Alkali pretreatment is employed for the removal of lignin from SB/SL. Alkaline pretreatment cleaves the intermolecular ester bonds between xylan, lignin, and other hemicelluloses present in the SB/SL cell wall contributing to removal of acetyl and uronic acids [27, 29]. The alkaline-based pretreatment methods constitute mainly sodium hydroxide (NAOH), calcium hydroxide (lime), potassium hydroxide, and aqueous ammonia hydroxide [10]. Use of alkali compounds increase the internal surface of cellulose promoting a decrement of its degree of polymerization and crystallinity with less sugar degradation and have been described as more effective on SB/SL that contain low lignin than wood materials [1, 37].

The alkali action is based on the lignin structure disruption by breaking of ester bonds between lignin, hemicellulose, and cellulose, and increasing the hydroly­sis efficiency of the carbohydrate portion during cellulase mediated enzyme action [10, 50]. Specific at higher temperatures, the alkali pretreatment can be performed at low temperatures with extended time of reaction and high concentration of the alkali [11,27].

A disadvantage of alkaline pretreatment is the formation of non-recoverable salts that can be incorporated into the biomass during the pretreatment reactions. Alkali pulping can result in formic and acetic acids, irrecoverable salts, and low-molecular — mass fragments as reaction products [10]. NaOH mediated pretreatment is one of the most common methods applied to pretreatment of SB. Wu et al. [51] evalu­ated the optimal low temperature NAOH pretreatment for SB hydrolysis in order to convert cellulose into ethanol using separate enzymatic hydrolysis and fermentation (SHF). Almost 98 % of glucan was hydrolyzed to glucose after 72 h using lower loading of commercial enzymes resulting in 88 % of fermentable sugars to ethanol by Saccharomyces cerevisiae strain. NAOH pretreatment can be more effective when combined to H2O2, a bleaching agent used in the paper and cellulose industry. In this alkaline/oxidative process, hemicellulose is solubilized in the first step, followed by solubilization and oxidation of lignin fraction from lignocellulosic material without formation of secondary products such as furfural and hydroxyl methyl furfural [14]. Some studies indicated that the use combination of both reagents promotes a total conversion of cellulose in glucose units [52]. Figure 16.2 shows the mechanistic demonstration of alkaline pretreatment applied to SB/SL.

Use of lime or calcium hydroxide promotes the removal of acetyl groups from hemicellulose and contributes to enhance the cellulose digestibility with lower cost and less safety requirements compared to sodium and potassium hydroxides despite their slow action on lignin [29]. Lime treatment can remove amorphous substances like lignin and hemicellulose increasing the crystallinity index and is also easily

recovered as calcium carbonate by neutralization with CO2 and regenerated [15, 53]. A comparison of the yield of total reducing sugars and glucose released from pretreated SB using lime or alkaline H2O2 pretreatments after enzymatic hydrolysis was developed. In this case, SB was previously hydrolyzed with H2SO4 (3 % v/v), followed by a low temperature alkali pretreatment method. Highest amounts of glucose were released from lime pretreated SB [54]. Lime and alkaline H2O2 pretreatments of SB were also compared aiming the evaluation of the potential of biogas production from the residues of second generation bioethanol. Results have shown that the highest methane production was obtained from SB residues pretreated in the presence of peroxide [55]. Rocha et al. [56] pretreated SB with steam explosion (1.3 MPa, 190 °C, 15 min) at pilot scale followed by the alkaline deligniflcation (1.0 % w/v NaOH). Almost 94 % of hemicellulose hydrolysis and 92 % of lignin solubilization was recorded during this strategy.

Combined alkali and microwave treatment of SB have been also described. Mi­crowave treatment (600 W) of SB in the presence of NaOH (1 % w/v) for 4 min followed by enzymatic hydrolysis resulted in 0.665 g reducing sugars/g dry biomass [28]. Xu et al. [57] realized two-stage pretreatments of SB with mild alkali (NaOH 1M) and acidic 1, 4-dioxane in HCl (2M). Alkali treatment resulted in 62.1 % of hemicelluloses (78-82.2 % of xylose) released after 18 h at 40 °C against only 10.6 % under acidic conditions (44.9-46.8 % of xylose). Deligniflcation of SB with alkali (NaOH 10 % w/v; 90 °C; 1.5 h) and per acetic acid (PAA) (15 % w/v; 75 °C; 3 h) in a two-stage process was developed for generation of enzymatic digestible pulp as well as to be used for simultaneous saccharification fermentation (SSF) to ethanol pro­duction. The alkali-PAA pulp showed more xylose concentration and higher ethanol conversion when compared to dilute acid pretreated SB [58].

Pretreatment of pine sawdust was conducted using various methods, including organosolv extraction, organosolv extraction followed by ultrasonic treatment, organosolv extraction followed by NaOH treatment, and combined organosolv ex­traction, ultrasonic treatment and NaOH treatment in sequence, to remove lignin and/or hemicellulose and disrupt the textual structure of the pine sample to increase the accessibility of cellulose to enzymes.

19.2.2.1 Organosolv Extraction

Pine sawdust (20 mesh) was dried in an oven at 105 °C for 12 h before use. The exper­iment was carried out in a 1L stirred autoclave reactor under nitrogen atmosphere. A sample of 50 g was weighed and mixed with an ethanol-water solution (1:1 v/v) at a biomass-to-solvent ratio of 1:10 (w/v). The reaction temperature was set at 190 °C and the stirring speed was controlled at 400 ± 20 rpm. The initial nitrogen pressure was 300 psi in the reactor and the maximum pressure during the reaction was 700 psi. After a reaction time of 4 h, the reactor was cooled down rapidly using cooling water. The liquid and solid phases in the reactor were then separated via filtration. The liquid phase was used to recover hemicellulose and lignin. The dried solid phase was considered to be cellulose-rich residue, which was divided into four parts and subjected to further treatments as described below. One part of the residue was further treated with ultrasound and NaOH in sequence and was designated as pine sample with (organosolv + ultrasound + NaOH) pretreatment. Another part was treated with ultrasound only and was denoted as pine sample with (organosolv + ultrasound) pre­treatment. The third part was treated with NaOH only denoted as pine sample with (organosolv + NaOH) pretreatment. The fourth part without any further treatment and was designated as pine samples with (organosolv) pretreatment. For comparison, the raw (untreated) pine sawdust sample was used as a reference for enzymatic hy­drolysis. Detailed experimental conditions and methods for the ultrasonic and NaOH treatment are described as follows.

Sugars were quantified by HPLC using two Rezex RPM columns in series (7.8 mm x 30 cm, Phenomenex, Torrance, CA, USA) and a Waters HPLC (Waters,

Milford, MA, USA) equipped with differential refractive index detector (ERC — 5710, ERMA), on elution with water (0.5mL/min) at 85 °C. Aliquot portions of hydrolysates (6 mL) were centrifuged and the supernatant (5 mL) was transferred to a test tube containing inositol as an internal standard (1 mL, 0.5 mg/mL), mixed, deionized (column-containing Amberlite IR-120 H+ (0.5 mL) and Amberlite IR-402 OH — (0.5 mL) resins), and filtered (0.45 ^m).

Acetic acid was quantified by HPLC using a Rezex ROA organic acid column (7.8 mm x 30 cm, Phenomenex, Torrance, CA, USA) and a Waters HPLC (Waters, Milford, MA, USA) equipped with differential refractive index detector (ERC-5710, ERMA), on elution with 0.005 N aqueous H2SO4 (0.5 mL/min) at 65 °C. An aliquot of hydrolysate (1 mL) was taken and filtered (0.45 ^m) into an HPLC vial.

The total reducing sugar yield (%) for each sample was calculated as Eq. 9.1. Since the maximum sugar yield was detected after 3 days hydrolysis, the third day total sugar yield was used in the response surface optimization analysis.

Total sugar yield (%)

sum of sugars concentrations (mg/mL) x buffer volume100 (mL) x 100 %

wood dry weight 5,000 (mg)

This type of gasifier is suitable for low ash fuels. Unlike downdraft and updraft types, it releases the product from its sides. Air at high velocity enters the gasifier through a nozzle at a certain height above the grate creating a very high temperature zone. The product gas exits from the opposite sides of the gasifier. Start-up time is much faster (5-10 min) compared to other moving bed gasifiers, which improves the response to load changes. Due to relatively high temperature zones on a cross-draft gasifier, the product gas is low in tar, high in carbon monoxide, and low in hydrogen and methane.

Heat transfer to the biomass particles is generally the main limitation in industrial py­rolysis. Biological and organic polymeric materials have poor thermal conductivity, but high specific heats. Therefore, a limitation is reached in pyrolysis processes when heating a feedstock to high temperatures due to the high temperature dependence of the reaction kinetics (expressed as an Arrhenius law). The limitation arises at the specific temperature where the decomposition rate becomes greater than the heating rate. Acknowledging that pyrolysis is an overall endothermic reaction, increasing temperature from that point is very difficult.

Conventional pyrolysis is also referred to as slow pyrolysis because of the low heating rates (6-60°C/min [36]). The peak pyrolysis rate will be reached at a rela­tively low temperature and the limited heat transfer will result in moderate pyrolysis temperatures (300-700°C). These reaction conditions promote bio-char production and minimize volatiles (non-condensable and condensable gases). As the tempera­ture of the pyrolysis process is increased, the weight fraction of volatile increases: this effect is governed by the resonance mechanism. The release of lighter molecules from a macromolecular matrix generates instabilities that are dispersed within this matrix in order to stabilize its structure. As temperature increases, the instability gains in magnitude. In slow pyrolysis, biomass is kept at constant moderate temperatures, such that the macromolecule has time to reach a new stable form with a new compo­sition that will handle higher internal energy without decomposing (thermodynamic equilibrium), hence limiting the release of volatile. This is where conventional (slow) pyrolysis differentiates from fast pyrolysis. Volatile and char yields therefore depend on this resonance kinetics.

13.2.3.1 High-pressure Gas Safety Act

In Japan, legal matters pertaining to high-pressure gas were established in the “High-pressure Gas Safety Act [13]”, which was enacted in 1951 to guarantee public safety. In order to prevent disasters, the Act regulates production, storage, sale, import, and transfer of high-pressure gas, and also promotes autogenous activities

concerning high-pressure gas through private business people and the High Pressure Gas Safety Institute of Japan.

Manufacture of high-pressure gas is classified according to high-pressure gas pro­cessing capacity. Biogas (a flammable gas) manufacturers are classified as “Class 1 producers” if daily high-pressure gas production volumes are 100 Nm3 or greater, and “Class 2 producers” if their daily output is below 100 Nm3. Significant differ­ences between regulations for “Class 1 producers” and “Class 2 producers” include the need for permission of gas production and for a resident safety controller. The refining-compressing-filling equipment developed in this research satisfied the “Transfer production facility” and “Class 2 producer” designations, which handle gas production volumes of 100 m3/day or less. Required administrative procedures include the “manufacturing report”, “maintenance of and compliance to technical standards”, “implementation and documentation of periodic self-inspections,” and “discontinuation report.”

Zeolites are widely used as catalysts in organic synthesis, as they are non-toxic and non-corrosive, and easy to recover for reuse. They can be synthesized with different crystal structures and definitive pore size, and adjusted acid centers to have some important catalytic properties. The number of B acid sites in H-form zeolites is related to the atomic ratio of Al/Si. Acidic zeolites have been successfully used for conversion of sucrose into mono-sugars at mild conditions. Zhang and Zhao [45] performed cellulose hydrolysis with H-zeolite in IL solvents heating at 100 °C by oil bath, and a glucose yield of 2.1 % was achieved after 10 h. After cellulose was milled, a glucose yield of 12 % was obtained using H-ZSM-5 catalyst, but the

catalytic efficiency is still lower than that of other solid acid catalysts. For cellulose hydrolysis with an H-zeolite catalyst, cellulosic materials need to be dissolved in a solvent, and be converted into short-sugar chains to make full use of B acid sites in internal channels of the zeolite. Four modifications were proposed to improve the catalysts [63-68]: loading of cations, synthesis of zeolites with super-large pores, loading of super acids, and synthesis of composite zeolites. Herein, other kinds of modification technologies are proposed further:

1. Synthesis of nanosized zeolites Confined space synthesis is a novel method in ze­olite preparation. It involves the crystallization of zeolite inside the pore system of an inert mesoporous matrix [69]. By using this technology, Schmidt et al. [69] synthesized ZSM-5 with Al/Si ratios of 0, 0.02 and 0.01 which have controlled- average crystal size in the range 20-75 nm. Nanosized zeolite suspension has characteristics of a fluid solution to provide more active sites per gram. By means of adjusting the particle size, some desired properties can be obtained for nanopar­ticles. However, recycling of nanoparticles can be seen as a research barrier due to the adsorption, agglomeration, and viscous effects of the reaction mixture.

2. Introduction of lanthanum La as an effective element to adjust acidity of zeolites can maintain a certain amount of acid strength, and significantly improve the water-tolerance and thermal-stability of the zeolites. Yu et al. [70] studied the influence of lanthanum and cerium cations on the stability of Y zeolite. It was confirmed that the stability of Y zeolite was enhanced by the introduction of La. A strong interaction between the rare earth species and Y zeolite clusters was also found. Their small pore diameters limited the accessibility of acid sites to |5-1,4-glucans in cellulose, leading to a poor performance in cellulose hydrolysis. Therefore, synthesis of zeolites by combination of La modification and aperture increase maybe a good solution. The tolerance to hot-water is also an important factor in catalytic applications of zeolites. Therefore, the combination of above two proposed technologies is necessary to synthesize stable zeolites for cellulose hydrolysis.

Biomass can be thermally pretreated through either pyrolysis or torrefaction process. Pyrolysis involved breakdown of large complex hydrocarbon molecules of biomass into relatively smaller and simpler molecules of gas, liquid, and char [35]. Torrefac­tion involves heating of biomass in inert condition at temperature between 200 and 300 °C. Both pyrolysis and torrefaction removes moisture from the biomass followed by decomposition of biomass. Thermally pretreated biomass has better handling and fuel properties. One of the most common disadvantages of biomass compared to coal is the bulk amount of moisture that lowered its heating value and combustion performance.

17.2.2.1 Pyrolysis

Most researchers conducted pyrolysis in the absence of air or in the presence of medium such as water or hydrogen. Pyrolysis process in vacuum at medium heating rate, temperature of 400 °C, and residence time of 2-30 s will result in bio-oil as the product [4, 35]. Figure 17.1 shows the weight loss (%) curve obtained during the pyrolysis of dried oil palm fronds (OPF) under inert atmosphere at a heating rate of 40 °C/min. OPF has been selected as an example among generated waste from oil palm plantation in Malaysia, since it is easily available due to pruning of palm tree. According to this figure, pyrolysis curves of dried OPF follow the usual shape for most of the lignocellulosic materials [36]. During thermal degradation of dried OPF, two distinct pyrolysis zones are observed. Once the loss of water and volatile compounds

occurred, there is a sharp drop in the weight loss of the samples up to 385 °C. Thereafter, a slight change in the weight loss curves is observed, indicating the initiation of a second reaction zone. This zone is referred to the passive zone. Further loss of weight occurs until 800 °C due to devolatization process, after which there is essentially no further loss of weight. The analysis of the curve of the weight loss rate shows that during the active pyrolysis zone, two different peaks appear and therefore, two decomposition processes corresponding to the degradation of hemicellulose and cellulose are observed. At the end of this stage, a slower decrease of weight loss rate is observed. This loss corresponds to the slow degradation of lignin.

Biomass offers some advantages as a feedstock due to its high volatile content and high char reactivity. Compared to fossil fuels, however, biomass is a solid with a low heating value, containing much less carbon and more oxygen. In general, it is important to remove oxygen when producing fuel from biomass because high oxygen content results in low energy density. Biomass contains about 40-60 wt % oxygen compared to less than 1 wt % for fossil fuel. Pyrolysis can be used as a pretreatment process to reduce the oxygen content in biomass to lower levels compared to torrefaction depending on the operating condition.

Biomass pyrolysis yields three products: semi-char solid, liquid bio-oil (condens­able gases), and a non-condensable gaseous fraction. The non-condensable gaseous fraction is composed mainly of hydrogen, carbon dioxide, carbon monoxide, and methane. Following the biomass feedstock pyrolysis, the solid and liquid products of pyrolysis can be mixed and fed as slurry to the gasifier, which is advantageous for entrained bed reactors [8]. However, bio-oils mainly contain double carbon bound chemicals. These materials tend to polymerize and become increasingly viscous, which is detrimental to its stability during storage.

It is also important to note that the pyrolysis conditions have an effect on the reactivity of the char produced during gasification. Therefore, the proper selection of pyrolysis conditions is the key to ensure the gasification process will benefit [9].

As discussed in this section, there are several methods for biomass pretreatment, which can be combined for the benefit of the gasification process. Each method has its specific advantages as well as disadvantages, and the best selection depends on the feeder characteristics and gasifier type.

Jean-Remi Lanteigne, Jean-Philippe Laviolette and Jamal Chaouki

Abstract Bioreflneries are small integrated plants aiming at the recovery of specific biomass wastes via their conversion to high-value biofuels and chemicals. Pyrolysis is among the promising technologies to achieve this goal. Three major factors influ­ence the development of a pyrolysis process: the type of biomass, process operating conditions and choice of reactor technology. In this chapter, pyrolysis as a solution to sustain biorefineries is reviewed. The chapter first discusses the various biomass feedstocks and their important characteristics. Secondly, the pyrolysis concepts and kinetics are reviewed in light of their importance in process design and modelling. The chapter also discusses the influence of several process conditions and reactor technologies on pyrolysis reaction and pyrolysis products behaviour. Finally, strate­gies for product optimization and avoiding purity issues are analyzed. The emphasis of this chapter is put on technologies that have been developed at commercial scale.

Keywords Biomass ■ Pyrolysis ■ Biorefinery ■ Pre-treatments ■ Bio-oil ■ Bio-char ■ Kinetics ■ Hydrodynamics

11.1 Introduction

In the present day, various technologies are presented as feasible to sustain biorefiner­ies [1, 2]. Two main pathways are often highlighted: the thermochemical [3, 4] and the biochemical pathways [5]. Thermochemical pathways involve the decomposition of matter at high temperature in the absence (pyrolysis) or presence (gasification) of oxygen. On the other hand, single — and multi-step alcoholic fermentations are the main focus of biochemical process development and involve the digestion of matter by microorganisms.

The development of both thermochemical and biochemical processes faces many challenges. Cellulose fermentation processes are characterized by slow reaction rates and low overall yield for non-genetically modified microorganisms [5]. On the other hand, reaching high yield and selectivity remains an issue for both gasification and py­rolysis [4]. However, the thermochemical pathway offers a significant advantage over

J. Chaouki (H) ■ J.-R. Lanteigne ■ J.-P. Laviolette

Chemical Engineering Department, Ecole Polytechnique de Montreal, C. P. 6079, succ. Centre-Ville, Montreal, QC, Canada H3C 3A7, e-mail: jamal. chaouki@polymtl. ca

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

Green Energy and Technology,

DOI 10.1007/978-3-642-32735-3_11, © Springer-Verlag Berlin Heidelberg 2013 the biochemical processes: reaction rates are high and offer the potential for high product throughput, which is essential to develop a commercially viable industry. Nevertheless, there is an increasing interest in using both pathways in bioreflneries such that their respective advantages are exploited.

Gasification is a multi-step process in the context of biorefineries: it yields a synthesis gas rich in hydrogen and carbon monoxide that requires further synthesis to produce ‘biorefinables’ [6]. The second recombination process is performed at mild temperatures with patented catalysts [3, 6], and achieving high conversion as well as high selectivity remains a challenge to this day.

On the other hand, pyrolysis potentially offers interesting techno-economic advan­tages over gasification since it is a single-step process operating at lower temperature that yields three products: non-condensable gas, condensable gas (oil) and char [6]. Pyrolysis processes may therefore require significantly less process equipment compared to gasification. Produced from biomass pyrolysis, bio-char have direct applications as activated carbon [7]. Furthermore, bio-oil can be further refined to produce specialty chemicals and/or biofuels in dedicated plants (biorefineries) that this chapter will discuss in more detail.

Together with products’ market value, the operating scale also determines the fea­sibility of biomass pyrolysis and gasification pathways for biomass pre-treatments for biorefineries. It has been repeatedly demonstrated that gasification is sustainable at very large scale. However, considering that biomass availability is geographically limited, pyrolysis may be better suited for smaller distributed biorefineries. This chapter will discuss biomass pre-treatments for pyrolysis processes as well as pyrol­ysis as a pre-treatment for further biorefining. The pyrolysis process products and operability depend on several factors including (1) the type of biomass (chemical and physical characteristics), (2) the pyrolysis process operating conditions and (3) the type of reactor (gas/solid hydrodynamics and heat/mass transfer).