Materials in Energy Conversion, Harvesting, and Storage

By Kathy Lu

First authored book to address materials' role in the quest for the next generation of energy materials

Energy balance, efficiency, sustainability, and so on, are some of many facets of energy challenges covered in current research. However, there has not been a monograph that directly covers a spectrum of materials issues in the context of energy conversion, harvesting and storage. Addressing one of the most pressing problems of our time, Materials in Energy Conversion, Harvesting, and Storage illuminates the roles and performance requirements of materials in energy and demonstrates why energy materials are as critical and far-reaching as energy itself. Each chapter starts out by explaining the role of a specific energy process in today’s energy landscape, followed by explanation of the fundamental energy conversion, harvesting, and storage processes.

Well-researched and coherently written, Materials in Energy Conversion, Harvesting, and Storage covers:

• The availability, accessibility, and affordability of different energy sources
• Energy production processes involving material uses and performance requirements in fossil, nuclear, solar, bio, wind, hydrothermal, geothermal, and ocean energy systems
• Issues of materials science in energy conversion systems
• Issues of energy harvesting and storage (including hydrogen storage) and materials needs

Throughout the book, illustrations and images clarify and simplify core concepts, techniques, and processes. References at the end of each chapter serve as a gateway to the primary literature in the field.

All chapters are self-contained units, enabling instructors to easily adapt this book for coursework. This book is suitable for students and professors in science and engineering who look to obtain comprehensive understanding of different energy processes and materials issues. In setting forth the latest advances and new frontiers of research, experienced materials researchers and engineers can utilize it as a comprehensive energy material reference book.

• Language: English
• Publisher: Wiley
• Release date: Aug 7, 2014
• ISBN: 9781118892381

Book preview

1

Energy Resources, Greenhouse Gases, and Materials

1.1 Energy Supply and Consumption

Energy has always played an important and inseparable role in human survival and civilization. In the nebulous stage of the early life, men learned to use fire from volcanoes, lightening, and other sources. With the ability to acquire, preserve, and care for fire, men started staying in secured shelters and eating cooked food. Fast forward, starting from the first century AD, steam engines were conceived, advanced, and remained the dominant source of power well into the twentieth century. They are the moving force behind the Industrial Revolution and enjoyed widespread commercial use driving machinery in factories, mills, and mines. By now, different energy sources have been put to our use: coal, nuclear, hydro, solar, etc. We relish in our abilities to make things bigger, faster, more comfortable, and cheaper. Today, with the fast economic growth in highly populated countries such as China and India and our desire to live and work wherever we please (we give it a catchy name globalization), suddenly our thirst for energy seems to generate some problems that have not been envisioned.

While the majority of us agree that energy is becoming a top priority in our standard of living, prosperity, and even national security, opinions still differ on the scope, severity, and urgency of the issue. Some extreme examples are as follows. In Out of Gas (2004), David Goodstein describes an impending energy crisis brought on by The End of the Age of Oil: the crisis will bite, not when the last drop of oil is extracted, but when oil extraction cannot meet demand—perhaps as soon as 2015 or 2025 [1]. The survival of the United States of America as we know it is at risk, Former Vice President Al Gore said in July 2008. In The Skeptical Environmentalist (2001), Bjørn Lomborg, however, claimed that declining energy resources, deforestation, species loss, certain aspects of global warming, and an assortment of other global environmental issues are unsupported by analysis of the relevant data [1]. In light of this, first we need to have a look at why we are debating energy sources and supplies and why we should examine these issues in an urgent sense.

The best starting point to understand the world energy problems is to look at where we acquire all our energy and where we expend them. Figure 1.1 shows the world total energy supply and consumption by source (2008) [2]. Because the energy consumption landscape is changing quickly, the data here may be slightly different from the numbers in the subsequent chapters for individual energy sources. Nonetheless, all sets of data show that fossil fuels are the primary energy source and take a huge majority in the overall energy consumption (Fig. 1.1a). In the total supply pie chart, coal, oil, and natural gases are the three major components of fossil fuels. They take up 27, 33, and 21%, respectively, with a whopping total of 81%. The second energy source comes from combustible renewables and wastes at 10%. This is mainly extracted from local biomass resources such as crop residuals, trees, and animal wastes. The main users of these resources reside in developing or underdeveloped countries as necessities for living. Nuclear energy, even with all the debate raging in the society, only takes about 6%. Hydropower takes only about 2% of the world energy supply; geothermal, solar, wind, and heat sources all together take only about 1%. Overall, we are either digging up the ground or drilling through it to obtain the energy needed to maintain and improve our living standards. The sources present on the Earth surface have been largely untapped or remain inaccessible to us.

c1-fig-0001c1-fig-0001

Figure 1.1 Worldwide energy supply (a) and consumption distributions (b) in 2008.

Data from Ref. 2.

From a different angle, global energy consumption offers a similar picture and includes 10% coal, 42% oil, and 15% gas, at a total of 67% (Fig. 1.1b). In addition, we consume 17% of energy in the form of electricity, which is mostly generated from coal. Overall, our vast majority of energy use is derived from fossil fuels. The rest of the energy consumption distribution mirrors that of the supply. In brief, we consume what we procure. The two sides of the energy coin (supply and consumption) tell the same story.

As to the energy usage in the United States, the data from the U.S. Energy Information Administration [3] presents a very similar picture to the world energy supply (Fig. 1.2) and needs no further discussion here.

c1-fig-0002

Figure 1.2 Energy usage in the United States.

Data from Ref. 2.

1.2 Energy Problems and Challenges

Part of the energy demand increase is fueled by the world population increase; part of it is fueled by the desire to improve living standards and expand economic growth. As of today, it is estimated that the world population is 7.096 billion [4]. When the entire population demands to be fed and clothed at a better quality, it is not surprising that energy demand dramatically increases. As of now, the world’s primary energy consumption has increased to 14 TW-years year−1, almost 50 times the preindustrial level of about 0.3 TW-years year−1. Since 1971, global energy use has risen nearly 70% and is poised to continue its steady increase over the next several decades [5, 6]. To state this in a different manner, demand has risen at over 2% year−1 for the past 25 years and will continue to climb at about the same rate over the next 15 years if current energy use patterns persist, according to the International Energy Agency.

The first problem related to the rising energy demand is the uneven distribution of energy sources (Fig. 1.3). Since coal energy conversion has various environmental complications, different countries lean more toward oil consumption. As a result, the energy markets have been shaken by the instability of Middle East oil. Moreover, developing countries like China are becoming bigger energy consumers, while energy producers like Russia see the opportunity to widen their influence. In this changing landscape, the energy dependency (especially from countries without many reserves) places developed and high energy consumption countries in a highly vulnerable position, both economically and politically. What is thought of as energy geopolitics today has come to encompass much more than physical control over resources to advance state interests. It also refers to the ability to influence or even set world prices for fuels and technologies, and to leverage energy power to achieve a larger political end or ward off interference from other states and nongovernmental bodies. Energy geopolitics can also mean expanding one state’s power and security at the expense of another’s in a zero-sum game.

c1-fig-0003

Figure 1.3 Opposing issues related to energy use.

Another major issue related to rising energy demand is the increasingly adverse, nonrepairable impact on the environment (Fig. 1.3). If we examine the energy deposits today (such as coal), running out of resources does not emerge as the major worry, if they can be redistributed as demanded. Yet there is another worry, greenhouse gas emission, which is becoming more insidious and urgent. Along with rising energy demand and use comes a concomitant increase in greenhouse gas emissions from fossil fuels. As shown in Figure 1.1a, fossil fuels supply >80% of the world’s energy; energy-related emissions account for >80% of the CO2 released into the atmosphere each year. In 2008, the CO2 emission worldwide was 29.4 billion tons with the contribution distribution shown in Figure 1.4. The International Energy Agency estimates that global energy consumption and annual CO2 emissions rise by almost 50% from 1993 levels. The average CO2 emissions from different energy sources are as follows: coal at 1000 g kWh−1, oil at 800 g kWh−1, natural gas at 400–500 g kWh−1, solar at 13–730 g kWh−1, wind at 7–124 g kWh−1, and nuclear at 2–60 g kWh−1 [7]. Again, the math is clear. Heavy reliance on coal, oil, and natural gas comes with a heavy price. CO2 enters the atmosphere mainly through burning of fossil fuels (oil, natural gas, and coal), solid wastes, and trees and wood products. When an overwhelming amount of CO2 is released into the atmosphere, even though a small portion of it can be absorbed by plants as part of the photosynthesis cycle, the rest will stay and accumulate as greenhouse gases.

c1-fig-0004

Figure 1.4 Worldwide CO2 contribution from different sources.

Data from Ref. 2.

Greenhouse gases absorb infrared radiation (heat) heading out from the Earth and re-emits it in a random direction; this random redirection of the atmospheric heat traffic impedes the flow of heat from the planet, just like a quilt. As a result, excessive CO2 emission has a warming effect on the climate and is termed global warming. There have been numerous publications and debates about global warming. We will not spend more efforts on that. It should only be noted that there is now more acceptance in the society that global warming is real; the climate change problem is principally an energy problem.

1.3 Current State of Improving Energy Efficiency

So what needs to be done to solve the world’s increasingly urgent energy problems? The first thing is to stop arguing whether there is an energy problem. Fortunately, there seems to be less of a debate about this nowadays, and a general consensus is in place about the needed efforts to face energy issues. Either from geopolitical point of view or carbon emission point of view, we generally agree that energy needs to be conserved and strategically consumed.

With this admission in place, the next question is how we can more effectively address the energy issues. Different strategies need to be examined and implemented simultaneously, which can be done in three ways: (i) by reducing our population, (ii) by changing our lifestyle, and (iii) by reducing energy intensity through efficiency and technology. Apparently, the first one is not an easy solution with the huge population the world already has, let alone different historical, cultural, political, and economical factors. The second can be done, but it would require difficult, if not painful, choices among our daily conveniences. The third one might enable us to enjoy what we already have in a sustainable way. This means more investment in energy innovation and technology, as well as continuous and long-term energy investment through generations.

Before discussing the strategies to address energy problems, energy sources and uses should be grouped and viewed carefully. There are seven major energy forms as alluded to earlier, namely, fossil, nuclear, solar, biomass, geothermal, hydro, and wind. Comparison of different energy sources can be made as follows [8].

Let’s first look at fossil fuels. The main features for coal are that the sources are finite and its use is rapidly expanding on a global basis. Even though coal is cheap, burning fossil fuels produces dust, smoke, and oxide impurities, which may lead to environmental pollution. Burning fossil fuels also produces CO2, which contributes to the greenhouse effect, warming the Earth. In addition, burning coals produces photochemical pollution from nitrous oxides and acid rain from SO2. As to the reserves, we currently have plenty of coal supplies, with 860 billion tonnes recoverable and available in >70 countries worldwide. As to oil, the total reserve has a different picture. Oil has a finite source, and there are several different categories of oil, each having different costs, characteristics, and, above all, depletion profiles. In terms of global consumption, crude oil remains the most important primary fuel, accounting for 36.4% of the world’s primary energy consumption. However, 47% of the total reserves of conventional oil discovered so far have been consumed. For the cumulative crude oil production until the end of 2005 (143 billion tonnes), half of it was produced within the last 23 years. If the crude oil consumption pace continues as it has been, we only have a few decades of oil left unless new technologies are developed for oil extraction.

Nuclear power is mainly generated using uranium through nuclear fission. The desirable features of nuclear energy are that there is no smoke or CO2 production, and huge amounts of energy can be generated from small amounts of fuel with small amounts of waste. However, waste produced is highly dangerous and must be sealed up and buried for thousands of years to allow the radioactivity to die away. A lot of investment is needed on nuclear reactor safe operation and waste disposal. If anything does go wrong, a nuclear accident can be a major disaster. Even though it is not currently an urgent issue, fuels for nuclear power are not renewable; once all the Earth’s nuclear fuel is dug up and used, there will not be any more nuclear fuel resources.

Solar energy from the Sun is the most abundant source of energy. The annual solar radiation reaching the Earth is over 7500 times the world’s annual primary energy consumption of 450 exajoules. However, currently the energy conversion efficiency for solar radiation is low, ~10%. Large investment is needed for photovoltaic conversion efficiency increase and collector cost reduction.

In its most primitive form, biomass is widely used throughout the world by burning up plant residuals for heating and cooking. Bioethanol extraction from corn and sugar is a mature technology as a fuel. Recently, biomass production through cellulosic decomposition and algae harvesting has generated renewed interests. It is widely believed that bioenergy reduces CO2 emission and is a renewable energy source. However, the competing use of arable land and the potential impact on the environment need to be addressed.

Winds are generated by complex mechanisms involving the rotation of the Earth, heat energy from the Sun, the cooling effects of the oceans and polar ice caps, temperature gradients between land and sea, and the physical effects of mountains and other obstacles. The world’s wind resources are vast. It has been estimated that if only 1% of the land area on the Earth were utilized, and allowance made for wind’s relatively low capacity factor, wind power potential would roughly equate to the current level of worldwide energy-generating capacity. Some of the windiest regions are to be found in the coastal regions of the Americas, Europe, Asia, and Australia. However, large investment cost is required for windmills, and energy storage capability has to be drastically developed for wind energy usage.

Geothermal energy utilizes the natural heat of the Earth’s crust. There are no major perceivable drawbacks from geothermal use. However, it is not a perpetual source of energy as solar, wind, and hydro energies are.

Hydro energy is currently the largest of all perpetual or so-called renewable energy resources. Total world hydro capacity is ~778 GW.

Based on the earlier understanding of different energy forms, energy can be utilized in three responsible ways: (i) We can invest in clean coal technology; (ii) We can invest in other energy sources that can satisfy our appetite for energy, such as nuclear fission; (iii) We can harness different renewable energies. Any of these energy generation strategies will require new and improved knowledge and facilities to happen. We either have to solve existing environmental issues, address new ones, or develop more efficient energy conversion processes.

Up to now, the criteria for energy choice have been based on their availability, accessibility, and affordability. To this list, we must now add three other imperatives: the sources must be sustainable, they should emit a minimum amount of CO2, and they should not pose dangers to global security. In addition, the no CO2 resources have to be made efficient, economical, and available. Unfortunately, primary energy sources are not always of a form that is suitable for the end use. Instead, the sources have to be converted, and this involves wastage of energy and adverse environmental impact.

For the converted (or secondary) energy sources, the most important, versatile, and controllable one is electricity. However, we only have limited ability in electricity storage. Even if we convert the primary energy sources into electricity, there is still an issue of how to store and release it as demand fluctuates.

To increase energy conversion efficiency, harness energy sources in a more responsible way, and open new, sustainable energy sources, many new technologies need to be developed. For example, CO2 capture techniques for coal-fired plants are being explored. For sustainable energy source use, solar energy, wind, and hydropower are active research topics. Different countries have provided varying degrees of subsidiaries to encourage the commercial use of renewable energy sources. There is also active research about more efficient energy storage systems. Such activities add up to efficient energy use efforts.

With this understanding in mind, this book is intended to take a closer look at the available energy conversion systems, examine current state and future development trends of more efficient energy systems, and discuss how renewable and sustainable energy sources can be more effectively utilized. New energy harvesting technology and energy storage options will be presented. The core is to discuss different material functions, usages, and challenges in improving energy conversion, harvesting, and storage performance.

1.4 Inseparable Links between Energy and Materials

Energy and materials have a continual and mutually enriching relationship (Fig. 1.5). In the complex web of energy resource, production, storage, use, and efficiency, materials play a critical role as diverse and far-reaching as energy itself. Materials enable the production of energy or the transformation of primary energy into useful forms. Energy, in turn, has made possible the production of a broad range of materials for the society: from liquid-state fluids, to solid-state devices, and to high-temperature components.

c1-fig-0005

Figure 1.5 The intertwining relationship between energy and materials.

Materials for energy come in a near continuum: naturally occurring materials releasing energy through chemical or nuclear reactions, refractory metals and ceramics used in energy conversion systems, and functional (sometimes nanoscale) materials for energy storage and use. Increasing demand for energy, diminishing stocks of fossil fuels, and the public’s desire to enhance environmental quality, particularly by reducing greenhouse gas emissions, all point to the need for improved materials. For example, generating electricity from the most abundant fossil fuel, coal, efficiently and with no environmental damage, presents notable challenges to developing higher-performance materials in harsh environments. New materials that increase the efficiency of the energy conversion and lower its cost would provide valuable flexibility in material use. Producing electricity with no CO2 emissions is a major frontier for materials research in combustion and gas separation. New nuclear fuels and cladding materials would realize a new generation of safer, more efficient nuclear reactors. Photovoltaic materials convert solar energy into electrical power. Wind turbine blades transform wind energy into mechanical or electrical power. Engineered thermoelectric and mechanoelectric materials can tap otherwise wasted energy and transform it into useful forms. Materials also store and deliver energy in the forms of batteries, supercapacitors, and biofuels. High-performance materials for storing hydrogen would enable more energy efficient vehicles and off-grid operation. It is these fascinating material behaviors and properties that give us the high hope of tackling the challenging energy problems.

1.5 Terms Related to Energy and Power

Before proceeding to different energy systems, it would be helpful to examine some common technical terms related to energy measurement, use, and conversion.

Energy is the capacity to do work; power is the rate at which work is done. Energy is the power used or generated within a given time period and can be defined as

(1.1)

Energy is conserved and measured in kWh or MJ. Energy density is simply the amount of energy per unit weight (gravimetric energy density, expressed in Wh kg−1) or per unit volume (volumetric energy density, expressed in Wh L−1). We cannot actually produce, create, or consume energy; we can merely convert it from one form to another. However, we loosely use the term energy creation, energy production, and energy consumption when we talk about a certain form of energy.

Power is the rate at which work is done and is measured by W (watts), kW (kilowatts), MW (megawatts), GW (gigawatts), or TW (terawatts). One joule per second is called 1 W. One 40 W light bulb, kept switched on all the time, uses 0.96 kWh·day−1. Rate capability is expressed as gravimetric power density (in W·kg−1) and volumetric power density (in W·L−1).

Energy density is the amount of energy stored in a given system or region of space per unit volume. It determines how much energy a given system can accommodate and often times the ability to run certain devices for an expected duration. Power density is defined as power per unit volume or per unit mass. Power density often determines how fast energy can be supplied to a specific demand, which is critical for the operation of a given device.

Capacitance is the ability of a body to store electrical charge. Any object that can be electrically charged exhibits capacitance. Capacitance is directly proportional to the surface area of the conductor plates and inversely proportional to the separation distance between the plates. If the charges on the plates are +q and −q, and V gives the voltage between the plates, then the capacitance C is given by

(1.2)

Specific charge (in Ah·kg−1) is the ratio of stored charge to its mass, and charge density (in Ah·L−1) is a measure of electric charge per unit volume of space, in one, two, or three dimensions.

1.6 Outline of This Book

The outline of this book can be presented as follows. This chapter outlines the energy resources, environmental impacts of rising energy demands, and relations between energy and materials. Chapter 2 will focus on material uses and demands in fossil energy conversion systems. Chapter 3 will address nuclear energy generation and nuclear materials. Chapter 4 will explain solar energy and solar cell material requirements. Chapter 5 will cover different forms of biomass and biofuels and related material issues. Chapter 6 will discuss wind energy and related materials. Chapter 7 deals with materials in hydrothermal, geothermal, and ocean energy conversion. Chapter 8 is focused on different types of fuel cells and material needs. Chapter 9 is related to mechanoelectric energy harvesting. Chapter 10 discusses materials in thermoelectric energy conversion. Chapter 11 examines materials from a different perspective: energy storage. Chapter 12 specifically focuses on hydrogen storage.

References

1. MacKay DJC. Sustainable Energy — Without the Hot Air, Version 3.5.2. Cambridge: UIT Cambridge Ltd; November 2008.

2. International Energy Agency. 2010 Key World Energy Statistics. Paris: International Energy Agency; 2010.

3. Fichman B, Repice R, Adler R, Berry J, Schmitt R, King R, Paduano N, Bonnar D, Lee A, Barrick J, Anderson M, Sweeney A, Young P, Wirman C, Mobilia M, Jacobs G, Lindstrom P, Ko N. Annual Energy Review 2010. Pittsburgh, PA: DOE/EIA; 2011.

4. United States Census Bureau. U.S. and World Population clock. U.S. Department of Commerce. Available at http://www.census.gov/population/popclockworld.html. Accessed July 2, 2013.

5. International Energy Agency. Energy Statistics and Balances. Paris: OECD; 1997.

6. International Energy Agency. World Energy Outlook 1996. Paris: OECD; 1996.

7. Arunachalam VS, Flelscher EL. The global energy landscape and materials innovation. MRS Bull 2008;33(4):264–276.

8. Babu BV. Biomass pyrolysis: a state-of-the-art review. Biofuels Bioprod Biorefin 2008;2(5):393–414.

2

Fossil Energy and Materials

2.1 Fossil Fuels

Fossil fuels refer to any hydrocarbon deposit, such as coal, petroleum, or natural gas, derived from living matter of a previous geologic time and used as a fuel. Overall, fossil fuels are the largest energy resources in today’s society and currently generate over 40% of the world’s electricity.

Historically, coal has been used since the industrial revolution, but only in the last 100 years have huge quantities of oil and gas been removed from underground reservoirs. Oil and gas are used as fuel energy in combustion engines and as feedstock for other industries—raw materials for the manufacture of chemicals, such as plastics and agricultural fertilizers. The energy stored in oil is significantly greater than in any other currently available source. There is no other equivalently cheap and powerful energy.

Regardless of the early use of the fossil fuels, neither coal nor oil is inexhaustible. It takes thousands of years and big geological events for fossil fuel formation; there is a limited amount of fossil fuel; it is not renewable; and there is no known way to make more.

The world benefits from a plentiful supply of coal. Recoverable coal reserves are estimated to be 860 billion tons. Three countries have nearly 60% of the global reserves, the United States being the largest at 240 billion tons, Russia at 157 billion tons, and China at 115 billion tons [1]. Australia and India are also in the top rank. Coal has many uses critically important to economic development and poverty alleviation worldwide—with the most significant being electricity generation, steel and aluminum production, cement manufacturing, as well as use as a liquid fuel. Since year 2000, global coal consumption has grown faster than any other fuel—at 4.9% year−1. The five largest coal users—China, the United States, India, Japan, and Russia—account for around 72% of total global coal use. Looking forward, the use of coal is expected to rise by over 60% by 2030, with developing countries responsible for around 97% of this increase. Most of the consumption is in the power generation sector, with coal’s share in global electricity generation set to increase from 41 to 44% by 2030, according to the International Energy Agency.

At the same time, the availability of oil presents a different scenario. The total crude oil reserves in the world are 1532 billion barrels. The top 10 countries with oil reserves are Saudi Arabia (265 billion barrels), Venezuela (209 billion barrels), Canada (174 billion barrels), Iran (151 billion barrels), Iraq (143 billion barrels), Kuwait (102 billion barrels), United Arab Emirates (98 billion barrels), Russia (60 billion barrels), Libya (48 billion barrels), and Nigeria (39 billion barrels). Significant supplies of oil fuels are found in only a handful of countries around the globe.

In a comparative sense, coal is cheap, widely available, and the cost of power generation from coal conversion is relatively low, at <5 cents kWh−1, approximately one-sixth that of oil or natural gas. Coal continues to be the most heavily used fuel in the world for electric power generation. If we make a bold assumption that all the coal reserves are accessible, by estimate, there is enough coal to last us around 112 years at current rates of production. However, in reality, the actual coal consumption is much higher, and the world population is also increasing, rapidly passing 7 billion [2]. In addition, not all this energy can be utilized. The first loss of energy occurs when coal is converted to other energy formats. For a standard coal-fired power plant, the efficiency to turn the chemical power in the coal into electricity is about 37% or lower, around 30–33% (steam turbine efficiency). Of course, with increasing environmental concerns, coal energy cannot be simply converted without addressing plant emissions, most importantly CO2. If a coal-fired power plant is equipped with carbon capture and storage, it would reduce the electricity generation capacity by about 25% at the same cost. If the coal consumption growth rate is 3.4% year−1 (the growth rate over the last decade), the end of business-as-usual is coming before 2072, <60 years [3]! The rest of the estimation may fluctuate, but we can leave those details for another time.

Different from the more scattered coal deposits, the distributions of oil and natural gas reserves are such that most of the quantities are concentrated in the largest fields and found in few selected countries. The problem of uneven distributions of energy supplies is compounded by the uneven demand for energy. Taking the United States as an example, it comprises only 4.5% of the world’s population but consumes about 22.5% of the world’s total electricity [4]. Because of the uneven oil and natural gas reserve distributions, many production, distribution, and pricing issues have risen over the years, often complicated by national and political issues. Traditionally, the United States has imported much of its energy resources from other countries. Oil from the Middle East is the largest energy import. This has created serious issues. In addition, the production and reported proven reserves have different relationships in different countries. About 66% of the global proven reserves are produced at a rate of about 1.2% year−1, at a reserves-to-production ratio of about 85 years from only six countries, while about 21% of global proved reserves are produced at a rate of about 6% year−1, at a reserves-to-production ratio of about 17 years. The remaining 13% is produced at a rate of about 3.2% year−1, at reserves-to-production ratio of about 32 years. This means these reserves will be exhausted at different times for different regions of the world. As a result, the consumption and export policies vary from country to country.

Even if fossil fuel reserve is not an imminent problem, there are a host of other issues with energy extraction from fossil fuels. The most important one is environmental impact, that is, the generation of greenhouse gases. New technologies are much needed in order to make power generation from coal more efficiently and environmentally friendly. Therefore, the question of fossil fuel use is twofold (Fig. 2.1): (i) how to best utilize the available resources and (ii) how to minimize the environmental impact, more specifically CO2 emission.

c2-fig-0001

Figure 2.1 Interrelationships between fossil fuels, environment, and resource availability.

In this chapter, we will focus on different issues for existing and future coal-fired power plants. Different types of coal-fired power plants will be discussed first and then the respective requirements on materials will follow. Since oil and natural gas industries mainly center around drilling and refining, material issues are primarily related to increasing corrosion resistance, wear resistance, and catalytic activity. As a result, the material discussion for natural gas use will be brief in this chapter. Carbon capture and storage from coal-fired power plants, by themselves, are processes that consume rather than generate energy. Nonetheless, they are closely related to power generation efficiency and sustainability and will be discussed because of their significance in future fossil fuel uses. It should also be pointed out that the exact term related to greenhouse gas emissions should be CO2 capture and storage. In brevity, carbon capture and storage are used widely in the literature, and this book follows the same norm.

2.2 Existing Coal-Fired Power Plants

In most coal-fired power plants, chunks of coal are crushed into a fine powder and fed into a combustion unit where it is mixed with hot air and burned. Heat from the burning coal is used to generate steam through the boiler, which spins one or more turbines to generate electricity. Figure 2.2 shows the schematic of a coal-fired power generation plant [5]. The essential parts are the boiler, the turbine, and the transformer. To avoid turbine corrosion, high-purity water is critical for steam generation. After doing its work in the turbine, the steam is drawn into a condenser, often a large chamber in the basement of the power plant, and converted back into water, which is then pumped back to the boiler. The closed-loop system allows the high-purity water to be used over and over again, preventing contaminants from corroding the turbines and other components. This essential cooling process requires large quantities of water; thus, most steam-electric stations are located on lakes or rivers.

c2-fig-0002

Figure 2.2 Schematic of a coal-fired power generation plant. © Cambridge University Press, 2008.

Reproduced with permission from Ref. 5.

The efficiency of coal-fired power plants ranges from 33% to just above 40% and is a strong function of the steam temperature and pressure [6]. Efforts to increase both the temperature and the pressure have been continuous. The need to reduce CO2 emissions has recently provided an additional incentive to increase energy conversion efficiency and push the limits of coal-fired power plant operating conditions. Broadly, there are three main types of coal-fired power plants based on the steam conditions: subcritical, supercritical, and ultrasupercritical (Fig. 2.3). The first two types are in operation; the last one is in the market entry stage.

c2-fig-0003

Figure 2.3 Historical evolution of steam conditions for coal-fired power plants.

The most common version is subcritical coal-fired power plants, operating at a steam temperature of 540°C and a steam pressure of 16.5 MPa. Subcritical coal-fired power plants were introduced in the 1950s and are a mature technology. However, their energy conversion efficiency is low, at 33–35% (Fig. 2.3). As environmental concerns mount on power generation from coal, a lack of carbon capture capability has arisen as a major problem.

In the 1960s, supercritical technology was introduced; the steam temperatures reached as high as 563°C and the pressures as high as 25 MPa. Generally, supercritical coal-fired power plants are those operating with a steam pressure at 22 MPa or higher and an operating temperature at 540–565°C. This is also a mature technology [7, 8]. Commercial supercritical power plants have 41% efficiency. However, such plants built in the 1970s and 1980s contain built-in redundancy and several levels of safeguards against unplanned downtime. In the 1980s, the level of redundancy and the design margins were decreased in an effort to reduce cost, yet maintain availabilities in the 82–86% range. At the same time, the operating temperature and pressure were continually increased for improved efficiencies. In the 1990s, modular plant designs were developed to enable users to pick and choose the plant configuration from predesigned modules with minimal engineering time. Thus, the steam temperatures of the most efficient coal-fired power plants are now in the 600°C range, representing an increase of about 70°C in 30 years. Nearly two dozen plants have been commissioned worldwide with main steam temperatures at 580–600°C and pressures at 24–35 MPa.

To reduce the adverse environmental impact of current coal-fired power plants, different approaches have been taken regarding clean coal technologies. These include pulverized coal-fired power plants with flue gas desulfurization, circulating fluidized bed combustion plants, and pressurized fluidized bed combustion plants.

For the pulverized coal-fired power plants, power is produced by a condensing steam turbogenerator, using heat released by combustion of finely pulverized coal in a high-temperature flame. The boiler flue gases are cleaned to remove particulates and treated in a flue gas desulfurization plant to remove most of the SO2. NOx emissions can be reduced through the staging of the combustion air within the pulverized fuel burners [9]. For circulating fluidized bed combustion plant, coal is burned in a bed of solids and fluidized by a high-velocity air stream. The off-gases and entrained solids are separated in a cyclone, and the solids are returned to the fluidized bed. Heat is extracted from the combustor and from a waste heat boiler that cools the combustion gas before final clean-up. Superheated steam in the boiler drives a conventional condensing steam turbine, which generates power. Limestone may be added with coal to reduce SO2 emissions, and NOx emissions are inherently low. In pressurized fluidized bed combustion plant, combustion of coal happens in the high-pressure air provided by the compressor section of a gas turbine. Sulfur removal is obtained by directly injecting limestone into the pressurized fluidized bed combustion chamber. A high burnout is achieved at a lower temperature than in a pulverized coal combustor, and this significantly inhibits NOx formation. A further advantage of the lower operating temperature is that furnace slagging and fouling are avoided [9].

While such systems are widely used, advanced combustion technologies seek to further maximize the energy conversion efficiency of power plants through increases in steam temperatures and pressures above the critical point of water. Power plants operating at >565°C are termed ultrasupercritical. Currently, ultrasupercritical technologies are available in the United States to increase the operating temperature to 600°C and the pressure to 28 MPa; Europe and Japan are also introducing new power plants with 620°C and 28 MPa operating conditions. Ultrasupercritical coal-fired power plants have reported efficiencies up to 46%. The John W. Turk, Jr. (Ultrasupercritical) Power Plant, dabbed as one of the cleanest, most efficient coal-fired plants in the United States and the first of its kind in operation in North America, began commercial operations on December 20, 2012 (Fig. 2.4).

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Figure 2.4 John W. Turk, Jr. Power Plant, the first ultrasupercritical coal-fired power plant in operation in North America. Courtesy of AEP Southwestern Electric Power Company

To further increase the energy conversion efficiency, Europe has ongoing efforts to increase the steam operating temperature to 650°C, and the United States aims to increase the operating temperature to 720°C and the pressure to 37.5 MPa. Furthermore, the United States intends to increase the ultrasupercritical steam temperatures and pressures to as high as 760°C and 37.9 MPa for Advanced Power System. This is a challenging goal and requires new materials for realization of an efficiency of at least 46% to make economic sense.

Similar to other systems, the low level of SO2 (0.41 kg MW−1 h−1) in the ultrasupercritical plant emissions is achieved by capture of the sulfur in the wet limestone flue gas desulfurization system. The nominal design of SO2 removal rate is set at 96%. The minimization of NOx production and subsequent emissions (0.29 kg MW−1 h−1) is achieved by zoning and staging of combustion in the low NOx burners, the overfire air staging employed in the design of the boiler, and selective catalytic reduction. Particulate discharge to the atmosphere (0.063 kg MW−1 h−1) is reduced by the use of a fabric filter, which provides a particulate removal rate of 99.9%. CO2 emissions (760 kg MW−1 h−1) are equal to those of other coal-burning facilities since a similar fuel is used. In addition, total CO2 emissions are lower than those for a typical pulverized coal plant at the same capacity due to the higher thermal efficiency. However, higher steam temperatures may result in thicker steamside oxide layers on superheater tubes, which will increase the metal temperature, and thereby the corrosion and creep degradation rate of the steel tubes. These issues are our next topic of discussion.

2.3 Materials for Existing Coal-Fired Power Plants

2.3.1 Material Issues

Material challenges for existing power plants depend primarily on the type of feedstock utilized. Two important aspects are fatigue and creep as well as corrosion.

2.3.1.1 Fatigue and Creep

The key issues for the subcritical and supercritical power plants, which are typically two shifts per day and operate at temperatures well into the creep range for low-alloy steels, are fatigue and creep damage, the latter being influenced greatly by the operating hours, which often times have exceeded their design lives. Steam turbine rotors are among the most critical and highly stressed components. The potential consequences of a rotor failure include blade loss due to disc head failure, spindle fracture from a thermally initiated circumferential crack, and, most detrimentally, fast fracture from a near-bore defect causing a catastrophic burst. There have been only a few instances worldwide of catastrophic bursts of rotors, but the consequences are invariably severe as the fragmented shaft is unlikely to be contained within the casing [10].

Superheater headers, drums, and steam pipes should be rigorously and thoroughly examined [10]. Internal ligament cracking is a common form of thermal fatigue and creep damage affecting superheater headers. Pipes, which are heavy-section components, are subject to fatigue induced by thermal stresses. High temperatures increase component distortion, short-term overheating failures, tube material degradation, disruption of protective oxides, degradation of transition joint integrity, fatigue-induced crack formation in tube walls, and tube misalignment. Thermal fatigue resistance dictates the use of high-strength ferritic or martensitic steels. This has resulted in ferritic steels that are capable of operating at temperatures up to 620°C with good weldability and fracture toughness. However, at higher temperatures, more creep-resistant Ni-based alloys are needed.

In ultrasupercritical plants, the key components are high-pressure pipes, waterwalls, and headers. All of them have to meet creep strength requirements. Austenitic stainless steels are candidate materials for reheaters and superheaters due to their combination of sufficient oxidation resistance, high creep strength, and reasonable price. Low-alloy steels are waterwall tubing materials because of their weldability and higher thermal conductivity. High supercritical pressures and high heat release furnaces can increase the waterwall temperatures to the point that low-alloy steels have insufficient creep strength. For example, STBA20 (0.5Cr0.5Mo) and STBA22 (1.0Cr0.5Mo) have only 35 MPa strength. Higher strength steels are available but require postweld heat treatments. 50Cr50Ni thermal spray and Ni-based alloy weld overlay are sometimes applied to waterwalls to reduce or avoid excessive sulfidation. High Cr ferritic/martensitic steels such as P91 (9Cr1MoVNb), P92 (NF616, 9Cr0.5Mo1.8WVNb), and P122 (HCM12A, 12Cr0.5Mo2WCuVNb) are used in pressure steam piping because of their high creep strength and lower coefficient of thermal expansion [6].

2.3.1.2 Corrosion

For high-temperature applications, steam oxidation of tubes, headers, and pipes are serious concerns. Thick steam oxidation scales formed inside the tubes of the high-temperature zone can increase the tube temperatures and unexpectedly shorten the tube lives. When the thickness of the steam oxidation scales grows significantly, hindered heat transfer can result in many difficulties, for example, a serious increase in outside tube metal temperature, clogging or blockage of tubes, and erosion damage to turbine blades by exfoliated oxide scales. A loss of tube cross section because of corrosion deposits and the resulting heat transfer decrease can cause premature creep failures of tubings. Excessive hot corrosion of ferritic steels caused by liquid iron-alkali sulfates in the tube deposits is an acute concern in the United States for superheaters and reheaters, where high-sulfur corrosive coals are used more frequently. Therefore, high-strength ferritic stainless steels such as P91 (9Cr1MoVNb) are infrequently used in the United States. The standard practice is to use T-22 (2.25Cr1Mo) steels for the lower temperatures and SS304H (8–11% Ni, 18–20% Cr, 0.04–0.10% C) or SS347 (9–13% Ni, 17–19% Cr, 1% Si, 2% Mn, 0.08% C) steel for the higher temperatures.

For hot corrosion of superheater/reheater tubes, the metal temperature, the sulfur content in coal, and the chromium content of the metal are the most important factors to be considered. Ash refers to the noncombustible mineral content of the feedstock, which, depending on the operating temperature of the combustor, will be present either as particulates at temperatures typically <1100°C or as a flowing slag at temperatures typically >1300°C. Ash deposition causes the formation of melting salts while destroying the oxide scales and exposing the metal to the molten salts, consisting of sodium-potassium-iron tri-sulfates (Na,K)3Fe(SO4)3 in the deposit ash.

For waterwall corrosion, the key factors are H2S concentration near the wall, the metal temperature, the sulfur content in coal, and the stoichiometric ratio in the burner zone. They often consist of pure iron oxide, a mixture of iron oxide and iron sulfide, or nearly pure iron sulfide. Although the basic corrosion mechanism is the sulfidation of iron, the process is influenced not only by the structure and composition of the scales and deposits but also by the surrounding gas compositions.

Steam oxidation of austenitic steels strongly depends on the Cr content and grain size. HR3C (25Cr20NiNb) with 25% Cr is the most oxidation resistant at higher temperatures. To avoid sulfur corrosion of headers and piping at ultrasupercritical conditions, stronger austenitic steels or Ni-based alloys would be needed. Weldable high-strength alloy clads or overlays with high Cr contents have to be used to reduce excessive corrosion for ultrasupercritical boilers [6]. For waterwall tubing, low NOx burner systems can cause the corrosion to change from oxidation to sulfidation, thus resulting in a significant increase in the tube wall corrosion, as high as 2 mm year−1.

To improve the steam oxidation resistance of 18Cr8Ni austenitic steels for ultrasupercritical conditions, a shot-blasting technique has been reported. The shot-blasting technique and mechanism of suppressing steam oxide scale growth are shown in Figure 2.5. Shot blasting on the inner tube surface introduces a cold-worked layer, and Cr diffusion is accelerated along the slip bands at high temperatures. The process results in a Cr-rich oxide layer in the early stage of the boiler operation, which can become a strong barrier against further scale formation [8].

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Figure 2.5 Principle of shot blast technique.

2.3.2 Material Development

An example of material development for coal-fired power plant applications is the evolution of ferritic steels. Figure 2.6 shows 100,000 h creep rupture strength at 600°C by year of development [11]. Four generations of ferritic steel evolution have taken place, increasing the creep rupture strength from a mere 35 MPa to nearly 180 MPa. The advancement consists of adding Mo, V, and Nb to simple 9–12% Cr steels in the 1960s and 1970s, with optimization of C, Nb, and V content occurring during the late 1970s and early 1980s. Partial substitution of W for Mo in the late 1980s and early 1990s was followed by additions of more W and Co in the current generation of steels. Cr contributes to solid solution strengthening, as well as to oxidation and corrosion resistance [6]. W, Mo, and Co are primarily solid solution strengtheners. V and Nb contribute to precipitation strengthening by forming fine and coherent precipitates of M(CN) type carbonitrides in the ferritic matrix. V also precipitates as VN during tempering or long-term creep. V and Nb are more effective in combination at levels of about 0.25% V and 0.05% Nb.

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Figure 2.6 Development progress of ferritic steels for boiler; the strength values are for 100,000-h creep strength at 600°C.

Reproduced with permission from Ref. 11. © ASM International (www.asminternational.org), 2005. All rights reserved. With color enhancement.

For low Cr alloys, HCM2S (T-23) is a low-C 2.25Cr1.6 W steel with V and Nb. It is a cost-effective steel with a higher creep strength than T-22. Due to its excellent weldability without preweld or postweld heat treatment, it is a good candidate for waterwall tubing [6].

Among the fully commercialized 9% Cr steels, the P91 (9Cr1MoVNb) steel has the highest allowable stress and has been extensively used as a material for headers and steam pipes in ultrasupercritical plants operating at steam temperatures up to 593°C. Alloy P92 (9Cr1.8W0.5MoNbV), which was developed by substituting part of the Mo in P91 steel with W, has an even higher allowable stress and can be operated at steam temperatures up to 620°C. E911 is a European alloy that is similar in composition to P92 and has similar high-temperature strength capabilities. Beyond 620°C, the 9% Cr steels are limited by their low oxidation resistance; the 12% Cr steels and austenitic steels have to be used. Ferritic steel developments are mostly aimed at their use for thick section pipes.

Among the 12% Cr steels, HT91 (12Cr1MoV) has been widely used for tubing, headers, and piping in Europe. The use of the HT91 steel in Japan and the United States has been limited due to its poor weldability. HCM12 (12Cr1Mo1WVNb) is an improved version of HT91 with 1% W and 1% Mo, and has a duplex structure of α-ferrite and tempered martensite, leading to improved weldability and creep strength. Further increases in creep strength are achieved by substituting more W for Mo and adding Cu. This has resulted in alloy HCM12A (P122, 12Cr2W0.5MoCuVNb), which can be used for headers and pipings up to 620°C. Two alloys, NF12 (12CrWCoNiVNb) and SAVE12 (12CrWCoVNb), have even higher creep strengths than HCM12A though they are still in the developmental stage. W, Co, and some minor elements contribute to strengthening by producing fine and stable nitride precipitates.

Nickel improves the toughness but at the expense of the creep strength of ferritic steels. Partial replacement of Ni by Cu helps to stabilize the creep strength. Carbon is required to form fine carbide precipitates, but the amount needs to be optimized for good weldability. Cobalt is an austenite stabilizer and delays recovery on tempering of martensitic steels. Cobalt also promotes the nucleation of finer secondary carbides on tempering. This is attributed to its effect on the activity of C. Cobalt also slows coarsening of alloy carbides in secondary hardening steels. This may be a result of Co in increasing the activity of C because Co is not soluble in alloy carbides. Cu addition increases precipitation strengthening through fine dispersion of a Cu-rich phase (SUPER304H) or through heat treatment to obtain