Candidate crops for biofuel production are generally chosen for their rapid growth rate, high resource-use efficiency (regarding water, radiation and nutrients) and broad tolerance of pests, diseases and stressful environments [104-107]. However, the traits that characterize an ideal biofuel crop are also the same traits that characterize many invasive species [93, 104-106].

The potential invasiveness of Miscanthus species has been assessed in various locations using either a weed risk assessment (WRA) or field observations (Table 4.1). WRA is a model adapted from the Australian weed risk assessment system or AWRA developed by Pheloung [108] to a local environment. The WRA uses an additive approach, with set scores ranging from -3 to 5 for each of 49 questions dealing with the species’ growth habit and persistence, distribution, reproductive system and whether it has become a weed elsewhere.

Using the WRA, M. x giganteus identifies as noninvasive (Table 4.4) due largely to its sterility and, thus, dramatically reduced risk of escaping into natural environments [92, 105, 106, 109]. This contrasts with M. sinensis, which identifies as invasive at most locations. However, not all genotypes of M. sinensis would be expected to present the same invasiveness risk, since sterile triploid clones exist within this species [110], but unfortunately the genotypes used in the M. sinensis study were not specified. Nevertheless, many invasive species do not produce fertile seeds but are serious invaders as Arundo donax, Polygonum cuspidatum and so on [104-106, 109].

In addition, M. x giganteus rhizomes have a slow rate of lateral growth (only spread at a space around 10 cm per year) compared to M. sacchariflorus, which has extensive creeping rhizomes (can spread several meters in a few years), which increased the risk of spreading due to erosion and water transport [93, 110]. Therefore, M. sacchariflorus is defined as invasive.

It is important to note that invasiveness evaluations using risk assessment systems are not absolute, as they must incorporate subjectivity and uncertainty [116]. For example, the WRA can produce incorrect answers and provides non definitive answers in almost one — third of all cases [106, 116]. Therefore, it is recommended that a combined assessment approach be used that incorporates pre-entry and post-entry evaluation tools [105,106, 116].

1. Pre-entry evaluation

• WRA to identify invaders and benign species and reject or accept them for intro­duction. The protocol used would be based on the biology of the target species and its ecology, climatic requirements, history and biogeography relative to the target regions. [106, 109]

• Climate matching analysis (e. g. CLIMEX model [119]) to determine the climatic and agronomic regions at risk of a potential invasion.

• Evaluate the potential for the proposed bioenergy crop to hybridize with related species or taxa.

2. Post entry evaluations

Any species conditionally accepted would then require in situ ecological analyses:

• Agronomic trials in quarantined field trials in the new environments.

• Long-term experiments to determine the competitiveness of the proposed bioenergy crop within native or managed ecosystems in the new environment.

• An efficient management plan covering the eradication of each feedstock in case of invasiveness after commercialization [106, 109].

Breeding and management programs will also help to minimize the invasive risk of Miscanthus [109] by minimizing seed production and rhizome spread.

Harvesting residues from corn and wheat will undoubtedly provide the most plentiful agri­cultural source of cellulosic biomass for the foreseeable future because of the extensive area upon which these crops are grown in the U. S.A. However, to achieve a sustainable harvest strategy only a portion of the total residue produced can be harvested and a suf­ficient amount must be left behind to meet all other critical ecosystem services and soil protection requirements. The ultimate challenge of balancing economic drivers favoring increased harvest to meet conversion demand with minimal transportation cost against the ecologically limiting factors (Figure 8.8) was well illustrated by Wilhelm et al. [20]. In fields where excess residue interferes with subsequent planting, stand establishment,

Economics

and nitrogen immobilization, partial residue harvest will likely increase subsequent yields. However, in more rolling and erosive landscapes most of the residue produced will likely be needed for soil protection. So, how can producers know whether or not they should consider harvesting their residues?

One strategy being developed with much of the REAP and RP data described above is the Residue Management Tool. This tool uses various databases and input information such as (1) the location and spatial extent of the potential harvest area, (2) crop rotations, (3) tillage management, (4) residue harvest methods, and (5) other land management practices to establish the potential for a safe and sustainable harvest. Every scenario involving these factors can be examined with the tool using an integrated systems model for which the input information can be defined. Using the location and spatial extent (which can be obtained directly from a combine using output files from the yield monitor), the site-specific crop yields, soils data, and climate data are assembled from the coupled databases. As the integrated residue removal tool executes its set of scenario runs, the data management modules are dynamically accessed to acquire and format the data needed for each of the models being coupled together. The integrated residue removal tool loops across the complete set of scenarios pushing each model output to the results database. The tool then aggregates the results calculated for each of the scenario runs.

Currently, the tool uses models such as RUSLE2 and WEPS to determine the amount of residue needed to mitigate water and wind erosion, and CQESTR or DAYCENT to monitor changes in the soil organic matter pool. Nutrient balance models (e. g. IFARM) and soil-test information help ensure those needs are being met and work is ongoing to
develop least-limiting water relationships between soil aeration, compaction, and plant response. By connecting all of these models and supporting input information, various soil and crop management scenarios can be created and used to develop and guide sustainable crop residue harvest programs.

The initial version of the Residue Management Tool has been developed and is currently being evaluated for use with corn stover feedstock systems. However, since the tool is simply a computer framework that connects user supplied information about the location and spatial extent to be investigated, crop rotations, tillage management practices, residue removal methods, and land management practices, it can be easily adapted for other cellulosic energy crops by changing or adding additional simulation models to those it currently connects. Also, by expanding the spatial scale, the tool could be used to design landscape management scenarios [21] that could utilize multiple cellulosic energy crops to achieve economically viable feedstock production goals while simultaneously providing other ecosystem services, such as erosion control, nutrient cycling, buffering and filtering, wildlife habitat, carbon sequestration, and opportunities for rural development. The need for such an integrated framework was recently recognized by the Chicago Council on Global Affairs in a report that examined not only agronomic crops but also various waste streams as potential cellulosic feedstock for sustainable bioenergy production.

We conclude that although crop residues may often be excluded from cellulosic energy crop discussions, they will undoubtedly be part of cellulosic bioenergy systems for many years. The best option from our perspective is to integrate them into an overall feedstock production and delivery system that will be economically, environmentally, and socially acceptable for many years to come.

Any purchaser of biomass is interested in the cost of the material as it enters their plant. The plant can be burning the biomass directly to produce heat and power, or it can be using the biomass as a feedstock for some more complex conversion process to produce a high-value product. In this chapter, the term “feedstock” is used to refer to any raw biomass before its chemical structure is modified by a conversion process, be it direct combustion, thermochemical, or biological.

Feedstock cost ($/dry ton) is defined as the cost of the stream of size-reduced material entering the reactor at the bioenergy plant for 24/7 operation. (The reactor is defined as the unit operation where an initial chemical change in the feedstock occurs.) The reason for choosing this end point for the biomass logistics system is twofold:

1. The plants that can operate continuously, 24/7, have an advantage. Maximum production (tons/yr) per unit of capital investment gives a competitive advantage in the market place — the cost to produce the product is lower.

2. Some logistics systems do size reduction with the harvesting machine, while some reduce size at a transfer point between in-field hauling machines and highway hauling machines (perhaps as a prerequisite to a densification step), and some reduce size at the entrance to the processing plant. In order to compare the several systems, it is necessary to have a consistent end point for the system analysis.

The reader should be aware that many studies in the literature select a different analysis end point than used here. A typical end point is the cost of feedstock when a truckload of raw biomass enters the plant gate. This end point is favored because of tradition. Some agricultural (and forest product) industries still pay the producer when the raw material is delivered to their plant. Other industries have moved away from this model.

Edward P. Richard, Jr.1 and William F. Anderson2

1Sugarcane Research Unit, USDA Agricultural Research Service, U. S.A.

2 Crop Genetics and Breeding Research Unit, USDA Agricultural Research Service, U. S.A.

5.4 Sugar and Energy Cane

The 2007 Energy Independence and Security Act mandates that 16 billion of the targeted 36 billion gallons of biofuels must be derived from cellulosic sources. Sugarcane as a biofuel feedstock has tremendous potential as a source of this biofuel [1,2]. Sugarcane is a major agronomic crop grown in approximately 80 countries within the latitudes of 30°N and 35°S [3,4]. Utilization of the entire aboveground sugarcane plant and the development of high-flber/low-Brix types of sugarcane as a potential bioenergy feedstock for cellu­losic conversion technologies has been reviewed [5-10]. Sugarcane grown solely for the production of energy is commonly referred to as energy cane [11]. For energy cane to be sustainable, it must economically produce high and consistent yields [7]. This chapter discusses the production of sugar/energy cane as a dedicated bioenergy feedstock with an emphasis to areas where sugarcane may not be traditionally grown. Information on the production of energy cane is limited; however, its production should be somewhat similar to the production of sugarcane for sugar and much of the information presented will be based on research conducted on the production of sugarcane for sugar.

Another planting configuration involves an intercropping system using wide rows of saw­timber crops that are interplanted with a biomass crop such as Panicum virgatum. The rows of pine are planted 6.1 m apart and pines are planted 1.5 m apart within each row. To capture the solar energy between rows, a biomass crop is established and harvested for fuel. This dual cropping system has an advantage in that some of the understory vegetation is

converted to a useful product (e. g. a liquid fuel). The company, Catchlight Energy, is currently working with the forest products company Weyerhaeuser in researching the opportunities for producing cellulosic biofuels from biomass crops in pine plantations.

Throughout the world, sorghum is known and grown for its ability to use water efficiently and to maintain productivity during drought [55]. This is important because biomass pro­duction from energy crops is expected to be rainfed. The water use efficiency and/or drought tolerance of sorghum is due to a number of morphological and physiological traits [56]. As would be expected, there is significant variation in the Sorghum genus for drought tolerance among the types of sorghum (i. e., energy, grain, or sweet) and the genotypes within each group [57]. This variation has been used by breeders to enhance drought tolerance of grain sorghums [58] and has created interest in deploying it in energy sorghum cultivars as well.

There are differences in drought tolerance between sweet sorghum and energy sorghum that are related to maturity of the two groups [1]. In essentially all crops, reproductive growth, from the initiation to completion, is more sensitive to water stress than vegetative crop growth [59]. Since energy sorghum does not initiate reproductive growth until very late in the season (and in some cases, not at all), it has an inherently greater level of tolerance to water stress during the growing season than either grain or sweet sorghum cultivars. When drought occurs, energy sorghum can essentially become dormant and resume growth when moisture is available. Because sweet and grain sorghum genotypes flower, post-anthesis drought tolerance (i. e., “stay-green”) is critical in these types [60]. Over the past 30 years, the stay-green trait has been integrated into many grain sorghum hybrids and cultivars. It is much less common in sweet sorghum because there has been a limited breeding effort in sweet sorghum until very recently. Given that stay-green is associated with increased non-structural carbohydrate accumulation in the stalk [61] and greater tolerance to charcoal rot lodging [62], it is likely that there will be little or no detrimental effect to introgression of this trait into sweet sorghum.

Poplar trees have been extensively cultivated in many countries and several different tech­nologies for using their biomass have been implemented. Using wood obtained from SRP poplar as a fuel has energy, economic and environmental advantages when compared to coal and other fossil fuels. When used for direct combustion in heat and power plants, wood biomass has advantages over herbaceous biomass because of the lower quantity and higher quality ash that, in many cases, can be returned and applied as a soil amendment. The quantity of ash is related to chemical composition and bark content. Therefore, Guidi et al. [58] conducted studies to determine allometric relationships to predict fuel quality of poplar biomass before harvesting was undertaken. They found a significant relationship between bark content and main stem diameter at 130 cm (diameter at breast height, DHB) and pointed out that for DHB classes between 1 and 4 cm there was a rapid reduction in bark content compared to stems with a DHB of less than 1 cm. This indicated that it is more rational to harvest SRP poplar in three or four-year cutting cycles or to use poplar clones that do not produce a high number of low DHB stems.

Poplar wood can also be treated as a feedstock for production of second and third gen­eration biofuels through conversion of lignocellulose into ethanol [59] and other fuels. However, it is important to recognize that lignocellulosic biomass is a complex matrix of hemicelluose, cellulose and lignin and, therefore, pretreatment (sometimes called prehy­drolysis) is required before the biomass can be converted into liquid fuels. Authors studying different methods of Populus nigra biomass pretreatment (steam explosion and hot water pretreatment) have found that the former process gave better cellulose recovery when mea­sured by enzymatic conversion of the biomass into bioethanol [60,61]. In an extensive review, Huang et al. [62] also reported numerous technologies designed to provide the most effective pretreatment of lignocellulosic biomass and conversion into ethanol. They concluded that the best results have been achieved when complex methods (i. e., chemical, physical, and/or biological pretreatments) were combined. For enzymatic hydrolysis and fermentation, the most important and efficient method utilized cellulase produced by the commercially available fungus Trichoderma reesei.

Zhang et al. [63] presented an interesting but challenging approach to utilize the lignin and hemicelluloses in addition to the cellulose components. According to their citations, economically sound and environmentally friendly technologies for processing these com­ponents, once considered waste, have been developed and are being used to produce marketable products. Among them is the potential to replace phenolic compounds from the oil industry with lignin-originated products, while hemicelluloses, because of their less stable nature, can be converted to a mixture of monosaccharides.

Van Acker et al. [64] reported that by using biotechnology, poplar biomass can be converted into liquid biofuels without costly and energy consuming pretreatment. This can be achieved by reducing the amount of lignin in the wood biomass or by changing its composition to obtain forms that are more susceptible to chemical degradation, thus making saccharification more efficient. The key enzyme in the phenylpropanoid pathway for lignin modification is cinnamoylo-CoA reductase (CCR). Trees that have been genetically modified in terms of CCR regulation were originally produced for the pulp industry, but this trait appears to be even more suitable for processing of poplar wood into second generation biofuels, since saccharification was increased by 50%.

Klasnja et al. [65] compared the calorific value of willow and poplar biomass, with special attention given to a comparison between old and young stems of both species. Bark was separated from wood. The higher heating values of oven dry poplar wood (calculated for the whole tree with an adjustment based on the proportion of bark) ranged from 15 787 to 24 275 kJ kg-1 for one and two-year old clones of hybrid I-214, respectively. The authors concluded that the calorific value of wood is more favorable than that of bark, and the highest calorific values refer to two-year-old trees. Their other conclusion was that woodchips from young SRPs harvested biannually could be used as biofuel without the bark separation needed when using older stems.

The canes of Miscanthus are harvested once each year, from the second year after estab­lishment using a self-propelled forage harvester, like that used for harvesting silage maize or by hay harvesting equipment, including cutters, conditioners and balers [92, 120]. Hay harvesting equipment is used more frequently as it generates a denser feedstock, thus decreasing transport volume [120].

distributions for the invasive species of agronomical origins are similar to the bioenergy crops.

M. sinensis — Large climate niche

(broad climatic tolerance) of bioenergy crops positively correlated with invasiveness.

M. sacchariflorus This does not indicate

invasiveness but broad climatic tolerance have to be considered in evaluation of invasiveness of bioenergy crops

Miscanthus is generally harvested in late winter when biomass quality is at its peak for combustion processes and before crop growth increases again from early spring. By late winter, the composition of biomass is more suited for bioenergy such as combustion (see section genetic diversity for biomass composition). Moisture content is also lower in late winter, enabling a higher production of net energy. In one study, moisture content was shown to decrease by more than half between early and late winter, from 47 to 52% in December to 16-20% in March [43].

A disadvantage of a late winter harvest is that there is less harvestable biomass at this time due to the loss of senescent leaves and the remobilization of nutrients from aboveground to belowground biomass [28]. More than a third of the aboveground biomass can be lost in M. x giganteus between the early harvest period of September or October and the late winter harvest time of February to March [28, 37, 38]. Similar yield losses have been observed for M. sinensis and M. sacchariflorus [24]. An early harvest might better suit cellulosic ethanol production because there should be more lignocellulose sugar available at this time [121]. However, it is likely that early harvested crops will subsequently require more nitrogen and other fertilizer, as considerably more nutrients are exported from the system with an early rather than a late harvest. For example, Strullu et al. [28] showed that M. x giganteus crops harvested in late winter mobilized 71% of their peak nitrogen content to belowground biomass compared with just 42% of the peak nitrogen content of early harvested plants. In the same trial, the nitrogen content of senescent leaves collected over winter from the soil of 2-3 year-old crops amounted to 15.5 ± 3.5 kgN ha-1 yr-1 in late harvested treatments but was negligible in early harvested treatments [50]. An early harvest is, therefore, likely to increase the crop’s requirement for added nitrogen and other fertilizer.

In addition, leaf fall during winter provides carbon to the soil and the litter layer by senescent leaves can also help control weeds. In the same field trial, senescent leaves correspond to about 3 tDM ha-1 yr-1, which correspond to about 1.3 tC ha-1 yr-1 [50]. This was smaller than the amount measured by Kahle et al. [122] for older plants (4.5 tDM ha-1 yr-1, corresponding to about 2 tC ha-1 yr-1). The weight of the litter formed by senescent leaves of Miscanthus x giganteus accumulated at the soil surface was measured every year during nine years of cultivation by Christian et al. [46]. It increased during the four first years and was then fairly constant in time (with about 6 tDM ha-1 on average), probably because the fall of new senescent leaves each year was compensated by leaves decomposition.

In summary, a winter harvest has advantages over an earlier harvest. It allows a remo­bilization of nutrient from aboveground to belowground biomass and it can contribute to enhance carbon sequestration in soil. Nevertheless, the quality of the canes at that date might reduce accessibility of the cellulose and hemicellulose, which would require appropriate processes for some end-uses such as biofuel.

Michael W. Cunningham and Bijay Tamang

ArborGen Inc., U. S.A.

9.1 Phylogeny, Growth, Yield and Chemical Composition

9.1.1 Introduction and Phylogeny

Eucalyptus belongs to the Myrtaceae family and is comprised of more than 700 species. Most Eucalyptus species are native to Australia but a few are also native to New Guinea, Indonesia and Philippines. Corymbia and Angophora are two closely related genera with Eucalyptus. These three genera are collectively known as eucalypts, but Corymbia and Angophora have been classified as subgenera of the Eucalyptus genus in the recent taxo­nomic classification [1].

Eucalyptus is one of the most widely planted genera in the world but large scale planta­tions are mostly limited to tropical areas. Eucalyptus plantations worldwide cover a total of

17.9 million ha [2]. About 11 million ha are located in Asia while South America has about 5 million ha [2]. Eucalyptus has been an attractive species to farmers in developing coun­tries because of its fast growth, straight form, coppicing ability and adaptation to various soil types.

Out of more than 700 species of Eucalyptus, only about 500 have the potential for commercial plantation [3]. E. grandis is one of the widely planted species because of its fast growth and higher productivity. Eighty percent of the Eucalyptus plantations worldwide are comprised of E. grandis, E. urophylla, E. camaldulensis, E. globulus, and their hybrids [4]. In the United States, there are approximately 50 000 ha of Eucalyptus planted in Florida, California and Hawaii [5]. Major species planted in the United States are E. grandis, E. urograndis (a hybrid between E. urophylla and E. grandis), E. benthamii, E. globulus, E. robusta and E. camaldulensis, but only the former three, as well as E. amplifolia, are commercially planted in the southeastern states.

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

Eucalyptus species have various uses. It is a major source of wood for fuel and con­struction material in developing countries. Some species have been used in windbreaks in agricultural farms to modify microclimate and increase yields [6,7]. In South America, Australia and South Florida, U. S.A., Eucalyptus is a major windbreak species to manage citrus canker. Lately, Eucalyptus has received wide attention because of its potential to supply the increasing wood demand from emerging biomass power plants. The pulp and paper industry has been using Eucalyptus for decades for fiber. Because of its fast growth and early canopy closure, some Eucalyptus species also have the potential to suppress light — dependent invasive species such as cogongrass (Imperata cylindrica) and restore degraded mined sites [8].

It is helpful to visualize the various unit operations in a logistics system as a chain with the links as the unit operations. The example in Figure 13.1 shows a logistics system to move round bales from Satellite Storage Locations (SSLs) to a bioenergy plant. The dotted lines show various segments of the chain that are assigned to the several entities in the business plan. In this example, the segment identified “Farmgate operation” is performed by the feedstock producer, the segment identified “Load-haul operation” is included in the operations performed by a load-haul contractor, and the segment identified as “Receiving Facility” is performed by the bioenergy plant.

The division shown by the dotted lines in Figure 13.1 is arbitrary; several other options are used in commercial practice. Three examples are given to show the reader the range of these options.