Crystallinity

Crystallinity represents the relative amount of cellulosic crystalline structure in the cellulose content of feedstock. Higher crystalline causes the lower saccha­rification rate of cellulose [42-44]. During the organosolv process, the crys­tallinity structure of feedstocks could be reduced due to the solvent attacking on the
cellulose by thermal and infiltration effects. Unfortunately, the effect of organosolv pretreatment on the cellulosic crystallinity cannot be precisely measured in the case of an actual substrate. The cellulosic crystallinity might be masked by the removal of amorphous lignin and hemicellulose, as well as selective hydrolysis of amorphous part of cellulose which are superior to the cellulosic decrystallization [45].

Crystalline Allomorph

Additionally, the most abundant crystalline polymorphs found in higher plants are cellulose Ia and IP [34, 46, 47]. They can be converted to allomorphic forms such as cellulose II, IIII, and IVI by ionic liquids (ILs), alkali, or ammonia-based pre­treatment [34,48-50]. Without any relevant loss of crystallinity, these polymorphs have been shown to promote their hydrolysis rates [34, 51, 52]. Accomplished by the thermochemical effect, organosolv pretreatment can also convert cellulose I to other allomorphic forms to enhance the hydrolysis rates. For example, pretreating B. davidii by aqueous ethanol resulted in the replacement of cellulose Ia and ip by para-crystalline and amorphous cellulose forms [23]. Glycerol can convert ramie cel­lulose IIII to cellulose IV at 260 °C [ 53]. Relative to the other factors, the respond of cellulosic allomorphic form transformation to the effect of organosolv pretreatment is less understood and needs more investigation.

Zhu [16] showed that lignin content, acetyl content, and cellulose crystallinity were key factors that determine biomass digestibility. However, it is still not con­clusive which the most important factors are to accelerate the biomass digestibility in the organosolv pretreatment, due to the various natural structural features and processing operation conditions. For example, removal of lignin and hemicellulose, reduction in DP, and decrease in the crystalline allomorphs (Ia and IP) increased the amenability of the ethanol pretreated B. davidii to enzymatic degradation [23]. But the success of ethanol pretreatment on enzymatic hydrolysis of Loblolly pine was alternatively contributed to a decrease in cellulose crystallinity, as well as removal of lignin and hemicellulose [54]. Therefore, studying the changes in the structure of cellulose during organosolv pretreatment is still an area of research that remains to be thoroughly investigated under certain conditions [23].

Alumina is now the most widely used catalyst support in the industry. The rapid development of nano-alumina in functionalization, controllable preparation, and conformation diversification has brought strong impact on the catalytic areas of traditional alumina. Nano-alumina is usually prepared by physical and chemical methods. Physical technology includes high-performance grinding, ball milling, vibration grinding, and ultrasonic methods. Nano-alumina prepared by chemical method involves the chemical reactions between ions and molecules, accompanied by the growth of nuclei. By using chemical method, the size and particle distribution could be effectively controlled to improve the structural and textural properties.

Nano-alumina catalysts have advantages of high selectivity, small particle size, and controllable size and aperture distribution. Boz et al. [117] prepared KF- impregnated nano-y-Al2O3 as heterogeneous catalysts for the transesterification of vegetable oil with methanol, and methyl ester yield of 98 % was obtained with 15 wt% KF loading. The relatively high basicity of the catalyst surface (1.68 mmol/g) and the high BET surface area (about 12 m2/g) are considered as the main reason for the high conversion rate. Nano-alumina could improve the thermal performance and acidity of the incorporated catalysts [118].

However, the nano-alumina crystal grain size had wide distribution and poor preparation repeatability. Furthermore, aggregation of nano-alumina affected its application in catalysis fields. Under high preparation temperature, the activity of nano-alumina-supported catalysts decreased due to the decline of surface area and the contraction of volume. There are several new preparation technologies to improve the catalyst properties, such as supercritical anti-solvent precipitation, ultrasonic-chemical precipitation, and combustion synthesis [119].

When the FFAs value is below 10 %, some companies remove such range of FFAs% by distillation. The distillation of crude fatty acids removes both the low and high boiling impurities typically present in such feedstocks. Fatty acids depending on their

degree of saturation are sensitive to heat, oxidation, and corrosion effects. Normally, the higher degree of saturation of FFAs is the higher resistance of FFAs toward heat and oxidation conditions. Distillation of FFAs from oil feedstocks is in general carried out under high vacuum and lower temperatures and with the shortest residence time allowable. Normally, feedstocks are pre-dried and degassed under vacuum and then fed to the distillation unit. Either tripping steam or high vacuum systems are provided to improve circulation and reduce partial pressure, thus lowering the distillation temperature and reducing degradation losses of FFAs [30].

Generally, the organosolv pretreatment was efficient on the bioconversion of soft­wood. After the pretreatment, the cellulose conversion yield during the subsequent enzymatic hydrolysis could be as high as 99 %, which is much higher than other chemical pretreatements, namely DAP, alkaline, and wet oxidation pretreatments (Table 8.5).

8.3.3 Ionic Liquids (ILs) Pretreatment

Ionic liquids (ILs) has recently received extensive research attention on the cel­lulose dissolution [137-142]. Some ILs show promise as efficient and “green”, novel cellulose solvents. They can dissolve large amounts of cellulose at consid­erable mild conditions, and feasibility of recovering nearly 100 % of the used ILs to their initial purity makes them attractive [143]. After the ILs pretreatment, the precipitated cellulose is washed thoroughly with water to remove the ILs. No negative effect of the residual ILs was reported on the subsequent cellulose hy­drolysis and fermentation [44]. As cellulose solvents, several ILs possess several advantages over regular volatile organic solvents of biodegradability, low toxicity, broad selection of anion and cation combinations, low hydrophobicity, low vis­cosity, enhanced electrochemical stability, thermal stability, high reaction rates, low volatility with potentially minimal environmental impact, and non-flammable property.

The dissolution mechanism of cellulose in ILs involves the oxygen and hydro­gen atoms of cellulose hydroxyl groups in the formation of electron donor-electron acceptor (DA) complexes which interact with the ILs [144]. Upon interaction of the cellulose-OH and ILs, the hydrogen bonds are broken, resulting in opening of the hydrogen bonds between molecular chains of the cellulose [144]. These interactions result in the dissolution of cellulose. Solubilized cellulose can be re­covered by rapid precipitation with some anti-solvents such as water, ethanol, methanol, or acetone. The recovered cellulose was found to have the same DP and polydispersity as the initial cellulose, but significantly different macro — and micro-structure, especially the decreased degree of crystallinity [145]. The previ­ously used ILs include 1-n-butyl-3-methylimidazolium chloride (BMIMCl) [146], 1-allyl-3-methylimidazolium chloride (AMIMCl) [147], 3-methyl-N-bytylpyridin — ium chloride (MBPCl), and benzyldimethyl (tetradecyl) ammonium chloride (BD — TACl) [143]. It should be noted that the presence of water significantly hampers the dissolution efficiency of ILs. Thus, the water content in the wood chips should be decreased prior to the pretreatment [148]. In addition, an IL can be recovered after regeneration of cellulose with water or water/acetone mixture. The solvent added to the IL should be evaporated prior to its reuse in the next extraction cycle [148].

Application of ILs has opened new ways for the efficient utilization of lignocel — lulosic materials in such areas as biomass pretreatment and fractionation. However, there are still many challenges in putting these potential applications into practical use, for example, the high cost of ILs, regeneration requirement, lack of detailed toxicological data and knowledge about basic physico-chemical characteristics and action mode on hemicellulose and/or lignin contents of lignocellulosic materials, and inhibitor generation issues. Further research is required to address such challenges.

In this method, water is sprayed on the gas, which makes the particles and tars collide, creating large droplets and separate from the gas stream by using cyclones. The tar liquid can be re-injected into the gasifier, and water may be regenerated by stripping the tar away. This method produces a large amount of waste water with a high organic content, removes a large fraction of the carbon and hydrogen stored in tars, and also reduces the gas temperature to near ambient temperatures, which results in a loss of thermal efficiency.

There are some commercial methods of wet scrubbing available, such as OLGA and TARWTC technologies, which use oil as a scrubbing liquid [59,43]. The oil with separated tars can be recirculated to the gasifier so the energy in tar can be recovered. In this method, however, cooling the gas prior to cleaning is still required.

Pyrolysis reaction kinetics are characterized by hundreds or thousands of parallel re­actions and in series (solid and gas phase). Pyrolysis is governed by several chemical mechanisms: resonance, bond breaking, rearranging, dehydrogenation, cyclization etc. Due to its complexity, the entire chemical pyrolysis reaction network has not been characterized and pyrolysis kinetics models currently available in the scientific literature are highly simplified.

Since biomass is a solid phase macromolecular system, it is impossible to characterize its reaction kinetics with conventional gas-phase Arrhenius equations containing partial pressures or concentrations. Generally, each reaction step that is considered in the conceptualization of pyrolysis reactions has its own kinetics and parameters to describe its rate: a specific order of reaction and enthalpy of reaction. Pyrolysis kinetics can be expressed in the following general modifiedArrhenius form [33]:

In Eq. (11.1), the rate of reaction is the function of a pre-exponential factor (A), an Arrhenius term containing an activation energy (E) and a linear function representing the weight of the decomposing sample (f (m)) to the power of the order of reaction (n). The weight function (f(m)) can be written in an absolute (n = 1) or normalized form. In the latter case, the non-dimensional term can be formulated in two ways: (1) normalized with respect to the weight of emitted volatiles or (2) the weight of decomposable material. In the first case (emitted volatiles), the rate of reaction will be referred to as the rate of devolatilization with the weight function f (m) being equal to (1-m). In the second case (decomposable material), the weight function f (m) will be equal to m. When expressed in an absolute form (not normalized), the weight function is also equal to m, but with appropriate weight units. The order of reaction (n) will depend on the reaction model and the biomass material. Generally, authors assume the reactions to be of first or second order. However, since most pyrolysis reaction models are global models, the apparent order of reaction is generally characterized by a value between 0.5 and 3. Few studies have experimentally evaluated pyrolysis kinetics by considering the order of reaction as an unknown [25, 34]. Similarly to the reaction order (n), the activation energy (E) and pre-exponential factor (A) depend on the biomass material as well as the characteristics of the reaction model.

Apart from the assumption related to the kinetic expression, several other factors may bias the measurement of kinetic parameters. In fact, heat and mass transfer are important phenomena to consider during experiments. Biomass is characterized by a very poor thermal conductivity combined with a high specific heat. Therefore, biomass particles may have a significant internal temperature gradient when heated at high rates [35]. When conducting laboratory scale pyrolysis kinetics experiments, static systems (thermogravimetric analysers (TGA)) are often employed where the biomass particles remain immobile. The use of TGA minimizes attrition such that the particles remain intact throughout the experiments and the internal mass transfer is limited. Unfortunately, this may not be representative of industrial pyrolysis systems and the derived reaction kinetics will not be accurate when applied at the industrial scale.

Now plasma technologies of treatment and gasification of organic substances develop in two directions differed by the way of energy transfer to the plasma-chemical reactor. One of them is based on the electric arcs burn directly in the reactor volume (transferred arc). Arcs close between the electrode injected into the reactor and electroconductive melt (molten slag) in the bottom part of the reactor. Electrodes can be made of graphite or metal. The advantage of this method is possibility to create a large single plasma-chemical reactors with power consumption of about 5-30 MW. Free-burning arcs are used in large-scale metallurgical installations which are almost ready to commercial operation.

The disadvantages are rapid wear of electrodes, considerable quantity of admix­tures, and low-energy transfer coefficient of a free-burning arc into the processing substance. The efficiency of such installations, as a rule, does not exceed 30-35 %.

Stationary plasma torches are used for plasma generation in another method. The plasma-forming gas gains energy from the electric arcs burning in the discharge chamber of the plasma torch (non-transferred arc), and then arrives into the reactor volume.

The efficiency of heat exchange between the generated plasma and processed substance is significantly higher in comparison with the first method. The efficiency of plasma torches optimized for industrial applications exceeds 90 % and energy of a plasma jet almost completely transfers to the processed substance. That rather simplifies control over the chemical composition of syngas. Now, the application of this method is restrained by absence of sufficiently powerful plasma torches capable of generating plasma from oxidizing media (air, steam, etc.) in prolonged modes with high efficiency and lifetime of electrodes.

Powerful stationary electric arc plasma torches, meeting the requirements of plasma-chemical technologies, can be classified by current type: alternating cur­rent (AC) plasma torches [67-81] and direct current (DC) plasma torches [82-86]. They also can be divided by a type of plasma forming gas: neutral, reducing, or oxidizing. Devices differ in design and other features: type of discharge chambers, material and form of electrodes, a way of working gas supply, a principle of the arc stabilization, etc. Most plasma torch designs utilize a combination of methods of arc discharge stabilization comprising gas stream organization, arc contraction by the insulated inserts, and magnetic field. Electrode designs of plasma torches are rod, toroidal, ring, and tubular; in some cases the electric arc chamber is one of the electrodes. We will present only basic types due to enormous variety of designs.

Pretreatment is the first step in the conversion of lignocellulosic biomass to biofuels and chemicals. The purpose of pretreatment is to break down the lignin that binds cellulose and to destroy the crystalline structure of cellulose and increase its surface area so that fragments become accessible to chemical or enzyme active sites [1, 13]. Pretreatment is the most expensive step in the production of cellulosic ethanol by enzymatic hydrolysis and fermentation. The conventional methods of pretreatment are physical, chemical, physical-chemical, and biological pretreatments. Typical physical pretreatment includes chipping, grinding, milling, and thermal methods. Mais et al. [19] pretreated cellulose by milling, and subsequently obtained up to 100 % hydrolysis yield with a relatively low enzyme loading. However, ball-milling consumes significant energy. It is widely recognized that chemical pretreatment is more economic.

Ionic liquids (IL) are salts containing large organic cations and small inorganic anions that exist as liquids at relatively low temperatures, and their solvent properties vary according the anion and alkyl constituents of the cation [63]. Both fractions from lig — nocellulosic materials can be dissolved in the presence of IL by formation of hydrogen bonds between the non-hydrated anions present in IL and the sugar hydroxyl protons portion from carbohydrates or lignin [29]. This non-covalent interaction causes the disruption of the strong biomass linkages minimizing undesirable products formation [63, 64]. IL, such as 1-butyl-3-methylimidazolium chloride [Bmim] [Cl], 1-allyl-3- methylimidazolium chloride [Amim] [Cl], and 1-ethyl-3-methylimidazolium acetate [Emim] [Ac], have been used as alternative solvents for pretreatment of lignocellu — losic materials mainly by its chemical inertness and thermal stability [11, 60]. Liu et al. [65] had used the [Bmim] [Cl] IL as reaction medium to modify native SB cellu­lose generating phthalated cellulosic derivatives. This compound was also evaluated as a pretreatment solvent for SB and demonstrated to increase in 8-fold the cellulose conversion in comparison to untreated material [24]. ILs can easily dissolve and regenerate the cellulose molecule and is also a promising technology forproducing

modified cellulose. The combined use of the [Bmim] [Cl] and ultrasound can be used for fractionating SB cellulose [66].

Ultraviolet-visible spectroscopy or ultraviolet-visible spectrophotometer (UV-Vis or UV/Vis) was employed to determine the characteristic absorption of the sawdust
samples prior to and after the treatment. The analysis was performed on a CARY- 5E-UV-VIS-NIR spectrophotometer. The UV wavelength range was setting from 240 to 380 nm. X-ray diffraction (XRD) is a method of determining the arrange­ment of component structure within a crystal. The carbon crystalline structure of the pine sawdust before and after pretreatment was determined by XRD using Rigooku DMAX-RB diffractometer operated at 45 kV and 50 mA. Following the method of Kim and Holtzapple [48] as well as Liu et al. [49], the carbon crystallinity index (CrI) was calculated with the Eq. 19.5:

CrI(%) = 7°02 ~ Iam x 100 % (19.5)

І002

where I002 is the intensity of the crystalline area of (0 0 2) plane at 20 = 22.4 °; Iam is the intensity of the amorphous region at 20 = 18.7 °. It shall be noted that I002 includes both crystalline and amorphous intensity (background). The scanning electron microscope (SEM) was used to characterize the morphological structure change in pine sawdust before and after pretreatment. All solid samples were milled to 20 mesh in particle size, mounted with gold coating before the SEM scanning. The images were taken by a JEOL5900LV SEM, Oxford INCA microanalysis full system with 4 “x5” analytical stage.