Solid Fuels Combustion and Gasification: Modeling, Simulation, and Equipment Operations Second Edition

488 Pages·2010·13.274 MB·English

by  Marcio L. de Souza-Santos

Second Edition
Solid Fuels
Combustion
and Gasification
Modeling, Simulation,
and Equipment Operations
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S e c o n d E d i t i o n
Solid Fuels
Combustion
and Gasification
Modeling, Simulation,
and Equipment Operations
Marcio L. de Souza-Santos

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Library of Congress Cataloging-in-Publication Data
Souza-Santos, Marcio L. de.
Solid fuels combustion and gasification : modeling, simulation, and equipment
operations / Marcio L. de Souza-Santos. -- 2nd ed.
p. cm. -- (Mechanical engineering)
Includes bibliographical references and index.
ISBN 978-1-4200-4749-3 (alk. paper)
1. Fuel--Combustion. 2. Coal gasification. 3. Fuel--Combustion--Equipment and
supplies. I. Title.
TP319.S68 2010
621.402’3--dc22 2009050557
Visit the Taylor & Francis Web site at
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Dedication
To the 200 years of Charles Darwin, one among the greatest liberators of
humanity.
The history of science—by far the most successful claim to knowledge
accessible to humans—teaches that the most we can hope for is successive
improvement in our understanding, learning from our mistakes, an asymp-
totic approach to the Universe, but with the proviso that absolute certainty will
always elude us.
Carl Sagan
(The Demon-Haunted World: Science as a Candle in the Dark)

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Contents
Preface.......................................................................................................................ix
Acknowledgments ..................................................................................................xiii
Nomenclature ...........................................................................................................xv
Chapter 1. Basic Remarks on Modeling and Simulation .......................................1
Chapter 2. Solid Fuels ..........................................................................................19
Chapter 3. Equipment and Processes ...................................................................43
Chapter 4. Basic Calculations ..............................................................................79
Chapter 5. Zero-Dimensional Models .................................................................99
Chapter 6. Introduction to One-Dimensional Steady-State Models ..................125
Chapter 7. Moving-Bed Combustion and Gasification Model ...........................141
Chapter 8. Chemical Reactions ..........................................................................167
Chapter 9. Heterogeneous Reactions .................................................................185
Chapter 1.0. Drying and Devolatilization .............................................................203
Chapter 1.1. Auxiliary Equations and Basic Calculations....................................235
Chapter 1.2. Moving-Bed Simulation Programs and Results ...............................271
Chapter 1.3. Bubbling Fluidized-Bed Combustion and Gasification Model ........299
vii
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viii Contents
Chapter 1.4. Fluidization Dynamics .....................................................................315
Chapter 1.5. Auxiliary Parameters Related to Fluidized-Bed Processes .............343
Chapter 1.6. Bubbling Fluidized-Bed Simulation Program and Results ..............361
Chapter 1.7. Circulating Fluidized-Bed Combustion and Gasification Model .....411
Chapter 1.8. Circulating Fluidized-Bed Simulation Program and Results ...........421
Appendix A The Fundamental Equations of Transport Phenomena ....................433
Appendix B Notes on Thermodynamics ..............................................................439
Appendix C Possible Improvements on Modeling Heterogeneous Reactions ......455
Appendix D Improvements on Various Aspects ...................................................463
Appendix E Basics on Turbulent Flow .................................................................467
Appendix F Classification of Modeling for Bubbling Fluidized-Bed
Equipment ........................................................................................473
Appendix G Basics on Techniques of Kinetics Determination ............................479

Index ......................................................................................................................483
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Preface
The general perception is to equate solid fuels with pollution. That is understandable,
but not necessarily correct or inevitable. There is no doubt that the burning of coal
in thermoelectric stations raises justified concerns regarding the emission of pollut-
ants. The abundance of coal, lack of alternative resources, and economic pressures
have led many countries to continue and even increase the application of this fuel
for power generation. Evidently, that picture is not unchangeable, first because spe-
cial attention has been given to biomasses because of their renewability and overall
zero carbon dioxide generation aspects, and second because of the development of
new techniques. These new techniques have allowed much lower emissions of sul-
fur and nitrogen oxides into the atmosphere, while raising efficiency. A good part
of that success is due to process optimization through mathematical modeling and
simulation.
Therefore, it is not surprising that the number of professionals and graduate
students entering fields related to combustion and gasification of solid fuels is
increasing. However, unlike focused researchers, many of these new professionals
are not interested in deep consideration based on exhaustive literature review of spe-
cialized texts on the subject. These publications are important, but most assume
an audience of accomplished mathematical modelers and are not preoccupied with
presenting the details of how it is possible to start from fundamental and general
equations and arrive at a final model for an equipment or process. Those just start-
ing out in the field are also generally not interested in the other extreme, i.e., simple
and mechanistic description of equipment design procedures or instruction manuals
for application of commercial simulation packages. They tend to have a few main
preoccupations:
• Being acquainted enough with the fundamental phenomena taking place in
the equipment or processes
• Gaining knowledge of basic procedures of modeling and simulation of
equipment and systems
• Developing mathematical models and simulation programs to predict the
behavior of this equipment or these processes, mainly for cases when no
commercial simulators are available
• Applying the available tools when simulators are commercially avail-
able, depending on the conditions of time and resources (in this case, one
should be able to properly set the conditions asked as input by the simulator,
evaluate the applicability of possible solutions, and choose among various
alternatives)
• Using simulation programs to improve operations of existing equipment or
for optimized design of new equipment
• Building confidence for decision making regarding process improvements
and investments
ix
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x Preface
Experience shows that a good route to acquiring real and testable understanding
of a subject in the area of processing is to develop models and computer simula-
tors. Developing a successful simulator is an accomplishment in which a student can
justly take pride, since it represents the accumulated knowledge of the subject.
It is important to emphasize the need for simple models. Of course, there are
several levels at which models can be built. Nonetheless, one should be careful
with too simple or too complex ones. The low extreme usually provides only super-
ficial information, while the other usually takes years to develop and involves
considerable computational difficulties because of convergence problems and
incoherencies.
In the present text, model complexity is extended only as far as needed to achieve
a reasonable representation of that equipment. For instance, the examples are lim-
ited to two dimensions, and most of the models are based on a one-dimensional
approach. This may sound simplistic; however—as demonstrated in the text—the
level of detail and usefulness of results from such simulations are not negligible. In
addition, this book can also be used as an introduction for more complex models.
The text is designed to be useful to graduate students, engineers, and profession-
als with degrees in any exact science and has been applied as such for many years in
courses at all levels.
Although the main concern of this book is with modeling combustion and gas-
ification processes, it is also an introduction to mathematical modeling and simula-
tion. The basic methods illustrated here can be used for modeling a wide range of
processes and equipment commonly found in industry.
Operations of boilers, furnaces, incinerators, gasifiers, or any other equipment
dealing with combustion or gasification phenomena involve a multitude of simul-
taneous processes, such as heat, mass, and momentum transfers; chemical kinetics
of several reactions; drying; pyrolysis, etc. Those should be coherently combined to
allow reasonable simulation of industrial units or equipment. To help accomplish
that, the text describes combustion and gasification processes in some detail.
Although the basic concepts of thermodynamics and transport phenomena can
be found in several texts, the respective fundamental equations are included. Thus,
the need to consult other texts has been minimized. Most of the time, the concepts
usually learned in engineering courses are sufficient.
In view of the practical approach, several correlations and equations are taken from

the literature without a preoccupation with mathematical demonstrations. References
are provided and should be consulted by those interested in further details.
The main strategy of the book is to teach by example. Besides the significant frac-
tion of industrial equipment operating with suspensions of pulverized solid fuels, the
specific cases of moving and fluidized beds have been selected because they have
several qualities:
• They cover a good fraction of processes found in industry involving com-
bustion or gasification of solid fuels. In the particular case of fluidized beds,
the fraction of equipment using that technique has continuously increased.
Actually, conventional boilers and furnaces operating with suspensions
have been retrofitted to fluidized beds.
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Preface xi
• They allow easy-to-follow examples on how simplifying assumptions regar-
ding the operation of real industrial equipment can be set.
• They permit relatively quick introduction of fundamental equations without
the need for too complex treatments.
• They provide simple examples of modeling and show how those examples
can be put together in order to write a simulation program.
To summarize, the book intends to accomplish the following objectives:
• Show several constructive and operational features of equipment dealing
with combustion and gasification of solid fuels, such as coal, biomass, solid
residues, etc.
• Present basic aspects of solid and gas combustion phenomena
• Introduce the fundamental methodology to formulate a mathematical model
of the above equipment
• Demonstrate possible routes from model to workable computer simulation
program
• Show comparisons between simulations and real operations
• Illustrate interpretations of simulation results that may be applied as tools
for improving the performance of existing industrial equipment or for opti-
mized design of new equipment
It is organized as follows:
Chapter 1 presents a few generally applicable notions concerning modeling
and simulation.
Chapter 2 shows the main characteristics of solid fuels, such as coals and
biomasses.
Chapter 3 introduces basic concepts of solid-gas systems and the main charac-
teristics of combustion and gasification equipment.
Chapter 4 provides formulas and methods to allow initial calculations regard-
ing solid fuel processing.
Chapter 5 describes the fundamental equations of zero-dimensional models
with the objective of allowing verification of overall relations between inputs

and outputs of any general process, including combustors and gasifiers.
Chapter 6 introduces a very basic and simple first-dimension model of a gas
reactor. Of course, it is not the intention to present any model for flames.
That is beyond the scope of this introductory book. However, it is useful
to introduce standard considerations regarding mathematical modeling and
the application of mass, energy, and momentum transfer equations.
Chapter 7 describes the first example of a model for solid fuel combustion and
gasification equipment. The case of a moving-bed combustor or gasifier is
used for this model.
Chapters 8 and 9 introduce the methods to compute gas–gas and gas–solid
reaction rates.
Chapter 10 introduces the modeling of drying and pyrolysis of solid fuels.
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xii Preface
Chapter 11 presents auxiliary and constitutive equations and methods that may
be used to build a computer program to simulate the model described in the
previous chapter.
Chapter 12 shows how to put together all the information previously described
in order to build a workable simulation program. The chapter also pres-
ents comparisons between simulations and real operations of a moving-bed
gasifier.
Chapter 13 repeats the approach used in Chapter 7, but now having in mind
bubbling fluidized-bed combustors and gasifiers.
Chapters 14 and 15 provide correlations and constitutive equations needed
to complete the simulation of bubbling fluidized bed combustors and
gasifiers.
Chapter 16 shows the strategy of assembling a simulation program based on
the model presented for bubbling fluidized beds, as well as comparisons
between simulations and real operations of boilers and gasifiers.
Chapter 17 repeats the approach used in Chapter 13 for the case of circulating
fluidized-bed combustors and gasifiers.
Chapter 18 is similar to Chapter 16, but now using examples related to circulat-
ing fluidized bed equipment.
Almost all chapters include exercises. They are intended to stimulate the imagi-
nation and build confidence in solving problems related to modeling and simulation.
The relative degree of difficulty or volume of work involved on solutions is indicated
by the increasing number of asterisks. Problems with four asterisks usually require
good training in solving differential equations or demand considerable work.

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