Handbook of Cellulosic Ethanol

Transcription

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3 Handbook of Cellulosic Ethanol

4 Scrivener Publishing 100 Cummings Center, Suite 541J Beverly, MA Publishers at Scrivener Martin Scrivener Phillip Carmical

5 Handbook of Cellulosic Ethanol Ananda S. Amarasekara Department of Chemistry, Prairie View A&M University, Texas, USA

6 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. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or other - wise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) , fax (978) , or on the web at Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) , fax (201) , or online at Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. For general information on our other products and services or for technical support, please contact our Customer Care Department within the United States at (800) , outside the United States at (317) or fax (317) Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic formats. For more information about Wiley products, visit our web site at For more information about Scrivener products please visit Cover design by Russell Richardson Library of Congress Cataloging-in-Publication Data: ISBN Printed in the United States of America

7 Contents Preface xvii Part 1 Introduction to Cellulosic Ethanol 1 1 Renewable Fuels Introduction Renewable Energy Biofuels Advantages of Biofuels Gaseous Biofuels Liquid Biofuels Renewable Energy in the United States Federal Agencies Promoting Renewable Energy Incentives for Renewable Fuels Renewable Fuel Legislature in the United States Renewable Fuel Standards of Energy Independence and Security Act of US EPA 2013 Renewable Fuel Standards 22 References 25 2 Bioethanol as a Transportation Fuel Introduction History of Bioethanol as a Transportation Fuel Alcohol Fuels Fuel Characteristics of Ethanol Disadvantages of Ethanol 33 v

8 vi Contents 2.4 Corn and Sugarcane Ethanol First Generation Ethanol Production Advantages of Cellulosic Ethanol 35 References 40 3 Feedstocks for Cellulosic Ethanol Production Introduction Cellulosic Ethanol Feedstock Types Potential of Agricultural Wastes Major Crop Residue Feedstock Corn Stover Wheat Straw Rice Straw Sugarcane Bagasse Barley Straw Forestry Residue, Logging and Mill Residue Grass Feedstocks Switchgrass Miscanthus Grass Prairie Cordgrass Arundo Donax or Giant Reed Reed Canary Grass Alfalfa Other Grasses Purpose-Grown Trees as Feedstock Poplar Willows Pines Eucalyptus Municipal and Other Waste as Feedstock for Cellulosic Ethanol Municipal Waste Feedstock Utilizing Cellulosic Ethanol Plants 107 References 108

9 Contents vii Part 2 Aqueous Phase Biomass Hydrolysis Route Challenges in Aqueous-Phase Biomass Hydrolysis Route: Recalcitrance Introduction Two Ways to Produce Cellulosic Ethanol Challenges in Aqueous-Phase Biomass Hydrolysis Structure of Plant Cells and Lignocellulosic Biomass Major Components of Lignocellulosic Biomass Cellulose Hemicellulose Lignin Cellulose Recalcitrance 140 References Pretreatment of Lignocellulosic Biomass Introduction Different Categories of Pretreatment Methods Physical Pretreatment Machinery Used in Physical Pretreatment Physical Pretreatment of Woody Biomass Physicochemical Pretreatment Steam Explosion or Steam Pretreatment Liquid Hot Water (LHW) Pretreatment Ammonia-Based Pretreatments Method Ammonia Fiber/Freeze Explosion (AFEX) Ammonia Recycle Percolation (ARP) Soaking Aqueous Ammonia (SAA) Supercritical Carbon Dioxide Pretreatment Organosolv Pretreatment Ionic Liquid (IL) Pretreatment N-Methyl Morpholine N-Oxide (NMMO) Pretreatment Chemical Pretreatment Aqueous Acid Pretreatment Sulfur Dioxide Pretreatment Alkaline Pretreatment Methods Lime Pretreatment 183

10 viii Contents Aqueous Alkali Hydroxide-Based Pretreatments Wet Oxidation Pretreatment Ozone Pretreatment Biological Pretreatment Conclusion 191 References Enzymatic Hydrolysis of Cellulose and Hemicellulose Introduction Enzymatic Actions on Lignocellulosic Biomass Enzymatic Hydrolysis of Cellulose Cellulose Hydrolysis Mechanisms Cellulase Preparation Methods In-Plant Cellulase Production Immobilization of Cellulases Immobilization on Nanoparticles Enzymatic Hydrolysis of Hemicellulose Hemicellulose Hydrolysis Mechanisms Hemicellulases Preparation Methods Future Directions in Enzymatic Cellulose Hydrolysis Research 237 References Acid Hydrolysis of Cellulose and Hemicellulose Introduction Concentrated Acid Hydrolysis Arkenol Process Mechanism of the Concentrated Acid Hydrolysis Recent Advances in the Concentrated Acid Process Dilute Acid Hydrolysis Percolation Reactors Plug Flow Reactors Bed-Shrinking Flow through Reactors 255

11 Contents ix Comparisons and Modeling Studies on Hydrolysis Reactors Recent Advances in Dilute Acid Process Different Acids Ionic Liquid-Based Direct Acid Hydrolysis Acid Catalysts in Ionic Liquid Solutions Solid Acid Catalysts in Ionic Liquid Solutions Acid Group Functionalized Ionic Liquids Solid Acid Hydrolysis Zeolites Cation-Exchange Resins Metal Oxide Supported Solid Acid Catalysts Carbonaceous Solid Supported Acid Catalysts 271 References Fermentation I Microorganisms Introduction Detoxification of Lignocellulosic Hydrolyzate Separate Hydrolysis and Fermentation (SHF) Microorganisms Used in the Fermentation Fermentation Using Yeasts Genetic Modification of Yeasts Fermentation Using Bacteria Genetic Modifications of Bacteria Simultaneous Saccharification and Fermentation (SSF) SSF Using a Mixture of Saccharification and Fermentation Microorganisms SSF Using Microorganisms that Can Do both Saccharification and Fermentation or Consolidated Bio-Processing (CBP) Heterologous Expression of Cellulase Genes in Yeast S. cerevisiae for the Development of CBP Surface-Engineered Yeast Strains for the CBP 312

12 x Contents Cell Recycle Batch Fermentation (CRBF) Comparison of Different Fermentation Configurations Immobilization of Yeast 317 References Fermentation II Fermenter Configuration and Design Introduction Batch Fermentation Examples of Batch Fermentation Fed-Batch Fermentation Advantages and Disadvantages of Fed-Batch Fermentation Examples of Fed-Batch Fermentation Types of Fed-Batch Fermentation Reactors Fixed Volume Fed-batch Reactors Variable Volume Fed-batch Reactors Control Techniques for Fed-Batch Fermentation Continuous Fermentation Types of Continuous Fermentation Reactors Advantages of Continuous Fermentation Examples of Continuous Fermentation Continuous Fermentation Using Immobilized Microorganisms New Directions in Fermenter Configuration and Design 352 References Separation and Uses of Lignin Introduction Structure of Lignin Separation of Lignin in the Cellulosic Ethanol Process Physical and Chemical Properties of Lignin Applications of Lignin Lignin-Based Phenol Formaldehyde Resins Lignin-Phenol co-polymers as Adhesives 366

13 Contents xi Lignin-Polyolefin Blends Lignin-Polyvinyl Polymer Blends Lignin-Rubber Blends Preparation of Vanillin from Lignin Synthesis of Vanillin-Based Polymers 371 References 373 Part 3 Biomass Gasification Route Biomass Pyrolysis and Gasifier Designs Introduction Chemistry of the Conversion of Biomass to Syngas Composition of the Syngas Classifications of Biomass Gasifiers Fixed-Bed Gasifier Fluidized-Bed Gasifier Bubbling Fluidized-Bed (BFB) Gasifier Circulating Fluidized-Bed (CFB) Gasifier Allothermal Dual Fluidized-Bed (DFB) Gasifier Entrained-Flow Gasifier Syngas Cleaning Hot Gas Cleaning Inertial Separation Using Cyclone Gas Cleaning Filters Electrostatic Separations Cold Wet Gas Cleaning or Conventional Gas Cleaning Tar Control and Treatment Methods 403 References Conversion of Syngas to Ethanol Using Microorganisms Introduction Advantages and Disadvantages of Biocatalysis Metabolic Pathways Microorganisms Used in Syngas Fermentation Biochemical Reactions in Syngas Fermentation 414

14 xii Contents 12.5 The Effects of Operation Parameters on Ethanol Yield The Effect of Culture Media Effect of Medium ph The Effect of Carbon Source The Effect of Inhibitors and Impurities in Syngas The Effect of Gas Pressure Syngas Fermentation Reactors Industrial-Scale Syngas Fermentation and Commercialization 426 References Conversion of Syngas to Ethanol Using Chemical Catalysts Introduction Homogeneous Catalysts Introduction to Heterogeneous Catalysts Heterogeneous Catalyst Types Rhodium-Based Catalysts Studies on Reduction of CO and CO Role of Promoters Role of Catalyst Support New Catalyst Supports Copper-Based Modified Methanol Synthesis Catalysts Copper Nanoparticles-Based Catalysts Modified Fischer-Tropsch Type Catalysts Molybdenum-Based Catalysts Catalyst Selection 459 References 461

15 Contents xiii Part 4 Processing of Cellulosic Ethanol Distillation of Ethanol Introduction Distillation of the Beer How Distillation Works Conventional Ethanol Distillation System Beer Column or Stripping Column Rectifying or Refining Column Side Stripper Column Steam Generation for Distillation Process Design Basis of the Boiler Studies on Development of Hybrid Systems for Ethanol Distillation 476 References Dehydration to Fuel Grade Ethanol Introduction Dehydration Methods Adsorption Method Dehydration of Ethanol Using Zeolite Molecular Sieves Dehydration of Ethanol Using other Adsorbents Azeotropic Distillation Method Extractive Distillation Methods Extractive Distillation Using High Boiling Solvents Extractive Distillation Using Salts Membrane-Based Pervaporation Methods Direct Pervaporation of Ethanol from Fermentation Broth Other Dehydration Methods Comparisons of Common Dehydration Methods 498 References 500

16 xiv Contents Part 5 Fuel Ethanol Standards and Process Evaluation Fuel Ethanol Standards, Testing and Blending Introduction Fuel Grade Ethanol Standards in the United States Quality Assurance and Test Methods Visual Clarity American Petroleum Institute (API) Gravity phe Level Sulfur Content Total Sulfate Inorganic Chloride ASTM D 5501 GC Analysis for Ethanol, Methanol and Denaturant European Fuel Ethanol Standards European Fuel Grade Ethanol Standard pren Material Safety Data Sheet (MSDS) for Denatured Fuel Ethanol Gasoline Ethanol Blends Ethanol Blends Used around the World Blender Pump Blends Engine Performance Using Gasoline Ethanol Blends 524 References Techno-Economic Analysis and Future of Cellulosic Ethanol Introduction Techno-Economic Aspects of Biomass Hydrolysis Process Techno-Economic Aspects of Biomass Gasification Process Comparison of Biomass Hydrolysis and Gasification Processes Some Cellulosic Plants around the World 540

17 Contents xv 17.6 Challenges in Cellulosic Ethanol Technological Challenges Future Prospects of Cellulosic Ethanol 553 References 554 Appendix Index 575

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19 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 legislations promoting the use of renewable energy resources and increasing government incentives for commercialization of renewable energy technologies. Development of science and technologies for efficient 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 first 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 first 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 feedstocks 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 produced from a variety of abundant lignocellulosic biomass feedstocks, and should be able to be produced in substantial amounts to meet the growing global energy demand. There are two fundamental routes to produce cellulosic ethanol from renewable biomass: the aqueous-phase biomass saccharification-fermentation route, xvii

20 xviii Preface and thermochemical gasification 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 cellulosic ethanol, divided into five parts: introduction to cellulosic ethanol, aqueous-phase biomass hydrolysis route, biomass gasification route, processing of fuel grade ethanol, and techno-economical evaluation of the processes. The first part covering Chapters 1 to 3 introduces the reader to cellulosic ethanol, presenting the advantages over first generation corn or sugarcane ethanol. In the United States, the gradual transition to renewable energy sources is supported 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 saccharification-fermentation route. Chapters 4 and 5 detail the challenges in biomass saccharification, or recalcitrance, as well as various pretreatment techniques such as physical, physicochemical, chemical, and biological pretreatments, and applications to different feedstocks. Chapter 6 covers the enzymatic saccharification, including cellulases, hemicellulase families, mechanisms, enzyme preparation methods, and immobilization of enzymes. Chapter 7 is dedicated to acid hydrolysis, or direct saccharification, 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 presented in Chapters 8 and 9. The microorganisms used in the fermentations, including recent advances in genetic modifications of microorganisms, separate hydrolysis fermentation (SHF), simultaneous saccharification and fermentation (SSF), consolidated bioprocessing (CBP), and surface-engineered and immobilized yeasts are covered in detail in Chapter 8. Fermentation configurations 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 biorefinery concept.

21 Preface xix The third part of the book (Chapters 11 13) is dedicated to the biomass gasification route, which is an alternative approach for producing ethanol from lignocellulosic biomass. Pyrolysis chemistry, gasifier 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 presented in Part 4 of the book. Concentration of ethanol to approximately 90% ethanol by distillation is the first step in purification of ethanol. Technologies used in the current first generation ethanol 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 provides 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

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23 Part 1 Introduction to Cellulosic Ethanol

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25 1 Renewable 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, firing clay pots, and processing metals. The industrial revolution, leading to the development of the internal 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 economic development are driven by energy consumption. The global demand for energy is expected to increase at a faster rate in upcoming 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 3

26 4 Handbook of Cellulosic Ethanol 800 Energy (quadrillion BTU) Non-OECD OECD Figure 1.1 World energy consumption from (quadrillion BTU) from International Energy Outlook report released in 2011 [1]. 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 consumption 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 fulfilled 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 traditional renewables is slightly higher than all the combined modern renewables. Traditional biomass energy sources such as firewood,

27 Renewable Fuels 5 8.5% Traditional renewables 8.2% Modern renewables 2.7% nuclear 3.3% Biomass/solar/ geothermal hot water/heating 3.3% Hydropower 0.9% Wind/solar/ biomass/ geothermal power generation 0.7% Biofuels 80.6% Fossil fuel Figure 1.2 Renewable energy share of global final energy consumption, from Renewable Energy 2012 Global status report, Paris [2]. which are used primarily for cooking and heating in rural areas of developing countries, could be considered renewable. These traditional renewables account for approximately 8.5% of total final 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%. Biomassderived 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 limited. 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 diminished 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 finite 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 predictions, 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

28 6 Handbook of Cellulosic Ethanol are expected to account for almost one-third of total electricity output [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, renewables will become the world s second largest source of power generation by 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 technologies grew at very rapid rates. Solar photovoltaics (PV) grew the fastest 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 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 renewable energy technologies far exceeds the global average [2]. 1.2 Renewable Energy A renewable energy source can be defined 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 definition of renewable energy resources includes many forms such as wind energy, solar energy, biofuels, geothermal energy, and ocean wave energy. It is natural to believe that human civilization is not prepared to make sacrifices in the quality of life and inhibit energy consumptiondriven growth due to the decline in finite 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 finite 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.

29 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 generating 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 TW, of which about 1000 TW could in principle be captured for energy conversion to forms we can use [4]. Of course, there are significant losses due to poor conversion efficiencies and land use constraints that need to be taken into account, but even so, there should be sufficient raw energy from the sun to meet our needs many times over. The challenge is development of efficient 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 influential. 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 demanding 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 figures 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 biofuels 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 methods [7, 8]. In general the use of unprocessed biomass forms like firewood for heating or cooking purposes are not included in this group. Chemically, many forms of biofuels contain oxygen as one

30 8 Handbook of Cellulosic Ethanol of the elements, whereas petroleum fuels are hydrocarbons free of oxygen. Another important difference is the sulfur level; all biofuels are very low in sulfur in comparison to petroleum fuels and many have low nitrogen levels as well 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 benefits or advantages of biofuels can be broadly classified 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 biorefinery 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 developing 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 political climate Ready availability Ability to set domestic targets for production and markets