The biomass feedstock moisture and ash content have an effect on the gasification product composition, the process energy balance, and the reactor operating condi­tion. Different gasification processes require different moisture levels; high moisture content in biomass will, however, reduce the gasifier temperature and have negative effects on gasification efficiency and the quality of the products. To reduce moisture, driers are used and the most common type is the rotary drier. Nevertheless, the use of steam drying techniques is also increasing due to its easy integration into existing sys­tems. Other techniques, such as biomass mechanical dewatering and leaching with water, have been demonstrated to reduce ash content efficiently [3]. Since water va­por plays an essential role in the chemical reactions of the gasification process, there is a trade-off between moisture removal (beneficial to the process energy balance) and syngas composition.

Biomass particle-size reduction techniques include milling, grinding, and pulver­ization, which make drying, transfer, and biomass storage easier. Their use depends mainly on the requirements of the gasification system as well as the biomass char­acteristics. For example, to pulverize particles to an average size below 0.2 mm, a vibration mill is recommended [4]. There are also some new methods being de­veloped, like freezing pulverization or explosion disintegration, which are suitable for materials that could not be pulverized by conventional methods; however, their power consumption is relatively high.

The reactor is considered as a one-dimensional plug flow reactor under steady-state conditions [94]. The gas phase is considered as perfectly mixed radially and the solid particles are distributed uniformly in the radial direction. The mass balance for solid and gas components can be described as follows [103]:

10.7.1 CFD Models

CFD modeling solves a set of equations for the conservation of mass, momentum, and energy simultaneously to give the gasifier temperature, the product concentra­tion, and the hydrodynamic parameters at different locations. Due to the complexity of the gasification process, however, there are not many CFD models available for this process and most of them must use fitting parameters and major assumptions for areas where accurate information is not available. Most of the CFD models are for coal gasification and combustion in entrained flow reactors since gas-solid flow is less complex compared to fluidized bed reactors [104] and [105]. A typical CFD model for gasification consists of a set of sub-models for different reactions and phe­nomena, such as drying biomass particles, devolatilization (pyrolysis), secondary pyrolysis, and char oxidation [106]. There are also other sophisticated subroutines for the destruction of solid fuels during gasification and combustion, which could be coupled with transport phenomena of the gasifier [107]. Due, however, to consider­able computational times for CFD models, particularly when chemical reactions are involved, this type of modeling is not very common for fluidized bed gasifiers.

Rotary kilns have been used for decades to generate energy from wastes. The opera­tion of this type of reactor has been demonstrated in a continuous mode at industrial scale and many problems have been reported. The very high temperatures during incineration promote NOx and SOx production as well as dioxins and furans, which are carcinogens. Moreover, leaching and slagging can affect rotary kiln incinerators operability. Notwithstanding these reports, the interest for using rotary drum reac­tors for pyrolysis applications is currently growing [15]. Operation of the reactor at lower temperature and without oxygen will likely decrease pollutant emissions as well as minimize risks of leaching and slagging. In most cement plants around the world, rotary kilns can have impressive dimensions (over 100 m long). For pyrolysis applications, some pilot-scale units were built and operated to process waste.

Typically, a rotary drum pyrolysis reactor processes raw materials such as MSW, RDF and waste tires. It consists of a horizontal cylindrical reactor rotating at a certain speed in order to mix the bed of materials and promote transport phenomena. For this application, as oxygen must be purged, heat supply is generally indirect, that is, gas burners are mounted underneath the rotary drum and the flue gases are circulated around the drum in a blanket-like chimney. Figure 11.3c shows a schematic of a rotary drum pyrolysis system with an electric heater.

Xiaofei Tian, Zhen Fang and Charles (Chunbao) Xu

Abstract Enzymatic saccharification of lignocellulosic biomass encounters many prohibitive factors which make it difficult to be developed on an industrial scale. Pretreatment has been found essentially effective for increasing the susceptibility of substrates to the enzyme, for example, by removing the lignin barrier and breaking down the crystal structure of cellulose in the raw materials. As a green and efficient technique, pretreatment of lignocellulosic biomass employing organic solvents and organic electrolyte solution (OES) is introduced in this chapter. Future prospects and recommended research work for developing these technologies for practical applica­tion, as well as coupling production of high-value bio-products from lignocellulosic biomass, are also discussed.

Keywords Aqueous organic solvent ■ Delignification ■ Decrystallized cellulose ■ Ionic liquids ■ Selective precipitation ■ Hydrolysis yield

14.1 Introduction

In biorefineries of lignocellulosic biomass, especially for cellulosic ethanol pro­duction, there are three key steps in the bioconversion process: (1) lignocellulose pretreatment, to separate the lignin and hemicellulose components from cellulose in the biomass and destroy the recalcitrant cellulosic structure to reactive intermediates; and (2) enzymatic hydrolysis, saccharification of the cellulose and hemicellulose to fermentable sugars (e. g., glucose and xylose) by cellulase-catalyzed hydrolysis; and

Z. Fang (H) ■ X. Tian

Biomass Group, Chinese Academy of Sciences, Xishuangbanna Tropical Botanical Garden,

88 Xuefulu, Kunming, Yunnan province, 650223, China e-mail: zhenfang@xtbg. ac. cn

X. Tian ■ C. (Chunbao) Xu

The Institute for Chemicals and Fuels from Alternative Resources, Faculty of Engineering, Western University, London, Ontario, N6A 5B9, Canada

Z. Fang (ed.), Pretreatment Techniques for Biofuels and Biorefineries, 309

Green Energy and Technology,

DOI 10.1007/978-3-642-32735-3_14, © Springer-Verlag Berlin Heidelberg 2013

(3) fermentation, to produce bio-ethanol or other bio-based chemicals (e. g., lactic acid and succinic acid) [1-3].

Effect of the nature of lignocellulosic substrate on the enzymatic attachment and activity of cellulases are two of the critical factors that influence the entire rate and yield during the enzymatic hydrolysis process, and also the big challenges that have to be overcome for the commercial production of lignocellulosic bio-ethanol [4-6]. Consequently, pretreatment is required prior to the hydrolysis to remove the recal­citrance, such as (1) disruption of the carbohydrate-lignin shield by reducing lignin and hemicellulose contents [6-9]; (2) concomitantly decreasing the crystallinity of the cellulosic structure [6, 7]; (3) extension of enzymatic accessible surface area by particle cracking and increasing the volume of micro pores [5]; (4) loosening of the structure of lignin or (and) cellulose by cleavage of beta-1-O-4-aryl-ether bond or (and) beta-1-O-4-glucosidic bond [8]. Since pretreatment accounts for the largest proportion (around 50 %) of bio-ethanol production cost excluding the enzyme and raw material costs [5,9-11], it has great potential for efficiency improvement and cost reduction through research and development [8]. Plenty of work on pretreatment has been conducted and published during the last 10 years. The reported pretreatment methods can be classified into the following categories: (1) physical treatments (i. e., milling, grinding, high energy radiation, etc), (2) chemical treatments (e. g., soaking and boiling by diluted acid or alkalis, ozone oxidation, etc.), (3) physiochemical treatments (steam explosion, ammonia fiber explosion, ammonia recycled percola­tion [8], pyrolysis, hydrothermal and organosolv methods), and (4) biological (white rot fungi) treatments [12].

The nomenclature of organosolv pretreatment is derived from organosolv, which is a pulping technique that uses an organic solvent to solubilize lignin and hemi — cellulose and was invented by Theodore Kleinert [13]. Organosolv pretreatment is generally classified as a physiochemical technique, which employs non-aqueous or aqueous organic solvents, such as various alcohols, acetone, glycerol, dioxane, ethylene glycol, triethylene glycol, and phenol [14], with (or without) catalysts to remove parts of noncellulosic components (mainly lignin), and (or) convert the form of cell walls, as well as structures of natural cellulose, providing effective frac­tions of lignocellulosic biomass for the subsequent enzymatic saccharification step. Though organic acid and organic peracid were used to pretreat biomass as well [15], they differ from the organosolv pretreatment of interest here due to their dissimilar fundamentals.

This chapter presents an overview of the status of organosolv pretreatment tech­niques and their corresponding fundamentals, as well as further processing of the organosolv by-products. In addition, a newly developed technique using organic electrolyte solution (OES) composed of organic solvent and cellulose-soluble elec­trolyte solvent in pretreatment is also introduced. Future prospects and recommended research work on developing these technologies for practical application, as well as coupling production of high-value bio-products from lignocellulosic biomass are also discussed.

Recently, novel synthesis methods for nano-catalysts and nanostructures have been widely reported. The synthesis of silica-based particles focuses on nanostructures of nano-ropes, nano-tubes, and paintbrushes [96]. Hydrothermal treatment with acid can be used to improve the order and stability of the nanostructures after synthesized with an aqueous ammonia solution. By sol-gel synthesis, silica nano-tubes can be bundled to form nanostructures known as paintbrushes. The sol-gel processing of nanoparticles is commonly performed by a so-called semi-alkoxide route in which inorganic compounds like hydroxides, acetates, carbonates, and chlorides, are used as sources to alkaline earth ions [97]. Nanosized transition metal components are usually synthesized using alkoxide and semi-alkoxide routes.

Hybrid nanocomposites will find applications in catalysts. Hybrid (inorganic — organic) nano objects and higher level nanostructured networks were obtained by novel equilibrium and non-equilibrium self-assembly approaches [98]. By control­ling over the morphology of hybrid materials, controllable particle size and size distribution were achieved. Dong et al. [99] proposed a feasible and effective self­assembly method to synthesize different scale coordination polymers in highly dilutes solution. The nano and microscale particles gave better catalytic conversion rate (73 %) and selectivity in the hydroxylation of phenols than the bulk crystals.

Reversed micelle technique has been reported on micellular HPA [84]. The syn­thetic method was based on the reaction between the components dissolved in the lyophilic media and the reversed micelles. Cellulose was hydrolyzed for three con­tinuous repeated runs under the same reaction conditions, and complete hydrolysis was achieved. The highest glucose yield reached with the micellular HPA was 60 % with 85 % selectivity for the three continuous runs. The catalyst was separated from the reaction mixtures by centrifugation.

Supercritical anti-solvent precipitation synthesis was recently used for synthesiz­ing MnOx-CeO2 hollow spheres. As reported by Jiang et al. [100], a mixed solution of manganese acetylacetonate, and cerium acetylacetonate in methanol was injected into the precipitator. As the solution droplets contacting with supercritical carbon dioxide, the nanoparticles were precipitated. Then, the supercritical CO2 was allowed to flow to remove the residual methanol. The system was further depressurized to atmospheric pressure, and the generated nanoparticles were collected. Finally, they were calcined in a muffle furnace, and the MnOx-CeO2 hollow nano-spheres with an average diameter of about 50 nm and a wall thickness of 10-20 nm were obtained.

Magnetic nano-catalysts with ordered or disordered array of nano-crystallites have attracted great attention recently because of their wide application poten­tials. However, it is still a big challenge to tune the magnetic nano-crystallites into three-dimensional regular aggregates with varied nanostructures. By ultrasonic — chemical precipitation synthesis, magnetite particles with 15 nm average diameter were obtained [101]. Under ultrasonic agitation, Fe3O4 precipitates were produced immediately by adding sodium hydroxide into mixture of FeSO4 and FeCl3 with Fe3+ and Fe2+ molar ratio of 1.5:1. Moreover, C12H25OSO3Na could be added as surface active agent, assisting to obtain Fe3O4 nanoparticles with homogenous size and shape distribution.

The development of highly acidic nano-solid catalysts that have special charac­teristics (e. g., paramagnetic properties) is an interesting area for developing practical systems for biomass hydrolysis. Through the combination of novel nano-catalysts preparation techniques, it is expected that chemical processes based on the hydrolysis of cellulose will be developed rapidly.

Crude plant oils typically contain FFAs < 3.0 % and gums in the range of 0.05-0.5 % [21]. The only pretreatment needed for such oil feedstocks is to remove the gums us­ing either conventional degumming methods or using enzymes [22]. Higher FFAs oil feedstocks, such as recovered yellow grease, are no longer viable for the production of biodiesel by the conventional alkaline process. Such feedstocks containing up to 9 % FFAs can be pretreated by different methods such as the SRS (see Sect. 18.6.1) continuous flow acid by SRS Engineering, removal of FFAs by distillation or adsorp­tion, or treated with enzymes (Table 18.1) [ 27]. Crude corn oil or yellow grease with FFAs >9% requires acidic pretreatment, enzymes or Ca(OH)2 (Table 18.1). High FFAs oil which is disposed in the drains “brown grease” is a potentially problem­atic waste stream and it clogs installations in waste-water treatment plants and thus, it adds to the cost of treating effluent [23, 24]. Such potential feedstock contains typically 30% and up to 100% FFAs, requires an enzymatic or acid-based esterification methods (Table 18.1).

Biodiesel producers always have to decide on what is the cutoff of the FFAs level in the feedstock and decide how to move forward with biodiesel production. Some producers work with feedstock streams containing FFAs lower than 1 % (virgin oil) where any known technology for making biodiesel can be applied (Table 18.1).

When the feedstock contains up to 3.5 % FFAs, the soap formation during the alkaline reaction will be challenging to work with. One way to deal with it, is blending such oil (FFAs <3.5 %) with a lower FFAs feedstocks (<1 %) to obtain oil feedstocks of FFAs less than 2 %. Also, feedstocks containing FFAs levels below 3 % can be pretreated by adsorbents which extract FFAs from the oil out into the matrix. Spent adsorbents can normally be disposed off in a landfill. Another approach would be to convert the FFAs to their potassium salts and be removed by either water — wash process or centrifugation. When oil feedstocks contain higher than 3.5 % FFAs, pretreatment methods should be applied for the removal of FFAs, simply because the alkaline biodiesel production process converts the FFAs to soaps which eventually result in complicating of the downstream processing of the final products [25].

Not all pretreatment methods are suitable for every biodiesel production plant. Several factors will have to be considered such as the process type (alkaline, acidic, heterogeneous, enzymatic, etc.), the type of existing equipment in the plant, the long-term availability of feedstocks used with variable contents of FFAs, and how the pretreatment technology ties into those systems.

Unlike DAP, dilute alkaline pretreatment, and wet oxidation pretreatment, SPORL was proved to be efficient for softwood species. Zhu et al. [109] investigated the combination of a sulfite treatment with mechanical size reduction by disk refining to enhance enzymatic hydrolysis of SW. This study was the first to establish this novel pretreatment process. Pretreatment conditions of spruce chips (20 %, w/v) that produced optimal cellulose conversion during enzymatic hydrolysis (>90 %) were treatment with 8-10 wt% bisulfite and 1.8-3.7 wt% H2SO4 for 30 min at 180 °C. Nearly all hemicellulose was removed, which exposed the underlying cellulose frac­tion to enzymatic attack. Additionally, furfural and HMF were produced in minimal concentrations, about 1 and 5 mg/g untreated wood, respectively. In addition, similar results were also observed with Lodgepole pine and red pine [109, 120].

The raw products of biomass gasification contain contaminants, including particles, organic impurities (tar) alkali metals, chlorine, nitrogen, and sulfur compounds (such as H2S, CS2, COS, AsH3, PH3, HCl, NH3, and HCN). These contaminations can block the downstream units, such as gas coolers and engines, and also interfere with the catalyst used in the production of synthetic fuels. Therefore, they have to be completely removed or significantly reduced before utilizing the gas depending on the application of interest. Also, the purpose of the conditioning system is to adjust the components to the appropriate ratio. Depending on the type of feedstock, its composition, and the type of gasification product application, there are different types of gas cleaning and conditioning that can be categorized as physical cleaning, such as cyclone, filters, and wet scrubbers’ application, or chemical cleaning, such as catalytic cracking, thermal reforming, shift hydrolysis, and hydrogenation.

Some feedstocks are characterized by low densities (such as bark and wood residues) and increasing their particle size may be necessary for some reactor technologies. In fluidized bed reactors, for example, particles with low terminal velocities may be rapidly entrained outside of the reaction zone resulting only in partial conversion. If the pyrolysis reactor technology requires larger particles, particle agglomeration techniques can be used. Pelletization, also referred to as densification, is a well — established process, and it is currently used to transform many MSW into denser particulate RDF (Refuse Derived Fuel) feedstocks to be directly used as fuels [10]. These same processes can also be used for biomass pre-treatment for biorefineries.

The most widely used equipment to produce pellets is the extruder [29]. It con­sists of one or two (partially overlapped) cylindrical ducts in which screws force
deformable solids to flow with very high shear forces. At the end of the extruder, a die controls the pellet average size and a binding agent can be introduced with the solids in order to consolidate the agglomerates. Depending on the objective of the extrusion process, the design can consider multiple outlets to remove water.

Since pulp and paper industry wastes contain significant quantities of water, drying can become very expensive. In this instance, some compacting technologies can mechanically remove water from biomass while forming pellets or briquettes. Some extruders and other hydraulic or pneumatic presses perform compaction as well as removal of liquid water. By reaching high pressures using extrusion, Edwards [30] was able to compact a mix of bark and wood residue and lower its moisture content from 56.5 wt% down to 34.8 wt%.

It is useful to perform numerical simulation of the process to now the prospects of plasma gasification of chicken manure. Calculation of equilibrium composition allows evaluating the yield limits of valuable gasification products at the set param­eters (oxidant/feedstock ratio, temperature, and pressure) [52]. Deficiencies of the method are the restrictions imposed by assumptions: ideal mixing and unlimited residence time [53]. Nevertheless, the given approach accords satisfactory with the experimental data [39, 54-56].

Calculations of the equilibrium composition were implemented by means of the software Chemical WorkBench (Kinetic Technologies Ltd., http://www. kintech. ru/).

Possibility to supply energy with plasma almost completely removes the kinetic restrictions imposed on the process by low temperatures in autothermal modes. Ac­tually, the plasma mode boundary lines are determined by the oxygen consumption required for oxidation of carbon of the feedstock to CO (so-called carbon boundary) and autothermal limit when additional energy is not required for achievement of the set parameters. For chicken manure, the carbon boundary is ~245g/kg (mode 1, Fig. 12.1) for air and ~63.9g/kg (mode 3) for steam plasma. Autothermal mode (temperature 1,500 K, pressure 1 atm) air gasification is achieved at oxidizer con­sumption ~2.16 kg/kg. Accordingly, all regimes between these two air consumptions are allothermal under equal conditions (temperature and pressure).

Let us examine modes of stoichiometric gasification of chicken manure by air and steam, and also a mode with an air consumption ~ 1.20 kg/kg (mode 2) that is between autothermal and stoichiometric modes, and a mode with the steam consumption ~0.314 kg/kg (mode 4), matching to the previous one by the amount of fed oxygen. Figure 12.1 shows the results of calculations. In calculations, the composition and heating value of a chicken manure specified in Table 12.1 were used and also the following composition of air was used: N2—78.09, O2—20.95, Ar—0.93, CO2—

0. 03 %mol.

Approximation of synthesis rate was used for calculation of space velocity of Fischer-Tropsch process on Co-Mn/TiO2 catalyst [57] at pressure 10 bar and tem­perature 523 K. Before Fischer-Tropsch synthesis, the syngas was cleaned from sulfur compounds and nitrogen oxides, part of CO was converted to H2 by water-gas shift reaction to provide stoichiometric relation of synthesis—H2/CO = 2 according to its chemical equation (12.1).

CO + 2H2 ^ -(CH2) — +H2O (12.1)

The synthesis continued until CO content in convertible gas decreased to 0.1 %mol. The process rate was calculated according to equation (12.2) [57].

rCO = kp x bCO x Pco x Ph2/(1 + bco x Pco) (12.2)

where, rCO is CO conversion rate (mmolCO/min gcat), kP and bCO are kinetic param­eters equal to 0.367 and 1.454, respectively, Pco and Ph2- are partial pressures of CO and H2.