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

F Dalena*, A Senatore*, A Tursi*, A Basile†

*University of Calabria, Rende, Italy,†Institute on Membrane Technology (ITM-CNR),Rende, Italy

Abbreviations

ABE acetone butanol ethanol

ADP adenosine diphosphate

ATP adenosine triphosphate

EMP Embden-Meyerhof-Parnas pathway

FAME fatty acid methyl ester

Fd ferredoxin

GHG green house gas

GRAS generally recognized as safe

HPR hydrogen production rate

IEA International Energy Agency

SHF separately hydrolysis fermentation

SSF simultaneous saccharification and fermentation

TAG triacylglyceride

VFA volatile fatty acids

WEO World Energy Outlook

WO wet oxidation

WtE 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.

17.1 Introduction

In the last few years, industrial research efforts have focused on low-cost large-scaleprocessing for lignocellulosic feedstocks originating mainly from agricultural res-idues and municipal wastes or, generically, lignocellulosic biomass for bioenergyproduction Different raw materials (feedstocks) have been employed for processingfirst into simple sugars and then into bioenergy (Dalena and Basile, 2014) Thesefeedstocks (principally crops such as sugar beet and sugarcane, corn, canola, andappropriate land crops) used for the production of first-generation biofuels haveaddressed the global markets to make more biofuels obtained from agricultureand forestry and inventories and from nonfood crops (second generation) However,the exploitation of these materials is in conflict with a balanced diet as competingdirectly with the food; therefore, they are not so promising even if they have permis-sion to maintain consumption water and the destruction of forests for intensivecultivation 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 theuse and maintenance of soils and on the expenditure of large amounts of energy thatreduce the economic advantage

So, the primary concern of researchers interested in biofuels is to find the idealproduction cycle through the use of microorganisms With this goal, the research isdeveloping biofuels obtained by a third-generation feedstock in order to minimizegreenhouse gas (GHG) emissions and disposal problems For example, the con-sumption of CO2by 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 orwastewater

Studies dating back to 2011 (Kim et al., 2011; Lee and Lazarus, 2011) suggest that

in order to cut CO2emissions, the demand for bioenergy will increase significantly by

2050 The International Energy Agency (IEA) has also suggested that the use of energy is expected to triple by 2050 to about 135 EJ/yr (IEA, 2010); screenings ofpotential bioenergy range from 100 to 300 EJ by 2050

bio-Production of energy from biomasses, that is, in the form of biodiesel orbiomethane, is one way to reduce both consumption of crude oil and environmentalpollution, because it can be mixed with the already present fuels due to its high octanenumber that impedes self-ignition in the gasoline engine (Demirbas et al., 2011).Waste-to-energy (WtE) technologies convert solid waste into various forms thatcan be used to supply energy (Demirbas and Balat, 2010) Energy can be derived fromwaste that has been treated and pressed into solid fuel and from waste that has beenincinerated In fact, WtE can be used to produce biogas (CH4and 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 tions and the potentiality of the bioenergy production from biomass treatmentproducts (renewable sources) In particular, the chapter is divided into two parts

focused on the production processes from different biomass feedstocks In fact, adays, it is possible to consider the production processes of biofuels as a function ofthe raw materials and the resulting experimental conditions of the processes.These generations are divided into three parts for two specific reasons: (a) thedifferent type of substrate and (b) the biofuel product (as it is shown inTable 17.1) In the first generation, the substrate consists mainly of seeds, grains,

now-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 mainlycomposed of lignocellulosic biomass, and biofuels produced are mainly bioethanoland biobutanol (via enzymatic hydrolysis), methanol, and biodiesel (by thermo-chemical processes) (Biomass Research, 2009); in the third generation, the sub-strates are algae, and biofuels produced are mainly biodiesel, bioethanol, andbiohydrogen (from green and blue algae) (Leite et al., 2013)

The main criteria of feedstock choose is price, hydrocarbon content, and gradability Simple sugars are preferred as substrate for bioenergy production becausethey can be easily and quickly decomposed by microorganisms However, from theeconomical viewpoint, feedstocks containing pure sugars are comparably expensive,for that reason: lignocellulosic biomass is the most profitable source for bioenergyproduction However, it should be noted that for more than half a century variousmaterials have been suggested as feedstock for the bioenergy production As intro-duced previously, these materials can be divided on three generations

biode-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 sequent conversions as shown inFig 17.1

sub-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 nol) shows the same metabolic pathway from glucose to acetyl coenzyme A (acetyl-CoA), but it triggers in several subsequent processes There are three major classes of

buta-Table 17.1 Summarization of substrates and products in first-, second-, and third-generation biofuels

BiofuelsFirst generation Second generation Third generationSubstrate,

Seeds, grains, or sugars

Substrate,Lignocellulosic biomass

Substrate,AlgaeProduct,

Bioethanol

Product,Bioethanol,biobutanol,biodiesel

Product,Bioethanol,biodiesel,biohydrogen

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 falls: The feed is foodstuff; therefore, alternative processes based on nonfood organicsubstrates should be applied for noncompetitive way of these production processes.This comes with the second and the third generations of starting materials that will

pit-be discussed in this review chapter

The alcohols produced in this way could be used as biofuel or reagents for quent chemical processes In the field of biofuels, biobutanol, resulting from the ABEprocess, is considered a valid substitute for bioethanol The advantages related to theuse of biobutanol compared with bioethanol (the main product of the first-generationfuels) are multiple both from the point of view of energy, compatibility with themotors and with the distribution systems, and from the point of view of agroforestryresources

subse-n-Butanol was produced starting from 1916 mainly as a solvent to feedstock pared with the methanol and ethanol, it is a more complex alcohol with significantadvantages: higher heat value (biobutanol has 110,000 BTU/gal, while bioethanol

OH

OH

O

CH2OH O

OH 300–600

OH

HO

OH OH

OH O

Fig 17.1 Conversion of complex sugars to simple sugars

Fig 17.2 From cellulose to acetone, butanol, and ethanol

only has 84,000 BTU/gal), low volatility, higher viscosity, and high concentrations inmixing with other fuels from petroleum distillates, and it allows a reduction of NOxemissions (Chen et al., 2009).

The limitation of the ABE process could be a self-inhibition caused by somebacteria The process could become more efficient through a genetic modification

of bacteria used or a specific search for other bacteria strain more tolerant to theproduction cycle The highest production capacity of n-butanol by bacterial fermen-tation 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 sametime, by means of other pretreatments (as described below), it can still produceglucose, the starting substrate of the ABE fermentation Therefore, the ABE processleads to the formation of n-butanol for both substrate composed of simple sugars (firstgeneration) and substrates composed of lignocellulosic biomass (second generation).Despite the fact that simple sugars are more easily used to convert into biofuels, theircost does not allow a large-scale use in industry The use of biomass (second gener-ation) has the dual advantage of being cost-effective than compared with thefirst-generation feedstock and of being able to transform biomasses Additionally,lignocellulosic biomass can be supplied on a large-scale basis from different low-costraw materials such as municipal and industrial wastes and wood and agriculturalresidues (Cardona and Sanchez, 2007)

17.2.1 From substrate to biofuel in ABE process

Lignocellulosic materials constitute a substantial renewable substrate for bioethanolproduction in the ABE process These cellulosic materials also contribute to environ-mental 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 verydifficult components to degrade, although both are rather heterogeneous polymersand differ considerably depending on their origin Lignin is an aromatic and rigid bio-polymer 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 plantcell wall (Mielenz, 2001) Hemicellulose is an amorphous and variable structureformed of heteropolymers including hexoses (D-glucose, D-galactose, andD-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 relativelyeasy to hydrolyze, but in raw material, these molecules are protected from hydrolysis

by a complex linkage with lignin and cellulose Glucose (simple sugars), lose, and cellulose are converted into pyruvate through three different pathways.Glucose (6C) is, initially, phosphorylated to glucose-6-phosphate, which is subse-quently converted to pyruvate (3C) via Embden-Meyerhof-Parnas (EMP) pathwaythrough some intermediates:

hemicellu-Glucose! Glucose  6  phosphate ! Pyruvate (17.1)Other fermentation substrates contain hemicellulose or cellulose (e.g., fibrous bio-mass such as rice straw or wheat straw) Hemicellulose is converted in xylose, viapentose phosphate pathway (PPP), and produces fructose-6-phosphate (intermediate

in the pathway that starts from glucose), and after, via EMP pathway, it can beconverted in pyruvate:

Hemicellulose! Xylose ! PPP pathway ! Fructose  6  phosphate

When the substrates contain cellulose, instead, become glucose via cellulose lysis that follow the metabolic pathway in the same manner as it earlier stated (Ranjanand Moholkar, 2012),

In all the described cases, the final product of the first part of the reactions is the vate that allows to produce ethanol, acetone, and n-butanol, as described inFig 17.3

Pyruvate Ferredoxin Oxidoreductase

Tiolase

Aldehyde/alcohol dehydrogenase

AcetoacetylCoA:acetate /butyrate:CoA transferase

1 NADH+3-Hydroxybutyryl-CoA Dehydrogenase;

2 Crotonase;

3 NADH + butyryl-CoA dehydrogenase

Aldehyde/alcohol dehydrogenase

Fig 17.3 Schematic representation of the ABE process

At this point, the pyruvate-ferredoxin oxidoreductase (PFOR) enters into functionthat cleaves pyruvate resulting from glycolysis, in the presence of coenzyme A, to pro-duce carbon dioxide and acetyl-CoA, by converting the oxidized ferredoxin simulta-neously in its reduced form (Menon and Ragsdale, 1997) In order to have aquantitative view, the ferredoxin of clostridia produces 1 mol of each acetyl-CoAand CO2per 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 phosphateacetyltransferase 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 theproducts of the preceding acidogenic phase are reassimilated and converted to acetoneand n-butanol

The enzyme catalyzing this conversion is Co-A transferase, which converts CoAfrom acetoacetyl-CoA either to acetate forming acetyl-CoA or to butyrate resulting inbutyryl-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., gradability), the first-generation biofuels have the highest carbon footprint com-pared with other generations of biofuel The production technologies adopted forthe production of first-generation biofuel are inclusive of transesterification processfor biodiesel production and fermentation process for bioethanol production.However, the physical characteristics of the raw biomass could greatly affect theefficiency of the conversion processes To overcome this problem, more compli-cated lignocellulosic biomass processing technologies, such as thermochemicaland biological conversion processes, are employed to produce second-generationbiofuel (Liew et al., 2014)

biode-These biofuels, also called simply biofuels, are produced by processing biomassand include bioethanol and biodiesel that can be used in vehicles and in industrialprocess

In fact (as shown inFig 17.4), despite the substrates are different, the conversion inacetone, butanol, and ethanol is the same, once transformed into simple sugars Thisconversion is managed by ABE fermentation in both generation feedstocks.Therefore, the conversion of lignocellulose to monomeric fermentable sugars in thenature is a quite prolonged process In order to receive enough amounts of fermentablesugars, it is necessary to use pretreatment methods for the destruction of interconnec-tions in the lignocellulosic biomass and cellulose and hemicellulose hydrolysis.Owing to the structural complexity of the lignocellulosic matrix, biofuel produc-tion from biomasses requires at least three major unit operations includingpretreatment, hydrolysis, and fermentation

17.3.1 Pretreatment of lignocellulosic biomasses

In general, there are four typical pretreatment processes: physical, biological, ical, and combinatorial pretreatment (physiochemical and biochemical) conversion(Agbor et al., 2011) The choice of the pretreatment method mainly depends onphysical-chemical properties of lignocellulosic biomass, and it is fundamental foroptimal successful hydrolysis and, consequently, to transform lignocellulosic polymerunits in the monomeric units of simple sugars The overall efficiency of the pre-treatment process is correlated to a good balance between low inhibitor formationand high substrate digeribility The goal of any pretreatment is characterized byseveral criteria: reducing the degree of polymerization of the lignocellulosic chain,preserving the pentose (hemicellulose) fractions, limiting the formation of degrada-tion products that inhibit the growth of the fermentative microorganism, and minimiz-ing energy demands and limiting cost (Council National Research, 2000)

chem-17.3.1.1 Physical pretreatment

These methods can be of two types: mechanical comminution and pyrolysis Theobjective of the mechanical pretreatment is a reduction of particle size and crystallin-ity of lignocellulose in order to increase the specific surface the degree of polymer-ization This can be produced by a combination of chipping, grinding, or milling

1st Generation

Starch

(corn, potato)

Sugars (sugarcane)

Lignocellulosic biomasses

Fig 17.4 Conversion of different types of feedstocks into acetone, butanol, and ethanol

depending on the final particle size of the material (10–30 mm after chipping and0.2–2 mm after milling or grinding) (Alvira et al., 2010; Mosier et al., 2005) Thepower requirement of mechanical comminution depends on the final particle sizeand the waste biomass characteristics (Cadoche and Lopez, 1989) Instead, pyrolysistreated biomasses at temperature greater than 300°C; cellulose rapidly decomposes toproduce gaseous products and residual char (Kilzer and Broido, 1965) The decom-position is much slower, and less volatile products are formed at lower temperatures.Mild acid hydrolysis (H2SO41 N, 97°C, 2.5 h) of the residues from pyrolysis pre-treatment has resulted in 80%–85% conversion of cellulose for reducing sugars withmore 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 ing agents Among these methods, dilute acid pretreatment using H2SO4is the mostwidely used Depending on the type of chemical used, pretreatment could have differenteffects on lignocellulose structural components Alkaline pretreatment, ozonolysis,peroxide (both techniques that used oxidizing agents), and wet oxidation (WO) pretreat-ments are more effective in lignin removal, whereas dilute acid pretreatment is moreefficient in hemicellulose solubilization (Galbe and Zacchi, 2002)

oxidiz-17.3.1.3 Physical-chemical pretreatment

The solubilization of lignocellulose components depends on temperature, pH, andmoisture content In lignocellulosic materials such as wheat straw, hemicellulosesare the most thermal-chemically sensitive fraction Hemicellulose compounds start

to solubilize into the water at temperature higher than 150°C, and among various ponents, xylan can be extracted the most easily (Sun and Cheng, 2002; Hendriks andZeeman, 2009) There are different types of solubilization of hemicelluloses byphysical-chemical production Every type employs the characteristics of pressureand temperature The most useful method uses the explosion of CO2to separatethe hemicellulose, that is, to reduce polymeric chains of glucosidic compounds tomost simple and fractionable sugar Conventional mechanical methods require70% more energy than physicochemical pretreatments to achieve the same amount

com-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 inthe hemicellulosic portion (Balat, 2011; Clark and Mackie, 1987)

17.3.1.4 Biological pretreatment

Biological pretreatment comprises using microorganisms, such as brown-, white-, andsoft-rot fungi, and seems to be, in our opinion, more effective than the other pretreat-ments The abovementioned pretreatment methods are harsh and cost-energy inten-sive; on the contrary, biological pretreatment processes are mild and environmentalfriendly

Microbial pretreatment consists of a solid-state fermentation process in whichmicroorganisms 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 sugarsnecessary for the production of biofuels in an autonomous way such as bacteria oralgae (third generation), but they are the perfect helpers in the degradation of ligno-cellulosic substrates and lipid accumulation.

The main fungi used as a pretreatment in the conversion of lignocellulosic biomassinto fermentable sugars are white-, brown-, and soft-rot fungi Brown rots mainlyattack cellulose, whereas white- and soft-rot fungi are usually preferred for the highselectivity in lignin degradation over cellulose loss (Wan and Li, 2012)

Lignin degradation by white-rot fungi, the most effective for biological treatment of lignocellulosic materials, occurs through the action of lignin-degradingenzymes such as peroxidases and laccases (Kumar et al., 2009)

pre-White-rot microbes typically secrete lignin peroxidases (as exposed below), alongwith various types of glycosyl hydrolases that cleave the C-C lignin backbone in thepresence of hydrogen peroxide Other enzymes involved in aerobically catalyzedlignin 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 phenolicrings to produce phenoxy radicals; in particular, it catalyzes the removal of an electronand a proton from phenolic hydroxyl and aromatic amino groups, to form freephenoxy radicals and amino radicals (Hatakka, 1994; Leonowicz et al., 2001; Wanand Li, 2012) as shown inFig 17.5

LiP is a hemeprotein that needs hydrogen peroxide H2O2from other enzymes to beactive It catalyzes the oxidation of nonphenolic aromatic lignin moieties and similarcompounds, 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 groupthat 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 finaleffect of these enzymes is to initiate wood decay and facilitate the penetration ofhydrolytic enzymes into cellulosic and hemicellulosic substrates The enzymatic reac-tion 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 arePhanerochaetechrysosporium, Ceriporiopsis subvermispora, and Daedalea flavida (Maurya

et al., 2015; Wan and Li, 2012) Biological pretreatment by white-rot fungi has beenadded together with organosolv pretreatment in an ethanol production process bysimultaneous saccharification and fermentation (SSF) from beech wood chips

Compound III Compound II

O Fe Fe

O

Compound I Native

H2O2 H2O

Substrate (RH or Mn2+) Substrate

(RH or Mn2+)

Product, radical (R • + H+ or Mn3+)

Product, radical (R • + H+ or Mn3+)

N N

N N

N N

Resting enzyme

Resting

enzyme

Peroxide-level intermediate

Peroxide-level intermediate

Native intermediate

Native intermediate

H

HO

O O

H

H

O O

H H

O O

H H

Fig 17.5 Laccase utilization of molecular oxygen to carry out reactions of oxidation withphenolic rings to produce phenoxy radicals

(Itoh et al., 2003) Another approach is to use minimally treated mushroom spent straw(MSS) as a feedstock for downstream thermochemical and biological processing.The advantages of biological pretreatment include low-energy requirement andmild environmental conditions However, the rate of hydrolysis in most biologicalpretreatment processes is very low.

17.3.2 Hydrolysis

Hydrolysis is performed to break down the complex structure of cellulose and cellulose into monomers (simple sugars) Glucose is obtained from cellulose, whilehemicellulose produces a mixture of pentoses and hexoses Hydrolysis is carriedout using either mineral acids (acid hydrolysis) or enzymes (enzymatic hydrolysis)

hemi-In acid hydrolysis, lignocellulosic biomass (LCB) is treated with mineral acids(e.g., sulfuric acid and hydrochloric acid) for a definite period of time at the specifictemperature to break cellulose and hemicellulose into monomer sugars as it follows:

Three main groups of cellulolytic enzymes were determined in white-rot fungi:(i) Endo-1,4-β-glucanases (EDG; EC 3.2.1.4) are involved in the initial cellulosebreakdown, by an attack to the amorphous regions of cellulose, forming new freechain ends, more accessible for cellobiohydrolases; (ii) cellobiohydrolases (CBH;

EC 3.3.1.91) are exocellulase enzymes responsible for cellobiose formation, lyzing preferably β-1,4-glycosidic bonds from chain ends; and (iii) β-glucosidases(BGL; EC 3.2.1.21) hydrolyze soluble cellobiose and cellodextrins to glucose(Dashtban et al., 2009; Goodell et al., 2008; Sun and Cheng, 2002) In addition, thereare many other enzymes that attack hemicellulose, such as xylanase, glucuronidase,acetylesterase, andβ-xylosidase (Duff and Murray, 1996; Sun and Cheng, 2002)

hydro-In literature, strains ofTrichoderma are considered among the best degrading fungi

in cellulase enzyme production, with other genera such asChrysosporium, Penicillium,andAcremonium (Gusakov, 2011;Zhang et al., 2014a,b)

17.3.3 Fermentation process

Wood cellulose ethanol production from biomasses refers to the use of special tation microorganisms to metabolize the abovementioned six (hexoses) and five (pen-toses) carbon sugars

fermen-In the fermentation process, the hydrolyzed sugars are mixed in a bath of water andmicroorganisms This microorganisms ferments sugars into bioethanol (as shown in

the reaction) via EMP pathway (Lin and Tanaka, 2006) under anaerobic conditionsand controlled temperature as seen in the ABE process:

There are three types of microorganisms that can be used for the production ofbioenergy, yeasts, bacteria, and fungi.Table 17.2shows some examples of microor-ganisms and their chemical-physical pretreatment conditions for an optimal fermen-tation It also shows the carbon source of the microorganisms (the substrate where themicroorganisms grow)

In particular, the best known microorganisms for ethanol production are the yeastSaccharomyces cerevisiae (i.e., the principal yeast used in the brewery and wineindustries and metabolizes glucose by EMP pathway) and the bacteriumZymomonasmobilis (that metabolizes glucose through Entner-Doudoroff (ED) pathway)(Claassen et al., 1999) They have, respectively, an ethanol yield of 130.12 and99.78 g/L The yeast S Cerevisiae remains the major industrial ethanol producer(Zaldivar et al., 2001), because it is generally recognized as safe (GRAS) microorgan-ism that can be produced by fermentation up to the 20% (v/v) ethanol from carbon(mainly C6 carbon sugars) (Cot et al., 2007) However, a major limitation, whichraises a serious industrial challenge, is the inhibition of the fermentation process

by accumulation of ethanol (Bayrock and Ingledew, 2001; Casey and Ingledew,2008; Hahn-H€agerdal et al., 2007) Instead, the bacteriumZ mobilis cannot fermentall forms of simple sugars but only glucose, fructose, and sucrose For this reason, it isnot well suited for all the feedstocks in the substrate

S cerevisiae and the Z mobilis offer high ethanol yields (90%–97% of the retical one) and high ethanol tolerance, up to ca 10% (w/v), in fermentation medium(Talebnia et al., 2010)

theo-Another suitable microorganism that can be used in ethanol production is the terium Escherichia coli; the engineering form can give higher yields in bioethanolproduction for the ability to ferment a wide spectrum of sugars

bac-Also the filamentous fungi, such as Neurospora crassa or Zygosaccharomycesrouxii, can produce bioethanol but with a poor yield (9.9 g/L) The reason is notyet entirely clear, but most probably, this low yield is due to the low resistance of thesefungi to higher concentration of ethyl alcohol formed in the batch

17.3.4 SSF and SHF process

The SSF and SHF processes turn out to be important processes for the production ofethanol from lignocellulosic substrates (the most common and popular are eucalyptus)and herbaceous substrates (sorghum, bagasse, and wheat straw) (Badhan et al., 2007)

In fact, the cellulosic materials are natural complexes at higher carbon content inthe form of plant biomass However, the production of ethanol from lignocellulosicraw materials is more difficult than from sugar or starch, although precise studies haveshown that over 100 types of microorganisms can metabolize sugar with five carbonatoms to produce ethanol, including bacteria, fungi, and yeast

Table 17.2 Different yields of ethanol in some of main yeasts, bacteria, and fungi

NCIM 870

Firstaerobicstep28

The aim of the first pretreatment (i.e., using explosion stream with dilute sulfuricacid or SO2) is to solubilize hemicellulosic sugars rendering the remaining celluloseavailable for enzymatic hydrolysis To convert the residual cellulose and hemicellu-lose into monomeric sugars (Parekh et al., 1988; Schell et al., 2003; Tucker et al.,

2003), pretreatment processes are implemented to improve the digestibility with tial solubilization or degradation of hemicellulose and lignin (Jan and Chen, 2003).The next steps to obtain bioethanol are enzymatic hydrolysis and fermentation thatmay occur separately (SHF) or simultaneously (SSF) (Fig 17.7)

par-The advantages of SHF surely reside in the possibility of control and optimization ofthe temperature of enzymatic hydrolysis and fermentation (Thomas-Pejo et al., 2008).The enzymatic hydrolysis is performed by cellulase (endoglucanase andexoglucanase normally) that break down cellulose into two glucose molecules bymeans of the ß-glucosidase (Olofsson et al., 2008) However, the activity is inhibited

by endoglucanase cellobiose while the ß-glycosidase from glucose (Palmqvist andHahn-H€agerdal, 2000)

An optimal fermentative microorganism should be able to utilize both hexose andpentose simultaneously with minimal toxic end-product formation By means of theSSF and of the SHF, the limits are exceeded preventing the inhibition by glucose infor-mation for the production of ethanol, optimizing the conversion of cellulose (Martin

et al, 2008)

The optimum temperature for saccharification is about 55°C while 30°C for thefermentation

Also the agitation requires an intermediate condition and is usually carried out at

150 rpm, and the microorganism used mainly in this process is theS cerevisiae (Adsul

et al., 2004)

Surely, these processes and in particular the SSF are consolidated techniques andwithout any problem from the point of view of the yields and of the reactions, but interms of economy, yet, many constraints have to be solved Then, to actually reach thefiber capable of producing ethanol, the industry has a long way to go

17.4 Third generation feedstocks

The third-generation biofuels (also calledadvanced biofuels) are sourced from food crops (mainly algae), but the resulting fuel is indistinguishable from its petro-leum counterparts (Fenton and Ohuallachain, 2012)

non-On the basis of current technology projections, third generation is considered to be

a viable alternative energy resource devoid of the major drawbacks (i.e., food-fuelcompetition) associated with first- and second-generation biofuels (Singh et al., 2011).Algae utilize enormous amounts of CO2for their growth and remove CO2frompower plant emissions, convert biomass via photosynthesis, and liberate more oxygen

to the atmosphere The algal biomass can be transformed into different types ofbiofuels according to three types of production processes (as shown inFig 17.8), ther-mochemical processes, biological processes, and chemical reaction

3.1 Algal biomass

3.2 Thermochemical conversion

3.3 Biological conversion

3.4 Chemical reaction

Fig 17.8 Schematic representation of main processes and products from algal biomass

17.4.1 The feedstock in the third generation: the algae

Algae can be divided into macroalgae and microalgae All these organisms are acterized by rapid growth on saline water, municipal wastewater, coastal seawater,and land unsuitable for farming (Chen et al., 2011; Pittman et al., 2011)

char-The total annual world production of algae biomass is about 12 Mt dry basis (around

16 Mt wet basis) for macroalgae and about 9200 t dry basis for microalgae, which wereharvested from wild habitats and aquaculture farms (Chen et al., 2015; Jung et al., 2013;Vassilev and Vassileva, 2016) The amount of the mass-cultivated macroalgae has con-tinuously increased over the last 10 years at an average of 10% (Vassilev and Vassileva,

2016;Jung et al., 2013) About 98% of commercial algae biomass production is rently with open ponds because this cultivation seems to be the most economicaland preferable way (Vassilev and Vassileva, 2016;Chen et al., 2015)

cur-17.4.1.1 Macroalgae

Macroalgae (also known asseaweed) are multicellular aquatic organisms ized by having low levels of cellulose and lipid and no lignin content in their structurebut high levels of structural polysaccharides (Allen et al., 2015; Ghadiryanfar et al.,2016; Goh and Lee, 2010) Macroalgae are comparatively large and photoauxotrophicorganisms that are able to grow up to 60 m in length (Raheem et al., 2015).Macroalgae are classified mainly into three major groups according to the thalluscolor derived from photosynthetic pigmentation variations, namely, green(Chlorophyta), red (Rhodophyta), and brown (Phaeophyta) (Chen et al., 2015;Demirbas, 2010;Vassilev and Vassileva, 2016)

character-17.4.1.2 Microalgae

Microalgae are unicellular photosynthetic microorganisms that are highly productiveand are able to produce large amounts of biomass more efficiently than current cul-tivation practices for terrestrial crops They have sized of<400 and of 1–30 μm indiameter (Vassilev and Vassileva, 2016) Microalgae have the possibility to convertalgal biomass, water, and CO2by sunlight energy (as it can be seen inSection 17.4.3)into various forms of bioenergy products

The photosynthetic efficiency of microalgae in engineered systems can reach4%–5% of the solar energy compared with 1%–2% for terrestrial plants (Shiltonand Guieyesse, 2010)

These conversions are much more efficient than those of crop plants On the onehand, there are many advantages for microalgae compared with lignocellulosic feed-stocks: (1) fast growing, (2) the need of lower amounts of water per kilogram ofbiomass produced, (3) grown in salt water, (4) the ability to sequester CO2from fluegas of industrial installations, and (5) the yield of a large amount of lipids and starchsuitable for the production of biodiesel, bioethanol, and others Biologists have cate-gorized microalgae in a variety of classes, mainly distinguished by their pigmentation,life cycle, and basic cellular structure The most important classes or categories ofmicroalgae in terms of their abundance are (1) diatoms (Bacillariophyceae), (2) green

(Chlorophyceae), (3) blue and blue-green cyanobacteria (Cyanophyceae), (4) golden(Chrysophyceae), and (5) red (Rhodophyceae) algae (Demirbas, 2010;Vassilev andVassileva, 2016;Ziolkowska and Simon, 2014).

It is estimated that 50000 species of microalgae exist; however, only few werepractically used (Vassilev and Vassileva, 2016) The most important microalgaefor each class are reported inTable 17.3

The differences reported in table between the various types of microalgae are interm of total lipid percent, the range in term of percentage from C16:0 to C18:3(long-chain fatty acids) This parameter is the lipid produced from microalgal speciesthat usually contains C16 and C18 fatty acids, which is similar to that of vegetable oilsand suitable for biodiesel production (Harrington, 1986; Ho et al., 2010; Miao and Wu,

2006) C16 and C18 are fatty acid groups that occupied up to 86% of total fatty acidswhen cultured in the nutrient-deficient medium This C16/C18 content is markedlyhigher than that obtained from cultivation under the nutrient-rich (67%) andnitrogen-deficient (78%) conditions (Ho et al., 2010) In addition, microalgae produc-tion has the potential to utilize CO2emissions and offers potential for a carbon neutralbiofuel

17.4.2 Thermochemical processes

In thermochemical processes, the biomass is gasified, liquefied, or heated (according

to the production process) to obtain a wide range of products: H2, CO, CO2, CH4, lighthydrocarbons biochar, and biotar (that are biohydrocarbons) (Kapdan and Kargı,

2006) as described in the following reaction:

Thermochemical conversion technique can further be divided into gasification andpyrolysis (as described inFig 17.9)

17.4.2.1 Pyrolysis

Pyrolysis is a thermochemical process in which biomass is converted into biochar,bio-oil, and syngas at high temperatures in the absence of oxygen The pyrolysis pro-cesses occur in the range of 400–1200°C Although the product yield dependsupon various operating parameters, generally, low temperature and high residencetime favor the char production (Tripathi et al., 2016) Depending on the values ofthese two conditions (temperature and residence time, whose experimental valuesare reported inTable 17.4), the pyrolysis can be further classified into slow pyrolysis,fast pyrolysis, and flash pyrolysis

Slow pyrolysis is principally used for the production of char, but liquid and gaseousproducts are also formed in a small quantity (Jahirul et al., 2012)

Instead, the fast pyrolysis is a procedure where the biomass is heated up rapidly to atemperature of 850–1250°C A typical fast pyrolysis produces 60%–75% of liquidproduct (tar and oils that remain in liquid form at room temperature like acetone

Table 17.3 Methane yield and biomass composition of some microalgae

oculata

and acetic acid (Ni et al., 2006)), 15%–25% of biochar (and almost pure carbon plusother inert materials), and 10%–20% of noncondensable gaseous products (H2, CH4,

CO, CO2, and other gases depending on the organic nature of biomass and pyrolysis(Bridgwater, 2006)), while the flash pyrolysis can be considered as an improved form

of fast pyrolysis: in this procedure, the temperature required for the degradation of thecomponents of biomass is archived by heating it with a very high rate of the order of

1000°C/s

17.4.2.2 Gasification

Gasification is a process in which carbonaceous content of the biomass is convertedinto the gaseous fuel in the presence of gaseous medium like oxygen, air, carbon diox-ide, steam, or some mixture of these gases at elevated temperature (between 700°Cand 900°C) (Tripathi et al., 2016)

As in the pyrolysis procedure, also the gasification process uses high temperatures,but in gasification process, the combustion takes place in the presence of O

Fig 17.9 Products obtained from pyrolysis and gasification in thermochemical processes

Table 17.4 Pyrolysis classes

Slow pyrolysis Fast pyrolysis Flash pyrolysisTemperature (°C) 550–950 850–1200 900–1200