Cellulosic Energy Cropping Systems

1

Introduction to Cellulosic Energy Crops

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].

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 phosphoenolpyruvate (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.

Table 1.1 Representative compositions, proximate analysis, ultimate analysis, and energy density for several lignocellulosic feedstocks.

1.2 Cellulosic Biomass Properties and Their Relevance to Downstream Processing

The choice of biomass feedstock is a critical driver in determining key performance metrics of bioenergy – including economic viability, scale of production (both at individual facilities and in aggregate), and environmental impact. For commodities such as fuels or electricity, feedstock cost typically represents two-thirds of the product cost, or more [26]; therefore, selecting a cost-effective feedstock is essential. As is discussed in Part IV of this book, the logistics of growing, harvesting, storing, and transporting biomass – unique for a given feedstock type – affects the feasible size of the processing facility, which, in turn, impacts the overall sector scale. Each feedstock also has a particular set of environmental attributes – for example, water use, wildlife habitat, soil quality, and so on – that significantly affects the environmental performance of the bioenergy system.

In assessing the suitability of a biomass feedstock for a given conversion process, several material properties are important to consider, including: (1) moisture content; (2) energy density; (3) fixed carbon/volatile matter ratio; (4) ash content; (5) alkali metal content; and (6) carbohydrate/lignin ratio. The first five properties are especially important in thermochemical processing. For biological conversion, the first and last properties are of primary concern.

1.2.1 Moisture Content

Biomass moisture content is defined as the amount of water in the biomass expressed as a percentage of the material's weight; reporting on a wet basis is most common. Moisture content at harvest for woody feedstocks is usually 40–60% (wet basis); for herbaceous crops, it typically ranges from 10 to 70% (wet basis) depending upon the species, climate, geographic location, and stage of maturation. Biomass net energy density per unit mass decreases with increasing moisture content. Transport efficiency of biomass feedstock, therefore, decreases as moisture content increases. Storage of high-moisture biomass is also less efficient, both because of reduced energy density and increased probability of biological degradation, fire risk, and mold formation. Moisture content also affects downstream processing, especially for thermochemical conversion. High-moisture feedstocks must be dried to levels of less than 50% for conventional combustion and less than 20% for gasification and pyrolysis. In biological processing for which some form of thermal pretreatment is used, moisture content can also significantly affect the energy efficiency of the process.

1.2.2 Energy Density

Energy density, often termed heating value, refers to the amount of energy released per unit fuel combusted, usually measured in terms of energy content per unit mass for solids (e.g. MJ/kg) and per unit volume for liquids (e.g. MJ/l). Energy density can be expressed in two forms, higher heating value (HHV) or lower heating value (LHV). HHV represents the total energy released when the fuel is combusted in air, including the latent heat contained in the resulting water vapor product – the maximum potentially recoverable energy from a given feedstock. The latent heat contained in the water vapor, however, typically cannot be used effectively. LHV, therefore, is the appropriate value to use when quantifying the energy available for subsequent use. As noted above, moisture content significantly affects biomass feedstock energy density. Freshly cut wood, for example, might have as much as 60% moisture and a relatively low energy content (e.g., 6 MJ/kg). In contrast, oven-dried wood with little moisture might have up to 18 MJ/kg. Representative LHV values for many of the biomass crops considered in subsequent chapters are listed in Table 1.1.

1.2.3 Fixed Carbon/Volatile Matter Ratio

Fuel analysis that quantifies the amount of chemical energy stored as volatile matter (VM) and fixed carbon (FC) has been developed for solid fuels such as coal. The VM of a solid fuel is the portion released as gas (including moisture) by heating to 950°C in the absence of air for seven minutes; the FC is the mass remaining after the volatiles have been driven off, excluding the ash and moisture contents. Fuel analysis based upon VM content, ash, and moisture, with the FC determined by difference, is termed the proximate analysis of a fuel. Elemental analysis of a fuel, presented as C, N, H, O and S, together with the ash content, is termed the ultimate analysis of a fuel. The ratio of FC to VM provides an indication of the ease with which the solid fuel can be ignited and subsequently gasified, or oxidized, depending on how the fuel is to be converted. Representative proximate and ultimate analyses for many of the biomass crops considered in subsequent chapters are listed in Table 1.1.

1.2.4 Ash Content

Conversion of biomass feedstock, either thermochemically or biochemically, results in a solid residue. In themochemical processing via combustion in air, the residue consists solely of ash. For biochemical processing, it contains both ash and other unconverted material, especially lignin. The bioprocess residue can be further processed thermochemically to yield ash as the final solid residue. The ash content negatively affects the energy density of the feedstock. Ash can also pose operational problems in thermochemical processing, such as slagging in which the ash melts and fuses together. Relatively low-cost control measures, such as leaching the raw feedstock with water and using different mineral additives (e.g. kaolinite, clinochlore, ankerite), can be used to reduce negative effects [27]. Potential end uses of ash include mineral agricultural fertilizer [28] and construction material additive [29]. Representative ash content values for many of the biomass crops considered in subsequent chapters are listed in Table 1.1. As can be seen from the table, herbaceous feedstocks tend to have higher ash contents (e.g. ≥5%) than woody feedstocks (e.g. <2%).

1.2.5 Alkali Metal Content

During thermochemical conversion, alkali metals (Na, K, Mg, P, Ca) present in the ash react with silica – originating both from the biomass itself and from soil introduced during harvesting – to produce a sticky, mobile liquid phase that can contribute to slagging, deposition, and corrosion of process equipment. As noted above, water leaching and fuel additives can be used to reduce the damaging effects of ash components, including alkali metals.

1.2.6 Carbohydrate/Lignin Ratio

In biological processing, carbohydrate present in cellulose (and potentially hemicellulose) is converted to fuels and/or chemicals, while the lignin fraction remains unaffected. Furthermore, the recalcitrance of cellulosic biomass to bioconversion typically increases with increasing lignin content, requiring more severe pretreatment, which decreases process efficiency. Bioconversion processes, therefore, favor feedstocks with high carbohydrate to lignin ratios. Representative cellulose, hemicellulose, and lignin values for many of the biomass crops considered in subsequent chapters are listed in Table 1.1.

1.3 Desirable Traits and Potential Supply of Cellulosic Energy Crops

Given the world's finite land resource, the most important trait for cellulosic energy crops is productivity – the annual dry matter produced per unit land area. As listed in Table 1.1, productivity of the crops considered in this book ranges from 0.1 to 1.75 Mg/ha/yr (dry basis) for wheat straw, to as high as 44 Mg/ha/yr (dry basis) for miscanthus. The best energy crops will also have few inputs and low production costs. Easily established, robust perennial crops having long life spans (e.g. ≥10 years) are favored over annual crops, as are those having low fertilizer, pesticide, and insecticide requirements. Native, non-invasive species that provide good habitats for wildlife are preferred.

Feedstocks used in thermochemical processing should be harvested when moisture content is relatively low to minimize preliminary energy intensive drying. Low moisture is not as critical in bioconversion feedstocks, for which wet storage can sometimes be a viable option. Ideally, ash content should be low (e.g. <1%), ash melting temperatures should be high (e.g. >1500°C), with low levels of particularly damaging elements, including alkali metals, alkaline earth metals, silicon, chlorine, and sulfur.

Conventional plant breeding – which involves manipulating the genes of a species via selection and hybridization so that desired genes are packaged together in the same plant and as many detrimental genes as possible are excluded – has traditionally been used to enhance desired agronomic traits such as productivity, water use efficiency, and crop lifespan. Breeding systems have been developed, and continue to be developed, that can be used to improve virtually all plant species. The productivity of corn, for example, has more than quadrupled since the 1930s largely through conventional breeding [30]. Biomass productivity can potentially be increased even further using more sophisticated biotechnology techniques. Recent molecular and genetic studies have identified a number of regulators of plant biomass production – for example, vegetative meristem activities, cell elongation, photosynthetic efficiency, and secondary wall biosynthesis – that might be manipulated to enhance energy crop yields [31].

The potential to produce viable energy feedstocks is vast. A detailed study led by the Oak Ridge National Laboratory estimates that the United States could produce 602–1009 million dry tons annually by 2022, and 767–1305 million annual dry tons by 2030, at a price of $60 per dry ton [32]. (The low value in the range assumes a 1% annual increase in yield; the high value, a 4% annual increase.) This excludes resources that are currently being used, such as corn grain and forest products industry residues. When currently used resources are included, the total biomass estimate jumps to over one billion dry tons per year for the lower productivity case – enough to displace about half of the country's current gasoline consumption (134 billion gallons/year) if converted to ethanol at a yield of 100 gallons/dry ton. Estimates for the global annual supply of biomass feedstocks range from 100 to 400 EJ/year – equivalent to 6 to 24 billion dry tons. If converted to ethanol, this represents 120–460% of current global gasoline consumption (338 billion gallons/year).

1.4 The Case for Cellulosic Energy Crops

With ever-increasing indications that resource use is exceeding the planet's biocapacity [33] – largely driven by non-renewable fossil fuel consumption – it is clear that humankind must shift to sustainable practices in order for a peaceful, equitable, and thriving future to be possible. Furthermore, given mounting evidence of climate change – to the point that some say we are now living in a new geologic epoch, the Anthropocene [34] – this transformation must begin now and be completed within decades, not centuries. Indeed, it is fair to characterize this transition, moving from finite resource capital to renewable resource income, as the defining challenge of our time.

Most sustainable paths from primary resources to human needs pass through either plant biomass or renewable electricity, with biomass being the only foreseeable source of organic fuels, chemicals, and materials, as well as food. In comparison, other large-scale sustainable energy sources are most readily converted to electricity and heat. Because liquid organic fuels have a greater energy density than batteries, both today and with anticipated improvements in battery technology, it is reasonable to expect that organic fuels will meet a significant fraction of transportation energy demand for the indefinite future. This is particularly true for long-distance travel via personal vehicles and for heavy-duty applications, such as aviation and long-haul trucking, which account for more than half of global transportation energy [35]. Biofuels would, therefore, appear to be an essential component of tomorrow's sustainable world rather than a discretionary option.

Cellulosic biomass energy potentially offers many environmental benefits that contribute to its sustainability, some of which are:

Fossil fuel displacement.

Lower emissions of greenhouse gases and other air pollutants.

Enhanced soil quality.

Reduced soil erosion.

Reduced nutrient run-off.

Enhanced biodiversity.

Demirbas [36], Rowe et al. [37], Arjum [38], and Skinner et al. [39] provide more detailed reviews and discussion of these and other potential benefits.

In addition to the environment, cellulosic biomass energy also has the potential to enhance energy security and rural economic development. Nations dependent upon petroleum face increasing security costs to ensure the steady supply of oil. The United States, for example, according to the RAND Corporation [40], spends about $75.5–93 billion per year – representing between 12 and 15% of its current defense budget – to secure the supply and transit of oil. Furthermore, major oil supplying countries hold leverage over nations relying upon imports, as the oil producers control price stability. This directly affects foreign policy, forcing import nations to prioritize stability over values such as democracy, transparency, and human rights. Even if a country could produce 100% of the oil it uses, its consumers would still be vulnerable to global price fluctuations based on supply disruptions in unstable regions. Beyond consumerism, modern militaries invest for the long term – new airplanes, ships, and vehicles are expected to last decades. This requires alternative energy sources to be able to accommodate infrastructure that is likely to be in place for years.

In recognition of this, the United States Department of Defense has developed an alternative fuels policy to ensure operational military readiness, improve battle space effectiveness, and increase the ability to use multiple, reliable fuel sources [41]. Consistent with this, the US Navy has plans to deploy a Great Green Fleet strike group of ships and aircraft running entirely on alternative fuel blends – including cellulosic fuels – by 2016 [42]. It also has a goal of meeting 50% of its total energy consumption from alternative sources by 2020. To help enable these goals, the Navy – together with the Departments of Energy and Agriculture – signed a Memorandum of Understanding (MOU) to assist the development and support of a sustainable commercial biofuels industry [43]. The MOU calls for $510 million in funding over three years to develop advanced biofuels that meet military specifications, are price competitively with petroleum, are at geographically diverse locations with ready market access, and have no significant impact on the food supply.

A cellulosic biofuels industry, by generating demand for agricultural products, has the potential to significantly increase employment in rural areas in sectors ranging from farming to feedstock transportation to plant construction and operation. Workers would be required in a variety of occupations, including: scientists and engineers conducting research and development; construction workers building plants and maintaining infrastructure; agricultural workers growing and harvesting energy crops; plant workers processing feedstocks into fuel; and sales workers selling the biofuels. Brazil's sugar/ethanol industry directly employs about 489 000 workers, with an additional 511 000 workers engaged in supporting agricultural activities [44]; the United States corn ethanol industry directly employs about 400 000 [45]. A study forecasting the impact of advanced biofuels on the US economy estimates that the industry could create over 800 000 jobs by 2022 [46].

Cellulosic biofuels also have the potential to promote rural economic activity within developing nations and improve the lives of the world's poor. Farmers would have increased demand for their products, including crop residues from existing crops, and employ additional workers to produce the energy feedstocks. They would also be able to make use of degraded or marginal land not suitable for food production. Care must be taken, however, to include small landholders in the sector's development and to adequately invest in local workforce training for feedstock production, production facilities construction, and process operation. In addition, to the extent possible, the sector should be developed around existing industries, such as sugarcane processing, to lower investment barriers [47]. Also, selection of feedstock supply chains that do not compromise food security is critical. Significant potential exists to actually enhance food security through bioenergy production – by using inedible crops grown on marginal land, for example, or integrating production of food, animal feed, and bioenergy. One can envision many benefits that might be realized: employment and development of marketable skills; introduction of agricultural infrastructure and knowledge; energy democratization, self-sufficiency, and availability for agricultural processing; and an economically rewarding way to restore degraded land. Bioenergy could potentially improve both food security and economic security for the rural poor [48].

Such benefits, however, are by no means guaranteed. The environmental impact of biomass energy very much depends upon how the given system is designed and implemented. Detractors of bioenergy have called into question its sustainability, citing a number of concerns, including:

Food versus fuel.

Land use change (direct and indirect).

Water use.

Invasive species.

Biodiversity.

This productive debate has prompted an expanding literature analyzing and discussing the keys to getting biofuels right, so that the promise of sustainable bioenergy can be realized [49–51]. To minimize both competition with food production and land use change effects, multiple classes of feedstocks are available, including energy crops grown on abandoned agricultural lands; food crop residues such as corn stover and wheat straw; sustainably harvested forest residues; double crops grown between the summer growing seasons of conventional row crops; mixed cropping systems in which food and energy crops are grown simultaneously; municipal and industrial wastes; and harvesting invasive species for bioenergy [49,50,52–54]. Water use can be minimized by selecting crops having low irrigation requirements, by using non-potable sources such as wastewater or high-saline water for any necessary irrigation [55,56], and using subsurface drip irrigation to minimize evaporative losses [57]. The potential for non-native energy crops becoming invasive can be limited by proper preliminary risk assessment, including test plots [58], regular monitoring and stewardship programs [59], and by using sterile plant varieties [60]. The impact of a given energy crop upon biodiversity depends strongly on specific regional circumstances, the type of land and land use shifts involved, and the associated management practices [61]. Herbaceous perennial crops, in particular, appear to be capable of providing suitable habitats for a variety of species, especially with careful attention to crop placement and when mixed cultures are used [62–65]. By incorporating many of the above strategies, Dale et al. [51] calculated that, using the 114 million hectares of cropland currently allocated in the United States for animal feed, corn ethanol, and exports, 400 billion liters of cellulosic ethanol (80% of current gasoline demand) could be made – all while producing the same amount of food. In summarizing their findings, the authors write:

Our analysis shows that the U.S. can produce very large amounts of biofuels, maintain domestic food supplies, continue our contribution to international food supplies, increase soil fertility, and significantly reduce GHGs. If so, then integrating biofuel production with animal feed production may also be a pathway available to many other countries. Resolving the apparent food versus fuel conflict seems to be more a matter of making the right choices rather than hard resource and technical constraints. If we so choose, we can quite readily adapt our agricultural system to produce food, animal feed, and sustainable biofuels.

Any human activity involving new technology can potentially be harmful if not thoughtfully planned and appropriately conducted. The early-generation Altamont Pass wind farm in California, for example, unwittingly located on a major bird migratory route, results in thousands of bird deaths every year [66]. To remedy the problem, the farm's owners are installing new, less destructive turbines and shutting down a significant fraction of the turbines during the migration season [67]. In the case of cellulosic biomass, if care is taken to address the key concerns noted above, the resource could very likely contribute substantially – indeed, uniquely and essentially, by accommodating energy services not easily met by other means – towards achieving a sustainable global energy future. Kline et al. [50] succinctly capture the promise of this vision:

When biofuel crops are grown in appropriate places and under sustainable conditions, they offer a host of benefits: reduced fossil fuel use; diversified fuel supplies; increased employment; decreased greenhouse gas emissions; enhanced habitat for wildlife; improved soil and water quality; and more stable global land use, thereby reducing pressure to clear new land.

This book – through detailed consideration of cellulosic energy crop production; the logistics of feedstock harvest, storage, and transport; and commercial deployment that is mindful of economic, environmental, and social concerns – seeks to disseminate knowledge that can help make large-scale, sustainable bioenergy a reality.

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2

Conversion Technologies for the Production of Liquid Fuels and Biochemicals

Sofie Dobbelaere, Tom Anthonis, and Wim Soetaert

Centre of Expertise for Industrial Biotechnology and Biocatalysis, Faculty of Bioscience Engineering, Ghent University, Belgium

2.1 Introduction

Until the last century, plant-based resources were largely focused towards food, feed, and fiber production. In addition, biomass has been a major source of energy for mankind worldwide. However, plant/crop-based renewable resources are also a viable alternative to the current dependence on non-renewable, diminishing fossil fuels, to alleviate greenhouse gas (GHG) emissions, and a strategic option to meet the growing need for industrial building blocks and bioenergy. Indeed, biomass seems a very promising resource for substituting fossil hydrocarbons as a renewable source of energy and as a sustainable raw material for various industrial sectors. Over the past decades, the use of biomass has increased rapidly in many parts of the world, mainly to meet the often ambitious targets for energy supply.

Developing biomass into a sustainable, domestic source of affordable biochemicals and biofuels requires the flexibility to use a wide variety of, preferably, non-food biomass resources. Lignocellulosic biomass such as agricultural and forestry residues and herbaceous energy crops can serve as low-cost renewable feedstock for many, next-generation, bio-derived products. However, the use of biomass as feedstock for the production of materials, products or energy requires new technologies well adapted to the physical cha-racteristics of the biomass. The use of plant/crop resources for energy, or as basic building blocks for industrial production, has been limited because of a poor fit with the hydrocarbon processing system that has been successfully developed to utilize fossil fuels [1]. Although biomass is a nearly universal feedstock, characterized by a high versatility, domestic availability, and renewability, at the same time it has also its limitations. Over the years, numerous research and development efforts have been undertaken to develop and apply new cost-efficient conversion processes for lignocellulosic biomass. This chapter gives an overview of the conversion technologies for liquid fuels and biochemicals.

2.2 Biomass Conversion Technologies

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):

Figure 2.1 Schematic representation of the two routes for the conversion of lignocellulosic biomass.

The (Bio)Chemical Route: Biochemical conversion makes use of the enzymes of bacteria 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].

2.3 (Bio)Chemical Conversion Route

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.

Figure 2.2 Schematic picture for the conversion of lignocellulosic biomass into bioethanol highlighting the major steps. Hydrolysis and fermentation can be performed separately (SHF, indicated by broken arrows) or as simultaneous saccharification and fermentation (SSF). In consolidated bioprocessing (CBP), however, all bioconversion steps are minimized to one step in a single reactor using one or more microorganisms. (Reproduced from Dashtban, M., Schraft, H. and Qin, W. Fungal Bioconversion of Lignocellulosic Residues; Opportunities & Perspectives. Int J Biol Sci 2009; 5(6):578–595. doi:10.7150/ijbs.5.578 © 2009, Ivyspring International Publisher [7]).

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.

2.3.1 Pretreatment

The following step is the pretreatment of the fractionated material. The main goal of pretreatment is to overcome this lignocellulosic recalcitrance, to separate the cellulose from the matrix polymers, and to make it more accessible for enzymatic hydrolysis. Reports have shown that pretreatment can improve sugar yields to greater than 90% theoretical yield for biomass such as wood, grasses, and corn [8, 9]. Pretreatment technologies for lignocellulosic biomass include thermal, (thermo)chemical, physical and biological methods or various combinations thereof [5, 9].

In general, pretreatment processes produce a solid pretreated biomass residue that is more amenable to enzymatic hydrolysis by cellulases and related enzymes than native biomass. Many pretreatment approaches, such as dilute acid and steam/pressurized hot water based methods, seek to achieve this by hydrolyzing a significant amount of the hemicellulose fraction of biomass and recovering the resulting soluble monomeric and/or oligomeric sugars. Other pretreatment processes, such as alkaline-based methods, are generally more effective at solubilizing a greater fraction of lignin while leaving behind much of the hemicellulose in an insoluble, polymeric form [10]. Most pretreatment approaches do not hydrolyze significant amounts of the cellulose fraction of biomass but enable more efficient enzymatic hydrolysis of the cellulose by removal of the surrounding hemicellulose and/or lignin along with modification of the cellulose microfibril structure [11]. Biological pretreatment uses microorganisms and their enzymes selectively for delignification of lignocellulosic residues and has the advantages of a low energy demand, minimal waste production and a lack of environmental effects [7, 12, 13]. It has been suggested that there will probably not be a general pretreatment procedure and that different raw materials will require different pretreatments [10]. Table 2.1 gives an overview of the different pretreatment technologies.

Table 2.1 Overview pretreatment methods [9, 14–17].

The choice of the optimum pretreatment process depends very much on the objective of the biomass pretreatment, its economic assessment and environmental impact. Technological factors, such as energy balance, solvent recycling and corrosion, as well as environmental factors, such as wastewater treatment, should all be considered carefully when selecting a method [5]. Diverse advantages have been reported for most of the pretreatment methods, which make them interesting for industrial applications. Only a small number of pretreatment methods has been reported as being potentially cost effective thus far. These include steam explosion, liquid hot water, dilute acid pretreatments, lime, and ammonia pretreatments [11, 16, 18,19]. The complete depolymerization of these renewable feedstock in a cost-effective manner with minimal formation of degradation products represents a significant challenge for microbiologists and chemical engineers. Obstacles in the existing pretreatment processes include insufficient separation of cellulose and lignin (which reduces the effectiveness of subsequent enzymatic cellulose hydrolysis), formation of by-products that inhibit microbial growth and fermentation (e.g. acetic acid from hemicellulose, furans from sugars and phenolic compounds from the lignin fraction [20]), high use of chemicals and/or energy, and considerable waste production. Research is focused on converting biomass into its constituents in a