Another technology in the extraction space is super­critical fluid extraction (SFE) whereby a solvent is subjected to temperature and pressure conditions to adjust the properties to those intermediate to a gas and liquid in a dedicated reactor setup. This in turn effects the solubilization of solutes in a matrix (Wenclawiak, 1992). The main supercritical solvent employed is carbon dioxide. Carbon dioxide (critical conditions: T = 30.9 °C and P = 73.8 bar) is cheap, envi­ronmentally friendly and has generally recognized as safe status from the US Food and Drug Administra­tion. Supercritical CO2 (SC-CO2) is also attractive because of its high diffusivity combined with its easily tunable solvent strength (Herrero et al., 2010).

However, due to its chemical nature, it possesses several polarity limitations. As mentioned previously, solvent polarity is particularly important when extracting polar solutes and when strong matrix inter­actions are present. To augment the process, organic solvents are commonly added to the carbon dioxide extracting fluid to alleviate the polarity limitations (Handa, 2008). CO2 is gaseous at room temperature and pressure, which makes recovery very simple and provides solvent-free products, i. e. once the liquid depressurizes, the CO2 returns to a gaseous state, and only the extracted products remain. SFE using CO2 can be operated at low temperatures, which allows the extraction and integrity preservation of thermolabile compounds (Mendiola et al., 2007).

Most of the clay minerals have low Bronsted and Lewis acidity. The Bransted acidity and surface area can be slightly improved during drying when interstitial and intercalated cation hydration water molecules are removed. Another way to improve the acidity is to exchange the intercalated species with highly polarizing species such M3+ cations, where the hydrolysis of the solvated water molecules release protons.

The Lewis acidity is normally associated with exposed Al3+ or Fe3+ at the broken crystalline edges, which can be increased by heating the clay material to temperatures above 300 °C or by acid treatment (Moronta, 2004). As the thermal treatment can lead to the collapse of the clay mineral structure, the acid treat­ment is the most effective way to improve both Lewis and Bransted acidity. This procedure was already
described in the 1960s (Ryland et al., 1960), when acid-modified smectites provided high gasoline yields when used as a petroleum-cracking catalyst.

Acid activation of clay minerals using mineral acids is not new and this procedure causes disaggregation of clay particles, elimination of impurities and improve­ment of their surface area, porosity and catalytic proper­ties. Acid-activated clays are broadly spread in different industrial processes, being especially used as bleaching agent and catalysts. Although this process was found to be dependent on several factors such as the type of mineral, its crystallinity, morphology and particle size, the effects of the selective acid leaching of a 2:1 clay min­eral, which is a first-order process, can be schematically seen as shown in Figure 16.2.

The treatment of clay minerals with mineral acids at room temperature tends to replace the intercalated cat­ions by hydrated protons and/or by the leached cations, improving Bransted acidity while the structure is mostly preserved (Figure 16.2(b)). By contrast, the thermal acid activation has several effects on the structure of the clay minerals, which depend on the temperature, time of treatment and concentration and strength of the acid used. Under mild temperatures, times and acid con­centration, the first effect is usually the removal of acid-soluble impurities and partial leaching of the octa­hedral coordinated metals from the octahedral sheet

(Figure 16.2(c)). During this process, not only the Lewis acidity is generated but also the anions of the acid can be incorporated in the structure.

As clays having Mg or Fe in their structure are more easily leached than octahedra occupied by Al, the acid activation must be optimized to extract the best charac­teristic of each clay mineral. Under extreme conditions, regardless of the type of clay mineral, all the octahedral metals are removed to produce inactive hydrated fibrous silica as reported previously (Figure 16.2(d)) (Wypych et al., 2005). The effect of an effective acid acti­vation is the broadening of the basal X-ray diffraction peak due to the damage of the layers and, finally, to the total collapse of the structure. The physical effects of this activation process are the improvement of the surface area and pore volumes up to a specific point from which these properties are decreased. The pore radii are also constantly reduced during the treatment. For each acid and activation conditions, a new optimiza­tion needs to be reported since each clay mineral has a characteristic that depends not only on the mineralog — ical classification but also on the mine from which it was extracted.

As an example for montmorillonites (Wilson and Clark, 2001; Zatta et al., 2012), Figure 16.3 shows a possible mechanism for the formation of Bronsted and Lewis acid sites after mineral acid treatment of a 2:1 clay mineral constituted basically of Al in the octahedral sites and Si in the tetrahedral sites.

Alternatively, instead of the use of metal catalysts and hydrogen for hydrogenation, solvolytic depolymeriza­tion reactions were performed in the presence of hydrogen donors such as tetralin or anthracene deriva­tives (Dorrestijn et al., 1999). However, the high costs of these solvents that are consumed during the process prevent practical implementation. A solution to this problem could be the use of formic acid or 2-propanol as hydrogen donors (Kleinert and Barth, 2008; Kleinert et al., 2009). In the presence of relatively large amounts of formic acid and a low chain alcohol the resulting phenolic oil contains substantial amounts of aliphatic hydrocarbons, indicating that extensive hydrogenation of the resulting depolymerization products occurs (Gellerstedt et al., 2008). Another advantage of this pro­cess is the negligible formation of char. Xu et al. (2012) used this approach to depolymerize lignin with a combi­nation of formic acid and a Pt/C catalyst in ethanol to further promote the production of lower molar mass fractions. After 4 h all lignin has been completely solubi­lized. The highest H/C and lowest O/C molar ratios were obtained with prolonged reaction times.

Lignin depolymerization in aqueous ethanol leads to a reduced formation of char, which might be attributed to the solubility power of ethanol and the hydrogen donation capability of ethanol to stabilize generated free lignin radicals (Ye et al., 2012).

Zakzeski et al. 2012 used ethanol/water mixtures that greatly enhanced the solubility of different technical lig­nins (e. g. kraft, organosolv and sugarcane bagasse lignin) and consequently led to higher yields of monoar­omatics in one-pot lignin liquid phase reforming (LPR) reactions. During solubilization extensive cleavage of various ether linkages in the macromolecule occurred. The Pt/Al2O3-catalyzed LPR reactions yielded up to 17% of monomeric guaiacol-type products for kraft lignin in the presence of H2SO4. Depending on the lignin source and the used cocatalyst, different product distri­butions and light gases such as hydrogen and methane were formed. Char formation was not observed in any of the reactions. HDO reduction of solubilized lignin us­ing transition metal catalysts led to the formation of alkyl-substituted guaiacol-type molecules with isolated yields of up to 6% for Pt/Al2O3.

Toledano et al. 2012 used a microwave-assisted bifunctional catalytic process using tetralin or formic acid as in situ hydrogen donating solvents lead to over 30% bio-oil yield mostly enriched in monomeric and dimeric phenolic compounds. However, the amount of biochar and residual lignin still needs to be reduced.

Organosolv and kraft lignin were depolymerized using a silica-alumina catalyst in a water/1-butanol mixture to a yield of 85—88 C-mol%. In a second step the lignin-derived slurry was cracked over a ZrO2— Al2O3—FeOx catalyst in water/1-butanol Total recovered phenols is 6.6—8.6% and the conversion of methoxy phenol reached 92—94% to phenol and cresol (Yoshi — kawa et al., 2013).

Extraction and Isolation from Specific Plants

Conventional productions involve various mostly physical but sometimes chemical methods to isolate and enrich phytochemicals from selected wild or pur­posely farmed plants. Representative methods consist of solid-liquor extraction (including steam distillation), liquid—liquid extraction, or membrane separation, whose choices are based on effectiveness (low cost) and effi­ciency (recovery, especially for labile or low-abundance phytochemicals). In principle, phytochemical produc­tions may involve mechanical grinding of feedstocks, sin­gle or multiple steps of extraction, and enrichment or purification of final products. The extraction (leaching) parts may be simple binary systems or assisted by enhanced energy inputs (ultrasound, microwave, high pressure, sub — or supercritical condition) (Huang and Ramaswamy, 2012). Proper selection of solvent, adsor­bent, and other conditions is critical. Chemical transfor­mation is also applied to convert phytochemical precursors to final products, as exemplified by sulfuric acid treatment of madder to yield alizarin or purpurin.

Plant-derived colorants are mostly produced by the methods listed above: betalains (including betanin) extracted from red beet (Beta vulgaris); bixin/norbixin (annatto) from the tree Bixa orellana (Chattopadhyay et al., 2008); gossypol from cotton seed; lutein from mari­gold (Tagetes erecta); capsanthin/capsorubin from paprika; capsorubin from Capsicum annuum; crocin from saffron (Crocus sativus) flower; anthocyanins from grape skin, apple or cranberry; acylated anthocyanins from black carrot; curcumin (turmeric) from Curcuma longa; carminic acid from Dactylopius coccus; alizarin or purpurin from madder (Rubia) plants; chlorophyll from spinach; and indigo from Indigofera or Isatistinctoria (woad) plants.

For plant-derived S-compounds, glutamylcysteine or (allyl)cysteine sulfoxide are prepared from Allium spe­cies (including garlic); betaine from sugar beet (Kripp,

2006) ; tannin from tea, quebracho, chestnut or barks; caffeine from coffee and tea plant; nicotine from tobacco; camphor from camphor laurel; bromelain from pine­apple; papain from papaya; essential oils from a variety of fruits, seeds, leaves, woods, barks and roots; and menthol from mint.

Plant-derived drugs or precursors have been prepared by combinations of the methods listed above: quinine from cinchona tree, artemisinin from sweet wormwood (Artemisia annua), paclitaxel from Pacific yew (Taxus brevi — folia)’s endophytic fungi, 10-deacetylbaccatin from a few yews, (—)-shikimic acid from shikimi tree, diosgenin from Dioscorea plants, cytisine from Cytisus laburnum, vinblastine from Madagascar periwinkle (Catharanthus roseus), salicylic acid from willow bark, salicin from meadow sweet (Filipendula ulmaria), galanthamine from Caucasian snowdrop (Galanthus caucasicus), digoxin from foxglove (Digitalis lanata), and ephedrine from Ephedra sinica (Simard et al., 2012; Braz-Filho, 1999).

Plant-derived phytochemicals active as plant protec­tion and other bioactive agents are also produced from specific plants. For instance, pyrethrum is prepared from Chrysanthemum cinerariifolium and Chrysanthemum coccineum, rotenone from jicama vine, thymol from thyme (O’Brien et al., 2009; Dayan et al., 2009), and flavo — noid glycosides or polymethoxylated flavones from citrus peels (juice-extracted residues) (Manthey, 2012).

When producing any molecule of commercial interest with a microorganism, prospecting for the key gene is as important as the choice of the host to be used. The evolu­tionary convergence of ethylene production is high­lighted by the three pathways delineated above: Yang cycle, KMBA and 2-oxoglutarate, with the last having been found to be the most efficient when overproduc­tion is desired. The evolutionary radiation of the mobile plasmid encoding the efe gene among the different species and strains might have produced a naturally optimized gene that could be used in commercial pro­duction. A comparison between 20 P. syringae strains revealed a high amino acid sequence similarity between five pathovars, with P. syringae pv. phaseolicola PK2 being the most efficient, giving a twofold higher production of ethylene (Weingart et al., 1999). However, these varia­tions are likely to be due to differences in regulation as the sequence of amino acids of efe of these five strains differs by only one codon.

Using the efe gene encoded by an indigenous plasmid from P. syringae pv. phaseolicola PK2, ethylene production was reported in E. coli with a tenfold increase when compared to the original strain, P. syringae (Fukuda et al., 1992a; Ishihara et al., 1995), showing that the efe gene alone was sufficient for ethylene production. When a high-copy-number plasmid containing efe was transconjugated into Pseudomonas putida and P. syringae, ethylene production was increased, but surprisingly, production was 27- and 8-fold higher, respectively, than the wild type, whereas the amount of protein pro­duced in the cloned P. syringae was 20-fold higher (Ishihara et al., 1996), suggesting the presence of a post­transcription regulatory system. Using cellulose as sub­strate, Tao et al. showed the production of ethylene in Trichoderma viride through the heterologous expression of efe from P. syringae pv. glycinea. Thus, the use of agri­culture wastes as substrate for ethylene production was proved to be feasible, but the recombinant filamentous fungus produced only very small amounts of ethylene (Tao et al., 2008).

So far, cyanobacteria have been shown to be the best model for the bioproduction of ethylene. Of course, many barriers still have to be crossed and commercial production is far from reality at present, but the last few years have seen encouraging reports where the pro­ductivity was increased several fold without compro­mising cell fitness, suggesting that the true production limit might be much higher. The efe (EFE) from P. syrin­gae was originally cloned into Synechococcus elongatus PCC 7942 (Fukuda et al., 1994; Sakai et al., 1997; Taka — hama et al., 2003). The first problem area, the production of only trace amounts of ethylene by the transformants (Fukuda et al., 1994), was later shown to be due to the nature of the promoter used. A systematic evaluation of different promoters showed the psbA1 promoter is more efficient for efe expression than those (lac and efe) previously used in other reports, achieving production rates up to 240nl/mlh or 451 nl/ml h OD730 (Taka — hama et al., 2003). However, these recombinants showed high genetic instability. Sequencing of the heterologous gene from mutants that had ceased to produce ethylene showed punctual mutations at a defined sequence of five nucleotides, suggested to be a possible hot-spot site for spontaneous mutagenesis (Takahama et al.,

2003) . Nevertheless, active ethylene-producing strains showed signs of metabolic stress, evidenced by their yellow-green color. When these strains had ceased ethylene production due to spontaneous mutation of efe (genetic instability), they recovered the normal blue-green phenotype.

In another strategy (Ungerer et al., 2012), Synechocys — tis sp. PCC 6803 was used as model organism. Toxicity to ethylene was tested, efe was codon optimized and artifi­cially synthesized, eliminating the bases at the putative mutational hot spot by conservative substitution. As well, efe was placed under the control of the psbA1 pro­moter. A semicontinuous culture using a clone contain­ing two copies of efe was sustained over a three-week period, reaching a constant production of 3100 nl/ml h, compared to the previous result of 240 nl/ml h (Taka- hama et al., 2003). The peak of the specific productivities was 380 nl/ml h OD730 for one efe copy and 580 nl/ mL h OD730 for two copies, respectively, and when in semicontinuous culture, the average rate was 200 nl/ ml h OD730. The additional copy of the efe gene pre­sented some production improvement when compared with the previous work from Takahama et al., (451 nl/ ml h OD730 compared to 580 nl/ml h OD730) but the real advance for the field can be seen from the healthy state of the culture. The growth rate, the color of the cul­ture and the growth curve were the same for wild type and the mutants containing one or two copies of efe. This shows that there is no toxicity either by the product or by the metabolic route used to produce ethylene. In addition to the zero toxicity, the release of five carbons per ethylene formed does not seem to present a burden to the cell, as shown by the growth pattern of the single and double mutant when compared with the wild type. Nevertheless, the metabolic consequences to the cell of a higher rate of ethylene production are unknown and a physiological approach would help to understand how far ethylene production can be pushed and what to target to improve the final yield.

In general, chemical pretreatments show a high degree of selectivity for the biomass component they degrade; they also involve relatively harsh reaction conditions, which may not be ideal in a biorefinery scheme due to the possible production of toxic substances and their possible effects on downstream biological processing (FitzPatrick et al., 2010). Degradation of lignin has been observed in most chemical pretreatments, and particularly in dilute-acid and lime pretreatments (Samuel et al., 2011).

Acid treatments solubilize the hemicellulose, and by this, make the cellulose more accessible. The main reac­tion that occurs during acid pretreatment is the hydroly­sis of hemicellulose, especially xylan as glucomannan is relatively acid stable. The condensation and precipitation of solubilized lignin components is an unwanted reac­tion, as it decreases digestibility (Hendriks and Zeeman,

2009) . Dilute-acid pretreatment is considered as one of the promising pretreatment methods despite its high — energy (steam or electricity) requirements and/or corrosion-resistant high-pressure reactors, and extensive washing, which increases the cost (Isroi et al., 2011). On the other hand, pretreatments with strong acids for the ethanol production is not an attractive option, because there is a risk of formation of inhibiting compounds (Hendriks and Zeeman, 2009). Other weak organic acids such as lactic acid and phosphoric acid have been also investigated (Monauari et al., 2011).

During alkaline pretreatment the first reactions taking place are solvation and saponification. This causes a swollen state of the biomass and makes it more accessible for enzymes and bacteria. At "strong" alkali concentra­tions, dissolution, "peeling" of end groups, alkaline hy­drolysis, degradation and decomposition of dissolved polysaccharides can take place. Alkali extraction can also cause solubilization, redistribution and condensation of lignin and modifications in the crystalline state of the cellulose (Hendriks and Zeeman, 2009).

ILs are generally defined as salts that melt at or below 100 °C, providing liquids exclusively composed of ions. Simple inorganic salts (e. g. NaCl) melt at very high tem­peratures (803 °C), rendering unfeasible their routine use as solvents for organic chemical processing (Tadesse and Luque, 2011). ILs have been termed green solvents due to their negligible vapor pressure (Patel and Lee, 2012). The application of ILs to biomass valorization and pretreatment recently started to attract a great deal of attention because they are capable of disrupting the hydrogen bonds between different polysaccharide chains, thus decreasing the compactness of cellulose and making the carbohydrate fraction more susceptible to hydrolysis (Tadesse and Luque, 2011). Additionally, the recovering and the recycling of ILs has been pro­posed for decreasing the cost of the pretreatment process (Tadesse and Luque, 2011). However, cost and energy­intensive recycling of the solvents are major constraints preventing ILs from commercial viability (Fu and Mazza, 2011). Another drawback of ILs is the fact that cellulases are inactivated even at low concentrations of ILs (Wang et al., 2011a). The ILs pretreatment has been tested using coadjuvant metal or acid catalysts to obtain higher conversion and/or yields of intermediates (Tadesse and Luque, 2011).

Organosolv pretreatment is the process to extract lignin from lignocellulosic feedstocks with organic sol­vents or their aqueous solutions. This process is similar to that used in industrial paper-making processes but the degree of delignification for pretreatment is not demanded to be as high as that of pulping. Generally, organosolv processes are conducted at high temperatures

(100—250 °C) using low boiling point solvents (methanol and ethanol), high boiling point alcohols (ethylene glycol, glycerol, tetrahydrofurfuryl alcohol) and other classes of organic compounds including ethers, ketones, phenols, organic acids, and dimethyl sulfoxide (Agbor et al., 2011). This pretreatment removes extensive lignin and nearly complete hemicellulose, enhancing the enzymatic digestibility as a consequence of the increase in accessible surface area and pore volume (Agbor et al., 2011; Zhao et al., 2009). The organosolv pretreatment is more expen­sive at present than the leading pretreatment processes; however, organosolv can provide some valuable by­products that might lead it to be a promising pretreat­ment for biorefining lignocellulosic feedstock in the future (Zhao et al., 2009).

The advantages of organosolv pretreatment includes, organic solvents are always easy to recover by distilla­tion and recycled for pretreatment; the chemical recov­ery in organosolv pulping processes can isolate lignin as a solid material and carbohydrates as syrup, both of which show promise as chemical feedstocks. However, there are inherent drawbacks to the organosolv pretreat­ment, such as air and water pollution, the pretreated solids always need to be washed with organic solvent before water washing in order to avoid the reprecipita­tion of dissolved lignin, which leads to cumbersome washing arrangements (Zhao et al., 2009).

Perennial grasses (lignocellulosic biomass) such as switchgrass (Panicum virgatum), Miscanthus, and Napier grass (Pennisetum purpureum) have been gaining atten­tion recently for use in biofuel production because of

their low energy requirement for production in the US and Europe, and high biomass yield (Khanna et al.,

2008) . Miscanthus species most often used in biomass research is the sterile hybrid Miscanthus x giganteus, a hardy and fast growing C4 grass that is cultivated via rhizomes (Lewandowski et al., 2000). Yields per acre vary depending on where the crop is grown. The typical yield is 4—10 tons/acre, but yields have been known to reach 16 tons/acre in southern Europe (Lewandowski et al., 2000). Recent research on M. x giganteus and switchgrass at the University of Illinois has produced an average yield of 12 tons/acre and a maximum of

24.7 tons/acre for M. x giganteus and approximately 5 tons/acre for switchgrass (Heaton et al., 2008). The harvestable biomass of Miscanthus is 190% greater than that of corn and could produce 742 more gallons of ethanol per acre (Heaton et al., 2008) or 600 more gallons of butanol per acre. Napier grass, which belongs to sug­arcane family and a native to Africa, is now found in most tropical and subtropical regions of the world (Pen — nisetum purpureum, 2013). It has a high moisture content of 70—80% and reaches maturation following 8 months of plantation/rationing. Approximately two-thirds of Napier grass biomass (dry weight) is composed of sugars: glucan (38.43%), xylan (20.20%), galactan (2.02%), arabinan (2.73%) and mannan (0.23%), and lignin accounted for 20.93% of the lignocellulosic mate­rial, with ash (7.75%) and extractives (1.76%) comprising the remaining fraction (Takara and Khanal 2011). Napier grass has a rapid and dense growth, which have attracted the attention of researchers as a potentially ideal source for lignocellulosic biomass. Napier grass is capable of producing 42 dry tons/acre/year, approximately double the biomass yields of sugarcane and switchgrass (McLaughlin and Kszos, 2005; Takara and Khanal, 2011).

Nonetheless, one of the key steps in the lignocellu — losic biomass-to-fermentable sugars conversion is pre­treatment. The goal of pretreatment is to disrupt the biomass structure and disentangle lignin—carbohydrate complex such that enzymatic hydrolysis of the carbohy­drate fraction of the lignocellulosic biomass-to-simple sugars can be achieved more rapidly and with greater yield (Mosier et al., 2005; Ezeji and Blaschek, 2010). Economic analysis of the current pretreatment methods has shown that the relatively high costs of biofuel (ethanol) production from lignocellulosic biomass arise mainly from costs associated with three factors: (a) harsh pretreatment conditions (high temperature, high pres­sure, use of acids or bases, long residence time, and so on, allowing for inhibitor formation); (b) overuse of expensive enzymes; and (c) recovery of end products (low ethanol concentration in beer; Eggeman and Elan — der, 2005; Ezeji and Blaschek, 2010). Technologies that lead to improvement in any of these areas will help to make isobutanol production using energy crops as feedstock more cost-effective. Moreover, energy crops can be genetically modified to improve biomass yield (per acre per year) without the risk of compromising grain yield or quality along with reducing their recalci­trance to efficient deconstruction to monomeric sugars.

While producing microorganisms have not been shown to directly utilize lignocellulosic biomass as a car­bon source for isobutanol production, Higashide et al.

(2011) recently demonstrated the first production of iso­butanol from crystalline cellulose using C. cellulolyticum. This breakthrough was accomplished after a couple of attempts. First, the activities of the first three enzymes in the isobutanol production pathway were examined by transforming plasmids expressing alsS or alsS ilvCD into C. cellulolyticum and no C. cellulolyticum alsS or alsS ilvCD transformants were obtained. Realizing that alsS and alsS ilvCD transformants could not be obtained, a second attempt wherein genes encoding B. subtilis a-acetolactate synthase, E. coli acetohydroxyacid isomer — oreductase, E. coli dihydroxyacid dehydratase, L. lactis KDC, and E. coli and L. lactis ADHs (alsS, ilvCD, kivd, and adhA, complete isobutanol production pathway genes) were expressed in C. cellulolyticum (Higashide et al., 2011). Despite a mutation in alsS, the alsS ilvCD kivd adhA strain produced 140 and 420 mg/l isobutanol from cellobiose and cellulose, respectively. When plas­mids expressing kivd yqhD alsS ilvCD, in which alsS was the third gene in the operon, was constructed and transformed into C. cellulolyticum, 364 and 660 mg/l isobutanol were produced from cellobiose and cellulose, respectively (Table 7.2). Given the fact that isobutanol production technology has been changing at a rapid pace, this accomplishment in which cellulose is used as a carbon source is significant because it opens the frontier for utilizing lignocellulosic biomass such as energy crops for isobutanol production.

In HTL, water is an important reactant and catalyst, and thus the biomass can be directly converted without an energy-consuming drying step, as in the case of py­rolysis (Bridgwater, 2004). As hot compressed liquid wa­ter approaches its thermodynamic critical point (Tc = 373.95 °C, Pc = 22.064 MPa), its dielectric constant decreases due to a decrease in hydrogen bonding be­tween water molecules (Figure 10.11). At these condi­tions, water is still in a liquid state, and has a range of exotic properties very different from liquid water at room temperature. Among them is increased solubility of hydrophobic organic compounds, such as free fatty acids (Holliday et al., 1997). Subcritical water can also sustain acid and base ions simultaneously and promotes radical-driven chemistry. These properties make subcrit­ical water an excellent medium for fast, homogeneous and efficient conversions of algal organics to biocrude. But this technology is not without challenges—the solu­bility of some salts in the reacting medium decreases significantly leading to excess precipitate in the system. Salts present in the HTL process are typically subdi­vided into two categories: Type I and Type II. Type 1 salts, such as NaCl, still exhibit relatively high solubility at subcritical conditions. Type 2 salts such as Na2SO4, on the other hand, have very limited solubility at these con­ditions (Hodes, 2004). If Type II salts are present in the

FIGURE 10.11 The critical point of water. (For color version of this figure, the reader is referred to the online version of this book.)

reaction medium, the decreased solubility can lead to what’s known as "shock precipitate" which can adsorb onto the walls of processing equipment causing fouling and eventually blockage. Technologies designed to remove or reduce salts from the production stream are currently being evaluated (Marrone, 2004).

HYDROTHERMAL CATALYTIC
LIQUEFACTION

The principal role of HTL is to fractionate organic macromolecules into simpler molecular units that can then be further upgraded to produce specific liquid fuels. The HTL environment promotes the hydrolytic cleavage of ester linkages in lipids, peptide linkages in proteins, and glycosidic ether linkages in carbohydrates. The speed and efficiency of these cleavage reactions can be improved by the addition of catalysts to the reaction medium. Catalysts are generally classified as homoge­neous and heterogeneous. In chemistry, homogeneous catalysis is a sequence of reactions that occur when a catalyst is codissolved in the same phase as the reac­tants. The most reported homogeneous catalyst for HTL processing of microalgae is Na2CO3 (Tekin, 2013; Zhang et al., 2013). While it has been reported that the addition of Na2CO3 to the HTL process increases the overall biocrude yield from microalgae, others have re­ported that Na2CO3 negatively impacts yields derived from lipids or proteins, but improves yields of precur­sors derived from carbohydrates (Biller et al. 2011). The effects of other homogeneous catalysts (e. g. KOH, HCOOH, and CH3COOH) on HTL of microalgae have been examined and ordered according to effectiveness Na2CO3 > CH3COOH > KOH > HCOOH. For HTL
processing of microalgae, heterogeneous catalysts may provide a more attractive option than homogeneous cat­alysts because heterogeneous catalysts can be more easily separated from the reaction products. Further, the yields of HTL biocrude using heterogeneous catalyst have been reported to be as high as 71% (Zhang et al., 2013).

(Zoca et al., 2012). Padua Ferreira et al. (2011) gives a value of 28%. Biodigestability is 70 l/kg (Frederiks,

2012) . Methane capture can be economic using a cheap biodigester and long retention times. The coffee factories can use the methane for electricity production. Coffee farmers can install small digesters and use the methane for cooking.

The residue from grain crops amounts to nearly 3000 million tons per year (Table 13.8). The most important are maize, wheat and rice. A fraction of this is used as animal fodder or animal bedding. Ani­mal fodder turns into manure and bedding becomes a bedding manure mixture. The fraction that is left in the fields can be used for anaerobic digestion. During threshing the straws are deposited in the fields in swaths and can be picked up after a few days. Losses are between 20% and 50%. The costs for collecting the residues are high. Schmaltschinski (2008) estimates 75 V per ton of shredded straw on the truck at the field in Germany. Mo et al. (2011) report a value of 50 V in Poland. Baled straw sells for 140 V in the Netherlands. Substrate costs are 0.30 V/m3 methane for shredded straw and 0.50 V/m3 for baled straw at 100 days reten­tion times (Table 13.8).

SPENT BEDDING

Straw used as animal bedding can be collected and digested. It is then an organic fertilizer. The costs for this are mainly transport costs. The use of spent bedding means that all straw can contribute to methane produc­tion. Barsega et al. (1994) digested cattle bedding from wheat straw.

Spent bedding of rice husks from piggery housings gave less than 20% reduction in VS (Tait et al., 2009). Weathering in a pig shed improved the biodegradability. Bonilla et al. (1985) successfully digested spent poultry litter of rice chaff.

Spent wheat bedding from ducks gave a methane yield of 310 l/kg VS (Buisonje, 2009).

KITCHEN AND GARDEN WASTE

Around 100 kg/person kitchen and garden waste are separately collected in Germany (Kern et al., 2008). VS are around 40%. Extrapolation to the world population would give 150 million tons of VS per year.

AQUATIC WEEDS

Sewage from most urban areas in tropical countries is discharged untreated in rivers and lakes. This leads to eutrofication. Fast growing aquatic weeds use the nutri­ents and form thick mats hindering fishing and naviga­tion. Lake Victoria is a prime example. Biological control of the waterweeds is seen as one method to reduce the problems, but does not address the root cause of

eutrofication. Controlling of one type of waterweed will give others the opportunity to become a pest. There are a number of instances where aquatic weeds are used as an organic fertilizer (Jandl, 2010), but this practice is not widespread.

Aquatic weeds are digested in Luzira prison Kampala, Uganda, (Lindsay et al., 2000) and at the Songhai agricul­tural training centre in Porto Novo Benin (Jandl, 2010).

Harvesting of aquatic weeds is expensive. Antunuassi et al. (2002) calculate a cost of 15,000 V/ha or 0.15 V/kg VS. Veitch (2007) suggest significant cost re­ductions using outboard motor-powered launches with a rake and a land-based backhoe.

Anaerobic digestion of whole plants is not common. There are a number of tests with chopped (10—60 mm) water hyacinth (Table 13.9). Gas yields are high for the experiments of Wolverton et al. (1979), Vaidyanathan et al. (1985), and Almoustapha et al. (2008). Swine manure is not well suited as a seed for water hyacinth digestion as it has little biogas bacteria and this explains the low yield of Moorhead et al. (1993). Duration of the tests by Ofoefule et al. (2009) is too low.

Moorhead et al., 1993 have done tests with ground (1.6 mm) water hyacinth and chopped water hyacinth (12.6 mm), resulting in a 15% lower gas yield for the chopped water hyacinth. The results for the digestion of whole plants will be also lower than for ground water hyacinth.

Hemicelluloses in various hardwood species differ from each other both quantitatively and qualitatively. The main hemicelluloses of hardwood are glucuronoxy — lans (O-acetyl-4-O-methylglucurono-b-(1,4)-D-xylan;

GXs), which can also contain small amounts of GMs. In hardwoods, GXs represent 15—30% of their dry mass and consist of a linear backbone of b-(1,4)-D-xylo- pyranosyl units. Some xylose units are acetylated at C2 and C3 and 1 in 10 molecules has an uronic acid group (4-O-methylglucuronic acid) attached by a-(1,2) linkages (Table 17.2). The percentage of acetyl groups ranges between 8% and 17% of total xylan (about 3.5—7 seven acetyl residues per 10 xylose units). The xylosidic bonds

between the xylose units are easily hydrolyzed by acids, but the linkages between the uronic acid groups and xylose are very resistant. Acetyl groups are easily cleaved by alkali, and the acetate formed during kraft (alkaline) pulping of wood mainly originates from these groups. Besides these main structural units, GXs may also contain small amounts of L-rhamnose and galactur — onic acid. The latter increases the polymer resistance to alkaline agents. The average degree of polymerization (DP) of GXs is in the range of 100—200 (Peng et al., 2012; Girio et al., 2010).