The term hemicellulose is a collective term. It is used to represent a family of polysac­charides that are found in the plant cell wall and have different composition and structure depending on their source and the extraction method. Unlike cellulose, hemicellulose is composed of combinations of pentose (xylose (Xyl) and arabinose (Ara)) and/or hexoses (mannose (Man), galactose (Gal), and Glu); and it is frequently acetylated and has side chain groups such as uronic acid and its 4-O-methyl ester. The chemical nature of hemicellulose varies from species to species. In general, the main hemicelluloses of softwood are galactoglucomannans and arabinoglucuronoxylan, while in hardwood is glucuronoxylan (Fig. 8.3) [ 28]. Table 8.2 summarizes the main structural features of hemicelluloses appearing in both softwood and hardwood.

Important aspects of the structure and composition of hemicellulose are the lack of crystalline structure, mainly due to the highly branched structure, and the presence of acetyl groups on the polymer chain. Hemicellulose extracted from plants possesses

a high degree of polydispersity, polydiversity, and polymolecularity (a broad range of size, shape, and mass characteristics). However, the DP does not usually exceed 300 units whereas the minimum limit can be around 50 monomers, which are much lower than cellulose.

In addition, most sugar components in the hemicellulose can take part in the formation of lignin-carbohydrate complexes (LCC) by covalent linkages between lignin and carbohydrates [31, 32]. The most frequently suggested LCC-linkages in native wood are benzyl ester, benzyl ether, and glycosidic linkages [33]. The benzyl ester linkage is alkali-labile and may, therefore, be hydrolyzed during the alkaline pretreatment. The latter two linkages are alkali-stable and would survive from the hydrolysis during alkaline pretreatment.

As per theoretical and practical views, it is elucidated that enzymes Lac and peroxi­dase (LiPs and MnPs) are larger than the pore size of the cell wall, and they cannot have direct contact with the lignin. Various low-molecular weight compounds are found in white-rot fungi, which play an important role in ligninolytic enzyme sys­tem of white-rot fungi. It has been studied the role of mediators or co-factors in various in vitro studies, which revealed optimum concentration of H2O2, lignin, O2, and suitable mediators [122]. Therefore, for effective biological removal of lignin components from lignocellulosic biomass, the fungi and/or bacteria should not fail to produce these mediators or co-factors.

When compared to the use of batch reactors, the development and application of con­tinuous high pressure steam reactors however could provide a better control of the
pre-treatment variables which are critical to obtaining optimal biomass degradation and conversions, and a higher purity of the valuable post-hydrolysis components. This is due to the partial overcoming of the heat transfer limitations usually encoun­tered in batch systems which would subsequently result in a lower accumulation of undesirable degradation by-products [13].

Pellets usually expand significantly right after the extrusion and their density decrease with time until reaching a constant volume; this is termed a spring-back effect. The relaxed density of the pellet is always lower than the initial pellet density (i. e., the pellet density measured right after the extrusion). This is due to the rheological behavior of the polymer components of the lignocellulosic biomass fiber. When the pellets extruded from the die, the individual biomass particles are free from the high compression force. The resulted pellets suddenly undergo dilation. The elasticity of polymers (i. e., cellulose, hemi-cellulose, and lignin) allows them to have a tendency to restore to their original un-deformed structure prior to compression. If the binding of the individual particles within a single pellet is not strong enough, the pellets will have inertia to expand and the resulted density decreases.

Pressure affects the density and durability of the pellets. High pressures and tem­peratures during densiflcation may develop solid bridges by a diffusion of molecules from one particle to another at the points of contact, thereby increasing the pellet density and durability. However, the pellet density initially increases significantly with the pressure and reaches a maximum point, and beyond that it does not increase with pressure. Fractures may occur in the pellets due to sudden dilation when the pressure exceeds the optimum level.

Retention/relaxation time refers to the hold times of the biomass inside the die. It is usually around 5-30 s. During this time, it allows enough time for the biomass particles to build up a certain pressure and forms a dense structure without a sig­nificant spring-back effect. The relaxation time has a significant effect on the final density of the pellet during low-pressure compaction. At high-pressure compaction, the relaxation time did not show a significant effect on pellet density.

Fermentation medium can represent almost 30 % of the cost for a microbial fer­mentation. Complex media commonly employed for growth and production are not economically attractive due to their high amount of expensive nutrients such as yeast extract, peptone, and salts. In order to reach high production titers at reasonable costs, fermentation medium should be carefully designed to make the end product compatible with its synthetic petrochemical counterpart.

Fermentation feedstock has been the most expensive constituent in microbial biopolymer production. Till the1990s, studies were generally focused on using de­fined culture conditions in order to recover ultra pure biopolymers with minimum batch-to-batch variation and free of impurities that would interfere with their chem­ical and biological characterization. However, to maximize the cost effectiveness of the process, recent work has shifted to use multi-component feedstock systems and the synthetic media were replaced by cheaper alternatives such as olive mill wastew­ater (OMW), syrups, and molasses [7, 16, 35]. Currently, a wide range of industrial and agricultural by-products and waste materials are used as nutrients for industrial fermentations. In Table 2.2, various biomass resources and the applied pretreatment methods have been listed for some microbial EPS producers together with the EPS yields obtained after a certain fermentation period.

As analyzed in Sect. 4.4.1, cells of materials pretreated with steam explosion are separated to some extent. However, a microscope observation demonstrates that cells from different tissues separate differently. That is, fiber cells and parenchyma cells separate into a single cell which is light. Several dermal cells connected with each other because there is cutin out of them. Connected dermal cells are heavier than a single cell. According to the different weight of cell, air flow super grinding technology is applied to materials steam exploded to realize fractionation in the cell level.

A fluidized bed opposed jet mill [10], FJM-200 super grinding equipment (Beijing, Jinxin Technology Ltd., China) is applied. Main parameters are as follows: working pressure 0.6-1.0MPa, air consumption rate 1.10-1.73 m3/min, particle size smaller than 60 mesh, throughput 2-10 kg/h, motor power 13-15 kW.

In the crushing process, air becomes high-pressure gas through high-pressure nozzle. Steam-exploded rice particles are accelerated to high velocity and collided with each other to become powder. Powder fraction is collected from the grinding cavity, and residue fraction is collected from the discharge opening. Heat produced in the process of grinding is taken away by air. Therefore, heating effect is avoided.

For rice straw pretreated with steam explosion severity logioR0 [min] = 3.1, the powder yield could reach 78 % when operating parameter is as follows: feed load 15 kg/h, rotational speed of classifier wheel 4,544 rpm, moisture content lower than 5 %, grinding time 25 min. With the same conditions, powder yield is just 28 % for raw rice straw.

Fiber cell is easy to be crushed by high-pressure air. To avoid deconstruction of fiber cell, steam explosion integrated wet grinding technology is researched. By this way, fiber cell yield could be enhanced.

The degree of fibrous tissue separation is defined as the ratio of fiber cell content to non-fiber cell content in powder divided by the ratio of fiber cell content to non-fiber cell content in rice straw pre-fractionated.

C /г’

Degreeof fraction = fibercell/ gonfibercdl

C fiber cell |Cnon fiber cell

where C is the cell content of post-fractionation, C0 is the cell content of pre­fractionation.

The conditions to fractionate rice straw is as follows [35]: steam explosion severity log10R0 [min] = 3.5, raw material size 3-6 cm, feed load (wet weight) 350-450 g, moisture content of raw materials 40-50 %, grinding time 30 min. Degree of fraction reaches 2.04 (degree of fraction for raw material is 1.00). Powder fraction yield is

70.4 %. Fiber cell content in powder fraction is 63.1 %, and parenchyma cell is

33.5 %. Fiber cell content in powder fraction is higher than raw rice straw by 37.8 %. Cellulose content in powder is 65.6 %, which is higher than that of raw rice straw by

74.9 %. For residues fraction, there is mainly dermal cell and silicon cell. For silicon cell, it could be applied for nano-SiO2 preparation. It reveals that steam explosion integrated wet super grinding technology is an effective way to fractionate rice straw in cell level (Fig. 4.2).

Michael Zviely

Abstract Concentrated hydrochloric acid-driven hydrolysis provides the most pow­erful and industrially proven technology for converting all cellulosic wastes-wood, solids from city sewage plants, bagasse, grasses, etc.—to sugars that can be fer­mented to ethanol or other biofuels as well as a large variety of bio-products and food and feed.

Our process begins by steam expansion of debarked chipped wood, which un­dergoes a pre-extraction stage to remove all extractives, for example, tall oils and ash. The pre-extracted wood continues into hydrolysis stage performed using highly concentrated HCl at low-temperature (10-15 °C), thus affording sugars hydrolyzate (98 % of the theoretically available sugars, composing 65 % of the dry weight of the wood chips for pine wood, are converted into sugars) with minimum degradation products, while simultaneously separating the solid lignin.

A key limitation to any concentrated acid hydrolysis is the difficulty in recovering the acid. In particular, Virdia 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.

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.

Keywords Biomass ■ Lignocellulose ■ Sugars ■ Hydrochloric acid ■ Saccharification ■ Hydrolysis ■ Extraction ■ Lignin ■ Tall oils

7.1 Introduction

Renewable energy can come from technologies using wind, sun, hydroelectric, and geothermal, as their source, and from technologies using liquid biofuels as source. Liquid fuels mostly originate from biological, thermo chemical, bio thermal, and chemical processes.

M. Zviely (H)

Research & Development Department, Virdia (Formerly HCL CleanTech), 46733 Herzlyia, Israel e-mail: michael. zviely@virdia. com

Z. Fang (ed.), Pretreatment Techniques for Biofuels and Biorefineries, 133

Green Energy and Technology,

DOI 10.1007/978-3-642-32735-3_7, © Springer-Verlag Berlin Heidelberg 2013

Biological processes mostly use enzymes to hydrolyze the polysaccharides con­tained in the biomass, and chemical processes use mostly inorganic acids for the polysaccharides hydrolysis.

Of the inorganic acids used as catalysts for the hydrolysis, hydrochloric acid (HCl) plays a major role due to its unique characteristics. Concentrated (fuming) HCl provides the most powerful and industrially proven technology for converting all cellulosic wastes into sugars. This highly concentrated HCl forms hydrates, and it is assumed these species are responsible to the efficient hydrolysis of cellulose.

In addition to the liquid fuels obtained by these technologies, many additional products, for example, lactic acid, amino acids, or terphthalic acid, can be produced from the intermediate materials, mostly from the sugars, both in biological and chemically catalyzed processes.

Cellulosic residues, considered as a chemical raw material, are a source of hexose and pentose sugars. The major portion, the hexoses, is equivalent to sugar from common sources. The pentoses, on the other hand, are unique, as they may be processed to furfural, a chemical which has not been produced from other sources. The pentosans in corncobs and bagasse are now the main source of furfural. The pentosan content of wood, however, is too low for an economic venture supported by this single product. The brief considerations above lead to the conclusion that while there are indirect benefits to be gained by the development of a technically feasible saccharification process, the important measure is economics. To be of value, the process must be capable of producing sugar at a price competitive with corn or cane sugar or molasses in the locality in which it operates. Wood saccharification is but one aspect of a larger problem—the chemical utilization of cellulosic residues. Sugar is but one of many products obtainable from cellulosic residues, and wood is but one of a variety of potential starting materials [1].

Most of the brown-rot fungi degrade cellulose and hemicellulose more rapidly than lignin in woods. But the lignin is modifled up to certain level and left as modifled brown lignin residue, hence collectively called as brown-rot fungi. Many brown — rot fungi such as Serpula lacrymans, Coniophora puteana, Meruliporia incrassata, Laetoporeus sulphureus, and G. trabeum are used in various investigations [91, 92]. Most of the brown-rot fungi prefer soft-wood to hard-wood as substrate, for example S. lacrymans (dry-rot fungus) and C. puteana are the most harmful fungi occurring in wood in temperate region.

Brown-rot fungi have a unique mechanism to break down the wood. In contrast to white-rot fungi that de-polymerize the cell wall carbohydrates only to the extent that they utilize degraded product in fungus metabolism, brown-rot fungi accumulates the de-polymerized cell wall cellulose and hemicellulose since the fungus does not utilize all the products in the metabolism [61]. Early in the decay process, these brown-rot fungal hyphae penetrate from one cell to another through existing pores in wood cell walls. The hyphae start penetration from the cell lumen, where they are in close connection with the S3 layer. The brown-rot affects the S2 layer of the wood cell wall first [49].

Although brown-rot fungi consume economically important materials in biomass, the potential biotechnology application of brown-rot fungus is used to produce cattle feed from pine dust through solid-state fermentation. The brown-rotted lignin is used as an adhesive as it reacts more rapidly than native lignin due to increased phenolic — hydroxyl groups, for example, to replace phenol-formaldehyde flake board resin [49]. G. trabeum is the most extensively used fungus for treatment of wood chips. For example, Monrroy et al. [46] pretreated bioorganosolv process of Pinus radiata wood chips by using bioorganosolv process. They used G. trabeum for 3 weeks followed by organosolv treatment with various ratios of ethanol-water mixture at pH 2 and optimized H factor (factor that combine time and temperature in one variable). They found significant improvement in solvent accessibility and H factor was found to be decreased from 6,000 to 1,156 for obtaining 161 g ethanol/kg of P radiata wood. Another example, Ray et al. [93] pretreated Scots pine (Pinus sylvestris) sapwood by C. puteana for 35 days and they found that glucose release from the wood increased by four to five folds after 10 days exposure with minimum loss of weight (5 %) and maximum sugar release occurred 15 days after exposure to C. puteana with 9% weight loss.

To some extent, brown-rot fungi have similar pathways to degrade the lignocel- lulose as white-rot fungi. The wood decay mechanisms of both types of fungi rely on radical formation, low pH, and the production of organic acids such as oxalic acid. The radical formation would maximize the solubility of lignin in alkali and the decay process is an oxidation reaction, hence decay can be enhanced by high oxygen supply. However, many proposed mechanisms are not fully proven experimentally [49].

The application of steam techniques for pre-treatment of lignocellulosic biomass can be broadly differentiated into two schemes (a) the uncatalysed steam pre-treatment and (b) catalysed steam explosion methods. With this chapter dealing specifically with the use of steam as a pre-treatment tool, liquid hot water pre-treatment sys­tems which are operated at high pressures using elevated temperatures (i. e. as demonstrated in [4-6]) will not be covered here.

The trade-off of producing fine grinds is to consume increased energy to produce finer particles. An optimum particle size should be defined that fulfills the need of pelletization process and downstream process (e. g., combustion, ethanol production, and gasification).

In general, manufacturing 1 t of dried pellets may use 100-180 MJ for grinding

[5] . It is the third highest energy consumption unit operations of the overall pellet production after drying and pelletization. It is reported that for a pellet mill that uses sawdust as feedstock and has a production rate of 51 (w. b) pellet per hour, a 110-kW hammer mill is needed [33]. The energy expense is around 18.7 kW h or 0.38 % of the energy content of the pellets, based on an assumed energy content of the pellets 4900 kW h/t (w. b) [33].