wide range of furnaces meeting the classification shown in Figure 1.3 follow later in this chapter We will also refer to Figure 1.3 when considering the design of furnaces in Chapter 12

1.1.3 Principle objectives of furnace designers and operators

1 Obtain a satisfactory product

2 Use minimum fuel and energy to achieve that product

3 Construct the furnace for the lowest capital cost

4 Operate with the lowest possible manning levels

5 Achieve a satisfactorily long life with low maintenance costs

Classification

number Example of type of furnace

5 8

9 10 13 7 6 12 16

3 14

11

18 19

4 Granular Fixed surface

Solid Liquid

SINK Gas

20 21 15

Objective 1 overrides all others because, if the product is tory, then it cannot be sold or must be sold for an inferior price While safety has not been mentioned in the above objectives, it can

unsatisfac-be taken as ‘ a given ’ in today ’ s environment and is covered sively in Chapter 8

The art of furnace design involves achieving the best combination

of these five objectives over the entire life of the furnace, in other words to produce a high quality product at the lowest achievable cost Although these five objectives may seem self-evident, in our experience they are not always achieved Furnace users have gener-ally purchased on the basis of capital cost; understandably so, given that the other objectives are much more difficult to assess at the ten-dering stage This approach has led to relatively little engineering effort being expended on improving existing designs because engin-eering time is expensive and the costs are difficult to recoup for the furnace builder However, an era of higher fuel and energy costs will put new demands on designers and operators of furnaces, forcing a new approach for many applications

This book aims to provide designers and operators with the tools

to achieve the best combination of the above five objectives for their particular process and hence meet the future challenges Our approach is to consider the basic issues that affect furnace perform-ance from first principles and then to build a design methodology

on these considerations We hope that this approach will ally supersede traditional techniques and achieve significant gains in practical furnace efficiency

1.2 Where are furnaces used? Brief review of current

furnace applications and technology

Before examining the principles behind furnace design, we have reviewed a few of the furnaces in common use, largely for the bene-fit of those new to the field but also for those who have worked in one specific industry and have not been exposed to the wide variety

of furnaces employed in other industries

1.2.1 Ceramics, brick making and pottery

Ancient pottery and brick kilns are the precursors of modern naces Pottery kilns arose to satisfy the need for durable containers for food and drink Pottery from as early as 6000 BC was found during excavation of a Neolithic site near Catal Huyuk in central Anatolia ( Raymond 1984 : 6–7) On this basis, high temperature fur-nace technology (pottery furnaces typically operate around 1000° C)

fur-is some 8000 years old These early furnaces were very inefficient