The dissolution of Norway spruce in [BMIM][Cl] or [AMIM][Cl] depended on the size of the biomass. Whereas ball-milled powder and spruce sawdust (size 0.1-2 mm) were completely dissolved at 80°C in several hours, it took several weeks to dissolve wood chips (5 x 5 x 1 mm3) at 130°C in the same ILs. In general, dissolution was fastest for ball-milled wood, followed by sawdust (particle size 0.1-2 mm), thermomechanical pulp fibers, and wood chips [7]. A similar size effect was observed for southern pine and red oak wood chips [36], and rice straw [46]. Ball-milling of Norway spruce TMP and southern pine increased the glucose yield after IL pretreatment and enzymatic hydrolysis, by opening access to the wood structure for enzymes. The same effect, which became more significant with milling time, was also observed for corn stovers. The molecular weight of ball — milled corn stovers decreased with increasing milling time [33].

The size reduction effect could be explained by the increase of effective surface area and the improved access of enzymes to the biomass cellulose. However, feedstock size reduction through mechanical grinding is energy-intensive [36]. Also, ball-milling for several days can lead to significant degradation and chemical modification of cellulose and lignin, as well as the generation of soluble species that reduces the recyclability of the IL [31, 33, 57]. It was reported that extensive ball-milling causes cleavage of aryl-ether linkages in lignin and the generation of phenolic hydroxyl groups [57].

Since the first report [10], until today, there is no available information about catalysts for HDO better than Ni-Mo and Co-Mo. Although, they are known as good deoxygenating catalysts for 20 years, recently, due to increasing interest in production of renewable diesel, numerous research groups have done studies on improving catalyst performance and understanding mechanism of HDO of fatty acids and their derivatives.

Nowadays the commonly used catalysts are sulfided Ni-Mo/y-Al2O3 and Co — Mo/y-Al2O3 [16-20]. Sulfided catalysts gave higher conversion and selectivity at lower temperatures, between 250 and 300°C (Table 6.1), compared to perfor­mance of the oxides which are more active at temperatures above 350°C [10]. Although sulfided catalysts are more active in deoxygenation, there is a threat of fast deactivation caused partially by leaching of sulfur from the catalyst (Table 6.1). Alternatively sulfur could be added to feedstock, but in that case it will cause contamination of the product—diesel fuel, which should be sulfur-free.

When comparing sulfided catalysts, in terms of activity and selectivity, sulfided Ni-Mo/c-Al2O3 outperformed Co-Mo/c-Al2O3 catalyst at lower temperatures (250-300°C) giving higher conversion and selectivity to hydrocarbons (Table 6.1) [11-13]. The data shows that the sulfided Ni-Mo/y-Al2O3 is the most suitable catalyst taking into account its selectivity and activity for HDO reaction at rela­tively low temperature.

Pineapple waste comprises the skin, seeds, and remaining parts after juice extraction. Cooked Sago is added to mill juice to enrich it with sugar to a level of 8% (w/w). The physicochemical characteristics of pineapple waste are given in Table 9.6 that shows its potential for ethanol production due to high carbohy — drates/total solids content [183].

9.2.4.3 Orange Peel Waste

Orange waste is another substrate used for ethanol production. The proximate composition of orange waste and orange filtrate is given in Table 9.7.

9.2.4.4 Potato Peel Waste

The proximate composition of potato waste and potato filtrate (Table 9.7). Apparently, indicates its suitability for the production of ethanol.

9.2.2 Coffee Waste

Use of coffee waste as a substrate for ethanol fermentation has also been reported earlier [16].

Orange waste coming from food industries is used in continuous fermenta­tion. It has been found that fixed bed immobilized cell reactor showed maximum ethanol production [50]. Use of citrus processing by-product mainly peel by fermentation by S. cerevisiae for ethanol production has been reported [58, 94]. The initial saccharification of polysaccharides by commercial cellulase and poly­galacturonase followed by removal of inhibitory compounds by filtration and pH adjustment of the hydrolysate was necessary for successful fermentation [29].

Ethanol has also been produced from lignocellulosic waste by employing recombinant bacterial strains of E. coli and Klebsiella oxytoca [91]. The bacterial strains had the capacity to produce ethanol from pentose sugars. The conversion of monosaccharides in orange peel hydrolysates into ethanol by recombinant E. coli (KOll) was in pH controlled batch fermentations that led to very high yields of ethanol. The microorganism was capable of converting all major monosaccharides in orange peel hydrolysates into ethanol and to a smaller amount of acetic and lactic acids [57]. Citrus molasses prepared by evaporation and concentration of the press liquor and molasses mixed with the citrus pulp have also been used by distillaries as an alcohol feedstock [50]. Initial moisture content of the solid medium has been shown to be a limiting factor for maximum ethanol production [130]. Industrial alcohol has also been produced from waste fruits such as apple, pear, and cherry through fermentation [11].

Maneesha Pande and Ashok N. Bhaskarwar

Rapid depletion of fossil fuels, compounded by the accompanying environmental hazards, has prompted the need for alternative sources of energy. Energy from biomass, wind energy, solar energy, and geothermal energy are some of the most promising alternatives which are currently being explored. Among these, biomass is an abundant, renewable, and relatively a clean energy resource which can be used for the generation of different forms of energy, viz. heat, electrical, and chemical energy. There are a number of established methods available for the conversion of biomass into different forms of energy which can be categorized into thermochemical, biochemical, and biotechnological methods. These methods have further been integrated into the concept of a biorefinery wherein, as in a petroleum refinery, a variety of biomass-based raw materials can be processed to obtain a range of products including biofuels, chemicals, and other value-added products. We present here an overview of how biomass can be used for the generation of different forms of energy and useful material products in an efficient and economical manner.

1.1 Introduction

The current major source of energy/fuel is fossil fuel, which, for all practical purposes can be considered to be nonrenewable. Fossil fuels are all petroleum derivatives and the use of these fossil fuels leads to the generation of greenhouse gases such as CO2, CH4, N2O. The transportation sector is responsible for the

M. Pande • A. N. Bhaskarwar (H)

Department of Chemical Engineering, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India e-mail: anbhaskarwar@gmail. com

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

DOI: 10.1007/978-3-642-28418-2_1, © Springer-Verlag Berlin Heidelberg 2012

highest rate of growth in greenhouse gas emissions (GHG) among all sectors. This concern as well as the current concern over the rapid depletion of fossil fuel, accompanied by the ongoing price increase of fossil resources and uncertain availability, combined with environmental concerns such as global warming has propelled research efforts toward generating alternative means of energy produc­tion using renewable resources. The solution to this problem seems to emerge in the form of bioenergy, i. e., energy generated from biomass.

Biomass is the only renewable organic resource. It is also one of the most abundant resources. It comprises all biological materials including living, or recently living organisms, and is a huge storehouse of energy. The dead biomass or the biological waste can be used as a direct source of energy like heat and elec­tricity or as an indirect source of energy like various types of fuels. The living biomass, or components thereof, like microorganisms, algae, and enzymes can be used to convert one form of energy into another using biofuel cells. Figure 1.1 gives the various sources of biomass which can be used for biomass conversion into energy. In the entire process of conversion of biomass into energy, a dual purpose of energy generation and environmental clean-up is achieved.

Sunlight is an infinitely abundant source of energy on this earth and all energy on this planet, in principle, is renewable. However, considering the factor of time frame, the present sources of energy such as coal, oil, and natural gas take mil­lennia to renew. Therefore, it is imperative that research in the field of energy generation should focus on reducing this time frame by cutting short the time required to turn sunlight into usable energy. Biomass is an excellent source of renewable energy and serves as an effective carbon sink. Plants and trees which constitute biomass can be considered as perpetual powerhouses capable of con­tinuously tapping the energy from sunlight and converting it via photosynthesis

Atmospheric carbon dioxide, water and sunlight

through photosynthesis

Plant material is harvested
ana burnt

Fig. 1.2 Renewable nature of biomass conversion into energy into carbon-rich compounds. These carbon-rich compounds which constitute the biomass can then be exploited as and when required to release the energy trapped from sunlight (Fig. 1.2).

It can be seen from Fig. 1.2 that the carbon which is released into the atmo­sphere as a result of burning biomass, returns to the biomass by way of photo­synthesis, which is again converted into carbon-rich compounds for reconversion into energy. This, process can thus be considered to be carbon neutral unlike fossil fuel, which is carbon positive, i. e., burning fossil fuel releases CO2 into the atmosphere which remains in the atmosphere, thus increasing the amount of CO2 indefinitely.

The current technology of biomass to energy conversion is at the most, carbon neutral but the amount of CO2 already present in the atmosphere as a result of use of fossil fuel for so many years, is so high that it cannot be absorbed by con­ventional sinks such as trees and soils. Thus there is a dire need to reduce the global CO2 emissions by energy generation technologies that are carbon negative in nature. These technologies, which are commonly termed as ‘‘Bioenergy with Carbon Capture and Storage’’ (BECCS) are expected to achieve the goal of cre­ating a global system of net negative carbon emissions. This carbon capture and storage (CCS) technology, serves to intercept the release of CO2 into the atmo­sphere and redirect it into geological storage locations. A similar alternative to achieve carbon negativity lies in fourth-generation fuels which are those fuels based on high solar efficiency cultivation. This chapter gives an overview of conversion of biomass into energy with special reference to the biorefinery con­cept. The recent developments in the area are also highlighted.

There are currently three established technologies for the production of electricity from biomass—pyrolysis, gasification, and direct combustion. These are already discussed at length in Sect. 1.2.1.1 Direct combustion is the oldest method for electricity generation from biomass where complete oxidation of biomass in presence of excess air is done to produce carbon dioxide and water. Hot flue gases are used to heat the process water to steam, which can be used to drive a turbine resulting in production of electricity. This is not a very efficient method of elec­tricity generation when compared to pyrolysis and gasification. Pyrolysis involves the thermal destruction of biomass under anaerobic conditions without the addition of steam or air resulting in the production of gases and condensable vapors. Combustion of these gases is done in a gas turbine resulting in generation of electricity. This method is more efficient than direct combustion but requires more process control and investment. The gasification method comprises controlled addition of steam to the biomass resulting in partial oxidation of the biomass to produce combustible gases which have a high calorific value. These gases are fed to a combined gas turbine to produce electricity. This method, like pyrolysis is
more efficient than direct combustion but requires more process control and investment. The carbon emissions produced as a result of electricity generation from biomass are much lower than the other energy counterparts. The highest carbon emissions during electricity generation from biomass is reported to be 60 g CO2 equivalent/kWh, which is less than one-third of the lowest CO2 emission during electricity produced from natural gas and one-fifth of the lowest CO2 emissions produced from a coal — fired power station [57].

First-generation technologies are well established, these include transesterification of plant oils, fermentation of plant sugars and starch for liquid biofuel production, anaerobic fermentation of organic residues to generate biogas, combustion of organic materials for heat recovery or combined heat and power (CHP) systems for the production of both heat and electrical power. Second-generation or advanced technologies often refer to the conversion of lignocellulose materials into fuels. These technologies comprise a range of alternatives such as enzymatic production of lignocellulose ethanol, syngas-based fuels, pyrolysis-oil based biofuels, gasifi­cation and others, but are not yet economically viable and technical aspects are still under development.

Much attention is currently focused on the production of liquid biofuels that are manufactured with first-generation technologies because they rely on feedstocks derived from food-crops, the so-called first-generation biofuel. Thus, this has heightened the needs to identify and work on agronomic potential of alternative bioenergy crops including non-edible oil crops such as jatropha, castor bean, jojoba, karanja that can be grown on land unsuitable for food crops and multi­purpose crops like sweet sorghum that can yield food in the form of grain, fuel in the form of ethanol from its stem juice, and fodder from its leaves and bagasse.

Deployment of second-generation technologies offers an opportunity to expand the type of feedstock and to take advantage of currently unused lignocellulose sources. It also facilitates the use of energy crops that can be grown on land unsuitable for food crops. These technologies offer a more efficient production making use of the entire plant beyond the carbohydrate component. Further research and development on bioenergy conversion technologies is required to overcome the technical barriers for them to become a viable option.

2.7 Conclusion

Various technology options are available from biomass which can serve many different energy needs from large-scale industrial applications to small-scale, rural end uses. Different types of solid, liquid or gaseous fuels exist in bioenergy. Such fuels can be utilized in transportation and also in engine and turbine electrical power generation. Chemical products can also be obtained from all organic matter produced. There are various conversion technologies that can convert biomass resources into power, heat and fuels for potential use. Biorefinery integrates bio­mass conversion processes and equipment to produce fuels, power and value — added chemicals from biomass.

First-generation biofuels can be derived from sources such as starch, sugar, animal fats and vegetable oil and can be produced through well-known processes such as cold pressing/extraction, transesterification, hydrolysis and fermentation, and chemical synthesis. The most popular types of first-generation biofuels are biodiesel, vegetable oil, bioethanol and biogas. Second-generation biofuels are not yet commercial on a large scale as their conversion technologies are still in the research and/or development stage. Second-generation biofuels are produced through more advanced processes, including hydro treatment, advanced hydrolysis and fermentation, and gasification and synthesis. A wide range of feedstocks can be used in the production of these biofuels, including lignocellulosic sources such as short-rotation woody crops. These produce biodiesel, bioethanol, synthetic fuels and bio-hydrogen.

Cellulose hydrolysis is the result of the synergistic action of three different types of cellulases: endoglucanases that cleave b-1,4-glycosidic bonds on cellulose chains, cellobiohydrolases that convert long cellulose chains into cellobiose, and b-glu — cosidases that convert cellobiose into glucose [118, 119]. The mechanisms underlying cellulase activity on a heterogeneous substrate, such as lignocellulosic biomass, is still under investigation [72, 119]. Multiple models have been devel­oped to understand the multiple steps involved in cellulose hydrolysis: adsorption of cellulases on the substrate, formation of the enzyme-substrate complex, loca­tion and hydrolysis of b-glycosidic bonds, desorption of the enzyme, synergy between endoglucanases, cellobiohydrolases, and b-glucosidases [119].

Once biomass is regenerated from its IL solution, it can still contain traces of IL that can reduce cellulase activity [72]. Several studies have focused on the stability of commercial cellulases in various ILs and their saccharification yields on purified cellulose substrates and native biomass. Celluclast 1.5L (cellulases from Tricho — derma reesei) and Novozyme 188 (b-glucosidase from Aspergillus niger) retained 76 and 63% of their original activity on carboxymethylcellulose after incubation at 50°C for 24 h in 15 and 20% [EMIM][OAc] solutions, respectively [120]. The activity of Celluclast 1.5L was also assessed on a-cellulose in [MMIM][DMP], [AMIM][Cl], [BMIM][Cl], and [EMIM][OAc] at a 10 vol.% concentration. The activity in these ILs was between 70 and 85% lower than the activity in sodium acetate buffer at pH 4.8 [67]. An increase in the IL concentration led to an increase in the IL viscosity by a factor of 4 [67]. The activity of cellulases from Tricho — derma reesei on cellulose azure was found to decrease dramatically with low concentrations (22 mM) of [BMIM][Cl] or [BMIM][BF4] [121]. No saccharifi­cation of Avicel cellulose was observed with cellulases from Trichoderma reesei in 60 vol.% [EMIM][DEP] [122]. The activity of cellulases from Aspergillus niger decreased with incubation time in [BMIM][Cl] and [BMIM][Cl] concentration

[123] . It is important to note at this point that variations of 20% in cellulase activity were observed between different Celluclast 1.5L lots from the same manufacturer [67].

Despite the partial deactivation of cellulases in ILs, reducing sugar yields were still higher after IL pretreatment with low residual IL concentrations, due to the improved access of enzymes to the cellulose in biomass. For example, the cellu­lase mixture of Celluclast 1.5L and Novozyme 188 still converted 45% of the cellulose contained in a solution of 0.6% [EMIM][OAc]-pretreated yellow poplar with 15% [EMIM][OAc]. The conversion rate was much higher than for the untreated yellow poplar (11%) [120]. The activity of the same mixture was also assessed on purified cellulose substrates: an Avicel solution in [EMIM][OAc] and untreated Avicel in citrate buffer. After enzymatic hydrolysis for 24 h at 50°C, 91% of the [EMIM][OAc]-pretreated Avicel was converted to glucose, while only 49% of the untreated Avicel was converted [120]. With cellulases from Trichoderma reesei in 20 vol.% [EMIM][DEP], 70% of the cellulose was con­verted to cellobiose or glucose, a conversion rate that was higher than the untreated Avicel (about 33%). A comparison with [EMIM][OAc] using the same procedure yielded conversion rates that were half of those with the diethylphosphate anion [122].

The stability of another commercial cellulase, GC 220, a mixture of endoglu — conases and cellobiohydrolases from Trichoderma reesei was assessed in eight different ionic liquids. With the exception of tris-(2-hydroxyethyl)methylammo — nium methylsulfate (HEMA), the fluorescence of the trytophyl marker on the cellulases was quenched in the other ILs that included several imidazolium-based ILs, suggesting denaturation of the enzymes. The cellulase activity was measured spectroscopically in a citrate buffer (pH 4.8) and in the eight ILs using cellulose azure as the substrate. Cellulase activity was detected only in the ILs 1-methyl — imidazolium chloride ([MIM][Cl]) and HEMA, but it was significantly lower than in the buffer. The cellulases remained active even after 2 h in these two ILs at 65°C [124].

The tolerance of cellulases produced by Penicillium janthinellum to ionic liq­uids was tested by incubating the extracted enzymes in an aqueous solution of [BMIM][Cl] of concentration ranging from 10 to 50%, and then measuring their residual activity on different substrates (filter paper Whatman no. 1, carboxy — methylcellulose, xylan solution or p-nitro phenyl b-D-glucopyranoside). After incubation in 10% ionic liquid for 5 h, the cellulases retained at least 80% of their activity on all substrates. At a higher concentration of 50%, the residual activity decreased significantly to reach below 20% for all substrates [125].

The tolerance of cellulase-producing bacteria from termites to [BMIM][Cl] was studied by characterizing their growth in [BMIM][Cl] at concentrations ranging from 0.1 to 10 vol.%. The three bacteria that were the most effective at cellulase production could tolerate [BMIM][Cl] at concentrations smaller than 1.0 vol.%. No growth was observed for concentrations larger than 5 vol.%. For two of the bacteria, the growth rates were unchanged for concentrations smaller than 1.0 vol.%. [118].

Cellulases are deactivated in ILs through multiple mechanisms. Stability and unfolding of the cellulases were studied by differential scanning calorimetry. Thermal unfolding was irreversible in the citrate buffer with a broad transition peak between 60 and 75 °C. The ILs [MIM][Cl] and HEMA improved the stability of the cellulases with the shift of the transition temperatures above 75 °C. The low activity in HEMA compared to the buffer was attributed to the high viscosity of HEMA [124]. Cellulase activity also decreased when the viscosity of the enzyme solution without IL was increased with polyethylene glycol [67].

Deactivation was attributed to the dehydrating environment introduced with [BMIM][Cl] that causes the denaturation of the enzyme. This conclusion was supported by fluorescence spectra of the cellulase in [BMIM][Cl] and various denaturants, such as the surfactant sodium dodecylsulfate and urea [121]. Cellulase deactivation in [BMIM][Cl] was similar to the deactivation in NaCl solutions at high concentrations above 0.35 M, suggesting that interactions between the enzymes and the IL charged species also play a role in the denaturation of the enzymes [67, 121]. Enzyme activity can be recovered when the IL was diluted with buffer solution [67].

The strains of clostridia genus are very common for butanol synthesis whereas their yield and productivity varies. Basically, strains of this genus are spore­forming, rod-shaped, obligate anaerobes, and Gram-positive bacterium. Broadly, the number of species of this genus (such as C. acetobutylicum, C. beijerinckii, C. saccharoperbutylacetonicum, Clostridium saccharoacetobutylicum, Clostrid­ium aurantibutyricum, Clostridium pasteurianum, Clostridium sporogenes, Clos­tridium cadaveris, Clostridium tetanomorphum) are found with the capability for synthesizing butanol. Although, C. acetobutylicum, C. beijerinckii, C. saccharo — perbutylacetonicum, and C. saccharoacetobutylicum have shown significant activity [4]. C. acetobutylicum was the first isolated and patented bacteria for butanol production, later this strain was used for industrial scale ABE fermentation from sugars and cereal grains [45]. Earlier, it was believed that C. acetobutylicum was the only species involved in ABE fermentation. Shortly (1990s), on the basis of 16S rRNA gene sequencing and DNA fingerprinting, it was observed that there were four species in this mixed culture, namely C. acetobutylicum, C. beijerinckii, C. saccharoperbutylacetonicum, and C. saccharoacetobutylicumm [13, 45-47].

The selection of the strains for biobutanol production depends on type of feedstock, productivity required additional nutrients needed, and bacteriophage and butanol resistance. More number of attempts are needed to isolate novel organism for better yield and productivity and to modify the isolated strains applying genetic engineering.

C6H12O6 +2ADP + 2Pi! 2CH3CH2OH +2CO2 + 2ATP

glucose ethanol

As shown in the above equation, one molecule of glucose produces two molecules each of ethanol and CO2, under anaerobic conditions. In other words, 180 g of glucose (1 mol) should yield 92 g of ethanol (2 mol) and 88 g of CO2 (2 mol). The theoretical yield of ethanol production, therefore, comes to 51%. Under practical conditions, a very high percent (i. e., 47%) yield can be achieved. The metabolism though yields equimolar quantity of CO2 and ethanol, the actual amount of CO2 liberated is less than theoretical. This is because of partial reuti­lization of CO2 in anabolic carboxylation reactions [138]. According to an esti­mate, about 85% of the sugars are metabolized to ethanol and CO2, and the energy produced is used for various cell functions. The rest of the sugars are channeled for biosynthetic reactions. Figure 9.9 shows the pathway of conversion of pyruvate into ethanol and CO2.

Fig. 9.9 Pathway of conversion of pyruvate into ethanol

CH3- CHO

СНзСН2ОН

Ethanol