The main reaction that occurs during acid pretreatment is the hydrolysis of hemicel — lulose. Hemicellulose mainly xylan is hydrolyzed to fermentable sugars during DAP [59]. Solubilized hemicelluloses (oligomers) can be subjected to hydrolytic reactions producing monomers, furfural, HMF, and other (volatile) products in acidic environ­ments [60, 61]. Recently, Sannigrahi et al. [62] have demonstrated that pseudo-lignin can be generated solely from carbohydrates without significant contribution from lignin during DAP especially under high severity pretreatment conditions. Further analysis indicates that pseudo-lignin is in spherical droplet form and has carbonyl, aromatic, methoxy, and aliphatic structures.

During DAP, it is generally accepted that the majority of the hemicellulose are removed initially, followed by the hydrolyzation of cellulose and subsequently some solubilization of Glu through the course of DAP [63-65]. Foston et al. [65] stated that cellulose degradation pathway can be viewed as acid catalyzed, thermally accelerated polysaccharide hydrolysis by chain scission within the fibril structure from either a crystalline or amorphous region of cellulose. This process consists of two major stages: The initial stage was regarded as rapid hydrolytic attack on the amorphous chain segments while the latter stage takes place on the crystal surfaces [66, 67]. Sannigrahi et al. [68] observed an increase in the relative proportion of cellulose Ip accompanied by a decrease in the relative proportion of both cellulose Ia and para- crystalline region from dilute acid pretreated Loblolly pine. This suggested that the types of lignocellulosic materials and pretreatment conditions influence cellulose crystalline allomorphs and para-crystalline contents during DAP.

DAP does not lead to significant delignification. Recent studies revealed an in­crease in the degree of condensation of lignin, during the DAP. The increase in degree of condensation is accompanied by a decrease in в-О-4 linkages which are frag­mented and subsequently recondensed during the high-temperature acid-catalyzed reactions [68, 69]. In addition, studies also indicated that lignin balls (or lignin droplets) were formed during DAP. These lignin droplets originated from lignins and possible lignin carbohydrates complexes [70, 71].

White-rot fungi produce a wide range of organohalogen metabolites. The most com­monly produced halogens are chlorinated anisyl metabolites (CAM) and chlorinated hydroquinone metabolites (CHM). CAM has an important physiological function in lignin degradation, contributing as substrates forAAO involved in extracellular H2O2 production. Among CHM metabolites, chlorinated 1,4-dimethoxybenzene such as

2- chloro-1,4-dimethoxybenzene, 2,6-dichloro-1,4-dimethoxybenzene, tetrachloro-

1.4- dimethoxybenzene, and tetrachloro-4-methoxyphenol are identified. 2-Chloro-

1.4- dimethoxybenzene (2-Cl-1,4-DMB) is another substrate for LiP, indicating a possible active function in the wood decomposition process. Like VA, it can also act as a redox mediator [98, 127].

Cells in stalk could be classed into two categories according to the thickness of cell wall. Category one is the cell that only has a primary cell wall, such as parenchyma tissue cell, sieve cell, companion cell. Category two is the cell that has both primary cell wall and secondary cell walls, such as sclerenchyma cell (including fiber and hardened cell), vessel cell, tracheid cell, and collenchyma cell. Though there is cellulose both in primary cell wall and secondary cell wall. Lignin content in the secondary cell wall is high. Lignin is regarded as recalcitrance of stalk hydrolysis [21]. For bioconversion of stalk, the property for two kinds of cells would be different.

Besides, there are special cells with a different cell wall structure [19]. For a silicon cell, the cell wall is often silicified, and for dermal cell, the wall is usually hornificated. There are also secretory tissue cells including secretory cell, glandular hair, nectar, secretory sac. Secretory cell belongs to parenchyma cell, so their cell walls are rich in cellulose. Though there are few special cells in vascular plant, they affect bioconversion process. For example, hornification dermal cell is regarded as recalcitrance for enzyme hydrolysis [21].

Therefore, stalk is heterogeneous in the level of cell because of different com­ponents for a cell wall. So it is necessary to research their different conversion properties.

When corn stalk is pretreated with steam explosion integrated Bauer screening, two fractions could be obtained. The fraction bigger than 28 mesh contains more than 89 % fiber cell and the fraction smaller than 200 mesh contains 64 % parenchyma cell. Therefore, these two fractions are chosen to analyze the hydrolysis property of fiber cell and parenchyma cell. The fraction bigger than 28 mesh is crushed first to smaller than 200 mesh to remove the effect of a particle size. When two fractions are hydrolyzed for 48 h, glucose concentration is 5.15 g/L for parenchyma cell, which is two times higher than that of the fiber cell, and the hydrolysis rate of parenchyma cell reaches 70 %.

If corn stalk is fractionated with steam explosion integrated super grinding, pow­der fraction and residues fraction could be obtained. Parenchyma cell in powder fraction is 26.6 % higher than that of residues (area percentage), and fiber cell con­tent in residue fraction is 26.4 % higher than that of powder fraction [10]. Dermal cell content is also different in two fractions. Therefore, two fractions obtained from corn stalk pretreated by steam explosion integrated super grinding could be used to analyze the hydrolysis property of different cells. After hydrolysis for 24 h, reducing sugar content is 61.4 % for powder fraction, which is 3.8 times higher than that of residues [10].

The cell content before and after 48 h hydrolysis for powder fraction is analyzed. It reveals that fiber cell reduces by 22.8 % after hydrolysis. Parenchyma cell percentage reduces from 54.2 % to 7.3 %, while dermal cell percentage increases from 10.4 % to 80.9 %. These changes demonstrate that different cells have different hydrolysis properties. The enzyme hydrolysis property could be arranged as parenchyma cell > fiber cell > dermal cell [10].

In the process of pretreatment with steam explosion integrated super grinding, moisture content could affect fractionation results. If moisture content of steam ex­ploded materials is 40 %, the fiber cell content in residue fraction would be more than 60 %. Therefore, residue fraction could be applied to analyze fiber cell conversion properties.

Ethanol self-catalyzing method is applied for pulping. Pulping process is carried out at 180 °C for 2 h with 50 % ethanol concentration and solid-to-liquid ratio 0.8/10 (g/mL). It reveals that crud pulp yield of residue fraction could reach 61.4 %. How­ever, pulp yields of steam exploded rice straw and rice straw are 35.5 % and 32.1 %, respectively. So it demonstrates that there is positive correlation between fiber cell content and pulping property. Therefore, it would be an effective way to fractionate different cells and convert them, respectively.

MW-based pretreatment approach utilizes both thermal and non-thermal effects gen­erated by an extensive intermolecular collision as a result of realignment of polar molecules with MW oscillations. Compared to conventional heating, electromag­netic field generated by MW has the ability to directly interact with the material to produce heat, thereby accelerating chemical, physical, and biological processes. The advantages of employing MW rather than the conventional heating include reduction of process energy requirements, selective processing and capability for instantaneous starting and ceasing of the process. This also offers enormous benefits such as en­ergy efficiency due to rapid and selective heating and the possibility for developing a compact process.

When MW is applied to pretreatment of lignocellulosic biomass, the unique fea­ture of selectively heating the more polar part will result in an improved disruption of the recalcitrant (treatment-resistant) structures of lignocellulose. With the nonthermal effects, electromagnetic field enhances the destruction of crystalline structures and changes the super molecular structure of lignocellulosic material thereby improving its reactivity.

MW pretreatment is also an energy-efficient and environmentally benign tech­nology that aids in the transport of chemicals into the substrates. The project team from the US Department of Energy in partnerships with research institutes including the Oak Ridge National Laboratory [10] has showed that by opening the cellular microstructures of wood, for example, MW pretreatment could permit pumping chemicals for easy access of even large sections (10 cm longx 10 cm diameter) of hardwoods. The project team has demonstrated that, for both hardwood and soft­wood chips, MW pretreatment can decrease both H-factor and chemicals required to pulp hardwoods and softwoods by greater than 40 % with acceptable quality. The steam pressure generated inside the wood breaks the pit membranes and vessel cell walls, thereby enhancing the woods permeability to chemicals and process liquors.

Other than the lignocellulosic biomass, the use of MW for pretreatment of samples for a more efficient oil extraction and pretreatment of FFAs for biodiesel conversion has also been proposed [11, 12].

Whey is the major by-product obtained during the preparation of dairy products such as cheese. The nutrient composition of whey is based on the nutrient composition of milk from which it is derived, which in turn is affected by many factors including how the milk was processed. Lactose is the major component comprising about 70 % of the total solids of whey. Whey also contains a pool of nutrients and growth factors that have the potential to stimulate the growth of microorganisms but the suitability of whey for EPS production highly depends on the ability of the microorganism to utilize lactose. Cheese whey has been used as carbon and nitrogen source for xanthan [59] and gellan [40] production. Mozzarella cheese whey has been used for xanthan production by two different X. campestris strains and although both strains reached comparable yields, the polymers were found to differ in their chemical characteristics [59]. The low yields were attributed to the low capacity of the X. campestris strains to utilize lactose. On the other hand, Fialho et al. [40] evaluated the gellan gum production by the S. paucimobilis ATCC 31461 strain in media containing lactose, glucose, and sweet cheese whey as substrates. The strain was known to grow on lactose and to produce highly viscous gellan directly from lactose [76]. Sweet cheese whey obtained from the industry was neutralized and disinfected by three cycles of heat treatment at 80 °C for 30 min. A maximum gellan yield of 7.9 g/L could be recovered from the flask cultures after 100 h of fermentation period [40]. Cheese whey has also been investigated as a potential substrate for dextran production by L. mesenteroides NRRL B512 cultures [37]. For this, proteins were removed from whey by precipitation through autoclaving and then centrifugation. Though lactose in the supernatant was found to repress the dextransucrase activity, 7.23 g/L dextran could be produced when carob extract was also present in the medium [37].

Pak Sui Lam, Zahra Tooyserkani, Ladan Jafari Naimi and Shahab Sokhansanj

Abstract Pretreatment is a first crucial step to modify the structure of wood via physical, chemical, and biological treatment for cost effective and sustainable fu­els and chemicals production. Different pretreatments would be selected to upgrade the characteristics of wood with respect to different applications and process effi­ciencies. High-temperature pretreatment (e. g., torrefaction) at the temperature range greater than 250 °C led to higher degradation rate of sugars and extractives, which is not preferable for fuel and chemicals production from ligno-cellulosic biomass. Instead, high-temperature pretreat-ment was used to upgrade the solid fuel for thermo-chemical conversion (e. g., combustion and gasification). It can remove the moisture and volatiles with a low-heating value of the native biomass, which favors for the ease of fuel combustion compared to the raw wood. In addition, it can in­crease the hydrophobicity of the biomass which improves their handling and storage performance. In this chapter, the production chain of the wood pellet production with incorporating recent novel pretreatment technologies (torrefaction, steam ex­plosion, and hydrothermal carbonization) were discussed. The resulted pellets are a uniform feedstock for producing chemicals, heat, and energy via biochem-ical and thermochemical conversion, respectively.

Keywords Pretreatment ■ Wood pellet ■ Drying ■ Grinding ■ Biomass preprocessing ■ Torrefaction ■ Steam explosion ■ Hydrothermal carbonization ■ Pellet quality

P. S. Lam (H) ■ Z. Tooyserkani ■ L. J. Naimi ■ S. Sokhansanj Biomass and Bioenergy Research Group, Clean Energy Research Center, Department of Chemical and Biological Engineering,

University of British Columbia,

2360 East Mall, Vancouver, B. C., V6T 1Z3, Canada e-mail: wilsonlam82@yahoo. com

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

Green Energy and Technology,

DOI 10.1007/978-3-642-32735-3_5, © Springer-Verlag Berlin Heidelberg 2013

5.1 Introduction

Renewable energy has been targeted as a strategic important area for many countries for both environmental and economic reasons [1]. The establishment of a clean energy supply can provide greater energy independence and security, has notable environmental benefits due to reduced CO2 emissions, as well as promoting positive economic growth for the local area. Among various types of renewable energy, bioenergy is attractive as biomass is considered to be carbon neutral that absorbs CO2 from the atmosphere during production [2]. Besides, bioenergy systems can create the highest job creation effect, particularly in the rural areas with high unemployment rate, and resulting in the stimulation of economic growth [2, 3]. Since biomass is dispatchable, it is economically preferable to deploy when required. The biomass feedstock supply logistic cost contributes around 30-50 % of the total bioenergy production cost [4]. An optimized pre-processing of the biomass into densified pellets is essential to achieve a cost-effective production process for bioenergy.

Wood pellets are a type of solid fuel made from sawdust with uniform shape and dimensions. Pellets are made by densifying the ground particles of woody biomass. Wood residues usually come as sawdust from saw mills. Their bulk densities are around 40-60 kg/m3 (wet basis) depending on species and moisture content (MC)

[4] . Drying is required to ensure that the size-reduced feedstock is good for pel­letization (densification) to produce durable wood pellets. The bulk densities of the biomass pellets are around 550-700 kg/m3 depending on the size of the pellets [4]. The volume reduction reduces the space required for storage and transportation. For storage, biomass densification helps to reduce the space required to store the materi­als. Biomass pellets improve the heating efficiency and have lower emissions during combustion than using the low bulk density and fluffy biomass. The typical example is the co-firing plant using wood pellets and coal as feedstock where the difference between these two materials’ densities cause difficulties in feeding due to the uneven, fluffy, and low bulk density of the biomass feedstock [4].

The typical production process of biofuel pellets is collecting the residues from saw mill and following by drying using a rotary drum dryer, further size reduction to the granular form by hammer mill, and finally pelletizing into fuel pellets using a pellet mill (Fig. 5.1). The fuel pellets are then cooled, screened, and transported to an export port by trains. They are usually transported on conveyor belts and dropped from the height of 10-15 m above the storage silos for temporary storage. They are stored under a well-monitored environment to prevent self-heating and off-gas accumulation. Pellets are then loaded into the ocean vessel. The details of each unit operation will be discussed in the following sections. The pellet quality needs to be maintained during transportation in order to meet the import specification of the European standard [5].

Biomass preprocessing is aimed at enhancing the energy density of a bulk biomass. Further optimization of the process can be achieved by enhancing the production yield and reducing the energy required for the preprocessing process. Two major technical problems during the preprocessing process need to be addressed. Poor mechanical strength of biomass pellets contributes to disintegration of pellets into fines during

transportation. This usually happens for the pellets transporting on the conveyor and loading from the top of the silo to form piles and pellets break into fines due to impact. The fines cause blockage of the conveyor or hopper during processing and also lead to an occupational health problem to the workers inhaling the fines [6]. Moreover, the fines also lead to dust explosion which causes severe fire damage to the expensive handling facilities. This is related to the lack of natural binding between the fibers of the pellets, and most biomass species including straws and stover are difficult to densify without any expensive binders [7-10]. Only wood pellet can be formed with good durability due to their binderless characteristics.

Pellets easily adsorb moisture and disintegrate into fines under high-humidity conditions. The high surface area of the small fines favors the susceptibility of the attack by the micro-organisms during storage [11]. Anaerobic conditions lead to local heat generation and generation of toxic off-gassing that may include terpenes [12]. Local heat generated may ignite the volatiles in the pellets to cause fire, and the off-gas accumulations inside the storage silos are toxic to the workers. High MC reduces pellets combustion efficiency at the power plant.

In the following, we will focus on discussing the development and optimization of the entire biomass pellet production chain by different pretreatment techniques. This not only aims to produce fuel pellets for ease of chemical conversion and energy production, but also to reduce the preprocessing cost and improve the safe handling of pellets during transport and storage.

As already mentioned, the main cell-wall components are cellulose, hemicelluloses, and lignin. Another polysaccharide found in the biomass, in relatively low concen­tration is pectin. In addition to these, the biomass contains extractives, mainly tall oil and volatile terpenes, ash, and some nitrogen containing molecules.

The main monomeric sugars in the high percentage polysaccharides, that is, cel­lulose and hemicelluloses, are: glucose, xylose, mannose, galactose, and arabinose.

7.3 Acid Catalyzed Hydrolysis of Biomass

Various methods for the hydrolysis of lignocellulosic materials have recently been described [9]. The dilute acid process is conducted under high temperature and pres­sure. For example, using a dilute acid process with 1 % sulfuric acid (H2SO4) in a continuous flow reactor at a residence time of 0.22 min and a temperature of 237 °C with pure cellulose provided a yield over 50 % sugars. Dilute acids lead to a limited hydrolysis called prehydrolysis. Dilute-acid hydrolysis is carried out using mineral acids such as H2SO4 or HCl, at temperatures of 122-202 °C [10]. The chief ad­vantages to using HCl over H2SO4 are that HCl permeates the wood more easily than H2SO4 and is a volatile compound, which assists in the crucial acid recov­ery steps. The Udic-Rheinau process was an attempt to make the Bergius-Rheinau process economically advantageous. The latter process will be further discussed. In the improved process, the wood was first prehydrolyzed in 1 % HCI at 130 °C to remove the hemicelluloses. The wood was then dried and subsequently hydrolyzed with 40 % HCl at 12 °C for 10 h. After washing the lignin residue with dilute HCl, the HCl was recovered by vacuum distillation. This process was more economical than the Bergius-Rheinau process [11]. The biggest advantage of dilute acid processes is their fast rate of reaction, which facilitates continuous processing [12].

To summarize, acid catalyzed hydrolysis of biomass can be performed for example by using HCl or H2SO4. The H2SO4 catalyzed hydrolysis process was developed in the US to yield sugar monomers, however, it seems that the sugar yields are relatively low, due to the formation of relatively high percentage of degradation products. Additional drawbacks are relatively high acid left in the lignin, high energy consumption, and formed gypsum as by-product.

Tien et al. [63] discovered LiP in the extracellular medium of P. chrysosporium grown under nitrogen limitation. The enzyme uses H2O2 as co-factor or mediator for activity and is capable of oxidizing and/or cleaving lignin and lignin model compounds. This was supposed to be the key reaction of lignin degradation. Very few fungi are found to produce extracellular LiP [98]. P. chrysosporium, T. versicolor, Bjerkhandera sp., and T. cervina are some fungi, which can produce LiPs [32]. Indeed, LiP was found to play only a minor role in lignin degradation by T. versicolor, at least as measured by bio-bleaching of kraft pulp [99].

LiPs are monomeric homo-protein and glycol protein belonging to oxidoreduc — tase family, which specifically act on peroxide as an acceptor (peroxidases). These enzymes have molecular weight of 40 kDa and isoelectric points (pI) ranging from

2.8 to 5.3. The absorption spectrum of the native enzyme in P chrysosporium has a very distinct maximum at 406-409 nm due to the presence of a single heme group, where Fe3+ pentacoordinates with four heme tetrapyrrole nitrogen and a histidine of LiPs (protoporphyrin IX) [32, 98]. The interaction of LiPs with its substrate follows ping-pong mechanism [100]. As shown in Fig. 1.2, LiPs are oxidized by H2O2 to two-electron oxidized intermediates (LiP I) along with iron ions as Fe4+ and freerad — ical residues on tetrapyrolle. LiP I then oxidises the donor substrate by one electron, where the donor substrate, VA (3,4-dimethoxybenzyl alcohol, VA) yields second intermediate LiPs complex (LiPs II) in which iron ion is found in same oxidation state, that is, Fe+4, but there is no free radical residue on tetrapyrolle of heme and a radical cation. LiP II then oxidises a second molecules of donor substrate (VA), confers another radical cation and native form of LiP. Here the reformation of native LiP mainly depends upon the LiP II reduction step, which is a rate limiting step in catalytic cycle. Because the reduction of LiP II is a relatively slow process and LiP II is less potent than LiP I complex. Consequently, LiP II complex is long available for reaction again with H2O2 leads to inactivation of enzyme and forms LiP III com­plex (Fig. 1.2), which is characterized as a complex between LiP and superoxide. The catalytic cycle of LiP is described in Fig. 1.2. VA radical cations act as redox mediators and are capable to reduce LiP III complex back to its native form, LiP. In this LiP catalytic cycle reaction, VA radical cations (VA’+) are usually restored back after its oxidation reaction with non-phenolic compounds of lignin.

As in this catalytic cycle reaction, VA plays an important role. Three major functions of VA have been investigated so far. Firstly, VA acts as a mediator in electron-transfer reaction. Secondly, VA is a good substrate for compound II, there­fore VA is essential for completing the catalytic cycle of LiP during the oxidation of terminal substrates. Furthermore, if the inactive LiP III complex forms, the interme­diate VA^+ will be capable of reducing LiP III complex back to its native form LiP

(Fig. 1.2). Thirdly, VA prevents the H2O2-dependent inactivation of LiPs by reducing LiP II complex back to its native form LiP. Almost all the white-rot fungi synthe­size VA via de novo glucose pathway during early stage of secondary metabolism in parallel with LiP production [98].

LiPs oxidize non-phenolic and phenolic units of lignin by removing one electron and creating free radicals, which lead to chemically decompose the polymer. LiP has been shown to oxidize fully methylated lignin, lignin model compounds as well as various polyaromatic hydrocarbons. LiPs cleave selectively Ca-C|5 bond, aryl Ca bond, aromatic ring opening and demethylation in the lignin molecule [32, 98].

The mode of action of the catalysed steam pre-treatment method is similar to the auto-hydrolytic process described in the preceding section with the major difference being the impregnation of the lignocellulosic biomass with acidic gases or liquids (i. e. sulphur dioxide (SO2), sulphuric acid (H2SO4), nitric acid (HNO3) and hydrochloric acid (HCl)) which act as process catalysts prior to applying the steam pre-treatment.

The implementation of acid catalysis has been demonstrated to exhibit a higher pre-treatment efficiency than the use of the steam pre-treatment method alone. Com­pared with the use of the steam pre-treatment alone, the catalysed route has been observed to lead to a greater hemicellulose (near complete) removal with a reduced generation of inhibitory compounds [32]. This in turn results in an increased con­version potential of the post-treated fractions via an improvement of the biomass digestibility using enzymatic methods. An improved sugar recovery, especially as pentoses in the post treated aqueous phases has been shown by [13].

As highlighted in Sect. 3.2.1, due to the characteristic low reactivity and perme­ability of the fibres of softwoods to the penetrating steam used in the auto-hydrolytic process [13], acidic conditions could thus be employed with the use of this particular type of lignocellulosic biomass to enhance the conversion efficiencies obtainable with the use of the steam pre-treatment method. In addition, the use of the acid catalysts on such biomass materials have been seen to facilitate the use of lower steam temperatures and shorter exposure times [33]. As demonstrated in [31], more than 85 % of the original hemicellulose derived sugars was recovered in the aqueous phase after an acid catalysed steam explosion of Douglas fir wood chips at relatively lower steam severity conditions (175 °C, 7.5 min.) with SO2 (4.5 %, w/w) used as the process catalyst. In that study, it was also observed that all of the hexose components obtained from the biomass using low — and medium severities were readily converted to ethanol [31].

Steam explosion is one of the hydrothermal treatments that subject the biomass to high-pressure saturated steam at the reaction temperature between 180 and 240 °C for several minutes followed by a rapid decompression. The residence time of steam treatment depends on the reactor type and the desired degree of treatment. The degree of steam treatment can be described by a severity equation developed by previous researchers [44]. This equation was developed based on modeling complex reaction systems by assuming that each reaction is homogenous, and Arrhenius dependence rate law and the temperature function were linearized by Taylor series [45,46]. This equation is developed based on the data from batch reactor.

where R0 = is the reaction severity, T is the reaction temperature (°C), t is the reaction time (min).

Another equation is also developed for scaling up from a batch process to a continuous process [47],

log R0,Batch = 1.50 X (log R0,Continuous!-)

where R,0,Batch is the reaction severity studied based on batch reactor, R,0,Contmuousis the reaction severity applied to the continuous large scale reactor.

chemistry

Under high pressure of saturated steam, the initial reaction that takes place inside the chemical components in presence of woody biomass is hydrolysis. Biomass is a mixture of polymer composite mainly made up of carbohydrates (cellulose, hemicelluloses) and lignin. In particular, hemicelluloses and lignin hydrolyze in the presence of acetic acid to release low molecular weight components: mono­sugars and acid-soluble lignin. Some mono-sugars will further degrade into other chemicals at high temperature by dehydration reaction. One typical example is the formation of furfural by dehydration reaction of xylose (mono-sugar of xylan, a type of hemicelluloses). For lignin, repolymerization or condensation reaction of low molecular weight lignin takes place as reaction coordinate proceed. This changes the structure and morphology of lignin, which is important for improving the binding ability of the wood fibers during pellet production. Detailed of chemistry has been reported previously [48].