The thermochemical direct liquefaction process involves converting biomass to an oily liquid by contacting the biomass with water at elevated temperatures (250-350°C) with sufficient pressure to maintain the water primarily in the liquid phase (12-20 MPa) for resi­dence times up to 30 minutes (Figure 2.3). It mimics the natural geological processes thought to be involved in the production of fossil fuels. Alkali may be added to promote organic conversion. In the liquefaction process, the carbonaceous materials are converted to lique­fied products through a complex sequence of changes in physical structure and chemical bonds [58]. The primary product is an organic liquid with reduced oxygen content (about 10%) and the primary by-product is water containing soluble organic compounds. The resulting intermediates can be converted to hydrocarbon fuels and commodity chemicals for products similar to those produced from petroleum [16]. Work done on the determina­tion of the reaction mechanisms of liquefaction, mainly with pure cellulose, suggests that the technique offers a potential alternative synthetic route to phenolics, furans and other chemicals [59]. Liquefaction is suitable for high moisture content biomass, such as aquatic biomass, garbage, organic sludge and so on.

This book was conceived and initiated by Dr. David I. Bransby, and it is to him that the final product is dedi­cated. David is a professor in the Agronomy and Soils Department in the College of Agriculture at Auburn Uni­versity in Auburn, Alabama, U. S.A. A native of South Africa, David arrived at Auburn in 1987 to teach and conduct research in forage and livestock management. Shortly thereafter, he was asked to provide oversight and leadership for a federal, multistate grant focused on high-yielding, low-input herbaceous plants that could be converted to bioenergy. David insisted he was not quali­fied because he knew nothing about converting biomass to energy and even thought “it was a crazy idea.” He was quickly reassured that “nobody else knew anything about it, either; renewable energy was a totally new area.”

David immediately began learning all he could about the production of energy from biomass while simultaneously educating himself, as an immigrant, about U. S. agriculture. Suddenly he realized that the two topics could provide a nearly perfect union. He sur­mised that the major commodities were often being overproduced and that the government response through decades of farm programs had created “stagnation in U. S. agriculture by discouraging new ideas and change.”

Nearly three decades later, David has built two research and outreach programs, one in forage and livestock management and one in energy crops and bioenergy, that have both received national and international recognition. A cornerstone of these programs has been David’s emphasis on outreach, built on a philosophy that “the ultimate goal of applied research should be to benefit society, and this goal cannot be achieved without getting involved in outreach.” Through his personal involvement with many different stakeholder groups, David concludes that he has “gathered valuable information that has helped me design more relevant research and improve the content of the courses I teach.”

David is convinced that biofuels made from switchgrass and other agricultural crops and by-products can reduce America’s dependence on foreign oil, strengthen farm economies and revitalize rural communities. “Energy crops, while not a total solution, would help by giving farmers new markets and reducing their dependence on farm subsidies.” He has continued his endeavors because “I believe this is really important stuff. It’s going to play a major role in our country’s future.”

Maryse Brancourt-Hulmel1,2, Charlotte Demay2, Emeline Rosiau1,
Fabien Ferchaud2, Linda Bethencourt1, Stephanie Arnoult1,3,
Camille Dauchy1, Nicolas Beaudoin2, and Hubert Boizard2

INRA (l’Institut National de la Recherche Agronomique), France
1Joint Research Unit INRA/USTL Abiotic Stress and Differentiation of Cultivated Plants

(UMR SADV)

2Research Unit Agro resources and environmental impacts (INRA UR AgroImpact)
3Experimental Unit Crops Innovation Environment — Picardy (INRA EU GCIE Picardy)

4.1 Introduction

Miscanthus, a tall, perennial, rhizomatous C4 grass of the Poaceae family [1,2] is a good candidate for a cellulosic energy crop. The name Miscanthus originates from the Greek mischos (pedicel) and anthos (flower) and refers to the stalked or pedicellate spikelets of the Miscanthus inflorescence. Several species belong to the genus with ploidy ranging from diploid to hexaploid [3]. The basic chromosome number corresponds to x = 19 [4]. Miscanthus is capable of high biomass production with minimal inputs [5]. Tropical and subtropical genotypes of Miscanthus grow to 3-4 m when cultivated in Europe and even higher in the warm and wet climates of south-east Asia. Miscanthus rhizomes, or microplants, are planted in spring with canes developing during the summer and harvested annually during the late autumn or winter, following the second or third growing season. The lifetime of the crop varies from 20 to 25 years [6]; long-term Miscanthus plantations can contribute to soil carbon storage [7]. Miscanthus spreads naturally via its underground

Cellulosic Energy Cropping Systems, First Edition. Edited by Douglas L. Karlen. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.

storage organs or rhizomes but some species can also be seed-propagated. As Miscanthus is propagated vegetatively, the clone is the most common variety type. This chapter provides details about Miscanthus as a cellulosic energy crop.

Generally, two main routes for the conversion of lignocellulosic biomass can be distin­guished, which can lead to the production of biofuels and other value-added commodity chemicals (Figure 2.1):

The (Bio)Chemical Route: Biochemical conversion makes use of the enzymes of bac­

teria or other microorganisms to break down and convert the biomass. In most cases the microorganisms themselves are used to perform the conversion processes, such as fermen­tation, anaerobic digestion or composting. Sometimes, only the isolated enzymes are used, also known as biocatalysis. Plant monomers can also be further converted chemically.

The Thermochemical Route: Thermochemical conversion includes processes in which

heat and pressure are the dominant mechanisms to convert the biomass into another chemical form.

The bioconversion of lignocellulosic residues to biofuels and biochemicals is more complicated than the bioconversion of sugar or starch-based feedstock. Plant cell walls are naturally resistant to microbial and enzymatic (fungal and bacterial) deconstruction. This recalcitrant nature of the lignocellulosic feedstock (resistance of plant cell walls to deconstruction) therefore poses a significant hurdle in the biochemical route and necessitates

extra pretreatment steps before this lignocellulosic biomass can serve as low-cost feedstock for the production of fuel ethanol and other value-added commodity chemicals. Plant cell walls are comprised of long chains (polymers) of sugars (carbohydrates such as cellulose and hemicellulose), which can be converted into common monomer sugars such as glucose, xylose, and so on, the ideal substrates for chemical, physical, and fermentation processes [2]. However, these polymers are bound together by lignin, which has to be degraded first before the sugar polymers become accessible to hydrolysis by chemical or biological means. Lignin is a complex structure containing aromatic groups linked in a three-dimensional structure that is particularly difficult to biodegrade [3]. Lignins perform an important role in strengthening cell walls by cross-linking polysaccharides, thus providing support to structural elements in the overall plant body. This also helps the plant to resist moisture and biological attack [4]. These same properties, however, constitute one of the drawbacks of using lignocellulosic material in fermentation, as they make lignocellulose resistant to physical, chemical, and biological degradation. The higher the proportion of lignin, the higher the resistance to chemical and enzymatic degradation [5]. Overcoming the recalcitrance of lignocellulosic biomass is a key step in the biochemical production of fuels and chemicals; it is the main goal of the pretreatment.

In the thermochemical conversion route, the recalcitrant nature of the lignocellulosic biomass poses no problems to the technology. However, other limitations of the biomass need to be taken into account in this case: the energy density of biomass is low compared to that of coal, liquid petroleum or petroleum-derived fuels. And most biomass, as received, has a high burden of physically adsorbed moisture, up to 50% by weight [6].

During the period 1982-1993, the Royal Dutch Shell Laboratory developed the HTU (Hydro-Thermal Upgrading) process to convert wet biomass such as wood, plants or organic waste into a liquid fuel, so-called biocrude. Biomass is firstly treated in an aqueous slurry at 200°C and 30 bar, followed by a treatment at 330°C and 200 bar. This process results in a biocrude, an oil with low oxygen content, which can be further upgraded by a catalytic hydrodeoxygenation step to a high-quality naphtha or diesel oil with very low oxygen, nitrogen and sulfur contents that can be blended in any ratio to fossil diesel [60, 61].

Renewable resources, their use and modification are involved in a multitude of important processes with a major influence on our everyday lives. Applications can be found in the energy sector, chemistry, pharmacy, the textile industry, paints and coatings, to name but a few.

The area interconnects several scientific disciplines (agriculture, biochemistry, chemistry, technology, environmental sciences, forestry…), which makes it very difficult to have an expert view on the complicated interaction. Therefore, the idea to create a series of scientific books, focusing on specific topics concerning renewable resources, has been very opportune and can help to clarify some of the underlying connections in this area.

In a very fast changing world, trends are not only characteristic for fashion and political standpoints; also, science is not free from hypes and buzzwords. The use of renewable resources is again more important nowadays; however, it is not part of a hype or a fashion. As the lively discussions among scientists continue about how many years we will still be able to use fossil fuels — opinions ranging from 50 years to 500 years — they do agree that the reserve is limited and that it is essential not only to search for new energy carriers but also for new material sources.

In this respect, renewable resources are a crucial area in the search for alternatives for fossil-based raw materials and energy. In the field of energy supply, biomass and renewable — based resources will be part of the solution alongside other alternatives such as solar energy, wind energy, hydraulic power, hydrogen technology and nuclear energy.

In the field of material sciences, the impact of renewable resources will probably be even bigger. Integral utilization of crops and the use of waste streams in certain industries will grow in importance, leading to a more sustainable way of producing materials.

Although our society was much more (almost exclusively) based on renewable resources centuries ago, this disappeared in the Western world in the nineteenth century. Now it is time to focus again on this field of research. However, it should not mean a ‘retour a la nature’, but it should be a multidisciplinary effort on a highly technological level to perform research towards new opportunities, to develop new crops and products from renewable resources. This will be essential to guarantee a level of comfort for a growing number of people living on our planet. It is ‘the’ challenge for the coming generations of scientists to develop more sustainable ways to create prosperity and to fight poverty and hunger in the world. A global approach is certainly favoured.

This challenge can only be dealt with if scientists are attracted to this area and are recognized for their efforts in this interdisciplinary field. It is, therefore, also essential that consumers recognize the fate of renewable resources in a number of products.

Furthermore, scientists do need to communicate and discuss the relevance of their work. The use and modification of renewable resources may not follow the path of the genetic engineering concept in view of consumer acceptance in Europe. Related to this aspect, the series will certainly help to increase the visibility of the importance of renewable resources.

Being convinced of the value of the renewables approach for the industrial world, as well as for developing countries, I was myself delighted to collaborate on this series of books focusing on different aspects of renewable resources. I hope that readers become aware of the complexity, the interaction and interconnections, and the challenges of this field and that they will help to communicate on the importance of renewable resources.

I certainly want to thank the people of Wiley’s Chichester office, especially David Hughes, Jenny Cossham and Lyn Roberts, in seeing the need for such a series of books on renewable resources, for initiating and supporting it and for helping to carry the project to the end.

Last, but not least, I want to thank my family, especially my wife Hilde and children Paulien and Pieter-Jan, for their patience and for giving me the time to work on the series when other activities seemed to be more inviting.

Christian V. Stevens, Faculty of Bioscience Engineering

Ghent University, Belgium Series Editor ‘Renewable Resources’ June 2005

Preface

As stated in the Dedication, this book was conceived and initiated by Dr. David I. Bransby, who strongly believes that “research should not be an end in itself, but the first step in a process for generating and transferring information or technologies that are of value to the communities we serve.” David chose to focus the book on plant biomass because even though fats and oils can be used for bioenergy production, plant biomass is more abundant than animal biomass and thus offers much greater potential for energy production. Plant biomass can provide a variety of inputs including starch, oil, and sugar, but it is the lignocellulosic (cellulosic) biomass itself that is most abundant. Composed of cellulose, hemi-cellulose, and lignin these cell wall components are renewed on an annual basis around the globe.

There are also numerous technologies that are ready or under development for converting cellulosic biomass to heat, electricity and/or liquid fuels. With that in mind, David set out to produce a book that provided comprehensive documentation of how cellulosic energy crops such as switchgrass, Miscanthus, and sorghum and the cellulosic fraction of sugarcane, maize and wheat residues could be sustainably produced and converted to affordable energy through liquid fuels and electricity. Unfortunately, due to an on-going battle with diabetes, David was unable to complete the project. I am very humbled to have been able to pick up the gauntlet and with the outstanding help of many of my friends and colleagues complete this very important project. It is our hope as editor and authors of this work that readers around the globe will catch hold of David’s inspiration and continue the ground-breaking work in the area, building new programs where none existed before, and continuing to build an awareness of the potential benefits of bioenergy to the public at large and to policy makers. The target audience for this book is society as a whole, but especially those elected officials who are often ultimately responsible for building new programs through their critical enabling legislation.

The book is divided into five sections. The first (I) provides general background related not only to the challenges and various potential cellulosic feedstocks (Chapter 1) but also to technologies for production of liquid fuels and biochemicals (Chapter 2) or production of heat and electricity (Chapter 3). Section II hones in on each of the herbaceous crops that have been identified as a potential cellulosic feedstock for not only bioenergy but also bioproduct development. Miscanthus (Chapter 4), switchgrass (Chapter 5), sugarcane and energy cane (Chapter 6), sorghums (Chapter 7) and crop residues (Chapter 8) are

examined in detail by reviewing their phylogeny, cultural practices, and opportunities for genetic improvement. Section III follows a similar format although the focus is on woody crops, including eucalyptus (Chapter 9), pine (Chapter 10), poplar (Chapter 11), and willow (Chapter 12).

Section IV moves toward David’s ultimate goal of commercialization by reviewing criti­cal logistical issues associated with both herbaceous (Chapter 13) and woody (Chapter 14) feedstocks. Alternate strategies for harvesting, transporting, and storing various cellulosic materials are examined. Finally, Section V tackles the challenge where “the rubber meets the road”, that is, moving the technology from the researchers to society as a whole.

To achieve long-term sustainability, emerging cellulosic bioenergy and/or bioproducts industries must meet three crucial and equally important challenges. One is that the new enterprise(s) must be economical (Chapter 15). The second is they must not have adverse environmental impacts (Chapter 16), and, finally, they must be socially acceptable (Chapter 17). The final two chapters are intended to provide readers with case study examples of an actual bioenergy commercialization project (Chapter 18) and a glimpse at activities in Brazil, China, and India (Chapter 19).

In summary, to meet ever increasing global needs for sustainable food, feed, fiber, and fuel supplies, greater attention must be given to soil, water, and air resources. Redirecting from an increased trajectory of expanded row crops to cellulosic energy crops and crop rotations is one component needed to achieve the intensified productivity required for high quality agricultural products that are economically viable, socially acceptable, and adaptable. This book is intended to help: (1) identify suitable cellulosic energy crops that are adapted to a wide range of climates and soils; (2) develop best management practices for sustainably growing, harvesting, storing, transporting and pre-processing these crops with minimal negative impacts on the environment and food production; (3) develop integrated cellulosic energy cropping systems for supplying commercial processing plants; and (4) educating landowners, technology owners, students, policy makers and the general public on how to use cellulosic energy crops to maximize the many benefits they offer. It is my hope that we have successfully provided the information in a format that will enable all of us to achieve this important twenty-first century goal.

Douglas L. Karlen USDA, Agricultural Research Service, National Laboratory for Agriculture and the Environment

U. S.A.

4.1.1 Phylogeny

Miscanthus is a C4 perennial grass of tropical and subtropical origins with a wide area of distribution (Figure 4.1). It belongs to the Poaceae family, a subfamily of Panicoidae, the tribe of Andropogoneae and the subtribe of Saccharineae. Among others, the gen­era Miscanthus, Saccharum and Erianthus belong to this subtribe [8]. The phylogeny of Miscanthus was first described by Andersson in 1856 [9]. Miscanthus was introduced into Europe by Aksel Olsen, who brought it from Japan to Denmark in 1935 (reported by Atienza et al. [10]). Miscanthus sensu lato (s. l) comprises more than 20 species [11] while Miscanthus sensu stricto (s. s.) contains about 12 species [3]. There is no consensus yet on the definition of Miscanthus (s. l. or s. s.), the taxonomic system to be used or the number of species, subspecies, varieties and forms to be recognized. This can be attributed to the existence of natural interspecific hybrids, the famous one being M. x giganteus, issued

from a cross between M. sacchariflorus and M. sinensis. Furthermore, the distribution of each Miscanthus species has not been fully investigated [9].

Miscanthus is closely related to other genera of the “Saccharum complex” (the Sac — charum genera belonging to this complex) and Saccharum-Miscanthus hybrids, that is, miscanes, are used to create varieties of miscane [12]. Alix et al. [13] (Table 4.1) con­cluded that Miscanthus was more similar to Saccharum than Erianthus while Cai et al. [14] (Table 4.1) placed Miscanthus between Erianthus and Saccharum. However, a phy­logenetic analysis of more than 57 species belonging to the tribe Andropogoneae showed that Saccharum was in fact more closely related to Miscanthus than to other species in the Saccharum complex [11].

The original taxonomy of Miscanthus first described by Andersson (1856) has been subsequently modified many times using morphological measurements as reported by Sun et al. [9]. Recently, Sun et al. [9] revised the taxonomy of 500 Miscanthus accessions according to 41 morphological characters, of which 24 were qualitative traits and 17 quantitative traits (Figure 4.2).

Molecular methods have enabled the phylogeny of Miscanthus to be even more clearly defined such as in situations where morphological characters were not efficient. Molecular data, especially DNA data, provide a direct assessment of genetic diversity and unlike morphological characters are not influenced by environmental factors [15]. Using nuclear DNA, Greef et al. [16] assessed the genetic diversity of European Miscanthus species with Amplified Fragment Polymorphism (ALFP) and found it was a powerful tool in evaluating genetic diversity and hybrid success and in identifying incorrect classifications. Kim et al. [17] developed a sequence-characterized amplified region (SCAR) marker that clearly distinguishes M. sacchariflorus, M. sinensis and M. x giganteus. Using chloroplast DNA, De Cesare et al. [18] identified six chloroplast Single Sequence Repeat (cpSSRs) markers capable of differentiating most Miscanthus species.

Biochemical conversion comprises breaking down or “cracking” biomass by using physical, chemical, enzymatic and/or microbial action, to make the polymeric carbohydrates of the biomass (hemicellulose and cellulose) available as (fermentable) sugars, which can then be converted into biofuels and bioproducts using microorganisms (bacteria, yeast, fungi, etc.) and their enzymes or chemically converted using specific catalysts. A general overview of the different process steps of the biochemical conversion of lignocellulosic biomass is given in Figure 2.2.

Firstly, a reduction in particle size is often needed to make material handling easier and to increase surface/volume ratio, so as to enable better accessibility of the processed material in the next pretreatment step. Size reduction is most often done by a mechanical process such as crushing, milling, chipping, grinding or pulverizing to the required particle size.

Lignocellulosic biomass is seen as an attractive feedstock for future supplies of renewable fuels and biochemicals. Their abundant supply makes them attractive candidates to replace oil-based liquid fuels and chemicals. Substantial investment is occurring in conversion technologies and in determining the most economic, practical and cleanest technology for the production of these lignocellulosic-based chemicals.

Two main routes can be distinguished for the conversion of lignocellulosic biomass: the biochemical route and the thermochemical route. The key bottleneck in the biochemical conversion of lignocellulosic biomass is the initial conversion of the biomass into sugars. Further improvement of the physicochemical pretreatment processes and new biotechno­logical solutions are needed to improve the efficiency of this conversion. This “biomass recalcitrance” remains one of the most significant hurdles to producing economically fea­sible chemicals from lignocellulosic biomass via the biochemical route [7]. Continued research and development is needed to develop and scale-up new biochemical routes.

Thermochemical processes can easily overcome this natural resistance of biomass due to the relative high temperatures that are used. Therefore, a broader range of feedstock can potentially be used. However, also for the thermochemical route technical and commercial barriers still exist [62]. Innovative R&D is needed to improve the energy efficiency and cost effectiveness of thermochemical conversion technologies. Gasification technology is considered to be ready for deployment between now and 2020. Other thermochemical routes like pyrolysis or liquefaction are not as well developed [63].

It is likely there will be no single preferred conversion technology for the production of cellulosic fuels or chemicals, but rather technologies appropriate for specific feedstock [52]. Feedstock restrictions for thermochemical conversion mostly pertain to particle size, moisture and ash content.

Overall it can be concluded that significant investment into research, pilot and demonstra­tion plants is ongoing and will be further needed to develop commercially viable processes utilizing biochemical and thermochemical conversion technologies for the production of biofuels and biochemicals from lignocellulosic biomass.

Acknowledgement

Sofie Dobbelaere works on the IWT project no. 080598, concerning the provision of Technological Services related to Industrial Biotechnology, set up by FlandersBio, Ghent Bio-Energy Valley and Essenscia Vlaanderen.

Mark Laser and Lee Lynd

Thayer School of Engineering, Dartmouth College, U. S.A.

1.1 Cellulosic Biomass: Definition, Photosynthesis, and Composition

Plants, through photosynthesis, convert solar energy, carbon dioxide, and water into sugars and other derived organic materials, referred to as biomass, and release oxygen as a by-product. Humans have long used plant biomass for a variety of applications, such as fuel for warmth and cooking, lumber and other building materials, textiles, and papermaking. More recently, plant biomass has been considered as a feedstock for biofuels production — the focus of this book — with first-generation fuels being made from edible portions of plants, including starch, sucrose, and seed oils. Next-generation biofuels will be produced from non-edible cell wall components (described below) that comprise the majority of plant biomass.

Photosynthesis consists of two stages: a series of light-dependent reactions that are independent of temperature (light reactions) and a series of temperature-dependent reactions that are independent of light (dark reactions). The light reactions convert light energy into chemical energy in the form of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). The dark reactions, in turn, use the chemical energy stored in ATP and NADPH to convert carbon dioxide and water into carbohydrate.

About half of the light energy falls outside the photosynthetically active spectrum; some of the available energy is reflected away and not captured. Further energy is lost during the light absorption process, and during carbohydrate synthesis and respiration. As a result, photosynthesis typically converts less than 1% of the available solar energy into chemical energy stored in the chemical bonds of the structural components of biomass [1].

Cellulosic Energy Cropping Systems, First Edition. Edited by Douglas L. Karlen. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.

Plants have evolved three photosynthetic pathways, each in response to distinct envi­ronmental conditions. One is called the C3 pathway because the initial product of carbon fixation is a three-carbon compound (phosphoglyceric acid, or PGA). When carbon dioxide levels inside a leaf become low, especially on hot dry days, a plant is forced to close its stom­ata (microscopic pores on the surface of land plants) to prevent excess water loss. If the plant continues to fix carbon when its stomata are closed, carbon dioxide is depleted and oxygen accumulates in the leaf. To alleviate this situation, the plant uses a process called photores­piration in which a molecule ordinarily used in carbon fixation (ribulose-1,5-bisphosphate, or RuBP) combines instead with oxygen, catalyzed by the enzyme RuBisCO, which also figures prominently in carbon fixation. This reduces photosynthetic efficiency in two ways: firstly, it creates competition between oxygen and carbon dioxide for the active sites of RuBisCO — sites that take up oxygen are not available for carbon dioxide; secondly, the process re-releases carbon dioxide that had been fixed. Photorespiration reduces photosyn­thetic efficiency by 35-50%, depending upon environmental conditions, with warm, arid habitats promoting greater photorespiration [1].

In response, many plant species in warm, dry climates have evolved two alternative photosynthetic pathways — the C4 pathway and crassulacean acid metabolism (CAM) pho­tosynthesis, both of which significantly reduce photorespiration and enhance efficiency. Both convert carbon dioxide into a four-carbon intermediate using the enzyme phospho — enolpyruvate (PEP) carboxylase — which does not react with oxygen — rather than RuBisCO. C4 plants fix carbon dioxide during the day; CAM plants, to keep stomata closed during the day, fix carbon dioxide at night [2].

The highest reported solar energy conversion efficiency is about 2.4% for C3 plants and 3.7% for C4 species [3]. CAM plants are estimated to be 15% more efficient than C3 plants, but 10% less efficient than C4 plants [4]. Zhu et al. [3] estimate the theoretical maximum efficiency to be 4.6 and 6% for C3 and C4 crops, respectively. The C3 pathway is the oldest — originating around 2800 million years ago — and most widespread, both taxonomically and environmentally, accounting for about 95% of total plant species [5]. C4 photosynthesis is found in about 1% of plant species [5] and is most prevalent in grasses, with about 50% of the species using the pathway [6]. CAM occurs in about 4% of total plant species [5].

The energy crops considered in this volume all have either a C3 or C4 photosynthetic pathway. They include:

• C3 pathway: wheat straw, eucalyptus, poplar, willow, pine

• C4 pathway: miscanthus, switchgrass, sugarcane, energy cane, sorghum, corn stover.

Though not considered here, examples of potential energy crops having the CAM path­way include agave and opuntia. More detailed treatments of photosynthesis are available elsewhere [2,7].

Each of the above plant species contains cellulosic biomass, that is, the fibrous, generally inedible portions of plants, rich in the polysaccharide cellulose, which make up the majority of all plant material. Cellulosic biomass can generally be grouped into four categories: herbaceous plants, woody plants, aquatic plants, and residual material such corn stover, sugarcane bagasse, paper sludge, and animal manure. Terrestrial cellulosic energy crops and agricultural crop residues are the primary focus of this book.

Cellulosic biomass contains varying amounts of cellulose, hemicellulose, lignin, pro­tein, ash, and extractives. Cellulose, a structural component of the primary cell wall in plants, generally comprises the largest fraction, with 40-50% on a dry weight basis being typical. The material is a polymer of glucose, a six-carbon sugar, joined by 1-4 beta-linkages. Linear cellulose chains, which have an average molecular weight of about 100 000, are generally arrayed in parallel and held together with extensive hydro­gen bonding forming macromolecular fibers 3-6 nm in diameter called microfibrils. The material is well ordered, largely crystalline, and highly recalcitrant to rapid reaction under many conditions.

Hemicellulose, another polysaccharide — one that binds tightly, but non-covalently, to the surface of each cellulose microfibril — usually comprises 20-35% of the dry mass of biomass. In contrast to cellulose, hemicellulose is composed of multiple sugars — the identity and proportion of which depend on the type of plant — and has a heterogeneous, non-crystalline branched structure. As a result, hemicellulose is generally more reactive than cellulose and is readily hydrolyzed by dilute acid or base as well as hemicellulase enzymes. Xylose, a five-carbon sugar, is the dominant constituent of hemicellulose in plants other than softwoods; for softwoods, mannose is often the most abundant sugar.

Lignin is an amorphous polymer of phenyl-propane subunits (six-carbon rings linked to three-carbon chains) joined together by ether and carbon-carbon linkages, and covalently bound to hemicellulose. The subunits may have zero, one, or two methoxyl groups attached to the rings, giving rise to three structures — denoted I, II, and III, respectively. The proportions of each structure depend on the plant type. Structure I is commonly found in grasses, structure II in softwoods, and structure III in hardwoods. Lignin both creates a net around carbohydrate-rich microfibrils in plant cell walls and penetrates the interstitial space in the cell wall, driving out water and strengthening the wall. The dry mass fraction of lignin in plants typically ranges from 7-30%. Leafy herbaceous plants are generally at the low end of this range, woody plants at the high end, with softwoods having more lignin than hardwoods.

Smaller amounts of protein and minerals are also present in plant tissues. As plants mature, wall composition shifts from moderate levels of protein and almost no lignin to very low concentrations of protein and substantial amounts of lignin. Protein content can be significant (e. g. 10% dry mass) in early-season herbaceous crops, but is relatively low in late-season harvests and minimal in most woody crops.

Plants require a variety of inorganic minerals for proper growth, including both macronu­trients (N, P, K, Ca, S, Mg) and micronutrients, or trace elements (B, Cl, Mn, Fe, Zn, Cu, Mo, Ni, Se, Na, Si). Plant roots, mediated by transport proteins, absorb mineral nutrients as ions in soil water. Each mineral participates in distinct biological functions within the plant. Nitrogen, for example, is involved in all aspects of plant metabolism, with its fore­most function being to provide amino groups in amino acids, the building blocks of every protein. Potassium, meanwhile, is essential for activating a multitude of enzymes, including pyruvate kinases involved in glycolysis, and is one of the most important contributors to cell turgidity in plants. Another vital macronutrient, calcium, is essential for providing structure and rigidity to cell walls, and is used as a signaling compound in response to mechanical stimuli, pathogen attack, temperature shock, drought, and changes in nutrient status. When plant biomass is converted to fuels, chemicals, electricity, and/or heat, inor­ganic minerals remain as ash, with the amount residual ash being dependent upon plant species. Herbaceous plant species typically have higher levels of ash (e. g. 5-10% dry mass) than do woody species (<2% dry mass).

The term “extractives” is also commonly used when characterizing the composition of plant biomass. Extractives are materials in the biomass that can be dissolved in a solvent (typically water and/or ethanol), including resins, fats and fatty acids, phenolics, phytosterols, salts, minerals, soluble sugars, and other compounds.

More detailed consideration of the composition of cellulosic biomass can be found elsewhere [8,9]. Representative compositions for many of the biomass crops considered in subsequent chapters are listed in Table 1.1.