A reduction in digestion plant size can be obtained by increasing the solids content. The amount of work in transporting and spreading the digestate is also reduced.

Ong et al. (2000) obtained an increase in gas yield compared to a continuously fed and stirred reactor in a continuously fed nonstirred reactor with the outlet in the middle. It seems that some solids removal from the bottom has to take place, as accumulation of inert solids will reduce the effective volume of the reactor.

Shyam (2001) demonstrated that cow manure can be digested at 18% total solids. The method increases the gas yield with 40% using practically the same equip­ment as before.

LOADING AND UNLOADING
OF DIGESTERS

Manure-based (wet) digesters use pumps for loading. Straw is mixed with digestate and pumped in the digester. An alternative is using an auger and pushing the straw to below the liquid level in the digester. Heavy solids are removed from the bottom of the digester using a rotating rake and pump. Straw disintegrates during digestion and is pumped off together with the digested manure.

In the scum layer plant straw can be loaded with front loaders using an inlet shaft with an outlet to below the liquid level. Shredded straw can also be blown into the inlet shaft. Solids are removed in the same way as in wet digesters. The remaining digestate is pumped out. Plug flow dry digesters are loaded and unloaded with augers.

Leachate recirculation digesters are operated in batch mode. The digesters are opened and the digestate is unloaded with front loaders. Capacity of these digesters is relatively low due to the loading procedure.

b-(1 / 3, 1 / 4)-glucans (1314Gs) consist of a linear chain of b-D-glucopyranosyl units linked by (1 / 3) and (1 / 4) bonds (Table 17.1). 1314Gs are present in Poaceae (grasses and cereals) as well as in eq — uisetum, liveworts and Charophytes. The mixed-linkage glucans are dominated by cellotriosyl and cellotetrasyl units linked by b-(1 / 3) linkages, but longer b-(1 / 4)-linked segments also occur. Cellulose is also b-D-glucan, which is linked by (1 / 4)-glycosidic bonds, and thus cellulose has high stiffness (crystal­linity) and is insoluble in most solvents. Contrary to cellulose the b-(1 / 3) linkages existing in 1314Gs make glucans flexible and soluble (Peng et al., 2012).

Complex Heteroxylans

Complex heteroxylans are present in cereals, seeds, gum exudates and mucilages and they are structurally more complex. In this case the b-(1,4)-D-xylopyranose backbone is decorated with single uronic acid and arabi — nosyl residues and also various mono — and oligoglycosyl side chains (Girio et al., 2010).

Conclusions on Carbohydrate Feedstocks

Storage carbohydrates are uniform in composition and relatively easily to isolate and purify. Therefore many fermentative and catalytic processes have identi­fied these feedstocks as their initial feedstock of choice. Because of costs and societal debates (food versus fuel and indirect land use debates) many researchers both from industry and academia are investigating the use of lignocellulose as feedstock. Pure cellulose has the same advantages as starch in that it is only build up from glucose and relatively easy to hydrolyze (although much more difficult than starch) when pure (and amor­phous). However, to make use of lignocellulose econom­ically also the hemicellulose needs to be used. This overview clearly shows that due to the heterogeneity of the monosaccharides incorporated and large diversity in linkages and side groups, both enzymatic hydrolysis system as well as a catalytic/fermentative conversion system needs to be quite robust to make optimal use of the cellulose and hemicellulose fractions.

LIGNIN

In addition to carbohydrates the major component of lignocelullulosic biomass is lignin. Lignins are major structural components of higher plants, and confer to woody biomass its mechanical structure and resistance to environmental stress and microbial decay. Lignin, the name of which is derived from the Latin word for "wood", accounts for 15—30 wt% of woody biomass and it is also available from agricultural residues such as straw, grass and bagasse.

Lignins are built in plants starting from three basic monolignols via oxidative phenolic coupling reactions to generate the three-dimensional lignin polymer (Ralph et al., 2007). The heterogeneity of lignin polymers exists in molecular composition and linkage types between
the phenylpropane monomers, p-hydroxyphenyl — (H), guaiacyl — (G), and syringyl- (S) units. These are derived from the monolignols sinapyl-, coniferyl-, and coumaryl alcohol, respectively (Table 17.1). Lignin composition will be different not only between species, but also between different tissues of an individual plant. In soft­wood lignin coniferyl alcohol is the predominant build­ing unit (over 95% guaiacyl structural elements), while in hardwoods (and dicotyl fiber crops) the ratio coniferyl/synapyl shows considerable variation. In lignins of cereal straws and grasses the presence of cou — maryl alcohol leading to p-hydroxyphenylpropane struc­tures is typical. The lignin content ranges (Table 17.1) and chemical structures of the three primary building blocks in lignocellulosic biomass are given in Table 17.3 and the occurrence and type of different interunit linkages in Table 17.4.

For the production of aromatic chemicals from bio­refinery lignin selection of suitable resources can be made on the occurrence of building blocks and interunit linkages next to the choice of pretreatment and isolation procedure. The presence of one or two methoxyl groups at the ring may for the production of some chemicals (e. g. guaiacol, syringol) be a requirement. For the con­version of lignin into benzene, toluene, xylene (BTX) or phenol the presence of coumaryl units (H-units) may be an advantage due to the lack of side groups next to the aromatic phenolic group (Table 17.3).

The complex structure of (isolated) lignins needs suit­able characterization methods and an ongoing effort for improvement of these methods have been performed during the last decades. However, results of these analytical procedures are not always consistent.

Methods for lignin characterization can be found in the literature (Gosselink et al., 2004; Baumberger et al., 2007; Tejado et al., 2007; Monteil-Rivera et al.,

2013) and via the International Lignin Institute (www. ili-lignin. com). Two-dimensional nuclear magnetic resonance and pyrolysis gas chromatography mass spectrometry are now established analytical techniques for detailed structural lignin analysis (Table 17.4).

From a chemical perspective, lignins are highly com­plex polyphenolic biopolymers with aromatic units in different configurations. Lignins are traditionally produced in the pulp and paper mills by extracting lignin upon liberation of cellulosic fibers used for paper making. Most kraft lignins are burnt within paper mills to generate heat and power, thus providing energy autonomy and lowered operating costs. The majority of lignosulfonates are used as additives in the building sector, where they provide plasticity and flowability to concrete. Lignosulfonates are also used as binders in animal feed, in road building, oil well drilling and as dispersants and coatings in pesticides used for agricul­ture applications. Sulfur-free lignins derived from soda pulping of annual plants such as grass and wheat straw are produced commercially and used among others in wood adhesives and in animal feed. More recently, biorefinery lignins are produced in so-called biorefinery or fractionation processes, for example for the manufacturing of cellulosic bioethanol. This side stream is for the short term used primarily as energy source, but for the medium to longer term utilization of these lignins for the production of biofuels, aromatic chemicals and materials are expected. So far limited industrial use of technical lignins is seen mainly due to the easy use as

TABLE 17.3 Lignin Content and Chemical Structures of Lignocellulosic Biomass

Source: Azadi et al., 2013.

TABLE 17.4 Frequencies of Different Interunit Linkage Types in Native Softwood and Hardwood Lignin per 100 C9 units

Name Structure Softwood Hardwood

Source: Henriksson et at, 2010.

energy source, the impurities in technical lignin sources, tendency to form condensed structures, inferior perfor­mance compared to synthetic compounds, unique reac­tivity, lack of availability of high-purity lignins and a large variety of different types of lignins (Vishtal and Kraslawski, 2011). Additionally, traditional (heteroge­neous) catalysts work inferior to biorefinery lignin and need to be redesigned (Zakzeski et al., 2010).

Upon depletion of fossil resources, the production of aromatic chemicals from these resources will come under stress. As lignin is by far the most abundant aro­matic renewable resource on earth, lignin is the only resource that could fulfill the quantities needed for the substitution of the main aromatic compounds used in in­dustry (Holladay et al., 2007). These are phenol, BTX, and terephthalic acid (Van Haveren et al., 2008). The annual global production of these largest aromatic chemicals is estimated at 103 million metric tons total, with benzene at 44, toluene at 22, and xylenes at 37 million metric tons (Nexant Chem. Systems, 2012).

With an estimated global biomass production of about 150 billion tons annually about 30 billion tons of lignin is generated each year globally (Balat and Ayar, 2005). This amount of lignin is far exceeding the need for the conver­sion to aromatic chemicals even at low conversion de­grees of about 10%. Currently there is a strong desire from major brand owners (e. g. Coca Cola, Pepsi, Heinz) to "green" their product portfolio by using biobased poly­mer building blocks. Lignin could play in the future an important role as a biobased feedstock. However, there are quite some challenges to overcome for the develop­ment of an economically viable process for the production of aromatic chemicals from lignin (Gosselink, 2011).

The traditional lignin applications include, primarily, lignin uses where lignin plays a replacement role for a relatively low-value chemical or material. As it was mentioned earlier, the largest current use of industrial lignin is fuel. However, there are a number of matured industrial lignin applications, constituting the bulk of the higher value lignin commercial chemical market, and this includes, primarily, lignosulfonates or sulfo — nated kraft lignins in large-medium-size markets such as additives for concrete admixtures, dust control, feed and food additives, dispersants, resin and binder com­positions, and oil well drilling. Examples of smaller mar­kets for technical lignins include carbon black, emulsifiers, water treatment, cleaning chemicals, leather tanning, battery expanders, and rubber additives (Higson, 2011). There are many comprehensive reviews in the literature which cover traditional lignin applica­tions (ACS, 1999; Holladay et al., 2007); therefore, we will not discuss them here in detail.

Cyanobacteria have different life forms: some species are unicellular, others form colonies and filaments, or live in symbiosis with eukaryotic organisms. Accordingly,
the protection of O2 sensitive enzymes from photosyn — hetically evolved oxygen has evolved through several different strategies.

In the absence of combined nitrogen, many filamen­tous N2-fixing cyanobacteria physically separate oxygenic photosynthesis and N2-fixing enzymes by differentiating specialized heterocyst cells, which are regularly spaced among vegetative cells. Mature hetero­cysts are unique cells providing a microaerobic environ­ment suitable for the enzymes involved in N2 fixation. The microaerobic environment inside of heterocysts is maintained by an elevated rate of respiration, lack of active PSII complexes resulting in an absence of photo­synthetic O2 evolution, and a thick cell wall (Wolk et al., 1994). These two cell types, the vegetative cells and heterocysts, depend on each other. During diazotro­phic growth the vegetative cells perform photosynthetic CO2 fixation and provide the heterocysts with organic carbon intermediates, like sucrose (Lopez-Igual et al.,

2010) , whereas heterocysts provide vegetative cells with fixed nitrogen required for cell growth (Figure 21.2). Since H2 production in heterocysts depends on carbohy­drates produced in vegetative cells, this process of H2 production has been classified as indirect water biophotolysis.

Heterocyst differentiation is tightly regulated by NtcA, a global transcription factor of carbon and nitro­gen metabolism (Zhao et al., 2010). HetR is another essential protein specifically involved in the initial steps of heterocyst development. The patterned differentia­tion of heterocysts is controlled by the ratio of activator, HetR, and suppressor molecules, peptides derived from PetS and HetN (Muro-Pastor and Hess, 2012; Risser and Callahan, 2009). For the heterocystous strain, Anabaena PCC 7120 the frequency of heterocysts is approximately 10% under optimal laboratory growth conditions. Such a low frequency of heterocysts in the filament might result in modest yields of net H2 production. Thus, one possible strategy to improve H2 production is to increase the number of heterocysts in the filaments. However, although the overexpression of HetR resulted in an over­all enhancement of heterocyst frequency up to 29% in

-(oxygenic photosynthesis)

Heterocysts

© [psii PSI

fixatioi

Anabaena PCC 7120 mutant, no increase in the nitroge — nase activity of the filaments took place (Buikema and Haselkorn, 2001). It is possible that the relative decrease in the number of vegetative cells makes them incapable of producing enough reducing power to be transferred to heterocysts for enhanced H2 production.

Unicellular and filamentous nonheterocystous N2-fixing cyanobacteria apply mostly temporal separa­tion mechanism, by performing photosynthesis during the daytime and N2 fixation at night (Compaore and Stal, 2010). The energy generated by photosynthesis is stored in glycogen granules, which are later subjected to oxidative breakdown.

Trichodesmium are unique cyanobacteria, because these filamentous nonheterocystous cyanobacteria are able to fix N2 simultaneously with oxygenic photosynthesis during the photoperiod (Berman-Frank et al., 2001). Nitrogenase is localized in subsets of cells in each trichome, which also contain photosynthetic complexes. During hours of high N2 fixation the cells can turn photo­synthetic activity down within 10 min, which is observed as unequally distributed inactive zones in whole fila­ments. Importantly, the PSII activity was shown to be essential for N2 fixation in Trichodesmium (Berman-Frank et al., 2001). According to the authors, Trichodesmium utilizes photosynthetic electron transport to support N2 fixation and concomitantly enhances the Mehler reaction, which efficiently eliminates the evolved O2. Recently published complete genome sequence of Trichodesmium erythraeum (http://www. ncbi. nlm. nih. gov) shows that the strain indeed possesses the genes encoding the Flv1 and Flv3 proteins, which are involved in the "Mehler — like" reaction in cyanobacteria. Thus, the nitrogenase enzyme in this organism is protected from O2 by a com­bined and modulated temporal and spatial segregation of N2 fixation and oxygenic photosynthesis within indi­vidual cells (Berman-Frank et al., 2001).

The N2-fixing, unicellular cyanobacteria Cyanothece has recently attracted lots of research interest as a highly efficient H2 producer under natural aerobic conditions. Cyanothece sp. ATCC 51142 is the best hydrogen pro­ducer among the known wild-type cyanobacterial strains (Bandyopadhyay et al., 2010). Also, the ability to grow phototrophically, mixotrophically, and hetero- trophically makes this strain an attractive organism for biotechnology. Cyanothece demonstrates temporal sepa­ration of oxygenic photosynthesis and N2 fixation by performing photosynthesis in daytime and N2 fixation at night. Moreover, these alternating processes are regu­lated by an intrinsic circadian rhythm. The genome sequence reveals the presence of the bidirectional [Ni—Fe]-hydrogenase, uptake hydrogenase, and the conventional Mo—Fe nitrogenase. Diazotrophically grown Cyanothece cells entrained in 12-h light/12-h dark cycles exhibit a light-induced H2 production (specific rate >150—300 mmol H2 mg/Chl h) under aero­bic conditions during "subject dark" (Bandyopadhyay et al., 2010). Interestingly, the robust circadian rhythm of Cyanothece allows cells to fix N2 and produce H2 at reasonably high rates even when grown under contin­uous light (Min and Sherman, 2010).

In the presence of combined nitrogen, Cyanothece pro­duces H2 at very low rates, 2—10 mmol H2 mg/Chl h. This H2 production is catalyzed by the bidirectional hydrogenase and is dependent on PSII activity. In diaz — otrophically grown cultures, the production of H2 is driven by the nitrogenase enzyme and the activity of the enzyme is linked to PSI and respiratory electron flow (Min and Sherman, 2010). Moreover, the rates of H2 production in Cyanothece 51142 could be greatly enhanced when cells were grown in the presence of additional carbon sources, as observed in cultures sup­plemented with high concentrations of CO2 or glycerol (Bandyopadhyay et al., 2010). Photoproduction of H2 can be significantly enhanced by increasing reductant availability via dark anaerobic preincubation. This indi­cates the tight coupling of H2 photoproduction to the dark, anaerobic metabolism (Skizim et al., 2012).

Recently, it was reported that Cyanothece can copro­duce H2 and O2 over 100 h under continuous illumina­tion and uninterrupted photosynthetic electron transport (Melnicki et al., 2012). Of course, Cyanothece has a very flexible metabolism and the existence of intra­cellular O2 gradient within the cells cannot be excluded. Despite many interesting papers describing the H2 production in Cyanothece, the molecular mechanisms behind the regulation of the nitrogenase and protection against oxygenic photosynthesis are still under debate.

The term "food waste" covers the wastes (and by­product streams) that are generated during the whole food supply chain starting from production of the raw material followed by the processing into edible products by the food industry and the final disposal by con­sumers, restaurants or catering services. Valorizing the waste derived from the food industry sector would result in the creation of novel biorefineries leading to restructured and advanced industrial plants that will not only satisfy the traditional market of food produc­tion but also other markets that are nowadays depen­dent on petroleum to provide the necessary feedstocks. Food processing waste streams constitute renewable re­sources enriched in carbohydrates, protein, oils and fats, phenolic compounds and various micronutrients.

PHA Production from Winery By-Products

Wine production constitutes an important industrial sector in many countries around the world, such as the South European countries, United States, Chile and Australia. Wine making generates both solid and liquid by-products. Residues from wine production involve mainly trimming wastes, grape stalk, grape pomace or marc, wine lees and winery wastewater. These by­products are currently supplied to ethanol distilleries (e. g. in the case of wine lees), used (if possible) as fertil­izers or processed as wastes in order to reduce the envi­ronmental impact caused by their disposal to the environment. However, given the fact that environmental policies are changing, new practices should be applied aiming at valorization of winery by-product streams.

Ongoing research focuses on valorization of residues from wine making. Trimming wastes are rich in cellu­lose, hemicellulose and lignin. Combined thermochem­ical treatment with enzymatic hydrolysis can be applied to convert cellulose and hemicelluloses into C5 and C6 sugars that can be assimilated by microorgan­isms. Delignification steps are usually required since the complex structure of lignin prevents hydrolysis of polysaccharide. Bustos et al. (2005) evaluated the use of trimming wastes and wine lees aiming at the produc­tion of lactic acid through simultaneous saccharification and fermentation carried out by Lactobacillus rhamnosus. Trimming wastes could be also used as solid support in solid-state fermentations for the production of various enzymes (Sanchez et al., 2002).

Grape pomace or marc is the solid fraction remaining after the extraction and it consists of skins, pulp, seeds and stems of grapes. Research has focused on efficientutilization of this waste stream, since it contains ligno — cellulosic fractions that can be hydrolyzed and further used in microbial bioconversions. Solid-state fermenta­tion for production of hydrolytic enzymes has also been reported using grape marc as solid support (Botella et al., 2005).

Wine lees is the remaining residue after the end of the fermentation stage. It is a rich source of ethanol, tartaric acid, phenolic compounds and yeast cells. Wine lees can be used for the production of potable alcohol (wine lees mainly produced by large wineries), as nutrient supple­ment for fermentation (Bustos et al., 2004; Salgado et al., 2010), for the production of tartaric acid (Versari et al., 2001; Rivas et al., 2006) and as raw material for compost­ing (Diaz et al., 2002; Nogales et al., 2005). A novel pro­cess has been developed at the Agricultural University of Athens targeting the creation of a novel biorefinery concept based on wine lees valorization (Figure 24.4). The process starts with centrifugation or filtration of wine lees in order to separate the liquid stream that can be used for ethanol production via distillation. The ethanol produced can be used as potable or fuel ethanol depending on the purity. Current processes produce potable ethanol. Ethanol could be also used as a plat­form chemical to supply the future sustainable chemical industry. Alternatively, ethanol could be also utilized as carbon source for microbial fermentation aiming to PHB production by the bacterial strain C. necator NCIMB 12080 (Senior et al., 1986). This, however, may not be a cost-competitive alternative when compared to the traditional potable ethanol market. The remaining liquid after ethanol extraction can be used in subsequent hydrolysis stages to increase the presence of nutrients.

The solid fraction that remains after centrifugation of wine lees contains phenolic compounds with antioxi­dant properties, tartrate salts and yeast cells. A phenolic-rich fraction can be easily isolated via sol­vent extraction. Tartrate salts can be subsequently sepa­rated from yeast cells via treatment with hydrochloric acid. Versari et al. (2001) extracted tartaric acid with pu­rity up to 99% from three different winery by-product streams, including wine lees. Moreover, Nurgel and Canbas (1998) investigated the production of tartaric acid from grape pomace. Use of tartaric acid is well established in wine making in order to adjust the pH of the must prior to fermentation. Tartaric acid could be also used as food additive.

After the extraction of phenolic compounds and tartrate salts, residual wine lees solids are subjected to enzymatic hydrolysis with the addition of crude en­zymes produced via solid-state fermentation of a fungal strain of A. oryzae on wheat bran. The ethanol-free me­dium that remains after the distillation step is used as liquid in the hydrolysis stage. In this stage, yeast cells are lysed and converted into a nutrient-rich supplement similar to yeast extract. This supplement is rich in various sources of nitrogen (e. g. amino acids and pep­tides), phosphorus and various trace elements. This nutrient supplement can be combined with a carbon source (e. g. crude glycerol from biodiesel industries) as fermentation media for the production of PHB with C. necator. Preliminary experiments with C. necator DSM 7237 and crude glycerol as carbon source showed that PHB production is feasible using wine lees hydroly­sates. However, supplementation with a low quantity of minerals is necessary showing that this nutrient supple­ment is deficient in some minerals. The wine lees hydro­lysate could be combined with a sugar-rich hydrolysate derived from treatment of lignocellulosic streams derived during wine production.

PHB Production from Confectionery and Bakery Industry Waste Streams

Significant quantities of waste streams are generated annually from confectionery industries and bakeries. The waste streams from the industrial sectors mentioned above produce flour-, starch — or sugar-rich waste streams generated either during processing or as end — of-date products returned from the market. Confection­ery waste streams are currently used as animal feed, for composting or are discarded to landfills. However, these low-cost materials constitute renewable feedstocks that could be used for the development of novel biorefinery schemes. Anaerobic digestion from various food waste streams and biodiesel production from cooking oils are predominant alternatives that have been proposed for the utilization of various food waste streams. Current research on confectionery waste streams and waste bread is rather limited, but in recent years research has started to focus on the valorization of such waste streams. Dorado et al. (2009) utilized hydrolysates derived from wheat milling by-products as fermentation media for the production of succinic acid (50.6 g/l). Leung et al. (2012) developed a two-stage bioprocess involving solid-state fermentation and enzymatic hy­drolysis of waste bread to produce a fermentation feed­stock for the production of succinic acid (47.3 g/l at a conversion yield of 0.55 g SA/g bread) using the bacte­rial strain Actinobacillus succinogenes.

A potential biorefining concept for the production of PHAs and biodiesel from confectionery industry waste streams is presented in Figure 24.5. In the case of confec­tionery wastes that contain high oil content, this could be removed via solvent extraction. The oil obtained from this step can be used for biodiesel production. Remaining fractions will be rich in directly assimilable sugars such as glucose, fructose, sucrose and lactose as well as starch and protein. Utilizing starch- and

protein-rich waste streams as sources of carbon and ni­trogen in fermentation processes demands the conver­sion of starch into glucose and protein into amino acids and peptides. The amylolytic and proteolytic en­zymes required for the hydrolysis of these macromole­cules could be produced via solid-state fermentation using the fungal strain Aspergillus awamori cultivated on wheat milling by-products. The fermented solids, rich in amylolytic and proteolytic enzymes, are subse­quently combined with confectionery waste to produce hydrolysates that can be used in fermentation processes for the production of platform chemicals, microbial oil or PHB. The production of PHB or PHAs from confec­tionery industry wastes could be employed for the pro­duction of biodegradable packaging materials for the same industry.

The proposed process is based on the results that were achieved for the production of PHB using wheat as the whole raw materials (Koutinas et al., 2007a, 2007b; Xu et al., 2010). In this biorefinery concept, wheat is fractionated into bran and gluten as value — added co-products, while remaining fractions are used for the production of fermentation media suitable for the production of PHB via fed-batch cultures using the microbial strain Wautersia eutropha NCIMB 11599. Xu et al. (2010) developed a fermentation process for the production of PHB from wheat-derived fermentation media during fed-batch cultures in a bioreactor. The highest PHB concentration achieved was 162.8 g/l. However, wheat is regarded a food resource and should not be used for chemical production. Starch — or flour- rich food wastes could be used, instead of wheat, as a renewable resource for the production of PHB.

PHB Production from Whey

Whey is the main by-product occurring from cheese manufacture and lactose is one of the primary compo­nents. Current whey valorization processes mainly focus on the production of whey powder, whey protein concentrate or whey protein isolate. Utilization of whey in fermentation processes has been widely investigated, given the fact that it is produced in many countries in significant quantities. Furthermore, whey valorization will also contribute to the improvement of the environ­mental impact of the cheese industry because whey disposal is a notorious environmental burden.

Future cheese industries could incorporate integrated processing schemes for the production of whey protein and PHAs. Koller et al. (2010) reviewed various biocon­versions that employed whey permeate as carbon source aiming at the production of PHAs. Different strategies were proposed concerning uses of whey permeate; direct conversion as substrate or hydrolysis of lactose to glucose and galactose were examined. Moreover, Wong and Lee (1998) presented PHB production from whey powder with recombinant E. coli in pH-stat cul­tures. In fed-batch cultures with additions of concen­trated whey solution, the corresponding dry cell weight and PHB concentrations were 87 and 69 g/l, respectively. The PHB content reached up to 80% (w/ w). These results established that PHB fermentation pro­cess from whey could be industrially employed, increasing the sustainability and market alternatives of traditional cheese producing plants.

Whey protein concentrate and isolate that could be extracted from whey by ultrafiltration and evaporation steps can be applied as food additives. Moreover, they are considered to possess therapeutic properties and for this reason, whey protein concentrate was applied for treatment of various clinical disorders. Furthermore, whey protein ingredients are added to food targeting to improve their functional or technological properties. Hence, keeping that in mind, biorefinery schemes based on whey utilization could be easily proposed.

CONCLUSIONS AND FUTURE
PERSPECTIVES

The necessity to eliminate our dependence on fossil resources will lead to an inevitable reconstruction of the current industry in order to introduce the utilization of renewable resources and produce chemicals, fuels and materials in a sustainable manner. Implementation of biorefinery concepts into existing industrial facilities provide an alternative processing option, taking into consideration that industrial by-products and waste streams are generated in significant quantities and currently, they are inadequately utilized. Consequently, production of value-added products from waste and by-product streams will enhance sustainability and diversify market opportunities. Furthermore, produc­tion of biofuels should coincide with chemical and biodegradable polymer production to enhance their sus­tainability. This study showed potential industries where biofuel and food production could coincide with PHA production. This research area is currently at the inception phase and significant effort is required in order to develop the technologies that will be imple­mented on industrial scale.

The lignocellulosic substrates include woody sub­strates such as hardwood (birch, aspen, etc.) and soft­wood (spruce, pine, etc.), agri residues (wheat straw, sugarcane bagasse, corn stover, etc.), dedicated energy crops (switchgrass, willow, hemp, Miscanthus, etc.), weedy materials (Eichhornia crassipes, Lantana camara, etc.), and municipal solid waste (food and kitchen waste, etc.). The diversity of raw materials will allow the decen­tralization of fuel production with geopolitical, eco­nomic, and social benefits (Van Dyck and Pletschke, 2012; Wyman, 2007). Despite the success achieved in the laboratory, there are limitations to success with lignocellulosic substrates on a commercial scale (Chan — del and Singh, 2011) as each source of biomass brings a unique technological challenge.

The advanced biomass-to-biofuels development platform has multiple goals, including the use of new enzymes to take full advantage of available carbohy­drates, the development of new lines of bioenergy crops with increased fermentation productivity (Carpita, 2012; Abramson et al., 2010), the development of new uses for coproducts, and the reduction of pro­cessing and energy costs. Lignocelluloses have three main components: cellulose, hemicelluloses, and lignin. Cellulose is the most abundant organic polymer on the earth. It is a homopolymer of sugars containing six carbon atoms linked together in a chain that constitutes the largest proportion of the plant cell wall. Hemicellu — loses are heteropolysaccharides consisting of short branched chains of hexoses, e. g. mannose units in mannans and pentoses such as xylose units in xylans (Chandel et al.,. 2010; Girio et al., 2010; Kuhad et al., 1997).

Table 2.4 summarizes the basic cell wall composition of some important lignocellulosic biomass used in bio­energy generation. In general, hardwoods contain 18—25% lignin, 45—55% cellulose, and 24—40% hemicel — luloses, while softwoods contain 25—35% lignin, 45—50% cellulose, and 5—35% hemicelluloses. Grasses normally contain 10—30% lignin, 25—40% cellulose, and 25—50% hemicelluloses (Balat, 2011; Sanchez, 2009; Howard et al., 2003; Malherbe and Cloete, 2003; Betts et al., 1991). Agri-biomass commonly comprises about 40% cellulose, 25% hemicellulose and 18% lignin. The structure and components of the cell walls of weeds are significantly different from those of most plant species, which may influence digestibility during the bioconversion process to bioethanol (Van Dyck and Pletschke, 2012; Chandel and Singh, 2011; Sarkar et al.,

2009) .

The hydrolytic breakdown of cellulose in nature involves the use of enzymes including cellobiohydro — lases, endoglucanases and b-glucosidases produced by microbes or other biological agents, alone or in combina­tion (Turner et al., 2010; Kuhad et al., 1997). More recent studies have shown that additional oxidoreductase enzymes (glycosyl hydrolase family 61 polysaccharide monooxygenases and cellobiose dehydrogenase) are essential components in a complete cellulose-degrading enzyme system (Horn et al., 2012; Kittl et al., 2012; Lang­ston et al., 2011). The sugar chains of cellulose can be hy­drolyzed to produce glucose and cellooligosaccharides, most of which can be fermented using ordinary baker’s yeast. To attain economic feasibility a high ethanol yield is a necessity. Producing monomer sugars from cel­lulose and hemicellulose at high yields is far more difficult than deriving sugars from sugar — or starch — containing crops, e. g. sugarcane or maize (Van Dyck and Pletschke, 2012; Tuohy et al., 1994). Therefore, although the cost of lignocellulosic biomass is far lower than that of sugar and starch crops, the cost of obtaining sugars from such materials for fermentation into bioethanol has historically been far too high to attract industrial interest. For this reason, it is crucial to solve the problems involved in the conversion of lignocellulosic biomass to sugar and further to ethanol (Agbor et al., 2011; Galbe and Zacchi, 2002).

The heterogeneity in feedstock and the influence of different process conditions on microorganisms and enzymes makes the biomass-to-ethanol process

(Continued)

complex. Ethanol can be produced from lignocellulosic materials in various ways. The main difference between the process alternatives is the hydrolysis steps, which as mentioned previously, can be performed by dilute acid, concentrated acid or enzymatically. Some of the process steps are more or less the same, independent of the hy­drolysis method used. For example, enzyme production will be omitted in an acid hydrolysis process; likewise, the recovery of acid is not necessary in an enzyme hy­drolysis process (Galbe and Zacchi, 2002).

To achieve lower production costs, the sustainable supply of cheap raw materials is a necessity. It is also essential to ensure that all components of the biomass are utilized and resulting by-products and wastes are used in a biorefinery system. When lignocellulosic raw materials are used, the main by-product is lignin, which can be used as an ash-free solid fuel for production of heat and/or electricity, for which there are no foresee­able market limits. However, in addition, lignin can be used for a range of additional high-value products that have the potential to enhance overall process economics significantly (Azadi et al., 2013; Lange et al., 2013; Doherty et al., 2011; Collinson and Thielemans, 2010). Accordingly, it will only be possible to produce large amounts of low-cost ethanol if lignocellulosic feedstocks such as fast-growing trees, grass, aquatic plants, waste products (including agricultural and forestry residues) and municipal and industrial waste are used
(Van Dyck and Pletschke, 2012; Wheals et al., 1999). The potential of using lignocellulosic biomass for energy production is even more apparent when one realizes that it is the most abundant renewable organic compo­nent in the biosphere (Claassen et al. 1999). Currently enzyme hydrolysis has high yields (70—85%) of biocon­version, and improvements are still possible (85—95%) (Van Dyck and Pletschke, 2012; Sills and Gossett, 2011; Redding et al., 2010; Hu and Wen, 2008).

Generally speaking, rational engineering refers to planned biochemical changes to a protein through the use of protein sequence and structure information, which in theory corresponds to a physiological or functional change in the proteins behavior. The engineered changes are usually predicted using computational biology and protein sequence data. However, there is limited struc­tural information available for enzymes, for example, in structure—function relationship—so predictions on behavioral changes after rational engineeringstill remain in a trial-like state (Maki et al., 2009). Nonetheless, with increasing knowledge of biomass substrates and a rigorous test of our knowledge about enzyme interac­tions with plant-based biomass, rational engineering can be a valuable tool in the economical production of biofuels and value-added by-products.

Briefly, rational design of proteins can be summed up in three simple steps: (1) a suitable enzyme is chosen based on desired characteristics, (2) using computational biology or a high resolution crystallographic structure, the amino acid sites to be changed are identified, and

(3) mutants produced from rationally engineered pro­teins are characterized (Percival Zhang et al., 2006).

Moreover, rational modifications to enzymes often include amino acids substitutions using site-directed mutagenesis, which can be used to increase the stability of enzymes (i. e. thermostability), substrate specificity, cofactor specificity, and the elucidation of enzymatic mechanisms (Bornscheuer and Pohl, 2001). In the field of biomass conversion to biofuels and bioproducts, the use of rational design has pioneering examples as out­lined here.

For the most part, there are numerous reviews that summarize studies that revealed the mechanism of cellulase and other biomass-converting genes through the use of site-directed mutagenesis (Schulein, 2000; Wilson, 2004; Wither, 2001). On the contrary, very few researchers have reported increasing cellulase and other biomass-converting activities or enhancing properties through site-directed mutagenesis. However, Baker et al. were able to improve the activity of endoglucanase

Cel5A of Acidothermus celluloyticus toward microcrystal­line cellulose by 20% (Baker et al., 2005). This was accomplished utilizing a high-resolution crystallo­graphic structure (Sakon et al., 1996) to determine a se­ries of mutations designed to alter the active cleft through a change in chemistry of the product-leaving side. As a result, structural information allowed end — product inhibition to be alleviated by a substitution of a nonaromatic residue at site 245; a Y245G mutant increased the KI of cellobiose by 15-fold.

In a similar study, site-directed mutagenesis was used to improve the catalytic activity of endo/exocellulase Cel9A in Thermobifida fusca by 40% with soluble and amorphous cellulose, such as carboxymethyl cellulose (CMC) and swollen cellulose. Through the use of com­puter modeling, the conserved phenylalanine residue F476 (one of three residues) was found at the end of the carbohydrate binding module and appeared to play an important role in the initial binding of the cellulase to substrate. Also, computer modeling was used to predict that a new hydrogen bond could be created as a result of mutating the conserved phenylalanine residue F476 to a tyrosine. Therefore, the observed increase in catalytic ac­tivity of mutant F476Y is thought to be attributed to better binding properties, which are key for placing the soluble and amorphous cellulose chains in the carbohydrate binding domain (Escovar-Kousen et al., 2004).

Rational engineering of enzymes can also be used to improve characteristics such as thermostability and alkalinity in addition to specific activity. The roles of highly conserved residues (Asp 60, Tyr 35 and Glu 141), near the catalytic site, were investigated in the pH — dependent activity of xylanase XYL1p from Scytalidium acidophilum using site-directed mutagenesis. In doing so, three single mutants, D60N, Y35W and E141A, were created and the activities of three combined xyla — nase mutants DN/YW, DN/EA and YW/EA were eval­uated at different pHs and temperatures. An increased pH optimum of 0.5—1.5 pH units and lower specific activities were observed in all the mutants except one. Mutant E141A exhibited a 50% increase in specific activ­ity at pH 4.0 and had an overall higher catalytic effi­ciency than wild-type enzyme (Al Balaa et al., 2009). This work presents some important knowledge in acid­ophilic adaptation and, at the same time, is a prime example of how rational engineering can lead to the development of enzymes more suitable for the biocon­version industry environment, with competitive cata­lytic efficiency maintained.

Finally, the possibility of using rational engineering to improve the pH optimum and catalytic efficiency of lac — case enzymes, involved in the oxidation of lignin, has been increasing as several researchers explore important residues conserved in laccases from fungi (Rogers et al.,

2009) . In one compelling example, researchers replaced an Asp residue in position 206 with an Asn residue in a laccase from T. versicolor, using site-directed mutagen­esis. Upon expression of mutants in the yeast Yarrowia lipolytica, it was noted that catalytic activity was signifi­cantly affected as the pH optimum was raised by 1.4 pH units (Madzak et al., 2006), highlighting the interac­tion between the reducing substrate and the binding pocket of laccase. This study, like those discussed previ­ously, pave the way for future development of efficient biomass-converting enzymes.

The microbial species in a biofilm covering an anode are important because they determine the mode of elec­tron transfer and the mechanism of electricity genera­tion as well as what forms of organic material can be utilized in the feed stream. Theoretically, a myriad of microorganisms may be useful for MFCs, but most of them have no direct electrochemical activity and thus cannot transfer electrons directly from the cytoplasm to the anode, i. e. they are not electrogenic. However, many microorganisms with the addition of a soluble redox mediator can act as electron transfer intermediates to transfer electrons. Table 9.1 shows the microbial spe­cies and the electron transfer mechanism in the anodic chamber that can perform such processes.

In MFCs, mixed cultures usually possess higher elec­tron transfer efficiency than the pure culture because its specificity to the microbe is very strong and its growth rate is relatively slow (Hassan et al., 2012). Mixed cul­tures are often found to perform better than pure stains. This is because a synergistic biofilm consortium contains various syntrophic species, with each organism contrib­uting specific roles. A consortium can adapt to substrate variations in wastewater and harsh environmental con­ditions because generally a biofilm consortium is far more robust metabolically than a pure-culture biofilm. The consortium is able to self-select the most efficient electron transfer mechanism if several are available.

Biocathodes

Biocathodes use biofilms as catalysts to improve the cathode reaction, avoiding using precious metal cata­lysts. Another unique advantage of biocathodes is that oxidants other than oxygen can be used, including sul­fate, nitrate, carbon dioxide, H+, Fe(III), Cr(VI), U(VI), Mn(IV), tetrachloroethene, fumarate, perchlorate, and trichloroethene (Huang et al., 2011c). In addition, the sustainability of MFC may be improved with the elimi­nation of problems such as sulfur poisoning of Pt and the requirement for electron mediators in the cathodic chamber (He and Angenent, 2006).

There are two types of biocathodes: aerobic and ana­erobic. Aerobic biocathodes reduce oxygen (electron acceptor). The biofilm on the cathode surface can catalyze the oxidation of transition metal compounds, such as Fe(II) and Mn (II), releasing the electrons to oxygen. MFCs with aerobic biocathodes can produce higher power density than that of anaerobic biocathodes (Srikanth and Venkata, 2012).

The use of a biocathode also means that an MFC can potentially be used to treat an additional wastewater stream in the cathodic chamber. It may be a wastewater stream containing sulfate or nitrate that can come from agricultural runoff (Srikanth and Venkata, 2012). How­ever, the accumulation of microbial metabolites in the cathode chamber can inhibit microbial activities. In addition, metabolites which act as electron donors for bacteria can also compete against the cathode, and there­fore reduce the MFC performance (Hamid et al., 2008).

Furthermore, Zhou et al. (2013) indicated that the voltage output for the combined redox reaction involving Eqn (9.3) and the oxidation of an organic car­bon such as acetate may be too small for MFC after sub­tracting various overpotentials.

As outlined above many biological sources can be used for the generation of biofuels (Demirbas et al., 2011; Vasudevan et al., 2005); however, one source of biomass for the production of biodiesel that is often overlooked is the waste fat from animals (e. g. (Ali et al., 2012; Duku et al., 2011; Feddern, 2011; Panneerselvam et al., 2011; Wisniewski Jr et al., 2010)). Generally three broad categories of waste animal fats are described—tallow and related raw fats from process­ing industries, yellow grease from waste cooking oil used to cook, for example, chicken, and brown grease that is obtained from traps used to prevent waste fats and oils being released into the environment. Animal fats can be sourced as room temperature solids or semisolids from a variety of animals and include tallow and suet (cattle and mutton), lard (pigs), schmaltz (poultry especially chicken and goose), duck, fish oil and dairy products (milk, butter) (Jayasinghe and Hawboldt, 2011; Kerihuel et al., 2005; Mrad et al., 2012; Panneerselvam et al., 2011; Wisniewski Jr et al., 2010). It is also possible to reclaim waste animal fats from wastewater (Awad et al., 2012a). Many of the properties of animal fats used put to specific uses have been known for a long time (Andes, 1898; Shahidi and Zhong, 2005). A significant percentage of waste animal fat can be con­verted to biodiesel using similar techniques to those used for plant oils, the main process being transesterifi­cation, described later (Proskova et al., 2009). The triglycerides in animal fats are saturated, compared to unsaturated plant triglycerides, and this has some impli­cations when used as biodiesel. In particular the cloud point, the temperature at which the oils solidify, is higher for animal fats. However, when used as additives to other sources of diesel, for example, 5% or 20% biodiesel (B5 or B20 blends), the high cloud point does not affect the blend overall.

Production of biodiesel from waste animal fats has been shown using a variety of methods including a novel, integrated method in which fat from lamb meat is continuously extracted by supercritical CO2 followed by enzymatic production of biodiesel (Schenk et al.,

2008) . Feedstocks containing high levels of FFAs require an additional preproduction step to convert the FFAs into esters, which can subsequently be converted into biodiesel. Waste sources that contain high levels of FFAs require a separate step (acid catalyzed pretreat­ment) before the base catalyzed reactions can be used to provide maximal yields of biodiesel (Canakci and Van Gerpen, 2001; Knothe et al., 2005; Popescu and Ionel, 2011). Multistep processes using waste restaurant oil and animal (pig) fat containing high levels of FFAs can achieve high yields of biodiesel of up to 80% by volume on a small scale (Math et al., 2010). Other high FFA content oils, including used cooking oils, rendered animal fat and some inedible plant oils (Mathiyazhagan et al., 2011) can be processed in a similar fashion (Bakir and Fadhil, 2011).

The feasibility and sustainability of using waste animal fats as feedstocks for biofuel production has been the subject of many studies in many areas, for example, general studies (Demirbas, 2009; Nigam and Singh, 2011), Australia (Puri et al., 2012), Ghana (Duku et al., 2011), the United States (Groschen, 2002), Brazil (Aranda et al., 2009), Ireland (Thamsiriroj and Murphy,

2010) and Hungary (Lako et al., 2008). In addition, the use of animal fats from waste tissue may also have envi­ronmental benefits, such as being considered as a waste management process and as a fuel source that does not compete with food resources (e. g. soybean), the food versus fuel debate. Table 12.1 shows typical values reported for triglycerides in several animal fats in com­parison to values for soy, a commonly used plant — derived feedstock. In all cases, waste animal fats contain high levels of the fatty acids that are capable of being converted to methyl esters by transesterification reactions to produce usable biodiesel. From a sustain­ability point of view an estimate of the total annual US production of animal fats as compared to plant — derived oils is shown in Table 12.2.

Vegetable oils tend to be produced for human con­sumption, whereas animal fats form part of a wide group of animal by-products that are rendered into many products that may be used in part for human con­sumption (e. g. production of gelatin). All animal by­products, including fats, are coded and classified (Alakangas et al., 2011) according to their intended use and animal fats not intended for human consumption are controlled in the European Union by Regulation (EC) No 1069/2009 and related legislation. Similarly,

TABLE 12.2 Total Annual Production of US Fats and Oils

Vegetable Oil Production (billion pounds per year)

Canola 1.04

Corn 2.49

Cottonseed 0.617

Soybean 19.61

Sunflower 0.731

Total Vegetable Oil 24.49

Animal Fats (billion pounds per year)

Edible Tallow 1.859

Inedible Tallow 3.299

Lard & Grease 1.63

Yellow Grease 1.40

Poultry Fat 1.42

Total Animal Fat 9.61

Source: U. S. Department of Agriculture, 2010; U. S. Census Bureau, 2010.

the storage of animal fats for use as fuels also needs to be addressed. The storage of raw animal fat under unsuit­able conditions can lead to oxidation and other undesir­able chemical and microbial processes that can affect the quality of the final biodiesel product. The stability of the final biodiesel:diesel blend can also be affected by long­term storage under unsuitable conditions, and additives such as antioxidants might be added to improve stability (Geller et al., 2008; Jain and Sharma, 2010).

With the advent of Bovine spongiform encephalopathy (BSE) and more specifically Transmissible spongiform encephalopathies (TSE), there is a greater need to monitor human health issues when using waste animal fats for the production of biofuel, at all stages of the production pro­cess. The rendering industry recognizes that safe product (fats) can only be supplied if certain standards are adhered to (Woodgate and Van Der Veen, 2004). The raw materials could well have microbial contamination including path­ogenic bacteria and possibly prion material (Baribeau et al., 2005; Brown et al., 2007; Bruederle et al., 2008; Greene et al., 2007). There is also concern that prions will survive the rendering process itself (Bruederle et al., 2008). These concerns have in part led to the publication of guidelines for the safe handling and use of biodiesels (National Renewable Energy Laboratory, 2009).

Many trials of waste animal fat biodiesel-powered engines have been published (Darunde Dhiraj and Deshmukh Mangesh, 2012; Kleinova et al., 2011; Panneerselvam et al., 2011; Varuvel et al., 2012). One trial using public transport buses (Proc, 2006) showed that the biodiesel does not have any harmful effects on the engines at B5 and B20 mixes and also shows environmental benefit by way of reduced exhaust pollutants. However, there are other potential health and environmental issues in using animal fats as a feed­stock for biodiesel production (Greene et al., 2007) and the production of safe biodiesel is in part dependent on a safe feedstock (Woodgate and Van Der Veen,

2004) . Finally, the processes involved (e. g. rendering, cleanup, transesterification, etc.) in the production of biodiesel will generate waste that also needs to be assessed (Ellis, 2007).

Table 15.1 shows some results from recent studies on the pyrolysis of lignocellulosic biomass in the presence or absence of catalysts. Earlier, catalysts such as carbon­ates and hydroxides were mainly tested for the catalytic pyrolysis of lignocellulosic biomass. The use of alkaline compounds like NaOH, Na2CO3 and Na2SiO3 resulted in bio-oils rich in acetol and to some extent favored H2 formation. The use of Fe2(SO4)3 as a catalyst favored the formation of furfural and 4-methyl-2-methoxy — phenol (Chen et al., 2008). Lu et al. revealed that SOl~/SnO2 were an effective catalyst to yield 5-methyl furfural (Lu et al., 2009). The selectivity varied signifi­cantly once the catalyst support was altered. For example, SO4~/TiO2 catalyst favored the formation of furfural; SO4/ZrO2 catalyst favored the formation of furan (Lu et al., 2009).

Liquid acids such as H2SO4, hydrochloric acid, phos­phoric acid and solid acids such as ZSM-5, Al-MCM-41 are also used as catalysts (Table 15.1), in addition to their uses in the pretreatment of lignocellulosic biomass (Lu et al., 2011). The typical products of liquid acid catalytic

pyrolysis of biomass are levoglucosenone, furfural and levoglucosane (0.1 wt% sulfuric or polyphosphoric acid as catalyst) (Dobele et al., 2005; Kawamoto et al., 2007). Taking the separation between catalysts and liquid products and corrosive into consideration, the catalytic pyrolysis of lignocellulosic biomass over zeolites and many other solid catalysts have recently received much attention. Microporous zeolite and mesoporous materials such as ZSM-5, Al-MCM-41 have been used catalysts in catalytic pyrolysis of ligno — cellulosic biomass. In particular, hydrocarbons can be produced in considerable quantities by fast pyrolysis of biomass over these catalysts (Pattiya et al., 2008). Ola — zar et al. conducted the fast pyrolysis of pine sawdust catalyzed by ZSM-5 in a spouted-bed reactor using nitrogen as the carrier gas. A 12% yield of aromatic com­pounds was obtained (carbon) (Olazar et al., 2000). French et al. revealed that over nickel, cobalt, iron, or gallium-substituted ZSM-5 at 673—873 K and with a catalyst-to-biomass ratio of 5—10 by weight, lignocellu — losic biomass was pyrolyzed to give an approximately 16 wt% yield of hydrocarbons (French and Czernik,

2010) . The solid catalysts have advantages over the liquid acid catalysts. However, for solid catalysts at high temperatures, the cracking and deoxygenating activity decreased with time because of the coke formed on them (Carlson et al., 2008).

Reactors

Several types of reactors have been designed for fast pyrolysis of biomass, and the reactor is very crucial to fast pyrolysis of biomass. There are entrained-flow reac­tors (EFRs), fluid bed reactors, rotating cone reactors, and ablative reactors.

Entrained-Fow Reactors

In EFRs, schematically shown in Figure 15.1, biomass particles are usually fed into the reactor in a stream of hot, inert gas. Reaction is typically completed at 973—1073K within a residence time of a few seconds. Dupont et al. used the mixture of two softwoods (sylvester pine and spruce) as a model of biomass to pyrolyze in an EFR (Dupont et al., 2008). The influence of the particle size (0.4 and 1.1 mm), temperature (1073—1273 K), the presence of steam in the gas atmo­sphere (0 and 20 vol%) and the residence time (between 0.7 and 3.5 s for gas) on conversion and selectivity were studied. Results showed that the particle size was the most crucial parameter that influenced decomposition and more than 70 wt% of gas was produced.

Ablative Reactors

An ablative pyrolysis reactor is considered as a possible alternative to an EFR. The surface is heated by

hot flue gas produced by combustion of pyrolysis gases or char and rotates while biomass is pressed onto the hot surface (873K).

However, in general, an ablative pyrolysis reactor has difficulty in getting sufficient heat transfer from hot gases to the ablative surface and in contacting feed­stock of diverse morphologies (particle shape, struc­ture, and density) with the ablative surface. In practice, relatively few feedstocks would be suitable for ablative pyrolysis.

Bubbling Fluid Bed Reactor and Circulating Fluidized Beds

A fluid bed reactor is very suitable for fast pyroly­sis, as the biomass is rapidly heated and there are high heat and mass transfer rates between gas, parti­cles and catalysts and any other objects in the reactor. Vapor and solid residence time are controlled by the fluidizing gas flow rate. Bubbling fluidized beds (Figure 15.2(a)) are usually referred to simply as fluid­ized beds, which provide good temperature control and very efficient heat transfer to biomass particles due to high density of solids in the bed. Jung et al. pyrolyzed rice straw and bamboo sawdust in a bubbling fluidized bed equipped with a char separa­tion system (Jung et al., 2008). They found that the maximum bio-oil yield was above 70 wt% and a higher feed rate and a smaller feed size were more favorable to the production of bio-oil.

While circulating fluid bed reactors have similar features to bubbling fluid bed reactors. A main differ­ence between them is the amount of gas used to flui­dize the bed. In the circulating fluid bed reactor (Figure 15.2(b)), the gas flow is intentionally set high enough to transport particles out of the bed, which are recovered by gas cyclones and then returned to the fluidized bed. All char is burned in the secondary reactor to reheat the circulating sand or is separated as a fine powder.

The circulating fluidized bed can be divided into two zones: pyrolysis zone; and reduction and cracking zone (Wu et al., 1992). In the pyrolysis zone, biomass is loaded into the bed and pyrolyzed very quickly to form char, tar, H2O, and gas (CO2, CO, CH4, CnHm and H2). In the reduction and cracking zone, pyrolysis char of contributes to secondary cracking in the vapor
phase. For example, with the circulating fluidized bed as reactor, Dai et al. pyrolyzed wood powder at 773K (Dai et al., 2000). The main effects were: (1) the higher temperature and longer residence time contributed to the secondary reactions and then lead to less liquids; (2) the lower heating rate favored the carbonization and reduced the liquid production; and (3) most com­pounds in bio-oil were nonhydrocarbons and alkanes, aromatics, and asphalt were relatively less.

Rotating Cone Reactor

Rotating cone reactor is designed to achieve the intense mixing and heat transfer between biomass and heat carrier without the use of a large amount of fluidizing gas (Figure 15.3). Gas is needed for char burn-off in a secondary bubbling fluid bed combustor and sand transport recirculated to the pyrolyzer. Flash

pyrolysis of wood dust was processed in a rotating cone reactor by Wagenaar et al. the cone geometry was spec­ified by a top angle of p/2 radians and a maximum diameter of 650 mm (Wagenaar et al., 1994). The rotating cone reactor model included the description of the particle flow behavior, the particle conversion and the gas-phase cracking of tar vapors. It appeared that the product distribution was affected by the gas — phase reaction kinetics and residence time and the gas-phase residence time was determined by the avail­able reactor volume and the feed rate of the wood particles.

New Systems

Recently, emerging technology is to couple a pyroly­sis reactor with other catalytic reactors such as steam reformer and hydrogenation. For example, the technol­ogy of hydroprocessing is intended to convert bio-oil to petroleum-refinery compatible feedstock (Elliott et al., 2012). The combination can also be used to build a microscale pyrolysis reactor coupled to the molecular-beam mass-spectrometer (Bahng et al.,

2009) . It can be used as a very efficient tool for studying mechanisms of thermal and catalytic processes and to optimize process conditions for different products from a variety of feedstocks.