The transesterification reaction of TGs with dimethyl carbonate (DMC),49,50 methyl acetate51,52 or ethyl acetate53 produces a mixture of FAMEs (or FAEEs) and cyclic glycerol carbonate esters of fatty acids (FAGCs) [or glycerol triacetate (triacetin)] (Fig. 7.3).

DMC-BioD® is a biofuel, patented by Notari et al.,54 that integrates glycerol as glycerol carbonate in a process that can be developed by enzymatic technology,55 but conventional basic catalyst (sodium methoxide — the same biodiesel obtained by vegetable oils and methanol, MeOH-biodiesel) can also be used.

The main problem of an enzymatic process is the inactivation of the enzyme (in this case of lipases) by some short-chain alcohol acyl acceptors such as methanol. In order to enhance the stability of lipases, the short-chain alcohols could be substituted by methyl acetate as acyl acceptors. But this solution needs a great amount of enzyme (three times more than in a normal alcoholysis) and an excessive amount of methyl acetate (1:12 of oil/methyl acetate) to obtain good conversion values. These drawbacks could be the main limitations for a potential industrial application of methyl acetate as acyl acceptor in the transesterification reaction of vegetable oils.

In this context, it is worthwhile exploring novel reagent as acyl acceptors to prepare esters from lipids. DMC is a potential candidate as a reagent for the transesterification of oils due to its eco-friendliness, chemical reactivity and physical properties.56 DMC is neutral, odourless, cheap, non-corrosive, non-toxic and exhibits good solvent properties. Pioch et al. were the first researchers that reported ethyl oleate production by ethyl carbonate and oleic acid reaction catalyzed by an immobilized lipase.57 The enzymatic transesterification of oil with DMC, as acyl acceptor, catalyzed by lipase, results in an irreversible reaction due to the decomposition of carbonic acid monoacyl ester into carbon dioxide and an alcohol, and consequently, the reaction is favoured towards its completion. Moreover, the DMC gives higher conversion than those of conventional acyl acceptors such as methanol or methyl acetate.

Different lipase sources and various vegetable oil feedstocks have been investigated. Some key parameters were explored to determine the optimal transesterification conditions, first of all the stability of the immobilized enzyme, in view of a potential scaling-up to industrial processes.55 The main results concerning lipase sources and vegetable oils are summarized in Table 7.5.

From the screening results shown in Table 7.5, it is noticeable that Novozyme 435 (immobilized Candida antarctica) shows better activity towards all selected

vegetable oils (81.2%, highest conversion with olive oil). Other lipases showed very little or no activity. Further results show that this lipase also exhibited high conversions in non-polar solvents (with the best performance using petroleum ether) and high activity with the optimum molar ratio of 1:4.5 for oil/DMC, using a DMC one-step addition. Concerning the optimum temperature reaction and the enzyme amount, Novozyme 435 strongly increases its activity with increasing quantities of the enzyme (optimum quantity was found to be 10% based on oil weight). Its performance gradually decreases above 50°C. Finally, concerning the more important parameter for an industrial application, the enzyme reusability, Su et al. showed that Novozyme 435 preserves up to 80% of its initial activity after five reaction cycles, if washed with acetone between each batch use.

The principal difference between DMC-BioD® and biodiesel produced from vegetable oil and methanol (MeOH-biodiesel) was the presence of FAGCs in addition to FAMEs. However, the mixture (FAMEs + FAGCs) has relevant physical properties to be employed as a fuel.54,58 Flow and combustion properties of DMC-BioD®, relevant for its applications as a biofuel, are reported in Table 7.6.

Differences with respect to conventional biodiesel can be attributed to the presence of the FAGCs, which have a molecular weight larger than those of the corresponding FAMEs (see flash point and density). Nevertheless, the cetane number is almost the same but always lower than that of fossil diesel. DMC — BioD® has a higher viscosity than MeOH-biodiesel, but if blended with petroleum diesel, for example in a ratio of 20:80 v/v, the kinematic viscosity decreases to

3.3 cSt, a value closer to that of conventional diesel.

Moreover, the addition of DMC-BioD® at 20% level to diesel not only does not affect the fuel performance but also improves the lubricity of the diesel blend, which is a crucial factor for low-sulfur petroleum diesel. The lubricity value does not change significantly between MeOH-biodiesel and DMC-BioD®.

Last, but not least, from an economical point of view, the use of DMC in the transesterification reaction of vegetable oils will bring a minor impact on the overall biofuel costs: a large fraction of glycerol (> 65%) is incorporated into the biofuel in the form of FAGCs and a minor fraction is converted into glycerol carbonate and dicarbonate. These latter compounds could find utilization as additive and chemical intermediates, while, introducing into the market, glycerol carbonate and its derivatives (characterized by a low toxicity) can mitigate the problem of glycerol overproduction due to the increasing biodiesel utilization.58

Since the ABE fermentation is already a mature process (see Section 10.3), the biggest potential for optimization is offered by metabolic engineering. Prerequisite is the knowledge of genome sequence and development of genetic tools, which are both available for C. acetobutylicum and C. beijerinckii. Electroporation has been established as a method of choice for gene transfer to C. acetobutylicum (Mermelstein and Papoutsakis, 1993; Nakotte et al., 1998; Tyurin et al., 2000) and C. beijerinckii (Birrer et al., 1994). For C. acetobutylicum, transformation efficiencies of up to 105-107 transformants/pg plasmid DNA are reported (Mermelstein and Papoutsakis, 1993; Tyurin et al., 2000), but methylation of the DNA proved to be essential prior to transformation (Mermelstein and Papoutsakis,

1993) . Recently, a modular system for Clostridium shuttle vectors was described (Heap et al., 2009), which comprises the most common origins of replication and selective markers for clostridia. During the last few years, major improvements in gene inactivation were achieved as well. Previously, it was only possible to silence genes by antisense RNA techniques (Tummala et al., 2005; Wagner and Simons,

1994) or inactivate (and respectively replace) genes by homologous recombination (Tomas et al., 2005; Tummala et al., 2005). However, the latter is very time­consuming, since the recombination frequency of clostridia is generally not very high. Another problem is the lack of temperature-sensitive plasmids or counterselectable markers for this genus that necessitates the use of non-replicative plasmids, which are rapidly degraded inside the cell by DNases and endonucleases.

To overcome this issue, Soucaille et al. (2008) designed a mutant strain of C. acetobutylicum with an inactivated restriction endonuclease system and a deleted upp gene. This gene encodes an uracil phosphoribosyl-transferase, which catalyzes transformation of 5-fluorouracil into a toxic product and can now be used as a counterselective marker on a respective plasmid. An even faster method is provided by the so-called ClosTron system (Heap et al., 2007; Heap et al., 2010; Shao et al, 2007). This system allows the rapid creation of integration mutants based on a sequence-specific group II intron from Lactococcus lactis.

Several genes involved in solventogenesis were already overexpressed or inactivated in C. acetobutylicum (Table 10.5). Highest butanol titers (238 mM) were reported for a strain with an overexpressed adhE and an inactivated orf5 gene (Harris et al., 2001). orf5 is located directly upstream of the sol operon (Fig. 10.3) and was proposed to encode the repressor of that operon (SolR) (Nair et al, 1994b). However, a more detailed study revealed that its gene product was actually localized extracellularly (which is in contrast to a transcriptional regulator) and is involved in glycosylation-deglycosylation reactions (Thormann and Durre, 2002; Thormann et al., 2002). The repressing effect observed stemmed from an intergenic region between orf5 and the sol operon (Thormann et al, 2002).

In addition to increasing butanol yields, some studies also focused on elimination of by-products and the improvement of substrate utilization and tolerance to a variety of stresses. Acetone formation was reduced by inactivation of acetone — producing genes ctfA/B (Sillers et al., 2009; Soucaille, 2008; Tummala, Junne, and Papoutsakis, 2003) and adc (Jiang et al, 2009). In this context, efforts are also ongoing to engineer the C. acetobutylicum mutant strain M5, which lost the megaplasmid pSOL1 and thus does not produce acetone at all (Lee et al., 2009b; Sillers et al., 2008). The production of the acids acetate, butyrate and lactate was decreased by inactivating the phosphotransacetylase gene pta (Green et al., 1996; Soucaille, 2008) and/or the acetate kinase gene ack (Sillers et al, 2008; Soucaille, 2008), the butyrate kinase gene buk (Green et al., 1996; Harris et al., 2000; Soucaille, 2008), and the lactate dehydrogenase gene ldh (Soucaille, 2008), respectively. However, the elimination of more than one by-product at the same time (in order to design a homo-butanol producer) still remains a challenging task. Especially, the elimination of ethanol as a by-product might be critical, since most butyraldehyde and butanol dehydrogenases also show activity with acetyl-CoA and acetaldehyde, respectively. To create a more robust strain, aerotolerance was prolonged by inactivation of perR (Hillmann et al., 2008) and tolerance to butanol was improved by overexpression of the groESL operon (Tomas et al., 2003b). The latter resulted in a strain that showed 85% less butanol inhibition and a prolonged metabolism that yielded 40% higher butanol titers (Table 10.5). Efforts to improve the substrate utilization of C. acetobutylicum only showed minor success so far. Xylose utilization was improved slightly by introduction of a transaldolase gene talA from E. coli (Gu et al, 2009), and overexpression of cellulosome components resulted in formation of a minicellulosome (see Section 10.2; Sabathe and Soucaille, 2003).

a C. acetobutylicum mutant strain M5, which lost the megaplasmid pSOLI (containing genes adhE, ctfA/B, adc, orf5 and adhE2) and does not produce solvents.

Meanwhile, metabolic engineering also allows butanol production in other organisms, which are easier to handle such as E. coli, Bacillus subtilis or the yeast Saccharomyces cerevisiae (Atsumi, Cann, et al, 2008; Atsumi, Hanai, et al, 2008; Dijk and Raamsdonk, 2009; Donaldson et al, 2007; Inui et al., 2008; Liao et al, 2008; Nielsen et al., 2009; Steen et al, 2008), or have a significantly higher tolerance against butanol such as Pseudomonas putida (Nielsen et al., 2009; Ruhl et al, 2009), or have the ability to grow on abundant substrates like synthesis gas such as Clostridium ljungdahlii (Kopke, 2009). However, the respective butanol yields are (still) insignificant compared to those of solventogenic clostridia (Table 10.6).

(Continued)

Note: acdh67 = acetylating aldehyde dehydrogenase, adh1/2 = alcohol dehydrogenase, adhE/adhE2 = alcohol/aldehyde dehydrogenase, ald = butyraldehyde dehydrogenase, bcd = butyryl-CoA dehydrogenase, bdhB = butanol dehydrogenase, ccr = butyryl CoA dehydrogenase, crt = crotonase, erg10 = thiolase, etfA/B = electron transferring flavoproteins, fnr = oxygen transcriptional regulator, frdB/C = fumarate reductase, hbd = 3-hydroxybutyryl-CoA dehydrogenase, ilvA = threonine deaminase, ldhA = lactate dehydrogenase, leuA = 2-isopropylmalate synthase, leuB = 3-isopropylmalate dehydrogenase, leuC/D = isopropylmalate isomerase, n. d.a. = no data available, pta = phosphotransacetylase, ter = trans-2-enoyl-CoA reductase, thlA = thiolase, yqhD = alcohol dehydrogenase.

Dark fermentation is an alternative method for biological hydrogen production from biomass. It is a process which is carried out in the dark, under anaerobic conditions, and it is directly related to the acidogenic stage of anaerobic digestion process. It has been considered as a viable and effective method, since it is carried out at ambient temperatures and pressures, without photoenergy, so that the cost of hydrogen production is estimated as 340 times lower than that of the photosynthetic processes (Morimoto, 2002). The hydrogen-producing enzymes (hydrogenases) can be utilized in dark fermentations by using pure microbial cultures or by a mixture of anaerobic microorganisms. Since no oxygen is produced or consumed in these reactions, hydrogenase is less likely to be inactivated by oxygen. Organic wastes from agriculture or sewage can be fed into large anaerobic bioreactors, achieving the dual goals of waste management and hydrogen production. Dark fermentation as a method of hydrogen production does not have the demand of expensive photo-bioreactors, which are necessary for direct biophotolysis and photo-fermentation. Fermentative hydrogen production is focused on this handbook, since it is considered as the most promising compared to all biological hydrogen production methods. Brief comparison of biomass materials that can be used for biohydrogen production, microorganisms available, factors limiting biohydrogen production, modelling and process optimization and lastly strategies for process improvement will be highlighted in the next chapter section.

Biological conversion technologies for the production of biofuels cover a range of fermentative and biological processes. These basic technologies have also been employed for decades in the production of ethanol (e. g. wine) from sugars via a two-step process of saccharification (hydrolysis of sugars)/fermentation using yeast (Scheme 1.2), followed by distillation of the alcohol produced to obtain a higher degree of alcohol purity.

Bioethanol is therefore the most common biofuel prepared by biological conversion.13 It is the most employed biofuel on a world level, with the USA currently being the world’s largest producer and Brazil the largest exporter, accounting together for 70% of the world’s production and 90% of ethanol used

as fuel.13

The common feedstocks employed for the production of bioethanol are energy food crops, including sugar cane, corn, wheat, maize and sugar beet, although research on lignocellulosics and woody biomass is under way and these feedstocks have a great potential for future biofuel production.

Through various steps, a wide variety of biofuels can be obtained, including bioethanol, biobutanol and other bioalcohols, biogas and biohydrogen. Biological conversion processes and technologies will be fully addressed in Part III of the book (Chapters 9-11), so we refer the readers to these chapters for further reading on biological conversion processes.

This common milkweed (Fig. 4.5) is native from the Northeast and North Central United States where it grows on roadsides and in undisturbed habitat (Holser, 2003). On the basis of the fatty acid profile, the oil is expected to provide an alternative source to biodiesel production (Adams et al., 1984). Milkweed oil contains more than 6% of palmitoleic acid that is a strong candidate to enhance fuel properties, besides methyl oleate (Knothe, 2008).

Engine performance tests using biodiesel have been analysed by few authors that found appropriate cold weather properties (Holser and Harry-O’Kuru, 2006). Highly unsaturated ester structures oxidise more rapidly than the saturated ones. These oxidative processes lead to degradation of the fuel and reduce its quality.

Enzymes are proteins that work as nature’s catalysts. They are specific in the reactions they catalyze and are very proficient in doing so. Enzymes consist of active sites where the substrates bind, in a favorable position and angle, and react. They lower the activation energies of reactions by large factors and, similar to mineral catalysts, they are not consumed in the process.

Typically, enzymes are named and classified according to the substrates they catalyze, or a word or phrase describing their activity. Accordingly, lipase would be the class of enzymes that hydrolyze triglycerides (or lipids) to produce fatty acids. However, lipases have also been found to display catalytic activity towards a large variety of alcohols and acids in ester synthesis reactions. Since the synthesis of methyl esters are of primary interest, lipase would be the appropriate enzyme to be used for biodiesel production. Lipase-catalyzed production of biodiesel has been proposed to overcome the drawbacks facing the conventional chemically catalyzed methods, and have shown promising results. Most importantly, glycerol can be easily recovered without any complex process, FFA contained in the oils can be completely converted to methyl esters and subsequent wastewater treatment is not required (Al-Zuhair, 2008). As shown in Fig. 6.2, the enzymatic process is less complicated and does not require as many upstream and downstream

(b)

6.2 Comparison between alkali (a) and enzymatic (b) processes.

operations, compared to conventional alkali-catalyzed processes. Furthermore, lipase-catalyzed transesterification is performed at low temperature and ambient pressure making it not only less energy intensive but also safer than chemically catalyzed reactions.

Ethanol, with the chemical formula C2H5OH is a colourless liquid with a boiling point of 78°C and has been used to large extent as a chemical compound in the medical and food industries. Ethanol is highly flammable and has a flame which is difficult to be seen. It is soluble in water and forms an azeotrope, so it is difficult to achieve 100% pure ethanol by distillation. Ethanol can be used as a pure fuel or blended with gasoline or diesel in a transport system. Ethanol has lower energy density (about 34% lower) and lower vapour pressure than gasoline which makes starts in cold weather difficult. Ethanol is less toxic than gasoline, diesel or methanol regarding safety and environmental issues. Ethanol can be broken down by bacteria to carbon dioxide and water and it can be produced from ethene obtained from fossil sources in oil refining and also from biomass as bioethanol.

The most important characteristic of ethanol which makes it suitable as a fuel for Otto engines is its high octane number. The octane number is a numeric representation of the anti-knock properties of a motor fuel. By definition, the octane number is zero for n-heptane and 100 for iso-octane (2,2,4-trimethyl pentane) and for other fuels the octane number is decided by comparison with a mixture of these two compounds. Liquid fuels with a high octane number have better properties during engine combustion. For ethanol, with a high octane number (129), it is possible to push more fuel-air mixture into the engine’s cylinders (higher compression ratio gives higher efficiency and less fuel consumption) without any risk of uncontrolled self-ignition which may cause ‘knocking’ and serious damage to the engine as a consequence.

One disadvantage of ethanol is its low cetane number (8) and it can be used in diesel engines only if some ignition improver (e. g. di-tert-butyl-peroxides) is added to it. These kinds of additives are often costly, but there are commercially feasible alternatives in the market. The cetane number is a numeric representation of a fuel’s ignition properties. By definition the cetane number is 15 for hepta — methyl-nonane and 100 for n-hexadecane. For other fuels, it is decided by comparison with a mixture of those two compounds. Too low a cetane number causes slow ignition and poor engine performance.

It is technically possible to add at least 10% bioethanol to gasoline without any need for changes in the engine of cars and this can reduce gasoline consumption and the net concentration of fossil CO2 in the atmosphere worldwide. One obstacle to mixing a higher percentage of ethanol in gasoline (petrol) is that car manufactures, in many cases, do not guarantee, for ethanol blends more than 5-10%, cars with ordinary gasoline engines. Several modifications are needed to minimise the risk of any damage to some parts of the engine if higher blends are used.

It is possible to use neat ethanol (99% pure, water free) or blended with petrol or diesel in Otto engines and diesel engines, respectively. There are two types of vehicles: one is the flexible fuel vehicle (FFV) in which it is possible to use up to 85% ethanol in petrol, the second group is the vehicles that use pure (neat) ethanol.

When it comes to blends, bioethanol with diesel in private cars or heavy vehicles (buses, trucks, etc), and the addition of emulsifying agents is essential to achieve a homogenous emulsion of ethanol and diesel (aliphatic hydrocarbons). In pure ethanol fuel for a diesel engine, addition of an ignition improver is necessary. The ignition improver will increase the production cost of the bioethanol as fuel in transport sector.

All bioethanol may not be used as transport fuel. In fact, ethanol is used in the production of other industrial chemical compounds such as ethylene, ethyl acetate, acetic acid and acetaldehyde by various chemical reactions, e. g. oxidation, esterification. Therefore, as the production of bioethanol increases, it will replace fossil sources for ethanol production in many aspects.

K. STAMATELATOU, G. ANTONOPOULOU and G. LYBERATOS, University of Patras, Greece

Abstract: Anaerobic digestion is a biological process that converts the organic matter present in various types of wastes (sewage sludge, agro-industrial wastes, OFMSW, energy crops) into: (1) biogas (rich in methane, suitable to be used for heat and/or electricity generation) (2) biosolids (microorganisms grown on the organic matter and unconverted particulate residues mostly fibres which can be used as soil conditioner), and (3) liquor (dissolved organic matter, recalcitrant to anaerobic degradation and nutrients, which may be used as liquid fertiliser). The vast improvement in various scientific fields (reactor engineering, modelling and control practices, molecular tools) helped to gain a better insight of the process. In addition, the policy to promote biogas utilisation contributed in boosting the application of the anaerobic digestion technology to achieve a two-fold goal: energy production and waste minimisation. All these aspects are discussed in what follows.

Key words: anaerobic digestion, biogas, control, modelling, utilisation.

12.1 Introduction: the anaerobic digestion process

Anaerobic digestion is a biochemical process conducted by the concerted action of a consortium consisting of several groups of microorganisms that degrade the organic matter into a gaseous mixture consisting of methane and carbon dioxide (biogas) in the absence of oxygen. It happens naturally in environments with lack of oxygen such as the bottom of lakes, swamps, the landfills or the intestine of animals. However, the term ‘anaerobic digestion’ usually describes the technology of accelerating the naturally evolved bioprocess in an artificial environment of a closed vessel.

Anaerobic digestion was first applied in the tenth century bc for heating bath water in Assyria. In the seventeenth and eighteenth centuries, a flammable gas mixture produced was correlated with the decay of organic matter and, moreover, the correlation became quantitative; the more organic matter is decayed, the more flammable gas is produced. It was in 1808, when Sir Humphrey Davy discovered that methane was a constituent of the gas produced by cattle manure. The anaerobic technology was first demonstrated in Bombay, India, in 1859, by building an anaerobic digester (Meynell, 1976). The biogas recovered from a sewage treatment plant was used to fuel street lamps in Exeter, England, in 1895 (McCabe, 1957). The development of microbiology science in the 1930s brought up further improvement in the anaerobic technology through identification of the anaerobic bacteria, and the conditions favouring the process efficiency and the limitations (Buswell and Hatfield, 1936). Since then, numerous anaerobic applications have been developed worldwide, mainly in the field of waste treatment, but also in manufacturing of chemicals, fibres, food, etc.

Emerging and developing economies:

Afghanistan, Republic of Albania, Algeria, Angola, Antigua and Barbuda, Argentina, Armenia, Azerbaijan, Bahamas, The Bahrain, Bangladesh, Barbados, Belarus, Belize, Benin, Bhutan, Bolivia, Bosnia and Herzegovina, Botswana, Brazil, Brunei Darussalam, Bulgaria, Burkina Faso, Burundi, Cambodia, Cameroon, Cape Verde, Central African Republic, Chad, Chile, China, Colombia, Comoros, Congo, Democratic Republic of Congo, Republic of Costa Rica, Cote d’Ivoire, Croatia, Djibouti, Dominica, Dominican Republic, Ecuador, Egypt, El Salvador, Equatorial Guinea, Eritrea, Estonia, Ethiopia, Fiji, Gabon, Gambia, The Georgia, Ghana, Grenada, Guatemala, Guinea, Guinea-Bissau, Guyana, Haiti, Honduras, Hungary, India, Indonesia, Iran, Islamic Republic of Iraq, Jamaica, Jordan, Kazakhstan, Kenya, Kiribati, Kuwait, Kyrgyz Republic, Lao People’s Democratic Republic, Latvia, Lebanon, Lesotho, Liberia, Libya, Lithuania, Former Yugoslav Republic of Macedonia, Madagascar, Malawi, Malaysia, Maldives, Mali, Mauritania, Mauritius, Mexico, Moldova, Mongolia, Montenegro, Morocco, Mozambique, Myanmar, Namibia, Nepal, Nicaragua, Niger, Nigeria, Oman, Pakistan, Panama, Papua New Guinea, Paraguay, Peru, Philippines, Poland, Qatar, Romania, Russia, Rwanda, Samoa, Sao Tome and Principe, Saudi Arabia, Senegal, Serbia, Seychelles, Sierra Leone, Solomon Islands, South Africa, Sri Lanka, St. Kitts and Nevis, St. Lucia, St. Vincent and the Grenadines, Sudan, Suriname, Swaziland, Syrian Arab Republic, Tajikistan, Tanzania, Thailand, Timor-Leste, Democratic Republic of Togo, Tonga, Trinidad and Tobago, Tunisia, Turkey, Turkmenistan, Uganda, Ukraine, United Arab Emirates, Uruguay, Uzbekistan, Vanuatu, Venezuela, Vietnam, Yemen, Republic of Zambia, and Zimbabwe

Advanced economies:

Australia, Austria, Belgium, Canada, Cyprus, Czech Republic, Denmark, Finland, France, Germany, Greece, Hong Kong SAR, Iceland, Ireland, Israel, Italy, Japan, Korea, Luxembourg, Malta, Netherlands, New Zealand, Norway, Portugal, Singapore, Slovak Republic, Slovenia, Spain, Sweden, Switzerland, Taiwan Province of China, United Kingdom, and United States

Rice straw is one of the more abundant lignocellulosic waste materials in the world (Fig. 4.12), reaching 731 million tons per year. This amount of rice straw can potentially produce 205 billion litres bioethanol per year, which would be the largest amount from a single biomass feedstock (Bohlmann, 2006). By selecting high — biomass yielding species, combined with high nutrient and water use efficiency, economically efficient production of biofuel feedstock may be realised on less optimal land without pressuring prime grain crop territories (Jakob et al., 2009).

Panicum virgatum

Panicum virgatum, also known as switchgrass (Fig. 4.13), is a native perennial warm-season (C4 plant) grass with deep roots, growing on relatively poor quality lands, where water and nutrient availability would prevent the successful production of conventional crops. A widely adapted endemic species, it is an important ecological component of North American native grassland ecosystems (Lewandowski et al., 2003).

One of the advantages of switchgrass is that it can be harvested and handled with conventional hay-making equipment (Cundiff and Marsh, 1996; Sokhansanj et al., 2009). Switchgrass combines more of the attributes desirable for bioenergy feedstock production than other grasses. These attributes include distribution and high productivity across a wide geographical range and on diverse agricultural sites, high water use and nutrient use efficiency, and positive environmental attributes — including effects on soil quality and stability, cover value for wildlife and relatively low inputs of energy, water and agrochemicals required per unit of energy produced (McLaughlin and Walsh, 1998). Comparing corn and switchgrass on marginal soils for biofuel production, Varvel et al. (2008) found that the potential ethanol yield from switchgrass was equal to or greater than the potential total ethanol yield from corn grain.