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].

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.

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