Handbook of Cellulosic Ethanol

Scrivener Publishing100 Cummings Center, Suite 541J

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Publishers at ScrivenerMartin Scrivener (martin@scrivenerpublishing.com)

Phillip Carmical (pcarmical@scrivenerpublishing.com)

Handbook of Cellulosic Ethanol

Ananda S. Amarasekara

Department of Chemistry, Prairie View A&M University, Texas, USA

Copyright © 2014 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 Russell Richardson

Library of Congr ess Cataloging-in-Publication Data:

ISBN 978-1-118-23300-9

Printed in the United States of America

10 9 8 7 6 5 4 3 2 1

v

Contents

Preface xvii

Part 1 Introduction to Cellulosic Ethanol 1

1 Renewable Fuels 31.1 Introduction 3

1.2 Renewable Energy 6

1.3 Biofuels 7

1.3.1 Advantages of Biofuels 8

1.3.2 Gaseous Biofuels 9

1.3.3 Liquid Biofuels 10

1.4 Renewable Energy in the United States 14

1.4.1 Federal Agencies Promoting Renewable Energy 14

1.4.2 Incentives for Renewable Fuels 15

1.5 Renewable Fuel Legislature in the United States 20

1.5.1 Renewable Fuel Standards of Energy

Independence and Security Act of 2007 22

1.5.2 US EPA 2013 Renewable Fuel Standards 22

References 25

2 Bioethanol as a Transportation Fuel 292.1 Introduction — History of Bioethanol

as a Transportation Fuel 29

2.2 Alcohol Fuels 31

2.3 Fuel Characteristics of Ethanol 31

2.3.1 Disadvantages of Ethanol 33

vi Contents

2.4 Corn and Sugarcane Ethanol 34

2.4.1 First Generation Ethanol Production 35

2.5 Advantages of Cellulosic Ethanol 35

References 40

3 Feedstocks for Cellulosic Ethanol Production 433.1 Introduction 43

3.2 Cellulosic Ethanol Feedstock Types 46

3.3 Potential of Agricultural Wastes 46

3.4 Major Crop Residue Feedstock 50

3.4.1 Corn Stover 50

3.4.2 Wheat Straw 57

3.4.3 Rice Straw 60

3.4.4 Sugarcane Bagasse 63

3.4.5 Barley Straw 66

3.5 Forestry Residue, Logging and Mill Residue 68

3.6 Grass Feedstocks 70

3.6.1 Switchgrass 71

3.6.2 Miscanthus Grass 79

3.6.3 Prairie Cordgrass 83

3.6.4 Arundo Donax or Giant Reed 84

3.6.5 Reed Canary Grass 86

3.6.6 Alfalfa 89

3.6.7 Other Grasses 91

3.7 Purpose-Grown Trees as Feedstock 92

3.7.1 Poplar 92

3.7.2 Willows 94

3.7.3 Pines 96

3.7.4 Eucalyptus 98

3.8 Municipal and Other Waste as Feedstock for

Cellulosic Ethanol 101

3.8.1 Municipal Waste Feedstock Utilizing

Cellulosic Ethanol Plants 107

References 108

Contents vii

Part 2 Aqueous Phase Biomass Hydrolysis Route 131

4 Challenges in Aqueous-Phase Biomass Hydrolysis Route: Recalcitrance 1334.1 Introduction – Two Ways to Produce Cellulosic

Ethanol 133

4.2 Challenges in Aqueous-Phase Biomass Hydrolysis 134

4.3 Structure of Plant Cells and Lignocellulosic Biomass 135

4.4 Major Components of Lignocellulosic Biomass 137

4.4.1 Cellulose 137

4.4.2 Hemicellulose 139

4.4.3 Lignin 139

4.5 Cellulose Recalcitrance 140

References 143

5 Pretreatment of Lignocellulosic Biomass 1475.1 Introduction 147

5.2 Different Categories of Pretreatment Methods 150

5.3 Physical Pretreatment 150

5.3.1 Machinery Used in Physical Pretreatment 151

5.3.2 Physical Pretreatment of Woody Biomass 153

5.4 Physicochemical Pretreatment 153

5.4.1 Steam Explosion or Steam Pretreatment 154

5.4.2 Liquid Hot Water (LHW) Pretreatment 159

5.4.3 Ammonia-Based Pretreatments Method 165

5.4.4 Ammonia Fiber/Freeze Explosion (AFEX) 165

5.4.5 Ammonia Recycle Percolation (ARP) 167

5.4.6 Soaking Aqueous Ammonia (SAA) 168

5.4.7 Supercritical Carbon Dioxide Pretreatment 169

5.4.8 Organosolv Pretreatment 171

5.4.9 Ionic Liquid (IL) Pretreatment 174

5.4.10 N-Methyl Morpholine N-Oxide

(NMMO) Pretreatment 176

5.5 Chemical Pretreatment 177

5.5.1 Aqueous Acid Pretreatment 178

5.5.2 Sulfur Dioxide Pretreatment 181

5.5.3 Alkaline Pretreatment Methods 182

5.5.4 Lime Pretreatment 183

viii Contents

5.5.5 Aqueous Alkali Hydroxide-Based

Pretreatments 184

5.5.6 Wet Oxidation Pretreatment 187

5.5.7 Ozone Pretreatment 189

5.6 Biological Pretreatment 190

5.7 Conclusion 191

References 197

6 Enzymatic Hydrolysis of Cellulose and Hemicellulose 2196.1 Introduction 219

6.2 Enzymatic Actions on Lignocellulosic Biomass 220

6.3 Enzymatic Hydrolysis of Cellulose 221

6.3.1 Cellulose Hydrolysis Mechanisms 222

6.3.2 Cellulase Preparation Methods 224

6.3.3 In-Plant Cellulase Production 228

6.3.4 Immobilization of Cellulases 230

6.3.5 Immobilization on Nanoparticles 232

6.4 Enzymatic Hydrolysis of Hemicellulose 233

6.4.1 Hemicellulose Hydrolysis Mechanisms 234

6.4.2 Hemicellulases Preparation Methods 236

6.5 Future Directions in Enzymatic Cellulose

Hydrolysis Research 237

References 239

7 Acid Hydrolysis of Cellulose and Hemicellulose 2477.1 Introduction 247

7.2 Concentrated Acid Hydrolysis 248

7.2.1 Arkenol Process 249

7.2.2 Mechanism of the Concentrated Acid

Hydrolysis 250

7.2.3 Recent Advances in the Concentrated

Acid Process 251

7.3 Dilute Acid Hydrolysis 252

7.3.1 Percolation Reactors 253

7.3.2 Plug Flow Reactors 254

7.3.3 Bed-Shrinking Flow through Reactors 255

Contents ix

7.3.4 Comparisons and Modeling Studies

on Hydrolysis Reactors 256

7.3.5 Recent Advances in Dilute Acid

Process — Different Acids 256

7.4 Ionic Liquid-Based Direct Acid Hydrolysis 262

7.4.1 Acid Catalysts in Ionic Liquid Solutions 262

7.4.2 Solid Acid Catalysts in Ionic Liquid Solutions 263

7.4.3 Acid Group Functionalized Ionic Liquids 264

7.5 Solid Acid Hydrolysis 269

7.5.1 Zeolites 269

7.5.2 Cation-Exchange Resins 270

7.5.3 Metal Oxide Supported Solid Acid Catalysts 271

7.5.4 Carbonaceous Solid Supported Acid Catalysts 271

References 275

8 Fermentation I – Microorganisms 2838.1 Introduction 283

8.2 Detoxifi cation of Lignocellulosic Hydrolyzate 284

8.3 Separate Hydrolysis and Fermentation (SHF) 288

8.4 Microorganisms Used in the Fermentation 288

8.5 Fermentation Using Yeasts 289

8.5.1 Genetic Modifi cation of Yeasts 289

8.6 Fermentation Using Bacteria 294

8.6.1 Genetic Modifi cations of Bacteria 295

8.7 Simultaneous Saccharifi cation and

Fermentation (SSF) 300

8.7.1 SSF Using a Mixture of Saccharifi cation

and Fermentation Microorganisms 300

8.7.2 SSF Using Microorganisms that Can Do

both Saccharifi cation and Fermentation

or Consolidated Bio-Processing (CBP) 307

8.7.3 Heterologous Expression of Cellulase

Genes in Yeast S. cerevisiae for the

Development of CBP 308

8.7.4 Surface-Engineered Yeast Strains for the CBP 312

x Contents

8.7.5 Cell Recycle Batch Fermentation (CRBF) 315

8.7.6 Comparison of Different Fermentation Confi gurations 316

8.8 Immobilization of Yeast 317References 322

9 Fermentation II – Fermenter Confi guration and Design 3399.1 Introduction 339

9.2 Batch Fermentation 340

9.2.1 Examples of Batch Fermentation 340

9.3 Fed-Batch Fermentation 340

9.3.1 Advantages and Disadvantages of

Fed-Batch Fermentation 342

9.3.2 Examples of Fed-Batch Fermentation 343

9.3.3 Types of Fed-Batch Fermentation Reactors 343

9.3.4 Fixed Volume Fed-batch Reactors 343

9.3.5 Variable Volume Fed-batch Reactors 345

9.3.6 Control Techniques for Fed-Batch

Fermentation 345

9.4 Continuous Fermentation 346

9.4.1 Types of Continuous Fermentation Reactors 347

9.4.2 Advantages of Continuous Fermentation 348

9.4.3 Examples of Continuous Fermentation 349

9.4.4 Continuous Fermentation Using

Immobilized Microorganisms 349

9.5 New Directions in Fermenter Confi guration

and Design 352

References 353

10 Separation and Uses of Lignin 35710.1 Introduction 357

10.2 Structure of Lignin 359

10.3 Separation of Lignin in the Cellulosic

Ethanol Process 360

10.4 Physical and Chemical Properties of Lignin 363

10.5 Applications of Lignin 365

10.5.1 Lignin-Based Phenol Formaldehyde Resins 365 10.5.2 Lignin-Phenol co-Polymers as Adhesives 366

Contents xi

10.5.3 Lignin-Polyolefi n Blends 368

10.5.4 Lignin-Polyvinyl Polymer Blends 370

10.5.5 Lignin-Rubber Blends 370

10.5.6 Preparation of Vanillin from Lignin 371

10.5.7 Synthesis of Vanillin-Based Polymers 371

References 373

Part 3 Biomass Gasifi cation Route 381

11 Biomass Pyrolysis and Gasifi er Designs 38311.1 Introduction 383

11.2 Chemistry of the Conversion of Biomass to Syngas 384

11.2.1 Composition of the Syngas 386

11.3 Classifi cations of Biomass Gasifi ers 387

11.4 Fixed-Bed Gasifi er 388

11.5 Fluidized-Bed Gasifi er 389

11.6 Bubbling Fluidized-Bed (BFB) Gasifi er 390

11.7 Circulating Fluidized-Bed (CFB) Gasifi er 392

11.8 Allothermal Dual Fluidized-Bed (DFB) Gasifi er 392

11.9 Entrained-Flow Gasifi er 395

11.10 Syngas Cleaning 396

11.10.1 Hot Gas Cleaning 397

11.10.2 Inertial Separation Using Cyclone 399

11.10.3 Gas Cleaning Filters 400

11.10.4 Electrostatic Separations 401

11.10.5 Cold Wet Gas Cleaning or Conventional

Gas Cleaning 401

11.11 Tar Control and Treatment Methods 403

References 403

12 Conversion of Syngas to Ethanol Using Microorganisms 40712.1 Introduction 407

12.1.1 Advantages and Disadvantages

of Biocatalysis 408

12.2 Metabolic Pathways 410

12.3 Microorganisms Used in Syngas Fermentation 414

12.4 Biochemical Reactions in Syngas Fermentation 414

xii Contents

12.5 The Effects of Operation Parameters on

Ethanol Yield 416

12.5.1 The Effect of Culture Media 416

12.5.2 Effect of Medium pH 417

12.5.3 The Effect of Carbon Source 420

12.5.4 The Effect of Inhibitors and Impurities

in Syngas 420

12.5.5 The Effect of Gas Pressure 423

12.6 Syngas Fermentation Reactors 424

12.7 Industrial-Scale Syngas Fermentation

and Commercialization 426

References 427

13 Conversion of Syngas to Ethanol Using Chemical Catalysts 43313.1 Introduction 433

13.2 Homogeneous Catalysts 434

13.3 Introduction to Heterogeneous Catalysts 437

13.4 Heterogeneous Catalyst Types 437

13.5 Rhodium-Based Catalysts 438

13.5.1 Studies on Reduction of CO and CO2 438

13.5.2 Role of Promoters 440

13.5.3 Role of Catalyst Support 444

13.5.4 New Catalyst Supports 446

13.6 Copper-Based Modifi ed Methanol Synthesis

Catalysts 449

13.6.1 Copper Nanoparticles-Based Catalysts 453

13.7 Modifi ed Fischer-Tropsch Type Catalysts 455

13.8 Molybdenum-Based Catalysts 456

13.9 Catalyst Selection 459

References 461

Contents xiii

Part 4 Processing of Cellulosic Ethanol 467

14 Distillation of Ethanol 46914.1 Introduction 469

14.2 Distillation of the Beer 470

14.3 How Distillation Works 470

14.4 Conventional Ethanol Distillation System 472

14.4.1 Beer Column or Stripping Column 473

14.4.2 Rectifying or Refi ning Column 474

14.4.3 Side Stripper Column 474

14.5 Steam Generation for Distillation Process 475

14.5.1 Design Basis of the Boiler 475

14.6 Studies on Development of Hybrid Systems

for Ethanol Distillation 476

References 479

15 Dehydration to Fuel Grade Ethanol 48115.1 Introduction 481

15.2 Dehydration Methods 482

15.3 Adsorption Method 482

15.3.1 Dehydration of Ethanol Using Zeolite

Molecular Sieves 483

15.3.2 Dehydration of Ethanol Using other

Adsorbents 487

15.4 Azeotropic Distillation Method 488

15.5 Extractive Distillation Methods 491

15.5.1 Extractive Distillation Using High

Boiling Solvents 491

15.5.2 Extractive Distillation Using Salts 493

15.6 Membrane-Based Pervaporation Methods 494

15.6.1 Direct Pervaporation of Ethanol from

Fermentation Broth 496

15.7 Other Dehydration Methods 498

15.8 Comparisons of Common Dehydration Methods 498

References 500

xiv Contents

Part 5 Fuel Ethanol Standards and Process Evaluation 507

16 Fuel Ethanol Standards, Testing and Blending 50916.1 Introduction 509

16.2 Fuel Grade Ethanol Standards in the United States 510

16.3 Quality Assurance and Test Methods 514

16.3.1 Visual Clarity 514

16.3.2 American Petroleum Institute (API) Gravity 514

16.3.3 pHe Level 514

16.3.4 Sulfur Content 515

16.3.5 Total Sulfate 515

16.3.6 Inorganic Chloride 515

16.3.7 ASTM D 5501 – GC Analysis for

Ethanol, Methanol and Denaturant 516

16.4 European Fuel Ethanol Standards 516

16.4.1 European Fuel Grade Ethanol

Standard prEN 15376 518

16.5 Material Safety Data Sheet (MSDS) for

Denatured Fuel Ethanol 518

16.6 Gasoline Ethanol Blends 520

16.6.1 Ethanol Blends Used around the World 523

16.6.2 Blender Pump Blends 523

16.7 Engine Performance Using Gasoline Ethanol Blends 524

References 528

17 Techno-Economic Analysis and Future of Cellulosic Ethanol 53117.1 Introduction 531

17.2 Techno-Economic Aspects of Biomass

Hydrolysis Process 532

17.3 Techno-Economic Aspects of Biomass

Gasifi cation Process 533

17.4 Comparison of Biomass Hydrolysis and

Gasifi cation Processes 539

17.5 Some Cellulosic Plants around the World 540

Contents xv

17.6 Challenges in Cellulosic Ethanol 550

17.6.1 Technological Challenges 550

17.7 Future Prospects of Cellulosic Ethanol 553

References 554

Appendix 1 557

Index 575

xvii

Preface

The inevitable decline in petroleum reserves and the rise in demand for oil from rapidly growing economies have caused soaring oil prices, and coupled with climate change concerns have contributed to the current interest in renewable energy resources. In some parts of the world this interest has resulted in the introduction of legisla-tions promoting the use of renewable energy resources and increas-ing government incentives for commercialization of renewable energy technologies. Development of science and technologies for effi cient conversion of lignocellulosic biomass to renewable liquid transportation fuels has become one of the high priority research areas of the day, and bioethanol is the most successful biofuel to date. Corn- and sugarcane-derived fi rst generation bioethanol is currently in wide use as a blend-in fuel in gasoline sold in the United States, Brazil, and in a few other countries. However, there are a number of major drawbacks to these fi rst generation fuels such as the effect on food prices as traditional food resources are utilized as raw materials, net energy balance, and poor greenhouse gas mitigation.

Cellulosic ethanol is a second generation biofuel produced from agricultural wastes, grasses, municipal wastes, and other feed-stocks that do not double as food, so unlike traditional corn-based ethanol, it promises to avoid encroaching upon and destabilizing the human food supply. In addition, cellulosic ethanol can be pro-duced from a variety of abundant lignocellulosic biomass feed-stocks, and should be able to be produced in substantial amounts to meet the growing global energy demand. There are two fundamen-tal routes to produce cellulosic ethanol from renewable biomass: the aqueous-phase biomass saccharifi cation-fermentation route,

xviii Preface

and thermochemical gasifi cation route. The thermochemical route can be divided into two paths as syngas produced from biomass can be converted to ethanol by chemical or enzymatic methods.

This handbook is a comprehensive up-to-date guide to cellu-losic ethanol, divided into fi ve parts: introduction to cellulosic etha-nol, aqueous-phase biomass hydrolysis route, biomass gasifi cation route, processing of fuel grade ethanol, and techno-economical evaluation of the processes. The fi rst part covering Chapters 1 to 3 introduces the reader to cellulosic ethanol, presenting the advan-tages over fi rst generation corn or sugarcane ethanol. In the United States, the gradual transition to renewable energy sources is sup-ported by a series of legislations and government incentives, and these aspects of bioethanol are also discussed in this part. Then, various types of cellulosic ethanol feedstocks are presented in the third chapter, including agricultural wastes, fast growing grasses such as switchgrass and trees like poplar, forestry residues and municipal wastes.

The second part of the book covering Chapters 4 to 10 presents the cellulolysis processes or aqueous-phase biomass saccharifi ca-tion-fermentation route. Chapters 4 and 5 detail the challenges in biomass saccharifi cation, or recalcitrance, as well as various pre-treatment techniques such as physical, physicochemical, chemical, and biological pretreatments, and applications to different feed-stocks. Chapter 6 covers the enzymatic saccharifi cation, including cellulases, hemicellulase families, mechanisms, enzyme prepara-tion methods, and immobilization of enzymes. Chapter 7 is dedi-cated to acid hydrolysis, or direct saccharifi cation, using various acid catalysts: concentrated, dilute mineral acids, progress in ionic liquid-based systems, acid group functionalized ionic liquids, and solid acids. Fermentation of the sugar solution to “beer” is pre-sented in Chapters 8 and 9. The microorganisms used in the fer-mentations, including recent advances in genetic modifi cations of microorganisms, separate hydrolysis fermentation (SHF), simulta-neous saccharifi cation and fermentation (SSF), consolidated bio-processing (CBP), and surface-engineered and immobilized yeasts are covered in detail in Chapter 8. Fermentation confi gurations and engineering aspects of fermenter design are presented in Chapter 9. In addition, separation and utilization of lignin byproduct is also covered in this section, under Chapter 10, introducing the total bio-refi nery concept.

Preface xix

The third part of the book (Chapters 11–13) is dedicated to the biomass gasifi cation route, which is an alternative approach for producing ethanol from lignocellulosic biomass. Pyrolysis chemis-try, gasifi er designs, and syngas cleaning are covered in Chapter 11, whereas the conversion of syngas to ethanol using microorganisms and their metabolic pathways are presented in Chapter 12. Syngas produced from biomass can be transformed into ethanol using metal catalysts such as Rh-, Mo- or Cu-based systems as well, and this route is presented in Chapter 13.

Processing of ethanol produced thorough various paths is pre-sented in Part 4 of the book. Concentration of ethanol to approxi-mately 90% ethanol by distillation is the fi rst step in purifi cation of ethanol. Technologies used in the current fi rst generation etha-nol industry for distillation of the “beer” to an azeotrope mixture, and then dehydration to > 99.5% fuel grade ethanol, are adoptable to cellulosic ethanol as well. Engineering aspects of the industrial three column distillation set up and recent technological advances like pervaporation are discussed in Chapters 14 and 15. Part 5 pro-vides the details of fuel ethanol standards and process evaluation. Fuel grade ethanol standards in the US and EU, testing methods, and quality control are some of the sections in Chapter 16. Finally, the techno-economic aspects of cellulosic ethanol, a list of current and under-construction cellulosic ethanol plants around the world as of June, 2013, and the future prospects of cellulosic ethanol are presented in Chapter 17.

It is my great pleasure to thank Scrivener Publishing and John Wiley & Sons for kindly agreeing to publish this book. Finally, I wish to thank my wife Preethika, daughter Hiruni, and son Hasun for their love, support, encouragement, and patience during the writing of this book.

Ananda S. Amarasekara June, 2013

Part 1

Introduction to Cellulosic Ethanol

3

1Renewable Fuels

1.1 Introduction

Since the beginning of civilization on earth, humans have used biomass for many of their energy needs such as cooking, heating dwellings, lighting, fi ring clay pots, and processing metals. The industrial revolution, leading to the development of the inter-nal combustion engine for transportation and coal power plants for electricity generation have caused a rapid shift in our energy dependence from renewable resources to non-renewable fossil fuel resources. The processes of industrialization and continuous eco-nomic development are driven by energy consumption. The global demand for energy is expected to increase at a faster rate in upcom-ing years due to rapidly developing economies and partly due to the exponential growth in the world’s population.

The energy demand predictions for the Organization for Economic Cooperation and Development (OECD) nations as well as for non-OECD nations are available in the International Energy Outlook report of the U.S. Department of Energy. The world energy consumption from 1990 with predictions till 2035 is shown in the

4 Handbook of Cellulosic Ethanol

bar graph in Figure 1.1 [1]. This study forecasts that total world energy use will rise to 619 quadrillion BTU (British thermal unit) in 2020, and 770 quadrillion BTU in 2035 from the 2008 value of 505 quadrillion BTU. Furthermore, much of the growth in energy con-sumption is expected to occur in countries outside the Organization for Economic Cooperation and Development (non-OECD nations) where demand is driven by strong long-term economic growth. Energy use in non-OECD nations increases by 85 percent in the reference case, as compared with an increase of 18 percent for the OECD economies as shown in Figure 1.1 [1].

At a time of rapid increase in global energy consumption, energy sources are a critical term in the energy equation. As of 2012, more than 80% of the world’s energy needs are fulfi lled by fossil fuels and the contributions to global energy demand from different resources are presented in the Global Renewable Energy Share Report; the current shares of principal resources are shown in the pie chart in Figure 1.2 [2]. Total renewable energy share is 16.7%, and these sources can be divided into two groups: traditional renewables and modern renewables. On the global scale, the share of tradi-tional renewables is slightly higher than all the combined modern renewables. Traditional biomass energy sources such as fi rewood,

Figure 1.1 World energy consumption from 1990–2035 (quadrillion BTU) from International Energy Outlook report released in 2011 [1].

01990 2000 2008 2015 2020 2025 2030 2035

200

400

600

800E

nerg

y (q

uadr

illio

n B

TU

)

Non-OECD

OECD

Renewable Fuels 5

which are used primarily for cooking and heating in rural areas of developing countries, could be considered renewable. These tra-ditional renewables account for approximately 8.5% of total fi nal energy consumption. Modern renewable energy is dominated by hydropower for electricity generation and accounts for 3.3%; heat generation using modern biomass-derived fuels such as biogas, geothermal and solar heating accounts for another 3.3%. Biomass-derived transportation biofuel such as bioethanol and biodiesel supplies only 0.7% of the current global energy requirement.

While fossil fuels have become the world’s main energy resource and are at the center of global energy demands, its reserves are lim-ited. There are varying estimates of fossil fuel reserves on earth. In spite of all the recent advances in oil exploration technologies, the frequency of new oil and coal discoveries has rapidly dimin-ished in the last twenty years. In cases like shale oil and fracking, much higher efforts and investments are required for extraction of fossil fuel from earth. As a fi nite resource depletion of petroleum reserves is inevitable, limitations in the supply have resulted in a rapid increase in fuel prices around the globe after the 1970s.

However, according to the World Energy Outlook 2012 predic-tions, a steady increase in hydropower and rapid expansion of wind and solar power has cemented the position of renewables as an indispensable part of the global energy mix. By 2035, renewables

80.6% Fossilfuel

8.5%Traditionalrenewables

8.2% Modernrenewables

2.7%nuclear

3.3% Biomass/solar/geothermalhot water/heating3.3% Hydropower0.9% Wind/solar/biomass/geothermal powergeneration0.7% Biofuels

Figure 1.2 Renewable energy share of global fi nal energy consumption, from Renewable Energy 2012 Global status report, Paris [2].

6 Handbook of Cellulosic Ethanol

are expected to account for almost one-third of total electricity out-put [3]. Solar power is expected to grow more rapidly than any other renewable energy technology. Furthermore, in accordance with International Energy Agency (IEA) 2012 predictions, renew-ables will become the world’s second largest source of power gen-eration by 2015.

Modern renewable energy can substitute for fossil fuels in four distinct markets: power generation, heating and cooling, transport fuels, and rural/off-grid energy services. During the last decade, total global installed capacity of many renewable energy technolo-gies grew at very rapid rates. Solar photovoltaics (PV) grew the fast-est of all renewable technologies during this period, with operating capacity increasing an average of 58% annually. It was followed by concentrating solar thermal power (CSP), which increased almost 37%, growing from a small base and wind power, which increased by 26%. The growth of liquid biofuels has been mixed in recent years, with biodiesel production expanding in 2011, and ethanol stable or down slightly compared with 2010. Hydropower and geothermal power are growing globally at rates of 2–3% per year, making them more comparable with global growth rates for fossil fuels. However, in several European countries the growth in these and other renew-able energy technologies far exceeds the global average [2].

1.2 Renewable Energy

A renewable energy source can be defi ned as an energy source that is continually replenished, is available over the long term at a reasonable cost that can be used with minimum environmental impacts, produces minimum secondary wastes, and is sustainable based on current and future economic and social needs. This defi nition of renewable energy resources includes many forms such as wind energy, solar energy, bio-fuels, geothermal energy, and ocean wave energy.

It is natural to believe that human civilization is not prepared to make sacrifi ces in the quality of life and inhibit energy consumption-driven growth due to the decline in fi nite fossil-fuel-based energy resources. Therefore, humans who have already come this far are smart enough to realize that renewable energy is the alternative to fi nite fossil energy sources. In addition to this, there are many encouraging points for the development and use of renewable energy sources like diversity in energy supply options, both for developed and developing nations.

Renewable Fuels 7

Except in the case of geothermal energy, the sun is the primary source of all renewable energy, and currently the total energy gen-erating capacity of all energy conversion systems built by mankind amounts to about 14 TW (terawatt). In comparison to this, the solar input is extremely large, and the continuous solar input is equivalent to 90000 TW, of which about 1000 TW could in principle be captured for energy conversion to forms we can use [4]. Of course, there are sig-nifi cant losses due to poor conversion effi ciencies and land use con-straints that need to be taken into account, but even so, there should be suffi cient raw energy from the sun to meet our needs many times over. The challenge is development of effi cient green technologies. Energy scenarios are widely used to describe possible paths ahead and the sustainable growth scenario produced by Shell International in 1995 has been very infl uential. It suggested that, by around 2060, renewables sources could meet about half of the world’s total energy needs. Subsequent studies have suggested that in principle, by 2100, renewables could perhaps meet over 80% of global energy needs, assuming that they were seen as a priority for environmental reasons. Inevitably, long-term projections like this are very speculative. In 2012, modern renewables supplied around 8.2% of the world’s energy, which included about 3.3% provided by hydropower electricity. The contribution is expanding rapidly, stimulated by some quite demand-ing targets. For example, the European Union aims to have 12.5% of its electricity produced from renewable sources by 2020, with some member countries aiming for even higher targets. Denmark aims for 29%, Finland 21.7%, Portugal 21.5% and Austria 21.1%, and these fi g-ures exclude the contribution from large hydropower plants [2].

1.3 Biofuels

Biomass-derived fuels or biofuels are an important contributor in the modern renewables slice of the energy source distribution pie chart shown in Figure 1.2. The use of biogas in heating houses, biogas-derived syngas in electricity generation and transport bio-fuels are some of the major applications in this type of sustainable energy. Biofuels are produced from bio-based materials through various paths such as biochemical [5, 6], and thermochemical meth-ods [7, 8]. In general the use of unprocessed biomass forms like fi rewood for heating or cooking purposes are not included in this group. Chemically, many forms of biofuels contain oxygen as one

8 Handbook of Cellulosic Ethanol

of the elements, whereas petroleum fuels are hydrocarbons free of oxygen. Another important difference is the sulfur level; all bio-fuels are very low in sulfur in comparison to petroleum fuels and many have low nitrogen levels as well.

1.3.1 Advantages of Biofuels

Common biofuels include bioethanol, biomethanol, vegetable oils, biodiesel, biogas, biosynthetic gas or biomass-derived syngas, bio-oil, bio-char, and bio-hydrogen. The benefi ts or advantages of biofuels can be broadly classifi ed into three groups: economic, environmental, and energy security and these factors are outlined below [9, 6].

Economic impacts:

• Sustainability in relation to economic growth• Increased number of rural manufacturing jobs in

biorefi nery• Opportunity for certain developing countries to

reduce their dependence on oil imports • Increased investments in plant and equipment• Fuel diversity• Agricultural development• International competitiveness, especially for develop-

ing countries with land resources

Environmental impacts:

• Greenhouse gas reductions• Reduction of air pollution—bioethanol’s high oxygen

content reduces carbon monoxide levels more than any other oxygenate

• Biodegradability• Improved land and water use• Carbon sequestration

Energy security:

• Supply reliability independent of international politi-cal climate

• Ready availability• Ability to set domestic targets for production and

markets