Keywords

Introduction

The constant increase in the utilisation of energy in transport and industrial sectors has led to an ever increasing pressure on the availability and cost of existing fossil fuels system. Also, increase in general awareness of environmental issues such as global warming and consequences of greenhouse gas (GHG) emissions, and continuous depletion of fossil fuel reserves have further initiated exploration of alternative resources of energy. Several agencies are looking upon biofuels, the environment-friendly alternative energy carriers to the petrochemical-based transportation fuels.

Biofuel, any fuel made from organic matter resulting from agriculture or forestry, is a multiple objective sustainable resource, promising to substitute fossil fuels with energy from agricultural sources while providing a range of other benefits (Lovett et al. 2011). Currently, several types of biofuels, viz. bioethanol, biodiesel, biohydrogen, biomethane, biomethanol, biopropanol and biobutanol, are under consideration as potential alternative to fossil fuels.

Earlier, the main feedstocks for biofuels were food crops (first-generation biomass); however, the complete dependence of first-generation biofuels on food crops has caused food vs. fuel competition. However, shifting the dependence on secondary agriculture biomass has shown potential to be used as a feedstock for the production of biofuels, which otherwise would be treated as waste. Thus, biofuels can also help to reduce waste as well as providing a source of fuel. As second-generation biofuels technologies advance, it will become a preferable source of energy to both first-generation biofuels and fossil fuels, because of its wide range of benefits (Table 8.1). Second-generation biofuels does not depend on a particular feedstock and does not require highly fertile land for agriculture. As biofuels begin to enjoy growing acceptance around the world and in international markets, they could lower down the problems of energy supply as well as GHG emissions.

Table 8.1 Benefits of biofuels
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Bioethanol

Bioethanol is the most widely accepted biofuel to be used as an alternative to the gasoline. Currently, the major producers of ethanol are Brazil and the United States, where it is produced from sugarcane juice and corn grain, respectively. In India, ethanol is mainly produced from sugarcane molasses.

The bioconversion of secondary agricultural biomass to ethanol consists of two main processes: hydrolysis of lignocellulosic carbohydrate to fermentable reducing sugars and fermentation of the sugars to ethanol (Fig. 8.1). The hydrolysis of biomass is usually catalysed by cellulase, and the fermentation is carried out by yeasts or bacteria. The main factors that affects cellulose hydrolysis are porosity, crystallinity, and lignin and hemicellulose content (Margeot et al. 2009; Alvira et al. 2010; Kuhad et al. 2011). The presence of lignin and hemicellulose in lignocellulosic materials reduces the hydrolysis efficiency (Himmel et al. 2007). Pretreatment of lignocellulosic biomass before hydrolysis can significantly improve the hydrolysis efficiency by removal of lignin and hemicellulose, reduction of cellulose crystallinity and increase in porosity (Mosier et al. 2005; Kumar et al. 2009; Kuhad et al. 2011). Pretreatment can be carried out in different ways such as mechanical comminuting, steam explosion, ammonia fibre explosion, and acid or alkaline and biological pretreatments (Gupta et al. 2009, 2011; Kumar et al. 2009; Kuhad et al. 2010a).

Fig. 8.1
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Schematic overview of bioethanol production process (a pretreatment, b enzymatic saccharification, c fermentation, d distillation, e blending)

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Enzymatic hydrolysis of cellulose by cellulases is highly specific, and the major product of the hydrolysis is glucose. The utility cost of enzymes is lower than acid or alkaline hydrolysis because it is usually conducted at mild conditions and has no corrosion of equipment (Kuhad et al. 2010b). Both bacteria and fungi can produce cellulases for the hydrolysis of lignocellulosic materials. Of all these organisms, Trichoderma has been the most widely studied for cellulase production. The cellulase system contains three major enzyme components: endoglucanase (EC 3.2.1.4), cellobiohydrolase (EC 3.2.1.91) and β-glycosidase (EC 3.2.1.21). All these cellulase components act in a synergistic manner (Boraston et al. 2004; Kuhad et al. 2010b). The exoglucanase (CBH) acts on the ends of the cellulose chain and releases β-glucosidase as the end-product; the EG randomly attacks the internal O-glycosidic bonds, resulting in glucan chains of different lengths; and the β-glycosidases act specifically on the cellobiose disaccharides and produce glucose (Kuhad et al. 2010b). Hemicellulases are another group of polysaccharide-degrading enzymes that are specific to the hemicellulose substrate.

Biological ethanol fermentation is a process in which sugars are fermented by microorganisms to produce ethanol and CO2. Compared to starch and molasses, the fermentation of lignocellulosic hydrolysates is a more complicated process. Two major steps of ethanol fermentation from lignocellulosic biomass involve chemical and enzymatic hydrolysis, where the enzymatic hydrolysis of biomass releases the hydrolysate that contains mostly hexose sugars, but the acid hydrolysis yields not only pentose and hexose sugars but also a few fermentation inhibitors, such as furans and phenolics. Thus, detoxification methods are required to improve the fermentability of acid hydrolysate, for example, overliming, ion-exchange adsorption, activated carbon adsorption, solvent extraction, steam stripping and enzymatic (laccase) treatments (Chandel et al. 2007; Mosier et al. 2005; Palmqvist and Hahn-Hägerdal 2000a, b).

A variety of microorganisms ranging from bacteria, fungi and yeasts are known to ferment hexose sugars (Table 8.2). The most common and efficient microbe used for hexose fermentation is Saccharomyces cerevisiae (Hahn-Hägerdal et al. 2007). Various studies have been carried out using S. cerevisiae for the fermentation of lignocellulosic hydrolysates (Lee et al. 1999; Wang et al. 2004; Chen et al. 2007; Rocha et al. 2009; Gupta et al. 2009; Kuhad et al. 2010a). As an additional approach, simultaneous saccharication and fermentation (SSF) process has also been used for improved ethanol production. In the SSF process, the stages are virtually the same as in separate hydrolysis and fermentation systems, except that both are performed in the same reactor. It has been shown that SSF reduces the processing time, which, in turn, leads to an increase in the productivity of ethanol (Alfani et al. 2000; Soderstrom et al. 2005; Ohgren et al. 2007). Further, in order to economise the ethanol production, both pentose and hexose sugars must be converted to ethanol; however, even the most promising fermenting microbes do not efficiently ferment pentoses. Among the most common yeast species identified so far for the pentose fermentation are Candida shehatae, Pichia stipitis and Pachysolen tannophilus (Abbi et al. 1996a, b; Hahn-Hägerdal et al. 2007; Mosier et al. 2005; Palmqvist and Hahn-Hägerdal 2000a; Talebnia et al. 2008). Some other microorganisms that can ferment pentose sugars are listed in Table 8.2.

Table 8.2 Ethanol-producing microorganisms from hexose and pentose sugars
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Although Candida shehatae, Pichia stipitis and Pachysolen tannophilus can ferment pentose sugars, their commercial exploitation for ethanol production is limited because of their low ethanol tolerance, slow rates of fermentation, difficulty in controlling the rate of oxygen supply and sensitivity to inhibitors generated during pretreatment and hydrolysis of lignocellulosic substrates (Hahn-Hägerdal et al. 2007; Kumar et al. 2009). In mixed sugar fermentation, the pentose uptake is inhibited by hexoses, and thus, the pentose fermentation is only possible at very low glucose concentrations. In the last decade, genetic engineering of microorganisms used in ethanol production has shown significant progress (Jeffries and Jin 2004). Besides S. cerevisiae, bacteria such as Zymomonas mobilis and Escherichia coli have been targeted through metabolic engineering for ethanol production from lignocellulosic biomass (Jeffries 2006; Karhumaa et al. 2005, 2007; Liu and Hu 2010; Matsushika et al. 2008; Runquist et al. 2009; Yang et al. 2009). Few potent recombinant microbes with bioethanol potential are listed in Table 8.3.

Table 8.3 List of pentose utilising recombinant yeasts and bacterial strains
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The first techno-economic study was carried out in 1987 by the US National Renewable Energy Laboratory (NREL). The second economical study was carried out in 2002 by Aden and coworkers. As an estimation by the Energy Information Administration (EIA 2009), the wholesale price of gasoline in 2012 will be $2.62/gal of gasoline (US$ of 2007). Assuming a conversion factor of 0.67 gal of gasoline per gallon of ethanol, the projected cost of ethanol is set at $1.76/gal of ethanol (US dollar of 2007). However, the ethanol cost projection of the nth ethanol plant is at $1.49/gal of ethanol (US dollar of 2007).

Besides the USA, European research institutions, mainly in Sweden, the Netherlands and Denmark, have also evaluated the economics of bioethanol production. In the REFUEL project, 7 European institutes have analysed biofuels in terms of resource potential, costs and impacts. The data for bioethanol production from cellulosic materials based on the enzymatic hydrolysis strategy was procured from the Energy Research Centre of the Netherlands (Kuijvenhoven 2006) and the Copernicus Institute for Sustainable Development and Innovation of Utrecht University (Hamelinck 2004). The evaluation (Londo et al. 2008) resulted in a net production cost (including the sales of electricity as a by-product) of €0.62/L in 2010, €0.59/L in 2020 and €0.50/L in 2030.

Now it has been understood that the increasing production capacity to commercial scale can only be done with confidence when a process is shown to be robust at an intermediate, pilot scale. An ideal pilot plant needs to be fully integrated and should be able to evaluate the complete system (e.g., enzymes and yeasts) while having sufficient flexibility to investigate alternative process configurations and test opinions for better heat integration and recycling of process streams.

Biodiesel

Biodiesel is defined as the alkyl esters of vegetable oils or animal fats. The vegetable oils and fats can be considered as alternative source of transportation fuels with viscosities ranging from 10 to 17 times higher than the existing fossil fuel. Biodiesels have properties closer to gasoline and can be blended at high levels in certain vehicles. Currently, biodiesel-powered flexible-fuel vehicles are widely available in many countries (Carraretto et al. 2004).

Biodiesel is produced by transesterification of plant oil or fat to achieve a viscosity close to that of petroleum diesel. The conversion has two main steps, transesterification and hydrogenation. Transesterification refers to a reaction between triglyceride of one alcohol and a second alcohol to form an ester of the second alcohol (methyl ester). Transesterification of oils and fats to generate esters and glycerin is a well-established process. The purpose of the transesterification process is to lower the viscosity of the oil. While hydrogenation produces renewable diesels of superior quality and free of particulates and by-products (Gerhard 2010).

Various studies have been investigated using several oils as well as different catalysts such as NaOH, KOH, H2SO4 and supercritical fluids and lipase enzyme (Marchetti et al. 2007). Among various oil-bearing crops, only soybean, palm, sunflower, safflower, cottonseed, rapeseed and peanut oils are considered as potential for bio­diesel (Goering et al. 1982; Pryor et al. 1982). However, any vegetable oil could be used for biodiesel production (Demirbas 2006). Moreover, other sources for biodiesel include fats and waste oils. Efforts have also been made to exploit various algal species that produce oils (Nagel and Lemke 1990). In addition to oils, fatty acids can also serve as a potential reactant for biodiesel production. Since fatty acid bio­synthesis is a natural energy storage molecule in micro­organisms, it can be further esterified in vivo to form fatty acid ethyl esters (FAEEs) known as microdiesels with similar properties to bio­diesels (Kalscheuer et al. 2006). Such a pro­duction pathway has been demonstrated at pilot scale (Elbahloul and Steinbohel 2010; Steen et al. 2010).

Similar to first-generation feedstocks for bioethanol, though production of biodiesel from vegetable oils is a potential and inexhaustible source of energy with energy content close to diesel fuel, the extensive use of vegetable oils may cause its shortage in developing countries. Alternatively, a variety of biolipids can also be used for biodiesel production. These are (a) vegetable oil plants, (b) waste vegetable oil and (c) nonedible oils. In few countries like Malaysia and Indonesia, palm oil has been used for biodiesel production. While, in Europe, Rape seed is the main oil resource, in India and Southeast Asia, the Jatropha seeds have been used for biodiesel production.

In addition to the land oil crops, algae represent an important and novel platform used for biodiesel production. The algae have several advantages such as rapid growth rate, high photosynthetic efficiency and high biomass production. The use of waste nutrients is an important factor for sustainable production of algal biodiesel. Wastewater rich in N and P can be used for algae cultivation (Demirbas and Demirbas 2011). The use of residual algal biomass after lipid extraction, for example, as feed (because of the high vitamin content), is a key factor in biorefinery concepts in order to improve economic feasibility (Table 8.4). Therefore, the R&D efforts should be focused on the improvement of cost-effectiveness and sustainability for the production of biodiesel. There are numerous algal species which have potential to be used as suitable candidate for oil production (Table 8.5).

Table 8.4 Lipid content of selected microalga species
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Table 8.5 List of algae used for the production of biodiesel
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The most significant difference between algal oil and other oils is in their yield. The yield (per acre) of algae oil is approximately 200-folds higher than the plant/vegetable oils (Sheehan et al. 1998). It has been estimated that a diatom algae can produce 46 t of oil/ha/year. However, no commercial scale plant has been developed on algal biodiesel so far in India and should be seriously considered by the government and the private industry.

Similar to bioethanol, the cost of biodiesel production varies significantly, depending on the feedstock source and the scale of the plant. A review of several economic evaluation studies showed that the costs of biodiesel ranged from US$0.30–0.69/L. Rough estimations of the cost of biodiesel from vegetable oil and waste grease are US$0.54–0.62/L and US$0.34–0.42/L, respectively. With pre-tax diesel priced at US$0.18/L in the US and US$0.20–0.24/L in some European countries, biodiesel is thus currently not economically feasible, and more research and technological development will be needed (Bender 1999; Demirbas 2003).

There are still many technical challenges to be overcome for the large-scale production of biodiesel. In particular, genetic tools may lead to the construction of strains with desired characteristics, such as high oil contents. Neverthe­less, the economic feasibility of biodiesel might be achieved progressively by combining the fuel production with high-value by-products for food and feed ingredients to hopefully meet the growing energy demand in the future (Brennan and Owende 2010; Wijffels and Barbosa 2010).

Biohydrogen

Hydrogen can serve as a significant alternate energy carrier to fossil fuel with high energy content per unit mass of any known fuel (143 GJ t−1) and easily converted to electricity by fuel cells and on combustion it gives water as the only by-product. However, hydrogen is gaseous even at very low temperatures, so the storage density is an issue in any potential vehicular fuel application. Hydrogen can be produced from different methods, but biological methods of hydrogen production are preferable as they utilise CO2, sunlight and organic wastes as substrates under moderate conditions and considered environ­mentally benign conversions (Redwood et al. 2009). On the other hand, chemical methods for hydrogen production are energy intensive processes requiring high temperatures (>850°C) (Kapdan and Kargi 2006).

Direct biophotolysis of H2 production is a biological process which utilises solar energy and photosynthetic systems of algae to convert water into chemical energy.

$$ \mathrm{2H_{2}O+solar\,energy \to 2{H}_{2}+O_{2} }$$

Photosystem I (PSI) and photosystem II (PSII) are used in process of photosynthesis. PSI produces reductant for CO2, and PSII splits water to evolve O2 or the reductant generated by photosynthesis directly transferred to hydrogenase via reduced ferredoxin (2H+  +  2Fd  →  H2  +  2Fd) (Schnackenberg et al. 1996). The green algae and cyanobacteria (blue-green algae) contain hydrogenase and thus have the ability to produce H2 (Ni et al. 2006). In these organisms, electrons are generated when PSII absorbs light energy, which is then transferred to ferredoxin. A reversible hydrogenase accepts electrons directly from the reduced ferredoxin to generate H2 in the presence of hydrogenase (Das et al. 2008). The reversible hydrogenase and nitrogenase are sensitive to the oxygen, which is a major barrier for sustained hydrogen evolution. In cyanobacteria (e.g., Anabaena strains), heterocyst provides an oxygen-free environment to the oxygen-sensitive nitrogenase that reduces molecular nitrogen into NH3 as well as protons into H2 (Smith et al. 1992). Some green algae, for example, Chlamydomonas reinhardtii, deplete oxygen during oxidative respiration (Melis et al. 2000) and converts up to 22% of light energy into hydrogen energy which is equivalent to 10% solar energy conversion efficiency (Benemann 1996).

The process of indirect biophotolysis is completed in two separate stages that are joined via CO2 fixation. CO2 serving as an electron carrier between water splitting reaction and hydrogenase reaction. In first stage, CO2 is fixed in the form of storage carbohydrates (starch or glycogen) followed by second stage in which carbohydrate convert to H2 by reversible hydrogenase by both in dark- and light-driven anaerobic metabolic processes (Fig. 8.2).

Fig. 8.2
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Production of biohydrogen via indirect photolysis method

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The photosynthetic bacteria or non-sulphur bacteria are being used for long for H2 production. They produce H2 using light energy and organic acids (lactic, succinic and butyric acids or alcohols) as electron donors by mainly nitrogenase in anoxic condition. These bacteria have been found suitable for hydrogen production by using organic waste as substrate in batch processes and continuous cultures or immobilised whole cell system using different solid matrices (agar, porous glass, and polyurethane foam) (Das et al. 2008). The overall reaction is as follows:

Photofermentation has advantage over biophotolysis due to lack of PSII, which eliminates the difficulties of H2 production by inhibitory action of oxygen. The major drawbacks of the process are low photochemical efficiencies and non-homogeneity of light distribution in bioreactor. The maximum reported production rate is 6.55 mL H2/L h using malic acid as substrate (Tang et al. 2008).

While dark fermentation is a process in which organic substrate is converted to hydrogen by diverse group of bacteria under anaerobic conditions. The oxidation of substrate by bacteria generates electron which under anaerobic con­dition accepted by protons and reduced to molecular H2 (Das et al. 2008). Although hydrogen production also reported by utilising glucose as substrate by pure cultures, but utilisation of industrial wastewater as a substrate has been drawing considerable interest in recent years due to simultaneous waste treatment and inexpensive energy generation from low-cost substrate. Dark fermentation by using mixed consortia is a complex process in which hydrolytic microorganisms hydrolyze complex organic polymers to monomers which further converted to a mixture organic acids and alcohols by H2 producing acidogenic bacteria (Pandu and Joseph 2012). Some of the hydrogen producing bacteria has been listed in Table 8.6.

Table 8.6 Hydrogen-producing microbes
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Interestingly, a hybrid fermentation technology of dark fermentation and photofermentation in which light-independent bacteria and photosynthetic bacteria provide an integrated system for maximising hydrogen yield has also been used for hydrogen production (Eroglu et al. 2006). It provides maximum substrate utilisation which was thermodynamically limited in single-stage processes. It is a two-stage fermentation in which anaerobic fermentation of organic wastes produces low molecular weight intermediate (organic acids), which are then converted to hydrogen by photofermentation in photo bioreactor. The maximum yield was reported with sucrose (14.2 mol/mol) by utilising Caldicellulosiruptor and photosynthetic bacteria of Rhodopseudomonas capsulata (Wang and Wan 2008).

The main problem in the commercialisation of Biohydrogen is its high cost; therefore, novel strategies should be developed to make it more economically feasible. One of the ways to decrease the Biohydrogen production cost is to use lignocellulosic biomass (Chang et al. 2011). Several types of dedicated energy crops have been identified for biohydrogen production such as switchgrass, willow crops, hybrid poplar, alfalfa and corn stover. However, lignocellulosic biomass is usually not easily degraded by microorganisms due to their structural complexity. Pretreatment is thus necessary to lower down the lignin content, reduce the crystal structure and increase the surface area of the substrate (Xia and Sheng 2004).

A comparison of energy costs for different biological processes and the non-biological processes has been made by several investigators. The direct production of H2 from biomass eliminates the need for electrolysis, resulting in higher system efficiencies. The cost at ∼€40/million of Btu of fermentative H2 production at 10% conversion efficiency is considered to be unattractive. However, by adopting immobilised systems, an increase in the conversion efficiency up to 28.34% has been proposed (Kumar and Das 1999, 2000). In contrast, photobiological H2 production at 10% conversion efficiencies estimated to cost  ∼  €10/million Btu. It is attractive when compared with the fermentative ethanol production that costs  ∼  €31.5/million Btu at 1–30% conversion efficiencies. In addition, lower costs due to fewer requirements for pretreatment in comparison to ethanol production from biomass offer a great advantage.

However, the diversion of biomass for H2 production by dark fermentation and/or electricity generation would greatly depend on the technological maturity and land availability. In addition short duration, high density, fast growing and easily biodegradable dedicated energy crops would be needed. Similarly, long-term R&D efforts would be essential for the better conversion efficiency.

Biogas and Biomethane

Uncontrolled waste dumping is a serious problem of contemporary human habitation today; its controlled landfill disposal and incineration of organic wastes are not even considered optimal practices. In this context, energy recovery and recycling of organic matter and its nutrients through anaerobic digestion has been prioritised nowadays as an economical, immensely useful and greener technology. Production of biogas through anaerobic digestion of animal manure and slurries as well as of a wide range of digestible organic wastes including lignocellulosic wastes of agriculture is a process of significant environmental importance (Frigon and Guiot 2010). Anaerobic digestion converts these substrates into biogas, containing about 60% methane and 40% other gases, mainly carbon dioxide, and traces of nitrogen, hydrogen and hydrogen sulphide (Fig. 8.3). Resultant combustible gaseous product is usually termed as biogas, while biomethane is a term used to describe a gas mixture that is predominantly methane (>97%) obtained after upgrading biogas.

Fig. 8.3
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Composition of biogas

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There are numerous benefits associated with this renewable technology. It offers a clean and particulate-free source of energy. Biogas technology is a particularly useful system in the rural economy of any developing country like India and can fulfil several end uses. The gas is useful as a fuel substitute for firewood, dung, agricultural residues, petrol, diesel and electricity, depending on the nature of the task. Moreover, the slurry that is formed after methanogenesis is superior in terms of its nutrient content as the process of methane production serves to narrow the C:N, while a fraction of the organic N is mineralised to ammonium and nitrate, the form which is immediately available to plants.

The biogas process is a natural biological process that requires cooperation between different microorganisms and groups of microorganisms to function properly. Biogas microbes consist of a large group of complex and differently acting microbe species, notable the methane-producing bacteria (Table 8.7). The whole biogas process can be divided into three steps: hydrolysis, acidification (fermentation and acetogenesis) and methane formation (Fig. 8.4). Methane and acid-producing bacteria act in a symbiotically under anaerobic digestion process. On one hand, acid-producing bacteria create an atmosphere with ideal parameters for methane-producing bacteria, while on the other hand, methane-producing microorganisms use the intermediates of the acid-producing bacteria.

Table 8.7 Methane-producing microorganisms
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Fig. 8.4
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Microbiology of biogas production during anaerobic digestion

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There are two prominent methods of biogas production: using anaerobic digester and landfill gas.

Anaerobic digestion is a biochemical process whereby organic biomass sources are broken down by a diverse population of microorganisms in a low-oxygen environment, thus producing biogas as a natural by-product. Since the microorganisms are already present in all organic material, the process is triggered once the biomass is placed in a low-oxygen environment. Almost any organic material is a potential source of biomass feedstock to produce biogas using anaerobic digester (Nallathambi 1997). Sewage, manure, forestry wastes, agricultural wastes, energy crops, and industrial food processing wastes may be the most common biomass feedstocks for biogas production (Chynoweth et al. 2001). Biogas from sewage treatment plant digesters usually contains 55–65% methane, 35–45% carbon dioxide and <1% nitrogen, while biogas from other organic waste digesters usually contains 60–70% methane, 30–40% carbon dioxide and <1% nitrogen (Ras et al. 2007).

Landfill disposal is a predominant method of waste management. However, landfilling is unsustainable due to its harmful effects on the environment and public health. Therefore, biodegradable municipal waste (BMW) from landfills is required to divert to be used for landfill gas production. This is prominent technology towards development of energy from waste and is functional in Europe, the USA and other countries (Raven and Gregersen 2007). Landfill gas is a water-saturated gas mixture containing about 40–60% methane, with the remainder being mostly carbon dioxide (Asgari et al. 2011). Landfill gas also contains varying amounts of nitrogen, oxygen, water vapour, sulphur and a hundreds of other contaminants. Inorganic contaminants like mercury are also known to be present in landfill gas.

Biomethane production involves upgrading, or ‘cleaning-up’ of raw biogas to a higher-quality gas containing primarily biomethane. As a raw gas, biogas doesn’t contain the energy potential to be used for a number of applications such as gas grid injection or as a vehicle fuel (Fig. 8.5). Biogas upgrading involves removal of carbon dioxide, hydrogen sulphide, water vapour as well as trace gases. The resulting biomethane usually have a higher content of methane and a higher energy content making it essentially identical to conventional natural gas. There are number of different upgrading method which can be used to increase CH4 concentration (Wellinger and Lindberg 1999; Ryckebosch et al. 2011). The primary steps in the biogas upgrading process are as follows:

Fig. 8.5
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Schematic diagram of biogas and biomethane production and their utilisation

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Since the CH4 content is directly proportional to its energy content, increasing the methane content results in higher calorific value. Membrane separation, pressure swing adsorption (PSA) and water scrubbing are some of the prominent methods used to increase CH4 by removing CO2 from biogas (Wellinger and Lindberg 1999). Scrubbing with water is one of the cheapest and most common techniques for this purpose. Organic physical scrubbing includes solvents such as polyethylene glycol instead of water. Membrane separation and pressure swing adsorption (PSA) are different to absorption scrubbing techniques. A membrane is used from which water, O2 and CO2 are able to permeate through while a very limited amount of CH4 and nitrogen is able to pass (Wellinger and Lindberg 1999; Makaruk et al. 2010). Activated carbon and other molecular sieves can be used where the gas is fed through a series of columns in pressure, by which the CO2 is adsorbed to column matrix and CH4 reaches to the top of the vessel (Petersson and Wellinger 2009). H2S is also a common contaminant present in biogas, which can be removed by in situ reduction of H2S within the digester vessel by adding metal ions. Removal of H2S can be carried out via metal oxides, oxidation with air, adsorption on activated carbon and biological approach (Wellinger and Lindberg 1999). Chemical-oxidative scrubbing is also a promising technique for the removal of hydrogen sulphide from raw biogas (Miltner et al. 2012). Raw biogas is saturated with water vapour. Since water is potentially damaging to natural gas pipeline equipment and engines, it needs to be removed. The removal of water is performed via a number of different methods at varying points in the biogas upgrading process. Refrigeration, adsorption and absorption are some of the most common methods used for removing water from biogas (Wellinger and Lindberg 1999). In addition to H2S, H2O and CO2, there may be other trace contaminants present in the biogas which are potentially harmful to equipment and/or people and must therefore be removed or reduced to acceptable levels.

There have also been new developments in upgrading process of biogas such as cryogenic separation which is based on the sublimation points of different gases. Compressing and cooling the gas down to different temperatures, separation of various contaminants occurs. For instance, at −25°C, sulphur dioxide, siloxanes, water and H2S are removed; between −50 and −59°C, up to 40% of the CO2 is removed as a liquid; and finally, the remaining CO2 get removed in solid form when biogas cools further.

Advanced Energy Carriers

Furans

DMF is also emerging as a potential transportation fuel alternative, which have several properties such as 40% higher energy density, a boiling point 20°C higher than the ethanol and a lower solubility in water. DMF can be produced from the dehydration of HMF, a dehydration product of hexose sugars. HMF can be produced from sugars and lignocelluloses via dissolution and dehydration. Nevertheless, it is an intermediate, having a boiling point too high to be used directly as a transportation fuel. Thus, it should be derivatised to other compounds, such as furan derivatives, particularly dimethylfuran (DMF), which is a suitable compound for gasoline-range fuel (James et al. 2010). HMF esters are considered as relatively stable (Gruter et al. 2009) and, consequently, can be used as such or further derivatised.

Several noble metal catalysts were applied in the hydrogenolysis of HMF. Since these catalysts are degraded by acids, a neutralisation step is needed. Thus, a two-step method for the production of DMF from glucose has been proposed (Chidambaram and Bell 2010). Initially, 12-molybdophosphoric acid (H3PMo12O40) was used as the catalyst in a [C2MIM][Cl] ionic liquid-acetonitrile for the dehydration of glucose to produce HMF. As the second step, hydrogenation and hydrogenolysis of HMF were carried out over Pd/C catalysts. However, before second step, the temperature of the reaction mixture was cooled down to 50°C and neutralised (Maki-Arvela et al. 2012).

Methanol

Similar to other conventional biofuels, biologically derived methanol has also garnered tremendous interest. Although traditionally methanol is produced via a non-sustainable and cost-intensive chemical process involving catalytic steam reforming of natural gas, it is possible to produce this fuel in an environmentally benign manner using biomass resources (Demirbas 2008; Zinoviev et al. 2010). The advantages of using methanol as a fuel were realised long time ago and were introduced into the fuel economy (Reed and Lerner 1973). Thereafter, research and development on methanol production gained momentum applying various strategies. Before modern production technologies were developed, methanol was obtained from wood as a coproduct of charcoal production and, for this reason, it was commonly known as wood alcohol. It is produced from hydrogen and mixture of oxides of carbon by means of the catalytic reaction. Biosynthesis gas (bio-syngas) is a gas rich in CO and H2 obtained by gasification of biomass, which is available in renewable basis (Fig. 8.6). Methanol is currently manufactured worldwide by conversion or derived from bio-syngas, natural gas, refinery off-gas, etc. (Demirbas and Gulu 1998.), according to following reaction:

Fig. 8.6
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Biomethanol production from lignocellulosic waste

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$$ 2{\text{H}}_{2}+\text{CO}={\text{CH}}_{3}\text{OH}$$

Biomethanol can be an indispensable fuel with multiple applications. First and foremost, it can be used as a motor fuel in conventional engines, in its pure form or as a blend with gasoline, with an excellent emission profile. It is also possible to directly covert methanol to gasoline (Stelmachowski and Nowicki 2003; Demirbas 2008; Zinoviev et al. 2010). Second, it can be converted to MTBE (methyl tert-butyl ether), an additive to gasoline. While MTBE is a formidable fuel additive and enhancer, its production process involves using isobutylene, a product derived from fossil fuels. Third, it can be dehydrated to produce DME (dimethyl ether), a suitable replacement for natural gas. Lastly, it can be used as a raw material in the production of biodiesels (as FAMEs, fatty acid methyl esters) (Demirbas 2008; Zinoviev et al. 2010).

A variety of catalysts are capable of causing the conversion, including reduced NiO-based preparations, reduced Cu/ZnO shift preparations, Cu/SiO2 and Pd/SiO2 and Pd/ZnO (Demirbas and Demirbas 2007). Since the methanol production is a costly process, waste biomass should be considered for as potential cost-effective substrate. Biomass and coal have been considered as a potential fuel for gasification and eventually for syngas production and methanol synthesis. Adding sufficient hydrogen to the synthesis gas to convert all of the biomass into methanol can double the methanol produced from the same biomass base (Phillips et al. 1990). The natural gas is converted to methanol in a conventional steam reforming/water gas shift reaction followed by high-pressure catalytic methanol synthesis:

$$ \begin{array}{lll} \mathrm{CH_{4}+H_{2}O=CO+3H_{2}} \\ \\ \mathrm{CO+H_{2}O=CO_{2}+H_{2}} \\ \\ \mathrm{Finally, \, CO+2H_{2}=CH_{3}OH} \end{array} $$

However, quite recently, efforts are being made to produces biomethanol from crude glycerine, a renewable by-product of biodiesel synthesis.

Butanol

The four-carbon alcohols, ‘butanol’, are the longest chain alcohols found as natural major end-products of microbial fermentation. With the exception of a relatively small proportion of butanol produced by fermentation in China and Brazil (Ni and Sun 2009), these alcohols are currently petrochemically synthesised. The annual production of butanol has been estimated to be 2.8 million tons (2006) with continuous increase in demand and capacity. Butanol has many characteristics that make it a better fuel than ethanol. Butanol has the following advantages over ethanol: (a) it has 25% more Btu per gallon, (b) it is less evaporative/explosive with a Reid vapour pressure (RVP) 7.5 times lower than ethanol, (c) it is safer than ethanol because of its higher flash point and lower vapour pressure, (d) it has a higher octane rating, and (e) it is more miscible with gasoline and diesel fuel but less miscible with water (Ramey and Yang 2004). It is produced via acetone-butanol-ethanol fermentation (ABE fermentation) with the strict anaerobic bacterium, Clostridium acetobutylicum and C. beijerinckii (Qureshi and Blaschek 2001; Ramey and Yang 2004; Survase et al. 2011). However, with a few exceptions, anaerobic fermentation processes for production of fuels and chemicals, including ABE fermentation, usually suffer from a number of serious limitations including low yields, low productivity and low final product concentrations. Efforts are being made to make fermentation route competitive with petroleum-based solvent synthesis because the limitations have also been overcome (Chauvatcharin et al. 1998). However, petrochemical route of butanol production is still dominated over ABE fermentation. Few butanol-producing microbes are listed in Table 8.7.

Isopropanol

Isopropanol is one of the simplest secondary alcohols which are produced by microbes. Several species of Clostridium have been evaluated for isopropanol production, including Clostridium beijerinckii and Clostridium isopropylicum IAM 19239. However, the obtained isopropanol titres of these strains were very low, because clostridia produce isopropanol together with butanol (Table 8.8). Accumulation of isopropanol drastically reduced production yields. In this context, Inokuma et al. (2010) have recently applied a gas-stripping recovery method into the fed-batch culture system. Applying this method, they have successfully obtained production of 2,378 mM (143 g/L) of isopropanol from a recombinant clone of E. coli after 240 h with a yield of 67.4% (mol/mol) which is very close to the theoretical maximum yield (73.2% (mol isopropanol/mol glucose)).

Table 8.8 List of microbes producing butanol and isopropanol
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Chemically, the isopropanol is primarily produced by combining water and propene in a hydration reaction at industrial level (Lee et al. 2003). There are two routes for the hydration process: indirect hydration via the sulphuric acid process and direct hydration. In the indirect process, propene reacts with sulphuric acid to form a mixture of sulphate esters. Subsequent hydrolysis of these esters by steam produces isopropyl alcohol, which is purified after distillation. In direct hydration, propene and water react at high pressures in the presence of solid or supported acidic catalysts either in gas or liquid phases. Higher purity propylene (>90%) tends to be required for this type of process.

Thermochemical conversion of lignocellulosic feedstock into various advanced primary and secondary alcohol is an emerging technology which is known as producer gas fermentation (also known as synthesis gas or bio-syngas fermentation). It involves the use of acidogenic biological catalysts (such as Clostridium ljungdahlii, Clostridium autoethanogenum, Butyribacterium methylotrophicum, ‘Clostridium ragsdalei’ and Clostridium carboxidivorans P7, Table 8.8) which have an autotrophic mechanism of converting syngas components (CO, CO2 and H2 primarily) into solvents such as ethanol and butanol and commodity chemicals such as acetic acid and butyric acid (Imkamp and Muller 2007; Ramachandriya et al. 2011).

Dimethyl Ether (DME)

DME (CH3OCH3) is the simplest ether. It represents a new renewable fuel that still needs further exploration. It does not occur naturally in petroleum, hence is produced synthetically. The physical properties of DME are similar to LPG, as it can be transported and stored as a liquid at low temperature. DME is a clean fuel and contains no S or N compounds. Its energy content is ∼65% that of CH4. When used as a replacement for diesel fuel, DME has a high cetane value, which makes it more suitable for application in CI ICE (compressed ignition internal combustion engine) rather than in SI ICE (spark ignited internal combustion engine) (Semelsberger et al. 2006; Swaina et al. 2011).

Originally DME has been manufactured via methanol dehydration, but more recently the direct DME production from syngas (mixture of CO and H2). The direct production route involves one process instead of two processes – methanol synthesis and methanol dehydration and appears to be more energy and cost efficient. The cost of producing DME from methanol is influenced by price and availability, as methanol itself is an expensive chemical feedstock. In contrast, producing DME directly from syngas has many economic and technical advantages over methanol dehydration. Thermodynamically, DME production from syngas is more favourable than from methanol, and thus, in principle, the costs for DME production from syngas should be lower, provided a suitable catalyst can be found. Considering the exothermic characteristic of DME synthesis and the scale of production, we explored the feasibility of utilising a micro-channel reactor for DME synthesis, in conjunction with a combined methanol synthesis/dehydration catalyst system (Hu CK 2005; Hu J 2005).

Dimethyl ether can address energy security, energy conservation, environmental concerns and the pragmatic realisation of depleting petroleum reserves as an alternative fuel. Besides it can be exploited as a nontoxic, noncorrosive, environmentally benign, produced from domestic resources.

Pyrolysis Oil

Biomass can be processed in a liquid media (typically water) under pressure and at temperatures between 300 and 400°C. The reaction yields oils and residual solids that have a low water content and a lower oxygen content than oils from fast pyrolysis (www.nabcprojects.org). Upgrading of the so-called bio-crude is similar to that of pyrolysis oil. Pyrolysis oil can be produced by fast pyrolysis, a process involving rapidly heating the biomass to temperatures between 400 and 600°C, followed by rapid cooling. Through this process, thermally unstable biomass compounds are converted to a liquid product. The obtained pyrolysis oil is more suitable for long-distance transport than for instance straw or wood-chips. As a by-product, bio-char is produced that can be used as solid fuel, or applied on land as a measure of carbon sequestration and soil fertilisation. The oil can be processed in ways similar to crude oil, and several research efforts are currently undertaken to upgrade pyrolysis oil to advanced biofuels (EBTP 2010).

Microbial Batteries

Microbial fuel cells (MFCs) open up new horizons for the sustainable energy production from biodegradable organic matters. The microbial batteries convert energy of bio-convertible substrate into electricity. This can be achieved when bacteria switch from the natural electron acceptor to an insoluble acceptor, such as the fuel cell’s anode. MFCs have operational and functional advantages over the technologies currently used for generating energy from organic matter such as high conversion efficiency, operation at ambient temperature, does not require gas treatment, do not need energy input for aeration and have potential for widespread application.

Many organisms identified in microbial batteries possess hydrogenase enzyme, for example, Enterobacter, Bacillus, Clostridium, Bacteroi­detes, Actinobacteria and 11 novel phylotypes closely related to Ethanoligenens harbinense, Clostridium thermocellum and Clostridium saccharoperbutylacetonicum were also there (Xing et al. 2008). Hydrogenases could be directly involved in electron transfer towards electrodes. Hydrogenase is thought to become active in order to excrete excess reducing power under specific conditions, such as anaerobic conditions. Although, hydrogenase enzyme is purely responsible for this hydrogen production, which catalyses the following reaction:

$$ 2{\text{H}}^{+}+2{\text{X}}_{\text{reduced}}\leftrightarrow 6{\text{H}}_{2}+2{\text{X}}_{\text{oxidised}}$$

as illustrated in (Fig. 8.7).

Fig. 8.7
figure 7

Hydrogenase-mediated hydrogen production

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Bacteria can use soluble components that physically transport the electron from an (intra)cellular compound, which becomes oxidised, to the electrode surface. In many studies, redox mediators were added to the reactor, which often seemed to be essential. However, bacteria can also produce redox mediators themselves in two ways: through the production of organic, reversibly reducible compounds (secondary metabolites) and through the generation of oxidisable metabolites (primary metabolites).

Conclusion

The potential to use available residues from the agricultural and forestry sector to produce biofuels underscores the need for technology development. The assessment of sustainable biomass potential and the evaluation of benefits of biofuels are important key factors for increasing rural energy access. Moreover, the investment to help build capacities in the field for feedstock supply and handling can create favourable conditions to establishing a biofuel industry.