Coal-Fired Power Generation Handbook...Coal-Fired Power Generation Handbook Scrivener Publishing 100...

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Page 1: Coal-Fired Power Generation Handbook...Coal-Fired Power Generation Handbook Scrivener Publishing 100 Cummings Center, Suite 541J Beverly, MA 01915-6106 Publishers at Scrivener Martin
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Coal-Fired Power Generation Handbook

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Scrivener Publishing 100 Cummings Center, Suite 541J

Beverly, MA 01915-6106

Publishers at Scrivener Martin Scrivener ([email protected])

Phillip Carmical ([email protected])

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Coal-Fired Power Generation Handbook

James G. Speight

Φ Scrivener Publishing äcriv Publi

W I L E Y

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Copyright © 2013 by Scrivener Publishing LLC. All rights reserved.

Co-published by John Wiley & Sons, Inc. Hoboken, New Jersey, and Scrivener Publishing LLC, Salem, Massachusetts. Published simultaneously in Canada.

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Cover design by Kris Hackerott Library of Congress Cataloging-in-Publication Data:

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Printed in the United States of America

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Contents

Preface xv

Occurrence and Resources 1 1.1 Introduction 1 1.2 Origin of Coal 8 1.3 Occurrence 12 1.4 Coal Utilization and Coal Types 14

1.4.1 Lignite 17 1.4.2 Subbituminous Coal 18 1.4.3 Bituminous Coal 19 1.4.4 Anthracite 21

1.5 Resources 23 1.6 Reserves 27

1.6.1 Proven Reserves 28 1.6.2 Inferred Reserves 30 1.6.3 Potential Reserves 31 1.6.4 Undiscovered Reserves 31 1.6.5 Other Definitions 33

References 34

Classification 37 2.1 Introduction 37 2.2 Classification Systems 40

2.2.1 Geological Age 43 2.2.2 Banded Structure 44 2.2.3 Rank 45 2.2.4 Coal Survey (National Coal Board, U.K.) 52

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2.2.5 International System 52 2.2.6 Coal as an Organic Rock 54 2.2.7 A Hydrocarbon Resource 55

2.3 Coal Petrography 58 2.3.1 Vitrinite Group 61 2.3.2 Liptinite Group 62 2.3.3 Inertinite Group 62

2.4 Correlation of the Various Systems 63 References 67

Recovery, Preparation, and Transportation 69 3.1 Introduction 69 3.2 Coal Recovery 71

3.2.1 Surface Mining 73 3.2.2 Underground Mining 77

3.3 Coal Preparation 79 3.4 Size Reduction 92

3.4.1 Rotary Breaker 95 3.4.2 Roll Crusher 95 3.4.3 Hammer Mill 96 3.4.4 Impactor 96 3.4.5 Tumbler 97

3.5 Coal Cleaning 98 3.5.1 Effect of Composition and Rank 99 3.5.2 Methods 100

3.6 Coal Drying 106 3.6.1 Rotary Dryers 108 3.6.2 Fluidized Bed Dryers 110 3.6.3 Microwave Dryers 111 3.6.4 Screw Conveyor Dryers 112 3.6.5 Superheated Steam Dryer 112

3.7 Desulfurization 113 3.8 Transportation 115

3.8.1 Unit Train 116 3.8.2 Barge 117 3.8.3 Pipeline 117

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3.8.4 Truck 3.8.5 Ocean 3.8.6 Conveyer Belt

References

Storage 4.1 Introduction 4.2 Oxidation and Spontaneous Ignition

4.2.1 Oxidation and Rank 4.2.2 Pyrite 4.2.3 Other Minerals 4.2.4 Coal Size and Stockpile Ventilation 4.2.5 Moisture Content 4.2.6 Time Factor

4.3 Mechanism of Spontaneous Ignition 4.4 Stockpiling

4.4.1 Stockpile Construction 4.4.2 Stockpile Management

4.5 Effect of Storage 4.5.1 Long-Term Storage 4.5.2 Short-Term Storage

4.6 Preventing Spontaneous Ignition References

General Properties 5.1 Introduction 5.2 Sampling

5.2.1 Methods 5.2.2 In Situ Sampling 5.2.3 Ex-Situ Sampling

5.3 Proximate Analysis 5.3.1 Moisture 5.3.2 Natural Bed Moisture 5.3.3 Volatile Matter 5.3.4 Ash 5.3.5 Fixed Carbon

5.4 Ultimate Analysis 5.4.1 Carbon and Hydrogen 5.4.2 Nitrogen

119 120 120 121

127 127 129 133 136 137 137 138 140 140 144 145 146 149 150 150 151 153

159 159 164 166 167 168 171 173 175 177 179 187 187 189 189

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5.4.3 Oxygen 5.4.4 Sulfur 5.4.5 Chlorine 5.4.6 Mercury 5.4.7 Trace Elements

5.5 Calorific Value 5.6 Reporting Coal Analyses References

Physical, Mechanical, and Thermal Properties 6.1 Introduction 6.2 Physical Properties

6.2.1 Coal Type 6.2.2 Density 6.2.3 Pétrographie Analysis 6.2.4 Porosity and Surface Area

6.3 Mechanical Properties 6.3.1 Strength 6.3.2 Hardness 6.3.3 Friability 6.3.4 Grindability 6.3.5 Dustiness Index

6.4 Thermal Properties 6.4.1 Heat Capacity 6.4.2 Thermal Conductivity 6.4.3 Plastic and Agglutinating Properties 6.4.4 Agglomerating Index 6.4.5 Free Swelling Index 6.4.6 Ash Fusion Temperature

6.5 Epilogue References

Combustion 7.1 Introduction 7.2 General Aspects

7.2.1 Coal Devolatilization and Combustion of Volatile Matter

7.2.2 Char Combustion

190 191 192 193 194 195 198 204

211 211 215 216 217 222 223 225 226 227 228 230 234 234 236 237 238 242 242 243 245 245

251 251 255

255 257

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7.3 Chemistry and Physics 7.3.1 Influence of Coal Quality 7.3.2 Mechanism 7.3.3 Ignition 7.3.4 Surface Effects 7.3.5 Reaction Rates 7.3.6 Heat Balance 7.3.7 Soot Formation 7.3.8 Conduction, Convection, and Radiation 7.3.9 Slagging and Fouling 7.3.10 Additives and Catalysts 7.3.11 Excess Air 7.3.12 Coal/Air Transport

7.4 Catalytic Combustion 7.5 Fuels

7.5.1 Coal 7.5.2 Coal Blends 7.5.3 Coal-Oil Fuels 7.5.4 Coal-Water Fuels 7.5.5 Coal-Biomass Fuels

References

Combustion Systems 8.1 Introduction 8.2 Combustion Systems

8.2.1 Stoker Systems 8.2.2 Fixed Bed Systems 8.2.3 Fluidized Beds 8.2.4 Entrained Systems 8.2.5 Miscellaneous Systems

8.3 Fuel Feeders References

Gasification 9.1 Introduction 9.2 General Aspects

9.2.1 Coal Devolatilization 9.2.2 Char Gasification

258 262 266 269 270 272 272 273 274 275 276 278 279 279 280 282 284 290 292 298 307

315 315 319 321 323 330 342 345 352 353

355 355 358 361 362

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364 365 368 370 370 370 371 371 373 374 376 378 381 383 383 383 385 385 386 387 390 390 391 393 396 397

References 401

10 Gasification Systems 407 10.1 Introduction 407 10.2 Fixed-Bed Processes 409

10.2.1 Foster Wheeler Stoic Process 409 10.2.2 Lurgi Process 410 10.2.3 Wellman Galusha Process 412 10.2.4 Woodall-Duckham Process 414

10.3 Fluidized-Bed Processes 416 10.3.1 Agglomerating Burner Process 416 10.3.2 Carbon Dioxide Acceptor Process 416

9.3

9.4 9.5 9.6

9.7

9.8

9.9

Chemistry and Physics 9.3.1 Influence of Coal Quality 9.3.2 Mechanism 9.3.3 Primary Gasification 9.3.4 Secondary Gasification 9.3.5 Shift Conversion 9.3.6 Hydrogasification Catalytic Gasification Plasma Gasification Gasification Systems 9.6.1 Fixed Bed Gasifier 9.6.2 Fluidized Bed Gasifier 9.6.3 Entrained Bed Gasifier 9.6.4 Molten Salt Gasifier Gaseous Products 9.7.1 Low Btu Gas 9.7.2 Medium Btu Gas 9.7.3 High Btu Gas 9.7.4 Methane 9.7.5 Hydrogen 9.7.6 Other Products Gasification of Coal with Biomass and Waste 9.8.1 Biomass 9.8.2 Waste 9.8.3 Reactors Underground Gasification

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10.3.3 Coalcon Process 419 10.3.4 The COED/COGAS Process 420 10.3.5 Exxon Catalytic Gasification Process 421 10.3.6 The Hydrane Process 422 10.3.7 The Hygas Process 422 10.3.8 Pressurized Fluid-bed Process 424 10.3.9 Synthane Process 425 10.3.10 The U-Gas Process 427 10.3.11 Winkler Process 428

10.4 Entrained-Bed Processes 429 10.4.1 Bi-Gas Process 429 10.4.2 Combustion Engineering Process 430 10.4.3 Koppers-Totzek Process 431 10.4.4 The Texaco Process 432

10.5 Molten Salt Processes 434 10.5.1 Atgas process 434 10.5.2 Pullman-Kellogg Process 435 10.5.3 Rockgas Process 436 10.5.4 Rummel Single-Shaft Process 437

10.6 Underground Gasification 437 10.6.1 Borehole Producer Method 439 10.6.2 Chamber Method 440 10.6.3 Shaftless Method 440 10.6.4 Stream Method 441

References 442

11 Electric Power Generation 445 11.1 Introduction 445 11.2 Electricity From Coal 448

11.2.1 Conventional Power Plant 448 11.3 Steam Generation 452

11.3.1 The Steam Generator 455 11.3.2 The Steam Turbines and the Electrical Generator 455 11.3.3 Steam Condensing and Cooling Towers 458

11.4 Control of Emissions 461 11.4.1 Carbon Dioxide Emissions 461 11.4.2 Particulate Matter Emissions 462 11.4.3 Sulfur Dioxide Emissions 463 11.4.4 Mercury Emissions 463

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11.5 Power Plant Efficiency 11.6 Combined Cycle Generation

11.6.1 Cogeneration 11.6.2 IGCC Technology

References

12 Gas Cleaning 12.1 Introduction 12.2 Environmental Legislation 12.3 General Aspects 12.4 Air Pollution Control Devices

12.4.1 Nitrogen Oxide Emissions 12.4.2 Sulfur Oxide Emissions

12.5 Particulate Matter Removal 12.5.1 Electrostatic Precipitators 12.5.2 Fabric Filters 12.5.3 Granular-bed Filters 12.5.4 Scrubber Systems 12.5.5 Cyclones

12.6 Acid Gas Removal 12.7 Removal of Sulfur-Containing Gases 12.8 Removal of Nitrogen-Containing Gases References

13 Environmental Aspects of Power Generation 13.1 Introduction 13.2 Coal Preparation

13.2.1 Water Treatment 13.2.2 Dust Control 13.2.3 Noise Control 13.2.4 Recent Legislation

13.3 Transportation and Storage 13.3.1 Transportation 13.3.2 Storage

13.4 Combustion 13.4.1 Effect of Coal Type 13.4.2 Gaseous Effluents

13.5 Gasification

465 470 470 471 474

477 477 482 485 490 490 491 493 494 495 496 497 504 505 511 515 517

521 521 525 526 526

. 527 527 529 529 531 532 533 534 540

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13.6 Power Plant Waste 547 13.6.1 Coal Ash 551 13.6.2 Hue Gas Desulfurization Waste 568 13.6.3 Waste Heat 570

13.7 The Future 571 References 575

14 Clean Coal Technologies for Power Generation 579 14.1 Introduction 579 14.2 Historical Perspectives 588 14.3 Modern Perspectives 590 14.4 Clean Coal Technology 593

14.4.1 Precombustion Cleaning 597 14.4.2 Cleaning during Combustion 600 14.4.3 Post-combustion Cleaning 606 14.4.4 Conversion and Added-value Products 617

14.5 Managing Wastes from Coal Use 624 14.6 Carbon Dioxide Capture and Sequestration 626 References 637

15 Coal and Energy Security 645 15.1 Introduction 645 15.2 Energy Security 649 15.3 National Energy Plan and Coal Utilization 656 15.4 Electric Power Generation 659 15.5 Hydrogen from Coal 661 15.6 Energy Security and Sustainable Development 664

15.6.1 Continuing Use of Coal 664 15.6.2 Management of Coal Wastes 666 15.6.3 Sustainable Development 668

References 673

Glossary 675 Common Conversion Factors Used in Coal Technology 723 Index 727

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Preface

Coal accounts for approximately one quarter of world energy con-sumption, and of the coal produced worldwide, approximately 65% is shipped to electricity producers and 33% to industrial consum-ers, with most of the remainder going to consumers in the residen-tial and commercial sectors. The total share of total world energy consumption by coal is expected to increase to almost 30% in 2035.

This book describes the steps and challenges by which electric-ity is produced from coal and deals with the challenges for remov-ing the environmental objections to the use of coal in future power plants. New technologies are described that could virtually elimi-nate the sulfur, nitrogen, and mercury pollutants that are released when coal is burned for electricity generation. In addition, technol-ogies for the capture of greenhouse gases emitted from coal-fired power plants are described and the means of preventing such emis-sions from contributing to global warming concerns.

The book is divided into chapters that introduce the reader to:

• The occurrence of coal and the various resources. • The origins of coal that cause differences in coal prop-

erties such as rank and classification. • The properties of coal and the properties that are

particularly relevant to combustion and electricity generation.

• The mechanism of combustion and the various com-bustion systems currently available.

• The conversion of coal to electric power either as a single fuel, a blended fuel, or as a fuel combined with biomass.

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• The manner in which coal properties can influence electricity production.

• The technologies available for cleaning the combus-tion off-gasses to reduce the potential for pollutant emissions.

• The environmental aspects of coal-fired power generation.

• The future of coal-fired electricity generation through upcoming clean coal technologies with an overview of the future of electricity generation from coal.

• The role of coal in energy security scenarios.

The book is written in an easy-to-read style and is also illus-trated by diagrams and tables. It describes the performance of power plants and power generation as influenced by coal proper-ties. Specifically, coal quality impacts not only coal cost, but also net power output, as well as capital and operating and maintenance costs and waste disposal costs.

There is also a comprehensive glossary that will help the reader to understand the various terminologies that are used in this impor-tant energy field.

Dr. James G. Speight, Laramie, Wyoming.

December 2012.

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1 Occurrence and Resources

1.1 Introduction

An ever-expanding human population relates to a corresponding ever-increasing demand for energy to the extent that the world is presently faced with a situation of energy demand exceeding the energy in circulation, even from a variety of sources (Speight, 2008, 2011a). The production and consumption of energy have been asso-ciated with adverse environmental impacts such that the United Nations conference in Kyoto, Japan, in 1997 had to have what is known as the Kyoto Protocol that sets limits on carbon dioxide emissions into the atmosphere (Hordeski, 2008; Irfan et ah, 2010).

Coal (the term is used generically throughout the book to include all types of coal), geographically spread across all inhabitable conti-nents of the world, is a black or brownish-black organic sedimentary rock of biochemical origin that is combustible and occurs in rock strata {coal beds, coal seams) and is composed primarily of carbon with variable proportions of hydrogen, nitrogen, oxygen, and sulfur.

Coal has been a vital energy source to human populations for millennia. For example, in approximately 1000 BC, the Chinese relied on coal to smelt copper that served as the basis for their

1

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2 COAL-FIRED POWER GENERATION HANDBOOK

currency, and the Greek philosopher Aristotle made reference to it in his writings when he alluded to a dark charcoal-like rock (World Coal Institute, 2008). Furthermore, the discovery of coal cinders among Roman ruins in England suggests that the Romans relied on coal as a source of energy prior to 400 AD.

The first written record of coal in the Americas was taken in 1673 by Louis Joliet, who noted carbon de terra while mapping out the Illinois River region. In more recent times, the Nanticoke Indians, a Native American tribe who lived in Pennsylvania, were using local anthracite coal as a source for energy and jewelry during the 1760s (Dublin and Licht, 2005). In the modern world, steam coal, metal-lurgical coal, and industrial coal all play a vital role in the economy of many counties, especially the United States.

Coal continues to power vital industries. The iron industry still relies on basic oxygen furnaces that require a special type of coal, known as metallurgical or coking coal, to produce steel. Coke from coking coal is combined with limestone in a furnace where iron ore is blasted with pure oxygen and converted to steel. However, more pertinent to the present text, the electricity that powers electric arc furnaces is usually generated by burning pul-verized thermal coal.

Coal was the key energy source for the Industrial Revolution, which has provided amenities that most of people take for granted today, including electricity, new materials (steel, plastics, cement, and fertilizers), fast transportation, and advanced communications. Coal replaced wood combustion because of coal's abundance, its higher volumetric energy density, and the relative ease of transpor-tation for coal (Ashton, 1969; Freese, 2003).

The Industrial Revolution itself refers to a change from hand and home production to machine and factory. The first industrial revo-lution was important for the inventions of spinning and weaving machines operated by water power, which was eventually replaced by steam. This helped increase growth and changed late-eighteenth century society and economy into an urban-industrial state. New fuels such as coal and petroleum were incorporated into new steam engines, which revolutionized many industries, including textiles and manufacturing.

The demand for coal decreased for transportation and heating purposes due to intensified competition from petroleum, but activ-ity increased in the post-World War II industrial sector as well as the electricity generation sector after the 1960s. As the demand for

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OCCURRENCE AND RESOURCES 3

power increased, the demand for coal has continued to rise over the years.

The 1973 oil embargo renewed interest in the vast domestic coal reserves of the United States. This sharp rise in coal production helped solve the growing problem of scarce oil resources that were in high demand.

The demand for coal was also impacted by the Power Plant and Industrial Fuel Use Act (FUA) of 1978, which required most oil or natural gas burning power plants to switch to coal. As a result, the energy of the United States became significantly more dependent on coal. After repeal of the Power Plant and Industrial Fuel Use Act in 1987, natural gas use in electric power plants increased by 119 percent between 1988 and 2002. Indeed, the spike in natural gas consumption goes to show the influence the Power Plant and Industrial Fuel Use Act had on increasing the reliance of the United States on coal as a source of energy.

As developing countries such as China and India require more energy to meet their rapidly growing demand, competition for coal will continue increase. The United States has 96% of the coal reserves in North America, which accounts for approximately 26% of the total known coal reserves. As a result, the United States will be expected to export more coal to meet the strong demand from the world market. In doing so, the price of coal will remain stable, and developing countries will be able to meet their energy needs.

Coal is currently responsible for generating approximately 50 percent of the world electricity. In fact, the demand for coal in the United States is primarily driven by the power sector, which con-sumes 90 percent of all domestic coal production. In 1950, however, only 19 percent of coal was used by the power sector due to its high demand by other sectors such as industry, residential and commer-cial, metallurgical coke ovens, transportation, and electric power, which all accounted for an amount on the order of 5 to 25% of the total coal consumption at the time. Of the coal produced worldwide, approximately 65% is shipped to electricity producers and 33% to industrial consumers, with most of the remainder going to consum-ers in the residential and commercial sectors. The total share of total world energy consumption by coal is expected to increase to one third (approximately 30 to 33%) in 2035, although growth rates of coal consumption are not expected to be even in all countries where coal is used as an energy source (International Energy Agency, 2010; Energy Information Administration, 2011,2012a, 2012b).

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4 COAL-FIRED POWER GENERATION HANDBOOK

Coal-fired power plants, also known as electricity generations plants and power stations, provide approximately 42% of U.S. elec-tricity supply and more than over 42% of global electricity supply. In fact, the electricity generation sector is essential to meeting cur-rent and future energy needs (MIT, 2007; Speight, 2008; EIA, 2012a, 2012b; Speight, 2013).

Furthermore, global demand for electricity will continue to rise steeply until at least 2040, as the fuels used for electricity genera-tion continue to shift to lower-carbon sources, such as natural gas, nuclear, and renewables. Even now, the demand for electricity con-tinues to rise in all parts of the world. Population and economic growth are two main reasons, just as they are for the projected demand growth in other fuels. But with electricity, there is the switch to electricity from other forms of energy, such as oil or bio-mass for lighting and heating in the home, or coal in the industrial sector. The key to this growing demand is to make electricity gen-eration more efficient than is currently observed.

Current trends in the electric power market have many coal-fired generators in the United States slated for retirement (Energy Information Administration, 2010, 2011, 2012a, 2012b). Most of the coal-fired power plants projected to retire are older, inefficient units, primarily concentrated in the Mid-Atlantic, Ohio River Valley, and Southeastern United States, where excess electricity generat-ing capacity currently exists. Lower natural gas prices, higher coal prices, slower economic growth, and the implementation of envi-ronmental rules all play a role in the retirements. Coal-fired genera-tors in these regions, especially older, less efficient ones that lack pollution control equipment, are sensitive to changing trends in fuel prices and electricity demand, which are two key factors that influence retirement decisions.

The coal-fired power plants vulnerable to retirement are older power plants generators with high heat rates (lower efficiency) that do not have flue gas desulfurization (FGD) systems installed. Approximately 43% of all coal-fired plants did not have flue gas desulfurization systems installed as of 2010, and such plants will be required to install either a FGD or a dry sorbent injection system to continue operating in compliance with the mercury and air toxics standards (MATS).

Coal capacity retirements are sensitive to natural gas prices. Lower natural gas prices make coal-fired generation less competi-tive with natural gas-fired generation. Because natural gas is often

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OCCURRENCE AND RESOURCES 5

the marginal fuel for power generation, lower natural gas prices also tend to reduce the overall wholesale electricity price, further reducing revenues for coal-fired generators.

Installation of environmental control systems will add internal energy requirements reducing the efficiency of the plant. There are some changes that can be employed to make an existing unit more efficient. However, these changes typically will only result in an improvement to efficiency of a percentage point or so. In order to produce higher efficiency ratings, higher pressure and temperatures are required. This increases the cost of the plant, as special alloy materials will be needed. Technology improvements can assist by lowering the cost of these special materials through discovery and better manufacturing process.

In the context of this book, the more efficient use of coal is the focus, since electricity from coal represents more than 50% of cur-rent electricity generation in the United States.

Electricity generation in coal-fired power station requires com-bustion of the coal, after which the energy released during the com-bustion is used to generate steam, which is then used to drive the turbine generators that produce electricity. The power station can be conveniently divided in five separate but, in reality, integrated operations, which are (1) the combustor or firebox, (2) the boiler, (3) the turbine generator, and (4) the condenser.

Before the coal is burned in the combustor, it is pulverized (often to an extremely small size that has been stated to have the appear-ance of a black talcum powder), after which the coal is mixed with hot air and blown into the combustor. The coal is burned in suspen-sion, which provides the most complete combustion and maximum heat possible.

In the boiler, purified water is pumped through pipes inside the boiler, which converts the water to steam. At temperatures up to 540°C (1000°F) and under pressures up to 3500 psi, the steam is piped to the turbine where it contacts a series of turbine blades and turns the turbine shaft. The turbine shaft is connected to the shaft of the generator, where magnets spin within wire coils to produce electricity. The steam is then drawn into a condenser which con-denses the steam back into water so that it can be used over and over again in the plant. Millions of gallons of cooling water are pumped through a network of tubes that runs through the con-denser, and after the steam is condensed, it is pumped to the boiler again to repeat the cycle.

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6 COAL-FIRED POWER GENERATION HANDBOOK

Coal quality is the term used to refer to the properties and characteristics of coal that influence its behavior and use. Among the coal quality characteristics that are important for coal-fired power plants are the concentrations, distribution, and forms of the many elements contained in the coal feedstock. Knowledge of these quality characteristics in coal deposits may allow us to use this essential energy resource more efficiently and effectively and with less undesirable environmental impact.

Thus, an essential part of power plant development is the rigor-ous analysis of information that should be internally consistent and verifiable, such as coal quality, coal consumption and electricity output. It is, therefore, necessary to understand operating informa-tion for units at coal-fired power plants, not only for the purposes of determining, monitoring, reporting, comparing and projecting coal-fired power plant efficiencies, but also for monitoring carbon dioxide emissions (as well as the emission of other noxious gases and particulate matter).

To develop combustion technology for efficient production of electricity, the influences of the coal properties, such as (1) the ele-mental composition of the coal, (2) rank of the coal, (3) the mineral matter content of the coal, (4) the size of the pulverized coal par-ticles, and (5) the tendency of the combustion system to produce fly ash and bottom slag, are all of considerable importance and need to be addressed in assessing the potential performance of a coal-fired electricity generating plant.

Briefly, the degree of alteration (or metamorphism) that occurs as a coal matures from peat to anthracite is referred to as the rank of the coal. Low-rank coal includes lignite and subbituminous coal, which have a lower energy content (because of the low carbon content) and relatively high moisture content. High-rank coals, including bituminous and anthracite coals, contain more carbon than lower-rank coals, which results in much higher energy content. The high rank coals also have a more vitreous (shiny) appearance and lower moisture content than lower-rank coals.

However, before turning to a fuller description of coal proper-ties and power generation (Chapter 5, Chapter 6), it is necessary to understand the occurrence of coal and whether or not present estimates are sufficient to produce the electric power necessary for the next several decades.

Production of steel accounts for the second largest use of coal. Minor uses include cement manufacture, the pulp and paper

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OCCURRENCE AND RESOURCES 7

industry, and production of a wide range of other products (such as coal tar and coal chemicals). The steel industry uses coal by first heating it and converting it into coke, a hard substance consisting of nearly pure carbon (Speight, 2013). The coke is combined with iron ore and limestone, and then the mixture is heated to produce iron. Other industries use different coal gases emitted during the coke-forming process to make fertilizers, solvents, pharmaceuti-cals, pesticides, and other products.

Finally, in order to generate electric power using the maximum energy in coal, all aspects of a coal need to be understood, includ-ing: (1) handling and storage characteristics, (2) pulverizing behav-ior, (3) combustion behavior, (4) mineral matter and ash chemistry interactions, in addition to the characteristics of the coal and its ash in terms of environmental factors such as dust, self-heating, and emissions components. In order to ensure that quality is controlled, the coal chain must be regularly sampled and adjusted in accor-dance with the analytical results (Chapter 5, Chapter 6). Key control parameters, when monitored, can provide a reliable indication of quality in terms of both specification and consistency requirements.

Finally, the International Energy Agency (IEA) predicts that world energy demand will grow around 60% over the next 30 years, most of it in developing countries. China and India are very large countries in terms of both population and land mass, and both have substantial quantities of coal reserves. Cumulatively, China and India account for 70% of the projected increase in world coal con-sumption. Strong economic growth is projected for both countries (averaging 6% per year in China and 5.4% per year in India from 2003 to 2030), and much of the increase in their demand for energy, particularly in the industrial and electricity sectors, is expected to be met by coal.

Even as demand grows, society expects cleaner energy with less pollution and an increasing emphasis on environmental sustain-ability. The coal industry recognizes it must meet the challenge of environmental sustainability; in particular, it must reduce its green-house gas emissions if it is to remain a part of a sustainable energy future. The quality of coal needs to be assessed so that it can be suit-ably used in different industries. The mineral matter content and its type will give an idea about the coal preparation practice that will be required to be adopted for coal cleaning and subsequent use.

Investigation of physical properties, such as Hardgrove grind-ability index, will help in deciding the type and capacity of

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crushing and grinding machine required in coal beneficiation plants. Spontaneous heating susceptibility studies of coal will help in deciding the type of coal to use in a judicious manner such that the coal is utilized before it catches fire. Keeping this in view of the current text, it will become obvious that determination of coal qual-ity and coal behavior are necessary to ensure that coal is utilized in the most optimal and environmentally acceptable manner.

1.2 Origin of Coal

Discussions of the origin of coal are typically restricted to geochem-ical texts or to more theoretical treatises that focus on coal chem-istry. However, combustion of coal (as performed in a coal-fired power station) involves knowledge of combustion chemistry and the behavior of different coals in coal-fired power stations. Thus, it is the purpose of this section to focus on the origin of coal as it influences coal chemistry, particularly the combustion chemistry and behavior (Chapter 7).

Coal is a combustible sedimentary organic rock that is formed from decayed plant remains and other organic detritus. Although coal forms less than one percent of the sedimentary rock record, it is of foremost importance to the energy requirements of many coun-tries, and the origin of coal as it influences behavior has received much attention (Speight, 2013 and references cited therein). However, coal is also a compact stratified mass of plant debris which has been modified chemically and physically by natural agencies, interspersed with smaller amounts of inorganic matter. The natural agencies causing the observed chemical and physical changes include the action of bacteria and fungi, oxidation, reduc-tion, hydrolysis, and condensation (the effect of heat and pressure in the presence of water).

Coal has also been considered to be a metamorphic rock, which is the result of heat and pressure on organic sediments such as peat. However, the discussion is in favor of coal as a sedimentary rock, because most sedimentary rocks undergo some heat and pressure, and the association of coal with typical sedimentary rocks and the mode of formation of coal usually keep low grade coal in the sedimentary classification system. Anthracite, on the other hand, undergoes more heat and pressure, and is associated with low grade metamorphic rocks such as slate and quartzite. Subducted coal

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OCCURRENCE AND RESOURCES 9

may become graphite in igneous rocks or even the carbonate-rich rocks such as carbonatites, which are intrusive or extrusive igneous rocks characterized by mineralogical composition and consisting of greater than 50% w/w carbonate minerals.

Coal is a sedimentary black or dark brown rock that varies in composition. Some types of coal burn hotter and cleaner, while others contain high moisture content and compounds that, when burned, contribute to acid rain and other pollution. Coals of vary-ing composition are used around the world as a combustible fos-sil fuel for generating electricity and producing steel. Because peat is not a rock and the unconsolidated plant matter is lacking the metamorphic changes found in coal, it is not typically classified as coal. Thus, coal is classified into four main types, depending on the amount of carbon, oxygen, and hydrogen present: (1) lignite, (2) subbituminous coal, (3) bituminous coal, and (4) anthracite.

The degree of alteration (or metamorphism) that occurs as a coal matures from lignite to anthracite is referred to as the rank of the coal, which is the classification of a particular coal relative to other coals, according to the degree of metamorphism, or progressive alteration, in the natural series from lignite to anthracite (ASTM D388).

Because of the chemical process involved in the maturation of coal, it is possible to broadly classify into three major types, namely lignite, bituminous coal, and anthracite. However, because of other differences, and the lack of other differences (with overlap between borderline coals), there is no clear demarcation between the differ-ent coals, and other classifications such as semi-anthracite, semi-bituminous, and subbituminous are also used.

There are two predominant theories that have been proposed to explain the formation of coal: (1) the plant remains which even-tually form coal were accumulated in large freshwater swamps or peat bogs during many thousands of years. This first theory sup-poses that growth-in-place of vegetable material (the autochthonous theory, also often referred to as the swamp theory), and (2) the coal strata accumulated from plants which had been rapidly transported and deposited under flood conditions (the allochthonous theory, also often referred to as the drift theory).

It is believed that major autochthonous (in situ) coal fields gener-ally appear to have been formed either in brackish or fresh water, from massive plant life growing in swamps, or in swampland inter-spersed with shallow lakes. The development of substantial in situ coal measures thus requires extensive accumulations of vegetable

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matter that is subjected to widespread submersion by sedimentary deposits.

However, the types of fossil plants found in coal do not clearly support the autochthonous theory: for example, the fossil lycopod trees (such as Lepidodendron and Sigillaria) and giant ferns (espe-cially Psaronius) that are common in Pennsylvanian coals may have had some ecological tolerance to swampy conditions, yet other Pennsylvanian coal plants (e.g., the conifer Cordaites, the giant scouring rush Calamités, the various extinct seed ferns) by their basic construction may have preferred existence in well-drained soils and not in the proverbial peat swamp. The anatomy of coal plants is considered by many coal geochemists to indicate that ini-tiation of the coalification lay down occurred in a tropical or sub-tropical climate, a conclusion which can be used to argue against autochthonous theory, for modern swamps are most extensive and have the deepest accumulation of peat in the higher-latitude cooler climates.

It was the difference in coal properties of Gondwana (Indian) coals that led to the formation of the drift theory. The mode of deposition of coal forming can be explained as: (1) coal is formed largely from terrestrial plant material growing on dry land and not in swamps or bogs, (2) the original plant debris was transported by water and deposited under water in lakes or in the sea, (3) the transported plant debris, by its relative low density even when waterlogged, was sorted from inorganic sediment and drifted to a greater dis-tance in open water - the sediments, inorganic and organic, settled down in regular succession, (4) the process of sedimentation of the organic and inorganic materials continues until the currents can deposit the transported vegetation in the locations, (5) these depos-its are covered subsequently by mineral matter, sand, and results in coal seams, (6) the depositions can also stop for a particular period and again begin to occur when tidal and current conditions are cor-rect, and (7) even within coal rank, coal properties vary widely due to the varied types of vegetation deposited.

It is also factual that marine fossils such as fish, mollusks, and brachiopods occur in coal. Coal balls, which are rounded masses of matted and exceptionally well-preserved plant and animal fos-sils (including marine creatures) are found within coal strata and associated with coal strata (Mamay and Yochelson, 1962). Since there is little anatomical evidence suggesting that coal plants were adapted to marine swamps, the occurrence of marine animals with

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OCCURRENCE AND RESOURCES 11

non-marine plants suggests mixing during transport, thus favoring the allochthonous model (Rupke, 1969; Cohen, 1970).

Many factors determine the composition of coal: (1) mode of accumulation and burial of the plant debris forming the deposits, (2) age of the deposits and the geographical distribution, (3) struc-ture of the coal forming plants, particularly details of structure that affect chemical composition or resistance to decay, (4) chemical composition of the coal forming debris and its resistance to decay, (5) nature and intensity of the peat decaying agencies, and (6) sub-sequent geological history of the residual products of decay of the plant debris forming the deposits. In short, coal composition is sub-ject to site-specific effects and is difficult to generalize on a global basis (Speight, 2013).

In summary, there are advantages and disadvantages of both theo-ries. While the coal purist may favor one or the other, there are the pragmatists who will recognize the merits of both theories. Whichever theory is correct (if that is possible) and whatever the origin of coal, there are expected to be differences in properties and behavior.

Finally, Hilt's law is a geological term that states the deeper the coal seam, the deeper the rank (grade) of the coal - i.e., anthra-cite would be expected to lie in deeper buried seams than lignite (Figure 1.1). The law holds true if the thermal gradient is entirely vertical, but metamorphism may cause lateral changes of rank, irre-spective of depth. Furthermore, increasing depth of burial results in a decrease in the oxygen content of the coal.

400

350

300

250

200

150

100

50

0

CD 0_ .

π c Ώ

3

c / 1 ^B K Ü

- j . L +Z—\ H Hr^H

ω 0 5,000 10,000 15,000 20,000

Max. burial depth (ft.) 25,000

Figure 1.1 Schematic showing tendency of coal rank to increase with depth of Burial. ^Numbers are Approximate and Used for Illustration Only. Peat is Included only for Comparison and not to Indicate that Peat is a Coal Type.

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1.3 Occurrence

Coal is the world's most abundant and widely distributed fossil fuel and possibly the least understood in terms of its importance to the world's economy. Currently, about five billion tons are mined in more than 40 countries. Coal provinces (clustering of deposits in one area) occur in regional sedimentary structures referred to as coal basins. More than 2,000 sedimentary, coal-bearing basins have been identified worldwide, but less than a dozen contain reserves of more than 200 billion tons.

Although the majority of mined coal continues to be consumed within the country of production, the value of traded coal is increasing. The United States and Australia account for about 50% of world coal exports. This figure increases to 70% if exports from South Africa and Indonesia are included. Japan is the largest recipi-ent of exported coal - approximately 25% of the world coal trade - and as such, agreements with coal suppliers and Japan can have a great influence the world coal price. Japan, Taiwan, and South Korea together import about 45% of all coal exports, and countries of the European Union accounts for another 30% of the total coal exports.

Coal is burned to produce energy: the United States coal still accounts for over 50% of the domestic electricity generat-ing industry requirements, all from domestic production. The European Union, on the other hand, must import approximately 50% of its energy requirements (in the form of oil, gas, uranium, and coal).

Production of coal is both by underground and open pit mining. Surface, large-scale coal operations are a relatively recent develop-ment, commencing as late as the 1970s. Underground mining of coal seams presents many of the same problems as mining of other bedded mineral deposits, together with some problems unique to coal. Current general mining practices include coal seams that are contained in beds thicker than 27 inches and at depths less than 1,000 feet. Approximately 90% of all known coal seams fall outside of these dimensions and are, therefore, not presently economical to mine. Present coal mine technology in the United States, for instance, has only 220 billion tons (220 x 109 tons) of measured proven recoverable reserves out of an estimated total resource of three to six trillion tons (3 to 6 x 1012 tons) tons.