Chapter 1 - Steam and Its Importance

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1. Steam and Its Importance

1.1. Introduction

We have become aware of a variety of terms nearly every day regarding our sources of energy andthe effects on our environment. Some of the most used terms are alternative energy and renewable

sources such as solar, wind, and biomass fuels. Discussions on these topics generally are combined

with the effects on our environment from traditional energy sourcesfossil fuels. These effects are

often in the spotlight as we hear concerns over global warming and climate change. Although

many of these references are related to our means of transportation and their use of oil-based

products, they also are directed at the various methods of producing a critical energy source

electricity, which is so vital to our everyday way of life.

In today's modern world, all societies are involved to various degrees with technological

breakthroughs that are attempting to make our lives more productive and more comfortable, both

at home and at the workplace. These technologies include sophisticated electronic devices, the

most prominent of which are computer systems, cell phones which can perform multiple tasks

including computer functions, and various mobile entertainment devices. All of these systems in

our modern world depend on a reliable and relatively inexpensive energy sourceelectricity. And

these relatively new systems are in addition to those we traditionally think aboutlights, motors,

air conditioners, etc. In addition we see advances in the automotive world where electric cars are

being developed in order to reduce the dependence on oil. These demands point out the fact that

inexpensive and reliable electricity is critical to the sustained economic growth and security of the

United States and of the rest of the world.

The United States depends on reliable, low-cost, and abundant energy. Energy drives the economy,

heats and lights our homes, pumps water, and provides security, among countless other activities.

The efficient use and production of electricity and effective and reasonable conservation measures

are paramount to ensuring low-cost energy.

Steam and Its Importance

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As an example, the United States uses about 20 percent more energy today than it did in 1973.

There are more than 50 million additional homes and more than 50 million vehicles, and the gross

national product (GNP) is over 10 times higher according to the U.S. Department of Energy. This

emphasizes the need for the United States to have a flexible and comprehensive energy plan with

policies necessary to provide long term reliable, secure, and environmentally acceptable energy

supplies at predictable and affordable costs.

More than 33 percent of the total energy used in the United States is for the generation of

electricity. The demand for electricity has generally increased in parallel with the country's GNP

and it is expected that this path will continue in the future. Over the next 10 years, predictions have

been made which will require nearly $200 billion for new investments to meet the increasing

electricity needs of the United States alone. This will require that all economically feasible energy

technologies must be carefully evaluated. This will demand not only new power plants of various

technologies but also energy conservation in the form of energy saving systems which will provide

energy efficient products and services.

This expansion of the electric production must also be accomplished with an accompanying

systematic reduction of emissions throughout the world. Although major reductions in pollutants

have been accomplished over the past 40 years, further realistic reductions must be achieved.

However, the goals for these further reductions must be both realistic and economical. Low-cost

electric energy is vital for the economies of the United States and the rest of the world.

With the availability of electricity providing most of the industrialized world a very high degree of

comfort, the source of this electricity and the means for its production are often forgotten. It is the

power plant that provides this critical energy source, and in the United States approximately 90

percent of the electricity is produced from power plants that use steam as the energy source

powering the turbine generators, with the remaining 10 percent of the electricity produced

primarily by hydroelectric power plants (approximately 6 percent), and 4 percent from sources

such as biomass, oil, wind, and geothermal as outlined in Table 1.1. In other parts of the world,

similar proportions are common for their electric production. Even electricity produced from the

energy sources of oil, biomass, and geothermal utilize steam to power the turbine generators.

Table 1.1. Energy Source for Electric Generation in the United States

Coal 48.2%

Natural gas 20.5%

Nuclear 20.3%

Source: Energy Information Administration (EIA).




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Hydroelectric 5.9%

Biomass 2.1%

Wind 1.2%

Oil 1.0%

Solar 0.4%

Geothermal 0.4%

Total 100.0%

Source: Energy Information Administration (EIA).

The power plant is a facility that transforms various types of energy into electricity or heat for

some useful purpose. The energy input to the power plant can vary significantly, and the plant

design to accommodate this energy is drastically different for each energy source. The forms of

this input energy can be as follows:

1.

Thepotential energy of an elevated body of water, which, when used, becomes a hydroelectric

power plant.

2.

The chemical energy that is released from the hydrocarbons contained in fossil fuels such as

coal, oil, natural gas, and biomass fuels which becomes a fossil-fuel-fired power plant.

3.

Thesolar energy from the sun, which becomes a solar power plant.

4.Thefission or fusion energy that separates or attracts atomic particles, which becomes a nuclear

power plant.

5.

The wind energy that is generated from our natural environment and becomes a wind farm.

With any of these input sources, the power plant's output can take various forms:

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Other than wind energy, heat for a process or for heating.

2.

Electricity that is subsequently converted into other forms of energy such as lighting, motor

drives, computer, safety systems, etc.

3.

Energy for transportation, such as powering ships.

In these power plants, the conversion of water to steam is the predominant technology, and this

book will describe this process and the various systems and equipment that are commonly used in

today's operating steam power plants.

The development of new energy sources is extremely important to complement our traditionalmethods of producing electricity. There are locations in the United States and in the world where

technologies such as solar, wind, and biomass may prove to make a vital contribution to our

overall source of energy. This ninth edition ofSteam Plant Operation incorporates information on

these relatively new technologies, and, other than wind energy, solar and biomass plants also use

steam to convert these energy sources into electricity. But the emphasis of this new edition will

continue to be on the use of steam, developed from various energy sources, as its use results in

nearly 90 percent of the electricity produced in the United States, with a comparable percentage in

other parts of the world.

As in previous editions of this book, we will see that each power plant has many interacting

systems, and in a steam power plant these can include fuel and ash handling; handling of

combustion air and the products of combustion; feedwater and condensate; steam; environmental

control systems; and the control systems that are necessary for a safe, reliable, and efficiently run

power plant. This ninth edition ofSteam Plant Operation continues to blend descriptions and

illustrations of both new and older equipment, since both are in operation in today's power plant.

The demand for electricity can fluctuate significantly in the short term due to economic conditions,

the weather, and the price of electricity. When looking at the demand over a longer period, a

different pattern emerges. In the 1950s, the demand for electricity was increasing at a significant

rate, nearly 10 percent per year. Over a 10-year period, this resulted in an electric demand which

doubled, requiring the significant addition of power plants of increasing size in a relatively short

period of time. The reason for this exceptional increase was primarily due to the addition of air

conditioning throughout the country. This rapid increase in demand gradually decreased to less

than 3 percent per year in the 1990s, and recently this has further declined to about 1 percent peryear.




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This decline in demand has resulted despite our population increasing, and it is expected to

continue this trend for the next 20 years. The major reason for this is new energy efficiency

standards for such things as lighting, heating and cooling, and various appliances. This reduction in

demand is not expected to change even with the expected hybrid automobiles using electric plug-

ins to regenerate their batteries. However, if electric cars become the choice of many in the next 20

years, the electric demand for this will have to be met with many more new power plants.

This does not mean that we can relax on our needs for power in the future. Even at a rate of 1

percent per year of increased demand, this means that new power plants will be required to meet

the more than 20 percent additional capacity necessary to satisfy the new demand as well as the

replacement of retired power plants. All energy sources will be necessary to meet this new

challenge: coal, natural gas, nuclear, and renewable energy.

By the year 2030, the U.S. Department of Energy has forecasted that the demand for electricity

will increase by nearly 40 percent. This relates to the need for approximately 300,000 megawatts

(MW) of some type of electricity from a variety of resources including coal, natural gas, nuclear

power, wind, and other renewables. If this forecast were to be met just from additional nuclear

plants, for example, 200 to 300 new plants would be required, approximately 10 or more each

year. In order to meet this demand, it is recognized by industry leaders that coal and nuclear power

are the only proven technologies that could provide the large amounts of electric power which will

be required to meet the large demand for electricity in the future.

There is an ongoing debate on what many are claiming as "global warming" (or often using the

term "climate change"), caused primarily from emissions from power plants and transportation that

utilize fossil fuelscoal, natural gas, or oil. Whether climate change is occurring will not be

determined in this book. And if it is occurring, whether it is man-made or a normal cyclical

weather event will also not be determined. However, it is necessary to realize the importance of

any future decisions and their impact on the economy of the United States and of the world. There

must be a carefully thought-out plan to properly balance power production with any impact on our

environment. There are consequences of "going green" and all factors must be seriously

understood and properly evaluated.

It is important to realize that the climate of the earth has been continually changing. There have

been periods of increasing and decreasing temperatures. Most of the information that we see from

experts predicts a continual warming trend, resulting in serious consequences. Then there are other

experts that suggest that earth has undergone changes, but for the past 30 years, a cooling trend has

emerged. So, the argument exists that with the many changes in our climate, can the earth's

temperature be predicted with any degree of accuracy for the next 100 to 200 years?




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What we do know is that there are many factors which influence our climate, many that are natural

and some that are man-made. There are variations in solar cycles, solar radiation, ocean currents,

depletion of rain forests, volcanic eruptions, and wild fires. (The eruption of the volcano in Iceland

in 2010 showed how this natural occurrence can disrupt our way of life as well as affect our

environment.) These all have some impact on our environment, but to predict the future

temperature change would be next to impossible with all of these varying and unpredictable

factors.

The earth has seen warm periods followed by ice ages and then warmer periods again where the

glacier ice has receded. These changes have occurred thousands of years ago and none of these

was caused by man-made activities which again creates the question of how we can be so sure of

our future predictions regarding "global warming." The question requires a rational analysis and

conclusion. There are too many "natural" occurrences which have an impact on our environment

and we should not jump to the conclusion that man is responsible. If we do, a serious impact on

our economy and way of life may occur.

Approximately 96 percent of the electricity generated in the United States is produced basically

from four primary sources: coal, natural gas, nuclear, and hydroelectric. The electricity generated

from each energy source is shown in Table 1.1. (Note that these percentages do change from year

to year.)

Carbon dioxide (CO ) results from the complete combustion of a fossil fuel, whether it be coal,natural gas, oil, or a biomass fuel. This gas has been considered to be the primary contributor to

our atmosphere which many believe results in "global warming" or "climate change." So, when the

discussion has focused on "going green" as a means to eliminate these emissions, wind and solar

power are the suggested solutions. But each of these has significant drawbacks.

Solar power is undergoing research and development programs to establish a cost-effective means

of utilizing this energy source. Thermal solar, using the sun's rays to generate steam in a steam

generator with the steam powering a turbine generator, is discussed later in this chapter as the solar

technology having promise on a large scale for the generation of electricity. But, because of the

intermittent source of its energy, the sun, it is expected to have a very low capacity factor of

approximately 20 to 30 percent, and this would be for plants located in the southwestern portion of

the United States. The availability of electricity from other locations would be less. The time of the

year would also have an impact on the electricity produced. Solar power may have a small role in

our overall demand for electricity because of this limitation.

The demand for electricity is not constant. It follows a cycle of peaking during the day and then

declining at night. The production of electricity must therefore meet this variable load demand.

There is a base load power demand which is a power requirement that is basically constant. Then

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there is a peak power demand which occurs at certain times of the day, such as when users turn on

their air conditioners and televisions when they come home. Wind and solar are not suitable to

either condition as their power is intermittent, but this source of power could certainly contribute

to the overall power demand. It is expected that conventional power plants will meet the base load

and peak power demands for the foreseeable future. The most efficient units are base loaded and

produce electricity at basically a constant rate. Other less efficient systems must meet the variable

or peak load demands. Since electricity cannot be stored, it must be produced when needed.

Wind power is also suggested as an energy source to combat the "global warming" fear. There is

no doubt that this has a place in the energy picture. But it also has its limitations. There are two

major issues: (1) electric demand is constant, however, wind power is intermittent and (2) electric

demand is local, but major wind power sources are remote.

All other sources of electricity, excluding solar, can run full time meeting the electric demand, but

wind power cannot. Wind farms, where significant amounts of electricity can be economically

produced, must be located at sites where wind is sustainable. In the United States, the best sites are

remote, located in the Great Plains. The intermittent power production and the remoteness of

potential sites create two major cost obstacles: (1) conventional power plants, using fossil fuels,

are still required to meet the electric demand when wind power is unavailable or reduced, and (2)

new and extensive transmission lines will be required from the remote wind farm sites to the areas

of demand.

Wind farms are operational today, but generally these have been located near centers of demand. It

is anticipated that any significant wind development will require that the wind farms be located in

remote locations and therefore extensive transmission lines will have to be constructed.

Transmission lines of 1000 miles are a possibility and this cost is estimated to double the overall

wind power cost.

Transmission lines are seen throughout the country and it is often assumed that electric power is

transmitted over a long distance. But typically, electricity is produced within 100 miles of where it

is needed. Thus the existing grid is primarily local. Not only are the transmission lines expensive

but also the "right of way" for these lines is often difficult to obtain.

Because of the intermittent nature of wind, wind farms are estimated to have a capacity factor of

30 to 40 percent, as compared to 90 percent for a fossil-fired or nuclear power plant. Thus a 70

percent backup power plant, coal- or natural-gas-fired, would be necessary to meet the power

demand. Due to the low availability of wind farms and the need for backup power, and because of

the need for adding extensive transmission lines, it is estimated by some that electricity from a

wind farm could cost 2 to 4 times more than electricity from a conventional power plant.




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Careful studies must be made to evaluate these additional costs against the potential environmental

impact of conventional plants. It is well to remember, as noted above, that because of the expected

low availability of wind or solar power plants, conventional fossil-fired power plants will still be

required to meet the electric demand.

Each alternative energy source has its difficulties to overcome. The impact on the environment is a

very important factor in the final choice of the energy used to produce the needed electricity. But

so is the economical choice of the energy source as the economy requires low-cost electricity in

order to be competitive in the world marketplace. Thus a proper blend of conventional power

plants and of the most economical and practical plants using alternative energy sources must be

made. This will be a significant challenge.

It also should be recognized that the only immediately available large-scale energy source for

electric energy, which does not have emissions associated with "global warming," is nuclear

energy.

The capture and storage of carbon emissions from fossil-fired power plants (if legislated) would

have a significant impact on our energy costs and thus our total economy. In addition, as estimated

by major energy associations, by the year 2025, the cost of natural gas is expected to increase over

50 percent, and electricity costs will increase over 40 percent. It is also estimated that even with

the addition of jobs associated with renewable energy, such as wind and solar, such legislation will

also result in job losses which could exceed over 3 million. If these estimates are in the realm ofpossibility, the costs and job losses are significant and reflect the necessity to carefully evaluate

our country's energy policies and plans for the future.

As noted earlier, approximately 90 percent of the electricity generated in the United States comes

from using coal, natural gas, or nuclear fuel as the energy source. There are those who want to

believe that most, if not all, of the electricity should come from renewable energy sources,

primarily wind and solar.

There are sites where wind and solar power are suitable, and these should be pursued. But we are

talking vast amounts of power generation that must be available to meet the demand. Wind and

solar, as we have seen, are not available constantly, and therefore traditional power plants must

maintain permanently on-line backup generation. A fact, not often considered, is that energy

demand is often at its highest during extreme cold weather in the winter and during high

temperatures in the summer. Both of these weather occurrences are accompanied by high pressure

systems where the wind velocity is minimum, definitely affecting any wind power generation.




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Another factor that must be considered is the location of wind farms and solar power plants. We

have seen areas of the country which have protested the siting of wind farms because of the

obstruction of the natural view of the area. Solar power plants require extensive land areas for their

solar panels. Thus, siting for both wind and solar energy may become a major issue as well.

These issues point out the fact that any human activity, including the production of electricity, will

have some impact on the environment. A careful analysis of the potential environmental impact

and the costs of electricity must be made and compared to the gains produced by any project. Thus

there is no energy source that does not have some impact on our way of life. Each energy source

will play a part to assure that the country will have electricity that is affordable, always available

when we need it, and produced in an environmentally acceptable manner.

Electricity is a major factor in our society as it has significantly improved the quality of life for all

of those who use it. For the foreseeable future, coal will be a dominant energy source for the

production of electricity throughout most of the world. Currently coal produces approximately 50

percent of the electricity generated in the United States and over 40 percent of that produced in the

rest of the world. Even with mandatory capture and storage of CO , electricity from coal is still

expected to be less expensive than other energy sources such as nuclear, natural gas, wind, or

solar. Coal has also cleaned up its act over the past 40 years, even though its use has increased by

nearly 200 percent. Emissions from coal have been reduced over 80 percent in that same time

frame based on regulated emissions.

1.2. The Use of Steam

Steam is a critical resource in today's industrial world. It is essential for the production of paper

and other wood products, for the preparation and serving of foods, for the cooling and heating of

large buildings, for driving equipment such as pumps and compressors, and for powering ships.

However, its most important priority remains as the primary source of power for the production of

electricity.

Steam is extremely valuable because it can be produced anywhere in the world by using the heat

that comes from the fuels that are available in the area. Steam also has unique properties that are

extremely important in producing energy. Steam is basically recycled, from steam to water and

then back to steam again, all in a manner that is nontoxic in nature.

The steam plants of today are a combination of complex engineered systems that work to produce

steam in the most efficient manner that is economically feasible. Whether the end product of this

steam is electricity, heat, or a steam process required to develop a needed product such as paper,

the goal is to have that product produced at the lowest cost possible. The heat required to produce

the steam is a significant operating cost that affects the ultimate cost of the end product.

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In every situation, however, the steam power plant must first obtain heat. This heat must come

from an energy source, and this varies significantly, often based on the plant's location in the

world. These sources of heat could be

1.A fossil fuelcoal, oil, natural gas, or a biomass fuel

2.

A nuclear fuel such as uranium

3.

Other forms of energy, which can include waste heat from exhaust gases of gas turbines; bark,

wood, bagasse, vine clippings, and other similar waste fuels; by-product fuels such as carbon

monoxide (CO), blast furnace gas (BFG), or methane (CH ); municipal solid waste (MSW);

sewage sludge; geothermal energy; and solar energy

Each of these fuels contains potential energy in the form of a heating value, and this is measured in

the amount of British thermal units (Btus) per each pound or cubic feet of the fuel (i.e., Btu/lb or

Btu/ft ) depending on whether the fuel is a solid or a gas. (Note: A British thermal unit is about

equal to the quantity of heat required to raise one pound of water one degree Fahrenheit.)

This energy must be released, and with fossil fuels, this is done through a carefully controlled

combustion process. In a nuclear power plant that uses uranium, the heat energy is released by a

process calledfission. In both cases the heat is released and then transferred to water. This can be

done in various ways, such as through tubes that have the water flowing on the inside. As the water

is heated, it eventually changes its form by turning into steam. As heat is continually added, the

steam reaches the desired temperature and pressure for the particular application.

The system in which the steam is generated is called a boiler, or often commonly called asteam

generator. Boilers can vary significantly in size and design. A relatively small one supplies heat in

the form of steam to a building, and other industrial-sized boilers provide steam for a process.

Very large systems produce enough steam at the proper pressure and temperature to result in the

generation of 1300 MW of electricity in an electric utility power plant. Such a large power plant

would provide the electric needs for over 1 million people.

Small boilers that produce steam for heating or for a process are critical in their importance in

producing a reliable steam flow, even though it may be saturated steam at a pressure of 200 psig

and a steam flow of 5000 lb/h. This then can be compared with the large utility boiler thatproduces 10 million pounds of superheated steam per hour at pressures and temperatures

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exceeding 3800 psig and 1100F. To the operator of either size plant, reliable, safe, and efficient

operation is of the utmost importance. The capacity, pressure, and temperature ranges of boilers

and their uniqueness of design reflect their applications and the fuel that provides their source of

energy.

Not only must the modern boiler produce steam in an efficient manner to produce power (heat,

process, or electricity) with the lowest operational cost that is practical, but also it must perform in

an environmentally acceptable way. Environmental protection is a major consideration in all

modern steam generating systems, where low-cost steam and electricity must be produced with a

minimum impact on the environment. Air pollution control that limits the emissions of sulfur

dioxide (SO ) and other acid gases, particulates, and nitrogen oxides (NO ) is a very important

issue for all combustion processes.

The systems that are required to meet the environmental emissions requirements are quite

complex, and many of these systems are described in Chap. 12. There is no question that

protecting the environment is very important and that it is a very emotional issue. Many media

reports and many environmental groups have presented information from which one could

conclude that there is a crisis in the United States regarding air quality and that additional coal

burning cannot be tolerated. The evidence definitively contradicts this misleading information.

In accordance with data from the Environmental Protection Agency (EPA), the emissions of most

pollutants peaked around 1970. Since this peak, the regulated emissions from coal-based electricitygeneration in the United States have decreased by nearly 40 percent. This improvement has come

about even though the population increased by nearly 50 percent, the GNP nearly doubled, and the

use of fossil fuels increased dramatically. In particular, coal use by power producers nearly

quadrupled from 320 million tons in 1970 to over 1.2 billion tons in 2009, yet the air became

dramatically cleaner.

According to the National Energy Technology Laboratory, power plants which are in operation

today emit 80 to 90 percent less pollutants of SO , NO , particulates, and mercury than the plants

which were operational in the 1970s. Recent data from the EPA shows a significant decline in SO

emissions. Between 1980 and 2008, there has been a 71 percent decrease in SO emissions. This is

a national average and air quality does vary from one area to another, but the trend for emissions is

definitely downward.

The older coal-fired boilers often have been mislabeled as gross polluters, but because of the

requirements imposed by the Clean Air Act, emissions from many of these plants are lower than

those mandated by law.

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2 x

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When power plant emissions have been evaluated for particulate matter and SO since 1970, the

statistics are quite impressive. Particulate emissions have been reduced nearly 94 percent, and SO

reductions are 70 percent. The dramatic reduction in particulates results primarily from replacing

older electrostatic precipitators (ESPs) with fabric filters or high-efficiency ESPs. The use of flue

gas desulfurization (FGD) systems has resulted in the reduction of SO emissions.

The resulting air quality improvements in the United States come with a significant price tag

because over $90 billion has been invested over the past 30 years in FGD systems, fabric filters,

high-efficiency ESPs, selective catalytic reduction (SCR) systems for the reduction of NO , and

other environmental systems. Because of these additions, the cost of electricity in many areas has

increased approximately 10 percent.

Yet, despite these significant improvements in air quality, additional restrictions may be imposed.

These may include restrictions on small particulate matter, mercury, and CO , and systems are

being developed to meet these potential new regulations.

Low NO burners, combustion technology, and supplemental systems have been developed for

systems fired by coal, oil, or natural gas. These systems have met all the requirements that have

been imposed by the U.S. Clean Air Act, and as a result, NO levels have been reduced

significantly from uncontrolled levels.

1.3. The Steam-Plant Cycle

The simplest steam cycle of practical value is called theRankine cycle, which originated around

the performance of the steam engine. The steam cycle is important because it connects processes

that allow heat to be converted to work on a continuous basis. This simple cycle was based on dry

saturated steam being supplied by a boiler to a power unit such as a turbine that drives an electric

generator. (Note: Refer to Chap. 3. Dry saturated steam is at the temperature that corresponds to

the boiler pressure, is not superheated, and does not contain moisture.) The steam from the turbine

exhausts to a condenser, from which the condensed steam, now feedwater, is pumped back into the

boiler. It is also called a condensing cycle, and a simple schematic of the system is shown in Fig.

1.1.

2

2

2

x

2

x

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Figure 1.1. Schematic diagram for a Rankine cycle.

This schematic also shows heat (Q ) being supplied to the boiler and a generator connected to the

turbine for the production of electricity. Heat (Q ) is removed by the condenser as the exhaust

steam is condensed to feedwater, and the pump supplies energy (W) to the feedwater in the formof a pressure increase to allow it to flow through the boiler.

A higher plant efficiency is obtained if the steam is initially superheated, and this means that less

steam and less fuel are required for a specific output. (Superheated steam has a temperature that is

above that of dry saturated steam at the same pressure and thus contains more heat content, called

enthalpy, Btu/lb.) If the steam is reheated and passed through a second turbine, cycle efficiency

also improves, and moisture in the steam is reduced as it passes through the turbine. This moisture

reduction minimizes erosion on the turbine blades.

When saturated steam is used in a turbine, the work required to rotate the turbine results in the

steam losing energy, and a portion of the steam condenses as the steam pressure drops. The

amount of work that can be done by the turbine is limited by the amount of moisture that it can

accept without excessive turbine blade erosion because of the high speed of the turbine (3600

rpm). This steam moisture content generally is between 10 and 15 percent. Therefore, the moisture

content of the steam is a limiting factor in turbine design.

With the addition of superheat, the turbine transforms this additional energy into work without

forming moisture, and this energy is basically all recoverable in the turbine. A reheater often is

used in a large utility plant because it adds additional steam energy to the low-pressure portion of

the turbine, thereby increasing the overall plant efficiency. (Note: The properties of steam are

discussed in Chap. 3.)

By the addition of regenerative feedwater heating, the original Rankine cycle was improved

significantly. This is done by extracting steam from various stages of the turbine to heat the

feedwater as it is pumped from the condenser back to the boiler to complete the cycle. It is thiscycle concept that is used in modern power plants, and the equipment and systems for it will be

described in this book.

in

out

p




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1.4. The Power Plant

The steam generator or boiler is the major part of the many systems that comprise a steam power

plant. A typical pulverized-coal-fired utility power plant is shown schematically in Fig. 1.2. The

major systems of this power plant can be identified as

1.

Coal receipt and preparation

2.

Coal combustion and steam generation

3.

Environmental protection

4.

Turbine generator and electric production

5.

Condenser and feedwater system

6.

Heat rejection, including the cooling tower




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Figure 1.2. Schematic of a typical pulverized-coal-fired utility power plant. Reheater, ash and

reagent handling, and sludge disposal are not shown. ( 2011 The Babcock & Wilcox Company.

All rights reserved.)

In this example, the fuel-handling system stores the coal supply, prepares the fuel for combustionby means of pulverization, and then transports the pulverized coal to the boiler. A forced-draft

(FD) fan supplies the combustion air to the burners, and this air is preheated in an air heater, which

improves the cycle efficiency. The heated air is also used to dry the pulverized coal. A primary air

fan is used to supply heated air to the pulverizer for coal-drying purposes and is the source of the

primary air to the burners as the fuel-air mixture flows from the pulverizers to the burners. The

fuel-air mixture is then burned in the furnace portion of the boiler.

The boiler recovers the heat from combustion and generates steam at the required pressure and

temperature. The combustion gases are generally calledflue gas, and these leave the boiler,

economizer, and finally the air heater and then pass through environmental control equipment. In

the example shown, the flue gas passes through a particulate collector, either an electrostatic

precipitator or a bag filterhouse, to a SO scrubbing system, where these acid gases are removed,

and then the cleaned flue gas flows to the stack through an induced-draft (ID) fan. Ash from the

coal is removed from the boiler and particulate collector, and residue is removed from the

scrubber.

Steam is generated in the boiler under carefully controlled conditions. The steam flows to the

turbine, which drives a generator for the production of electricity and for distribution to the electric

system at the proper voltage. Since the power plant has its own electrical needs, such as motors,

controls, and lights, part of the electricity generated is used for these plant requirements.

After passing through the turbine, the steam flows to the condenser, where it is converted back to

water for reuse as boiler feedwater. Cooling water passes through the condenser, where it absorbs

the rejected heat from condensing and then releases this heat to the atmosphere by means of a

cooling tower. The condensed water then returns to the boiler through a series of pumps and heat

exchangers, calledfeedwater heaters, and this process increases the pressure and temperature of

the water prior to its reentry into the boiler, thus completing its cycle from water to steam and then

back to water.

The type of fuel that is burned determines to a great extent the overall plant design. Whether it be

the fossil fuels of coal, oil, or natural gas, biomass, or by-product fuels, considerably different

provisions must be incorporated into the plant design for systems such as fuel handling and

preparation, combustion of the fuel, recovery of heat, fouling of heat-transfer surfaces, corrosion of

materials, and air pollution control. Refer to Fig. 1.3, where a comparison is shown of a natural-gas

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-fired boiler and a pulverized-coal-fired boiler, each designed for the same steam capacity,

pressure, and temperature. This comparison only shows relative boiler size and does not indicate

the air pollution control equipment that is required with the coal-fired boiler, such as an

electrostatic precipitator or fabric filter and an SO scrubber system. Such systems are unnecessary

for a boiler designed to burn natural gas.

Figure 1.3. Comparison of (a) a natural-gas-fired boiler and (b) a pulverized-coal-fired boiler,

each producing steam at the same capacity, pressure, and temperature. ( 2011 The Babcock &

Wilcox Company. All rights reserved.)

In a natural-gas-fired boiler, there is minimum need for fuel storage and handling because the gas

usually comes directly from the pipeline to the boiler. In addition, only a relatively small furnace is

required for combustion. Since natural gas has no ash, there is no fouling in the boiler because of

ash deposits, and therefore the boiler design allows heat-transfer surfaces to be more closely

spaced. The combination of a smaller furnace and the closer spacing results in a more compactboiler design. The corrosion allowance is also relatively small, and the emissions control required

relates primarily to the NO that is formed during the combustion process. The boiler designed for

natural gas firing is therefore a relatively small and economical design. As we shall see later, the

use of natural gas for the production of electricity is now more commonly used in a combined

cycle cogeneration plant.

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The power plant becomes much more complex when a solid fuel such as coal is burned. Coal and

other solid fuels have a high percentage of ash, which is not combustible, and this ash must be a

factor in designing the plant. A coal-fired power plant must include extensive fuel handling,

storage, and preparation facilities; a much larger furnace for combustion; and wider spaced heat-

transfer surfaces. Additional components are also required:

1.

Sootblowers, which are special cleaning equipment to reduce the impact of fouling and erosion

2.

Air heaters, which provide air preheating to dry fuel and enhance combustion. (Air heaters are

also part of a natural-gas-fired boiler design to improve the efficiency of the plant.)

3.

Environmental control equipment such as electrostatic precipitators, bag filterhouses, and SO

scrubbers

4.

Ash handling systems to collect and remove ash

5.

Ash disposal systems including a landfill

The units shown in Fig. 1.3 are designed for the same steam capacity, but one is designed for

natural gas firing and the other is designed for pulverized coal firing. Although the comparison of

the two units shows only a relative difference in the height of the units, both the depth and the

width of the coal-fired unit are proportionately larger as well.

The operators of power plants are continually investigating various means to increase their

revenues by increasing the efficiency of their plants, by reducing their costs, and by creating other

salable products. This all must be accomplished by reducing the impact of the operation on the

environment. For example, one utility has taken unique steps in the handling and disposing of fly

ash.

This utility has constructed a storage dome that holds approximately 85,000 tons of fly ash, which

is the amount of fly ash produced from this plant in 2 months of operation. The storage dome is

filled in the winter and early spring so that the maximum amount of fly ash is available and used in

place of cement for the production of concrete in the summer months when many construction

projects are active.

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Fly ash is an excellent substitute for cement in concrete. With its use, the following improvements

are found in concrete: strength, durability, permeability, and susceptibility to thermal cracking and

sulfate attack. In the past, small amounts of fly ash have been used in concrete, but recent studies

conclude that concrete containing 50 percent fly ash can be used, and the results show the

significant improvements identified above.

The use of fly ash not only reduces the cost of the concrete but also reduces the landfill costs for

this waste product, which must be disposed in some manner. Therefore, the use of fly ash in

concrete and other unique ideas will continue to be investigated.

1.5. Utility Boilers for Electric Power

Both in the United States and worldwide, the majority of electric power is produced in steam

power plants using fossil fuels and steam turbines. Most of the electric production comes fromlarge electric utility plants, although many of the newer plants are much smaller and owned and

operated by independent power producers (IPPs).

Until the 1980s, the United States and other Western nations developed large electrical networks,

primarily with electric utilities. Over the past several decades in the United States, the increased

annual electricity demand has been met through independent power producers. However, the

United States is not dependent on this IPP capacity. The average electricity reserve margin is

approximately 15 percent. This allows the opportunity to investigate the possible changes of

established institutions and regulations, to expand wheeling of power to balance regional supply,

and to demand and satisfy these low incremental capacity needs in less expensive ways. (Note:

Wheelingis the sale of power across regions and is not restricted to the traditional local-only

supply. Wheeling is a term used to describe the act of transporting electric power over

transmission lines. Electric power networks are divided into transmission and distribution

networks, where transmission lines move electricity from the power plant to a substation, and the

distribution network moves the electricity from the substation to the ultimate users.)

Because power plants have become, in many cases, remote from the electricity user, a more

demanding electrical grid is required, as well as a managing computer and distribution complex to

ensure that electricity is transmitted to the user reliably and efficiently.

Electricity is generated at power plants. It is moved to local substations by transmission lines

which are high-voltage lines supported by metal towers. The United States has a network of these

high-voltage lines which encompasses over 150,000 miles, and these are known as the "electrical

grid." High voltage is used [generally 110,000 volts (110 kV) or above] as an efficient means of

transmission to local substations where transformers are used to reduce the voltage to that required

by the user of the electricity.




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Although there is a focus on developing renewable energy sources such as wind and solar power,

the means of transmitting this electric power to its ultimate users will require the installation of

new high voltage power lines. With the most probable sites for wind and solar being in somewhat

remote areas, a major investment in new power lines must be made to bring the generated power to

where it is needed. The cost for doing this is very high and requires the coordination of federal,

state, and local authorities, as well as the operating utilities.

Many developing countries do not have the luxury of having a reserve margin. In fact, their

electric supply growth is just meeting demand, and in many cases, the electric supply growth is not

close to meeting demand. Power outages are frequent, and this has a serious impact on the local

economy.

As an average for large utility plants, a kilowatt-hour (kWh) of electricity is produced for each

8500 to 9500 Btus that are supplied from the fuel, and this results in a net thermal efficiency for

the plant of 36 to 40 percent. These facilities use steam-driven turbine generators that produce

electricity up to 1300 MW, and individual boilers are designed to produce steam flows ranging

from 1 million to 10 million lb/h. Modern plants use cycles that have, at the turbine, steam

pressures ranging from 1800 to 3500 psi and steam temperatures from 950 to 1100F.

Table 1.1 shows the energy sources that contribute to the electricity generated in the United States.

These sources produce approximately 4200 billion kWh of electricity, and this amount is projected

to be 5000 billion kWh by the year 2030.

In analyzing Table 1.1, the electricity generated from burning the fossil fuels, coal, natural gas, and

oil, contribute approximately 70 percent of the electric production. A portion of the energy from

natural gas powers gas turbine generation plants that incorporate a steam cycle as part of a

cogeneration system. Since nuclear plants also use steam to drive turbines, when added to the

fossil-fuel plant total, over 90 percent of the electricity production comes from steam power plants,

which certainly reflects the importance of steam. Biomass plants also use steam which adds to this

total.

The overwhelming dominant fossil fuel used in U.S. power plants is coal, since it is the energy

source for approximately 50 percent of the electric power produced. There are many types of coal,

as discussed in Chap. 4, but the types most often used are bituminous, subbituminous, and lignite.

Although it is expected that natural gas will be the fuel choice for some future power plants, such

as gas turbine combined-cycle facilities, and that renewable energy sources, such as wind and

solar, will make some inroads, coal will remain the dominant fuel for the production of electricity

in the foreseeable future.

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It is important to understand that the United States has more coal than any other fuel. In fact, over

25 percent of all the known coal in the world is found in the United States. On an energy basis, the

United States has more coal than the Middle East has oil, and its resources are capable of meeting

the demand for more than 200 years.

As discussed later, the use of natural gas will continue to depend on its availability and its cost.

Assuming that availability and cost are favorable, the natural gas share of electricity production is

expected to be relatively constant at 20 percent through the year 2030 with electricity production

from coal reducing slightly to about 47 percent.

Renewable energy is part of Table 1.1 and comes from a variety of sources: solar, wind, municipal

solid waste, and biomass fuels, totaling approximately 3.7 percent. Although the potential for this

energy is quite substantial, they each have their problems and challenges. These renewable energy

sources can be summarized as follows for their current energy production to the total electricity

generated, along with the challenges that each must overcome.

1.

Solar

Electricity production: 0.4 percent

Potential: Estimates from some project that 10 percent of the electricity could be provided

by solar by the year 2030. Others predict very little change to the current total.

Problems: Solar energy is not cost competitive with other forms of energy and relies heavily

on government subsidies. Because of their general remote locations, significant expansion of

the electrical grid would be required. Extensive land areas are required to accommodate

required solar panels and this could be an environmental issue as well as an extensive

maintenance and reliability issue. Conventional power plants still must be available to meet

the electricity demand when the sun is not shining.

2.

Wind


Potential: Estimates project the increase in electricity production could jump to 20 percent of

the U.S. requirements by the year 2030. More conservative estimates show a production

share of 3 to 4 percent of the total electricity generated.




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Problems: Because of the remote locations of probable wind farms, a significant expenditure

will be required for the expansion of the electrical grid. Wind farms, with their many

required wind turbines, may not be acceptable to many in certain areas as they would impact

the natural environment. As with solar energy, conventional power plants must be available

to meet the electrical demand when the wind diminishes.

3.

Geothermal


Potential: Estimates project that power output from this source could double by 2030. Other

estimates predict no increase in its share of the total electric production.

Problems: Possible heat sources are limited and development is expensive.

4.

Solid waste

Electric production: 0.3 percent

Potential: As discussed in Chap. 13, there is an increasing amount of municipal solid waste

that must be disposed of in some manner. The potential energy in this waste is significant,

and waste-to-energy plants have been successfully operating and producing electricity as

well as reducing the landfill requirements. The number of these facilities, however, is

relatively small and any expected growth will be slow in evolving.

Problems: There is a resistance to the siting of new plants, even though dealing with ever

increasing waste is an ongoing problem. Putting waste into landfills is thought by many

politicians as the simple solution, leaving any long term solutions to the future for others to

solve. Recycling is also a "feel good" solution and has had only limited success in various

communities, often leading to additional costs because of few markets for the recyclables,

and thus the recycled material ends up in the landfill.

In Chap. 13, a waste-to-energy facility in Palm Beach County, Florida, is described where

MSW is used as an energy source to produce electric power which, at the same time, reduces

the quantity of waste material that must be disposed in a landfill.




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The total facility, however, is more than a waste-to-energy plant as it encompasses a truly

integrated waste management program incorporating composting, recycling, resource

recovery, and ultimately landfilling those items which cannot be recycled or recovered.

The major elements of this facility include:

a.

Composting: Yard waste is processed into compost for use in horticultural applications.

b.

Ferrous processing: This facility processes ferrous metals such as tin cans and white

goods (e.g., appliances) and this material is sent to manufacturers who melt it down for

use in making new steel products. This facility alone recovers over 30,000 tons of ferrous

metals each year.

c.

Recovered materials: The facility also recovers other materials that can be recycled.

These include containers of various types such as plastic containers; aluminum cans,

plates, and foil; milk and juice cartons; and glass bottles and jars. Paper and cardboard of

various types are collected and recycled. This processing facility can handle nearly 1000

tons per day of these types of materials.

Therefore, the overall complex is a variety of facilities which has been solving the municipal

solid waste problems in that area for many years.

5.

Biomass fuels

Electric production: 1.8 percent

Potential: Growth is expected using various agricultural food crops and wood waste. MSW

is often included in this category.

Problems: Critics complain that biomass fuels put too much demand on food sources and do

nothing to reduce the emissions of greenhouse gases.




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The generation of electricity from renewable sources such as wind, hydroelectric power, and

biomass is expected to increase significantly by the year 2030 to nearly 14 percent of the total

electric generation. Many of these sources of energy are supported by federal tax incentives and

mandatory state renewable programs.

By the year 2030, it is predicted that the amount of electricity generated from nuclear power plants

will increase over 10 percent. The addition of new units and the upgrading of existing nuclear

plants will be necessary to meet this increase. However, it is expected that even with this addition,

the overall share of the total electric generation will decrease slightly from 20 percent to 18

percent.

Operating costs, which are greatly affected by fuel costs, as well as environmental requirements,

will also play a role in determining the future mix of energy sources. The choice of a technology

for a new unit includes the lowest cost for the generation of electricity and the meeting of all

emission standards, both local and federal. For example, coal fired, nuclear, and renewable energy

plants, such as those firing biomass products, are all capital cost intensive with relatively low fuel

costs. Natural-gas-fired plants, such as a cogeneration facility, have lower capital costs; however,

their fuel costs are much higher. Additional transmission line costs are expected to be significant to

support the potential remote wind farms. Future emission requirements may also be imposed, such

as the capture and storage of CO , and this could have a significant impact on the cost and the

eventual selection of an energy source to meet the electrical demand.

The generation of electricity from wind is predicted to nearly triple by the year 2030 to nearly 3

percent of the total generation. The generation from biomass sources is expected to quadruple in

that same time frame to 4.5 percent of the total. The major energy source for biomass is expected

to come from biorefineries which produce ethanol from biomass feedstock. (Biomass is defined as

organic matter which includes wood, wood waste, sludge, agricultural byproducts, and various

types of crops.)

Experts predict that solar power will not make a significant impact in the total electricity generated

because of its cost and limitations on available sites. However, technologies are being developed

and demonstration plants are being built. Thus, solar technology is very possibly an economic and

environmental choice in certain parts of the United States and the world. A solar plant using steam

is described later in this chapter for a facility located in California.

Also in the renewable energy area, hydroelectric power is not expected to add significant

additional capacity because of limited available sites, and therefore, its share of the total electricity

generated will decrease. At present, hydroelectric power provides approximately 6 percent of the

total electric power in the United States. The damming of rivers is the most conventional means of

producing hydroelectric power, and additional major dams are not expected to be built in the near

[2]

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future because optimum sites are not available and environmental concerns make such a plant

prohibitive. However, studies are being made for nonconventional hydropower plants which could

involve the harnessing of the energy from waves, tides, and rivers. With the energy available from

the flow of water, underwater turbines are being investigated for use at various sites. Although this

technology may be applicable for small local use, it is not expected to have a significant impact on

the overall electric power supply.

Hydroelectric power is not without its critics. Public outrage has existed over fish kills and this has

limited new plant construction, and existing dams may be forced to be removed for this same

reason.

On a worldwide basis, a similar pattern for energy sources is present as it is in the United States,

with coal being the predominant fuel for the production of electricity. The approximate share of

these energy sources is as follows:

Coal 41%

Oil 6

Natural gas 20

Nuclear 15

Hydroelectric 16

Other 2

Total 100.0%

Includes solar, wind, biomass, and geothermal.

Although many newer and so-called sophisticated technologies often get the headlines for

supplying the future electricity needs of the world, electric power produced from generated steam,with the use of fossil fuels or with the use of nuclear energy, results in the production of 80 to 90

percent of the electricity required to meet the world's electric demand. Therefore, steam will

continue to have a dominant role in the world's economic future.

Electricity is essential in our modern way of life. Other than food, electricity is the largest

commodity which is purchased in the United States. And, as noted previously, approximately 50

percent of the electricity is generated from coal-fired power plants.

[3]

[3]




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It is estimated that coal reserves in the United States are sufficient to meet the demands of coal-

fired power plants for over 200 years. Although this is a significant energy source, all energy

resources must be utilized to meet the electricity demands of the future. Coal, natural gas, nuclear

power, and renewables, such as wind and solar power, will all be required to contribute in meeting

our energy demands and our environmental standards in a reliable and economical manner.

A low-cost source of electric energy is essential to growing the economy. When energy costs

increase, whether it be electric costs or the cost of gasoline, the impact on other goods and services

becomes very apparent. The U.S. Department of Energy has estimated that a high percentage of

households in the United States pay approximately 25 percent of their total income for energy

related items such as home energy bills and gasoline costs for transportation. This is a significant

cost and it emphasizes that maintaining low energy costs as low as possible is an extremely

important factor in our everyday life. Obviously, more expenditures on energy results in less

amounts for other very important areas such as education, health care, and many other areas which

improve our standard of living.

With the cost of coal expected to be lower than other energy sources, and because of its

availability, coal will remain a dominant fuel for the foreseeable future. Research will continue on

the most effective and economical technologies which are required to capture and store CO , a gas

that is considered to be a primary pollutant in the global warming issue. Even with the possible

additions of those systems and their added costs, coal will remain a viable energy source.

1.5.1. Coal-Fired Boilers

Coal is the most abundant fuel in the United States and in many other parts of the world. In the

United States, the supply of coal resources is estimated to be over 200 years when used at the

current demand. The benefit of its high availability, however, is offset by the fact that it is the most

complicated fuel to burn. Nevertheless, because of the availability of coal and because of its lower

fuel costs, over 600 power plants, comprised of approximately 1600 boilers, use coal in the United

States.

Many problems occur with the systems required to combust the fuel efficiently and effectively as

well as the systems that are required to handle the ash that remains after combustion. Even with

similar coals, designs vary from even one boiler designer because of operating experience and

testing. For different boiler designers, significant differences in design are apparent because of the

designers' design philosophy and the experience gained with operating units.

Despite all the complications that the burning of coal involves, it presents some very interesting

statistics, as developed by the International Energy Agency. Approximately 25 percent of the

world's coal reserves are located in the United States. This represents 90 percent of the total of

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U.S. energy reserves, which include natural gas and oil. As noted previously, approximately 50

percent of the total electricity production in the United States is generated from coal. Coal

production in the United States has increased from 890 million tons in 1980 to over 1200 million

tons in 2008. By the year 2020, coal production is expected to be nearly 1400 million tons.

Low energy costs and protecting the environment are two very important criteria that have to be

reasonably balanced in order to maintain economic growth. The use of coal provides a basis for the

low cost of electricity in the United States as well as other parts of the world where coal is

available. As a comparison of the costs of electricity from fossil fuel energy sources, the U.S.

Energy Information Administration has identified that coal generates electricity at a cost over 2.5

times less than the electricity generated from the use of natural gas and 8 times less than that

generated by oil. (Note that oil-fired power plants produce only slightly more than 1 percent of the

electricity generated.) On a cost per million Btu basis, this comparison looks like this and these

cost comparisons can vary significantly based on the current cost of the energy source:

Coal $2.00

Natural Gas $5.50

Oil $16.70

The economic benefits of having low-cost electricity are significant.

However, the environmental control aspects of coal firing present complexities. These include both

social and political difficulties when trying to locate and obtain a permit for a coal-fired plant that

has atmospheric, liquid, and solid emissions that have to be taken into consideration in the plant

design. Also, there are a wide variety of coals, each with its own characteristics of heating value,

ash, sulfur, etc., that have to be taken into account in the boiler design and all of its supporting

systems. For example, coal ash can vary from 5 to 25 percent by weight among various coals. Of

the total operating costs of a coal-fired plant, approximately 60 to 80 percent of the costs are for

the coal itself.

The large coal-fired power plant utilizes pulverized coal firing, as described in detail in Chaps. 2

and 5. An example of a medium-sized modern pulverized-coal-fired boiler is shown in Fig. 1.4 and

incorporates low NO burners to meet current emission requirements on nitrogen oxides (see Chap.

5). This unit is designed to produce 1,250,000 lb/h of steam at 2460 psig and 1005F/1005F

(superheat/reheat). This unit has the coal burners in the front wall and, as part of the NO control

system, has secondary air ports above the burners. This unit has a two gas pass, three air pass

tubular air heater (see Chap. 2). The FD fan also takes warm air from the top of the building

x

x




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(above the air heater) by means of a vertical duct. This design of the combustion air intake

improves the air circulation within the building as well as using all available heat sources for

improving plant efficiency. The environmental control equipment is not shown in this illustration.

Figure 1.4. Medium-sized pulverized-coal-fired boiler producing 1,250,000 lb/h of steam at 2460

psig and 1005F/1005F (superheat/reheat). (Riley Power, Inc., a Babcock Power, Inc., company.)

A larger pulverized-coal-fired boiler is shown in Fig. 1.5. This illustration shows a boiler system

and its environmental control equipment that produces approximately 6,500,000 lb/h of steam for

an electrical output of 860 MW. This is a radiant-type boiler that is designed to produce both

superheated and reheated steam for use in the turbine. For air heating, it incorporates a

regenerative air heater instead of a tubular air heater. For environmental control (refer to Chap.

12), it uses a dry scrubber for the capture of SO and a baghouse for the collection of particulates.

The boiler shown is designed for indoor use (see building enclosing equipment), but depending on

location, many boilers and their auxiliary systems are designed as outside installations.

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Figure 1.5. Large utility pulverized-coal-fired radiant boiler and its environmental control systems

designed to produce 6,500,000 lb/h of steam for a plant electric output of 860 MW. ( 2011 The

Babcock & Wilcox Company. All rights reserved.)

Coal has a dominant role as a critical fuel in the production of electricity both in the United States

and throughout the world. The use of this fuel brings with it environmental concerns that

encompass the development of cost-effective and efficient systems for the control of pollutants.These pollutants include emissions of solid, liquid, and gaseous wastes.

Coal piles can create fugitive dust problems, as well as storm water runoffs. Following the

combustion of coal, emissions of NO , SO , and particulates all must be controlled within

operating permit limits and the ash from the coal must be properly disposed and contained.

There are many projects in development, under construction, and in operation that will

demonstrate innovative ways to use coal efficiently while meeting strict environmental standards.

One such project, located in Jacksonville, Florida, is shown in Figs. 1.6 and 1.7. This plant consists

of two circulating fluidized bed (CFB) boilers (refer to Chap. 2) with each boiler designed to

produce approximately 2 million lb/h of steam at 2500 psig and 1000F when burning high-sulfur

coal and petroleum coke. The steam flows to a turbine generator, where each unit produces

approximately 300 MW of electricity.

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The ever-increasing demand for electricity and the abundance of coal in the world require that

clean burning technologies be developed and improved upon to ensure that our environment is

protected and that a critical energy resource, coal, is used effectively.

1.5.2. Oil- and Gas-Fired Boilers

The use of oil and gas as fuels for new utility boilers has declined except for certain areas of the

world where these otherwise critical fuels are readily available and low in cost. Large oil-

producing countries are good examples of places where oil- and gas-fired boilers are installed. In

other areas of the world, their use as fuels for utility boilers has declined for various reasons: high

cost, low availability, and government regulations. However, there have been significant

improvements in combined cycle systems that have made the use of oil and more often natural gas

in these systems more cost-effective. In addition, plants that have these gas turbine cycles are more

easily sited than other types of power plants because of their reduced environmental concerns.

However, in the majority of cases, they depend on a critical fuel, natural gas, whose availability for

the long term may be limited.

1.5.3. Steam Considerations

The reheat steam cycle is used on most fossil-fuel-fired utility plants. In this cycle, high-pressure

superheated steam from the boiler passes through the high-pressure portion of the turbine, where

the steam reduces in pressure as it rotates the turbine, and then this lower-pressure steam returns to

the boiler for reheating. After the steam is reheated, it returns to the turbine, where it flows through

the intermediate- and low-pressure portions of the turbine. The use of this cycle increases the

thermal efficiency of the plant, and the fuel costs are therefore reduced. In a large utility system,

the reheat cycle can be justified because the lower fuel costs offset the higher initial cost of the

reheater, piping, turbine, controls, and other equipment that is necessary to handle the reheated

steam.

1.5.4. Boiler Feedwater

When water is obtained from sources that are either on or below the surface of the earth, it

contains, in solution, some scale-forming materials, free oxygen, and in some cases, acids. These

impurities must be removed because high-quality water is vital to the efficient and reliable

operation of any steam cycle. Good water quality can improve efficiency by reducing scale

deposits on tubes, minimize overall maintenance, and improve the availability of the system. All of

this means lower costs and higher revenues.

Dissolved oxygen attacks steel, and the rate of this attack increases significantly as temperaturesincrease. By having high chemical concentrations or high solids in the boiler water and feedwater,

boiler tube deposition can occur, and solids can be carried over into the superheater and finally the




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turbine. This results in superheater tube failures because of overheating. Deposits and erosion also

occur on the turbine blades. These situations are serious maintenance problems and can result in

plant outages for repairs. The actual maintenance can be very costly; however, this cost can be

greatly exceeded by the loss of revenues caused by the outage that is necessary to make the repairs.

As steam-plant operating pressures have increased, the water treatment systems have become more

important to obtaining high availability. This has led to more complete and refined water treatment

facilities.

1.6. Industrial and Small Power Plants

Various industries require steam to meet many of their needs: heating and air conditioning; turbine

drives for pumps, blowers, or compressors; drying and other processes; water heating; cooking;

and cleaning. This so-called industrial steam, because of its lower pressure and temperature ascompared with utility requirements, also can be used to generate electricity. This can be done

directly with a turbine for electric production only, or as part of a cogeneration system, where a

turbine is used for electric production and low-pressure steam is extracted from the turbine and

used for heating or for some process. The electricity that is produced is used for in-plant

requirements, with the excess often sold to a local electric utility.

Another method is a combined cycle system, where a gas turbine is used to generate electric power

and a heat recovery system is added using the exhaust gas from the gas turbine as a heat source.

The generated steam flows to a steam turbine for additional electric generation, and this

cogeneration results in an improvement in the overall efficiency. The steam that is generated also

can be used as process steam either directly or when extracted from the system, such as an

extraction point within the steam turbine.

One of the most distinguishable features of most industrial-type boilers is a large saturated water

boiler bank between the steam drum and the lower drum. Figure 1.8 shows a typical two-drum

design. This particular unit is designed to burn pulverized coal or fuel oil, and it generates 885,000

lb/h of steam. Although not shown, this boiler also requires environmental control equipment to

collect particulates and acid gases contained in the flue gas.




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Figure 1.8. Large industrial-type pulverized coal- and oil-fired two-drum boiler. ( 2011 The

Babcock & Wilcox Company. All rights reserved.)

The boiler bank serves the purpose of preheating the inlet feedwater to the saturated temperature

and then evaporating the water while cooling the flue gas. In lower-pressure boilers, the heating

surface that is available in the furnace enclosure is insufficient to absorb all the heat energy that is

needed to accomplish this function. Therefore, a boiler bank is added after the furnace and

superheater, if one is required, to provide the necessary heat-transfer surface.

As shown in Fig. 1.9, as the pressure increases, the amount of heat absorption that is required to

evaporate water declines rapidly, and the heat absorption for water preheating and superheating

steam increases. See also Table 1.2 for examples of heat absorption at system pressures of 500 and

1500 psig.




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Figure 1.9. Effect of steam pressure on evaporation in industrial boilers. ( 2011 The Babcock &

Wilcox Company. All rights reserved.)

Table 1.2. Heat Absorption Percentages for Water Preheating, for Evaporation, and of Steam

Superheating

500 psig 1500 psig

Water preheating 20% 34%

Evaporation 72 56

Steam superheating 8 10

TOTAL 100% 100%

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The examples shown in the table assume that the superheat is constant at 100 higher than the

saturated temperature for the particular pressure (see Chap. 3).

It is also common for boilers to be designed with an economizer and/or an air heater located

downstream of the boiler bank in order to reduce the flue gas temperature and to provide an

efficient boiler cycle.

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It is generally not economical to distribute steam through long steam lines at pressures below 150

psig because, in order to minimize the pressure drop that is caused by friction in the line, pipe sizes

must increase with the associated cost increase. In addition, for the effective operation of auxiliary

equipment such as sootblowers and turbine drives on pumps, boilers should operate at a minimum

pressure of 125 psig. Therefore, few plants of any size operate below this steam pressure. If the

pressure is required to be lower, it is common to use pressure-reducing stations at these locations.

For an industrial facility where both electric power and steam for heating or a process are required,

a study must be made to evaluate the most economical choice. For example, electric power could

be purchased from the local utility and a boiler could be installed to meet the heating or process

needs only. By comparison, a plant could be installed where both electricity and process steam are

produced and utilized from the same system.

1.6.1. Fluidized Bed Boilers

There are various ways of burning solid fuels, the most common of which are in pulverized-coal-

fired units and stoker-fired units. These designs for boilers in the industrial size range have been in

operation for many years and remain an important part of the industrial boiler base for the burning

of solid fuels. These types of boilers and their features continue to be described in this book.

The fluidized bed boiler has been operational for over 40 years and has become more popular in

modern power plants because of its ability to handle hard-to-burn fuels with low emissions. As a

result, this unique design can be found in many industrial boiler applications and in small utility

power plants, especially those operated by IPPs. Because of this popularity, this book includes the

features of some of the many designs available and the operating characteristics of each.

In fluidized bed combustion, fuel is burned in a bed of hot particles that are suspended by an

upward flow of fluidizing air and flue gas. The fuel is generally a solid fuel such as coal, wood

chips, etc. The fluidizing gas is a combination of the combustion air and the flue gas products of

combustion. When sulfur capture is not required, the fuel ash may be supplemented by an inert

material such as sand to maintain the bed. In applications where sulfur capture is required,

limestone is used as the sorbent, and it forms a portion of the bed. Bed temperature is maintained

between 1550 and 1650F by the use of a heat-absorbing surface within or enclosing the bed.

Fluidized bed boilers feature a unique concept of burning solid fuel in a bed of particles to control

the combustion process, and the process controls the emissions of SO and NO . These designs

offer versatility for the burning of a wide variety of fuels, including many that are too poor in

quality for use in conventional firing systems.

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Chapter 1 - Steam and Its Importance