As a result of increasing gasoline prices, the use of ethanol as a biofuel has been introduced on a large scale in Brazil, the United States, and various European nations. As stated in the Introduction section, it is essential to use lignocellulosic biomass in order to meet the global biofuel demand. Prices of agricultural residues and energy crops are much lower than those of corn (wheat straw $24, barley straw $26, corn stover $50, grass hay $50, and switchgrass $60/ton). In the past few months corn prices have reached $230/ton. While agricultural residue substrates are available at a much lower cost, their processing faces a number of challenges before they can be converted to ethanol. These challenges include (1) pretreatment and enzymatic hydrolysis of lignocellulosic biomass to monomeric sugars; (2) generation of fermentation inhibitors during pretreatment; and (3) fermentation of mixed sugars (hexoses and pentoses) to ethanol. Pretreatment of agricultural residues or energy crops is essential to soften the fiber structure and make it available for enzymatic action and hence for further hydrolysis. Although there are numerous pretreatment technologies available, including dilute sulfuric acid, sodium hydroxide, steam expansion, and ammonia treatment, dilute sulfuric acid treatment still offers higher sugar yield than other methods. Pretreated and hydrolyzed biomass solutions contain hexoses (e. g., glucose, galactose, and mannose) and pentoses (e. g., xylose and arabinose). Fermentation of hexoses can be achieved efficiently using traditional strains such as S. cerevisiae and Zymomonas mobilis. However, the fermentation of pentose sugars poses problems when using parental microbial strains and will require engineering of these traditional strains with new metabolic pathways.

An opposite result for different mixing intensities emerged for low-rate, completely stirred anaerobic digesters that do not rely on biomass settling for advanced VSLR—mixing intensity did not affect long-term reactor performance. For this study with low-rate systems, microbial flocs had not developed in any of the four lab-scale bioreactors, which were continuously mixed with an impeller at different rotational speeds of 50, 250, 500, and 1500 revolutions per minute (RPM; Hoffmann et al. 2008). In the system with continuous mixing, diffusion limitations for hydrogen and acetate were overcome and this circumvented the requirement for a juxtaposed position of bacteria and their syntrophic methanogenic partner in flocs. Thus, a higher methanogenic activity of biomass was not dependent on floc formation and a juxtaposed position could, therefore, not be destroyed by a more intense mixing scheme. During the initial start-up period, however, the performance of the most intensely mixed digester was hampered. Based on this result, we concluded that mixing intensity should be low during initial start-ups. In addition, we found that the acetoclastic methanogenic com­munity structure was affected by the mixing intensities applied to the bioreactors. An inverse correlation between mixing intensity and relative 16S rRNA levels for M. concilii was found. At the higher mixing intensities, acetate was converted to methane by Methanosarcina spp., while this was converted by M. concilii at the lower mixing intensities (Hoffmann et al. 2008). On one hand, this may be important in regard to long-term stability because Methanosarcina spp. (with a higher substrate utilization rate at elevated acetate concentra­tions) can handle shock loads better than Methanosaeta spp. On the other hand, the affinity for acetate is higher for Methanosaeta spp. than Methanosarcina spp., resulting in lower effluent acetate concentration, and therefore higher methane yields.

Succinic acid is an important chemical used as an additive in food and pharmaceutical products, surfactants, detergents, solvents, and biodegradable plastics. Succinic acid can

Organism

Clostridium butyricum Clostridium butyricum (Engineered)

Klebsiella pneumoniae Anaerobiospirillum succiniproducens Actinobacillus succinogens Mannheimia sp

Enterobacter aerogenes Escherichia coli Escherichia coli (Engineered) Enterobacter aerogenes Escherichia coli (Engineered) Methanogens

Escherichia coli (Engineered) Me thylobac terium rhоdesianum MJ126-J

Ralstonia eutropha DSM11348

Gluconobacter oxydans (Engineered)

Propionibac terium acidipropionici

Clostridium pasteurianum

n-methylpyrrolidone, and linear aliphatic esters (Zeikus et al. 1999). Succinic acid is generally produced under anaerobic conditions through carboxylation of phosphoenol pyru­vate, a glycolytic intermediate. The carboxylation of PEP can occur by two mechanisms, either by phosphoenolpyruvate carboxylase (PPC) or by phosphoenolpyruvate carboxykinase (PEPCK). Conversion by PEPCK is energetically favored because an ATP molecule is produced during the reaction, thus regenerating the ATP used up during glycolysis, while in the case of PPC, this energy is dissipated via release of inorganic phosphate molecules. Anaerobiospirillum succiniciproducens is one of the most efficient succinate producers. It uses the PEP carboxylation pathway catalyzed by PEP carboxykinase (or PEP carboxylase), malate dehydrogenase, fumarase, and fumarate dehydrogenase (Lee et al. 2004). This microbe produced 19 g/L of succinate when glycerol was used as the sole carbon source in the culture medium (Table 6.1). Actinobacillus succinogenes, another natural producer of suc­cinate, was found to produce 4.9 g/L of succinate with a specific yield of 1.3 g/g glycerol (Lee et al. 2001). This organism uses PEPCK, an energy-efficient enzyme, to convert PEP to oxaloacetate in contrast to E. coli and many other microbes that use PPC. Overproducing A. succinogenes PEPCK enzyme in E. coli strain improved succinate production in media containing 120 mM NaHCO3 (Kim et al. 2004). In another study, succinate production was evaluated in strains overproducing PPC, and it was found that the level of succinate was enhanced by the simultaneous overexpression of PPC and pantothenate kinase (Lin et al. 2004). The overexpression resulted in an increased level of acetyl coenzyme A (acetyl-CoA), an activator of PPC (Lin et al. 2004). A novel succinic acid-producing bacterial strain designated as DD1 was isolated from the rumen of a cannulated Holstein cow (Scholten and Dagele 2008). This facultative anaerobe, which belongs to the family Pasteurellaceae and resembles the genus Mannheimia, produced 5.8 g/L succinic acid from glucose or sucrose with a yield of 0.6 g/g and a productivity of 1.5g/L/h. When crude glycerol was used as the carbon source in continuous fermentations, the amount of succinic acid produced was 8.4g/L; the yield was 1.2g/g glycerol; and the productivity was 0.9g/L/h. The high yield of succinic acid obtained from the fermentation was attributed to the highly reduced nature of carbons in glycerol molecules.

This section discusses existing commercial logistics operations of agricultural feedstocks that are similar to lignocellulosic biomass feedstocks. The principles required for an efficient logistic system for biomass are illustrated by several commercial operations. Two examples of the collection of herbaceous fiber are explained in detail, the cotton harvest throughout the Southern United States, and the sugarcane harvest in South Florida. Throughout the discus­sion on the cotton and sugarcane systems, the reader will be reminded of certain features of each system that relate to the design of a logistic system to deliver herbaceous biomass to a bioenergy plant.

The sugarcane harvest in South Florida illustrates an efficient system for a high- yield, high-moisture content crop and is a commercial example for tropical grasses with high yield, (55-125 t/ha) and high moisture content (80%). On the other end of the spectrum, the cotton harvest is the commercial example for a lower yield (2-4.5 t/ha) and lower moisture content (20%) crop.

The largest harvest of biomass in the United States is the wood harvest for the forest products industry. A byproduct of the harvest of logs for this industry is the harvest of “fuel chips.” This harvest is a major commercial example of a biomass harvest for bioenergy, thus it is an important reference point for our discussion of herbaceous biomass logistics. Anyone seeking to design a herbaceous biomass logistic system should first look at the fuel chip industry and learn the “tricks of the trade.”

The two herbaceous fiber examples, cotton and sugarcane, are contrasted with a grain example, the corn harvest in the U. S. Midwest. Grain is a flowable material, and the system for handling, storage, and transport is mature technology that was simply “plugged into place” for the corn ethanol industry. A fiber material is not flowable, certainly not to the degree grain is flowable, thus the material handling is more difficult, and the solutions are more costly. Although the technologies are not directly transferable, it is instructive to compare the performance parameters for the grain logistics system with the systems for handling herbaceous biomass as fiber.

Parameters compared for the three examples are (1) capacity of harvesting unit (t/hour); (2) highway hauling equipment; (3) location of storage; and (4) operation of a receiving facility at the processing plant. A harvesting unit is defined as a harvester and the infield hauling equipment required to keep it operating. Typical productivity and cost parameters are included to facilitate comparison between the three systems.

The presence of liquid water is essential in the HTL of lignocellulose feedstock, even more important than for other types of biomass (manure and algae, for example). Aside from its role as a vehicle and catalyst carrier for the feedstock, water also serves as a solvent and reactant. The use of water as the solvent for HTL presents several advantages over other solvents. Water is simple to use, is relatively low cost, and is environmentally benign.

Water is an excellent medium for the intermediate hydrolysis of cellulose and other high — molecular weight carbohydrates to water-soluble sugars. The primary reaction in the conver­sion to oil likely involves the formation of low-molecular weight, water-soluble compounds such as glucose. Hydrogenation of sugars at mild temperatures produces polyols (hexitols and xylitol), which undergo further transformation into mixtures of glycerol, ethylene glycol, and propylene glycol (Chornet and Overend 1985). In another account, monomers such as glucose are further reduced with the presence of reducing compounds (Houminer and Patai, 1969). The sugar is de-oxygenated producing high carbon-hydrogen compounds.

Most organic compounds do not react with water under normal conditions. However, at temperatures between 250°C and 350°C, molecules in liquid water undergo chemical reac­tions. Previously, these reactions were only expected to occur in the presence of strong acid or base, but recent research indicates otherwise (Siskin and Katritzky 1991( . Siskin and Katritzky (1991) have shown the geochemistry of the reactivity of organic molecules in hot water. Ester groups, which are bound in to the network of resource structures and serve as crosslinks, although thermally unreactive, are easily cleaved in water at 250-350°C (Siskin et al. 1990a). Similarly, benzyl aryl ethers were found to be more susceptible to cleavage under aqueous thermal conditions at 250°C. Cyclohexyl phenyl compounds with oxygen, sulfur, and nitrogen links are relatively unreactive thermally, but they readily cleave in water at 250°C to form methylcyclopentene together with phenol, thiophenol, or aniline, respec­tively (Siskin et al. 1990b). Benzonitriles, pyridinecarbonitriles, benzamides, and pyridinecar — boxamides are almost unaffected by thermolysis, but are rapidly hydrolyzed in water at 250°C to the corresponding ammonium carboxylates (the nitriles via the amides). The ammonia formed autocatalyzes these hydrolysates and the subsequent decarboxylations (Katritzky et al. 1990). In the formation and depolymerization of resource materials, autocatalysis appears to be a major mechanistic pathway. During the diagenesis of kerogens, oxygen functionalities such as carboxylic acids, aldehydes, and alcohols are lost directly by cleavage and indirectly by condensation reactions that form methylene-bridged, ether, and ester cross-links. The cleavage reactions release water-soluble products such as carbon dioxide, formic acid, and formaldehyde.

HTL can convert lignocellulose into oil, but the yield is relatively low when no catalyst or solvent is used. Wang et al. (2008) compared the oil produced from different biomass— including legume straw, corn stalk, cotton stalk, and wheat straw—under hydrothermal condi­tions of temperature 350°C, residence time 2-3 hours, solid content 15% (dry mass), and pressure 10-13 MPa without using any catalyst. Hydrothermal experiments were carried out with 5.0g biomass in a stainless tubular reactor which is 100mm length by 10mm internal diameter. The heating rate of reactor is about 10°C/min. The oil product was separated from reaction mixture by distillation at 101-405°C and atmospheric pressure. Water and oil auto­matically separated into two phases in their study. Experimental results showed that oil yield is in the range of 5.2%-10.5% and both char and gas yield are more than 35% as the total biomass was almost completely converted. In addition to CO2, gas products contained about 4.4%-8% H2 and 5.5%-13.3% CO. Analysis of the oil product indicated that oil mainly consists of alkanes, cycloalkanes, and aromatic hydrocarbons. Based on these results, they concluded that the component of starting material had little effect on oil composition.

Tracing the origin of polyphenols, an abundant class of natural compounds, may contribute to awareness of reactions taking place upon high temperature treatment of waste materials. Luijkx et al. ( 1991) reported on the high yield of 1,2,4-benzenetriol from conversion of aqueous HMF as well as D-fractose. In a heated open tube reactor with 1.43 mm ID x 3.18 mm length, a 0.05 mole of HMF in water was converted 80%—90% with maximum 25% yield of 1,2,4-benzenetriol at about 330-350°C for 250 seconds. At 330°C for 185 seconds the 1,2,4-benzenetriol yield is about 9% from D-froctose. Those observations indi­cate that 1,2,4-benzenetriol is directly produced from HMF. Other identified compounds are less than 2%, including 4-oxopentanoic acid (levulinic acid), furaldehyde (furfural), and

1.4- benzenediol (hydroquinone).

To understand the formation mechanism of oil-l ike or tarry compounds, it is logical to start with the chemistry of HTL using model compounds which are present during the reaction. Luijkx et al. (1993) converted model compounds such as HMF derived from biomass to1,2,4-benzenetriol with a yield of up to 46% at 50% HMF conversion at 290-400°C and 27.5 MPa. Experiments were conducted on a continuous process with a 4.8 mL tube reactor. It was found that 1,2,4-benzenetriol yield increased with residence time as HMF was converted at 290-380°C and that selectivity of 1,2,4-benzenetriol increased to 46% at 50% HMF conversion at 300-350°C followed by a decrease due to decomposition of

1.2.4- benzenetriol at high temperature and long residence time. In addition, distribution of products derived from HMF can be altered by pH changes, although 1,2,4-benzenetriol could be detected in the aqueous phase. It was concluded that HMF was a precursor of hydroxylated aromatics such as 1,2,4-benzenetriol in the HTL of biomass. However, a small amount of tar derived from HMF, which has a very high oxygen content (apparently 30%, and the usual oxygen content of hydrothermal oil products is about 18%) and was unstable due to polymerization. For that reason, HMF may not be considered as a representative compound for simulation of hydrothermal conversion of biomass.

Since the hydroxylated benzenes were identified in the aqueous product from hydrother­molysis of carbohydrates (Suortti 1983), the link between furan and hydroxylated benzenes was examined to further explore chemistry involved in HTL. Luijkx et al. (1994) studied the origin of hydroxylated benzenes from these furan derivatives from biomass under hydro­thermal conditions. Hydrothermal conversion of furan derivatives was performed in a con­tinuous tube reactor at 340°C, 27.5 MPa, residence time of 1-33 minutes, and feed concentration 0.01 M or 0.05 M with or without hydrochloric acid. Their results show that furan derivatives such as 2-acetylfuran, 2-prpoinylfuran, 5-methyl-2-furaldehyde can be directly converted to 1,2-benzenediol, 3-methyl, 1,2-benzenediol, and 1,4-benzenediol, respectively, without experience of smaller molecular fragments since only a few types of hydroxylated benzenes were obtained. Otherwise, a whole range of products would be col­lected. With catalysis of hydrochloric acid, acetic acid and propionic acids were produced as major products from conversion of furan due to cleavage of the furan ring. However, for 5-methyl-2-furaldehyde (which is found in hydrothermolysis as well as pyrolysis of carbon — hydrolysis of carbohydrates and HMF), 1,4-benzenediol was increased to about 30% due to the presence of hydrochloric acid. For the conversion of HMF, it was found that it was unlikely to get 1,2,4-benzenetriol via electrocyclic mechanism. The formation of hydroxyl — ated benzenes from furans such as HMF is probably via a hydrolytic furan ring opening followed by an intramolecular aldol condensation which is catalyzed by strong alkaline or sometimes by acid and subsequent dehydration.

With observation of a significant amount of CO2 formed from HTL, it is interesting to know how CO2 evolved from biomass under the hydrothermal condition. Luijkx et al. (1995) examined the role of deoxyhexonic acids in the hydrothermal decarboxylation of carbohy­drates. Hydrothermal conversion was conducted at 340°C and 27.5 MPa in a continuous tubular reactor for residence times of 1-3 minutes. It was found that the temperature exerted an effect on reaction pathway of hydrothermolysis since almost no 3-deoxy-D-erythrohex-2- ulose was detected at temperatures less than 250°C with the presence of alkaline. Only small amounts of 3-deoxy-d-hexonic acid were observed in hydrothermolysis of a mixture of D2glucose and oligomers. Although З2 and 22deoxyhexonic acid can decarboxylate, their contribution to CO2 formation during hydrothermal conversion is limited when small amounts of deoxyhexonic acids formed during hydrothermal conversion of carbohydrates was consid­ered. In addition, it seems that the addition of NaOH increases gas formation of CO but does not favor the formation of CO2.

The tar and char formation has been speculated by Chornet and Overend (1985). Chornet and Overend stated that the accessibility to cellulose chains was hindered due to surround­ing the compounds. “2n a pure pyrolytic context (i. e., carbonization), thermally induced rapid breakdown of the cellulosic chains results in intermediate compounds which are steri — cally hindered within the rigid lignin-rich structure of the compound, middle lamella, since lignin decomposes at higher temperatures than cellulose (Beall and Eickner 1970). The net effect is the random recombination of the intermediate compounds leading to tar and char formation” (Chornet and Overend 1985).

In the presence of a solvent, catalyst, and hydrogen, the oil yield from hydrothermal con­version of biomass could increase significantly. Kaufman et al. (1974) studied the conversion of cellulosic feed materials to liquid hydrocarbon fuels with newspaper as feedstock and nickel hydroxide as catalyst. The 20 wt % of powdered newspaper in mineral oil was pro­cessed in a 1-L continuous stirred tank reactor (CSTR) at 400-455°C, hydrogen pressure 34-102atm with the presence of 0.2 wt % of Ni(OH)2 catalyst. Results show that decrease in temperature can lower both the carbon conversion and oil yield, while an increase of hydrogen pressure from 36.7 to 70.8atm promotes oil yield from 5.2% to 46.1% at 453°C and 17.5 minutes space time and carbon conversion is constant (69.4% vs. 74.1%). The oil product would decompose to gas if exposed to 452-463°C more than 20 minute space time.

Complete liquefaction and gasification of biomass can be achieved with a catalyst present in water and the degree of gasification changed with the catalyst to biomass ratios (Boocock et al. 1979). Boocock et al. (1980b) hydrothermally liquefied 150g of air-dried poplar with particle size of a 0.5 mm mesh and 750 mL water in a 2-L magnedrive packless autoclave under catalyst effect. Catalyst (20 g) causes the complete liquefaction and gasification of wood with 33.8% oil yield at 350°C, holding time 1-2 hours, and H2 initial pressure10.7MPa. The catalyst obviously increased the consumption of H2 and formation of CH4, but inhibited CO2, which implies that Raney Ni could catalyze formation of CH4 from CO2 and H2. With the decrease of activity of the Raney Ni catalyst, CO2 selectivity increased while CH4 forma­tion decreased and biomass was completely converted. Results suggest that the catalyst was modified during the liquefaction process and that the spent catalyst did not promote the for­mation of methane, but enhanced the formation of carbon dioxide. It was thought that the oil product was responsible for catalyst modification and found that reaction was not influenced by water to oil ratio. The spent (modified) catalyst retained its activity as there were no signs of solid residue in the autoclave. In addition, using a lower H2 pressure slightly increases CO2 formation. Without H2 , the spent catalyst can also completely convert wood to viscous liquid at 350°C and 2 hours, which is flowable at room temperature except for a much lower hydrogen content in the oil product. The oil product shows viscosities from 700-8000 mPa. s at room temperature, oxygen content about 10%-13%, specific gravity of 1.1, heating value about 35.3 MJ/kg, 97% benzene solubility, 55% diesel solubility, and 33% aromatic carbon content. The residence time was reduced to 30 minutes or less, and the catalyst was upgraded from Raney nickel to nickel from nickel salts.

Raney nickel catalyst can be substituted by a less exotic and hence less expensive, nickel catalyst. Boocock et al. (1982) investigated and reported the effectiveness of the Raney cata­lyst in detail. Wood (7-year-old hydrid poplar) was thermohydrolyzed in a 22L packless magnedrive autoclave with Raney catalyst at 340°C for 2 hours. As temperature was more than 375°C, excessive char formation was noted. In a typical experiment 150 g dry wood and 20g Raney catalyst were used with 750mL water pressured under 1.7-8.3MPa H — . The oil product tends to be more viscous, darker, and denser with the stabilization of a fresh catalyst taking place. Their results imply that fresh Raney Ni seems not able to catalyze the conver­sion of CO2 and H2 to CH4 in the aqueous phase because the change in H2 consumption is not significant. Therefore, CO2 may be catalytically produced and CH4 is probably formed directly by cracking processes in the presence of a catalyst. Raney Ni does oxidize and move to the aqueous phase, presumably to produce H2 , but the amount is not large. Higher initial H2 pressures favor hydrogen consumption and most of H2 is consumed in methane formation. H2 consumption increases with stirring rate (1300-1750rpm) as initial pressure increases from 1.7 to 8.3 MPa (250-1200psi). At 2300rpm, oil viscosity can be lowered without a significant increase in H2 usage. At the same time, H2 uptake reduces the viscosity of oil and oil yield (separated by centrifuge and acetone) varies from 36.5% to 41%. The lower pH modified the Raney Ni and favored the CO2 formation. In general C, H, and O content of the oil product were 73%, 8%, and 17.5%, respectively. Heating value of oil production was about 34 MJ/kg. The H/C is lightly lower in the oil than in the wood. Only 10% H appeared in methane and using H — is not theoretically required; its major function is to prevent the nickel from being oxidized and passing to the aqueous phase. With nickel carbonate as the catalyst, promising results of 80% C remained in the oil product and 55% wood H was retained in the oil. Nickel carbonate was reduced in situ to finely divided nickel, which pre­sumably functioned as the catalyst (Boocock et al. 1980a). However, commercially available nickel powders did not appear to be as effective as Raney nickel or nickel produced in situ (Boocock et al. 1980a).

A recent study has also shown that a modified catalyst theoretically contributed to selectiv­ity of catalyst that potentially makes conversion of biomass to biofuel more efficient in the presence of a catalyst (Teschner et al. 2008). Although heterogeneous catalytic conversion is a surface process, there is accumulating evidence, particularly from experiments applying in situ functional analysis, that the bulk and especially the subsurface region (the few layers below the surface) can play a key role in surface events. Reaction conditions (such as tem­perature and the ambient reactive gas) may not only reconstruct the top surface layer, but also may create added rows and valleys of atoms or even massively change the whole mor­phology of the catalytic particles (Teschner et al. 2008). Atoms that are part of the catalytic feed can dissolve in metallic particles, and can change the electronic structure of the surface, and dissolved species can even participate in the reaction; for example, alkyne hydrogenation on palladium.

Pd itself is usually even more active in hydrogenating the corresponding alkene to alkane (Teschner et al. 2008). The typical explanation (a thermodynamic view) is that the differ­ence in the heat of adsorption of the feed alkyne and of the partial hydrogenation product alkene forces the intermediate product alkene to desorb and become replaced by the incom­ing alkyne of the feed (Teschner et al. 2008). A contrary example, ethylene could be adsorbed on a catalyst of Pd supported on silica while acetylene was present in the gas phase. This is possibly because the surface of catalysts is usually heterogeneous and can have discrete sites that facilitate selective adsorption (Teschner et al. 2008) . Another fact is that carbonaceous deposits formed during reaction might substantially affect selectivity (Teschner et al. 2008). In addition, alkyne hydrogenation usually goes through an activation period, which strongly suggests that the catalyst is not identical to its “as-introduced” form. It was found that selectively hydrogenate 1-pentyne, the active state of Pd is a Pd-C surface phase (PdC), approximately three Pd layers thick (Teschner et al. 2008).

The in situ X))ay photoelectron spectroscopic measurement and )n situ prompt gamma activation analysis (PGAA) were used to observe the hydrogenation process (Teschner et al. 2008). The amount of C incorporated within the top layers was 35-45 atomic % based on XPS investigation (Teschner et al. 2008). PGAA experiments show that the surface properties are necessarily decoupled from the bulk. The high concentration of dissolved carbon excludes H from populating the subsurface region and hence prevents total hydrogenation of alkynes. They are aware that many other factors, such as promoters in the form of a second metal or selective poison, can strongly modify the hydrogenation selectivity (Teschner et al. 2008). Their aim was to shed some light on the importance of subsurface chemistry in hydrogenation processes. They believed that a critical level of understanding of both surface and subsurface dynamics in these and other complex processes of heterogeneous catalysis is required. Although gas-phase alkynes hydrogenation on palladium catalysts is a surface process, they have shown that the population of the subsurface region by either C or H will determine the surface events.

Nickel catalyst has been shown to be helpful in stabilizing the liquid products and prevent­ing charring. A relative cheaper catalyst, nickel carbonate, was examined. Beckman and Boocock (1983) used an 8-mL tubular reactor with adequate mixing and rapid heat-up rate to convert seven-year-old hybrid poplar in the presence of NiCO) . Results indicate that the oil yield was highest at a short residence time. At the same time, oxygen content of oil decreases from 25% to17% during this period. The pH of the aqueous phase dropped due to carboxylic acids and phenolic compounds followed by an increase. The addition of nickel carbonate is deleterious to the liquefaction process under rapid heat-up in terms of C and H percentages in the wood that is retained oil. NiCO3 and H2 may not have a significant effect on oil yield, oil compositions, and pH of the aqueous phase. Degradation of wood begins at 230°C and 267°C because no oil was produced at 230°C. The results were interpreted that as slow heat-up proceeds, the wood is converted from a solid to a viscous tarry which is in turn liquefied in the presence of Ni or carbonized in the absence of Ni. At fast heat-up, wood is liquefied directly.

Besides nickel, other commercially available catalysts with different active sites such as Pd, Fe. O3 . NiO, and MoO. were used to convert lignin to oil in the presence of hydrogen. Meier et al. ) 1992) directly hydrogenated lignin in the aqueous phase over to a catalyst without using any solvents and pasting oil. In their experiments, 40 g of dry lignin with 3.5 g moisture inside mixed with 15 g of catalyst in a 250 mL autoclave under hydrogen pressure and stirring rate 1500 rpm. An oil product was extracted with dicholoromethane in a Soxhlet apparatus. Screening tests show that Fe2 O3 has a negligible effect on oil yield and zeolitic support is unsuitable for hydrogenation of lignin because its channel system makes lignin

difficult to access the active site. Pd/C exhibits highest activity among Ni/Mo/Al2O3/ SiO2 > Raney Ni, = NiO/MoO3/Al2O3/SiO2 in terms of oil yield. The composition of oil depends on the catalyst. Mo catalyst mainly gives monophenols while Pd/C produced ethyl — cyclohexanones and catechols, and so on. Ni/Mo was found possessing a high demethoxylat — ing power, and Pd or noncatalyst tests showed demethylation to catechols. It was also noted that sulfided NiMo was more active than the oxidized form when much higher oil yield was observed with kraft lignin than that from organocell lignin. The acetyl group in acetosolv lignin lowered the oil yield due to blockage phenomena. Surprisingly, oil composition from different types of lignin shows no difference.

Iron compounds (FeS or FeSO4) used in coal liquefaction were also examined for HTL of biomass to oil. Xu and Etcheverry (2008) treated 1 g of Jack pine wood with particle size smaller than 20 mesh (~0.8 mm) in a 14-mL reactor with 5wt % catalyst and 13mL ethanol in the presence of 2.0-10.0 MPa hydrogen. Generally, oil yield (15%—35%) increases with reaction time of 15—60 minutes, temperature of 200—260°C, and initial H2 pressure of 2.0— 10.0MPa without a catalyst. At the supercritical condition of ethanol, 45% oil yield can be obtained at 350°C, but more char formed at a temperature more than 300°C. In the presence of 5% FeS or FeSO4, the oil yield shows no significant improvement. It was noted that ethanol consumption and contribution to oil were considered.

An organic solvent layer may extract oil molecules from the aqueous phase during HTL in a two-2ayer reaction system. Miller and Fellows (1981) reported that wood or cellulose can be almost totally converted to liquids or gases at 350°C in pressurized phenol and water with catalyst. In a typical reaction, 2 g biomass mixed with 2 g phenol, 2.5 g water and 0.5g catalyst was heated to 350°C for a few minutes to several hours in a pressurized glass vessel. The phenol in a two-layer reaction system, which can be produced from lignin and recycled, intends to provide a solvent to slow down the higher order solid state condensation reaction. The recovered yield of neutral product was 0.55—0.76g per 2.5g dry aspen. About 3% aro­matic hydrocarbons, including toluene, ethyl benzene, and xylene were found in nonphenolic products when zinc chloride and nickel were used with hydrogen. In addition, the observed recoveries showed that a net phenol could be produced.

Except for phenol, other organic solvents such as acetone, methyl ethyl ketone, 1-, and 2-propanol, and 1-butanol were used for biofuel production with marked effect on the direct formation of a fluidized product. Ogi et al. (1990) examined the role of butanol solvent in direct liquefaction of wood. The wood chips in 80 mesh were heated with water, sodium carbonate, and organic solvent in an autoclave under nitrogen pressure. They observed that there were three phases, that is, butanol layer, water layer, and tar-like product on the reactor wall when butanol was used as a solvent. (It is interesting to know that t-butanol is different from 1-, 2-, and i-butanol, which mixes with water freely.) Blank tests indicated that butanol, especially t-butanol, degraded under experimental conditions of 270°C, 90 atm, and 60 minutes retention time. They found that isomers of butanol have no effect on oil yield as high as 45%—55% assuming that no isomer was converted to oil. The function of sodium carbonate was thought to be an agent inhibiting hydrolysis of biomass. The use of hydrogen donors is believed to be one of the most efficient ways to reduce these undesirable reactions. They clarified that butanol did not function as a hydrogen donor solvent, but only acted as an extraction solvent/stabilizer, in which undesirable reactions such as repolymerization were retarded.

Instead of direct TCC of biomass to oil in one reactor, a two-stage HTL was employed to produce the alternative oil product from lignocellulosic biomass. Roman-Leshkov et al. (2007) developed a process to produce dimethylfuran for liquid fuels from biomass-derived carbohydrates. The sugar solution was converted to HMF in high yield by the acid-catalyzed dehydration of fructose in a biphasic reactor using a low boiling point solvent (e. g., butanol) that continuously extracts the HMF product with NaCl presented in the aqueous phase. The extracting solvent containing HMF was then purified in an evaporator at low temperature (e. g., 89.9°C). Next, HMF is converted to dimethylformamide (DMF) over a copper-based catalyst such as CuCrO4 or CuRu/C by hydrogenolysis. Finally, DMF was separated from the solvent and intermediates via a distillation process. Sugar dehydrates at 180°C for 3 minutes with ~75% conversion and 5 wt % HMF can be converted to DMF with 61%-71% yield over CuRu/C catalyst in a flow reactor at 220°C, 6.8 bar hydrogen, and feed rate of 0.2cm3/min.

Minowa et al. (1998) applied the HTL process to 18 kinds of agricultural and forest residues in Indonesia. Tests were run in a 300mL stainless steel autoclave with a magnetic mixing, at 300oC, and 30 minutes retention time. N2 was used as the initial gas and added to 3 MPa at the beginning. Five wt % Na2CO3 was used as the catalyst and acetone was used to extract the oil product. Oil yields were in the range of 21%-36%, depending on the species and parts of feedstock. All oils had almost the same elemental properties, C ~70%, H ~7%, N <1%, O~20%. Heating values were around 30kJ/g and viscosity was greater than 105 mPa. s. Gas yields were around 20%, consisting mainly of CO2 with a range of 78-86 mol %. Other gases included CO (11-19mol %) and H2 (~2mol %). More than 35% of the energy in raw materi­als was recovered in the form of oil, and for some materials, this number was as high or more than 50%. Residue also had energy content, indicating it is possible to use residue as a process energy source, especially for coconut husk and oil-palm shell.

A high correlation between lignin and residue was observed (Minowa et al. 1998). The phenoxy radicals could be formed from the lignin under high temperatures, and a higher lignin content resulted in a higher residue yield by condensation and repolymerization.

The correlation between lignin and residue was also studied by Demirbas (2000) and a similar conclusion was drawn. In his study, nine species of biomass with different lignin contents were thermohydrolyzed in an autoclave with and without a KOH catalyst (20 wt %). All tests were performed in a 250mL cylindrical autoclave. Tests were run at 575K over 30 minutes, using N4 as the initial gas. There was a strong correlation between lignin content and oil yield. With increasing lignin content, the oil yield decreased and the char yield increased.

Nelson et al. (1984) studied the mechanisms of direct liquefaction of cellulose. At 250- 400oC, a pressure up to 20.7 MPa, and with the presence of Na2 CO3 , pure cellulose was converted in a 300-mL autoclave, to a mixture of phenols, cyclopentanones, and hydroqui — nones as well as other components. At 300o C for 1 hour, most of the oil components are present in amounts of 0.1 wt % or less in the oil, which makes it very complicated to analyze the oil product. Phenolic products have been observed as products of the alkali treatment of saccharides. Biacetyl and acetoin are precursors for aromatic components during cellulose liquefaction. A scheme of the formation of biacetyl and acetoin from cellulose was discussed. Faster heating rates would be useful to reduce the inevitable degradation and recombination of the initial products. The use of alkaline catalysts at 300°C was shown to shift the mechanism from one involving aqueous pyrolysis (predominant furan formation) to one incorporating aldol and related condensations.

Russell et al. (1983) used selected aldehydes and ketones which might have formed from cellulose degradation as model compounds to study the formation of aromatic compounds during cellulose liquefaction, under the same conditions as those of cellulose liquefaction. Many of the same aromatic compounds were formed from these reactions as were found in cellulose derived oils. The condensation and cyclisation of aldehydes and ketones is appar­ently involved in the formation of aromatic compounds in cellulose liquefaction oils. Eight aromatics were identified in both cellulose oils and model compound products. Mechanisms were proposed for five of the eight aromatics.

Maldas and Shiraishi (1997) studied the liquefaction of biomass in the presence of phenol and water, using alkalies and salts as catalysts. A closed pressure-proof tube was used as a reactor at 150-250oC, 0-90 minutes reaction time. A higher temperature was required to reduce residue. A reaction time of 45 minutes was sufficient to maintain low residue and to keep constant other parameters, such as nonreacted phenol, combined phenol, and pH of liquefied mixtures. Aqueous alkali was more effective than water for the liquefaction of biomass in phenol. The ultimate pH of liquefaction was always acidic whether the starting pH was alkaline or acidic. Dissolution of wood varied significantly with the variation of reaction conditions, for example, pH, temperature, and nature of metal ions.

Inoue et al. (1999) used ammonia and cellulose as feedstock to study the effect of nitrogen/ carbon ratio in the feedstock on the oil production. Ammonia water of 25 wt % and micro­crystalline cellulose were charged into 500 mL autoclave, and liquefied at 300oC, 2MPa N2 initial pressure, and 1 hour retention time. Alkali acted as a catalyst for hydrolysis of cellulose into small fragments and for prevention of undesirable reactions, such as polymerization. Ammonia acted both as reactant and a basic catalyst. The liquefaction process resulted in the creation of C-N bonds. Yield and nitrogen content of the oil increased with increasing N/C ratio. Excess ammonia did not react with cellulose under high N/C ratio. Productive mecha­nism of nitrogen-containing oil from protein-containing feedstock might be the decomposi­tion of proteins to water-soluble materials, which are then combined and converted into oil. Not only amine and amide were contained in the oil, but also heterocyclic compounds. Oil derived from this liquefaction consists of aldehydes, ketones, and aromatic compounds, containing many unidentified compounds.

Acid pretreatment can be conducted with concentrated or dilute mineral acids. Because con­centrated acid is very corrosive, hazardous, and requires expensive reactors that are resistant to corrosion before it can be used, dilute acid is usually preferred for biomass pretreatment. Typically in dilute acid hydrolysis, biomass is soaked in a 1.0 wt% solution of sulfuric acid at a 10 wt% solids concentration for 24 hours. Excess liquid was removed by filtration, leaving particles of a 20%-30% solids concentration. The solids are loaded in a reactor heated at 120-160°C for a predetermined length of time. The main reaction that occurs during dilute acid pretreatment is the disruption of lignocellulosic structure and the hydrolysis of hemicel — lulose, especially arabinoxylan. The solubilization of hemicellulose and lignin by acid treat­ment exposes the cellulose component for enzymatic hydrolysis. A dilute acid pretreatment process is typically conducted in two primary treatment regimens: high temperature short time (HTST) or low temperature long time (LTLT). Dilute acid pretreatment processes have been used to pretreat corn fiber and dried distillers’ grains and solubles (DDGS), which upon enzymatic hydrolysis was used to conduct ABE fermentation using various solventogenic Clostridium spp. (Qureshi et al. 2007; Ezeji and Blaschek 2008a). Dilute acid pretreated corn fiber (8.4% total solid loading) generated 54.3 g/L total sugars upon enzymatic hydrolysis (Qureshi et al. 2007), while DDGS (15% total solid loading) generated 52.6 g/L total sugars upon enzymatic hydrolysis (Ezeji and Blaschek 2008a).

The major impediment to facile enzymatic degradation of plant cell wall biomass to biofuels is its inherent recalcitrance (Himmel 2008). The main contributions to biomass recalcitrance are the following:

• Epidermal tissue of the plant body, that is, cuticle and epicuticular waxes

• Arrangement and density of the vascular bundles

• Relative amount of sclerenchymatous (thick wall) tissue

• Degree of lignification

• Structural heterogeneity and complexity of cell wall constituents, that is, microfibrils and matrix polymers

• Challenges for enzymes acting on an insoluble substrate

• Inhibitors to subsequent fermentations that exist naturally in cell walls or are generated during conversion processes

For the above reasons, plant biomass has to be subjected to pretreatment processes in order to enable the enzymes to gain access to the cell wall polysaccharides, notably cellulose (Mosier et al. 2005; Wyman et al. 2005; Galbe and Zacchi 2007; Brunecky et al. 2009).

Current dilute acid biomass pretreatment processes hydrolyze cell wall hemicellulose in order to expose the cellulose fibers for simultaneous saccharification and fermentation (SSF). The resultant cellulose slurry is easily manipulated to the correct concentration in the bioreac­tor and can be enzymatically converted to free glucose. Acidic pretreatment liquor contains hemicellulose-derived sugars (monomers and oligomers), inhibitory compounds, and other soluble components and must be neutralized, concentrated, and possibly detoxified before microbial conversion. More severe biomass treatments that hydrolyze the hemicellulose and cellulose to free monomeric sugars, such as two-stage dilute or strong acid cooking, result in dilute sugar solutions that must be concentrated before conversion; as well as the uncon­trolled production of sugar degradation products. In general, pretreatments that retain poly­meric cellulose, such as a single-stage dilute acid or alkali pretreatments, and utilize conditions of moderate to low severity are preferred for subsequent microbial conversion (Brunecky et al. 2009).

Biomass Yield

For agricultural residues, especially straws and stover, the quantity of biomass is estimated from grain yield and the ratio of straw or stover to grain. In converting yield from grain to biomass, we need to consider definition of bulk density (test weight) of grain and moisture content at which the bulk density is given. Once the gross yield of biomass is calculated (usually and preferably expressed in dry mass t) then we discount this value depending on the following factors:

• Minimum recommended mass to be left on the field for soil conservation. This amount can be calculated from detailed soil loss models or from recommended soil coverage.

• Field losses of equipment working on the biomass during harvest and postharvest opera­tions. These losses result in reduction of mass because of breakage and leaf loss.

• Losses because of elements in the field and in storage. These losses can be physical and biochemical reactions in the biomass that is left in the field or in storage.

In the case of a dedicated energy crops such as switchgrass, total biomass yield depends upon the time of harvest. In summer the biomass is green and high in moisture; later in the fall the biomass is in a mature stage or senescence and usually low in minerals (ash content); and in the following spring the biomass is often dry, brittle, and has lost some of its mass because of snow and wind. For instance, if the land is furrowed the height of cut must be kept high (more than 150 mm) in order to minimize contamination resulting from dirt and surface soil.

Values listed in Table 7.1 are not precise and are given here for demonstration purposes. The estimated net yield of straw is 1.8 t/ha, stover 3.7t/ha, and switchgrass 6.75 t/ha.

Supply Area

The supply area is calculated from annual demand for biomass and the net yield of biomass. Table 7.2 shows the area of cultivated land to provide 500,0001 of biomass annually. To calculate the total gross area, we then have to apply at least three factors: (1) fraction of the land under biomass cultivation; (2) how often (harvested every year, every other year, etc.) the producer will supply the biomass; and (3) the sector ratio assuming a circle for the supply area. Table 7.3 shows that the total area and the radius of supply circle increases substantially depending upon the supply factors (1)-(3). In this analysis, the competition of biomass for animal bedding and feeding and other industrial usage (press board, biofuel production, etc.) has not been considered.

aTest weight for wheat at 60lb/bu at 14% m. c. Test weight for corn at 56lb/bu at 15.5% m. c. Weights are in dry mass.

bThe straw/grain ratio is the yield of grain to that of its biomass (stalks and leaves); that is, dry weight of grain/dry weight of plant biomass.

Supply Schedule

Harvest of crop residue follows grain harvest. The grain moisture content at its physiological maturity may be in the range of 30%-40%. As soon as grain reaches this moisture, harvest will start, but not at once. Initially, a few harvests will start but the pace of harvest will pick up as the season progresses. Once the peak harvest passes, the pace of harvest slows down. In northern climates and for corn that often grows in summer, the harvest is completed before the cold temperatures set in and the work in the field becomes impossible due to rain or snow. The harvest season for grain crops ranges from 4 to 10 weeks.

In the case of switchgrass, the crop can be harvested twice a year with roughly 70% of the yield obtained from the first cut and 30% of the yield from the second cut. The yield and mass ratio of the first and the second cut drop for mid-western and northern regions of the United States (Vogel et al. 2002). In biomass supply analysis, harvesting twice a year would be expensive due to machinery use and low biomass yield. Therefore, it is appropriate to use a single harvesting approach for switchgrass, which usually commences from August 1 and continues for the next 3 months (August, September, and October). The harvest activity in the Midwest stops when daily average temperature is below -5oC. In the Upper Southeast, harvest can proceed through the winter until the end of March on days when soil conditions are such that equipment can operate.

It is common to design and analyze feedstock systems in discrete unit operations of harvest (mowing, raking, windrowing, and baling), storage, and transport, as done in certain sections of this chapter. However, looking at processes with a systems approach will give the best option. This involves examining the complete system to see what processes can be combined together for synergy of resources, reduction in waste, and cost reduction. For example, current technology looks at optimizing operations that harvest agricultural residues, or energy crops, and put the raw biomass into storage. The raw biomass is then put into the densest package practical for transport to a biorefinery for processing.

Another way to approach delivering biomass at least cost is to develop options that inte­grate systems together, thus achieving multiple tasks in one unit operation. For example, grain and fiber can be harvested together and brought to a farm-level location for subsequent sepa­ration of the grain and fiber. This system was discussed in the Biomass Harvest and Collection section. Another example is the development of a mobile pelleting machine that can densify feedstocks as they are harvested. Integrated systems provide new ways of viewing the problem and formulating the correct design questions.

tntegrated systems are increasingly becoming the way to design systems and analyze pathways. In agriculture and life sciences, this is made possible by the advances made in genetic engineering. Biomass plants engineered with enzymes in their tissue can undergo pretreatment while in storage, thus eliminating harsh pretreatment and saving energy at the plant. These innovative ideas are often not easy to put together in reality. A bottle­neck to developing integrated systems is the lack of close collaboration between all expertise in the production chain. With each discipline focused on their own aspect of the chain, a lot of opportunities for synergies are lost. Collaboration allows everyone to see the bigger picture of how everything fits in place and therefore harness the strengths of the production chain as a whole. An integrated systems approach can lead to optimal design of systems and processes. Because of the complexity and diverse disciplines involved in the production and use of agricultural feedstocks and waste for energy, it is even more important to approach problem formulation within an integrated systems approach.

Palmqvist et al. (1997) proposed the use of a hemicellulose hydrolysate obtained after steam pretreatment of willow for enzyme production by the cellulolytic fungus T. reesei. The sugars in the pentose fraction were almost completely utilized, and simultaneously the hemicellulose hydrolysate was detoxified. A reduction by more than 30% in A280 after a 6-day fermentation indicated the degradation of phenolic components by T. reesei. A change in the content or structure of the phenolics may have taken place by the fungus and might as well have decreased the inhibitory action of the lignin-derived compounds. This result was confirmed by a gas chromatography-mass spectrometry (GC-MS) analysis, which showed that there was an almost complete depletion of benzoic acid derivatives during the fermentation with T. reesei. After fermentation, the remaining hydrolysate can be recirculated to minimize the need for freshwater. The ethanolic fermentation of the detoxified hydrolysate with S. cerevi — siae resulted in as good and even better yields and productivities than in the fermentation containing only glucose and nutrients (Palmqvist et al. 1997). The fact that the steam- pretreated hardwood hydrolysate was detoxified by T. reesei could be essential for the process because the resulting enzyme-containing solution would be used to hydrolyze the cellulose fraction. The cellulase activity was 0.2 IU/mL in the T reesei fermentation where no willow was added and 0.6 IU/mL where willow was added.