Industrial Boilers and Heat Recovery Steam Generators Design, Applications, and Calculations

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.

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 300°F exit gas temperature from a single pressure unfired gas turbine HRSG generating saturated steam at 600psig using feedwater at about 230°F. A simple analysis reveals that only about 340-350°F is thermodynamically feasible.

• Supplementary firing in gas turbine HRSGs is an efficient way to generate

Steam compared with steam generation 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.

• 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 steam plant 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 chapters deal with calculations that should be of interest to steam plant 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 custom — designed 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 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 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.

V. Ganapathy

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