Many cellulose solvents were employed to treat biomass and accelerate its enzymatic accessibility and digestibility by disrupting linkages among cellulose, hemicellulose, and lignin component as well as breaking up the hydrogen-bond linkages in the or­derly crystalline cellulose to amorphous forms [84,85,107-111]. But this method is still challenging in dealing with real lignocelluloses due to the low efficiency in removal of lignin and hemicellulose from cellulose, resulting in lower cellulose digestibility and slower hydrolysis [42]. It was revealed that cellulose, hemicellu- lose, and lignin have different solubilities in OES composed of aqueous acetone or ethanol with concentrated phosphoric acid or ILs [89,97,112-114]. Utilization of this lignin-soluble OES to deposit cellulose and/or hemicellulose from the lignocellulosic biomass solutions can be a practical approach to resolve the problem.

Zhang et al. [97] developed a lignocellulose pretreatment technique to fractionate lignocelluloses to amorphous cellulose, hemicellulose, lignin, and acetic acid. It is featured by using 8 ml cellulose solvent (pre-calibrated concentrated phosphoric acid 83-85.9 %) to dissolve 1 g biomass under a modest reaction conditions (50 °C and atmospheric pressure). After the reaction, OES was immediately generated by the addition of 20 ml organic solvent (i. e., acetone) into the biomass-phosphoric acid solutions and then well mixed. Coupled with the formation of OES, the cellulose and hemicellulose were rapidly precipitated. Followed by subsequently centrifug­ing, the OES supernatants were collected for separation of lignin, acetic acid, and acetone. With the removal of acetone from the OES by simple evaporation as well as acetic acid, the low molecular lignin was precipitated from aqueous phosphoric acid. In this way, partial lignin was removed from the biomass combined with the decrystallization of cellulose. Consequently, the regenerated amorphous cellulose without lignin was hydrolyzed efficiently. The regenerated corn stover, switchgrass, and poplar, were hydrolyzed, respectively, where a high-hydrolysis efficiency ~94 % and ~96- ~97 % was achieved at the 12th and 24th hour with an enzyme loadings of 15 FPU cellulase and 60 IU beta-glucosidase per gram of glucan, respectively. The data of hydrolysis rates and digestibility were the highest in the literature [2, 8, 115]. Alternatively, although this method seems not to be efficient for treating Douglas Fir (~73 % digestibility at 12 h and ~75 % at 24 h), it was still ~1.7 times of the sample pretreated by SO2 steam explosion [97].

Similar study of OES-precipitating pretreatment for bamboos was conducted us­ing 85 % (w/v) concentrated phosphoric acid as a cellulose solvent and 95 % (v/v) ethanol as an organic solvent [113]. After dissolving by concentrated phosphoric acid and followed by precipitation with simultaneous formation of OES, the glucan recovery yield of the sample was 93.9 % and the delignification yield was 15.3 %. The elevation of hydrolysis yield was due to the increase in cellulose accessibility to cellulase from 0.27 to 9.14 m2 per gram of biomass. Glucan digestibility attained

88.2 % at the cellulase loading of 1 glucan in 72h. The overall glucose and xylose yields were 86.0 % and 82.6 %, respectively.

The advantages of OES-precipitating pretreatment by employing a non-volatile cellulose solvent and a highly volatile organic solvent are:

1. Decrystallized cellulose and hemicellulose can be separated efficiently from dis­solved biomass by precipitation because they have poor solubility in the OES mixture that is able to partially dissolve lignin.

2. Acetone-soluble lignin can be easily recovered after adding water or evaporation of organic solvent from the OES because it is insoluble either in water or specific cellulose solutions, such as phosphoric acid [97].

3. Organic solvents such as acetone and ethanol can be easily recycled and reused by fractional distillation due to their low boiling point against non-volatile ability of the specific cellulose solutions, such as ILs or concentrated phosphoric acids.

16.3.3.1 Acid Pretreatment

Among different types of pretreatments, the acid hydrolysis is one of the most com­monly used methods for SB/SL. This method is usually employed to solubilize the hemicelluloses fraction of the cell wall which eventually aids the accessibility of cellulolytic enzymes action [12, 29,44]. In the acid pretreatment, the hemicellulose can be hydrolyzed keeping the biomass in contact with diluted acid or concentrated acid under high temperature and low temperature respectively [10, 45]. Figure 16.3 shows the mechanistic demonstration of acidic pretreatment applied to SB/SL.

The hemicellulose fraction of SB/SL is depolymerized primarily into pentose sugars (xylose and arabinose) and hexose sugars (glucose, galactose, mannose, etc) along with inhibitory compounds [12, 46,47]. Recently, Moutta et al. [46] reported

56.5 g/L total reducing sugars from the hemicellulosic fraction of SL under the opti­mized set of conditions (130 °C, 2.9 % w/vH2SO4, 1:4 solid:liquid ratio, and 30 min of residence time).

Multiple methods showed their feasibility to eliminate the inhibitors from the hydrolysates prior to fermentation [47, 48]. The hemicellulosic hydrolysate after detoxification can be converted into value-added products such as xylitol, ethanol, lactic acid, etc [2, 49]. The advantage of the dilute acid hydrolysis process is the low operation and energy costs [14, 15,49]. Concentrated acid hydrolysis of SB/SL leads to the issues such as equipment corrosion and expensive costs of maintenance [29,45]. The diluted acid pretreated SB/SL is subsequently hydrolysed with cellulase to depolymerize the cellulosics fraction into glucose [44, 37].

19.2.1 Materials

The Pinus bank siana (Jack pine) sawdust was supplied by a local lumber mill (Northern Wood Ltd). The sawdust was grounded with a Wiley mill and screened using a 20-mesh to a particle size of 0.75 mm for the experiments. The particles were dried in an oven at 105 °C for 12 h before use. The results of proximate and ultimate analysis of the pine sample and the chemical compositions of the ash from pine wood samples are summarized in Table 19.1. Wherein, the proximate analysis (ash, volatile matters, and fixed carbon contents) was performed in accordance with the standard of ASTM D 5142 by thermogravimetric analysis (TGA), and the results are reported in Table 19.1 on a dry basis. The ultimate analyses or elemental compo­sitions were conducted according to ASTM D 5373 on a CHNS elemental analyzer, while the oxygen content was calculated by difference on a dry basis (including the ash content). The cellulose, hemicelluloses, and lignin content were determined by the Analytical Laboratory of FPInnovations, Montreal, Canada. The samples were extracted with acetone to obtain extractive-free test specimens. Carbohydrates were determined according to the Technical Association of the Pulp and Paper Indus­try (TAPPI) test method T249 cm-85 and the acid-soluble and acid-insoluble lignin was determined according to the TAPPI test method T222cm-88. The results are summarized in Table 19.2.

A RSM was used to determine the optimal pretreatment condition for producing maximum total reducing sugars. The method has been described in other studies [8, 17, 18]. The design was based on a 23 full factorial central composite design (CCD) and was conducted using Design Expert 8.0 software (Stat-Ease, Inc. MN, USA). The experiment conditions with corresponding codes are listed in Table 9.1. The three variables were temperature, reaction time, and solid loading with six re­peated experiments in the central point (185 °C, 20 min, 30 %). Since the total sugar was a dependent variable, all the three variables were coded to real independent vari­ables. The independent variables were calculated as (condition of the run-condition at central point)/ step change of the variable. Therefore, the coded values were X 1(temp-185)/15, X2(time-20)/10, and X3(solid-30)/10.

The problem of tar entrainment in the gas stream had been solved by designing downdraft gasifiers where the gasifying agent enters from the top and the gaseous products leave the reactor through a bed of hot ash at the bottom. A schematic of this reactor is shown in Fig. 10.1b. The main advantage of downdraft gasifiers lies in the possibility of producing a tar-free gas from high volatile fuels which will be suitable for engine applications. This is due to the fact that most of the tar is cracked when passing through the hot ash before exiting the reactor. A major drawback of this type of gasifier is its inability to operate with a wide range of fuels. In particular, fluffy, low density materials can cause flow problems, like an excessive pressure drop, requiring the solid fuel to be pelletized or briquetted before use. Downdraft gasifiers also suffer from slagging when operating with fuels characterized by high ash content. Compared to updraft gasifiers, downdraft systems show lower efficiencies resulting from a lack of internal heat exchange as well as the LHV of the gas.

As previously discussed, the pyrolysis conditions affect the global kinetics by promot­ing specific elemental reactions. Pyrolysis processes have therefore been classified with respect to the prevailing conditions during the reaction used to maximize the yield of one or more of these products. Conventional or slow pyrolysis, fast or ul­trafast pyrolysis and vacuum pyrolysis are the three main categories of pyrolysis process operation.

Figure 13.2 shows a biogas utilization system (BGUS). As a measure to handle new energy supply and surplus biogas in agricultural regions, the system has an

Farm <Owner of the Biogas

Delivery

Fig. 13.2 Biogas utilization system (BGUS)

RCF component to produce refined gas. The BGUS also comprises equipment that consumes the refined gas. The system can be used to fill storage cylinders with surplus biogas and supply general households and businesses in the region with fuel.

Metal oxides are usually used to synthesize solid superacids as introduced in Sect. 15.3.1. Active sites supported on single metal oxides performed high cat­alytic activity in many organic reactions [54-56]. Sulfonated metal oxides, such as SO42-/Al2O3, SO42-/TiO2, SO42-/ZrO2, and SO42-/V2O5 [57, 58], can supply much acid species, which function the same as [H+] in sulfuric acid for cellulose hydrolysis. Such acid solid catalysts are widely used in biodiesel synthesis, but it is generally very difficult to retain a strong B acid site as sulfonic acid in the framework. Jitputti et al. [57] studied the transesterification of crude oil by different solid acids, such as SO42-/SnO2, SO42-/ZrO2, and resulted in the production of over 90 % fatty acid methyl esters (FAMEs; biodiesel). It was found that the spent SO42-/ZrO2 could not be directly reused for the transesterification. Such solid acid catalysts cannot be used for cellulose hydrolysis at the current structure and composition. Kulkarni and Muggli [59], reported that an apparent increase in B acidity was found upon treat­ing SO42-/TiO2 with H2O, which proved that H2O displaced isopropylamine from approximately one-third to one-half of the B acid sites.

In order to improve the catalytic efficiency and stability of sulfonated metal oxides, some modifications are usually proposed as below: (1) Introducing other metals or metal oxides. Metals that promote the catalytic activity include aluminum, iron, and manganese, and platinum can also increase the stability of solid catalysts [60, 61]. (2) Introducing lanthanum. Lanthanun was widely used to improve the stability of active sites [62]. (3) Synthesizing nanoparticles. Nano-catalysts have the advantage of supplying larger surface area, good stability, and high effect active, which will be introduced in detail in Sect. 15.4.

Various physical treatment of biomass prior to its conversion into liquid, solid or gaseous products are available. The main purpose of the selected treatment depends

on the requirement of the conversion process, that is, to have the biomass at appropri­ate size for easiness or strength and durability in handling and processing. Specifically biomass conversion can be enhanced via increased digestibility [18] from increased specific surface area and reduced degree of polymerization and cellulose crystallinity [19].

Generally, different biomass may have different density and resistance that may hinder in handling, transportation, and storage. These limitations, however, can be overcome via densification of the biomass to improve its properties such as bulk density, that is, from 40-200 to 600-800 kg/m3 [20], abrasion resistance, im­pact resistance, compressive strength, water resistance, and long-term performance [21, 22].

Process for biomass densification includes agglomeration that increases the parti­cle size. This can be performed using pressure [23] in such techniques like extrusion, pelletizing, roll briquetting [24-26], and compaction [20, 22]. Another technique is tumble agglomeration that uses binding agents [23] that chemically or physically adhere to solid surfaces and form a bridge between the biomass particles.

On the other hand, appropriate sizing of biomass can also improve its physical properties for enhanced combustion efficiency via higher burning rates [27]. Full automatic operation and complete combustion in furnaces can also be achieved when the biomass is homogenously densified [28].

Grinder and sieve shaker are used to provide homogeneous particle size for the biomass feedstock. This eases the feeding process of biomass into the gasifica­tion system. For more efficient feed preparation, biomass is dried and subsequently ground and sieved to a range of 250-500 ^m to enable smooth feeding and minimum fluidization velocity [2, 29].

Moreover, biomass ash may have high inorganic trace elements that can cause slagging, fouling, and corrosion in combustion equipment. However, the quantity of these trace elements such as potassium, sodium, and chlorine content can be reduced via leaching, that is, washing [30].

As for hydrolysis of biomass into fuels, different mechanical size reduction tech­niques can be applied mainly to reduce cellulose crystallinity. The techniques and its uses are listed in Table 17.1.

Torrefaction is a biomass thermal treatment process at low temperature (200-300 °C) in the absence of oxygen and mostly at near atmospheric pressure. It is a mild py­rolysis process that destroys the fibrous structure of biomass, increasing its calorific value as well as its hydrophobic nature to improve biomass stability during storage. Since torrefied biomass is more friable compared to its original state, energy con­sumption for biomass particle-size reduction is lower. It has also been shown that torrefied biomass particles can be fluidized more smoothly compared to untreated biomass [5]. There are two main torrefaction methods: (1) the wet process [6] and

(2) the dry process. In wet torrefaction, biomass is treated with hot compressed water resulting in three groups of products: solid fuel, aqueous compounds, and gases. Dry torrefaction is an intensive drying at higher temperatures [7]. One of the drawbacks of the wet process is the necessity to separate the excess water from the torrefied biomass and liquid by-products with relatively high organic and mineral contents. Dry torrefaction is typically performed at temperatures in the range of 230-300 °C in the absence of oxygen, at near atmospheric pressure and a relatively low particle heating rate (lower than 50 °C/min). The mass and energy yield from the original biomass to the torrefied biomass is strongly dependent on torrefaction temperature, reaction time, and biomass type.