Industrial Boilers and Heat Recovery Steam Generators

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Industrial Boilers and Heat Recovery Steam Generators

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Preface

The role of boilers and heat recovery steam generators (HRSGs) in the industrial
economy has been profound. Boilers form the backbone of power plants,
cogeneration systems, and combined cycle plants. There are few process
plants, refineries, chemical plants, or electric utilities that do not have a steam
plant. Steam is the most convenient working fluid for industrial processing,
heating, chilling, and power generation applications. Fossil fuels will continue to
be the dominant energy providers for years to come.
This book is about steam generators, HRSGs, and related systems. There
are several excellent books on steam generation and boilers, and each has been
successful in emphasizing certain aspects of boilers and related topics such as
mechanical design details, metallurgy, corrosion, constructional aspects, maintenance,
or operational issues. This book is aimed at providing a different
perspective on steam generators and is biased toward thermal and process
design aspects of package boilers and HRSGs. (The terms ‘‘waste heat boiler’’
and ‘‘HRSG’’ are used in the same context.) My emphasis on thermal engineering
aspects of steam generators reinforced by hundreds of worked-out real-life
examples pertaining to boilers, HRSGs, and related systems will be of interest
to engineers involved in a broad field of steam generator–related activities such as
consulting, design, performance evaluation, and operation.
Copyright © 2003 Marcel Dekker, Inc.
During the last three decades I have had the opportunity to design hundreds
of package boilers and several hundred waste heat boilers that are in operation in
the U.S. and abroad. Based on my experience in reviewing numerous specifications
of boilers and HRSGs, I feel that consultants, plant engineers, contractors,
and decision makers involved in planning and developing steam plants often do
not appreciate some of the important and subtle aspects of design and performance
of steam generators.
 Many engineers still feel that by raising the exit gas temperature in boilers
with economizers, one can avoid acid dew point concerns. It is the feed water
temperature—not the gas temperature—that determines the tube wall
temperature (and hence the corrosion potential).
 Softened water is sometimes suggested for attemperation for steam temperature
control, even though it will add solids to steam that can cause problems
such as deposition of solids in superheaters and steam turbines.
 To operate steam plants more efficiently, plant engineers should be able to
understand and appreciate the part load characteristics of boilers and HRSGs.
However while specifying boilers and HRSGs, often only the performance at
100% load is stressed.
 HRSG steam generation and temperature profiles cannot be arbitrarily arrived
at, as pinch and approach points determine this. For example, I have seen
several specifications call for a 300F exit gas temperature from a single
pressure unfired gas turbine HRSG generating saturated steam at 600 psig
using feedwater at about 230F. A simple analysis reveals that only about
340–350F is thermodynamically feasible.
 Supplementary firing in gas turbine HRSGs is an efficient way to generate
steamcompared with steamgeneration in a packaged boiler. The book explains
why this is so, with examples in Chapters 1 and 8. Cogeneration engineers can
make use of this information to minimize fuel costs in their plants.
 A few waste heat boiler specifications provide the flue gas flow in volumetric
units instead of mass units, leading to confusion. Lack of information on
molecular weight or gas pressure can lead to incorrect evaluation of density
and hence the mass flow. Also, volume of flue gas is often given in cfm (cubic
feet per minute) and one is not sure whether it is acfm (actual cubic feet per
minute) or scfm (standard cubic feet per minute). The difference in mass flow
can be significant depending on the basis.
 Although flue gas analysis affects gas specific heat, heat transfer, boiler duty,
and temperature profiles, these data are often not given in specifications for
waste heat boilers. For example, the ratio of specific heats of flue gases from
combustion of natural gas and fuel oil is about 3.5%, which is not insignificant.
This is due to the 18% volume of water vapor in natural gas products of
combustion versus 12% in fuel oil combustion products.
Copyright © 2003 Marcel Dekker, Inc.
 A few consultants select boilers and HRSGs based on surface area, although
it can vary significantly based on tube geometry or fin configuration. With
finned tubes, as can be seen from several examples in this book, the variation
in surface areas could be in the range of 200–300% for the same duty.
 Operating cost due to fuel consumption or gas pressure drop across heating
surfaces is often ignored by many consultants in their evaluation and only
initial costs are compared while purchasing steam generators or HRSGs,
resulting in a poor selection for the end user. A few plants are now realizing
that the items of steamplant equipment they purchased years ago based on low
initial costs are draining their cash reserves through costly fuel and electricity
bills and hence are scrambling to improve their design and performance.
 Many engineers are not aware of recent developments in oil- and gas-fired
packaged boilers and are still specifying boilers using refractory lined furnace
walls and floors!
 Plant engineers often assume that a boiler designed for 600 psig, for example,
can be operated at 200 psig and at the same capacity. The potential problems
associated with significant changes in steam pressure and specific volume in
boiler operation are discussed in Chapters 1 and 3.
 Condensing exchangers are being considered in boilers and HRSGs not only
for improvement in efficiency but also to recover and recycle the water in the
flue gases, which is a precious commodity in some places.
 Emission control methods such as flue gas recirculation increase the mass
flow of flue gases through the boiler; yet standard boilers are being selected
that can be expensive to operate in terms of fan power consumption. Many are
not aware of the advantages of custom-designed boilers, which can cost less
to own and operate.
 A few steam plant professionals do not appreciate the relation between boiler
efficiencies and higher and lower heating values, and thus specify values that
are either impossible to accomplish or too inefficient.
As a result of this ‘‘knowledge=information gap’’ in process engineering
aspects of boilers or HRSG, the end user may need to settle for a product with
substandard performance and high costs. This book elaborates on various design
and performance aspects of steam generators and heat recovery boilers so that
anyone involved with them will become more informed and ask the right
questions during the early stages of development of any steam plant project.
This will give the best chance of selecting the steam generator with the right
design and parameters. Even a tiny improvement in design, efficiency, operating
costs, or performance goes a long way in easing the ‘‘energy crunch.’’
The first four chapters describe some of the recent trends in power
generation systems, a few aspects of steam generator and HRSG design and
performance, and the impact of emissions on boilers in general. The remaining
Copyright © 2003 Marcel Dekker, Inc.
chapters deal with calculations that should be of interest to steamplant engineers.
I authored the Steam Plant Calculations Manual (Marcel Dekker, Inc.) several
years ago and had been thinking of adding more examples to this work for quite
some time. This book builds on that foundation.
Chapter 1 is an introductory discussion of power plants and describes some
of the recent developments in power systems such as the supercritical Rankine
cycle, the Kalina cycle, the Cheng cycle, and the integrated coal gasification and
combined cycle (IGCC) plant that is fast becoming a reality.
The second chapter describes heat recovery systems in various industries.
The role of the HRSG in sulfur recovery plants, sulfuric acid plants, gas turbine
plants, hydrogen plants, and incineration systems is elaborated.
Chapter 3, on steam generators, describes the latest trends in customdesigned
package boilers and the limitations of standard boilers developed
decades ago. Emission regulations have resulted in changes in boiler operating
parameters such as higher excess air and FGR rates that impact boiler performance
significantly. It should be noted that there can be several designs for a
boiler simply because the emission levels are different, although the steam
parameters may be identical. If an SCR system is required, it necessitates the
addition of a gas bypass system, adding to the cost and complexity of boiler
design. These are explained through quantitative and practical examples.
Chapter 4, on emissions, describes the various methods used in boilers and
HRSGs to limit NOx and CO and how their designs are impacted. For example,
the HRSG evaporator may have to be split up to accommodate the selective
catalytic reduction (SCR) system; gas bypass dampers may have to be used in
packaged steam generators to achieve the optimal gas temperature at the catalyst
for NOx conversion at various loads. Flue gas recirculation (FGR) adds to the fan
power consumption if the standard boiler is not redesigned. It may also affect the
boiler efficiency through higher exit gas temperature due to the larger mass flow
of flue gases. Other methods for emission control, such as steam injection and
burner modifications, are also addressed.
Chapters 4–8, which present calculations pertaining to various aspects of
boilers and HRSGs and their auxiliaries, elaborate on the second edition of the
Steam Plant Calculations book. Several examples have also been added. Chapter
5 deals with calculations such as conversion of mass to volumetric flowrates,
energy utilization from boiler blowdown, general ASME code calculations, and
life cycle costing methods. (ASME has been updating the allowable stress values
for several boiler materials and one should use the latest data.) Also provided are
ABMA and ASME guidelines on boiler water, for evaluating the blowdown or
estimating the steam for deaeration. Life cycle costing is explained through a few
examples.
Chapter 6 deals with combustion calculations, boiler efficiency, and
emission conversion calculations. Simplified combustion calculation procedures
Copyright © 2003 Marcel Dekker, Inc.
such as the MM Btu method are explained. Often boiler efficiency is cited on a
Higher Heating Value basis, while a few engineers use the Lower Heating Value
basis. The relation between the two is illustrated. The ASME PTC 4.1 method of
calculating heat losses for estimating boiler efficiency is elaborated, and simplified
equations for boiler efficiency are presented. Examples illustrate the relation
between oxygen in turbine exhaust gases and fuel input. Correlations for dew
point of various acid vapors are given with examples.
Chapter 7 explains boiler circulation calculations in both fire tube and water
tube boilers. Fluid flow in blowoff and blowdown lines, which involve two-phase
flow calculations, can be estimated by using the procedures shown. The problem
of flow instability in boiling circuits is explained, along with measures to
minimize this concern, such as use of orifices at the inlet to the tubes.
Calculations involving orifices and safety valves should also be of interest to
plant engineers.
Chapter 8 on heat transfer has over 65 examples of sizing, off-design
performance calculations pertaining to boilers, superheaters, economizers,
HRSGs, and air heaters. Tube wall temperature calculations and calculations
with finned tubes for insulation performance will help engineers understand the
design concepts better and even question the boiler supplier. HRSG temperature
profiles are also explained, with methods described for evaluating off-design
HRSG performance.
The last chapter deals with pumps, fans, and turbines and examples show
the effect of a few important variables on their performance. The impact of air
density on boiler fan operation is illustrated, and the effect of elevation and
temperature on flow and head are explained. With flue gas recirculation being
used in almost all boilers, the effect of density on the volume is important to
understand. The effect of inlet air temperature on Brayton cycle efficiency is also
explained and plant engineers will appreciate the need for inlet air-cooling in
summer months in large gas turbine plants. The efficiency of cogeneration is
explained, as are also power output calculations using steam turbines.
A simple quiz is given at the end of the book. Its purpose is to recapitulate
important aspects of boiler and HRSG performance discussed in the book.
In sum, the book will be a valuable addition to anyone involved in steam
plants, cogeneration systems, or combined cycle plants. Many examples are based
on my personal experience and hence, the conclusions drawn do not reflect the
views of any organization. It is possible, due to lack of information on my part or
to the rapid developments in steam plant engineering and technology, that I have
expressed some views that may not be current or may be against the grain; if so, I
express my regrets. I would appreciate readers bringing these to my attention. The
calculations have been checked to the best of my ability; however if there are
errors, I apologize and would appreciate your feedback. It is my fervent hope that
Copyright © 2003 Marcel Dekker, Inc.
this book will be the constant companion of professionals involved in the steam
generation industry.
I would like to thank ABCO Industries for allowing me to reproduce
several of the drawings and photographs of boilers and HRSGs. I also thank other
sources that have provided me with information on recent developments on
various technologies.

Contents

Preface
1 Steam and Power Systems
2 Heat Recovery Boilers
3 Steam Generators
4 Emission Control in Boilers and HRSGs
5 Basic Steam Plant Calculations
6 Fuels, Combustion, and Efficiency of Boilers and Heaters
7 Fluid Flow, Valve Sizing, and Pressure Drop Calculations
8 Heat Transfer Equipment Design and Performance
9 Fans, Pumps, and Steam Turbines
Copyright © 2003 Marcel Dekker, Inc.
Appendix 1: A Quiz on Boilers and HRSGs
Appendix 2: Conversion Factors
Appendix 3: Tables
Glossary
Bibliography