Manish Kumar and Kalyan Gayen

7.1 Introduction

According to the United Nations Conference on Trade and Development, petroleum industry will face severe crisis after next three decades and increasing production rate can shorten this period. Energy crisis may start 5-10 years before the declination of the oil reserves due to the fluctuation in oil prices (www. unctad. org/ en/docs/ditcted20064_en. pdf). In addition of decrement of petroleum reserves, environmental issues such as green house effect, global warming, etc. are also the problems to be solved worldwide [1]. Moreover, combustion of petroleum fuels leads to raise the concentration of carbon dioxide and other greenhouse gases (methane, nitrous oxide) in atmosphere [2]. Therefore, depletion of petroleum fuel and adverse effect on climate changes enforce to pay attention on renewable sources of energy [3-5]. In this direction, biofuels is attracting the attention as the replacement or extender of petroleum fuel [1, 6, 7].

Other benefits of biofuels include energy security, mitigated environmental impact, foreign exchange saving, and socioeconomic issues which are equally significant and directly associated with the development of rural areas [7, 8]. The common examples of biofuels are biobutanol, bioethanol, and biodiesel, which show the potential to compensate the demand of petroleum-originated fuels [6, 9, 10]. Bioethanol has been already introduced as biofuels and is being consumed in automobiles with gasoline in different blending proportions. Brazil and U. S. [11] establish trademark for economic production of bioethanol on account of huge availability of sugarcane and corn, respectively in these countries. Likewise, the blending of 5% of bioethanol in gasoline has been started in India since the year of

M. Kumar • K. Gayen (H)

Department of Chemical Engineering, Indian Institute of Technology Gandhinagar, VGEC Campus, Chandkheda, Ahmedabad 382424, Gujarat, India e-mail: gkalyan@iitgn. ac. in

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

DOI: 10.1007/978-3-642-28418-2_7, © Springer-Verlag Berlin Heidelberg 2012 2008 [11, 12]. Similarly, biobutanol is the recently introduced organic alcohol, which can be used as biofuel [1]. Furthermore, biobutanol continuously emerges the attention of researchers and industrialists because of its several advantages such as high energy contents, high hydrophobicity, good blending ability, does not required modification in present combustion engines, and less corrosive over other biofuels [13, 14].

Industrial synthesis of biobutanol was started during 1912-1914 using Clos­tridium acetobutylicum (weizmann’s organism) from molasses and cereal grains in acetone-butanol-ethanol (ABE) fermentation [15]. However, first-time synthesis of biobutanol at laboratory level was reported by Pasteur in 1861 [16]. Later, promising strains (Clostridium beijerinckii, Clostridium saccharoperbutylaceton — icum, and Clostridium saccharobutylicum), which are capable to produce high yield of biobutanol were identified [17]. Before 2005, butanol was used as solvent and precursor of other chemicals. However, at a later stage David Ramsey reported the application of butanol as biofuel after his traveling across the U. S. with 100% butanol. Shortly, two giant companies DuPont and BP have declared to restart the ABE fermentation at industrial scale and they have also applied several patents for their work [18-20].

Many economical studies on ABE fermentation have suggested that raw material cost contributes more than 60% in the total cost of the biobutanol pro­duction. Therefore, the selection of the raw material is a vital step for establishing the industrial-scale plant for ABE Fermentation. In the raw material point of view, main concerns entail cost and availability, cost of pretreatment, long-term sus­tainability, and product yield. Sugar and starchy grains contributed as raw mate­rials for this production during First World War. At that time ABE fermentation was developed at industrial level to produce acetone [21]. Currently, these raw materials are cost-intensive, which can impact the on food cost and demand. Therefore, there is emerging interest to find out cheaper and highly available raw materials. In this order, at laboratory level some researchers investigated alter­native raw materials, namely lignocellulosic materials (biomass) such as agricul­ture wastes (corn stover [22], wheat straw [23, 24], corn fibers [25], barley straw [26], and switchgrass [22]) and woody residues, which can illustrate the possibility for economic production of biobutanol [2, 27, 28].

In the direction of process development, various fermentation processes have been examined, viz. batch, fed-batch, and continuous processes (free cells, immobilized cells, and cells recycling) [22-26, 29-38]. On the basis of these studies, continuous processes have shown more efficient results over batch and fed-batch as it illustrates several advantages such as saving sterilization and re-inoculation time, yield similar to batch process but superior productivity, and reduction in butanol inhibition [36]. However, some demerits of continuous process have also been counted such as high capital cost and more chances of contamination, and nitrogen-limiting conditions. Further, research work is being done to find optimum conditions for process [15].

Apart from above progress in the ABE fermentation, the major problems in this field are (i) selection of sustainable biomass (ii) constraints caused by severe butanol inhibition, and (iii) high recovery cost [39-44]. Recent research progress is focusing on butanol production by lignocellulosic materials, different recovery techniques to reduce the final butanol cost, and decrease the butanol inhibition. However, genetic engineers have great opportunity to engineer the microorganism for making it resistant to butanol and improving the yield of product. This chapter includes the challenges of biosynthesis of butanol, recent development, and future prospective. It also emphasizes on utilization of lignocellulosic materials as raw material.

These are components of cell walls associated with cellulose and are the second largest available organic renewable resource [36]. Hemicellulose consists of xy — loglucans with a chain of D-xylose linked through b-1-4 glycosidic bond (Fig. 9.8). The xylose polymer normally contains side chain branches of a-1-3 linked D-mannose and b-1-2 linked D-galactose, b-1-4 linked D-mannose and a-1-2 linked D-glucose. In hardwood hemicelluloses, the xylose units are intermittently esteri — fied with acetic acid at the hydroxyl group of carbon 2 and/or 3 [112].The xylan of softwood, however, is not esterified. The presence of side groups, protruding from the linear b-1, 4 configuration, increases the solubility and thus, renders the sub­strate easily to hydrolysis.

Due to the complex structure of hemicelluloses, several enzymes are needed for their enzymatic degradation. The main glucanase depolymerizing the hemicellulose

Fig. 9.7 Enzymes involved in cellulose degradation. a Endo-b-1,4 glucosidase. b Exo-b-1,4 glucosidase. c b-glucosidase. d Cellobiose phosphorylase. e Cellobiose kinase and phospho-b — glucosidase

backbone is endo-1,4-b-D-xylosidic linkages in xylans resulting in the production of small oligosaccharides. The enzyme does not hydrolyze xylobiose and xylotriose. The xylan-oligosaccharides are further hydrolyzed by the action of exo-1,4-b-D- xylosidase which removes successive D-xylose residues from the non-reducing terminal. The action of xylanase is, however, restricted due to side chains. Never­theless, the accompanying arabinosidase, galactosidase, glucuronidase, and man — nosidase remove the branch points allowing xylanase action. The monomeric xylose molecules are fermented to ethanol or can be utilized to produce single cell proteins or single cell oil [44, 46].

Fast pyrolysis is usually the process of choice for the production of liquid biofuels such as bio-oil. It involves heating of biomass at temperatures of 300-1,300°C under steam or other non-oxidizing gases at pressures ranging from atmospheric pressure to pressures up to 3 MPa, to produce pyrolytic oils and/or medium to high energy value gases. The product of pyrolysis comprises a dark brown homoge­neous liquid. This liquid has a heating value which is about half of that of con­ventional fuel oil. The process of fast pyrolysis thus “concentrates” the biomass to a higher energy density liquid product which can significantly improve the logistics (because liquids can be easily transported from one place to another), and the economics (because energy density of biomass increases) of biomass con­version. The process parameters can be adjusted to give different types of process variants and consequently, different proportions of liquid solid and gas.

In the process of fast pyrolysis, high liquid yields can be obtained under the following process conditions:

• high heating and heat transfer rates

• careful control of pyrolysis reaction temperature of * 500°C and vapor phase temperature of *400-500°C

• short hot vapor residence time (<2 s)

• rapid cooling of pyrolysis vapors to give liquid bio-fuel.

As can be seen from the above conditions, fast pyrolysis requires rapid heating as well as rapid cooling, i. e., it requires efficient heat transfer. Therefore, the reactor equipment, process, and raw material should be such that efficient heat transfer is possible. Lower temperatures favor formation of charcoal hence, this is to be avoided. Table 1.5 gives the different process variants of fast pyrolysis with respect to their reactor systems and their salient features.

A detailed description of these systems, and the locations where these are operational on an industrial scale, are given by Bridgewater [12]. Pyrolysis, being an endothermic process requires process heat to be provided by some means. A commercial process design typically provides this heat from by-products obtained from within the process. About 50-75% of the energy content of feed­stock is required to drive the process. The energy content in char—one of the by-products of fast pyrolysis is about 25% of the energy in the feedstock, whereas gas—the other byproduct, contains only about 5% of the energy in the feed. Thus, the net heat content of the by-products is insufficient to provide the heat of pyrolysis hence, process heat for fast pyrolysis is required to be provided by other external means:

• hot reactor wall

• tubes through which hot char and air are circulated

• hot fluidizing gas

• recycled hot sand

• addition of hot air.

The internal utilization of energy from within the process, i. e., utilization of char, or gas or utilizing energy from fresh biomass, or from the product itself, for providing process heat can be done by the following ways:

• combustion of fresh biomass to provide process heat, instead of using the energy content in the by-product—char (especially where there is a good market for char)

• gasification of the by-product char and subsequent combustion of the resultant synthesis gas

• use of the by-product—gas along with external supplementation (because the energy content of the process by-product gas would be insufficient in itself)

• use of the main product of the process, i. e., bio-oil.

The use of fossil fuel can also be done to supplement the above sources of energy to get by-products with high energy value. A properly designed fast pyrolysis process and process equipment has no waste products other than clean flue gas and ash [12].

The liquid product of the fast pyrolysis process can be obtained by condensing the gaseous products of the process which are in the form of aerosols, true vapors and non-condensable gases, by rapid cooling of the gases. The aerosols may be coalesced or agglomerated to obtain the liquid product. Usually, the char gets entrained in the gaseous product of fast pyrolysis and acts as a vapor-cracking catalyst. This char is removed from the vapor by means of cyclones. In spite of the use of cyclones to remove char from the gases, some char still gets away in the vapor, and on condensation of this vapor, remains in the liquid product. The liquid product of fast pyrolysis is hence a microemulsion, and the char remaining in the liquid product causes destabilization of this microemulsion. The elaborate nature of this liquid microemulsion and its characteristics has been described by Bridgewater [12]. The char remaining in the liquid product is removed by a modified pressure filtration process where the particulates up to <5 pm can be removed. Balat et al. [14] give the chemical composition of fast pyrolysis liquid and list the chemicals obtained from biomass oil produced by a fast pyrolysis process.

High temperatures and a controlled environment lead to virtually all the raw material being converted into gas. This takes place in two stages. In the first stage, the biomass is partially combusted to form producer gas and charcoal. In the second stage, the CO2 and H2O produced in the first stage are chemically reduced by the charcoal, forming CO and H2. The composition of the gas is 18-20% H2, an equal portion of CO, 2-3% CH4, 8-10% CO2 and the rest nitrogen. These stages are spatially separated in the gasifier, with gasifier design very much dependant on the feedstock characteristics. Gasification requires temperatures of about 800°C and is carried out in closed top or open top gasifiers. These gasifiers can be operated at atmospheric pressure or higher. The energy density of the gas is generally <5.6 MJ/m3, which is low in comparison to natural gas at 38 MJ/m3, providing only 60% of the power rating of diesel when used in a modified diesel engine. Gasification technology has existed since the turn of the century when coal was extensively gasified in the UK and elsewhere for use in power generation and in houses for cooking and lighting. Gasifiers were used extensively for transport in Europe during World War II due to shortages of oil, with a closed top design predominating.

A major future role is envisaged for electricity production from biomass plantations and agricultural residues using large-scale gasifiers with direct cou­pling to gas turbines. The potential gains in efficiency using such hybrid gasifier/ gas turbine systems make them extremely attractive for electricity generation once commercial viability has been demonstrated. Such systems take advantage of low grade and cheap feedstocks (residues and wood produced using short rotation techniques) and the high efficiencies of modern gas turbines to produce electricity at comparable or less cost than fossil fuel-derived electricity. Net atmospheric CO2 emissions are avoided if growth of the biomass is managed to match consumption. The use of BIG/STIG (biomass integrated gasifier steam injected gas turbine) initially and BIG/GTCC (biomass integrated gasifier gas turbine combined cycle) as the technology matures, is predicted to allow energy conversion efficiencies of 40-55%. Modern coal electrical plants have efficiencies of about 35% or less. Combined heat and power systems could eventually provide energy at efficiencies of from 50 to 80%. The use of low-grade feedstocks combined with high conversion efficiencies makes these systems economically competitive with cheap coal-based plants and energetically competitive with natural gas-based plants.

It has been observed that it takes a little under 1,000 acres (400 ha) of poplar (grown as a short-rotation crop at a usable yield of 5 dry U. S. tons/acre, or 11 metric tons/ha) to supply an electric power plant with a capacity of one megawatt (1 MW). A typical small biomass-fired power plant (25 MW) with 80% availability (i. e., actually operating 80% of the time) would produce about 175 million kWh per year, or approximately the electricity needs of 25,000 peo­ple. The required 25,000 acres of land (about 10,000 ha) would occupy about 2% of the total land area within a radius of 25 miles (40 km). These calculations are based on a 30% conversion efficiency from heat to electricity, and an energy content for dry poplar wood of 17 Btu/U. S. ton (19.7 GJ/metric ton).

The cost of electricity from two contrasting technologies (one present-day, one future), for a biomass-fired power plant from 10 to 50 MW in size are given in Table 2.1.

2.4.2.2 Catalytic Liquefaction

This technology has the potential to produce higher quality products of greater energy density. These products should also require less processing to produce marketable products. Catalytic liquefaction is a low temperature, high pressure thermochemical conversion process carried out in the liquid phase. It requires either a catalyst or a high hydrogen partial pressure. Technical problems have so far limited the opportunities of this technology.

Marcel Lucas, Gregory L. Wagner and Kirk D. Rector

4.1 Introduction

Lignocellulosic biomass could become an abundant source of liquid fuels and commodity chemicals that could satisfy energy needs in transportation and alleviate concerns about rising greenhouse gas emissions. The variety of potential feedstocks, which includes wood, agricultural wastes, forest products, grasses, and algae, reduces the pressure on food crops, in particular corn, for the production of ethanol [1,2]. Wood is composed of three main components: cellulose, hemicellulose, and lignin. Cellulose is a polymer of glucose. Hemicellulose is a branched polymer of different monosaccharides. Lignin is a branched polymer with p-hydroxyphenyl, guaiacyl, and syringyl units [3]. The conversion of lignocellulosic biomass to biofuels consists in the hydrolysis of cellulose and hemicellulose into fermentable sugars, followed by the fermentation of these sugars into ethanol and commodity chemicals. The access of enzymes to cellulose is severely restricted by the complex structure of the wood cell wall and the recalcitrance of lignin.

Conversion of native biomass to biofuels therefore requires a pretreatment step that should separate the three main components of wood, improve access of enzymes to cellulose, and decrease cellulose crystallinity. Kraft pulping has been the dominant process to produce purified cellulose substrates for papermaking, but it involves toxic chemicals and requires large amounts of water [4]. Other biomass pretreatments, such as acid hydrolysis, steam explosion, alkaline hydrolysis, and ammonia fiber explosion, are energy-intensive and also involve toxic chemicals [5]. Pretreatment is the most expensive step in the biomass conversion process, and could represent a fifth of the total cost [1, 2].

M. Lucas • G. L. Wagner • K. D. Rector (H) Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA e-mail: kdr@lanl. gov

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

DOI: 10.1007/978-3-642-28418-2_4, © Springer-Verlag Berlin Heidelberg 2012

Recently, room-temperature ionic liquids (ILs) have been considered as potential solvents for the dissolution and pretreatment of biomolecules and biomass [4, 6-11]. It was found that the solubility of cellulose in 1-butyl-3- methylimidazolium chloride ([BMIM][Cl]) could reach 25 wt% [9]. ILs are salts with melting temperatures below 100°C, characterized by an extremely low vapor pressure, high thermal stability, and low flammability. Their physicochemical properties, such as glass transition and melting temperatures, thermal stability, refractive index, and polarity depend on their chemical composition and structure [12-14]. The multitude of possible anion-cation combinations and the blending of multiple ILs provide great flexibility when tailoring an IL for a specific application [15]. ILs have numerous promising applications in catalysis [16-20], electro­chemistry, separations of gases, liquids, and impurities [21]. The positive effect of ionic liquids on catalysis was partially attributed to the stabilization of reactive intermediates and catalytically active oxidation states [17]. Promising studies on cellulose dissolution and regeneration led to an intense effort to develop an effective IL pretreatment for the direct pretreatment and dissolution of native biomass [4, 6-11].

In this chapter, the dissolution of native biomass in ILs will be reviewed. In Sect. 4.2, the different factors affecting biomass solubility in ILs will be reviewed. The mechanisms involved in the biomass delignification and cellulose dissolution will be discussed in Sect. 4.3. Section 4.4 will focus on the compatibility of ILs with cellulases and the different strategies developed for the stabilization of enzymes in ILs. Section 4.5 will deal with the recycling and biodegradability of ILs. Finally, in Sect. 4.6, applications of biomass pretreatment with ILs (other than fuel production) in the making of composite materials, the biomedical field, the production of commodity chemicals, and biochemical sensing will be reviewed.

The main application of the pretreatment of native biomass remains the pro­duction of liquid fuels. However, the valorization of lignin and hemicellulose may enhance the economic viability of IL-based processes through the produc­tion of commodity chemicals [106]. Ionic liquids have been used as the solvent for the synthesis of 5-hydroxymethylfurfural. Corn stovers were converted into 5-hydroxymethylfurfural in [EMIM][Cl] using CrCl2 as a catalyst [163]. Pine wood and rice straw were also used as feedstocks using [BMIM][Cl] under microwave irradiation with CrCl3 as the catalyst [35]. Acorns, with a high starch content, were successfully converted into 5-hydroxymethylfurfural in 1-octyl-3- methylimidazolium chloride, using mixtures of chromium halides (CrCl2, CrCl3, CrBr3, CrF3) as catalysts [164].

Chemical functionalization of native wood sawdust was performed by disso­lution of the biomass in an IL, followed by its reaction at high temperature or room temperature. For example, poplar sawdust reacted with octanoyl chloride, butyryl chloride and lauroyl chloride in [BMIM][Cl] to produce esterified wood [165, 166]. The addition of an acetic anhydride-pyridine mixture to a solution of spruce led to wood acetylation [7]. Milled fir wood was dissolved in [BMIM][Cl] and also reacted with acetic anhydride with pyridine for acetylation [40]. Norway spruce sawdust and thermomechanical pulp reacted with acetyl chloride, benzoyl chlo­ride, acetic anhydride, phenyl isocyanate, and lauroyl chloride after dissolution in [BMIM][Cl] to produce acetylated, benzoylated, lauroylated, and carbanilated wood derivatives. Thermogravimetric analyses and differential scanning calo­rimetry showed that the thermal properties of spruce were affected with the appearance of a clear glass transition [30, 167].

Fig. 4.4 Scanning electron micrographs of aerogels produced from spruce wood, coagulated in baths containing a 10 wt% ethanol and b 90 wt% ethanol. Reprinted from [168], copyright (2011), with permission from Elsevier

In another study, spruce thermomechanical pulp reacted with benzoyl chloride and lauroyl chloride in [BMIM][Cl] with pyridine to produce benzoylated and lauroylated spruce. The spruce derivatives were then added to poly(styrene) and poly(propylene) to extrude composite fibers and sheets. Thermogravimetric anal­yses showed an enhanced thermal stability of the composites compared to the spruce thermomechanical pulp [167]. The modifications of the chemical and thermal properties could improve the processability of wood and increase its compatibility to other polymers for the fabrication of composite materials.

Aerogels from milled spruce wood were prepared by dissolving the wood in [BMIM][Cl] at 130°C for more than 4 h. The hot solution was immersed in an ethanol bath at room temperature. The obtained gel was then transferred into a cell where it is immersed in ethanol and liquid CO2 at 70 bars for 2-3 h. The mixture was heated above the supercritical temperature when the pressure was released to obtain the dry aerogel. Scanning electron micrographs of the aerogels revealed an

open pore structure, with pore sizes ranging from 100 nm to 4 pm depending on the feedstock and the reaction conditions (Fig. 4.4) [168].

Composite fibers derived from native Southern yellow pine, oak, and bagasse were prepared by dissolving them in [EMIM][OAc] at temperature above 175°C for 10-30 min. The solution was spun into fibers by extruding the solution into a water bath. The selection of the feedstock affected the thickness and surface roughness of the resulting fiber. The thickness was higher when the biomass dissolution in IL was incomplete. Fibers made from pine pretreated with NaOH were thinner and their surface was smoother, possibly due to the lower hemicel — lulose and lignin content. Dissolution of bagasse at higher temperatures (185°C for 10 min) improved the processability and the maximum tensile stress applied to the fibers before breaking. The fibers with the highest tensile stresses at failure had the highest cellulose content and were derived from biomass with the highest cellulose content: bagasse with a 58% cellulose content (wt% of biomass) and oak with 49% cellulose content [169]. Wool keratin fibers, a polymer of amino acids, and cel­lulose were also dissolved at high temperatures (above 100°C) in [BMIM][Cl] and successfully extruded into composite fibers using methanol as the anti-solvent. The composite fibers were less brittle than the pure regenerated wool keratin fibers [170].

All applications mentioned above required the dissolution of biomass at high temperatures, typically above 100°C. A more economical and less energy-inten­sive strategy is to exploit the swelling of the biomass upon exposure to IL. Poplar wood was shown to swell at room temperature when exposed to [EMIM][OAc], with cell walls cross-sectional areas expanding by 60-100% in 3 h. After rinsing with deionized water, the wood structure contracted almost immediately [70, 71]. The rinsing of the swollen biomass with a suspension of nanoparticles allowed for the incorporation of materials inside the wood structure without prior dissolution. As a proof of concept, gold nanoparticles of 100 nm diameter were incorporated into poplar and confocal surface-enhanced Raman microscopy showed that the nanoparticles were up to 4 im deep into the cell wall structure (Fig. 4.5). The incorporation of materials/chemicals into natural and paper products have numerous applications in the development of effective biomass pretreatments, isotope tracing, sensing, and imaging [70].

4.6 Conclusions

In the past decade, numerous ILs have been synthesized to improve the pretreat­ment and dissolution of native biomass. The application of advanced analytical techniques have provided an insight into the mechanisms involved in the biomass dissolution and the improved access of enzymes to cellulose for a more efficient conversion to fermentable sugars. Advances on the development of IL-tolerant cellulases would enable the pretreatment of biomass and the hydrolysis of cellu­lose in one step, and therefore improve the economic viability of IL pretreatment of biomass. Biomass dissolution in ILs also has potential applications in com­posites, tissue engineering, chemical functionalization and sensing. Improved extraction processes are still necessary to optimize efficiency and recycle the ILs. Issues, such as corrosion due to ILs, their full environmental impact and disposal, remain unresolved.

Acknowledgments This study was funded by a Laboratory Directed Research and Development grant from Los Alamos National Laboratory (20080001DR).

Plant architecture is one of the important points to be considered for biomass enhancement. It is clear that different plant species grow to different heights, sizes and shapes. The final size and shape are determined by genetic and environmental factors. Thus, it would be appropriate to conclude that plant architecture is determined and influenced by the genetic information and the environmental factors, respectively. The final shape of a mature plant is established by post­embryonic growth of the shoot apical meristem (SAM) and root apical meristem (RAM). SAM activity involves development of lateral organs such as leaves, flowers and branches as well as maintenance of the meristem identity in a pool of stem cells within the meristem. Recent data show that SAM is controlled by several genes such as SHOOTMERISTEMLESS, CLAVATA and WUSCHEL in dicotyledonous plants (e. g., Arabidopsis) and OSH1 and MOC1 in monocotyle — donous plants (e. g., rice) (see [11] for detailed review). The involvement of

various phytohormones such as cytokinin, gibberellin, auxin and abscisic acid in regulating shoot development has been well recognized by plant physiologists and developmental biologists. Therefore, it is interesting to note that besides the genes listed above, several key regulatory genes that influence shoot development have been identified, among which are phytohormone signaling intermediates such as ARR5, ARR6 and ARR7 [11].

A number of other genes are known to be involved in regulating branching. Table 8.1 lists some of the known mutants with increased or decreased branching (for review see [12, 13]). The process of branching could be viewed as a multi­pronged developmental event, because it will involve establishment of axillary meristem, development of axillary bud, promotion of the outgrowth of the branch by overcoming the apical dominance [13]. Therefore, one can expect to find genes regulating the various steps in this developmental program, and they can be the targets of genetic modification of branching.

Manipulation of selected genes that are involved in plant growth and devel­opment may lead to the increase in the biomass. For example, mutation in a cytochrome P450 gene called SUPERSHOOT resulted in significantly increased axillary bud growth and led to profuse branching and significant increase in bio­mass [14].

Likewise mutations in the MAX1 and MAX2 loci resulted in bushy shoots in Arabidopsis [15]. The presence of OsMAX gene family in rice suggests that similar functions may be conserved in monocotyledonous plants as well. Also, overex­pression of a gene called OsSPL14 in rice increased shoot branching in the veg­etative stage and panicle branching in the reproductive stage [16]. The feasibility of modifying plant architecture was demonstrated with the bahiagrass (Paspalum notatum), which is a low input requiring turf grass, but with the undesirable trait of tall seedheads. Application of plant growth retardants can lead to shorter stature, but long-term use of chemicals may lead to phytotoxicity and environmental pollution. Hence, in an attempt to modify the architecture to shorter tillers with shorter leaves, transgenic plants expressing ATHB16 gene were generated [17]. These transgenic plants expressing the repressor of cell expansion (ATHB16 gene) exhibited the more desirable shorter tiller phenotype, likely to be conferred by the transgene. The teosinte branched1 (tb1) gene in maize, and homologs in wheat, rice and Arabidopsis regulate tillering or branching [13]. The loss of function of the probable rice ortholog OsTB1 gene (fine culm 1) leads to increased tillering in rice, and its overexpression leads to decreased tillering [18]. Similarly, overex­pression of the wild-type form of maize tb1 gene in wheat leads to decreased tillering, suggesting that this gene function is conserved among a variety of plant species [18]. The action of tb1 gene in sorghum (SbTB1) has been demonstrated to be under the control of phytochrome B, with suppression of the gene by the active Pfr form leading to promotion of tillering [19]. Conversely, when light conditions cause inactivation of phytochrome B, SbTB1 expression is increased and tillering is inhibited, which explains the light-mediated control of branching

This is supportive of the proposal of a combinatorial model of shoot devel­opment proposed according to which a series of independently regulated but overlapping programs modify a common set of processes leading to change from juvenile to mature phase [20]. This concept holds good and the identification of various regulatory genes and the complex genetic interactions among these genes as well as their interactions with biochemical (phytohormones) as well as envi­ronmental factors are beginning to emerge. Thus, a recent study showed that growing maize in clumps rather than equidistant planting under dryland conditions results in less tillering and biomass accumulation [21]. Due to the fact that plant architecture is significantly influenced by the phytohormones we will discuss how they may be used to enhance biomass in selected species.

S. cerevisiae is the preferred industrial microorganism for ethanol production because of its excellent fermentability and higher tolerance to industrial condi­tions. However, S. cerevisiae has some problems in producing ethanol from lig — nocellulosic materials, which are different from that of starch. Hemicelluloses, the second most common polysaccharide in nature, represent about 20-35% of lig — nocellulosic biomass. However, S. cerevisiae cannot utilize pentose released from hemicelluloses of lignocellulosic materials, thus decreasing the yield of ethanol production. In addition, although S. cerevisiae is robust, it cannot adequately resist the inhibitors derived from the process of pretreatment of lignocellulose [119].

Pentoses such as xylose and arabinose are the second most abundant fer­mentable sugars in the hydrolysate from agricultural residues. S. cerevisiae cannot utilize them due to the absence of enzymes in the first steps of the metabolic pathways. It is desired for xylose and arabinose to be fermented into ethanol by the industrial S. cerevisiae yeast strains to improve ethanol production efficiency and reduce the cost of the production [198].

Metabolic engineering technologies have been widely developed to set up the new pathways in S. cerevisiae. Wang [191] constructed the recombinant plasmids containing the genes that encode xylose reductase (XR) and xylitol dehydrogenase (XDH) from P. stipitis. Xylulokinase (XK) from S. cerevisiae has been trans­formed into the industrial strain of S. cerevisiae for the co-fermentation of glucose and xylose. This recombinant strain NAN-127 consumed twice as much xylose and produced 39% more ethanol than the parent strain in shake-flask fermentation [191]. However, the expression of so many enzymes in a single microorganism may represent a metabolic burden that negatively influences the fermentation capacity [54]. Most of the efforts in lignocellulosic ethanol production with

S. cerevisiae has been directed to improve the pentose fermentation. The expression of the P. stipitis genes XYL1, encoding a xylose reductase (XR), and XYL2, encoding a xylitol dehydrogenase (XDH), was the first successful approach for D-xylose utilization by S. cerevisiae [99, 185]. The first recombinant strains produced xylitol from D-xylose rather than ethanol. It was then suggested that the endogenous xylulokinase (XK), encoded by XKS1, could be limiting the perfor­mance of S. cerevisiae on D-xylose.

A biorefinery is a facility that integrates biomass conversion process and equip­ments to produce fuel, power, heat, and value-added chemicals from all types of feedstock comprising of biowaste and/or renewable biomass in a manner that it is zero waste producing. Thus, the bioconversion methods discussed in the previous sections are integrated to give multiple products. Biorefineries, since 1990s have evolved through a number of stages: The first stage of development can be con­sidered to be the Phase I biorefinery which used a single feedstock and was processed to give a single product. The Phase II biorefinery uses again a single feedstock but has flexible processing capabilities which finally result in a range of products. A conventional sugar plant, which produces sugar, ethanol, and biode­gradable plastics is an example of this type of a biorefinery. Thus, a phase II biorefinery incorporates a flexibility in terms of processing methods as well as products. A phase III biorefinery is the present “integrated biorefinery’’, where a variety of feedstocks can be handled via different conversion processes simulta­neously to produce a range of products. A Phase III biorefinery offers complete flexibility in terms of type of feedstock that can be used, conversion processes as well as product mix. Feedstock flexibility has, however, the first priority [33]. A present biorefinery can be considered to be analogous to a petroleum refinery which produces multiple fuels and products from petroleum. An excellent analogy with respect to the flow chart for both types of biorefineries is given in a report by the U. S. Department of Energy [34].

In the interest of mankind, it is absolutely essential that dependence on fossil fuels be reduced.

Integration of green chemistry into biorefineries and use of low environmental impact technology is the focus of all research efforts in the area of biorefinery. This, in concurrence with economically viable processes, will successfully replace petroleum refinery and products resulting from it, by biorefinery products.

The distinctive feature of a screw type briquetting machine is that heat is applied to the die ‘bush’ section of the cylinder. This brings about two important operational advantages. The machine can be operated with less power and the life of the die is prolonged. Further, the surface of the briquette is partially carbonized/torrified to a dark brown color making the briquette resistant to atmospheric moisture during storage. The temperature of the die should be kept at about 280-290°C. If the die temperature is more than the required one, the friction between the raw material and the die wall decreases such that compaction occurs at lower pressure which results in poor densification and inferior strength. Conversely, low temperature will result in higher pressure and power consumption and lower production rate.

2.6.2.2 Effect of External Additives

The briquetting process does not add to the calorific value of the base biomass. In order to upgrade the specific heating value and combustibility of the briquette, certain additives like charcoal and coal in very fine form can be added. About 10-20% char fines can be employed in briquetting without impairing their quality. Further, only screw pressed briquettes can be carbonized. When carbonized with additives in the briquette to make dense charcoal, the yield is remarkably increased. However, depending upon the quality of charcoal and coal powder, various formulations can be evolved for optimal results.

In piston press technology the effect of particle size and moisture content is similar to that of the screw press. But in this case preheating of raw material is not employed and the die is not heated. In fact the die needs cooling for smooth briquetting.