Keywords

14.1 Introduction

Providing sustainable energy sources for the modern industrialized world is becoming more difficult day by day. Today up to 80% of the world’s energy is provided by fossil fuels (Lund 2007). Recently there have been alterations in rainfall patterns in many regions because of global climate change brought about by increases in temperature and atmospheric CO2 levels. Global climate change is the main factor triggering worldwide droughts and will represent a significant challenge to the communities of the world in the near future. Studies have revealed there is a direct relationship between atmospheric CO2 levels and global warming (Salehi-lisar and Bakhshayeshan-agdam 2016). Although technologies, especially motorization, have facilitated human life at many levels, they have taken their toll on the natural ecosystem. One of the most important effects has been the intensive climatic change widely believed to be due to high CO2 generation by fossil fuel consumption (Voloshin et al. 2016; Zabed et al. 2016).

CO2 generation can be lessened by capturing and sequestering CO2 during the consumption of fossil fuels and utilizing renewable energy sources such as wind, solar, nuclear, and geothermal, as well as various biomass sources (Demirbas 2010; Nakagawa et al. 2007). In order to reduce their energy dependence on fossil fuels many countries are today focusing on alternative energy production strategies. This has led to biofuels becoming one of the most important fuel sources since they produce cheap energy, release low amounts of (or even no) greenhouse gases, and bring about foreign exchange savings related to socioeconomic benefits (Azad et al. 2015; Bahadar and Khan 2013; Sarkar et al. 2012). Lignocellulosic biomass consisting of cellulose, hemicellulose, and lignin is the most abundant raw material for biofuel production, especially bioethanol. The combustion of bioethanol produced from lignocellulosic biomass adds no net carbon dioxide to the earth’s atmosphere. It is a green fuel. The CO2 released due to the combustion of bioethanol is in fact that captured by photosynthetic plants from the atmosphere. This chapter presents an overview of lignocellulosic biomass, its sources, and lignocellulosic biofuel production technologies.

14.2 Biofuel: Future Energy

Liquid fuel and other components produced from biomass (organisms like plants) are called biofuels. All biofuels are renewable in that they involve photosynthetic conversion of solar energy to chemical energy thus setting them apart from fossil fuels. In recent years different arguments have been raised against the use of biofuels as future energy supply despite their contributing to a reduction in carbon dioxide emissions (Kour et al. 2019b; Kumar et al. 2019). There is increasing pressure today to reduce the use of fossil fuels. They are not only the main source of energy but also the main source of CO2 emissions (Alalwan et al. 2019). Biofuel is classified into two types: primary and secondary. Primary biofuels are fuels used essentially in their natural forms and come from organic material such as firewood, wood chips, and pellets. Although secondary biofuels also come from organic materials, they do not naturally exist in nature. Such biofuels include charcoal, ethanol, biodiesel, biooil, biogas, synthesis gas (syngas), and hydrogen and can be used for a wider range of applications such as transport and high-temperature industrial processes (Azad et al. 2015; Doshi et al. 2016; Yadav et al. 2019). Secondary biofuels are today based on primary sources and production techniques and classified in a number of categories (Fig. 14.1) such as first-, second-, third-, and even fourth-generation biofuels (Alalwan et al. 2019).

Fig. 14.1
figure 1

Structure of lignocellulosic biomass and its biopolymers: cellulose, hemicellulose, and lignin (Hernández-Beltrán et al. 2019)

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14.2.1 First-Generation Biofuels

First-generation biofuels are conventional biofuels produced from food crops. Such biofuels include bio-ethanol or butanol that are produced by the fermentation of starch (from crop residues of barley, corn, potato, wheat, etc.) or sugar (from sugar beet and sugarcane). Biodiesel is another first-generation biofuel produced directly from vegetable oils of oleaginous crops (such as rapeseed, palm, soybean, coconut, and sunflower) by transesterification (Alalwan et al. 2019; Jorgensen 2011).

14.2.2 Second-Generation Biofuels

Second-generation biofuels are fuels produced from various types of biomass, especially lignocellulosic biomass. Biomass means any source of organic carbon rapidly renewed by the carbon cycle of plants. Biomass used to produce second-generation biofuels is more efficient than that used for first-generation biofuels due to the low cost of feed biomass (Alalwan et al. 2019; Dar et al. 2018; Doshi et al. 2016).

14.2.3 Third-Generation Biofuels

Microalgal biomass is the material used for the production of third-generation biofuels. Ever since 1978 aquatic species of algae have been introduced as a biofuel source. Oil-rich algae can be used for biofuel production and their dried residue can be reprocessed to create ethanol. The microalgae typically targeted include Dunaliella salina, Chlorella vulgaris, and Chlamydomonas reinhardtii because of their high lipid content (Alalwan et al. 2019; Bahadar and Khan 2013; Carere et al. 2008; Demirbas 2010; Kong et al. 2010; Gouveia and Oliveira 2009; Voloshin et al. 2016).

14.2.4 Fourth-Generation Biofuels

Fourth-generation biofuels are the result of the biotechnological manipulation of algae and cyanobacteria. This is a young but strongly evolving research field (Heimann 2016). Unlike the first three generations of biofuels, fourth-generation biofuels do not require the biomass used to be destroyed. This class of biofuels includes electrobiofuels and photobiological solar fuels. Fourth-generation biofuels are today produced by photosynthetic microorganisms designed to produce photobiological solar fuels (Alalwan et al. 2019).

14.3 Lignocellulosic Biomass: Renewable Resource for Liquid Biofuels

14.3.1 Concept of Lignocellulosic Biomass

Lignocellulosic biomass is precisely defined as the dry matter of plants (biomass) such as cellulose, hemicellulose, and lignin that are the most abundant raw materials on earth for biofuel production (Fig. 14.2). Cellulose is a linear homopolysaccharide consisting of glucose units (500–15,000) linked by β(1–4) glycosidic bonds. Hydrogen bonds make the cellulose very rough and crystalline and provide protection from enzyme activity. Hemicellulose is an amorphous and variable polymer that originates from heteropolymers including hexoses (D-glucose, D-galactose, and D-mannose) as well as pentose (D-xylose and L-arabinose) and might contain sugar acids (uronic acids).

Fig. 14.2
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The four generations of secondary biofuels (Alalwan et al. 2019)

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Hemicellulose plays a key role in the linkage between lignin and cellulose. Lignin is an aromatic polymer and is the second major constituent of biomass after cellulosic matter. When burnt lignin produces a lot of energy and could even be used as a better source for the production of heat and power than the cost-effective yield of bioethanol. Since the carbohydrate polymers of a plant’s cell wall are tightly bound to lignin the biomass obtained from the cell walls is called lignocellulosic biomass (Hernández-Beltrán et al. 2019; Limayem and Ricke 2012). Lignocellulosic biomass is classified into three major categories that consist of (1) intact biomass that includes all terrestrial plants such as trees, shrubs, bushes, and grasses; (2) waste biomass that is produced as a low-value by-product of various industrial sectors such as agriculture (corn stover, sugarcane bagasse, straw, etc.) and forestry; and (3) energy crops defined as those crops that produce a high yield of lignocellulosic biomass and serve as a raw material for the production of second-generation biofuels (Jorgensen 2011; Kour et al. 2019c; Limayem and Ricke 2012).

Lignocellulosic biomass is the raw material used in the pulp and paper industry where the focus is on separating the lignin and cellulose of the biomass of plants. The fermentation of lignocellulosic biomass to ethanol is an attractive alternative to fossil fuels for the production of fuel. The combustion of lignocellulosic ethanol adds no net carbon dioxide to the earth’s atmosphere because the CO2 involved is photosynthetically fixed (Nakagawa et al. 2007).

14.3.2 Lignocellulosic Biomass Sources

Numerous groups of raw materials are classified by their origin, composition, and structure the most important of which are woody feedstock, agricultural residue, municipal solid waste, and marine algae (Fig. 14.3).

Fig. 14.3
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Lignocellulosic biomass sources and bioethanol production technologies that come from them

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14.3.2.1 Woody Feedstock

Primary and secondary industries take wood and convert it into products such as furniture, kitchen cabinets, flooring, building products, pallets, containers, and paper products. The high volume of woody residue produced during such manufacture is called woody feedstock (Organization 2015). Secondary manufacturing industries use products manufactured by primary industry. The residue produced by secondary industries is less than that of primary industries (Rooney 1998; McKeever 1998) because any residue produced is used to meet their energy needs for heat, especially in the winter. Wood residues have long been considered significant energy sources worldwide. Woody feedstock consists of sawdust, wood chips, and wood bark all of which have been found to be favorable for bioethanol production (Huzir et al. 2018). Such materials have the potential to be an additional resource for biological-based products for bioenergy. This potential should be properly managed using forest management techniques for bioenergy to be a sustainable source of fuel for the future.

14.3.2.2 Agricultural Residue

Agricultural residue represents a large, renewable, and rich source of lignocellulose biomass for the production of bioethanol. In countries whose economies are based on agriculture large quantities of agricultural field residues are available such as residues from growing beet, corn, fruit, sugarcane, and cobs of corn. Such residues include corn stover, leaves, orchard trimmings, rice husk, rice straw, and stalks (Sims 2004). Even weeds are available in large quantities. Although usually ultimately burned or left in the fields, such weeds can be better used for biofuel production (Kim and Dale 2004; Kumar et al. 2017; Sarkar et al. 2012). Agricultural biomass typically comprises ash, cellulose, hemicellulose, lignin, and protein. Moreover, crop residues offer a cheap and sustainable resource that can be used to produce biofuels worldwide (Bhatia and Paliwal 2011; Braide et al. 2016; Cheng et al. 2012; Voloshin et al. 2016).

14.3.2.3 Urban Residue

The two principal sources of urban lignocellulosic biomass are municipal solid waste and construction or demolition debris (Ahmed and Ahmaruzzaman 2016). Municipal solid waste such as sewage, any industrial leftover that is organic, and waste of modern urban life are low-cost sources of lignocellulosic biomass containing significant amounts of CO2 (Nakagawa et al. 2007). Construction and demolition debris make up the other principal source of urban residue and are considered separate from municipal solid waste because they come from different sources. The amount of such residue is highly dependent on economic activity, population, demolition activity, and recycling programs. Construction debris is potentially usable, while demolition debris is not since it tends to be contaminated. There is not enough data currently available about the volume of urban wood residue that could be used. Industrial residues have excellent potential to be recycled as cellulosic materials irrespective of whether they come from residential or nonresidential sources. However, there has been limited research into the utilization of municipal solid waste to generate cost-efficient biofuels profitably and on a large scale worldwide (Ahmed and Ahmaruzzaman 2016).

14.3.2.4 Marine Algae

Algae are aquatic organisms capable of converting solar energy into energy-rich chemicals such as starch and lipids. Consisting of macroalgae and microalgae (Falkowski and Raven 2004; Ho et al. 2013; Koutra et al. 2018) marine algae are highly cost-effective, abundant, and sustainable raw materials for the production of biofuels such as alcohol, diesel, methane, and hydrogen. Marine algae biomass is getting a lot of attention today as a third-generation biofuel as a result of the setting up of a number of quick biorefineries. Algal species such as Chlorococcum spp., Chlamydomonas reinhardtii (Choi et al. 2010; Kong et al. 2010), Schizocytrium spp. (Kim et al. 2012), Dictyochloropsis splendida (Abd El-Moneim et al. 2010), Spirulina spp. (Markou et al. 2013), Stichococcus bacillaris (Olivieri et al. 2011), and Chlorella vulgaris (Lee et al. 2011) are the best candidates for biofuel production. Algal biomass could also be used as a raw material for the production of aircraft fuel and rocket fuel, biocrude oils, bioplastics, and improved livestock co-products (Bahadar and Khan 2013).

14.4 Lignocellulosic Biomass-Based Biofuel

Lignocellulosic biomass holds out the greatest potential when it comes to production of biofuels such as biohydrogen, bioethanol, biomethanol, biobutanol, biodiesel, and biogas in an eco-friendly manner. Bioethanol is particularly popular and the technology behind its production is well-known (Azad et al. 2015; Almodares and Hadi 2009; Banerjee et al. 2010).

14.4.1 Bioethanol

Bioethanol is the principal petrol substitute for transport vehicles. It is largely produced from the fermentation of sugar, although it can also be produced when ethylene reacts with steam. The basic sources of sugar required to produce ethanol come from energy crops such as corn, maize, and wheat and their residue such as straw and stover. Other sources include willow and poplar, sawdust, reed canary grass, cord grasses, Jerusalem artichoke, Miscanthus spp., and sorghum (Jorgensen 2011; Weijde et al. 2013). Ethanol or ethyl alcohol (C2H5OH) is a clear and colorless liquid that is biodegradable with low toxicity and causes little environmental pollution. Ethanol is a chemical that can be used for a number of different purposes such as anti-freeze, beverages, solvents, depressants, germicides, and fuel (Braide et al. 2016).

Bioethanol has a number of advantages over traditional fuels the most important of which are: (1) resources are renewable in that energy crops are used for its production rather than finite resources; (2) greenhouse gas emissions are reduced; (3) blending bioethanol with petrol will help extend the life of oil supplies; (4) widescale production of bioethanol would give the rural economy a boost by growing the necessary crops; (5) bioethanol is not only biodegradable but also far less toxic than fossil fuels; (6) bioethanol can be easily inserted into the existing road transport fuel system; and (7) bioethanol will eventually be produced using well-known methods such as fermentation. Looked at collectively these advantages make the production and use of bioethanol eco-friendly (Perlack et al. 2005). Bioethanol can be produced from lignocellulosic biomass using hydrolysis and sugar fermentation processes described in detail in the following sections.

14.5 Lignocellulosic Biofuel Production Technologies

The first challenge to overcome in producing fuels from lignocellulosic biomass is releasing the fermentable sugars trapped inside the biomass. The fermentable sugars can be extracted by first disconnecting the celluloses from the lignin and then using acid or enzymatic methods to hydrolyze the celluloses to break them down into simple monosaccharides (Rastegari et al. 2019a). Another challenge that needs to be overcome associated with biofuel production is the high percentage of pentoses in the hemicellulose such as xylose. Unlike hexoses such as glucose and mannose, pentoses are difficult to ferment (Alvira et al. 2010). Synthesizing biofuels from lignocellolusic biomass is generally dependent on the yield of fermentable sugars available and on the effectiveness of the various steps involved in the production of biofuels from lignocellulosic materials such as pretreatment, hydrolysis, fermentation, and product separation or distillation (Banerjee et al. 2010; Sarkar et al. 2012) (Fig. 14.3).

14.5.1 Pretreatment

In order to hydrolyze lignocellulosic biomass into fermentable sugars a standard pretreatment method capable of removing lignin is required. Pretreatment is the most important, costly, and complex step in the biofuel production process. The cellulose–hemicellulose complex acts as a chemical barrier and affects the biofuel production process by restricting cellulase enzyme activity. Lignin physically encapsulates the cellulose-hemicellulose complex and is an important barrier to cellulase enzyme activity (Alvira et al. 2010; Procentese et al. 2017). The pretreatment processes that have been used can be classified into four categories: physical, chemical, solvent, and biological.

14.5.1.1 Physical Pretreatments

Physical pretreatment methods do not involve using chemicals of any kind for biomass transformation. Alterations in the biomass material that take place during physical pretreatment include increasing the surface area of the material to facilitate enzyme penetration and action, reducing the degree of crystallinity and polymerization of the cellulose content, hydrolyzing hemicelluloses, and disrupting the lignin structure albeit incompletely. Physical pretreatment methods include chipping, milling, grinding, and even freezing. Lignocellulosic biomass pretreatment using radiation such as microwaves also fits into this category.

However, the major drawback with physical pretreatment is its limitations when it comes to large quantities of feedstock as a result of the high amount of power requited for the radiation process (Kumari and Singh 2018; Sheikh et al. 2015). This has led to other physical pretreatment methods being developed such as hydrothermal processes including steam explosion and liquid hot water treatment. Biomass material in such pretreatment methods is hydrolyzed by applying high temperature (160–290 °C) and pressure (20–50 MPa) over a short period of time. Although a number of degrading compounds that inhibit microbial growth and are detrimental to ethanol fermentation such as furfural and carboxylic acid are generated in these methods, high xylose recovery (up to 90%) and lack of acid or any chemical requirement makes this method very economic (das Neves et al. 2007).

14.5.1.2 Chemical Pretreatments

One of the most important chemical pretreatment techniques is acidic pretreatment in which H2SO4 or HCl is used to extract sugar (Kumar et al. 2009). Concentrated and dilute acids can both be used for this purpose. Acid recovery is the biggest challenge facing application of this process on a commercial scale. This means dilute acid with a concentration lower than 2% is favored since it can easily be neutralized by alkaline compounds such as ammonium and lime. Despite the economic advantages of dilute acid pretreatment, there are some limitations to this technique that have led to new alternative pretreatment process techniques being developed such as alkaline pretreatment (Harun et al. 2011). Alkaline pretreatment using a number of alkalis such as sodium hydroxide, calcium hydroxide, potassium hydroxide, lime, and aqueous ammonia has some advantages such as bringing about delignification, decreasing cellulose crystallinity, and facilitating enzyme action on cellulose by increasing its surface area. In addition, alkaline pretreatment can be done under ambient conditions and does not require high temperature and pressure.

Alkaline pretreatment is typically chosen for lignocellolusic materials that contain low amounts of lignin. There are a number of other unusual chemical pretreatment methods such as ozone pretreatment (ozonolysis), ionic liquid pretreatment, CO2 explosion pretreatment, liquid hot-water pretreatment, wet oxidation pretreatment, steam explosion pretreatment, and ultrasonication (Kumar et al. 2009; Kumari and Singh 2018). Chemical pretreatment processes also suffer a number of limitations such as the time required for process completion can vary from hours to weeks and salt production during pretreatment not only inhibits microorganism growth but also affects the fermentation process raising concerns related to the environment. In addition, according to the high cost of this pretreatment method there is little chance of it being applied on a commercial scale (Procentese et al. 2017).

14.5.1.3 Solvent Pretreatments

Solvent pretreatment is a fractionation technique in which aqueous organic solvents with or without catalysts are used to bring about lignocellolusic material delignification. Methanol, ethanol, trimethyleneglycol, tetrahydrofurfuryl alcohol, ethylene glycol, glycerol, acetone, phenol, and n-butanol have been used in this method for lignin extraction (Kumar et al. 2009; Zhao et al. 2011). One of the advantages solvent pretreatment has over other pretreatments is that it recovers lignin as a by-product. However, organic solvents are expensive and difficulties in solvent recovery make this technique costly and impracticable commercially (Procentese et al. 2017; Zhao et al. 2011).

14.5.1.4 Biological Pretreatments

Biological pretreatment offers a solution to the disadvantages that plague physical and chemical pretreatment methods such as their need for expensive equipment, chemicals, and high energy usage for biomaterial processing (Kumar et al. 2009; Xu et al. 2016). Biological pretreatments are generally carried out by growing microorganisms directly on feedstocks or by using the microorganism’s enzymes. Microorganisms are typically chosen to hydrolyze lignocellulosic biomass under common conditions since there is no need for specific equipment. Bacteria and fungi can both be used for this purpose. Rot fungi in particular are rich in lignin-degrading enzymes such as lignin peroxidase, laccases, and manganese peroxidase. They are considered the best candidate for biological pretreatment (Bak et al. 2009; Kirk and Moore 2007; Zhang et al. 2007). Although cellulase plays the most important role in biomass hydrolysis, there are other enzymes such as hemicellulase, ligninase, and pectinase (Binod et al. 2010). Accessory enzymes are also a crucial part of the biological pretreatment process. Important accessory enzymes involved in the hydrolysis of lignocellulosic biomass are α-arabinofuranidase, endoxylanases, exoxylanases, and β-xylosidases (Sindhu et al., 2016). Major challenges facing biological pretreatment in commercializing the use of lignocellulosic biomass in bioethanol production are the cost of hydrolyzing enzymes and such cocktails (Rodionova et al. 2017; Sassner et al. 2008).

Although the biological method is very energy efficient, it also has several disadvantages such as (1) it is extremely slow; (2) a significant amount of biomass is lost; and (3) much of the fermentable sugar available for bioethanol production is used by the microbes themselves for their own growth. Nevertheless, biological pretreatment is necessary because it brings about an increase in digestibility and fermentation rates (Steffen et al. 2000). When it comes to biofuel production this means it needs to be merged with other pretreatment technologies and that novel strains of microorganisms with rapid and effective hydrolysis capabilities and low growth rates need to be screened in order to make this method commercially viable.

14.5.1.5 Combined Pretreatments

No single pretreatment method to degrade lignocellulosic biomass provides suitable results on its own because of the influence of many factors such as lignin content, cellulose crystallinity, linkages between lignin and cellulose, and even intrinsic disadvantages. Studies show that incorporating two or more pretreatments from different categories, called the combined pretreatment method, can be more effective than single pretreatment processes. The combined pretreatment method can include a variety of combinations such as alkali and electron beam irradiation, alkali and ionic liquid, alkali and photocatalysis, biological and dilute acid, biological and steam explosion, dilute acid and microwave, dilute acid and steam explosion, enzyme hydrolysis and superfine grinding with steam, ionic liquid and ultrasonic, organosolvent and biological, SO2 and steam explosion, supercritical CO2 and steam explosion, microwave-assisted acid, and microwave-assisted alkali (Kumari and Singh 2018; Procentese et al. 2017).

14.5.1.6 Hydrolysis

Hydrolysis is a stage in which complex carbohydrate is degraded to monomeric sugars that are usually called fermentable sugars. Hydrolysis can lead to the complete breakdown of carbohydrates into simple monomeric sugars and ethanol or incomplete breakdown into oligosaccharides that require further hydrolysis before fermentation takes place (Alvira et al. 2010). Fermentable sugars produced during hydrolysis include mannose in softwood; xylose, arabinose, and galactose in hardwood and agricultural residues; glucose; and even fructose (Taherzadeh and Karimi 2008). Lignocellulosic biomass hydrolysis into fermentable sugars is generally carried out using either acids or enzymes.

Sulfuric acid (H2SO4) is the acid currently used for hydrolysis. However, a number of other acids have also been used for hydrolysis such as hydrochloric acid (HCl), nitric acid (HNO3), trifluoroacetic acid (TFA), and phosphoric acid (H3PO4). Hydrolysis using acids is carried out either as dilute acid (< 1%) or as concentrated acid (30–70%). High temperature and pressure that have a low reaction time and low temperature that has a high reaction time (up to several hours) are required for dilute acid and concentrated acid treatment, respectively (Gírio et al. 2010).

Carbohydrates (hemicellulose and cellulose) in lignocellulosic biomass can be converted into fermentable sugars by enzymatic hydrolysis. This can be done either using degrading enzymes produced by microorganisms during their growth in media or using commercial enzymes. Enzymatic hydrolysis of cellulose into fermentable sugars is carried out by cellulase enzymes that consist of different enzymes including endoglucanase, exoglucanase, cellobiohydrolase, β-glucosidase, acetylesterase, glucuronidase, xy-lanase, β-xylosidase, galactomannanase, and glucomannanase (Kour et al. 2019a; Nigam and Singh 2011; Yadav et al. 2016; Zabed et al. 2016). Many microorganisms are capable of cellulase production such as Clostridium, Bacillus, Cellulomonas, Ruminococcus, Bacteroides, Erwinia, Thermomonospora, Acetovibrio, Streptomyces, Microbispora, Sclerotiumrolfsii, Phanerochaete, Trichoderma, Schizophyllum, Aspergillus, and Penicillium (Alvira et al. 2010; Kour et al. 2019c; Yadav et al. 2018). Several factors influence the enzymatic hydrolysis of lignocellulosic biomass the most important of which are temperature, pH and mixing rate, substrate concentration, cellulase loading, surfactant addition, and even pretreatment approach (Rastegari et al. 2019b, c; Sarkar et al. 2012; Taherzadeh and Karimi 2008).

Low amounts of energy and moderate conditions are generally required for enzymatic hydrolysis making it advantageous over acid hydrolysis. Enzymatic hydrolysis is one of few methods available that are advantageous as a result of it being less toxic, very cost-effective, and not generating any inhibitory by-products.

14.5.1.7 Fermentation

Fermentation is the final step of lignocellulosic biomass alteration in which microorganisms convert six-carbon sugars such as glucose, galactose, and mannose into ethanol (Harun et al. 2010). Finding microbial strains available in sufficient numbers that have ideal traits such as holding the potential for broad substrate utilization, high ethanol generation capacity, ability to tolerate high ethanol concentration and heat, and resistance to inhibitors is the main limitation to bioethanol production on an industrial scale (Singh et al. 2010). Saccharomyces cerevisiae, Zymomonas mobilis, Pachysolen tannophilus, Escherichia coli, Candida shehatae, Pichia stipitis, Mucor indicus, and Candida brassicae are the most frequently used microorganisms in the fermentation process (Lee et al. 2011; Sarkar et al. 2012).

14.5.1.8 Product Separation

The separation of biofuel, especially ethanol recovery from fermentation broth, is traditionally conducted using distillation (alone or in combination with adsorption). Ethanol existing in the fermentation broth can be concentrated depending on membrane selectivity via hydrophobic pervaporation before transferring it to distillation thus reducing the energy load on distillation (Sushil et al. 2013). During distillation the fermentation broth is distilled by separating ethanol from water in order to reach an ethanol concentration above 95%. Lignin, unreacted cellulose, hemicellulose, ash, enzymes, living cells, and other components are leftovers of this process that remain in the waste water. All remaining components can be concentrated and used either to provide energy or to be transformed into other co-products (Sarkar et al. 2012). Although most types of lignocellulosic biomass result in the ethanol produced being highly concentrated, this leads to a couple of problems: (1) the concentrations of inhibitors such as acetic acid and furfural are increased thus suppressing the performance of yeast and enzymes; and (2) high viscosity leads to the fermentor consuming more power and to a decline in mixing and heat transfer efficiency (Georgieva et al. 2007).

14.6 Lignocellulosic Biomass: Sustainable Renewable Resource for the Future

Lignocellulosic biomass has been getting a lot of attention in recent years as a renewable resource for the economical production of biofuel in the near future. This is because such raw materials are widely available, cheap, and have a high carbohydrate content (Lund 2007; Rastegari et al. 2020; Yadav et al. 2020a). Despite all these advantages, there are a number of challenges that need to be overcome to utilize lignocellulosic biomass for fuel production: (1) ethanol production from lignocellulosic biomass is not commercially viable because of low yields that result from the production of less fermentable sugars from different biomasses and technical limitations; (2) the contents of lignocellulosic biomasses differ one from another and depend on the source and type of raw material; (3) incomplete fermentation of pentose and hexose sugars present in the hydrolysate; (4) technological barriers existing in this technology such as high viscosity, inhibitor production, reaction temperature, and sugar availability in hydrolysate; and (5) a number of other factors such as lignocellulosic biomass nature, pretreatment methods, enzyme type, enzyme source and amount, microorganisms used, process conditions and reactor type influence biofuel production (Hernández-Beltrán et al. 2019; Rodionova et al. 2017). Biofuel production, especially ethanol from lignocellulosic biomass, is considered so complex that its utilization on a commercial scale is limited. However, as a result of biofuel science developing and technology improving there are new approaches showing great promise for fuel generation in the future.

Although lignocellulosic biomass is abundant, it is largely unutilizable as a resource for biofuels. However, biotechnology and gene engineering should come up with smarter strategies to facilitate the production of renewable raw materials and secure the future of the biofuel industry worldwide. Lignocellulosic biomass is mainly derived from crop residues and from the cultivation of perennial energy crops. The challenges facing biotechnology at the moment are hence increasing crop yield sustainably and developing crops with a suitable set of chemical and physical traits for biofuel production. Plant growth can be improved by increasing photosynthesis. The most successful approaches to improving plant growth include: (1) transferring genes from photosynthetic bacteria into plants without affecting the activity of plant-specific genes (traditional breeding techniques here are unsuitable for the development of crops for biofuel production); (2) manipulating genes involved in the metabolism of nitrogen (an essential element in proteins) and DNA has been successful as shown by the overexpression of a glutamine synthesis gene (GS1) in plants; and (3) extending the growth phase of plants by reducing seed dormancy or delaying flowering such that plants appropriate much of their energy in vegetative growth (Welker et al. 2015).

Abiotic and biotic stresses are the main cause of crop loss worldwide (Yadav et al. 2020b). Accordingly, developing crops with higher resistance to stresses is the prime focus of crop yield improvement either by traditional breeding methods or by biotechnological approaches. BT cotton has been engineered (genetically modified) with an insecticidal gene from the soil bacterium Bacillus thurengiensis and represents a very successful example of the use of biotechnology to develop crops with higher resistance to stresses (Welker et al. 2015).

Furthermore, the switch to renewable biomass sources will require the development of energy crops with desirable chemical (especially) and physical traits. The biosynthesis of cellulose and lignin is co-regulated; hence reducing the proportion of lignin will also increase the proportion of cellulose. Moreover, using techniques that can alter the properties of the cell wall could be key to facilitating sugar accessibility in the fermentation process (Alalwan et al. 2019; Jorgensen 2011; Welker et al. 2015).

Consideration should also be given to a number of concerns about utilizing lignocellulosic biomass derived from crops for biofuel production: (1) socioeconomic concerns about field management and choice of biomass source that should be carefully considered to ensure biofuel production does not negatively impact food production or biodiversity; (2) environmental concerns about the effects of fertilizers and herbicides used during the production of energy crops, especially in terms of human health; and (3) concerns about the environmental impact the combustion of specific types of biofuels could have in terms of emissions (Hernández-Beltrán et al. 2019).

14.7 Conclusion and Future Prospects

Lignocellulosic biomass has a lot of potential to be converted into biofuels such as bioethanol, biodiesel, and biogas. It has shown itself to be a good candidate to provide a solution to the world energy crisis in an ecofriendly manner, especially since the biofuels it produces are widely available as indigenous resources. Although extensive research has been carried out into the development of production technologies for biofuels from lignocellulosic biomass worldwide, there are still many limitations in such technologies to be overcome for future biofuel production to be commercially viable. Moreover, understanding the effects of unexpected events such as climate change on biofuel production from lignocellulosic biomass and its management is vital to the availability of sustainable biofuels in the future.