Organosolv pretreatment processing involves the use of aqueous organic solvents such as ethanol, methanol, hexane, acetone and inorganic acid catalysts such as hydrochloric acid (HCl) or sulfuric acid (H2SO4) to break the internal lignin and hemicellulose bonds. Organosolv pretreatments, as with other chemical pretreatments, often produce microbial inhibitory agents because it is usually performed at higher temperature (above 180°C) and can require large amounts of pretreatment reagents (Zhu et al. 2010). The organic solvents dissolve lignin and hemicellulose and leave the biomass residue with high cellulose content. The lignin and hemicellulose recovered from the organic solvent have a potential market value (see Table 2).

A plan or listing of activities for establishing and harvesting switchgrass is an essential prerequisite for preparing a switchgrass enterprise budget. The most economical method for establishing stands of switchgrass will differ across regions and soil types. The plan that follows is appropriate for cropland in the U. S. Southern Plains that is harvested in the summer or fall or for cropland pasture.

1. Conduct primary tillage in the fall prior to the spring in which the crop is to be planted.

2. Test the soil and if necessary apply the appropriate levels of phosphorus and potassium fertilizer and agricultural lime.

3. Conduct secondary tillage in late winter and use a cultipacker to firm the seedbed.

4. Wait for rainfall to germinate annual weeds.

5. If weeds are present, apply glyphosate within three days after planting. In some regions some pre-emerge selective herbicides may be registered for use. For example, if registered, s-metolachlor may be applied to fields prior to switchgrass emergence if the seed has been safened with fluxofenin (Vogel et al. 2002). Vogel et al. (2002) also report using atrazine as a pre-emerge herbicide. Mitchell et al. (2012) recommend a combination of quinclorac and atrazine as pre-emerge herbicides for switchgrass establishment.

6. In April, without additional tillage, plant 5.6 kg/ha pure live switchgrass seed 0.6 to 1.3 cm deep in the firm seedbed.

7. If broadleaf weeds are present, apply a labeled post-emerge herbicide.

8. In the summer, if weeds are excessive, a mowing activity with a rotary mower may be warranted before the weeds start to canopy the switchgrass. Clipping the weeds at the top of the switchgrass may increase the probability that sunlight can reach the young switchgrass plants.

9. Exercise patience and permit the young plants to become firmly established. Do not harvest during the establishment year.

10. In late winter after the establishment year, a prescribed burn may be conducted to facilitate new growth.

11.In year 2 and all subsequent years, fertilize with an appropriate level of nitrogen at spring green up and harvest once per year. Late in the growing season, nutrients (including nitrogen, phosphorus, and potassium) translocate from the above ground foliage to the plant’s crown and rhizomes. If harvest is delayed until after the first frost and the initiation of senescence, biomass yield will be maximized and nutrients will have translocated, reducing the quantity of fertilizer needed for biomass production in subsequent years (Madakadze et al. 1999; Sanderson et al. 1999; Reynolds et al. 2000; Vogel et al. 2002; Adler et al. 2006; Guretzky et al. 2011).

Weed control is an important factor in crop production and especially in switchgrass establishment. Weed competition can be reduced by using labeled herbicides or by tilling in the fall and again in the spring prior to planting. Broadleaf selective herbicides may be used to control most broadleaf weeds in stands of young switchgrass. However, grassy weeds can be more problematic. As noted in step 8, if weeds are excessive, a summer mowing activity with a rotary mower may be warranted before the weeds start to canopy the switchgrass. Clipping the weeds at the top of the switchgrass is a strategy designed to enable sunlight to reach the young switchgrass plants. Table 1 includes a listing of the field operations budgeted for switchgrass establishment with conventional tillage. Steps for establishing switchgrass in fields previously used to produce winter annuals such as wheat, barley, or rye are also included in Table 1.

Table 2 includes a listing of field operations that may be used to establish switchgrass without tillage. Adequate soil fertility, weed control, and an effective no-till drill are critical components of successful no-till switchgrass establishment.

Use of syngas for electrical power and heat production is more adaptable and direct as compared to its use for fuels and chemicals production. Several demonstrations of heat and power generation using syngas generated from biomass and wastes are available in literature (Young and Pian 2003; Bengtsson 2011; Son et al. 2011). Technologies used for heat and power productions include external combustion engines such as steam engine, internal combustion engines such as reciprocating gas engines and gas turbines and fuels cells. However, challenges remain in using existing equipment because of low volumetric energy content of biomass-generated syngas (4-15 MJ/m3) as compared to that of fossil fuels such as natural gas (38 MJ/m3) (Wang et al. 2008), for which the engines are designed for. Use of gas turbines was demonstrated in a week-long tests for power production from wood chips-generated syngas in Sweden at the scale of 18 MW (thermal) (Bengtsson 2011). In developing countries, gasification of locally available biomass and wastes has potential to provide electricity (Abe et al.

2007) . However, the challenges in commercialization of biomass gasification for power generation are several: robust syngas cleaning technologies are needed; power is lower value product than liquid fuels; and infrastructure and equipment to use biomass is not well-established unlike fossil-based feedstocks such as coal and natural gas.

Biochemical pathways such as aerobic respiration, anaerobic respiration and fermentation within microoganisms efficiently convert organic substrates into chemicals or biofuels such as ethanol. Aerobic respiration pathways convert carbon source such as glucose to produce ATP through series of the Embden-Meyerhof pathway, the tri-carboxylic acid pathway and the electron transport chain, with oxygen acting as the terminal electron acceptor. In anaerobic respiration (absence of oxygen), the terminal electron acceptors are replaced with inorganic compounds such as sulfate or nitrate to produce ATP. In fermentation, internally balanced oxidation and reduction of organic compounds occur with the biochemical pathway under anaerobic conditions, but without utilization of the electron transport system. However, bioprocessing industries often call both aerobic and anaerobic respiration fermentation processes where the term generally entails any bioconversion process.

Cellulosic ethanol fermentation may be performed using a wide range of microorganisms. Yeast such as Sacharomyces cerevisiae and bacteria such as Zymomonas mobilis are well known for utilizing glucose, fructose and sucrose for ethanol fermentation under anaerobic conditions with higher ethanol tolerance. Z. mobilis has a higher metabolic rate with less biomass production through the Entner-Doudoroff pathway (Fig. 3) compared to S. cerevisiae through the Embden-Meyerhof-Parnas (EMP) pathway (Fig. 4). The faster rates occur from decoupling energy generation from ethanol production with the absence of the highly-regulated enzyme phosphofructokinase (PFK) present in the EMP pathway. However, a number of disadvantages

exist for use of Z. mobilis, mainly in the production of byproducts such as levan catalyzed by levansucrase and other fructose polymers that tend to foul distillation columns downstream (Drapcho et al. 2008). S. cerevisiae is the most widely used microorganism for cellulosic ethanol production due to high ethanol tolerance and the remaining biomass being more suitable for use as animal feed than biomass from Z. mobilis fermentation. The hydrolysis of lignocellulosic biomass generates a mixture of both sugars (hexoses and pentoses) during the process. The simultaneous utilization of both sugars is the most challenging part for the cellulosic ethanol production. Therefore, other strains of yeast, bacteria and fungi have been explored or genetically modified for simultaneous utilization of both glucose and xylose in cellulosic ethanol fermentation. In literature, numerous microorganisms have been studied using xylose as the carbon source (Table 4). However, the performance of these microorganisms varies on hydrolyzed lignocellulosic broth due to variations in sugar utilization from the presence of inhibitors that depends on the chemical composition of lignocellulosic feedstock, chemical pretreatment and the extent of recirculation in the process (Table 5). A preprocessing or detoxification of these inhibitors from the hydrolyzed broth before or after the fermentation could be an energy-intensive step (Olsson and Hahn-Hagerdal 1996). However, these inhibitory effects could be resolved using fermenting microorganisms with high cell densities (Olsson and Hahn-Hagerdal 1996). A list of different microorganisms with their optimal ethanol yield and productivity is given for ethanol fermentation using enzymatic hydrolysate of lignocellulosic feedstock (Table 6).

In addition, fermentation can be performed in batch, semi-batch and continuous mode. The selection of the fermentation mode for optimal ethanol yields is based on the kinetic properties of microorganisms used and the integration of the cellulosic ethanol production process.

Many second generation bioenergy crops are lauded for their contribution to climate change mitigation efforts, particularly those involving minimizing greenhouse gas emissions and enhancing carbon sequestration. As a bioenergy crop, switchgrass rates well in a number of climate mitigation metrics (see Vadas et al. 2008). It is a perennial crop with deep roots (Lemus and Lal 2005), which are often implicated in its carbon sequestration abilities. Switchgrass performs better than maize in its carbon sequestration rates (Searchinger et al. 2008; Davis et al. 2012; but see Follett et al. 2012), and when combined with human health costs associated with fine particulate matter emissions in biofuel feedstock growth and processing, switchgrass comes across as far superior to maize (Hill et al. 2009). Not only does switchgrass perform better than a number of other bioenergy crop alternatives with respect to CO2 emissions (Monti et al. 2009), but production of agronomic switchgrass has also been tied to low NO2 and CH4 emissions when compared to most alternatives (Monti et al. 2012). Surely, different crop management strategies will contribute to variability in greenhouse gas emissions metrics (Monti et al. 2012), but overall it would appear that switchgrass is a leading bioenergy crop candidate in this critical area.

The high regard bestowed on switchgrass for its production-associated greenhouse emissions metrics may be tempered, however, by repercussions of landscape and land-use change that would be necessary to provide mandated amounts of ethanol in the U. S. The estimated amount of current non-agricultural land that would need to shift to second generation bioenergy crops to reach government mandates in ethanol production is upwards of 200,000 km2 to possibly three times that number (McDonald et al. 2009). This shift is typically not integrated into life cycle analyses of climate mitigation aspects of switchgrass production, but its impact cannot be overlooked. The clearing of forests and the changeover of range-, hay-, and pasture-lands to accommodate dedicated bioenergy crops, like switchgrass, would immediately result in substantial net CO2 emissions, which some studies have estimated to amount to approximately 350 Mt/ converted ha (Searchinger et al. 2008). Future conversion of non-arable land worldwide for crop production could result in > 3 Gt/yr of greenhouse gas emissions by 2050 (Tilman et al. 2011). The difficult and multifaceted challenge here would be to have quick-establishing (e. g., rapid growth in the early growing season), highly productive perennial bioenergy crops capable of substantial carbon storage in their roots. Currently, no CO2- related cap-and-trade laws are in effect, and treatment of CO2 emissions as pollutants by the U. S. Environmental Protection Agency is in its infancy; how these issues may affect future landscape conversion to switchgrass crop fields is unknown.

Those involved with improvement efforts in switchgrass will also need to be cognizant of climate forecasts in the areas it will be grown. For one, increased atmospheric CO2 levels may not necessarily lead to increased productivity (i. e., no "CO2 fertilization" effect) in switchgrass (Fay et al.

2012) . In addition, habitat suitability and climate envelope models already have switchgrass incapable of growing in a number of regions where some of today’s highest-yielding lowland cultivars originated in the southcentral U. S. by as early as 2040 in some of the "best case" scenarios (Barney and DiTomaso 2010; Tulbure et al. 2012). The hotter conditions forecast for the southeastern U. S., which include current areas of highest predicted switchgrass biomass yields, will dictate that breeding improvements be dedicated towards better water-use efficiency (see Le et al. 2011) and related traits aimed at "climate proofing" (sensu Oliver et al. 2009) the crop. This is noteworthy and may seem perplexing, given that switchgrass currently is noted for exhibiting high water-use efficiencies, higher than some alternatives (VanLoocke et al. 2012).

Bio-oil can also be used in furnaces and boilers to produce heat and power after moderate upgrading (Czernik and Bridgwater 2004). Recently, researchers have focused specifically on converting bio-oil to hydrocarbon fuels which are compatible with petroleum fuels. Bio-oil consists of several hundred compounds and separation is very challenging because the concentrations of the chemicals are low and any separation techniques must take into consideration interactions by many other functional groups present in the bio-oil.

Acknowledgements

We appreciate Madhura Sarkar, a graduate student, for helping collect some of the data for this chapter. Financial support is provided, in part, by Oklahoma Agricultural Experiment Station and National Science Foundation under Grant No. EPS-0814361.

There are two major components to mechanistic modeling: model parameterization and model validation using empirical data. Each model should be parameterized and validated at many locations to ensure that the plant parameters accurately reflect plant growth across space and time. Model parameterization for a site can be performed using detailed field data collected for functionally important plant traits or by gleaning reasonable values from previous findings in the literature. These functional traits may be morphological (i. e., leaf area), physiological (i. e., stomatal conductance or proportion of N in tissue) and phenological (i. e., date of green-up, senescence, and maximum growing degree days) characteristics. Model parameterization often requires significantly more data collection over time than model validation. Therefore, many studies do not independently parameterize each model but only change key variables known to vary between locations (Kiniry et al. 2008).

Model validation, comparing measured yields to simulated yields for many sites is becoming increasingly common as more field trials managed for biofuel production are being performed (Wullscheleger et al. 2010). A review by Wullschleger et al. (2010) revealed 17 switchgrass studies managed for biofuel production at 39 different locations. Management varies widely across these studies and can greatly impact biomass production. For example, yearly N fertilizer application in these studies ranges from 0 to 896 kg N ha1. Changes in management practices need to be carefully considered when validating model output using yields from several studies.

To our knowledge, there are currently 14 studies that have used mechanistic models of switchgrass growth to predict biomass production. The number of times each model has been parameterized and validated for switchgrass varies (Table 1). The ALMANAC model has been most extensively parameterized and validated with five studies parameterizing the model and six studies validating the estimated yields.

Biomass may require preprocessing before fed into a thermochemical conversion reactor. Drying, densification, pelletization and torrefaction are three most commonly used pre-preprocessing methods. Drying is needed to reduce biomass moisture content. Densification and pelletization increase the bulk density and flow characteristics; whereas torrefaction increases energy density and improves grindability. The degree and type of preprocessing depend on the thermochemical conversion process to be used. For example, pyrolysis may demand feedstock with relatively small particle size and moisture content compared to gasification process. However, since lignin is utilized in the thermochemical conversion processes, unlike biological conversion processes, removal or separation of lignin is not needed for the thermochemical conversion processes.

The oxidative pretreatment involves the use of a strong oxidizing reagent such as hydrogen peroxide to achieve delignification. Sometimes sodium silicate of magnesium sulfate is added with hydrogen peroxide to make the solution more stable (Guald 1984). The concentration of hydrogen peroxide used is in the range of 1-10% by volume. More precisely, effective pretreatment requires the ratio of hydrogen peroxide to substrate of at least 0.25 g H2O2/g substrate under alkaline conditions (Guald 1984).

In alkaline conditions, hydrogen peroxide decomposes to form hydroxyl ions that further react with phenolic groups of lignin. Hydrogen peroxide also plays an important role in stabilizing the ends of cellulose and hemicellulose structures (Gupta 2008). Guald (1984) mentioned that alkaline peroxide pretreatment can achieve 50% or more delignification of lignocellulosic biomass with greater than 90% overall theoretical saccharification efficiency. However, the degree of saccharification for oak was 52.5% compared to 93.0% for wheat straw (Guald 1984).

Table 3 includes a switchgrass conventional tillage establishment budget. Table 4 includes a no-till establishment budget. Custom rates are used to reflect the cost of budgeted machine operations (Doye and Sahs 2012). These cost estimates depend on the assumption that a sufficient quantity of custom operators could be hired to perform the operations in a timely manner. Both establishment budgets include a mowing operation designed to clip weeds that extend over the top of the switchgrass. If weed pressure is minimal, this operation would not be necessary.

Establishment costs are estimated to be $498/ha for no-till and $580/ha for conventional tillage. Stands of established switchgrass are expected to thrive for a minimum of ten years. The establishment costs are amortized over ten years at a 7 percent rate. This estimated amortized cost of establishment is $83/ha/year for conventional tillage and $71/ha/year for no-till. The $83/ha/year charge is included on the maintenance and harvest budget (Table 5).