Flash pyrolysis involves heating of biomass rapidly to temperatures around 450-600°C at very high heating rates, in the absence of oxygen. The product obtained depends on the conditions of pyrolysis. Temperatures of around 500°C with high heating rates and short vapor residence times (1 s or 500 ms) maxi­mizes liquid yield at up to 80% on weight basis with minimum gas and char production, whereas very rapid heating to temperatures around 700°C and vapor residence times similar to the above maximize the gas yields (up to 80% on weight basis), with minimum liquid and char production. The liquid produced from the flash pyrolysis process has a relatively low viscosity (51 cp) and a high water miscibility capacity (up to 35-50% w/w water can be mixed). The char­acteristics of bio-oil obtained from flash pyrolysis process are given by Bridgewater [13]. The conversion efficiency of biomass conversion into crude oil in a flash pyrolysis process can reach up to 70%. However, the quality and stability of the oil produced as a result of pyrolysis is a major problem with the flash pyrolysis process as flash pyrolysis of biomass invariably results in the production of pyrolysis water [14]. The gaseous product obtained from flash pyrolysis has a low to medium heating value (5-15 MJ/Nm3). This gas has a relatively high oil content. It is either used as such for drying feedstock or as a fluidizing medium in fluid bed reactors (however, its specific energy content is somewhat low). The gas from high temperature flash pyrolysis can also give non­equilibrium products such as alkenes. However, the yields (of around 15%) are not very economical.

2.5.1 Updraught or Counter Current Gasifier

The oldest and simplest type of gasifier is the counter current or updraught gasifier where the air intake is at the bottom and the gas leaves at the top. The combustion reactions occur near the grate at the bottom, which are followed by reduction reactions somewhat higher up in the gasifier. In the upper part of the gasifier, heating and pyrolysis of the feedstock occur as a result of heat transfer by forced convection and radiation from the lower zones. The tars and volatiles produced during this process are carried in the gas stream. Ashes are removed from the bottom of the gasifier. The major advantages of this type of gasifier are its sim­plicity, high charcoal burn-out and internal heat exchange leading to low gas exit temperatures and high equipment efficiency, as well as the possibility of operation with many types of feedstock (sawdust, cereal hulls, etc.).

Major drawbacks result from the possibility of “channeling” in the equipment, which can lead to oxygen breakthrough and dangerous, explosive situations and the necessity to install automatic moving grates, as well as from the problems associated with disposal of the tar-containing condensates that result from the gas cleaning operations. The latter is of minor importance if the gas is used for direct heat applications, in which case the tars are simply burnt.

4.2.1 Cellulose and Lignin Composition in Biomass

Great variability in lignocellulosic biomass feedstocks is observed in wood or non­wood plants: differences in fiber dimensions, lignin, and cellulose content across different species [22]. Enzymatic hydrolysis of 1,100 natural Populus trichocarpa trees resulted in a wide range of sugar yields that depended on the lignin content and the ratio of syringyl and guaiacyl units in lignin. Among the 1,100 samples, the lignin content ranged from 15.7 to 27.9 wt%, while the syringyl-to-guaiacyl unit ratio ranged from 1.0 to 3.0 [23]. Even in the same plant, differences were observed between the mature sections at the base and the younger sections at the top [24].

Due to this great diversity of chemical composition and the complex structure of native biomass, effective methods for the dissolution or hydrolysis of purified

Aromatic products

Fig. 4.1 Generalized chemical structure of lignin and schematic for its conversion into monomeric aromatic products. Reactions which cleave aryl-ethers and aryl-alkyl linkages would enable conversion of lignin into valuable aromatic chemicals. Reprinted from [28], copyright (2011), with permission from Elsevier cellulose or glucose oligomers can fail to translate to native biomass. In lignin, each type of linkages in the constituting monolignols provides a possible pathway for biomass delignification (Fig. 4.1) [25-28]. Developing a unique IL pretreat­ment that would be suitable for multiple feedstocks represents a tremendous challenge.

Ali Sinag

5.1 Thermochemical Biomass Conversion

Thermochemical biomass conversion methods can be divided into three main groups as combustion, gasification and Pyrolysis (Fig. 5.1). Combustion is thermal conversion of organic matter with an oxidant to produce mainly carbon dioxide and water. Combustion of biomass is the most direct and technically easiest process. However, the overall efficiency of generating heat from biomass energy is low. Gasification of biomass provides power generation for the technical appli­cations needing energy. The process generates valuable gaseous products (CO, CO2, H2O, CH4, C2—C6) and char depending on the design and operating conditions of the gasification reactor. Pyrolysis is thermal heating of the materials in the absence of oxygen, which results in the production of three categories: gases, pyrolytic oil and char [1-3]. Pyrolytic oil, also known as ‘‘tar or bio-oil’’, cannot be used as transportation fuels directly due to the high oxygen (40-50 wt%) and water contents (15-30 wt%) and also low H/C ratios. However, pyrolytic oil is viscous, corrosive, relatively unstable and chemically very complex [4-6].

The main advantages of these methods for biomass conversion over other conversion methods such as biochemical conversion technologies are the feed­stock used. All plant-based residues can be converted into value-added products such as transportation fuels (diesel), hydrogen, methane, syngas and chemicals [7]. However, the undesirable products like alkali compounds and the cost of cleaning the gaseous products and drying of biomass are the major problems. There are many attempts such as catalyst usage, co-firing of biomass with coal in order to improve product quality and the optimization of the experimental conditions.

A. Sinag (H)

Department of Chemistry, Science Faculty, Ankara University, Bejevler-Ankara 06100, Turkey e-mail: sinag@science. ankara. edu. tr

C. Baskar et al. (eds.), Biomass Conversion,

DOI: 10.1007/978-3-642-28418-2_5, © Springer-Verlag Berlin Heidelberg 2012

This chapter chiefly deals with the role of catalysts during thermochemical biomass conversion since usage of different types of catalysts for this conversion leads to the tar and oxygen removal, increasing calorific value of the products and reduction in the amount of undesirable contaminants. The effects of new types of nanocatalysts together with known types of catalysts on the process conditions and the product quality will also be discussed.

Phytohormones control every aspect of plant growth and development, including seed germination, seedling growth, branching, plant height, flowering, seed development and senescence. A few major phytohormones and their roles in regulating plant growth and development are listed in Table 8.2.

Table 8.2 Selected phytohormones and the growth and developmental responses influenced by them

Growth/developmental responses

Maintenance of meristem identity in shoot and root apical meristems, organogenesis of leaves, flowers, floral organs and lateral roots Seed germination, leaf expansion, induction of flowering, flower development and seed development Seed germination, root and shoot development and senescence Cell expansion, vascular differentiation, reproductive development, leaf inclination

Seeds germination, hypocotyl growth and shoot branching

Also, phytohormones such as auxins, gibberellins, cytokinins and ethylene can modify fiber and wood formation during growth [22]. Auxin is required for cell division and axial plant growth and it helps to enforce apical dominance (where the shoot tip exerts inhibitory action on the axillary bud outgrowth). The primary site of biosynthesis of auxins is at the shoot tip. It is transported basipetally to other parts of the plant via an elaborate transport mechanism involving a number of members of the PIN family of proteins [23]. The involvement of auxins and cytokinins is proven in organ development and controlling organ size. Cytokinins help to break apical dominance and promote the outgrowth of lateral shoots. Hence, interactions of auxin and cytokinin control the shoot branching in plants [24, 25]. More recently, another phytohormone strigolactones has been shown to be necessary to inhibit shoot branching, and mutants in the biosynthesis pathway exhibit plants with more branches [26]. Increased gibberellin biosynthesis by ectopically expressing AtGA20ox promotes growth rate and biomass increase in hybrid aspen [27] and tobacco [28]. Furthermore, in a recent study silencing of AtGA2ox homolog in tobacco was demonstrated to enhance plant biomass [29].

The effect of phytohormones can be examined from the biosynthesis and their biological actions. Thus, plants exhibiting wide variations in structure have been observed when key genes involved in phytohormone biosynthesis and signaling have been mutated. The classic examples of gibberellin-deficient plants (e. g., Arabidopsis ga1-3 mutant; [30]) showing extreme dwarfism is a good illustration of the importance of this hormone in regulating plant architecture. This mutant arose from a deletion in the ent-kaurene synthase enzyme that catalyzes an early step in gibberellic acid biosynthesis. However, it retains the ability to respond to exogenously added gibberellins to grow to normal size. Mutants in other phyto­hormone biosynthetic pathways are also known to result in similarly striking changes in plant morphology.

The discovery of specific receptors for the different phytohormones and their elaborate signaling pathways [31-33] is another area of interest for this discussion. The signaling cascade for cytokinins involves sequential phosphorylation and activation of intermediate proteins [34]. There are generally multiple receptors and intermediate proteins for the phytohormones. Thus, for cytokinin signaling, more than three receptors, five phosphotransfer proteins (cytoplasm to nucleus shunting)

Downstream target genes regulating shoot and root development branching etc.

Fig. 8.1 Schematic representation of cytokinin signal transduction pathway (based on [34, 37]). This is an example of the signaling intermediates of one of the phytohormones. Similarly, the signaling pathways of other phytohormones have many intermediates, genes for which can be the targets of biotechnological improvements of biomass yield in selected plants. AHK2, 3, 4 Arabidopsis Histidine Kinase2, 3, 4 are cytokinin receptors on cell membranes. Dimers of the receptors bind cytokinins such as zeatin. AHP Arabidopsis Histidine Phosphotransfer proteins serve as phosphate shuttle from the cytoplasm to nucleus. ARR Arabidopsis Response Regulator proteins are the response regulators that affect the transcription of downstream target genes that are activated by cytokinins and over 20 response regulator proteins are known (Fig. 8.1). Similarly, auxin signaling cascade has multiple receptors and effector proteins [33]. Another major aspect of phytohormone signaling is the crosstalk between different phytohor­mones [35], which adds a new dimension of control of plant development by this group of rather simple chemical molecules. Mutants in various intermediates along the signaling pathway can lead to interesting agronomic traits such as altered organ size, altered branching and overall changes to plant architecture. Thus we observed that suppression of AtHOGl expression, which is a putative cytokinin signaling intermediate, leads to enhanced branching in Arabidopsis and petunia [36]. It is not our intention to review phytohormone signaling in detail here, but this brief description is used to illustrate the genetic complexity of phytohormone signaling. Therefore, the various intermediates of the phytohormone signaling pathways may be explored as targets for genetic modification to achieve desired plant architecture.

The term ‘fermentation’ is derived from the Latin verb fervere, to boil, thus describing the appearance of the action of yeast on extracts of fruit or malted grain. The appearance of boiling is due to the production of carbon dioxide bubbles caused by the anaerobic catabolism of the sugars present in the extract. However, fermentation has different meanings according to biochemists and to industrial microbiologists. Biochemically, it relates to the generation of energy by the catabolism of organic compounds, whereas its meaning in industrial microbiology tends to be much broader.

In alcoholic fermentation, the substrates that are mainly sugars are fermented, with ethanol as the main product. It is widely distributed among microorganisms. Even plants switch to this pathway for a short period under anaerobic conditions. However, the yeast cell, especially the species of Saccharomyces is the main alcohol producer. Some bacteria, particularly Z. mobilis, which only utilize hex — oses, can also produce ethanol from glucose [150]. In other bacteria, the alcohol is not a predominant end product. Certain yeasts including S. cerevisiae can also ferment pentose sugar, xylose to ethanol though the yield is lower compared to the fermentation of hexoses. The production of alcohol by the action of yeast on malt or fruit extracts has been carried out on a large scale for many years and was the first ‘industrial’ process for the production of a microbial metabolite. Thus, industrial microbiologists have extended the term fermentation to describe any process for the production of a product by the mass culture of a microorganism. It may be noted that the fermentation equipment makes upto 10-25% of the total fixed capital cost of an ethanol plant depending upon its design.

Bio-based products can be classified into three categories: Biofuels (biodiesel, bioethanol, biogas), bioenergy (heat and power), and bio-based chemicals and materials (fine chemicals, cosmetics, polymers, plastics, composites). In a biore­finery, the biomass conversion processes are integrated in such a manner that almost all types of feedstocks can be converted to the above-mentioned products. Biofuels are produced in higher volumes and are responsible for increasing the carbon credits of the industry, whereas other products such as fine chemicals, pharmaceuticals, and polymers are produced in comparatively lower quantities, but, being high value products, increase the profitability of the biorefinery.

Biomass processing in a biorefinery involves two major transformations [3] the first transformation involves a bulk separation or extraction of the biomass using processes such as grinding, followed by fractionation or cracking by biological or physicochemical techniques. This step leads to release of such molecules from the biomass which are capable of undergoing second transformation involving pro­cesses such as fictionalization to yield a variety of molecules. These transforma­tions give rise to a large number of bio-based products in which the most important one can be considered to be biofuels/energy (Fig. 1.21).

Integrated biorefineries employ various combinations of feedstocks and con­version technologies to produce a variety of products, with the main focus on producing biofuels. Side products can include chemicals (or other materials) and heat and power.

This section focuses only on the energy aspects of biorefinery, though, other valuable products are also available from biorefinery [36, 37].

The above factors illustrate that biomass feed preparation is very important and forms an integral part of the briquetting process. The unit operations of the piston press and the screw press are similar except where the latest development in screw press technology has been adopted, i. e., where a preheating system has been incorporated to preheat the raw material for briquetting to give better performance commercially and economically to suit local conditions. In the present piston press operating briquetting plants, the biomass is briquetted after pre-processing the raw material but no preheating is carried out. Depending upon the type of biomass, three processes are generally required involving the following steps:

A. Sieving—Drying—Preheating—Densification—Cooling—Packing

B. Sieving—Crushing—Preheating—Densification—Cooling—Packing

C. Drying—Crushing—Preheating—Densification—Cooling—Packing

When sawdust is used, process A is adopted. Process B is for agro- and mill — residues which are normally dry. These materials are coffee husk, rice husk, groundnut shells, etc. Process C is for materials like bagasse, coir pith (which needs sieving), mustard and other cereal stalks.

Few studies have directly compared the efficiency of IL pretreatments to other pretreatments, such as the ammonia or organosolv pretreatment. Rice straw particles were pretreated with [EMIM][OAc] (1 g biomass in 20 ml of [EMI- M][OAc] at 130°C for 24 h) or ammonia (1 g biomass in 10 ml of 10 vol.% ammonia at 100°C for 6 h). In these conditions, the amount of cellulose regen­erated was comparable for the two pretreatments. However, the conversion rate of cellulose to glucose was significantly higher with the IL pretreatment and the improvement due to IL was most remarkable for larger particles (>10 mm) [46].

In another study, switchgrass was subjected to an acid pretreatment (3 wt% biomass in 1.2% sulfuric acid heated at 160°C for 20 min) or an IL pretreatment with [EMIM][OAc] (3 wt% biomass heated at 160°C for 3 h). Analysis of the recovered biomass after IL pretreatment showed lower lignin content and higher hemicellulose content, compared to the recovered biomass after acid pretreatment. X-ray diffraction measurement of the cellulose crystallinity showed a significant decrease in crystallinity after IL pretreatment, whereas the acid pretreatment caused an increase in crystallinity, which was attributed to the preferential breakdown of the amorphous cellulose during the acid pretreatment. Scanning electron microscopy showed that the cell wall structure was mostly preserved during the acid pretreatment, while the IL pretreatment left no fibrous structure. For the same enzyme loading, the enzymatic hydrolysis had faster kinetics and higher reducing sugar yields after the IL pretreatment. After a 24 h saccharifica­tion process, 96% of the cellulose was hydrolyzed for the IL-pretreated sample, while only 48% were hydrolyzed for the acid-pretreated sample [41].

Catalytic HDO over sulfided Ni-Mo/y-Al2O3 and Co-Mo/y-Al2O3 catalysts was proven to be a complex reaction [14], in which not only active metal is responsible for conversion of fatty acids and their derivatives, but also alumina support and sulfur compound exhibit activity as well.

The reaction pathway and mechanism for transformation of fatty acids esters over sulfided catalysts was proposed by [14]. Figure 6.2a shows a complete

pathway of reactions that can take place over sulfided catalyst. The fatty acid esters can be transformed to fatty acids by hydrolysis and thereafter converted directly to hydrocarbons containing one less carbon number by decarboxylation/decarbony — lation or transform indirectly via aldehyde as an intermediate. Hydrocarbons with the same carbon number can be produced by reduction of fatty acids via aldehyde and alcohol. Separately, esterification of the fatty acids and their alcohols can occur, forming esters with the same number of carbons in the chain on both sides of the ester group.

Apart from elucidating deoxygenation pathways of fatty acid esters over Ni-Mo and Co-Mo catalysts, influence of sulfur on the reaction mechanism was also shown [14]. Dehydration reaction can be catalyzed by acid. Therefore, it was proposed that, in conversion of alcohols to alkenes, elimination reactions (E1, E2) can occur (Fig. 6.2b). Although elimination reaction E1 is unlikely to occur, elimination reaction E2 is the plausible mechanism which produces alkenes. The supports-catalyzed acid reactions were, however, excluded because in the experi­ment using only y-Al2O3 no hydrocarbons were formed using fatty acids esters as a feedstock at 300°C and under 1.5 MPa hydrogen pressure. Fatty acids and alcohols originating from the transformation of esters were the only products found [11].

The elimination reaction does not explain the presence of sulfur compounds in the reaction mixture. Hence mechanism of substitution was proposed (Fig. 6.2c). Although substitution of SN1 is unlikely to occur due to the unstable carbenium ion as an intermediate, SN2 substitution occurs which is confirmed by the formation of thiol compounds [14].

The role of sulfur in the deoxygenation mechanism over sulfided Ni-Mo and Co-Mo catalysts is still unclear. Although elimination and substitution reactions can explain the formation of sulfur compounds and n-carbon alkenes, by dehy­dration of alcohols, the role of reactions on overall conversion has not yet been clarified. However, effect of the addition of the sulfur can be stated [12]. With increasing content of H2S in the reaction, the following was observed:

• increase of the conversion of aliphatic esters to hydrocarbons

• increase of selectivity toward decarboxylation/decarbonylation products

• increase of unsaturation of hydrocarbon products

To avoid sulfidation of the catalyst and addition of the sulfur to the feedstock, metal nitrides catalysts supported on alumina were tested for the deoxygenation of oleic acid and canola oil [15]. Although oxygen removal from canola oil in a continuous reactor, at 400°C and 8.35 MPa hydrogen pressure, reached 90% level, the yield of hydrocarbons which can be used as diesel fuel did not exceed 50/100 g of oleic acid.

When investigated separately Ni, Mo and Ni-Mo/y-Al2O3 catalysts, it was found that decarboxylation/decarbonylation reaction occur at the nickel surface, where reduction of the carboxylic group occurs on the Mo sites [16]. Therefore, Ni-Mo alloy is selective toward decarboxylation/decarbonylation depending on the proportions between the active metals. When the proportion of Ni to total amount of Ni-Mo on y-Al2O3 was varied between 0.2 and 0.4, the selectivity to

hydrogenation of carboxylic group was in the range of 60-80% (at 100% con­version, temperature between 260 and 2800C and under 3.5 MPa of hydrogen pressure). Although Ni and Mo on c-Al2O3 show lower conversion of triglycerides than Ni-Mo catalysts, their selectivity within the hydrocarbon products was close to 100% for Ni to decarboxylation/decarbonylation reaction and for Mo to hydrogenation of carboxylic group.

HDO of rapeseed oil has been recently studied over sulfided Ni-Mo catalyst using mesoporous alumina as a support. When compared to y-Al2O3, the sulfided Ni-Mo catalyst with mesoporous alumina structure showed higher activity in temperature range of 260-280°C. The catalyst with mesoporous support outper­formed the catalyst with micropores alumina by 50% yield at 260°C [17]. These results can be explained by good accessibility of long-chain fatty acids to the active sites of Ni-Mo catalyst.

Influence of hydrogen pressure and volume ratio between hydrogen and sun­flower oil was investigated over a non-sulfifed Co-Mo/Al2O3 catalyst in the temperature range between 320 and 380°C [18]. The results from the experiments indicate that with an increase of temperature, an increase in conversion and increase of ratio, between decarboxylation/decarbonylation and hydrogenation of carboxylic group, occur. On the other hand, an increase of hydrogen pressure favors hydrogenation reactions of triglycerides as well as slightly increases the conversion level. The results of experiments with different volume ratio between hydrogen and sunflower oil indicate that the highest conversion of sunflower oil was obtained at the ratio between 400 and 600 Nm3/m3.