Bioenergy: Biofuels Process Technology

Abstract

Bioenergy is a renewable source of primary energy, and its sustainable use does not emit carbon dioxide. Two main drivers have pushed renewable energy production to the top of global agendas: climate change and energy security. Rising concern over depleting fossil fuel and greenhouse gas limits has resulted in a high level of interest in non-conventional fuel originating from bio-renewable sources including sugars, starches, lignocellulosic materials and algal biomass. Bioenergy crops can be grown for two contrasting markets: power generation (electricity, heat, and combined heat and power) and liquid transport fuels. Although over one billion tons of biomass per year would be potentially available to meet the 30 % replacement of petroleum derived gasoline in 2030, the high cost of biomass could be a serious hindrance if potential lands and feedstocks are not managed and utilized efficiently. Biofuels such as Ethanol, Butanol, Hydrogen gas etc. produced from various lignocellulosic materials such as wood, agricultural and forest residues has the potential to be a valuable substitute for, or complement to, gasoline. Oil-seed crops are also by far the largest group of exploitable renewable biomass resource for liquid fuel such as biodiesel and energy generation. Biodiesel is an environmentally friendly fuel that can be used in any diesel engine without modification. A technology using microbial fuel cells (MFCs) that convert the energy stored in chemical bonds in organic compounds to electrical energy achieved through the catalytic reactions by microorganisms has generated considerable interests among academic researchers in recent years. Currently, real-world applications of MFCs are limited because of their low power density level of several thousand mW/m².

Introduction

There is an increasing interest in the production of chemicals and fuels from renewable resources due to the continuing price increase of fossil resources, the insecurity of the availability of fossil resources in the future, and additionally environmental concerns and legislations (García et al. 2011; Baskar et al. 2012). In recent years, growing attention has been devoted to the conversion of biomass into biofuel such as ethanol, butanol, biodiesel etc. considered the cleanest liquid fuel alternative to fossil fuels (Lin and Tanaka 2006). Moreover, biomass energy can play an important role in reducing greenhouse gas emissions; since CO₂ that arises from biomass wastes would originally have been absorbed from the air, the use of biomass for energy offsets fossil fuel greenhouse gas emissions (Lynd 1996). Currently ethanol is the main bio-fuel used in the world and its use is increasingly widespread, the worldwide prospects are the expansion of the production and consumption of ethanol (Bastos 2007). Fermentation-derived butanol is a possible alternative to ethanol as a fungible biomass-based liquid transportation fuel (Pfromm et al. 2010). The transesterification of vegetable oils (VOs) with short-chain alcohols is used to produce biodiesel or by the esterification of fatty acids. During the past few years biodiesel has attracted attention as an environmentally friendly and renewable fuel because of uncertainties concerning petroleum availability and recent increases in petroleum prices (Berchmans and Hirata 2008). Similarly Hydrogen production from biomass conversion plays a very important role in the development of hydrogen economy (Ni et al. 2006). The quality of energy crops, used for biogas production, is determined on the field. Methane production from organic substrates mainly depends on their content of substances that can be degraded to CH₄ and CO₂ (Amon et al. 2007). About 95 % of ethanol produced in the world is from agricultural products (Walter et al. 2008). Ethanol production from sugar crops such as sugarcane and sugar beet account for about 40 % of the total bioethanol produced and nearly 60 % corresponding to starch crops (Biofuels Platform, 2010a, b). Biobutanol is on the agenda of several companies and may be used in the near future as a supplement for gasoline, diesel and kerosene (Antoni et al. 2007). Assuming an oil price of US$60 per barrel, both biodiesel and bioethanol produced from wheat are not profitable in Europe. At the assumed oil price, only bioethanol and biobutanol produced on a large scale from lignocellulose-containing raw materials have the potential to be produced competitively (Festel 2008; Kumar et al. 2012). The U.S. has become the dominant ethanol producer (corn-based), although Brazil has started an ambitious program to increase production by 50 % by 2009 (sugar-based). Biodiesel production has increased at 20–100 % annual rates in recent years, particularly in Germany, France, Italy, Poland, and the United States (Renewables 2005). About half of all the hydrogen as currently produced is obtained from thermo catalytic and gasification processes using natural gas as a starting material, heavy oils and naphtha make up the next largest source, followed by coal. Currently, much research has been focused on sustainable and environmental friendly energy from biomass to replace conventional fossil fuels (Balat and Kırtay 2010). Current total annual worldwide hydrogen consumption is in the range of 400–500 billion Nm³ (Demirbas 2009a, b). Present utilization of hydrogen is equivalent to 3 % of the energy consumption and with a growth rate estimated at 5–10 % per year (Mohan et al. 2013). Only a fraction of this hydrogen is currently used for energy purposes; the bulk serves as a chemical feedstock for petrochemical, food, electronics and metallurgical processing industries. The global market for hydrogen is already greater than US$40 billion per year (Kraus 2007); including hydrogen used in ammonia production (49 %), petroleum refining (37 %), methanol production (8 %), and miscellaneous smaller-volume uses (6 %) (Konieczny et al. 2008).

Biofuel Feedstocks

Fermentation substrate is an important factor influencing the cost of ethanol, butanol, hydrogen gas etc. production (Qureshi and Blaschek 2000). Lignocellulose is the most abundant renewable resource on the planet, and has great potential as a substrate for fermentation. Hemicelluloses are the second most abundant polysaccharides in nature, and represent about 20 to 35 % of lignocellulosic biomass (Koukiekolo et al. 2005). Xylan or hemicellulose may contain arabinan, galactan, glucuronic, acetic, ferulic, and rcoumaric acids as well as xylose. The occurrence and quantity of these compounds depend on the sources of xylan (Olsson and Hahn-Hägerdahl 1996; Koukiekolo et al. 2005). The ethanol yield and productivity obtained during fermentation of lignocellulosic hydrolysates is decreased due to the presence of inhibiting compounds, such as weak acids, furans and phenolic compounds produced during hydrolysis (Palmqvist and Hahn-Hägerdal 2000).

Lignocellulosic Feedstocks

Lignocellulosic biomass is generally composed of hemicellulose (25–35 %), cellulose (4–50 %), and lignin (15–20 %), and these structures are illustrated in Fig. 10.1. Cellulose hydrolysis can also be achieved under harsher conditions using solutions of mineral acids (H₂SO₄) at elevated temperatures; however, the harsh conditions required for non-enzymatic deconstruction of cellulose favor the formation of degradation products such as hydroxymethylfurfural (HMF), levulinic acid, and insoluble humins (Alonso et al. 2010; Rinaldi and Schüth 2009).

Fig. 10.1

Lignocellulose composition: cellulose, hemicellulose and lignin (Alonso et al. 2010)

Pretreatment is necessary to make cellulose more prominent to be attacked by the enzymes which ultimately convert it into fermentable sugars (Fig. 10.2).

Fig. 10.2

Schematic of goals of pretreatment on lignocellulosic material (Mosier et al. 2005)

Lignocellulosic biomass normally comprises cellulose, hemicellulose and lignin i.e. near about 55–75 % carbohydrates on dry weight basis. Percent dry weight compositions of different lignocellulosic feedstocks were determined by Mosier et al. (2005) as shown in Table 10.1.

Table 10.1 Percent dry weight composition of lignocellulosic feedstock’s (Mosier et al. 2005)

Unfortunately, neither commercial ethanol-producing cultures, nor butanol-producing cultures can hydrolyze these substrates. Hence, they need to be hydrolyzed prior to fermentation using a combination of pretreatment (acid, alkali, organosolvent, supercritical extraction or ammonia explosion) and hydrolysis (enzymes: cellulase, β-glucosidase, and xylanase) techniques (Galbe and Zacchi 2002). It should be noted that in contrast to ethanol production by yeasts, hexose and pentose sugars obtained as a result of pretreatment and hydrolysis of these residues can be used by butanol-producing cultures (Qureshi et al. 2008a, b, c).

Algal Biomass Feedstocks

Algae capable of accumulating high starch/cellulose can serve as an excellent alternative to food crops for bioethanol production, a green fuel for sustainable future. Certain species of algae can produce ethanol during dark-anaerobic fermentation and thus serve as a direct source for ethanol production. Of late, oleaginous microalgae generate high starch/cellulose biomass waste after oil extraction, which can be hydrolyzed to generate sugary syrup to be used as substrate for ethanol production. Macroalgae are also harnessed as renewable source of biomass intended for ethanol production (John et al. 2011; Nguyen et al. 2009). The use of marine algal biomass with high carbohydrate contents of Ulva lactuca and other macroalgae like Saccharina spp. Laminaria, Durvillaea, Ecklonia and Homosira (brown algae) (Figueira et al. 2000) indicates that a more cost effective strategy might be to ferment the carbohydrates like glucose, mannitol and laminarin from these algal species to either ethanol or butane (Potts et al. 2012; Huesemann et al. 2012).

Fate of different feedstocks for ethanol and biobutanol production are shown in Fig. 10.3.

Fig. 10.3

Different feedstocks generally used for ethanol and biobutanol production

Microbial Modeling of Biofuel Production

A wide variety of biofuels can be produced through the bioconversion of substrates contained in agricultural crops and residues (Fischer et al. 2008). Bioconversion of the sugars, starches, and other organic substrates contained in agricultural residues can be converted to ethanol by a variety of yeasts, to hydrogen by a variety of fermentative bacteria and archae, to methane by a consortium of bacteria and archae, and to oils for biodiesel production by fungi and algae (Drapcho et al. 2008). All of these microbial processes can be described mathematically to simulate the bioprocess (Kumar and Murthy 2013). First generation biofuels, such as ethanol and biodiesel are already widely used, but they were selected mainly for convenience rather than their properties as fuels. In a microbial biofuel production process, bioengineered microbes are grown inside a reactor in a solution that is rich in cellulose-derived sugar (glucose and pentose) (Turner 2014).

Bioreactor modeling and design based on microbial growth and product formation kinetics may be used to optimize production of high-value biofuels or maximize utilization of feedstock nutrients. Kinetic models are normally divided into two classes: structured and unstructured one. Structured models take metabolic pathways into consideration and are generally complicated. A structured model for acetone–butanol fermentations was established by Votruba et al. (1985).

The Monod model is a widely applied model used to describe microbial growth. Suitable microbial hosts for biofuel production must tolerate process stresses such as end-product toxicity and tolerance to fermentation inhibitors in order to achieve high yields and titers (Fischer et al. 2008)

$$ \mu =\frac{\mu_{max}\ S}{K_s+S} $$

(10.1)

where μ = Specific growth rate co-efficient h⁻¹

μ_max = Maximum Specific growth rate co-efficient h⁻¹

S = Substrate concentration mg/L

K_s = half-saturation constant, mg/L

Kinetic expressions for product formation must account for growth associated and maintenance-associated production, as in the following equation:

$$ {r}_p={Y}_{PX}{r}_x+{m}_pX. $$

(10.2)

Where r_x is the volumetric rate of biomass formation, Y_PX is the theoretical or the true yield of product from biomass, m_p is the specific rate of product formation due to maintenance, and X is biomass concentration (g/L), r_p is the volumetric rate of product formation (Doran 1995).

It is also possible that two or more substrates may simultaneously be growth-limiting, thus, a model that can describe such a system is given by:

$$ \mu =\frac{\mu_{max}\ {S}_1}{K_1+{S}_1}\left(\frac{S_2}{K_2+{S}_2}\right) $$

(10.3)

Where, μ is the specific growth rate (1/h), μ_m is the maximum specific growth rate (1/h). The specific growth rate could be inhibited by medium constituents such as substrate or product. In a case of substrate inhibition, the term is given by:

$$ \mu =\frac{\mu_{max}\ S}{Ks+S+\left[\frac{S^2}{K_I}\right]} $$

(10.4)

For a case that exhibits, product inhibition such as ethanol fermentation and ABE fermentation, the specific growth rate is written as:

$$ \mu =\frac{\mu_{max}\ S}{Ks+S}\left(\frac{Kp}{Kp+P}\right) $$

(10.5)

Where K_p is product inhibition constant and P is the concentration of the product

Bioreactor for Biofuel Production

Basic bioreactor designs for suspended growth cultures are batch, continuous (flow) stirred tank reactor (CSTR), and CSTR with external or internal biomass recycle (Fig. 10.4). If the means of cell separation (filtration, centrifugation, settling) removes the compound with the biomass, then the product is considered particulate. Hydrogen and ethanol are examples of soluble, extracellular products, while oils produced by filamentous fungi Pythium are intracellular products (Drapcho et al. 2008).

Fig. 10.4

Basic bioreactor types. (a) Batch. (b) Simple CSTR. (c) CSTR with external biomass recycle. (d) CSTR with internal biomass recycle. Dashed lines indicate system boundary used for developing mass balance equations

Batch Bioreactor

When plotted on arithmetic paper, batch growth cure assumes a sigmoidal shape, this can be predicted by combing the Monod equation with growth equation (Shuler and Kargi 2002).

$$ \frac{dX}{dt}=\frac{\mu_m\ S}{K_s+S}X $$

(10.6)

The relationship between microbial growth yield and substrate is

$$ X-{X}_0={Y}_{X/S}\left({S}_0-S\right) $$

(10.7)

Where X₀ and S₀ are initial values and Yx/s is the cell mass yield based on limiting nutrient.

$$ \frac{dX}{dt}=\frac{\mu_m\left({Y}_{X/S}{S}_0+{X}_0-X\right)}{K_s{Y}_{X/S}+{Y}_{X/S}{S}_0}X $$

(10.8)

Continuous Stirred Tank Reactors

Theoretically, a continuous process can be described with the following equations:

$$ \frac{dX}{dt}=\left(\mu -D\right)X $$

(10.9)

$$ \begin{array}{ll}D=\frac{F}{V},\hfill & \left(\frac{1}{h}\right)\hfill \end{array} $$

(10.10)

$$ \begin{array}{ll}{T}_R=\frac{V}{F},\hfill & (h)\hfill \end{array} $$

(10.11)

$$ {M}_S={R}_S-\frac{\mu }{Y_{SX}} $$

(10.12)

where

μ is the specific growth rate (1/h); D is dilution rate (1/h); X is biomass concentration (g/L); F is the feed flow rate (L/h); V is the volume of the bioreactor (L); T_R is the residence time (h); M_s is the maintenance value (C-mol/C-mol/h); R_s is the rate of substrate consumption (C-mol/C-mol/h); Y_sx is the yield of biomass per unit mass of the substrate.

Ethanol Production

Bio-ethanol is ethyl alcohol, grain alcohol, or chemically C₂H₅OH or EtOH. Bio-ethanol and bio-ethanol/gasoline blends have a long history as alternative transportation fuels. Bio-ethanol has a higher octane number (108), broader flammability limits, higher flame speeds and higher heats of vaporization. Disadvantages of bio-ethanol include its lower energy density than gasoline (bio-ethanol has 66 % of the energy that gasoline has), its corrosiveness, low flame luminosity, lower vapor pressure (making cold starts difficult), miscibility with water, toxicity to ecosystems (Spatari et al. 2005) increase in exhaust emissions of acetaldehyde, and increase in vapor pressure (and evaporative emissions) when blending with gasoline. Some properties of alcohol fuels are shown in Table 10.2.

Table 10.2 Some properties of ethanol (alcohol fuel)

Ethanol Production from Sugar, Starch and Lignocellulosic Feedstocks

Bio-ethanol is a fuel derived from biomass sources of feedstock; typically plants such as wheat, sugar beet, corn, straw, and wood. The conversion of lignocellulosic biomass to ethanol is a three step process that involves pretreatment followed by polysaccharide hydrolysis to simple sugars followed by sugar fermentation to ethanol (Mielenz 2001). The presence of lignin in cell walls negatively impacts these conversion steps (Keating et al. 2006; Li et al. 2008).

The effect of pretreatment of lignocellulosic materials has been recognized for a long time (McMillan 1994). Pretreatments for lignocellulosic materials include mechanical comminution, alkali swelling, acid hydrolysis, steam and other fiber explosion techniques, and exposure to supercritical fluids. Mechanical comminution. Waste materials can be comminuted by a combination of chipping, grinding and milling to reduce cellulose crystallinity. Pyrolysis has also been used for pretreatment of lignocellulosic materials. When the materials are treated at temperatures greater than 300 °C, cellulose rapidly decomposes to produce gaseous products and residual char (Sun and Cheng 2002). Mild acid hydrolysis (1 N H₂SO₄, 97 °C, 2.5 h) of the residues from pyrolysis pretreatment has resulted in 80– 85 % conversion of cellulose to reducing sugars with more than 50 % glucose (Sun and Cheng 2002a, b). Steam explosion is the most commonly used method for pretreatment of lignocellulosic materials (McMillan 1994). In this method, chipped biomass is treated with high-pressure saturated steam and then the pressure is swiftly reduced, which makes the materials undergo an explosive decompression. Steam explosion is typically initiated at a temperature of 160–260 °C (corresponding pressure 0.69–4.83 MPa) for several seconds to a few minutes before the material is exposed to atmospheric pressure. The process causes hemicellulose degradation and lignin transformation due to high temperature, thus increasing the potential of cellulose hydrolysis. Ammonia fiber explosion (AFEX) is another type of physico-chemical pretreatment in which lignocellulosic materials are exposed to liquid ammonia at high temperature and pressure for a period of time, and then the pressure is swiftly reduced. Dilute acid hydrolysis such as H₂SO₄ and HCl has been successfully developed for pretreatment of lignocellulosic materials. The dilute sulfuric acid pretreatment can achieve high reaction rates and significantly improve cellulose hydrolysis (Esteghlalian et al. 1997). The mechanism of alkaline hydrolysis is believed to be saponification of intermolecular ester bonds crosslinking xylan hemicelluloses and other components, for example, lignin and other hemicellulose. The porosity of the lignocellulosic materials increases with the removal of the crosslinks (Tarkow and Feist 1969). Dilute NaOH treatment of lignocellulosic materials caused swelling, leading to an increase in internal surface area, a decrease in the degree of polymerization, a decrease in crystallinity, separation of structural linkages between lignin and carbohydrates, and disruption of the lignin structure (Fan et al. 1987).

Enzymatic hydrolysis of cellulose is carried out by cellulase enzymes which are highly specific (Beguin and Aubert 1994). The products of the hydrolysis are usually reducing sugars including glucose. Utility cost of enzymatic hydrolysis is low compared to acid or alkaline hydrolysis because enzyme hydrolysis is usually conducted at mild conditions (pH 4.8 and temperature 45–50 °C) and does not have a corrosion problem (Duff and Murray 1996).

Fermentation

Ethanol can be produced from lignocellulosic materials in various ways. The main features of the different ethanol processes are outlined in Fig. 10.5. All processes comprise the same main components: hydrolysis of the hemicellulose and the cellulose to monomer sugars, fermentation and product recovery and concentration by distillation (Galbe and Zacchi 2002).

Fig. 10.5

Current process to produce biofuel from lignocellulose

The most frequently used microorganism for fermenting ethanol in industrial processes is S. cerevisiae, Zymomonas mobilis can ferment glucose to ethanol with higher yields. Since lignocellulosic hydrolysates contain pentoses, which are not readily fermented by these microorganisms, several attempts to genetically engineer S. cerevisiae (Walfridsson et al. 1996; Hahn-Hägerdal et al. 2007), Z. mobilis (Panesar et al. 2006) and the bacteria Escherichia coli (Decker et al. 2007) have been performed (Fig. 10.6).

Fig. 10.6

Redox balance in biosynthetic routes to glycerol and ethanol in S. cerevisiae (Baskar et al. 2012a, b)

Yeast convert hexose to ethanol and carbon dioxide by glycolysis as shown by the following reaction:

$$ {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\to 2{\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{H}}_2\mathrm{O}\mathrm{H}+2{\mathrm{C}\mathrm{O}}_2 $$

Theoretically, 1 kg of glucose will produce 0.51 kg of bio-ethanol and 0.49 kg of carbon dioxide. However, in practice, the microorganisms use some of the glucose for growth and the actual yield is less than 100 % (Demirbas 2009a, b).

Usually by products such as glycerol, succinic acid and acetic acids are produced. Optimum temperature and pH values for yeast are 30–35 °C and 4–5 respectively (Shuler and Kargi 2002). Most notably, C. thermohydrosulfuricum strain 39E has the highest reported ethanol yield (1.9 mol of ethanol produced per mol of glucose fermented) of any taxonomically described thermophilic anaerobe (Ng et al. 1981). For thermophilic organisms optimum temperature may range from 50 and 60 °C. Ethanol production is triggered by anaerobic conditions. Glucose concentrations above 100 g/l are inhibitory for yeast. Ethanol and some of the other by-products are inhibitory to yeast above concentration of 5 % (v/v). Ethanol tolerance yeast strains are being developed to avoid ethanol inhibition (Balat and Balat 2009). Simultaneous removal of ethanol from fermentation broth is another alternative for ethanol inhibitions.

Downstream Processing

Ethanol can be separated from the culture vessels during fermentation using low temperature vacuum distillation, adsorption, or membrane separation.

Butanol Production

Butanol (butyl alcohol and 1-butanol) is a four carbon primary alcohol having the molecular formula of C₄H₉OH (MW 74.12). Butanol is a colorless liquid with a distinct odor. Butanol is completely miscible with organic solvents and partly miscible with water (Lee et al. 2008a, b). Butanol represents a biofuel extender or replacement with properties clearly superior to ethanol (higher mileage, not hygroscopic, usable without engine modifications, not corrosive). In addition, it is a valuable feedstock for the chemical industry (Dürre 2011).

A sustainable bacterial fermentation route to produce biobutanol is poised for re-commercialization. Biobutanol may be produced by the acetone–butanol–ethanol (ABE) fermentation (Kumar and Gayen 2011). Today, biobutanol can compete with synthetic butanol in the chemical market (Green 2011).

Several countries have initiated new alternatives for biobutanol production from renewable feedstocks like sweet sorghum bagasse, rice bran (RB), de-oiled rice bran (DRB), corn stover, and wheat straw (Swana et al. 2011; Al-Shorgani et al. 2012; Zhang et al. 2011). By sustainable harvest based on current yields, these materials can be converted to 8.27 billion gallons of biobutanol replacing 7.55 billion gallons of gasoline annually (Swana et al. 2011). Common feedstocks used for biobutanol fermentation process are mentioned in Table 10.3.

Table 10.3 Feedstocks used for biobutanol fermentation process (Niemisto et al. 2013)

Researchers have been re-directing their interests in biomass based fuels, which currently seem to be the only logical alternative for sustainable development in the context of economic and environmental considerations. Renewable bioresources are available globally in the form of residual agricultural biomass and wastes, which can be transformed into liquid biofuels (Nigam and Singh 2011).

Although research on genetics, fermentation, upstream processing, and downstream processing has progressed significantly, the Clostridia are not able to efficiently hydrolyze fiber-rich agricultural residues. For this reason, agricultural biomass must be hydrolyzed to simple sugars using economically developed methods. Dilute sulfuric acid pretreatment can be applied to agricultural residues to bring about hydrolysis. Unfortunately, during acid hydrolysis, a complex mixture of microbial inhibitors is generated. Examples of the inhibitory compounds include furfural, hydroxymethyl furfural (HMF), and acetic, ferulic, glucuronic, rho-coumaric acids, etc. (Varga et al. 2004).

Formerly, ABE fermentation was operated as a batch process followed by distillation to recover the products. Sugars (molasses) or starch (corn, wheat, and potatoes) was used as substrates. In this process the price of the substrate accounts for up to 60 % of the cost, dramatically affecting the economic viability of ABE fermentation (Claassen et al. 1999).

The use of marine algal biomass with high carbohydrate contents of Ulva lactuca and other macroalgae like Saccharina spp. Laminaria, Durvillaea, Ecklonia and Homosira (brown algae) (Figueira et al. 2000) indicates that a more cost effective strategy might be to ferment the carbohydrates like glucose, mannitol and laminarin from these algal species to either ethanol or butane (Potts et al. 2012; Huesemann et al. 2012).

Important characteristics of butanol are summarized in Table 10.4. The market for biobutanol is currently worth US$5 billion and is estimated to rise to $247 billion by 2020 (Kretzers 2012).

Table 10.4 Characteristic properties of butanol

Unfortunately, neither commercial ethanol-producing cultures, nor butanol-producing cultures can hydrolyze these substrates. Hence, they need to be hydrolyzed prior to fermentation using a combination of pretreatment (acid, alkali, organosolvent, supercritical extraction or ammonia explosion) and hydrolysis (enzymes: cellulase, β-glucosidase, and xylanase) techniques (Galbe and Zacchi 2002). It should be noted that in contrast to ethanol production by yeasts, hexose and pentose sugars obtained as a result of pretreatment and hydrolysis of these residues can be used by butanol-producing cultures (Qureshi et al. 2008a, b, c).

Feedstock’s biomass pretreatment can be achieved by air dry the biomass, dry, grind, and then hydrolyze with dilute acid such as sulfuric acid with different concentrations (at 0.5, 1.0, 2.0, and 5.0 % by weight) (Potts et al. 2012). After hydrolysis the pH was adjusted to a value deemed suitable for fermentation (approximately 4.5–5). Various lignocellulosic feedstocks have been claimed for maximum solvent production by Clostridium (Table 10.5).

Table 10.5 Different feedstocks and strains used along with maximum solvents and productivities achieved (Jurgens et al. 2012)

In addition, during the last years the use of ionic liquids (ILs) such as [BMIM]Cl, [BMIM][PF6], [BMIM][TFSI], etc. for dissolving lignocellulosics has been examined intensively (García et al. 2011; Holm et al. 2012).

Pretreatment of lignocellulosic biomass in a microwave oven is also a feasible method which uses the higher heating efficiency of a microwave oven and it is also easy to operate (Bjerre et al. 1996). Microwave treatment utilizes thermal and non-thermal effects generated by microwaves in aqueous environments (Sun and Cheng 2002).

ABE (Acetone, Butanol and Ethanol) Fermentation

ABE hetero-fermentation produces acetate, butyrate, ethanol, and acetone, as well as butanol. The metabolism of ABE producing clostridia can be divided into the following two distinct phases: acidogenesis (acid-production) and solventogenesis (solvent-production) during the exponential and stationary phases of growth (Jones and Woods 1986).

Biobutanol is a biofuel that can be produced from renewable resources using special strains of bacteria such as Clostridium acetobutylicum or Clostridium beijerinckii (Qureshi et al. 2007).

In a normal batch culture, solvent-producing Clostridium species produce hydrogen, carbon dioxide, acetate, and butyrate during the initial growth phase (acidogenic phase), which results in a decrease in the pH of the culture medium. As the culture enters the stationary growth phase, the metabolism of the cells undergoes a shift to solvent production (solventogenic phase). During the second phase of the fermentation the reassimilation of acids, this occurs concomitantly with the continued consumption of carbohydrate, normally results in an increase in the pH of the culture medium. The relationship between the breakpoint in the pH of the fermentation and the onset of solvent production, which occurs at the beginning of the second phase of the fermentation, was identified early on in the development of the industrial fermentation process (Jones and Woods 1986). Major redox reactions in acetone–butanol–ethanol fermentation by the bacterium Clostridium is shown in Fig. 10.7.

Fig. 10.7

Major redox reactions in acetone–butanol–ethanol fermentation by the bacterium Clostridium (Liu et al. 2013)

$$ \begin{array}{l}12\;{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\to 6{\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2\mathrm{O}\mathrm{H}+4{\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{O}\mathrm{CH}}_3+2{\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{H}}_2\mathrm{O}\mathrm{H}+18{\mathrm{H}}_2+28{\mathrm{C}\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\hfill \\ {}\begin{array}{llll}\mathrm{Glucose}\hfill & \kern3.5em \mathrm{n}-\mathrm{Butanol}\hfill & \kern5em \mathrm{Acetone}\hfill & \kern2.7em \mathrm{Ethanol}\hfill \end{array}\hfill \end{array} $$

Acidogenesis

Bacteria grows exponentially in the first phase of fermentation (acidogenesis phase) along the formation of acids (mostly acetate and butyrate), leading to decrease of pH to 4.5 (Gheshlaghi et al. 2009). Two moles each of pyruvate, ATP and NADH are produced from one mole glucose consumed through the glycolytic pathway in the acidogenic phase. In this phase, glycolysis pathway is active to produce pyruvate consuming glucose, which is converted to Acetyl-CoA. Acetyl-CoA is the prime precursor for synthesis of acetate, butyrate, ethanol, butanol and acetone anaerobically. Acetate and butyrate are produced in acid producing phase through two analogous steps from acetyl-CoA and butyryl-CoA respectively (Kumar and Gayen 2011). An update review on key enzymes for butanol production is available (Gheshlaghi et al. 2009). When acids accumulate to sufficiently high levels, cells cannot maintain the pH gradient across membranes, and a dramatic decrease in growth occurs (Huang et al. 2010). Therefore, the shift to solvent production in Clostridia is an adaptive response to toxic effect of acidic metabolites through their re-assimilation and induced expression of genes for the stress response (Grimmler et al. 2011; Grupe and Gottschalk 1992).

Solventogenesis

As intracellular ATP is consumed by biosynthesis, solventogenesis is initiated to consume NAD(P)H accumulated during the acidogenesis (Grupe and Gottschalk 1992). The acetyl-CoA and butyryl-CoA are the key intermediates in synthesizing ethanol and butanol (Sillers et al. 2008). The reduction of acetyl-CoA and butyryl-CoA to acetylaldehyde and butyraldehyde is catalyzed by acetaldehyde dehydrogenase and butyraldehyde dehydrogenase, respectively, followed by the further reduction of acetylaldehyde and butyraldehyde to ethanol and butanol by ethanol dehydrogenase and butanol dehydrogenase (Gheshlaghi et al. 2009; Jones and Woods 1986). In both C. acetobutylicum and C. beijerinckii, the activity of butanol dehydrogenase was NADPH dependent rather than NADH dependent (Dürre 2008).

The use of excess carbon under nitrogen limitation is required to achieve high levels of solvent production (Madihah et al. 2001). Iron is one of the essential factors for the production of solvent (Kim et al. 1984). When Clostridium acetobutylicum was grown in batch culture under iron limitation (0.2 mg l⁻¹) at a pH of 4.8, glucose was fermented, to butanol as the major fermentation end product, and small quantities of acetic acid were produced. The final conversion yield of glucose into butanol could be increased from 20 to 30 % by iron limitation (Junelles et al. 1988). However, if the pH decreases below 4.5 before enough acids are formed, solventogenesis will be brief and unproductive. Increasing the buffering capacity of the medium is a simple way to increase growth and carbohydrate utilization as well as butanol production (Bryant and Blaschek 1988).

The fermenter is inoculated with a 5 % inoculum from a 24 h culture. The batch fermentation period is usually 2–2.5 days. First rapid growth and production of acetic/butyric acids and carbon dioxide and hydrogen occur. The initial pH of the medium drops from 6.5 to nearly 4.5 during this phase. In a second phase, growth ceases and the organisms convert acetic and butyric acids to neutral acetone and butanol. The acidity of the medium decreases and gas production increases. At the end of the fermentation the pH is approximately 5 (Shuler and Kargi 2002). The final total concentration of solvents produced ranges from 12 to 20 g/L in batch fermentation, which can be separated from the fermentation broth by distillation. Classical fed-batch and continuous cultivation do not seem to be economically feasible, because of solvent toxicity and the biphasic nature of acetone–butanol fermentation, respectively. To overcome this problem, fed-batch culture has been coupled with an in situ recovery process (Ezeji et al. 2004a, b), and multistage continuous fermentation has been conducted (Godin and Engasser 1990). Pilot plant for biobutanol production has been shown in the Fig. 10.8.

Fig. 10.8

Pilot plant for biobutanol production (Source: Butyl fuels, 2010 Korean Institute of Science and Technology)

Solvent Toxicity is one of the most critical problems in ABE fermentation, which ceases Clostridial cellular metabolism in the presence of 20 g/L or more solvents (Woods 1995). Moreira et al. (1981) and Jones et al. (1982) had attempted to elucidate the mechanism of butanol toxicity in C. acetobutylicum.

Downstream Processing

During the past two decades a significant amount of research has been performed on the use of alternative fermentation and product recovery techniques (e.g. adsorption, gas stripping, ionic liquids, liquid–liquid extraction, pervaporation, aqueous two-phase separation, supercritical extraction, and perstraction, etc.) for biobutanol production (Ezeji et al. 2007a, b, c). The various bioprocess stages and unit operations along with pretreatment are shown in the Fig. 10.9.

Fig. 10.9

Bioprocess stages and unit operations (Moo-Young and Chisti 1994)

The application of some of these techniques to the ABE fermentation process is described below.

• 1. Distillation: The cost of recovering butanol by distillation is high because its concentration in the fermentation broth is low due to product inhibition. In addition to the low product concentration, the boiling point of butanol is higher than that of water (118 °C). The usual concentration of total solvents in the fermentation broth is 18–33 g/L (using starch or glucose) of which butanol is only about 13–18 g/L. This makes butanol recovery by distillation energy intensive (Ezeji et al. 2004a, b).

• 2. Liquid–liquid extraction is another efficient technique to remove solvents from the fermentation broth. This approach takes advantage of the differences in the partition coefficient of the solvents. As butanol is more soluble in the extractant (organic phase) than in the fermentation broth (aqueous phase), it is selectively concentrated in the extractant. Common extractants employed include decanol and oleyl alcohol (Lee et al. 2008a, b).

• 3. Pervaporation is a membrane-based process that is used to remove solvents from the fermentation broth by using a selective membrane. The liquids or solvents diffuse through a solid membrane, leaving behind nutrients, sugar, and microbial cells. The application of pervaporation to batch butanol fermentation has been described by several investigators (Ezeji et al. 2004a, b).

• 4. Gas stripping is a simple but efficient way to recover butanol from the fermentation broth. The fermentation gas is bubbled through the fermentation broth, and then passed through a condenser for solvent recovery. The stripped gas is then recycled back to the fermentor and the process continues until all the sugar in the fermentor is utilized (Lee et al. 2008a, b; Ezeji et al. 2003). Butanol recovery is based on the principle along with their advantage and disadvantage is shown in Table 10.6 and integrated systems for fermentation and in situ solvent recovery are shown in Fig. 10.10.

  Table 10.6 Biobutanol recovery (Kumar and Gayen 2011; Heitmann et al. 2012)

  Fig. 10.10

  

  Integrated systems for fermentation and in situ solvent recovery: fermentation coupled with (a) gas stripping; (b) liquid–liquid extraction (perstraction); (c) pervaporation (Lee et al. 2008a, b)

Biodiesel

The transesterification of vegetable oils (VOs) with short-chain alcohols is used to produce biodiesel or by the esterification of fatty acids. During the past few years biodiesel has attracted attention as an environmentally friendly and renewable fuel because of uncertainties concerning petroleum availability and recent increases in petroleum prices. Its chemical structure is that of fatty acid alkyl esters. The production of biodiesel by transesterification employing acid (H₂SO₄, HCl, etc.) or base catalyst (NaOH, KOH, NaOCH₃, etc.) has been industrially accepted for its high reaction and conversion rates. Biological catalyst (lipase) is also sufficient to carry out the reaction at lowest amount, since it is faster.

Biodiesel is defined as fatty acid methyl or ethyl esters (FAME) from vegetable oils or animal fats when they are used as fuel in diesel engines and heating systems (Marchetti and Errazu 2008). Nowadays, it is used as an alternative fuel due to depleting petroleum reserves (Sujan et al. 2009). Fatty acid methyl esters are products of the transesterification (also called methanolysis) of vegetable oils and fats with methanol in the presence of a suitable catalyst to form alkyl esters (biodiesel) and glycerin. The main chemical process to produce biodiesel is the alkaline transesterification with methanol and KOH, where the alcohol reacts in the presence of the catalyst to form alkyl esters (biodiesel) and glycerin (Neha et al. 2013). Beside these products, traces of unreacted glycerides (mono-, di- and tri-acylglycerides) can also be found. If methanol is used in this process it is called methanolysis. Methanolysis of triglyceride is represented in Fig. 10.11.

Fig. 10.11

General scheme for transesterification of triglycerides

However; biodiesel has a higher cetane number, no aromatics, and contains 10 %–11 % oxygen by weight. These properties of biodiesel reduce the emissions of carbon monoxide (CO), hydrocarbons (HC), and particulate matter (PM) in the exhaust gas (Math et al. 2010). Preferred methods of production of biodiesel typically consist of reaction of oil sources with alcohols with aid of either acid or base.

Oil Sources and Methods of Biodiesel Production

Biodiesel is usually produced from food-grade vegetable oils using transesterication process. Therefore, it is said that the main obstacle for commercialization of biodiesel is its high cost. Waste cooking oils, restaurant greases, soapstocks and animal fats are potential feedstocks for biodiesel production to lower the cost of biodiesel (Canakci and Sanli 2008). The feed stock for biodiesel production is mainly soybean oil, sunflower oil, jatropha oil, canola oil, rapeseed oil, rubber seed oil and micro-algae etc. (Demirbas 2005). The cost of biodiesel is slightly higher than the petroleum based diesel mainly due to cost of edible oils which makes it more costly than the diesel fuel (Aworanti et al. 2013). Biodiesel obtained from vegetable oils has been considered a promising option but its higher viscosity is major problem which can reduce the fuel atomization (Pratas et al. 2011a, b). The petroleum based diesel fuel emits more carbon dioxide, greenhouse gases and hydrocarbon particulate matter, these are humiliation of the entire environment, regarding environmental concern, biodiesel has received more attention worldwide due to its properties such as clean, biodegradable, safe and eco-friendly (Atadashi et al. 2011). Presently many countries such as Germany, Australia and United State are already using biodiesel in replacement of traditional petroleum based diesel. In the United States, soybean oil is the most common biodiesel feedstock whereas rapeseed and palm oil are the most commonly used in Europe (Singh and Singh 2010). One of the most important disadvantages of using biodiesel is their cost. Biodiesel purification is carried out at the end of the reaction; the glycerin formed is separated from the methyl esters in a decantation funnel. The purification of methyl ester is done by washing with preheated distilled water (at 55 ˚C for 1 h). The pH of biodiesel should be approximately neutral (Hossain et al. 2010). The less dense phase, composed by esters, are removed and stored for further analysis and purification. The process flow diagram for biodiesel production is given in Figs. 10.12 and 10.13 represents enzymatic production process of biodiesel with immobilized lipase.

Fig. 10.12

Process flow schematic for biodiesel production

Fig. 10.13

Enzymatic production process of biodiesel with immobilized lipase (Source: Zhang et al. 2012)

ASTM International, recognized from 2001 as the American Society for Testing and Materials, is worldwide standards organization that holds properties values of biodiesel (Table 10.7).

Table 10.7 Fuel properties of biodiesel from soybean oil

The combined vegetable oil and animal fat production in the United States totals about 35.3 billion pounds per year (Perlack et al. 2005). This production could provide 4.6 billion gallons of biodiesel. Methyl ester is analysed by gas chromatography.

Production of Hydrogen

Hydrogen production plays a very important role in the development of hydrogen economy. Biomass and water can be used as renewable resources for hydrogen gas production. Biological production of hydrogen gas has significant advantages over chemical methods (Ni et al. 2006). The major biological processes utilized for hydrogen gas production are bio-photolysis of water by algae, dark and photo-fermentation of organic materials, usually carbohydrates by bacteria (Kapdan and Kargi 2006). Carbohydrate rich, nitrogen deficient solid wastes such as cellulose and starch containing agricultural and food industry wastes and some food industry wastewaters such as cheese whey, olive mill and bakers yeast industry wastewaters can be used for hydrogen production by using suitable bio-process technologies (Ghirardi et al. 2010). Conventional hydrogen gas production methods are steam reforming of methane (SRM), and other hydrocarbons (SRH), non-catalytic partial oxidation of fossil fuels (POX) and autothermal reforming which combines SRM and POX (Kapdan and Kargi 2006). Integrated biohydrogen system is shown in Fig. 10.14.

Fig. 10.14

Schematic representation of integrated biohydrogen system

Hydrogen Production by Fermentation

Biological hydrogen production can be classified into five different groups: (1) direct biophotolysis, (2) indirect biophotolysis, (3) biological water–gas shift reaction, (4) photofermentation and (5) dark fermentation (Levin et al. 2004). Comparative biological hydrogen production process is given in Table 10.8.

Table 10.8 Major advantages and disadvantages of biological hydrogen production process

Bio-hydrogen production from cellulose/starch containing agricultural wastes and food industry wastewaters is represented in Fig. 10.15.

Fig. 10.15

A schematic diagram for bio-hydrogen production from cellulose/starch containing agricultural wastes and food industry wastewaters (Source: Kapdan dan Kargi 2006)

All processes are controlled by the hydrogen-producing enzymes, such as hydrogenase and nitrogenase. The major components of nitrogenase are MoFe protein and Fe protein. Nitrogenase has the ability to use magnesium adenosine triphosphate (MgATP) and electrons to reduce a variety of substrates (including protons) (Fig. 10.16). This chemical reaction yields hydrogen production by a nitrogenase-based system (Hallenbeck and Benemann 2002):

Fig. 10.16

Z scheme of photosynthetic electron flow in green plants and algae showing links to carbon metabolism and hydrogen production: Q, A, primary electron acceptors in Photosystems II and I, respectively; dotted arrow signifies cyclic electron flow (Melis and Happe 2001a, b)

$$ 2{\mathrm{e}}^{-}+2{\mathrm{H}}^{+}+4\mathrm{A}\mathrm{T}\mathrm{P}\to {\mathrm{H}}_2+4\mathrm{A}\mathrm{D}\mathrm{P}+4{\mathrm{P}}_{\mathrm{i}} $$

where ADP and P_i refer to adenosine diphosphate and inorganic phosphate, respectively.

Hydrogen Detection and Quantification

The H₂ concentration in the gas phase is commonly measured with gas chromatography (GC) with thermal conductivity detector (TCD), using argon or nitrogen as the carrier gas. Typical GC operating conditions include temperature of 100 °C for the TCD and pressure of 151 kPa (22 psi) for the carrier gas. Silica columns (at 25 °C) or microcapillary columns may be used for separation (Drapcho et al. 2008). Fermentative hydrogen yield by different organisms is reported in Table 10.9.

Table 10.9 Reported fermentative hydrogen yield by different organisms

Microbial Fuel Cells

One of the most exciting technologies for biological production of energy is the microbial fuel cell (MFC). A microbial fuel cell is a mimic of a biological system in which bacteria do not directly transfer their produced electrons to their characteristic electron acceptor. Instead, the transport process is subsequently conducted over an anode, a resistance or power user, and a cathode. Thus way, bacterial energy is directly converted to electrical energy.

Fuel Cell Design and Fabrication

Bacterial reactions can be carried out over several different temperature ranges depending on the tolerance of the bacteria, ranging from moderate or room-level temperatures (15–35 °C) to both high temperatures (50–60 °C) tolerated by thermophiles and low temperatures (<15 °C) where psychrophiles can grow. Virtually any biodegradable organic matter can be used in an MFC, including volatile acids, carbohydrates, proteins, alcohols, and even relatively recalcitrant materials like cellulose (Fig. 10.17) (Logan et al. 2006).

Fig. 10.17

(a) Diagram of two-chamber microbial fuel cell with aqueous cathode and anode chambers with solid graphite electrodes. (b) Diagram of single-chamber microbial fuel cell with aqueous anode chamber and air cathode chamber. The anode and cathode chambers are separated by a membrane. The bacteria grow on the anode, oxidizing organic matter and releasing electrons to the anode and protons to the solution. The cathode is sparged with air to provide dissolved oxygen for the reactions of electrons, protons and oxygen at the cathode, with a wire (and load) completing the circuit and producing power. The system is shown with a resistor used as the load for the power being generated, with the current determined based on measuring the voltage drop across the resistor using a multimeter hooked up to a data acquisition system (Drapcho et al. 2008)

Hydrogen ions (protons, H⁺) can accept reducing equivalents (conventionally represented as electrons, e⁻) generated either photosynthetically or by the oxidation of organic and inorganic substrates inside microbial cells:

$$ 2{\mathrm{e}}^{-}+2{\mathrm{H}}^{+}\to {\mathrm{H}}_2 $$

The terminal electron donor (e.g., reduced ferredoxin) could donate electrons to the anode of a battery. Protons could then, in the presence of O₂, complete the electric circuit at the cathode by the reaction:

$$ {\mathrm{O}}_2+4{\mathrm{e}}^{-}+2{\mathrm{H}}^{+}\to 2{\mathrm{H}}_2\mathrm{O} $$

Thus, forming a highly environmentally friendly source of electric power (a battery), fueled by microbial metabolic activity (Logan and Regan 2006). That, in essence, is the definition of a microbial fuel cell (MFC).

The voltage across the external resistor or load in an MFC can be measured using a multimeter. Voltage measurements are converted to current values using Ohm’s law:

$$ \begin{array}{llll}V=IR\hfill & \mathrm{where}\kern0.24em V=\mathrm{Voltage}\;\left(\mathrm{V}\right),\hfill & I=\mathrm{Current}\;\left(\mathrm{A}\right),\hfill & R=\mathrm{Resistance},\hfill \end{array} $$

The power output from an MFC is calculated as

$$ \begin{array}{ll}P=IV\hfill & \mathrm{where}\ P=\mathrm{Power}\;\left(\mathrm{W}\right)\hfill \end{array} $$

Power density is used to relate power output to the anode surface area or anode chamber volume. Power density is calculated based on anode surface area as follows:

$$ P{\mathrm{D}}_{\mathrm{A}}=\frac{IV}{A_A} $$

where PD_A = power density on area basis, W/m²; and A _A  = anode surface area, m².

Methane Production

During anaerobic digestion, organic matter is converted to methane and carbon dioxide by way of a series of interrelated microbial metabolisms, including hydrolysis, acetogenesis, and methanogenesis. The value and stability of the pH in an anaerobic reactor are extremely important because methanogenesis proceeds only at a high rate when the pH is maintained in the neutral range (van Haandel and Lettinga 1994; Zinder 1994). Biogas production from maize along the production process are shown in Fig. 10.18.

Fig. 10.18

Influences on biogas production from maize along the production process (Source: Amon et al. 2007)

Hydrolysis

Many of the potential biomass sources for methane production are high molecular weight, insoluble polymers such as polysaccharides, proteins, and fats that are too large to be transported across bacterial cell membranes. Polysaccharides such as cellulose and hemicellulose are hydrolyzed to glucose and xylose by cellulase and hemicellulase enzymes. Proteins and lipids are hydrolyzed to their constituent amino acids and long-chain fatty acids by proteases and lipases, respectively. The rate of hydrolysis is a function of several factors, such as pH, substrate composition, and particle size.

Fermentation (Acidogenesis)

The second phase of the overall process is fermentation that begins with the conversion of the sugar monomers to pyruvate (C₃H₄O₃), ATP, and the electron carrier molecule NADH by central metabolic pathways. The central metabolic pathways found within most bacteria are the Embden-Meyerhof pathway (glycolysis) and the pentose phosphate pathway. Next, these fermentative bacteria convert pyruvate and amino acids to a variety of short-chain organic acids—primarily acetate, propionate, butyrate, and succinate—and alcohols, CO₂, and H₂ through various fermentation pathways. Acid producing organisms are a mixture of facultative anaerobes, such as enteric bacteria and clostridial species which are called acid formers. The optimum temperature and pH values for this step are T = 35 °C and pH = 4–6 (Shuler and Kargi 2002).

Acetogenesis

The short-chain organic acids produced by fermentation and the fatty acids produced from the hydrolysis of lipids are fermented to acetic acid, H₂, and CO₂ by acetogenic bacteria. Syntrophic bacteria that oxidize organic acids to acetate, H₂, and CO₂ are reliant on the subsequent oxidation of H₂ by the next group, the methanogens, to lower the H₂ concentration and prevent end-product inhibition.

Methanogenesis

In the final phase, methane is produced through two distinct routes by two different microbial groups. Among methanogenic bacteria used for this purpose are Methanobacterium (nonspore-forming rods), Methanobacillus (spore forming rods), and Methanococcus and Methanosarcina.

The optimum temperature and pH range for methanogenic bacteria are T = 35–40 °C and pH = 7–7.8 (Shuler and Kargi 2002). The relationships of the three general metabolic groups of bacteria or stages of fermentation involved in methane production are shown in Fig. 10.19. One route is by the action of the lithotrophic H₂-oxidizing methanogens that use H₂ as electron donor and reduce CO₂ to produce methane. In the second route, the organotrophic acetoclastic methanogens ferment acetic acid to methane and carbon dioxide.

Fig. 10.19

A scheme showing the relationships of the three general metabolic groups of bacteria or stages of fermentation involved in methane production (Source: Bryant 1979)

$$ \begin{array}{l}4{\mathrm{H}}_2+\mathrm{C}{\mathrm{O}}_2\to \mathrm{C}{\mathrm{H}}_4+2{\mathrm{H}}_2\mathrm{O}\hfill \\ {}\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}\to \mathrm{C}{\mathrm{H}}_4+\mathrm{C}{\mathrm{O}}_2\hfill \end{array} $$

Conclusions and Future Prospects

Bio-fuels are being promoted in the transportation sector. More recently, ethanol produced from sugar and starch-based feedstocks has become another important biofuel. Other biofuels such as lignocellulosic ethanol, biodiesel, biohydrogen, and bioelectricity have been the focus of vigorous research, and the technologies for their production are being developed, although most of these are not quite ready for commercialization. Currently, a large amount of studies regarding the utilization of lignocellulosic biomass as a feedstock for producing fuel ethanol is being carried out worldwide (Balat and Balat 2009). Bioconversion of lignocellulosic biomass to ethanol is significantly hindered by the structural and chemical complexity of biomass, which makes these materials a challenge to be used as feedstocks for cellulosic ethanol production (Zheng et al. 2009). But in addition to that, the technology of recombinant DNA will provide important advances for the development of fuel ethanol industry. The development of genetically modified microorganisms capable of converting starch or biomass directly into ethanol and with a proven stability under industrial conditions will allow the implementation of the consolidated bioprocessing of the feedstocks (Cardona and Sánchez 2007). The willingness of mankind to pay high prices for energy in the future is a great uncertainty. Hence, the biobutanol production can aid in extending the life of petroleum oil reserves and diminish environmental concerns (Kumar and Gayen 2011). The acetone-butanol fermentation was the first large-scale fermentation process developed which is sensitive to contamination. Therefore this fermentation contributed much to the knowledge of how to run sterile processes on an industrial scale. As an alternative plan, many research projects have been initiated for the efficient use of lignocellulosic biomass, algal biomass, etc. which should be accomplished in the future (Mosier et al. 2005; Kumar et al. 2009). Sucrose from sugar cane is also an excellent substrate in certain regions of the world. An optimal Bioprocess for butanol production can be developed by integrating the fermentation and downstream processes with strain development (Lee et al. 2008a, b). Researchers are also attempting an aerobic production of biobutanol using genetically engineered organisms like E. coli, S. cerevisiae etc. (Atsumi and Liao 2008; Steen et al. 2008). It will be the milestone to attract the attention of government, commercial, and research organizations for further support in implementing the innovative fermentation and extraction technology. The study on biodiesel synthesis showed that the quantity of catalyst, the temperature and reaction time are the main factors affecting the production of methyl esters both for short chain methyl esters and long chain methyl esters (Riadi et al. 2014). Biological methods offer distinct advantages for hydrogen production such as operation under mild conditions and specific conversions. However, raw material cost is one of the major limitations for bio-hydrogen production. Utilization of some carbohydrate rich, starch or cellulose containing solid wastes and/or some food industry wastewaters is an attractive approach for bio-hydrogen production (Kapdan and Kargi 2006). Microbial production of electricity may become an important form of bioenergy in future because MFCs offer the possibility of extracting electric current from a wide range of soluble or dissolved complex organic wastes and renewable biomass. A large number of substrates have been explored as feed. The major substrates that have been tried include various kinds of artificial and real wastewaters and lignocellulosic biomass (Pant et al. 2010). Currently, biogas production from energy crops is mainly based on the anaerobic digestion of maize. Among them, methane produced by anaerobic digestion has been used by the human race for hundreds, if not thousands, of years. In the near future, biogas production from energy crops will increase and it has to be considered that energy crops are grown in versatile, sustainable crop rotations (Strauß et al. 2012).