Glycerolysis is a process in which the content of FFAs is reduced in feedstocks by combining FFAs with glycerin to create mono-, di-, and tri-glycerides as shown in the process flow diagram of Albemarl Corp (Figs. 18.4 and 18.5) [20]. The glycerin

is routed back to a reactor where it is combined with the high FFAs feedstock. The effluent of the glycerolysis process is a low FFAs feedstock, typically less than 1 %, suitable for alkaline transesteriflcation process under normal operating conditions. JatroDiesel, USA, offers a unit for the glycerolysis of acid feedstocks with different levels of FFAs. Glycerolysis with up to 100 % FFAs has been demonstrated at indus­trial scales while acid esterification process has been used for feedstocks with FFAs below 25 %, and stripping of FFAs has been used with 15 % FFAs or lower, prior to alkaline transesterification process for the production of biodiesel [20].

Biological pretreatment of lignocellulosic biomass changes the physico-chemical characteristic of biomass. Among the changes, lignin degradation is the most at­tractive and most studied. For example, lignin loss in wheat straw was found 25 % after 1 week [128]; lignin loss in corn straw was up to 54.6 % after 30 days pre­treatment with T. vericolor [129]; lignin loss increased from 75.67 % to 80 % when corn stalk treated with Irpex lacteus [130]; lignin extractability and glucose yield could be improved in canola straw with fungus strain T. vericolor and cellobiose dehydrogenase-deficient strain (m4D) [44]. Degradation of lignin by microbes is mainly due to a non-specific oxidative reaction, which leads to complete oxidation of lignin. Among bio-delignifier, white-rot fungus is one of the mostly studied microbes, as discussed earlier, which has unique capability to cleave carbon-carbon linkages of lignin and oxidizes with the help of various lignolytic enzymes. The changes in terms of the ratio between p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units of lignin were analyzed using pyrolysis-gas chromatography-mass spectrometry (Py — GC-MS) and concluded that the susceptibility of lignin units are in the following order: S > G > H. This order indicates that the biomass with S-rich lignin is more susceptible to fungal degradation than the biomass with other lignin units [32].

During fungal attack on biomass, hemicellulose and cellulose are also consumed and among biomass components, hemicellulose is easier to degrade. White-rot fungi such as P. chrysosporium [131], P. citrinopileatus and P. florida [132], Trametes ochracea and E. taxodii 2538 [77], C. subvermispora [133] have been found to degrade hemicellulose along with lignin loss (Table 1.1) and showed the multiple endoxylanase activity. This effect results in reduction of recalcitrance of lignocellu — lose but increases the risk of loss of cellulose or lowering the all sugar recovery in bioconversion process [130].

White-rot fungi also secrete cellulase enzyme with different specificity and synergistic characteristics during biological treatment of lignocellulose. Cellulase hydrolyzes P-1,4-linkage of cellulose to glucose and the hydrolyzed products are utilized by same fungi or other microbes. As mentioned earlier, non-selective white — rot fungi mineralize all lignocellulosic components equally. Selective white-rot fungi generally degrade negligible amount of cellulose and have promising role in the bi­ological pretreatment of lignocellulose. Cellulose loss can be analyzed by X-ray diffraction (XRD) method in terms of crystallinity index (CrI). When the corn stover was treated with brown-rot fungi Fomitopsis sp. IMER2, the crystallinity degree of treated biomass could be increased from 33.22 % to 46.06 % and crystalline por­tion from 59.96 % to 94.96 % [134]; and it was found that the crystalline change of the treated biomass is due to Fomitopsis sp. IMER2 preferential degradation of the amorphous region of cellulose. In contrast, crystallinity decreased from 68.4% to 64-65.9 % after the biological pretreatment of Japanese red pine (Pinus densiflora) with three white-rot fungi [5].

Further, Xu et al. [31] investigated the surface morphological changes dur­ing white-rot fungus I. lacteus CD2 attack on corn stover by scanning electron microscopy (SEM). SEM images showed some physical changes after biologi­cal treatment and resulted in irregular holes in the corn stover. The functional group changes and bond arrangement in the treated corn stover were analyzed by Fourier transform infrared (FTIR) spectroscopy [31], wheat straw biodegradation by P chrysosporium [131] and bamboo culms (Phyllostachys pubescence), which was treated by E. taxodii 2538 and T. versicolor G20 [135]. The various characterization results, obtained by distinguished researchers, indicate that biological treatment in­creases the pore volume, pore size and remarkably enhance the surface area of the lignocellulose. A more-defined surface area obtained from wheat straw treated by P chrysosporium supplemented with Tween 80 inorganic salts, indicating removal of lignin and making more accessible the surface of hemicellulose and cellulose [128]. Xu et al. [31] also indicated that biological treatment of corn stover with I. lacteus CD2 enhanced the pore size and pore volume of corn stover, resulted more accessible surface area for enzymatic saccharification.

Raw material Strain name Weight loss (%) Lignin loss (%) Cellulose loss (%) Hemicellulose loss (%) Reference Softwood, Pinus densiflom Ceriporia laceratalacerate 9.5 ±0.5 13.1 ±0.4 8.0 ±0.5 — [5] Polyporus bnnnalis 9.9 ±0.4 11.6 ±0.3 10.6 ±0.3 — Stereum hirsutum 10.7 ±0.7 14.5 ±0.4 7.8 ±0.3 — Sugarcane trashes Celhdomonas cartae 15.5 ±3.83 5.5 ±0.26 25.4 ±0.66 — [13] Celhdomonas и da 24.3 ±2.06 5.5 ±0.25 21.8 ±1.25 — Bacillus macerans 17.5 ±0.49 5.5 ±0.22 30.4 ±0.51 — Zymomonas mobdis 17.9 ±0.54 8 ±0.51 26.8 ±0.63 — Prosopis juliflora wood Pycnopoms cinnabarinus 18.87 ±1.11 8.87 ±0.22 4.06 ±0.18 — [136] Lantana camara wood Pycnopoms cinnabarinus 15.4 ±1.88 13.13 ±1.32 2.34 ± 0.54а — Chinese willow {Salbe baby- Echinodontium taxodii 32.5 ±1.7 45.6 ±2.0 26.7 ±0.2 50.8 ±1.8 [77] lonica, hardwood) China-fir (Cunninghmnia Echinodontium taxodii 24.1 ±0.9 39.8 ±1.2 12.6 ±0.1 31.4 ±2.7 lanceolata, softwood) Com stover Ceriporiopsis subvennispora 18.8 39.2 4.8 ±0.25 28 ±0.5 [137] Ceriporiopsis subvennispora 14.59 ±0.28 31.33 ±1.01 4.49 ±1.29 22.45 ±0.54 [138] Bamboo clurns Echinodontium taxodii 10.58 24.28 1.64 28.46 [135] Flammulina velutipes 2.27 3.14 3.88 4.82 Ganodenna lucidum 12.1 10.56 12.83 15.16 Trametes ochracea 15.21 18.63 10.79 29.22 Trichaptum bifonne 11.04 12.54 8.48 32.7 Water hyacinth Pleurotus citrinopileatus 31.9 ±0.2 19.1 ±0.4 30.1 ±0.5 37.5 ±0.1 [132] Pleurotus plorida 28.8 ±0.4 19.7 ±0.3 28.5 ±0.8 30.5 ±0.7 Moso Bamboo Irpex lacteus — 17.87 ±0.83 48.20 ±0.92 18.50 ±0.97 [139] Wheat straw Fames fomentarius — 35 ±1 45 ±1 51 ±27 [140] Com stover Auricularia polytricha, Irpex — 17.8 ±1.0 31.5 ±0.8 16.8 ±0.9 [141] lacteus

The effect of biological pretreatment of lignocellulose in terms of weight loss, cellulose, hemicellulose, and lignin losses is summarized in Table 1.1.

Stalk is composed of cellulose, hemicellulose, and lignin. Cellulose connects with hemicellulose and lignin with hydrogen bond, while hemicelluloses and lignin con­nects with each other by covalent bond. Therefore, the structure of cell wall is tight and complex. Cellulose, lignin, and hemicelluloses are composed of glucose, phenyl propane units, and pentose, respectively. It is obvious that three main component are different from each other. Cellulose and hemicellulose are carbohydrate, and lignin is aromatic.

Heterogeneous bioconversion property of stalk in component level presents in two processes: hydrolysis and fermentation.

In the hydrolysis process, hemicelluloses and cellulose could be hydrolyzed by cellulase. However, lignin is regarded as recalcitrance for cellulose enzymatic hy­drolysis [21]. On one hand, lignin prevents cellulase contacting to substrate. On the other hand, lignin absorbs cellulase non-productively.

In the fermentation process, glucose from cellulose could be used as the main carbon source. Pentose from hemicellulose could also be used by several microor­ganisms [8]. However, the hydrolysate of lignin, especially small molecular [22] is proved to be inhibitors for fermentation.

Therefore, if stalk is converted as a whole, cellulose conversion rate would be low because of lignin effect. It would be necessary to fractionate stalk into different components and then convert them, respectively.

For pulping, cellulose is extracted in pulping process with different chemicals pretreatment. However, lignin is removed to improve the property of pulp. Chemical structures of cellulose, hemicellulose, and lignin change differently during pulping.

A typical scientific MW apparatus normally works at a frequency of 2.45 GHz. It consists of a magnetron that generates the MW and introduces it into the cavity, where samples to be treated are placed. Figure 6.1 shows the typical apparatus for MW experiments (Ethos, Milestone Co. Ltd.). The MW power is programmable from 0 to 1000 W. In this apparatus, the temperature, time, and MW power output can be monitored and automatically controlled.

The reactors being used in the experiments are usually made of MW transparent materials such as glass or Teflon. However, for high-temperature experiments, high — pressure build-up inside the reactor is expected and specially designed reactor should be used. For this purpose, tough material such as those made from poly-ether ether ketone (PEEK) could serve as a casing for the reactor to resist high pressure. Till date, reactors made for use up to a maximum temperature of 250 °C and maximum pressure of 2.5 MPa are readily available but demand high costs.

Another commercially available laboratory scale MW apparatus for high — temperature and high-pressure experiments is the MW Accelerated Reaction System (MARS-5, CEM Corporation). This apparatus, comparable with the one shown in Fig. 6.1, can be operated at a programmable MW power up to 1250 W at 2.45 GHz

О о

Reactor components
Tmax = 250°C, Pmax = 2.5 MPa

Microwave Oven

Fig. 6.1 Typical microwave apparatus for high temperature and high pressure experiments (Ethos, Milestone Co. Ltd.) frequency. This is equipped with fiber optic temperature and pressure probes within the cavity and a turning carousel with a maximum of 14 pressure sealed 100 mL vessels (XP-1500), which can be used up to a maximum temperature of 260 °C and maximum pressure of 3 MPa. This equipment operates with a focused MW irradiat­ing beam and is capable of heating and holding at desired cooking (or temperature ramp) rates and holding times.

Pretreatment is the first and the most crucial step for effectively using biomass and for developing new routes to produce biofuels and value-added products. Pretreatment is a process intensive step and, for example, it is the single most expensive processing step in cellulosic ethanol production, making up approximately 20-40 % of the product cost. Although there are many research articles that focus on pretreatment techniques, it was felt by the authors that there was a lack of a comprehensive source where one could turn to understand the many possible methods and their range of application.

This text includes 19 chapters contributed by world-leading experts on pretreat­ment methods for biomass. It gives an extensive coverage for different types of biomass (e. g. molasses, sugar beet pulp, cheese whey, sugarcane residues, palm waste, vegetable oil, straws, stalks and wood), for different types of pretreatment approaches (e. g. physical, thermal, chemical, physical-chemical and biological) and for methods that show subsequent production of biofuels and chemicals such as sugars, ethanol, extracellular polysaccharides, biodiesel, gas and oil. In addi­tion to traditional methods such as steam, hot-water, hydrothermal, diluted acid, organosolv, ozonolysis, sulfite, milling, fungal and bacterial, microwave, ultrasonic, plasma, torrefaction, pelletization, gasification (including biogas) and liquefaction pretreatments, novel techniques (e. g. nano — and solid-catalysts, organic electrolyte solutions and ionic liquids) are introduced and discussed.

Each chapter was strictly reviewed externally by experts in biofuels listed in the Acknowledgement. The chapters are categorized into seven parts:

• Part I: Biopretreatment

• Part II: Thermal Pretreatment

• Part III: Chemical Pretreatment

• Part IV: Physicochemical Pretreatment

• Part V: Gasification, Liquefaction and Biogas

• Part VI: Novel Pretreatment Techniques

• Part VII: Treatment of Different Types of Biomass

This book offers a review of state-of-the-art research and provides guidance for future paths for developing pretreatment techniques of biomass for biofuels in the fields of biotechnology, microbiology, chemistry, materials science and engineering. It is our intention to provide a systematic introduction to pretreatment techniques. It is an accessible reference book for students, researchers, academicians and industrialists in bioreflneries.

Acknowledgement

First and foremost, I would like to thank all the contributing authors for their many efforts to insure the reliability of the information given in the chapters. Contributing authors have really made this project realizable.

Apart from the efforts of authors, I would also like to acknowledge the refer­ees listed below for carefully reading the book chapters and giving constructive comments that significantly improved the quality of the book:

Prof. Nicolas Abatzoglou (Universite de Sherbrooke, Canada), Dr. Muhammad T. Afzal (Univ. of New Brunswick, Canada), Dr. Taku Michael Aida (Tohoku Univ., Japan), Dr. Zainal Alimuddin Zainal Alauddin (Universiti Sains Malaysia), Prof. Jacques E. Amouroux (LGPPTS/UPMC/ENSCP, France), Dr. Anju Arora (Indian Agricultural Research Institute), Dr. Hassane Assaaoudi (McGill Univ., Canada), Dr. Hassan Azaizeh (Tel Hai College, Israel), Prof. Nicolas Brosse (Nancy Univ., France), Prof. Rafael B. Mato Chain (Universidad de Valladolid, Spain), Dr. Pascale Champagne (Queen’s Univ., Canada), Prof. Hongzhang Chen (Chinese Academy of Sciences), Prof. Andrzej G. Chmielewski (Warsaw Univ. of Technology), Prof. Jae-woo Chung (Gyeongnam National Univ. of Science and Technology, South Korea), Dr. Daniel Ciolkosz (Pennsylvania State Univ.), Dr. Louis J. Circeo (Ap­plied Plasma Arc Technologies, LLC, Atlanta), Dr. Rudolf Deutschmann (Lakehead Univ., Canada), Dr. Tim Dumonceaux (Agriculture and Agri-Food Canada), Dr. Animesh Dutta (Univ. of Guelph, Canada), Prof. Elena Efremenko (The M. V. Lomonosov Moscow State Univ.), Prof. Xu Fang (Shandong Univ., China), Prof. Toshi Funazukuri (Chuo Univ., Japan), Prof. Pag-asa Gaspillo (De La Salle Univ., Philippines), Dr. Anli Geng (Ngee Ann Polytechnic, Singapore), Prof. Ashwani K. Gupta (Univ. of Maryland), Prof. Rick Gustafson (Univ. of Washington), Prof. Michikazu Hara (Tokyo Institute of Technology), Prof. Adriaan van Heiningen (Univ. of Maine), Dr. Kazuhiko Ishikawa (Advanced Industrial Sciencr and Technology, Japan), Dr. Keikhosro Karimi (Isfahan Univ. of Technology, Iran), Prof. Han — sie Knoetze (Stellenbosch Univ., South Africa), Prof. Gunnur Kocar (Ege Univ., Turkey), Prof. Ramesh Chander Kuhad (Univ. of Delhi South Campus, India), Dr. Christopher Lan (Univ. of Ottawa), Mr. Jean-Remi Lanteigne (Ecole Polytechnique de Montreal, Canada), Dr. Jean-Michel Lavoie (Universite de Sherbrooke, Canada), Dr. Jianjun Li (Chinese Academy of Sciences), Dr. Yebo Li (Ohio State Univ.), Prof.

Yin Li (ChineseAcademy of Sciences), Dr. Lin Lin (Jiangsu Univ., China), Prof. Yun Liu (Beijing Univ. of Chemical Technology), Dr. Poupak Mehrani (Univ. of Ottawa), Dr. FelipeAlatriste Mondragon (Instituto Potosino de Investigation Cientiflca y Tec — nologica, Mexico), Dr. Antonis Mountouris (National Technical Univ. of Athens), Dr. Naim Najami (The Academic Arab College of Education, Israel), Prof. Yonghao Ni (Univ. of New Brunswick), Prof. Lucia Garcia Nieto (Universidad de Zaragoza, Spain), Dr. Abdul-Sattar Nizami (Univ. of Toronto), Dr. Melek Ozkan (Gebze In­stitute of Technology, Turkey), Prof. Igor Polikarpov (Universidade de Sao Paulo, Brazil), Prof. Xinhua Qi (Nankai Univ., China), Dr. Wensheng Qin (Lakehead Univ.), Dr. Armando T. Quitain (Kumamoto Univ., Japan), Dr. R. Michael Raab (Agrivida, Massachusetts), Dr. Mala Rao (National Chemical Lab., India), Prof. Joseph P. Roise (North Carolina State Univ.), Dr. Guus van Rossum (Univ. of Twente, the Nether­lands), Prof. Roger Ruan (Univ. of Minnesota), Prof. Elio Santacesaria (Complesso Universitario di Monte S. Angelo, Italy), Dr. Anton Sonnenberg (Wageningen UR, the Netherlands), Dr. Andy Soria (Univ. of Alaska), Dr. Wolfgang Stelte (Technical Univ. of Denmark), Dr. Chia-Hung Su (Ming-Chi Univ. of Technology, Taiwan, ROC), Dr. Lee Keat Teong (Universiti Sains Malaysia), Mr. Xiao-fei Tian (Chinese Academy of Sciences), Prof. Montserrat Zamorano Toro (Universidad de Granada, Spain), Mr. Satriyo Krido Wahono (Indonesian Institute of Sciences), Dr. Haisong Wang (Chinese Academy of Sciences), Dr. Chunbao (Charles) Xu (Western Univ., Canada), Dr. Jing Yang (Southwest Forestry Univ., China), Dr. Wennan Zhang (Mid Sweden Univ.), Dr. Xiao Zhang (Washington State Univ.), Prof. Xiao-yu Zhang (Huazhong Univ. of Science and Technology, China), Dr. Y.-H. Percival Zhang (Virginia Tech), Dr. Xuebing Zhao (TsinghuaUniv., China), Dr. Junyong Zhu (USDA Forest Service).

I am also grateful to Ms. June Tang (Associate Editor, Springer) for her encouragement and guidelines during my preparation of the book.

Finally, I would like to express my deepest gratitude towards my family for their kind cooperation and encouragement, which help me in completion of this project. Especially, I would like to dedicate this book to my Mother (Ms. Shubi Yu) for her love and support during her final illness. She passed away at 9:30 am on April 9, 2012 in Taining, Fujian.

May 30, 2012 ZhenFang

Kunming

Only few researchers have published work on grape pomace, which today is a very significant waste product in agriculture industries. Grape pomace is the residue left af­terjuice extraction by pressing grapes in the wine industry. Globally, about 10 million tons of grape pomace (seeds, skin, and stem) is produced each year. In Spain alone, over 250 million kg of this by-product are used every year either as animal feed (with low nutritional value) or for ethanol production by fermentation and distillation (low level benefit). This material is underexploited and most of it is generally disposed in open areas, leading to serious environmental problems [77]. Israilides et al. [46] ex­tracted the sugars in the grape pulp by using hot water at 65-70 °C and then clarified the solution and used for pullulan production by A. pullulans NRRLY-6220 cultures. Since the grape pomace extract mainly contained sugars and very low amounts of protein, the polymer produced was very similar in its amount as well as molecular weight to the pullulan produced in defined medium. Moreover, the pullulan yields were high reaching 22.3 g/L after 7 days of fermentation period. Stredansky and Conti [53] tested grape pomace, tangerine peels, and apple pomace as substrate for xanthan production by solid state fermentation (SSF) with X. campestris NRRL B — 1459 cultures. These substrates were soaked in alkaline solution to neutralize the organic acids and then added to the fermentation media. Performances of these feed­stocks were also evaluated in the presence and absence of spent malt grains as inert support and apple pomace proved to be a superior substrate yielding high amounts of xanthan under both conditions. Low xanthan yields with grape pomace were at­tributed to the low sugar content used and the low absorption capacity of the solid material.

5.2.1 Drying

The biomass, in the forms of wood chips, sawdust, bagasse, grass, and agricultural residues is bio-origin material that initially contains moisture from 50 % to over 150 % (dry basis) in the fresh form. In order to increase energy efficiency, improve energy product quality, and reduce emissions in its thermochemical energy conversion, drying of the biomass to the required MC is important in the development of energy production systems [13, 14]. In addition, it was found that uniformity of drying also significantly affects the energy efficiency in a combined heat and power (CHP) plant [15]. Other issues needed to be considered in biomass drying include forms of feedstock, energy conversion technology, environmental impact, risk of fire and explosion, available energy source, and drying costs [13].

Three common types of dryers widely used in industry are packed moving bed (PMB) dryer [16, 17], rotary dryers [18-20], and pneumatic or flash dryers [19, 21]. For the development of biomass drying technologies, multistage drying, exhaust air recycling, heat recovery, and the optimization of the drying conditions have been explored [13, 19, 22]. In particular, superheated steam drying has attracted great interest in order to prevent the risk of fire, reduction of emissions and increase the energy efficiency in drying [13, 23, 24].

The Bergius-Rheinau wood saccharification process has its origin in Willstatter’s

[13] discovery that the cellulose of wood is easily hydrolyzed by highly concentrated HCl at low temperature into dextrose. Karl Goldschmidt, one of the leading men in the German chemical industry of that time, who had an open mind for its great future problems, devoted himself to the industrial application of this process. For this the work of Hagglund [14] was decisive, who found that the obtained solution of dextrose in HCl can dissolve fresh cellulose several times over and, besides, that the HCl can be evaporated in a vacuum from the sugar and then recycled. However, from the beginning in the first half of the twentieth century, the technical execution of this process was considered extremely difficult [15].

During the first 20 years, a manufacturing process was developed in the Rheinau pilot plant in Germany under Friedrich Bergius. It represented in these days an ex­cellent technical achievement; especially the problem of the technical manipulation of the highly concentrated HCl was well solved.

In 1927, Bergius was able to conclude his own work on the liquefaction of coal, after the practical possibilities had been proved on a large scale. The I. G. Farbenindustrie and Imperial Chemical Industries then took up the work on an industrial scale. From that time onward Bergius devoted himself to a process of obtaining sugar from cellulose in wood, on which he had already worked during the First World War. He succeeded after 15-year work and an industrial plant was set up, also in the Rheinau works. It is amazing with what intensity Bergius took up the second part of his life’s work, namely this hydrolysis of cellulose in wood and similar substances to sugar. It seems as if the well-known difficulties of working with highly concentrated HCl had presented a special challenge to Bergius. Initially the process was taken up only in England and only during the 1930s of the twentieth century did Bergius manage to continue these experiments in Germany; his main concern was to rationalize the process and to ensure complete recovery of the HCl used by constructing intricate devices.

The capacity of the Rheinau plant was raised to 400 tons of raw sugar a month, and at Regensburg a manufacturing plant of 1,600 t of raw sugar a month was erected for food-yeast production. However, a profit with all other wood saccharification processes was only reached as long as the products were protected by the government [16]. At the end of the war, both plants had to be closed down. In Russia, a plant using HCl technology was active for 20 years in Siberia and in the US Dow Chemicals erected a pilot plant in the 1980s. All plants were closed; the main problem was the inability to recycle the HCl efficiently, thus causing the process to be uneconomical.

Concentrated (fuming) HCl-driven hydrolysis provides the most powerful and industrially proven technology for converting all cellulosic wastes—wood, solids from city sewage plants, bagasse, grasses, etc. into sugars that can be fermented to ethanol or other biofuels as well as a large variety of chemicals and bio-products and food and feed. HCl permeates the wood more easily than H2SO4. HCl makes the cellulose more susceptible to hydrolysis and it is a volatile compound, which assists in the crucial acid recovery steps.

The Virdia process begins by steam expansion of debarked chipped wood, which undergoes a pre-extraction stage to remove all extractives, for example, tall oil and ash. The pre-extracted wood continues into hydrolysis stage performed using highly concentrated HCl (42 %) at low-temperature (10-15 °C), thus affording sugars hy — drolyzate with minimum degradation products (e. g., furfurals), while simultaneously separating the solid lignin. Approximately 98 % of the theoretically available sugars, composing ca. 65 % of the dry weight of the wood chips for pine wood are converted into sugars, which are dissolved in the hydrolyzate. The sugars hydrolyzate is further treated by extracting of the acid for recycling. The soluble oligo-saccharides formed to some extent are converted into the more desired mono-saccharides mixture of glu­cose, mannose, galactose, xylose, and arabinose, thus removing any impurities that may remain or may have been created during the course of the hydrolysis process.

The hydrolysis catalyst—HCl, forms hydrates, for example: HCl ■ 2H2O; HCl ■ 3H2O; HCl ■ 4H2O (fuming HCl). It is assumed these species are responsi­ble to the efficient hydrolysis of cellulose. These hydrates are formed mostly at high-HCl concentration in water, that is, 40-42 %; below this concentration, the uniqueness of the HCl hydrates as dispersants of lignocellulose presumably drops sharply.

Another view [17] describes the following structures for hydrated HCl:

• For the dihydrate, (H2O-H+-OH2)(Cl-);

• For the trihydrate, (H2O-H+-OH2)(H2O)(Cl-);

• For the hexahydrate, (H3O+)(H2O)5(Cl-).

Or as shown by Botti et al. [18] where these species are so-called Eigen and Zundel — type complexes:

• Eigen [H9O4]+

• Zundel [H5O2]+

In the above descriptions are not shown any chloride ions that might be in the vicinity of the complex or water molecules that might be bonded to the other end of the Zundel ion. Not shown are also any chloride ions that might be in the vicinity of the Eigen ion complex.

Figure 7.3 shows a model of wood, into which the 42 % HCl succeeds to go through and separate between the lignin and the saccharides and probably penetrates the crystalline cellulose structure.

A key limitation to any concentrated acid hydrolysis is the difficulty in recovering the acid. In particular, HCl solution forms an azeotrope at between 21 and 25 % depending on the pressure; simple distillation cannot concentrate a dilute solution beyond the azeotropic point. The efficiency of acid recovery is a key condition to making acid hydrolysis of lignocellulosic materials an economically viable source of fermentable sugars.

It is important in minimizing the need for make-up HCl, for neutralizing chemi­cals, and for costly disposal and negative impact on the environment. Full recovery of HCl at high acid concentration and its reuse yields very minor waste stream, no complicating air emissions, and favorable life cycle analysis. As most of the HCl

Fig. 7.4 The extractant roles: removal of HCl and obtaining highly concentrated sugars aqueous solution

recovery happens at relatively low temperatures, this also permits highly efficient energy integration. In addition to the HCl removal from the sugars hydrolyzate, it is also removed from the remaining lignin through a proprietary de-acidification pro­cess at low temperatures, thus being almost fully recycled into the process, leaving a very pure lignin. The main innovation in Virdia process is based on the problem the Germans had—recovering the acid, that is, evaporating water from the dilute acid at azeotrope concentration means breaking the bond between the acid and water at ~23 %. This is performed by using a medium, that is, an extractant composition that can perform two contradictory roles at two different circumstances (Fig. 7.4):

1. Taking the HCl out of the water at a relatively low temperature, thus yielding highly concentrated sugars solution;

2. Recovering the acid at high concentration.

Virdia developed several processes for the recovery of HCl from a dilute solution [19]. The following process describes the extraction of the acid by bringing a dilute aqueous HCl solution into contact with a substantially immiscible extractant, thatis, comprising of tris-2-ethylhexyl amine (TEHA; Fig. 7.5), which is substantially water insoluble in both free and salt forms, an oil soluble weak organic acid, for example,

Fig. 7.6 Carboxylic acid O

enhancer to a secondary amine extractant

octanoic acid which is substantially water insoluble, in both free and salt forms; and a solvent for the amine and organic acid, for example, dodecane, as a result of which HCl selectively transfers to the extractant to form an HCl-carrying extractant. See the following scheme:

The role of the carboxylic acid, which is used an enhancer, is to form a stable complex with the amine and the HCl, as seen in the following (Fig. 7.6):

Table 7.2 shows equilibrium data of HCI extraction with TEHA/octanoic acid in dodecane extractant:

This HCl-loaded extractant is further treated to obtain gaseous HCl.

Stripping of HCl is performed by passing, for example, xylene vapors stream through the HCl-loaded extractant. The recovered HCl during the stripping is shown in Table 7.3:

The whole extraction/stripping process is presented by the following scheme (Fig. 7.7):

The final product consists of the following sugars (Figs. 7.8 and 7.9):

The HPLC of a typical final sugar product is seen in Fig. 7.10 and a typical final product in Fig. 7.11:

The full Virdia process is presented in the following scheme (Fig. 7.12):

As can be seen in the above scheme, in addition to the sugars produced, two more products are obtained, that is, lignin and tall oil.

The lignin (Fig. 7.13) is separated as solid from the process, the HCl is stripped and recycled, and the lignin is dried.

Lignin is used for binders, activated carbon, carbon fibers, fire-retardants, motor fuel, dispersants, sorbents, surfactants, and as starting material for vanillin.

An additional by-product of the Virdia process is tall oil—a generic name for a group of compounds which consist of resin acids, fatty acids, fatty alcohols, some sterols, and other alkyl hydrocarbon derivates. Resin acids occur in pine in a number of isomeric forms having the molecular formula of C20H30O2 and some related structures. The most prevalent are abietic-type acids, such as levopimaric, palustric, abietic, and neoabietic acids; and pimaric-type acids, such as pimaric and isopimaric acids (Fig. 7.14).

The fatty acids include more than 10 different acids: both saturated and unsat­urated. The most common are palmitic and stearic acids, which are saturated, and

oleic and linoleic, which are unsaturated. The unsaponiflables present in tall oil in­clude higher fatty alcohols, esters, plant sterols, and some hydrocarbons. The most common sterol present is в-sitosterol (Fig. 7.15) [20].

The resin acids part of the tall oil is used for inks, adhesives, paper-making, road­marking, and tyres. The fatty acids are applied to paints and coatings, bio-lubricants, fuel-additives, and performance polymers. Sterols are used as health-enhancing food additives and for pharmaceuticals.

Manganese peroxidase (EC 1.11.1.13, Mn(II):hydrogen-peroxide oxidoreductase, MnP) also require H2O2 as an oxidant in the Mn-dependent catalyzing reaction in which Mn2+ is converted to Mn3+ by MnP. Mn3+ then oxidizes phenolic rings to phenoxyl radicals, which leads to decomposition of compounds. Both LiPs and MnPs are heme-containing glycoproteins [49, 101, 102]. But LiPs are not as widespread as MnPs, and major difference between MnPs and LiPs in lignin degradations are as LiPs generally oxidize nonphenolic lignin substructures and MnPs oxidize phenolic

rings of lignin [49]. MnPs have an important role in lignin depolymerization, chloro — lignin, and demethylation of lignin. Therefore, MnPs have a very essential role in biological pretreatment of lignocellulosic biomass. So far, many researchers have reported that P. chrysosporium, Pleorotus ostreatu, Trametes sp., and several other species, which belong to Meruliaeiae, Coriolaceae, and Polyporaceae produce MnP [32].

MnPs contain one molecule of heme as iron protoporhyrin IX and comprise with 357 amino acid residues, three sugar residues (Glc Nac, Glc Nac at Asn 131, and a single mannose at Ser 336), two structural calcium ions, a substrate Mn2+ and 478 solvent molecules. For MnP, the acidic amino acids, aspartic acid, and two glutamic acids have been proposed as manganese-binding residues [32, 98]. MnPs act on its substrate almost similar to LiPs action. Thus, the native form of MnP is oxidized by addition of H2O2 to form MnP I complex (Fig. 1.3). Then this catalytic cycle involves in the oxidation of Mn2+ to Mn3+ by MnP I and MnP II complexes. Finally, Mn3+ oxidizes the lignin compounds by diffusing into the lignifled cell wall and attacks it from inside. Indeed, MnP I can directly involve in the oxidation of phenolic compounds such as 2,6-dimethoxyphenol, guaiacol, and phenolic tetrameric lignin model compounds. This oxidation reaction clearly elucidates that MnP oxidizes the phenolic part of the lignin indirectly via Mn ions. But MnP naturally does not oxidize aromatic compounds of lignin directly as LiP. Because they do not have tryptophan residue, required for electron transfer to non-phenolic substrates [98,103]. Recently, MnPs have been isolated from Bjerkandera sp. BOS55 and P eryngii that are found to be oxidized Mn2+ as well as aromatic compounds [98]. Hence, it is very clear that addition of Mn2+ may play further enhancement in the bio-oxidation of phenolic compounds of lignin and may induce MnP production in fungi.

The reaction parameters (i. e. steam temperature, reaction time and particle size) which have been previously highlighted to influence the auto-hydrolytic process similarly have been demonstrated to affect the catalysed steam pre-treatment method [21, 12]. In addition to those factors, the selection of the type and concentration of acid catalyst employed, as well as the biomass type and its moisture content are also major considerations which must be addressed with the use of this method.

Although a variety of acid catalysts prior to steam pre-treatment have been demon­strated in the literature (i. e. the use of aqueous phosphoric acid (H3PO4) for the pre-treatment of sugarcane baggase [34]), the use of H2SO4 and SO2 as the process acid catalysts currently predominates acid catalysts investigated and in practice. This section will, thus, concentrate on the use of these acid catalysts. A study carried out in [35] compared the use and influence of the choice of H2SO4 (0-3 %, w/w) and SO2 (1 %, w/w) for the pre-treatment of Salix caprea (Willow) wood chips with a steam temperature range of 160-230 °C investigated with a fixed reaction time of 10 min (with a 15 min treatment of the biomass samples with saturated steam at 1 bar prior to the acid catalyst addition). With an increase in the H2SO4 acid concentrations (and steam temperatures), a reduction in the fibrous material yields were observed. This was attributable to the improved solubilisation of the extractives and hemicelluloses under these conditions [35]. The xylose yields (based on the calculated biomass xy­lan content) were also seen to increase with an increase in the H2SO4 concentrations. With a steam temperature of 190 °C and using the highest study H2SO4 concentration of 3 %, a xylose yield of 80 % was obtained, the glucose yields (after enzymatic hydrolysis) were however seen to be reduced under these conditions [35]. This was considerably higher than the <15 % xylose yields observed when no acid catalysts were used for the steam pre-treatment [35]. With the use of SO2 as the process cat­alyst, a maximum xylose yield of 62 % was observed using a steam temperature of 200 °C [35]. Correspondingly, a glucose yield of 95 % (on the basis of the of the biomass glucan availability) was obtained after enzymatic hydrolysis with the use of the same reaction parameters [35]. With glucans being the main constituents of willow biomass and with higher glucose yields upon hydrolysis considered to be more beneficial than xylose yields in the water soluble fractions, the results of that study led to the conclusion that the SO2 acid catalysts were preferable for use as the acid catalyst candidate for use in the catalysed steam pre-treatment method.

The use of SO2 concentrations (1-4 %, w/w) as the process catalyst for the impregnation of wood prior to the steam pre-treatment was seen to significantly reduce the overall process temperature and time requirements to achieve optimum biomass solubilisation, recovery and hydrolysis of the post-treated substrates [13].

The use of SO2 as the process catalyst over H2SO4 has also been discussed to be preferred in industrial catalysed steam pre-treatment systems since the former was reported to require a comparatively less expensive reactor materials, leads to the generation of less gypsum as a process by-product, and less process steam re­quirements than the former [13, 36]. The ease in which SO2 is incorporated evenly within lignocellulosic materials has also been a factor favouring its use over H2SO4, especially with regards to the handling inconveniences encountered with the soaking process necessary with the use of aqueous H2SO4 [13].

Regarding the influence of the type of lignocellulosic biomass employed in the catalysed steam pre-treatment process, softwoods have been demonstrated to require more harsh pre-treatment conditions for the production of suitable substrates aimed at effecting high product yields upon hydrolysis [23]. Based on this, the use of SO2 as an acid catalyst was shown to be beneficial when used for the pre-treatment of hardwood, but highly essential for use for the impregnation of softwood, that is, spruce and fir prior to their steam pre-treatment [23, 31]. Furthermore, with an increasing moisture content of the wood chips, it was observed that a higher efficiency of the acid catalyst (expressed in relation to the dry weight of the wood chips) was retained within the chips, subsequently resulting in an enhanced biomass pre-treatment [23].