Bioenergy systems for the future 17 bioenergy production from second and third generation feedstocks

Bioenergy systems for the future 17 bioenergy production from second and third generation feedstocks Bioenergy systems for the future 17 bioenergy production from second and third generation feedstocks Bioenergy systems for the future 17 bioenergy production from second and third generation feedstocks Bioenergy systems for the future 17 bioenergy production from second and third generation feedstocks Bioenergy systems for the future 17 bioenergy production from second and third generation feedstocks

Bioenergy production from second- and third-generation feedstocks 17 F Dalena*, A Senatore*, A Tursi*, A Basile† *University of Calabria, Rende, Italy, †Institute on Membrane Technology (ITM-CNR), Rende, Italy Abbreviations ABE ADP ATP BGL CBH CoA ED EDG EMP FAME Fd GHG GRAS HPR IEA LAC LCB LE LiP MnP MSS PNS PPP PSI PSII SHF SSF TAG VFA WEO WO WtE acetone butanol ethanol adenosine diphosphate adenosine triphosphate ß-glucosidases cellobiohydrolases coenzyme A Entner-Doudoroff endo-1,4-ß-glucanases Embden-Meyerhof-Parnas pathway fatty acid methyl ester ferredoxin green house gas generally recognized as safe hydrogen production rate International Energy Agency laccase lignocellulosic biomass ligninolytic enzymes lignin peroxidase manganese peroxidase mushroom spent straw photosynthetic nonsulfur pentose phosphate pathway photosystem I photosystem II separately hydrolysis fermentation simultaneous saccharification and fermentation triacylglyceride volatile fatty acids World Energy Outlook wet oxidation waste to energy Bioenergy Systems for the Future http://dx.doi.org/10.1016/B978-0-08-101031-0.00017-X © 2017 Elsevier Ltd All rights reserved 560 17.1 Bioenergy Systems for the Future Introduction In the last few years, industrial research efforts have focused on low-cost large-scale processing for lignocellulosic feedstocks originating mainly from agricultural residues and municipal wastes or, generically, lignocellulosic biomass for bioenergy production Different raw materials (feedstocks) have been employed for processing first into simple sugars and then into bioenergy (Dalena and Basile, 2014) These feedstocks (principally crops such as sugar beet and sugarcane, corn, canola, and appropriate land crops) used for the production of first-generation biofuels have addressed the global markets to make more biofuels obtained from agriculture and forestry and inventories and from nonfood crops (second generation) However, the exploitation of these materials is in conflict with a balanced diet as competing directly with the food; therefore, they are not so promising even if they have permission to maintain consumption water and the destruction of forests for intensive cultivation of plants for the second-generation production cycle (Talebnia et al., 2010; Demirbas et al., 2011) Unfortunately, the first and the second generations have a material impact on the use and maintenance of soils and on the expenditure of large amounts of energy that reduce the economic advantage So, the primary concern of researchers interested in biofuels is to find the ideal production cycle through the use of microorganisms With this goal, the research is developing biofuels obtained by a third-generation feedstock in order to minimize greenhouse gas (GHG) emissions and disposal problems For example, the consumption of CO2 by algae (present in the whole ecosystem) for growing allows a removal of this substance to the air, improving the environment Microalgae have a high growth ratio, duplicate the population in 24 h, and can grow in salt water or wastewater Studies dating back to 2011 (Kim et al., 2011; Lee and Lazarus, 2011) suggest that in order to cut CO2 emissions, the demand for bioenergy will increase significantly by 2050 The International Energy Agency (IEA) has also suggested that the use of bioenergy is expected to triple by 2050 to about 135 EJ/yr (IEA, 2010); screenings of potential bioenergy range from 100 to 300 EJ by 2050 Production of energy from biomasses, that is, in the form of biodiesel or biomethane, is one way to reduce both consumption of crude oil and environmental pollution, because it can be mixed with the already present fuels due to its high octane number that impedes self-ignition in the gasoline engine (Demirbas et al., 2011) Waste-to-energy (WtE) technologies convert solid waste into various forms that can be used to supply energy (Demirbas and Balat, 2010) Energy can be derived from waste that has been treated and pressed into solid fuel and from waste that has been incinerated In fact, WtE can be used to produce biogas (CH4 and CO2), syngas (H2, CO2, and CO), liquid biofuels (ethanol and biodiesel), or pure hydrogen This chapter presents the results of critical analysis of published data on applications and the potentiality of the bioenergy production from biomass treatment products (renewable sources) In particular, the chapter is divided into two parts Bioenergy production from second- and third-generation feedstocks 561 focused on the production processes from different biomass feedstocks In fact, nowadays, it is possible to consider the production processes of biofuels as a function of the raw materials and the resulting experimental conditions of the processes These generations are divided into three parts for two specific reasons: (a) the different type of substrate and (b) the biofuel product (as it is shown in Table 17.1) In the first generation, the substrate consists mainly of seeds, grains, or simple sugars, and biofuel (mainly bioethanol) (Dias et al., 2012) is produced by fermentation of starch or sugars; in the second generation, the substrate is mainly composed of lignocellulosic biomass, and biofuels produced are mainly bioethanol and biobutanol (via enzymatic hydrolysis), methanol, and biodiesel (by thermochemical processes) (Biomass Research, 2009); in the third generation, the substrates are algae, and biofuels produced are mainly biodiesel, bioethanol, and biohydrogen (from green and blue algae) (Leite et al., 2013) The main criteria of feedstock choose is price, hydrocarbon content, and biodegradability Simple sugars are preferred as substrate for bioenergy production because they can be easily and quickly decomposed by microorganisms However, from the economical viewpoint, feedstocks containing pure sugars are comparably expensive, for that reason: lignocellulosic biomass is the most profitable source for bioenergy production However, it should be noted that for more than half a century various materials have been suggested as feedstock for the bioenergy production As introduced previously, these materials can be divided on three generations The first generation of feedstock consists in simple sugars or more complex sugars as corn or potato starch that undergo a treatment that makes them available for subsequent conversions as shown in Fig 17.1 Sustainable path from this foodstuff was to use acetone-butanol-ethanol (ABE) fermentation The biosynthesis of the main products of this synthesis (acetone, ethanol, and butanol) shows the same metabolic pathway from glucose to acetyl coenzyme A (acetylCoA), but it triggers in several subsequent processes There are three major classes of Summarization of substrates and products in first-, second-, and third-generation biofuels Table 17.1 Biofuels First generation Second generation Third generation Substrate, Seeds, grains, or sugars Product, Bioethanol Substrate, Lignocellulosic biomass Product, Bioethanol, biobutanol, biodiesel Substrate, Algae Product, Bioethanol, biodiesel, biohydrogen 562 Bioenergy Systems for the Future CH2OH CH2OH O CH2OH O OH OH OH O OH O OH OH O OH HO HO OH 300–600 O OH OH OH Fig 17.1 Conversion of complex sugars to simple sugars products during fermentation process: (i) solvents (acetone, ethanol, and butanol), (ii) organic acids (acetic, lactic, and butyric acids), and (iii) gases (CO2 and H2) (Datta and Zeikus, 1985) First generation of the starting materials for bioenergy production has the main pitfalls: The feed is foodstuff; therefore, alternative processes based on nonfood organic substrates should be applied for noncompetitive way of these production processes This comes with the second and the third generations of starting materials that will be discussed in this review chapter 17.2 ABE process As introduced above, the principal production process for the bioenergy production is the ABE Nowadays, this process is adapted for the optimization of the two products that are considered the fuels of the future, biobutanol and bioethanol This process tends to transform simple sugars in acetone, butanol, ethanol (in stoichiometric ratio of 6:3:1), H2, CO2, and a mixture of butyric, lactic, and acetic acid (as by-products) (Liu et al., 2005) as shown in Fig 17.2 The alcohols produced in this way could be used as biofuel or reagents for subsequent chemical processes In the field of biofuels, biobutanol, resulting from the ABE process, is considered a valid substitute for bioethanol The advantages related to the use of biobutanol compared with bioethanol (the main product of the first-generation fuels) are multiple both from the point of view of energy, compatibility with the motors and with the distribution systems, and from the point of view of agroforestry resources n-Butanol was produced starting from 1916 mainly as a solvent to feedstock Compared with the methanol and ethanol, it is a more complex alcohol with significant advantages: higher heat value (biobutanol has 110,000 BTU/gal, while bioethanol CH2OH CH2OH O CH2OH O OH OH O OH OH O OH OH O OH OH 300–600 OH HO HO O OH Acetone + Butanol + Ethanol + H2 + CO2 OH Fig 17.2 From cellulose to acetone, butanol, and ethanol Bioenergy production from second- and third-generation feedstocks 563 only has 84,000 BTU/gal), low volatility, higher viscosity, and high concentrations in mixing with other fuels from petroleum distillates, and it allows a reduction of NOx emissions (Chen et al., 2009) The limitation of the ABE process could be a self-inhibition caused by some bacteria The process could become more efficient through a genetic modification of bacteria used or a specific search for other bacteria strain more tolerant to the production cycle The highest production capacity of n-butanol by bacterial fermentation is of 3.0 wt% The most common substrate for the ABE fermentation is lignocellulosic material (mainly starch or simple sugars for the production of the first-generation biofuels), which are converted to glucose following acid/enzyme hydrolysis But at the same time, by means of other pretreatments (as described below), it can still produce glucose, the starting substrate of the ABE fermentation Therefore, the ABE process leads to the formation of n-butanol for both substrate composed of simple sugars (first generation) and substrates composed of lignocellulosic biomass (second generation) Despite the fact that simple sugars are more easily used to convert into biofuels, their cost does not allow a large-scale use in industry The use of biomass (second generation) has the dual advantage of being cost-effective than compared with the first-generation feedstock and of being able to transform biomasses Additionally, lignocellulosic biomass can be supplied on a large-scale basis from different low-cost raw materials such as municipal and industrial wastes and wood and agricultural residues (Cardona and Sanchez, 2007) 17.2.1 From substrate to biofuel in ABE process Lignocellulosic materials constitute a substantial renewable substrate for bioethanol production in the ABE process These cellulosic materials also contribute to environmental sustainability (Demirbas, 2003) Lignocellulose is composed of three parts: cellulose (30%–50%), hemicellulose (15%–35%), and lignin (10%–20%) (Petersen, 1984) Lignin and cellulose are very difficult components to degrade, although both are rather heterogeneous polymers and differ considerably depending on their origin Lignin is an aromatic and rigid biopolymer with a molecular weight of 10,000 Da bonded via covalent bonds to xylans (hemicellulose portion) conferring rigidity and high level of compactness to the plant cell wall (Mielenz, 2001) Hemicellulose is an amorphous and variable structure formed of heteropolymers including hexoses (D-glucose, D-galactose, and D-mannose) and pentose (D-xylose and L-arabinose) and may contain sugar acids (uronic acids), namely, D-glucuronic, D-galacturonic, and methylgalacturonic acids (McMillan, 1994; Ranjan and Moholkar, 2013) Pentoses and hexoses are relatively easy to hydrolyze, but in raw material, these molecules are protected from hydrolysis by a complex linkage with lignin and cellulose Glucose (simple sugars), hemicellulose, and cellulose are converted into pyruvate through three different pathways Glucose (6C) is, initially, phosphorylated to glucose-6-phosphate, which is subsequently converted to pyruvate (3C) via Embden-Meyerhof-Parnas (EMP) pathway through some intermediates: 564 Bioenergy Systems for the Future Glucose ! Glucose À À phosphate ! Pyruvate (17.1) Other fermentation substrates contain hemicellulose or cellulose (e.g., fibrous biomass such as rice straw or wheat straw) Hemicellulose is converted in xylose, via pentose phosphate pathway (PPP), and produces fructose-6-phosphate (intermediate in the pathway that starts from glucose), and after, via EMP pathway, it can be converted in pyruvate: Hemicellulose ! Xylose ! PPP pathway ! Fructose À À phosphate ! EMP pathway ! Pyruvate (17.2) When the substrates contain cellulose, instead, become glucose via cellulose hydrolysis that follow the metabolic pathway in the same manner as it earlier stated (Ranjan and Moholkar, 2012), Cellulose ! Glucose ! Pyruvate (17.3) In all the described cases, the final product of the first part of the reactions is the pyruvate that allows to produce ethanol, acetone, and n-butanol, as described in Fig 17.3 Pyruvate Pyruvate Ferredoxin Oxidoreductase Aldehyde/alcohol dehydrogenase Acetate Acetyl-CoA Tiolase Ethanol (1) AcetoacetylCoA:acetate /butyrate:CoA transferase Acetoacetyl-CoA Acetone (6) NADH+3-Hydroxybutyryl-CoA Dehydrogenase; Crotonase; NADH + butyryl-CoA dehydrogenase Butyrate Butyryl-CoA Aldehyde/alcohol dehydrogenase Fig 17.3 Schematic representation of the ABE process n-Butanol (3) Bioenergy production from second- and third-generation feedstocks 565 At this point, the pyruvate-ferredoxin oxidoreductase (PFOR) enters into function that cleaves pyruvate resulting from glycolysis, in the presence of coenzyme A, to produce carbon dioxide and acetyl-CoA, by converting the oxidized ferredoxin simultaneously in its reduced form (Menon and Ragsdale, 1997) In order to have a quantitative view, the ferredoxin of clostridia produces mol of each acetyl-CoA and CO2 per mole of ferredoxin reduced This process sees the consequent transfer of two electrons (Uyeda and Rabinowitz, 1971) Conversion of acetyl-CoA to acetate is permitted by the enzyme phosphate acetyltransferase and acetate kinase, whereas conversion of butyryl-CoA to butyrate is catalyzed by the enzyme phosphate butyltransferase and butyl kinase The next step of the process occurs in acid condition (solventogenic phase), and the products of the preceding acidogenic phase are reassimilated and converted to acetone and n-butanol The enzyme catalyzing this conversion is Co-A transferase, which converts CoA from acetoacetyl-CoA either to acetate forming acetyl-CoA or to butyrate resulting in butyryl-CoA Out of these, acetyl-CoA can be converted to acetone, butanol, and ethanol, whereas butyryl-CoA can only be converted to butanol (Ranjan and Moholkar, 2012) 17.3 Second generation feedstocks Despite the advantages of the conversion of simple sugars or starches (i.e., biodegradability), the first-generation biofuels have the highest carbon footprint compared with other generations of biofuel The production technologies adopted for the production of first-generation biofuel are inclusive of transesterification process for biodiesel production and fermentation process for bioethanol production However, the physical characteristics of the raw biomass could greatly affect the efficiency of the conversion processes To overcome this problem, more complicated lignocellulosic biomass processing technologies, such as thermochemical and biological conversion processes, are employed to produce second-generation biofuel (Liew et al., 2014) These biofuels, also called simply biofuels, are produced by processing biomass and include bioethanol and biodiesel that can be used in vehicles and in industrial process In fact (as shown in Fig 17.4), despite the substrates are different, the conversion in acetone, butanol, and ethanol is the same, once transformed into simple sugars This conversion is managed by ABE fermentation in both generation feedstocks Therefore, the conversion of lignocellulose to monomeric fermentable sugars in the nature is a quite prolonged process In order to receive enough amounts of fermentable sugars, it is necessary to use pretreatment methods for the destruction of interconnections in the lignocellulosic biomass and cellulose and hemicellulose hydrolysis Owing to the structural complexity of the lignocellulosic matrix, biofuel production from biomasses requires at least three major unit operations including pretreatment, hydrolysis, and fermentation 566 Bioenergy Systems for the Future 1st Generation Starch (corn, potato) 2nd Generation Sugars (sugarcane) Lignocellulosic biomasses Pretreatment Simple sugars ABE fermentation Acetone Butanol Ethanol Fig 17.4 Conversion of different types of feedstocks into acetone, butanol, and ethanol 17.3.1 Pretreatment of lignocellulosic biomasses In general, there are four typical pretreatment processes: physical, biological, chemical, and combinatorial pretreatment (physiochemical and biochemical) conversion (Agbor et al., 2011) The choice of the pretreatment method mainly depends on physical-chemical properties of lignocellulosic biomass, and it is fundamental for optimal successful hydrolysis and, consequently, to transform lignocellulosic polymer units in the monomeric units of simple sugars The overall efficiency of the pretreatment process is correlated to a good balance between low inhibitor formation and high substrate digeribility The goal of any pretreatment is characterized by several criteria: reducing the degree of polymerization of the lignocellulosic chain, preserving the pentose (hemicellulose) fractions, limiting the formation of degradation products that inhibit the growth of the fermentative microorganism, and minimizing energy demands and limiting cost (Council National Research, 2000) 17.3.1.1 Physical pretreatment These methods can be of two types: mechanical comminution and pyrolysis The objective of the mechanical pretreatment is a reduction of particle size and crystallinity of lignocellulose in order to increase the specific surface the degree of polymerization This can be produced by a combination of chipping, grinding, or milling Bioenergy production from second- and third-generation feedstocks 567 depending on the final particle size of the material (10–30 mm after chipping and 0.2–2 mm after milling or grinding) (Alvira et al., 2010; Mosier et al., 2005) The power requirement of mechanical comminution depends on the final particle size and the waste biomass characteristics (Cadoche and Lopez, 1989) Instead, pyrolysis treated biomasses at temperature greater than 300°C; cellulose rapidly decomposes to produce gaseous products and residual char (Kilzer and Broido, 1965) The decomposition is much slower, and less volatile products are formed at lower temperatures Mild acid hydrolysis (H2SO4 N, 97°C, 2.5 h) of the residues from pyrolysis pretreatment has resulted in 80%–85% conversion of cellulose for reducing sugars with more than 50% glucose (Fang et al., 1987; Balat, 2011) 17.3.1.2 Chemical pretreatment Chemical pretreatment employs different chemicals such as acids, alkalies, and oxidizing agents Among these methods, dilute acid pretreatment using H2SO4 is the most widely used Depending on the type of chemical used, pretreatment could have different effects on lignocellulose structural components Alkaline pretreatment, ozonolysis, peroxide (both techniques that used oxidizing agents), and wet oxidation (WO) pretreatments are more effective in lignin removal, whereas dilute acid pretreatment is more efficient in hemicellulose solubilization (Galbe and Zacchi, 2002) 17.3.1.3 Physical-chemical pretreatment The solubilization of lignocellulose components depends on temperature, pH, and moisture content In lignocellulosic materials such as wheat straw, hemicelluloses are the most thermal-chemically sensitive fraction Hemicellulose compounds start to solubilize into the water at temperature higher than 150°C, and among various components, xylan can be extracted the most easily (Sun and Cheng, 2002; Hendriks and Zeeman, 2009) There are different types of solubilization of hemicelluloses by physical-chemical production Every type employs the characteristics of pressure and temperature The most useful method uses the explosion of CO2 to separate the hemicellulose, that is, to reduce polymeric chains of glucosidic compounds to most simple and fractionable sugar Conventional mechanical methods require 70% more energy than physicochemical pretreatments to achieve the same amount of sugar reduction These methods are useful principally for agricultural residues, but they are less effective for softwoods due to the low content of acetyl groups in the hemicellulosic portion (Balat, 2011; Clark and Mackie, 1987) 17.3.1.4 Biological pretreatment Biological pretreatment comprises using microorganisms, such as brown-, white-, and soft-rot fungi, and seems to be, in our opinion, more effective than the other pretreatments The abovementioned pretreatment methods are harsh and cost-energy intensive; on the contrary, biological pretreatment processes are mild and environmental friendly 568 Bioenergy Systems for the Future Microbial pretreatment consists of a solid-state fermentation process in which microorganisms grow on the lignocellulosic biomass selectively degrading lignin (and in some cases hemicellulose), while cellulose is expected to remain intact For their heterotrophic character, these organisms are not able to produce the sugars necessary for the production of biofuels in an autonomous way such as bacteria or algae (third generation), but they are the perfect helpers in the degradation of lignocellulosic substrates and lipid accumulation The main fungi used as a pretreatment in the conversion of lignocellulosic biomass into fermentable sugars are white-, brown-, and soft-rot fungi Brown rots mainly attack cellulose, whereas white- and soft-rot fungi are usually preferred for the high selectivity in lignin degradation over cellulose loss (Wan and Li, 2012) Lignin degradation by white-rot fungi, the most effective for biological pretreatment of lignocellulosic materials, occurs through the action of lignin-degrading enzymes such as peroxidases and laccases (Kumar et al., 2009) White-rot microbes typically secrete lignin peroxidases (as exposed below), along with various types of glycosyl hydrolases that cleave the C-C lignin backbone in the presence of hydrogen peroxide Other enzymes involved in aerobically catalyzed lignin degradation include Mn-dependent peroxides, laccases (monophenol oxidase), and superoxide dismutase (Leonowicz et al., 1999) In the oxidation part of lignin, the ligninolytic enzymes (LE) are laccase (LAC) (EC 1.10.3.2), lignin peroxidase (LiP) (EC 1.11.14), and manganese peroxidase (MnP) (EC 1.11.13) (Leonowicz et al., 1999; Novotny´ et al., 2004; Wan and Li, 2012) Laccase is a copper binder enzyme, with four copper atoms in the active sites, which utilizes molecular oxygen to carry out reactions of oxidation with phenolic rings to produce phenoxy radicals; in particular, it catalyzes the removal of an electron and a proton from phenolic hydroxyl and aromatic amino groups, to form free phenoxy radicals and amino radicals (Hatakka, 1994; Leonowicz et al., 2001; Wan and Li, 2012) as shown in Fig 17.5 LiP is a hemeprotein that needs hydrogen peroxide H2O2 from other enzymes to be active It catalyzes the oxidation of nonphenolic aromatic lignin moieties and similar compounds, by one-electron oxidation of the aromatic ring (Leonowicz et al., 1999, 2001; Wan and Li, 2012; Wesenberg et al., 2003) The role of LiP in ligninolysis could be the further transformation of lignin fragments that are initially released by MnP (Wesenberg et al., 2003) (Fig 17.6) MnP is glycosylated glycoproteins with an iron protoporphyrin IX prosthetic group that oxidizes different phenolic compounds, thanks to the oxidation of Mn2+ to Mn3+ (Leonowicz et al., 1999, 2001; Wan and Li, 2012; Wesenberg et al., 2003) The final effect of these enzymes is to initiate wood decay and facilitate the penetration of hydrolytic enzymes into cellulosic and hemicellulosic substrates The enzymatic reaction should be described by the same mechanism of the LiP (Fig 17.6) Some of the best white-rot fungi in the degradation of lignin are Phanerochaete chrysosporium, Ceriporiopsis subvermispora, and Daedalea flavida (Maurya et al., 2015; Wan and Li, 2012) Biological pretreatment by white-rot fungi has been added together with organosolv pretreatment in an ethanol production process by simultaneous saccharification and fermentation (SSF) from beech wood chips Bioenergy production from second- and third-generation feedstocks C6 H12 O6 + 2H2 O ! 2CH3 COOH + 4H2 + 2CO2 585 (17.18) Changes in Gibbs free energy (ΔG0 ¼ À171.1 kJ/mol) and in enthalpy (ΔH0 ¼ 90.69 kJ/mol) of the reaction indicate that the reaction would occur spontaneously with no external energy requirement (Sinha and Pandey, 2011) Theoretically, a maximum of mol of H2 can be produced per mole glucose when acetic acid is the only VFA product When the end product is butyric acid, mol of H2 is produced: C6 H12 O6 + 2H2 O ! CH3 CH2 CH2 COOH + 4H2 + 2CO2 (17.19) When both acetic and butyric acids are produced in dark fermentation of glucose, theoretically 2.5 mol H2 is formed per mole glucose (Argun and Kargi, 2011; Bakonyi et al., 2014) To optimize the yield of hydrogen is necessary place particularly attention to two important parameters: the pretreatment of biomass (such as acid or alkaline treatment) and the operating conditions (such as pH control or temperature control) to eliminate H2 consuming undesirable species (such as methanogens and homoacetogenes) to which are capable to inherently cut the H2 turnout (Bakonyi et al., 2014) These pretreatments can be performed by exposing the biomass to unfavorable conditions, for example, to elevated temperature The method utilizing high temperature is one of the most used as it is relatively fast, easy to conduct, and highly reliable; it is called heat shock (Wang and Wan, 2008) Bakonyi et al (2014) have reported the exact treatment circumstances Conventional thermal curing was carried out in an ordinary water bath with occasional mechanical stirring (30 s in each min) resulting in obtaining data reported in Table 17.5 Moreover, it is demonstrated that increasing of pH during dark fermentation resulted in lower and longer lag time for both controlled and uncontrolled Table 17.5 H2 productivity in function of the pretreatment time and temperature Pretreatment T (°C) Pretreatment time (min) H2 productivity (mL H2/ mL-d) 75 75 85 85 85 95 95 30 50 40 40 40 30 50 0.78 0.76 0.73 0.65 0.82 0.57 0.07 From Bakonyi, P., Nemesto´thy, N., Belafi-Bako´, K., 2013 Biohydrogen purification by membranes: an overview on the operational conditions affecting the performance of non-porous, polymeric and ionic liquid based gas separation membranes Int J Hydrogen Energy 38 (23), 9673–9687 586 Bioenergy Systems for the Future conditions Zagrodnik and Laniecki (2015) have demonstrated that biohydrogen production by dark fermentative process could be improved by applying systems with pH control They found that the optimum, controlled pH for C acetobutylicum (indicated for photofermentation) and R sphaeroides (indicated for dark fermentation) were 5.5 and 7.5, respectively They concluded that the creation of efficient combined system to optimize the hydrogen production rate (HPR) requires pH control conditions that can support dark and photofermentation 17.4.3.5 Integrated process (dark-photo fermentation) Integration of photofermentative and dark fermentative bacterial metabolism has been reported as the maximum efficiency for the biohydrogen production (Eroglu and Melis, 2011a,b; Kapdan and Kargı, 2006; Sinha and Pandey, 2011) Eq (17.17) indicates the actual reaction with the real yield of the products In reality, the VFAs are undesired products, and the theoretical yield for the perfect performance of the dark fermentation would be of 12 mol of H2 for one mole of glucose This is not possible because the reaction is thermodynamically unfavorable: C6 H12 O6 + H2 O ! 12 H2 + O2 (17.20) In fact, the ΔG0 of the reaction is +3.2 kJ/mol To overcome this manufacturing problem, the dark fermentative system has been used in parallel with the photofermentation to optimize the quantity of hydrogen produced by creating a two-stage fermentation process (Sinha and Pandey, 2011; Ueno et al., 2007) In this integrated process system, biomass (in particularly lignocellulosic biomass, i.e., agricultural wastes) is processed into sugar in an anaerobic fermentation process; these sugars are then transformed into H2, CO2, and VFA (as seen in Section 17.4.3.4) by dark fermentation process In the second step, the VFA obtained are converted into hydrogen by photosynthetic bacteria (as seen in Section 17.4.3.3) The overall process includes two steps represented as follows: Stage 1—dark fermentation in anaerobic condition C6 H12 O6 + 2H2 O ! 2CH3 COOH + 4H2 + 2CO2 (17.21) Stage 2—photofermentation by photosynthetic bacteria CH3 COOH + 2H2 O ! 4H2 + CO2 (17.22) This hybrid system has been summarized in Fig 17.14 Thus, theoretically it is evident that by using glucose as a sole source of substrate in dark fermentation producing acetic acid as a major product, a total of 12 mol of hydrogen could be expected in a combined process from one mole of glucose Bioenergy production from second- and third-generation feedstocks CO2, H2 Dark fermentation (stage 1) Biomass H2 Hydrogen separation VFA Photofermentation (stage 2) 587 CO2, H2 CO2 Fig 17.14 Schematic representation of the integrated process However, the differences in organic acid production/consumption rates and therefore potential accumulation of organic acids in the media decrease in light penetration because of suspended solids that are the major problems in mixed fermentation process (Kapdan and Kargı, 2006) 17.4.4 Transesterification Techniques such as anaerobic digestion appear to be necessary for the biochemical conversion of the algal biomass However, one of the most common methods for producing biodiesel from algae is a chemical reaction called transesterification that allows to produce biofuels In fact, the transesterification reaction involves the introduction of a triacylglyceride (TAG) from the biomass with an alcohol to produce a different alcohol (in this case glycerol) and a fatty acid methyl ester (FAME)—more commonly known as biodiesel (Arnold et al., 2012) Transesterification, also called alcoholysis, therefore, is the process that leads to the exchange of a group of an ester (R0 ), with a group of an alcohol (R00 ), as described in the Eq (17.23): RCOOR0 + R00 OH ! RCOOR00 + R0 OH (17.23) In the biodiesel formation, methanol is particularly preferred among the alcohols, for its low cost and its physical and chemical characteristics such as the polarity and the shorter chain that makes it highly reactive; instead, fatty acids in the form of triglycerides derived from vegetable oils or microorganisms such as microalgae are usually 588 Bioenergy Systems for the Future preferred for the low viscosity and the highest cetane number compared with triglycerides derived from fossils (Clark et al., 1984; Varese and Varese, 1996) Thus, fatty acid methyl esters (known as biodiesel fuel) can be used as an alternative to the common diesel, for different reasons, such as the reduced levels of particulates, carbon monoxide, and nitrogen oxides, during combustion (Yamane et al., 2001) Below, there is a rapid view of the main transesterification processes for biodiesel fuel production that are divided into chemical and enzymatic processes (1) Chemical processes: In chemical processes, acid or basic catalysts are utilized; these convert triglycerides in esters and glycerol, where di- and monoglycerides are formed as intermediates The stoichiometric reaction requires mol of a triglyceride and mol of the alcohol However, an excess of the alcohol is used to increase the yields of the alkyl esters and to allow its phase separation from the glycerol formed (Freedman et al., 1986; Noureddini and Zhu, 1997): Triglycerides + 3MeOH ! Diglycerides ! Monoglycerides ! Methyl Esters + Glycerol (17.24) Acid transesterification This transesterification process is catalyzed by acids, preferably by sulfonic and sulfuric acids (Harrington and D’arcy-Evans, 1985a,b; Freedman et al., 1986; Graille et al., 1985) These catalysts give very high yields in alkyl esters, but the reactions are slower of alkali transesterification, requiring typically temperatures above 100°C and more than h to reach complete conversion For these reasons, such catalysts are usually rejected (Freedman et al., 1984) Acid transesterification is suitable for glycerides that have relatively high free fatty acid contents and more water (Ma and Hanna, 1999; Srivastava and Prasad, 2000) The reaction mechanism is shown in Fig 17.15 Alkali transesterification In the alkali transesterification (about 4000 times faster than that catalyzed by acids), the glycerides and alcohol should be anhydrous because water causes a partial reaction change that produces soap At the moment, this is the method that is generally employed in biodiesel O OH OH H+ + R1 OR R1 + + R1 OH R2 OH OR H Fig 17.15 Acid transesterification reaction OR OR R2 O −H+ + O R1 O R1 OR + ROH H R1 OR2 Bioenergy production from second- and third-generation feedstocks R'COO CH2 R''COO CH + R'COO CH2 R''COO CH OR −OR O H2C OCR''' H2C 589 CH2 R''COO CH OR O H2C R''' O− O R'COO C C R'COO CH2 R''COO CH R''' H2C + ROOCR''' O− O− R'COO CH2 R''COO CH H2C + BH+ O− R'COO CH2 R''COO CH H2C + B OH Fig 17.16 Alkali transesterification reaction production, and the recommended amount of base used for this process is between 0.1% and 1% w/w of oils and fats (Formo, 1954; Demirbas, 2009) The reaction mechanism consists of three steps as shown in Fig 17.16: (1) Nucleophilic attack on the carbonyl carbon atom of the trigliceride molecule by the anion of the alcohol (formed reaction of the base with the alcohol, producing an alkoxide and the protonated catalyst) to form a tetrahedral intermediate (Guthrie, 1991) (2) Rearrangement of the tetrahedral intermediate from which the alkyl ester and the corresponding anion of the diglyceride are formed (3) The latter deprotonates the catalyst, thus regenerating the active species, which is now able to react with a second molecule of the alcohol, starting another catalytic cycle Diglycerides and monoglycerides are converted by the same mechanism to a mixture of alkyl esters and glycerol Other types of processes that always lead to the same reaction product are the following: Supercritical methanol transesterification Supercritical transesterification is a chemical-physical process that would like to avoid the use of any catalyst for the transesterification process At temperatures of 350°C and with times of reaction of about 240 s in supercritical methanol, it is possible to convert oils to methyl esters (Saka and Kusdiana, 2001; Kusdiana and Saka, 2001) 590 Bioenergy Systems for the Future (2) Enzymatic processes: Recently, enzymatic transesterification using lipases enzymes has become more attractive for biodiesel production In this process, triglycerides are first hydrolyzed by lipase to partial glycerides that are then converted into methyl esters with methanol as shown below (Ban et al., 2001): Lipase Triglycerides + MeOH , Methyl Esters + Glycerol (17.27) In terms of production cost, there are two aspects to be considered in the industrial processes: the transesterification process and the by-product (glycerol) recovery A continuous transesterification process and the recovery of high-quality glycerol are choices to lower the production cost, and at the moment, alkali transesterification is considered the best for the shorter reaction time and the greater production capacity (Demirbas, 2009) 17.5 Conclusions and future trends 2016 2015 2014 2013 2012 2011 2010 2009 2008 2007 2006 2005 2004 2003 2002 2001 2000 1999 7500 7000 6500 6000 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 1998 Number of publications Nowadays, the production of so-called bioenergy is one of the most focused searches from the point of view of both scientific research and industrial development; in fact, this technology remains a sure and economically viable approach for sustainability and reduction of fossil fuel consumption This can be seen in Fig 17.17 It shows the behavior of the number of publication in function of the year A total number of 47,000 research publications were found on the keyword “biofuel” using the search in ScienceDirect The analysis gives an idea about the actually importance of the biofuels Majority of the published work, that is, 91% was in the form of the research articles published in various journal, followed by books with Year Fig 17.17 Graph that show the number of publications (using the keyword “biofuel”) in function of the year Bioenergy production from second- and third-generation feedstocks 591 about 8% It can be seen that since 2004 (the year of accession of Russia to the Kyoto Protocol) publications have increased exponentially up to 2016 with a number of publications for this year amounted to 6796 In particular, there is an evident increase between 2008 and 2009 The number of publications increased from 1617 to 2937 This is mainly due to the edition of the World Energy Outlook (WEO) of 2009 that presented in-depth analysis of three special topics: financing energy investment under a post-2012 climate framework, prospects for global natural gas markets, and energy trends in Southeast Asia Recent studies have assessed the technical global biomass potential at ranges between 30 EJ and 1500 EJ in 2050, which is between 10% and 300% of current global energy consumption At last, the assessment of biofuel production, in energy terms can vary depending on whether the biofuel is first, second, or third generation For first-generation biofuels, production costs are too expensive and require specific crops This makes this generation feedstocks already obsolete Second-generation biofuels derived from lignocellulosic feedstocks are a promising, environmental friendly, alternative energy source that currently draws considerable attention Moreover, compared with current leading thermal or chemical pretreatment processes, biological (fungal) pretreatment is an environmental friendly and energy-efficient process In particular, white-rot fungi have high selectivity on lignin degradation over cellulose loss Fungal pretreatment prior to mild physical and chemical pretreatment is shown as synergism on the improvement of cellulose digestibility with advantages similar to that of the biopulping process Microalgae as third-generation feedstocks were found to alleviate much of the shortcomings that plague its predecessors, but high production and energy costs represent major limitations The sustainability of the algal feedstocks and its susceptibility to energy conversion are the main desirable attributes that makes it suitable to be used in the production of bioenergy It can be expected from algal bioenergy its contribution to the decrease in the consumption of fossil fuels for a cleaner and sustainable earth in the future References Adsul, M.G., Ghuleb, J.E., Singhb, R., Shaikhb, H., 2004 Polysaccharide from bagasse: applications in cellulase and xylanase production Carbohydr Polym 57, 67–72 http://dx.doi org/10.1016/j.carbpol.2004.04.001 Agbor, V.B., Cicek, N., Sparling, R., Berlin, A., Levin, D.B., 2011 Biomass pretreatment: fundamentals toward application Biotechnol Adv 29 (6), 675–685 Akkerman, I., Janssen, M., Rocha, J., Wijffels, R.H., 2002 Photobiological hydrogen production: photochemical efficiency and bioreactor design Int J Hydrogen Energy 27 (1–2), 1195–1208 Allakhverdiev, S.I., Thavasi, V., Kreslavski, V.D., Zharmukhamedov, S.K., Klimov, V.V., Ramakrishna, S., Los, D.A., Mimuro, M., Nishihara, H., Carpentier, R., 2010 Photosynthetic hydrogen production J Photochem Photobiol 11 (2–3), 101–113 Allen, E., Wall, D.M., Herrmann, C., Xia, A., Murphy, J.D., 2015 What is the gross energy yield of third generation gaseous biofuel sourced from seaweed? 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