Advances in Biofuels Production, Optimization and Applications

Advances in Biofuels Production, Optimization and Applications discusses the optimization of chemical, biochemical, thermochemical and hydrothermal processes for biofuels. With a strong focus on applications, the book bridges the gap between technological developments and prospects of commercialization. Initial chapters review efficient hydrolysis and biofuel and bio-alcohol production before reviewing key processes such as biomass gasification, syngas conversion to biofuel, and pyrolysis techniques. Several biofuel applications are presented, including those within the transport industry as well as domestic and industrial boilers.

The book then finishes with a review of the circular economy, biofuel policies and ethical considerations. This will act as a systematic reference on the range of biomass conversion processes and technologies in biofuels production. It is an essential read for students, researchers and engineers interested in renewable energy, biotechnology, biofuels production and chemical engineering.

• Provides recent advances in the processes and technologies currently used for biofuel production
• Addresses the technology transfer of integrated biofuel upgrading and production at large scale
• Highlights policy and economics of biofuel production, biofuel value chains, and how to accomplish cost-competitive results and sustainable development
• Examines recent development in engines and boiler technologies for the eco-friendly applications of these biofuels in the industry and transport sectors

• Language: English
• Publisher: Elsevier Science
• Release date: Sep 8, 2023
• ISBN: 9780323950770

Book preview

Chapter 1: Lignocellulose biomass pretreatment for efficient hydrolysis and biofuel production

Chukwudi O. Onwosia,b,c; Flora N. Ezugworiea,c,d; Chioma L. Onyishia,c; Victor C. Igbokwec,e    a Department of Microbiology, Faculty of Biological Sciences, University of Nigeria, Nsukka, Enugu State, Nigeria

b Karen M. Swindler Department of Chemical and Biological Engineering, South Dakota School of Mines and Technology, Rapid City, South Dakota, United States

c Bioconversion and Renewable Energy Research Unit, University of Nigeria, Nsukka, Enugu State, Nigeria

d Department of Applied Sciences, Faculty of Pure and Applied Sciences, Federal College of Dental Technology and Therapy, Enugu, Enugu State, Nigeria

e Bio-Inspired Materials, Universite de Pau et des Pays de l’Adour, Pau, France

Abstract

Global interest has shifted to environmentally friendly and renewable biofuel due to the eco-related impacts of fossil-based energy. Despite the giant strides taken globally to make biofuel more competitive in the energy market, the recalcitrant nature of the lignocellulosic biomass (LCB) required for biofuel production has become a mounting task to surmount. Different pretreatment protocols are currently deployed to deconstruct the complex LCB toward making it more accessible during enzymatic hydrolysis to fermentable sugars. The various pretreatment steps—generally categorized as physical, chemical, physicochemical, and biological pretreatments and currently used in LCB-based biofuel production—are discussed in this chapter. The strengths, weaknesses, and opportunities of the techniques are highlighted herein. Emerging pretreatment protocols and integrated approaches that hold promise in biofuel production are also discussed.

Keywords

Renewable energy; Biofuel; Lignocellulosic biomass; Pretreatment; Enzymatic hydrolysis; Bioethanol

1: Introduction

The environmental and public health challenges associated with fossil-based fuels have repositioned global interest in the last decades toward developing alternative bio-based fuels (e.g., bioethanol, biobutanol, biomethane, biodiesel) that are eco-compatible and sustainable [1–6]. Biofuel serves as a major fuel for vehicles or in blending fossil-based gasoline in different countries including Brazil, India, the United States of America, etc. [2,7]. Despite holding huge promises in emerging and industrialized economies, biofuel has commanded a minimal share of the energy market [8]. This point could be attributed to the fact that transitioning from fossil-based to bio-based fuel resources has faced a daunting hurdle, especially, given that the production of the latter relies on recalcitrant lignocellulosic biomass (LCB) for its sustainability.

LCB is considered an eco-compatible, carbon-neutral, and renewable bioresource for the sustainable generation of biofuels [2,4,9–11]. According to Saini and Sharma [6], the LCB comprises a long tuft of cellulose (30%–60%) and hemicellulose (20%–40%) bound with complex lignin blocks (15%–24%). However, these structural components of LCB not only vary among different plant species but also with similar species as a result of environmental conditions or geographical location [6]. The stratified and multidimensional orientation of the lignocellulose cell wall hinders the efficient hydrolysis of the LCB by limiting the mass transfer of various catalysts for the conversion of LCB to biofuels and related products [6,11].

LCB-based biofuels production chain (Fig. 1) comprises the pretreatment, removal of toxic byproducts, saccharification (enzymatic hydrolysis), fermentation, and product recovery (downstream processing). Although currently at an immature stage, pretreatment of LCB is the initial and considered the most determining factor [12]. To utilize LCB for biofuel production, a pretreatment step is usually undertaken to disintegrate the packed structure of LCB thereby alleviating its recalcitrance [6,10,12–15]. Pretreatment, generally, paves the way for cellulolytic enzymes during the saccharification stage (enzyme hydrolysis) before the hydrolyzed sugars are converted to biofuel via fermentation [16]. However, LCB-based biofuel production has been hindered by the lack of efficiency of the current pretreatment protocols as well as poor process economics [4,17]. The efficiency of a pretreatment protocol depends on the quality of the recovered fermentable sugar, the amount of wastewater generated, eco-friendliness, scalability, energy cost, and the level of toxic byproducts [12,13]. A broad spectrum of traditional pretreatment protocols (e.g., acid, ammonia fiber explosion, steam explosion, etc.) has been widely reported for effective hydrolysis of LCB. The available report also shows that the current pretreatment procedures effectively disrupt the LCB cell wall but yield low sugar, consume high energy, yield a high level of inhibitory compounds, and generate a high volume of wastewater [12]. Newly developed pretreatment protocols including deep eutectic liquid, supercritical liquid, and ionic liquids are emerging but are still in the laboratory stage [12]. Due to low efficiency, the pretreatment step incurs huge costs limiting the maximal transformation of complex LCB into functional biofuel at the commercial scale [2,4]. It has been estimated that the pretreatment step constitutes more than 40% of the overall processing cost in LCB-based biorefinery [13]. Fine-tuning the existing pretreatment protocols has been affirmed as a vital step toward unlocking the present stagnation of commercialization of LCB-based biorefinery [12,18]. Commercial biofuel production from LCB requires the adoption of specific and innovative technologies during the pretreatment steps. This could be realized through a foundational understanding and merging of various pretreatment processes [13].

Fig. 1

Fig. 1 LCB-based biorefinery processing chain.

Considering these variables, the current chapter explores recent advances in the pretreatment of LCB toward biofuel production. Emphasis was made on the merits, and demerits of current and emerging protocols, as well as their overall impacts on biofuel production.

2: Composition of lignocellulosic biomass

LCB had provided different sources of fuel to man and has been generally seen as the future carbon-neutral bioresource [9]. The major source of LCB is not limited to agro-industrial waste but includes municipal solid wastes, grass, animal manure, forest products, paper mill sludge, algal biomass, and bioenergy crops [9,19,20]. Although LCB is a sustainable replacement for fossil carbon sources, it is also a complex legion of polymers, strongly intertwined and recalcitrant to enzymatic hydrolysis by default. However, understanding the chemical components and structures of the LCB will enhance the effectiveness of its conversion to biofuel and other bioproducts [21]. As shown in Fig. 2, the major components of the LCB cell wall include cellulose (35%–50%), hemicellulose (20%–35%), and lignin (10%–25%). The remaining fraction is made up of ash, oil, and protein. The species, tissues, and maturity of plants determine the structure and the amount of these cell wall constituents [22].

Fig. 2

Fig. 2 Structure of lignocellulosic biomass and its biopolymers: cellulose, hemicellulose, and lignin. G, H, and S correspond to guaiacyl, p -hydroxyphenyl, and syringyl monomeric units in lignin. Reproduced from F.H. Isikgor, C.R. Becer, Lignocellulosic biomass: a sustainable platform for the production of bio-based chemicals and polymers. Polym. Chem. 6 (25) (2015), 4497–4559 with permission from the Royal Society of Chemistry.

2.1: Cellulose

Among the LCB components, cellulose (C6H10O5)n is the most abundant (30%–50%), containing linear glucan chains comprising glucose units (>10,000), which are held by β-(1→4)-glycosidic linkage [13,23,24]. Whereas in other related glucan polymers, the cellulose chain comprises repeating units of cellobiose-disaccharide [22,23,25,26]. By combining with hydroxyl groups, these cellobiose units transform at different degrees of polymerization into microfibrils with crystalline features due to the considerable network of intramolecular hydrogen bonds and intermolecular van der Waals forces [23,24,27–30] These cellulose fibrils hinder enzymatic hydrolysis [30]. The extent of polymerization modulates the crystallinity (i.e., high stiffness) and porosity of cellulose. Thus, cellulose with long chains possesses stronger hydrogen bonds, becoming resistant to hydrolysis, while short chains possess weaker hydrogen bonds facilitating enzymatic hydrolysis [31,32]. This β-(1→4)-glucosidic linkage is responsible for the arrangement of the crystalline array and is different from that of hemicellulose [33].

2.2: Hemicellulose

Hemicelluloses (HC) (C5H8O4)m, constituting about 15%–35% of the LCB, are heteropolymers, rarely flexible, and made up of sugar compounds (galactose, arabinose, xylose, and fructose) connected by many monomer linkages [13,22,34]. HC has a combination of both branched and linear polysaccharides with several varieties among plant tissues and species, and its monosaccharide constituents serve as a carbon source for several microbial fermentation processes. They are easily pretreated and removed by steam explosion or dilute acid, and this action enhances and facilitates the enzymatic hydrolysis of cellulose [30,35]. Although HC can be effectively acetylated by the acetyl group, this action will engender changes in the cellulose hydrophobicity and diameter, so cellulase would be unable to bind to cellulose. According to its backbone, HC is divided into classes (xylan, mannans, galactans, noncellulose glucans, and xyloglucans). Xylan is the most abundant hemicellulose, with glucuronic acid, xylosyl, and arabinosyl as substituents and a β-(1→4)-linked xylosyl unit as a backbone [21]. Phenolic compounds, ferulic acids, and p-coumaric acid are possible side chains, which affect hydrogen bonding and the solubility of HC in polar solvents or water. Mannans, abundant in softwood species, have (1–4)-linked mannosyl units as their backbone combined with galactosyl and o-acetyl groups. Galactans have either β-(1→3) or β-(1→4) galactosyl units, but because β-(1→4) galactosyl linkage is abundant and has a planar structure that enhances hydrogen bonding, it is considered HC. Non-cellulosic glucans exist as monopolymers or mixed-linkage polymers. β-(1→4) glucosyl linkage as the monopolymer and interspersed β-(1→3) linkage with β-(1→4) glucosyl linkage as a mixed-linked polymer. The best for biofuel production is the xyloglucan, which has β-(1→4) linkage combined with substituents and branched patterns, is easily fermentable, and is abundant in vegetative tissues. Although HC can be abundant in a particular vegetative species at a particular stage in its development or growth, it can be absent, decrease in abundance, or increase in abundance in other growth stages of that particular vegetative species. For instance, early pine wood has more glucomannans which easily adsorbs cellulose, while late pine wood has more glucuronoarabinoxylan which has less adsorption capacity for cellulose. The type of linkage present in the cell wall determines the reaction of the biomass to pretreatment processes. Mild acid pretreatment leads to complete depolymerization of glycosidic linkage in HC, but the resultant arabinose and xylose sugars undergo dehydrogenation in the presence of dilute acid. Thus, furaldehydes, which are toxic to the fermentative microbes, are formed. Furthermore, there is an issue of diauxic growth when microbial cells are presented with two carbon sources. Other acids released during acid pretreatment include acetic, coumaric, uronic, and ferulic acids. On the contrary, oligomers of HC are solubilized and ester linkage hydrolyzed with little effect on glycosidic linkage under dilute alkaline pretreatment. HC is completely saccharified under alkaline pretreatment, but due to the multiple linkages embedded in HC, more than one enzyme is required to completely saccharify HC. Although genetic modifications are made in plants for better results, these modifications weaken the stems of the plants.

2.3: Lignin

Lignin [C9H10O3(OCH3)0.9–1.7]x with a relative abundance of 10%–25% exists as an irregular (three-dimensional) polymer of phenylpropanoid, commonly found in higher plants and can be referred to as natural plastic [13,22]. Lignin is very persistent and exists in the environment for a very long period. Lignin causes the rigidity and hydrophobicity of biomass, binding cellulose to HC and absorbing cellulase and other enzymes during enzymatic hydrolysis due to its hydrophobic nature [30]. The lignin structure is formed via ester (oxidative) linkage of three unique phenylpropane components comprising coniferyl alcohol, monolignols: p-coumaryl alcohol, and sinapyl alcohol corresponding to guaiacyl, p-hydroxyphenyl, and syringyl monomeric units [22]. The abundance of lignin differs among species of higher plants, grasses (guaiacyl, syringyl, and hydroxybenzyl units), softwoods (more guaiacyl units), and hardwoods (more syringyl units). Syringyl units have methoxyl substituents at positions C3 and C5 of the phenyl ring, thus forming a few carbon-carbon bonds that are recalcitrant. This promotes the formation of more linear chains with β–β resin structure. On the other hand, guaiacyl units are more branched and hinder fiber swelling during the pretreatment process due to the presence of methoxyl subunit only on the C3 of the phenyl ring. Hydroxylphenyl units with no methoxyl subunit (hence, more carbon-carbon bonds), reduce the formation of β-O-4′ aryl ether bonds and increase the formation of low molecular weight polymers by tilting toward the stability of radicals generated during oxidation [21,36]. There is a need for its delignification for effective and efficient enzymatic hydrolysis and biofuel production to occur. Phenolic hydroxyl groups also hinder cellulose hydrolysis, but this action can be attenuated by the drastic reduction of this compound by hydroxypropylation [13,37,38]. Due to the infiltration of monomer units into the polysaccharide matrix in plant development and their polymerization by free radicals, randomized structures (lignin) that protect cellulose fibers from depolymerization are formed, thus hindering saccharification of biomass during biofuel production. Biomass pretreatments are introduced during biofuel production to eliminate, transform, or reduce lignin from LCB during the process. It requires extreme conditions for alkaline, acid, heat, solvent, and oxidative agent pretreatment for its removal during biofuel production. To achieve this task, some factors have to be properly understood; constituents, polymerization level, and the nature of lignin. However, the nature of lignin might be influenced by the cell wall condition and the composition of both lignin and carbohydrate during the polymerization process [21]. Most of the covalent bonds such as ester, β-aryl ether, and carbon-carbon bonds are cleaved by the pretreatment process. β-O-4′ aryl ethers are more abundant in grasses than in woods and are easily cleaved even by dilute acids, while ester linkages abundant in grasses are cleaved under alkaline pretreatment [27]. The strongest linkage is the carbon-carbon bonds, which are cleaved by oxidative catalysts.

3: Overview of pretreatment techniques

Pretreatment processes are the techniques used to enhance the potential of LCB, which are optimized before biofuel production (Table 1). The main aim of biomass pretreatment is to open up the biomass structure and enhance the accessibility of enzymes into the complex structure of LCB, breaking hydrogen bonds of the cellulose crystal, increasing the surface area of cellulose, and for easy cleavage into smaller and simpler monomers during hydrolysis [50]. This can be done before or during hydrolysis [2]. The selection of a pretreatment technique for a particular feedstock is dependent on the enzyme utilized for the hydrolysis of cellulose and the sugar-release pattern [50]. Many pretreatment methods (physical, chemical, physicochemical, and biological) are applied before the production of biofuel with LCB (Fig. 3).

Table 1

Fig. 3

Fig. 3 Different pretreatment processes on lignocellulosic biomass.

The method of pretreatment used directly affects the enzymatic processes involved by affecting the fermentative microbial species involved during biofuel production [51]. For instance, the pretreatment method used for the production of biohydrogen may not be suitable for the production of bioethanol; hence, the type of biofuel to be produced determines the pretreatment method to be applied [2].

3.1: Physical pretreatment process

Physical pretreatment processes for LCB include milling, extrusion, microwave, and freezing. In all these processes, the main aim is to enhance LCB enzyme digestion by limiting the level of polymerization and crystallinity, enlarging LCB pore sizes and accessibility, and decreasing LCB waste residue. The milling process reduces the particle size and the crystal structure of cellulose. Some milling techniques carried out mechanically are expensive and also consume more energy and time while others are cheap but labor-intensive. The various milling techniques used include colloid milling, agitated bed milling, hammer milling, disk milling, ball milling, vibratory ball milling, compression milling, and two-roll milling. The particle size of LCB depends on the milling technique used, but generally, the particle size after milling is usually within the range of 0.2–2 mm [2]. Extrusion involves shearing, mixing, and heating of LCB to engender both chemical and physical change, which enhances enzymatic activities, reduces fiber, and improves depolymerization. The extrusion process can be improved by the addition of enzymes and chemicals and the use of ultrasonic waves [52]. Freezing and thawing involve allowing the biomass to suck water and freezing it. A large amount of water taken up by the biomass is then converted to solid thus cleaving the cellulose structure and enhancing enzymatic hydrolysis. Microwave pretreatment changes the structure of the biomass by using an electromagnetic field. Heat is generated without contact causing a drastic change in the polar, physical, and chemical structure of the biomass. The process is highly efficient with reduced byproducts, operational time, and resources [2].

3.2: Chemical pretreatment process

Due to the limitations in physical pretreatment methods, chemical biomass pretreatment methods were adopted to enhance the efficiency of biomass during biofuel production by eliminating lignin from the LCB and enhancing enzymatic hydrolysis. It can also be used for chemical hydrolysis, thus cleaving the complex structure of LCB into smaller sugar monomers [2]. These chemical methods include ionic liquid, acidic, carbon dioxide, oxidative, and alkaline pretreatments [53]. Chemical pretreatments are usually done during the hydrolysis process, and for effective pretreatment, certain factors should be considered, such as the type, price, and capacity of the chemical, type of bioreactor, byproducts formed, level of energy demand, type and structure of biomass, and efficiency and tolerant level of fermentative microorganisms employed [53–56]. These factors are very vital as the same quality or quantity of end-product might not be achieved using the same pretreatment techniques and chemicals on different LCBs. Due to this fact, different chemical pretreatment techniques will be discussed bearing in mind their effects on biomass and fermentative microorganisms.

3.2.1: Oxidative pretreatment

In this process, chemical agents like peroxide, NaOH/urea, N-methylmorpholine N-oxide, cadmium ethylenediamine solvent, LiCi/N, N-dimethylacetamide, carboxyl-methylcellulose, and peracetic acid are used to separate hemicellulose from cellulose and generate crystalline cellulose by dissolving lignin. This can be carried out through certain reaction processes such as alkyl aryl cleavage, electrophilic substitution, and side chain replacement. Among all the oxidative agents used for biomass pretreatment, many researchers have reported hydrogen peroxide as the most effective as it enhances enzymatic hydrolysis, increases biofuel yield, and causes partial cleavage on lignin with no effect on cellulose. Limitations of this process include the cleavage of the intermolecular bond linking two glucose monomers (between OH-2 and O-6) and the β-O-4 dissociation rate [53,57]. Ozonolysis is another oxidative pretreatment process that is mostly seen in hydrogen and methane (biogas) production. Ozonolysis is carried out before enzymatic hydrolysis. It increases biogas yield and reduces the generation of toxic byproducts but due to the large volume of ozone required, the process is expensive [53,58]. Generally, the oxidative pretreatment process is expensive and inefficient with the saccharification process, which makes it less preferable than the acidic or alkaline pretreatment process.

3.2.2: Alkaline pretreatment

In this process, lignin, xylan, and hemicellulose are extracted from cellulose, thus enhancing the stability of the cellulose thermodynamics, increasing porosity, hydrolyzing hemicellulose, and reducing the density and crystallinity. This method can be used to treat biomass such as sugar cane, Ipomea carnea, cattail waste, cotton stalk, corn stover, etc. Alkaline pretreatment helps to increase the surface area of hemicellulose and cellulose fibers and eliminate lignin. It is highly efficient in breaking hemicellulose bonds such as uronic and acetyl groups [2]. The alkalines used include lime, different percentages of NaOH, a mixture of oxidant and alkaline (NaOH/urea/thiourea), ammonia, KOH, etc. The limitation of this process includes the formation of salts, prolonged hydrolysis, and high operational cost [53,59]. The alkaline pretreatment method can be combined with physical methods such as hydrothermal treatment.

3.2.3: Acid pretreatment

The utilization of acid in the hydrolysis of LCB effectively yields structural orientation appropriate for conversion during fermentation [60]. An essential operational variable during acid pretreatment is the reaction temperature. Acid hydrolysis could be accomplished either at high temperatures with dilute acids or at low temperatures with concentrated acids. While the latter process effectively hydrolyzes LCB, it is an energy-demanding process coupled with costly used operational materials. Thus, diluted acid is generally more suitable and has become one among the preferred chemical treatment options during biofuel production [60].

Cellulose structure and orientations are also modified by dilute acid and the hydrolytic process occurs at a temperature range of 180–200°C [60,61]. Świątek et al. [61] reported that hemicellulose and cellulose fractions are completely hydrolyzed at 180 and 200°C, respectively, when LCB was subjected to a diluted sulfuric acid pretreatment for 2 h. Apart from temperature, another important factor during acid hydrolysis is the retention time. Diluted acid pretreatment could be performed either at lower temperatures and long retention time or at high temperatures and short retention time. Weak acids such as acetic acids could be utilized for acid hydrolysis. In particular, acetic acid-mediated hydrolysis of LCB is marked by the formation of hydronium ions (H3O+) that accelerates the breakdown of polysaccharides [60]. Acid pretreatment completely solubilizes cellulose, decomposes hemicellulose to glucomannan or xylan, and achieves about 80% lignin solubilization [62].

However, the combination of acid pretreatment with heat (high temperature) or enzymes goes a long way to achieve the desired result, especially with dilute acid. The use of carbon dioxide as acid pretreatment at 200°C in the hydrolysis of hemicellulose under acidic conditions can form carbonic acids, which enhances enzymatic hydrolysis. This can be observed with biomass such as coconut with HCl acid pretreatment. Adverse effects of acid pretreatment include increased formation of toxic byproducts, high cost of acid, and corrosion, which is mainly created by concentrated strong acid [60]. It also involves many steps, thus increasing operational costs and time. Despite these limitations, it is still widely used for several biofuel products due to its high effectiveness.

3.2.4: Ionic liquid pretreatment

In this method, cellulose is separated from lignocellulose by ionic solvents but a further separation technique is required to separate the ionic liquid from cellulose. These ionic solvents made up of anionic and cationic substances are easily modified by substitution of the vital functional groups. Its efficiency is higher than chemical pretreatment and it is eco-friendly [2]. Some of these solvents include 3-allyl-1-methylimidazolium chloride ([AMIM][Cl]), 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]), 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM][HSO4]), protic acid, 1-butyl-3-ethylimidazolium chloride ([BEIM][Cl]), 1-butyl-3-methylimidazolium hydroxide ([BMIM][HO]), and 1-ethyl-3-methylimidazolium acetate ([EMIM][AC]). Some of these ionic solvents have high capacity (up to 75%) in solubilizing cellulose effectively within a temperature range of 50–100°C but some require additional boost (microwave radiation or acids, etc.), to attain above 25% solubility. Some can also attain higher solubility with an increased temperature of above 100°C. Apart from stable temperature, other advantages include easy recovery of both product and ionic solvent, reduced vapor pressure, and enhances microbial and enzymatic activities thus improving biofuel production and yield, and it serves more operation cost due to its non-corrosive nature, which preserves the equipment more than other methods [53,63]. For complete cleavage and solubilization of complex structures such as lignin, a combination of ionic solvents can be used. However, this process comes with many limitations such as pH compatibility of different ionic solvents, toxicity, and complexity. Deep eutectic solvents (DESs) considered green solvents are suitable alternatives to ionic solvents due to their rapid biodegradation rate, less toxicity, and cost-effectiveness. DESs could selectively solubilize lignin (>90%) with high saccharification yields under mild temperature and pressure without disrupting cellulose and hemicellulose [64]. Despite this important advantage, the viscous nature of DESs hampers their effective utilization in biomass processing [65]. Other ionic pretreatment methods used for the removal of hemicellulose and lignin include co-solvent enhanced lignocellulosic fractionation (CELF), low temperature steep delignification (LTSD), and supercritical fluids (SCFs). These ionic pretreatments increase glucose yield with reduced cost and waste generation [66]. For selective hemicellulose and lignin degradation, increased polymer purification, and successful solvent recovery, organosolv pretreatment is employed. This process involves the use of organic acids, polyhydric alcohol, alkylene carbonates, short-chain aliphatic alcohols, and ionic solvents for selective lignocellulosic fractionation with high efficiency of recycling and recovery of solvents [67].

3.3: Physicochemical pretreatment

This is a combination of physical and chemical pretreatment methods for effective lignocellulose degradation. It helps rupture and open up biomass fiber. Examples of this pretreatment method include ammonia fiber explosion (AFEX), carbon dioxide explosives, steam explosion, and liquid hot water (LHW) [2]. The steam explosion pretreatment process is a hydrothermal process involving the use of high-pressure steam on LCB for a short time. Onto releasing the pressure, the fibers of the LCB appear ruptured and split without the use of a chemical. The benefits of this process are reduced contact time and energy consumption while the limitations include inefficiency in the degradation of xylan to hemicellulose and lignin removal, and formation of inhibitors [2]. To improve the removal of lignin and the degradation process, LHW was employed. In this process, water is heated at the temperature range of 170–230°C with a pressure of about 5 MPa and used on the LCB. In the AFEX pretreatment process, there is an increase in the surface area and body mass of the biomass due to the high temperature and pressure of about 2.7 MPa applied to the biomass. This process alters the structural features of lignin and converts hemicellulose to smaller sugar monomers. However, some adverse effects like corrosion and low efficiency are associated with the AFEX pretreatment process. The best physicochemical pretreatment process is the carbon dioxide explosive process because it serves as an environmental purification process by utilizing greenhouse gas compound (CO2) alongside steam at high pressure (1071 psi) and a critical temperature of 31°C with zero vapor release [68].

3.4: Biological pretreatment

This pretreatment technique includes the use of microorganisms such as bacteria and fungi to degrade LCB. This process is eco-friendly and relatively cheap with less energy consumption. The same species of microorganism or a combination of different species of microorganisms can be used for this process. However, a microbial consortium containing bacteria and fungi species (Aspergillus niger, Pseudomonas aeruginosa, Streptomyces badius, Coriolus versicolor, Gloeophyllum trabeum, Bacillus sp., Thermomonospora sp., Thermobifida fusca, Phanerochaete chrysosporium, and Trichoderma viride) can be more effective during the biological pretreatment process. In choosing microorganisms for this process, certain factors should be considered; first, the ability of the organism to produce the required enzymes (xylanase, ligninase, hemicellulase, and cellulase) for the degradation of xylan, lignin, hemicellulose, and cellulose [2]. The second factor is the ability of the organisms to withstand harsh environmental conditions (temperature, pH, etc.) during the pretreatment processes. Benefits of this process include an increase in the surface area of the biomass and the production of lignocellulolytic enzymes. The disadvantages of the biological pretreatment process are prolonged operational time, low hydrolytic rates, low utilization of the sugars by microorganisms, and lack of space for the pretreatment process [39,50]. Bioprospecting of novel lignocellulolytic enzyme cocktails from fungi and extremophiles could offer a huge advantage toward making biological pretreatment more economical and effective.

4: Byproducts of lignocellulose pretreatment

The main purpose of lignocellulose biomass pretreatment is to reduce the crystalline nature of the holocellulose to make available the sugar components for biodegradation. However, certain side reactions occur, leading to the simultaneous production of some byproducts during this process [69,70]. These byproducts are known to be inhibitory to the fermentation process as their build-up leads to significantly lower productivity of the fermenting microbes and, ultimately, decreased biofuel yields [69]. The nature and degree of byproducts formed are usually a function of the pretreatment method and environmental conditions [71]. Van der Pol et al. [70] also noted that the structural composition of the lignocellulose can affect byproduct formation.

4.1: Categories of byproducts

Different authors have categorized the inhibitory byproducts originating from the pretreatment of LCB on a different basis. van der Pol [72] grouped the byproducts as furans, phenols, and organic acids. Jönsson and Martín [71] described the byproducts formed as furans, organic acids, and phenylic components that include phenolic compounds and non-phenolic aromatics. The byproducts were classified by Basak et al. [69] as short-chain carboxylates, furan derivatives, and phenolic compounds. According to Bhatia et al. [66], furan aldehydes, phenolics, and weak acids, alongside fermentable sugars, are the various byproducts of hydrolysate pretreatment. Most of these categorizations are based on the chemical functions, origins, and effects of these byproducts on fermenting microorganisms [69].

In the following, the byproducts are discussed as categorized in the literature [69,73].

4.1.1: Furan derivatives

The exposure of monomeric sugars to high temperatures in an acidic environment leads to the formation of furan derivatives [70]. These acidic conditions are essential in certain pretreatment processes such as acid hydrolysis, the use of ionic liquid, and sulfite pulping. While pentoses undergo dehydration in an acidic environment to form furfural, a very common byproduct of lignocellulose pretreatment, 5-hydroxymethyl-2-furaldehyde (5-HMF), another economically important byproduct, is formed from the dehydration of hexose sugars [71]. Both furfural and 5-HMF are unstable, thus, can be degraded to other byproducts under higher temperatures [74], prolonged reaction time, and acid concentration [71]. For example, furfural is degraded to formic acid while 5-HMF is degraded to formic and levulinic acids. Kabel et al. [75] also noted that furoic acid is another byproduct of monomeric sugar degradation.

4.1.2: Aliphatic acids

The aliphatic acids include acetic acid, formic acid, and levulinic acid. As discussed earlier, formic and levulinic acids are products of extensive monosaccharide degradation [71,74]. On the other hand, acetic acid results from the hydrolysis of acetyl groups [73]. Acetyl groups are components of hemicellulose.

4.1.3: Aromatic compounds

Various phenolic and non-phenolic aromatic compounds are produced as byproducts during lignocellulose pretreatment depending on the type of biomass and operating conditions [76]. These are chemical species formed as a result of the degradation of lignin [73]. During acid pretreatment, the β-O-4 ether and other acid labile linkages in the lignin molecule could split open, leading to the formation of phenolic compounds [71]. The most common phenolic byproducts of lignin degradation are vanillin, syringaldehyde, syringic acid, 4-hydroxybenzoic acid, 4-hydroxybenzaldehyde, p-coumaric acid, and ferulic acid [76,77]. Non-phenolic aromatic compounds include benzoic acid, benzyl alcohol, cinnamaldehyde, cinnamic acid, and para- and ortho-toluic acid [76,78].

4.1.4: Metal ions and cations

An adverse condition such as the release of heavy metals (lead, iron, copper, nickel, etc.) in the already pretreated biomass occurs during acidic pretreatment [71]. Similarly, cationic elements such as magnesium, calcium, and sodium that can be toxic to fermentative microorganisms are released during chemical pretreatment, thus hindering fermentation processes.

4.2: Impact of byproducts on the hydrolysis step

Lignocellulose pretreatment byproducts can be inhibitory to fermenting microorganisms, enzyme activities, and the general fermentation process [70]. The presence of lignocellulose degradation byproducts has been confirmed to inhibit enzymes and, consequently, impede the hydrolysis process [79]. In a study by Mathibe et al. [80], the effect of some pretreatment-derived phenolics on xylanase was studied using Thermomyces lanuginosus VAPS-24 xylanase, XynA, whose structure is similar to that of other GH11 xylanases. The study indicated that the phenolics showed no competitive edge over each other while inhibiting the enzyme. The carbonyl groups, however, had a higher inhibitory effect on the enzyme than the hydroxyl groups of the phenolics. A total of eight phenolic compounds were used in the study at concentrations of 1–2 mg/mL with gallic acid and vanillic acid being the most inhibitory products.

Furfural is tagged as the most inhibiting byproduct of lignocellulose pretreatment. This could be because of its ability to diffuse easily into microbial cells as a result of its low molecular weight [81]. However, at low concentrations, furfural can be converted to less toxic compounds. Meanwhile, when it is present with 5-HMF, the rate of conversion is reduced for both, with 5-HMF only degraded after the complete conversion of furfural [82,83].

Studies show that formic acid and levulinic acid are formed at the expense of sugars [71]. Therefore, if a particular type of pretreatment is used to maximize the formation of these acids, sugar hydrolysis and biomass formation are drastically affected. For instance, Singh et al. [84] produced xylose hydrolysate (16%) by pretreating spent aromatic biomass with p-cymene-2-sulfonic acid (p-CSA), a Brønsted acid formed from d-limonene from citrus waste, under autoclaving conditions (Fig. 4A). However, the authors noted that further processing of the pretreated biomass with p-CSA in the presence of aqueous HCl under refluxing at 180°C resulted in selective production of levulinic acid (∼22% yield) from the cellulosic biomass (Fig. 4B).

Fig. 4

Fig. 4 Comparative fermentable sugar yield (A) and cellulose degradation products (B) from different spent aromatic biomass through pretreatment with p -CSA. Reproduced from M. Singh, N. Pandey, P. Dwivedi, V. Kumar, B.B. Mishra, Production of xylose, levulinic acid, and lignin from spent aromatic biomass with a recyclable Brønsted acid synthesized from d-limonene as renewable feedstock from citrus waste. Bioresour. Technol. 293 (2019) 122105 with kind permission from Elsevier.

Dutta et al. [85] also demonstrated that various paper wastes could be effectively processed toward a selective synthesis of desired products using an H2SO4-aided catalytic system. They hinted that H2SO4 accelerated the hydrolysis of LCB (e.g., paper wastes) to various C5 and C6 sugars that subsequently undergo dehydration to furfural, 5-HMF, and levulinic acid (Fig. 5). However, given that there is sufficient acidity in the reactant solution, the HMF could be rehydrated to levulinic acid over the Brønsted acidic sites. This made levulinic acid the primary product in the initial phase of the proceeding with a maximum yield of 22–29 mol%. However, replacing H2SO4 with FeCl3 (0.25 M, 5 min, 180°C) catalyst system led to the selective formation of 5-HMF (11.7–14.8 mol%) from the paper wastes. This outcome strongly suggested that metal chlorides promoted the production of HMF.

Fig. 5

Fig. 5 Product yields from H 2 SO 4 -aided catalytic conversion of various paper waste substrates: (A) H 2 SO 4 (0.25 M) at 180°C; (B) H 2 SO 4 (0.5 M) at 160°C; and (C) H 2 SO 4 (0.5 M) at 180°C. Reproduced from S. Dutta, Q. Zhang, Y. Cao, C. Wu, K. Moustakas, S. Zhang, K.H. Wong, D.C. Tsang, Catalytic valorisation of various paper wastes into levulinic acid, hydroxymethylfurfural, and furfural: influence of feedstock properties and ferric chloride. Bioresour. Technol. 357 (2022) 127376 with permission from Elsevier.

4.3: Potential use of byproducts as platform chemicals in other bioprocesses

The use of polymeric materials has always been important in industrial production processes, but its dependence on fossil resources has begun to jeopardize the future of the polymer industry [86]. Lignocellulose biomass appears to be one of the most promising alternative solutions to produce sustainable polymers [87,88]. The pretreatment of lignocellulose biomass has led to the production of over 200 value-added byproducts essential to producing polymers (Fig. 6) and using them in other biobased processes [22,89].

Fig. 6

Fig. 6 Byproducts from pretreatment processes used as platform chemicals for the production of other goods.

Furfural, also known as furan-2-aldehyde was initially isolated by Döbereiner in 1832 and has been produced since 1922 for industrial use [90]. As a renewable platform chemical, furfural has shown multiple purposes for use as a solvent, selective extraction agent, flavoring agent, agent of vulcanization, component of nematicides, pesticides, fungicides, herbicides, antiseptics, and rust removers. It has also found use in the production of cosmetics, resins, fragrances, and pharmaceutical products [90]. A US Department of Energy report included furfural as one of the top 30 added-value chemicals from biomass [91]. Furfuryl alcohol, furan, tetrahydrofurfuryl alcohol, tetrahydrofurans, and diols are some of the added-value chemicals that furfural can be converted to using platinum and palladium catalysts under appropriate reaction conditions [92].

Due to its use in a variety of industries, acetic acid has also begun to attract the interest of many researchers. Acetic acid is used traditionally as a solvent, as a food preservative, and as an ingredient in the production of certain commercial-grade chemicals [93]. Currently, the need for acetic acid as a platform chemical in the production of many useful products is growing. A report by Grand View Research [94] stated that the demand for acetic acid is expected to grow at a compound annual growth rate of 4.9% from 2022 to 2030.

4.4: Various mechanisms to control the formation of byproducts during pretreatment

Basak et al. [69] discussed extensively the different pretreatment methods and their usefulness in increasing the bioavailability of substrates for bioenergy production but also noted that the production of inhibitory byproducts has continued to be a key limiting factor in bioenergy production.

The following points summarize mechanisms that can be employed to control byproduct formation.

4.4.1: Development of alternative pretreatment methods

Conventional physicochemical pretreatment methods could be projected as being more effective than other methods. However, the cost of operation, including loss of carbohydrates and formation of inhibitors may not be escaped. All these factors add to the production cost of bioethanol and lead to problems with large-scale production. Therefore, an economically feasible pretreatment method is desired.

Biological approaches have been perceived as better alternatives to physicochemical methods of pretreatment [95,96]. This method is environmentally friendly and does not form inhibitory byproducts in the process. The approach consists of the lignocellulolytic activity of microorganisms on biomass, either in a submerged or solid state, leading to the disintegration of the structure and saccharification of the holocellulose [97]. One of the most widely studied is the white rot fungi [97].

The use of enzymes for the pretreatment of biomass is another emerging alternative. These pretreatment strategies result in the generation of an insignificant or no amount of inhibitory byproducts and should be further developed as cost-effective strategies for biomass pretreatment [98,99].

4.4.2: Detoxification with physical and chemical methods

During the pretreatment procedure, furfural, 5-HMF, and other inhibitors are precipitated out of the released sugar [66]. Detoxification or conditioning is one of the important methods of counteracting inhibition problems. The detoxification of LCB hydrolysates has been achieved with various chemical additives such as alkali (KOH, NH4OH, NaOH, Ca(OH)2), polymers, and reducing agents (sulfite, dithionite, and dithiothreitol) [71,100–102]. Other techniques include the physical methods of evaporation via heating, liquid-solid extraction (e.g., adsorption with activated charcoal, ion exchange), and liquid-liquid extraction [71]. LCB hydrolysate containing compounds of elevated hydrophobicity (e.g., phenolic compounds, furan) than sugars could be detoxified with activated charcoal [66]. Overall, these processes require more operational steps, cost, and time [71,103–105].

4.4.3: Feedstock selection

The nature of LCB plays an essential role in efficient biotransformation to biofuel. Thus, low recalcitrant substrates that require pretreatment under less severe conditions are more suitable for biofuel production at an industrial scale [71,100,101]. The feedstock selected should be low in lignin content as it limits the accessibility of hydrolytic enzymes to cellulose and hemicellulose and hinders their conversion to fermentable sugars [71]. Other unique attributes of feedstock that should be considered include high biomass output per dry weight, cost-effectiveness, less effluent production, low greenhouse gas emission, and desirable chemical concentrations. The plant should have excellent above-ground foliage with denser growth ability that enhances the quantity of biomass produced in an acre of land. A broad spectrum of energy crops and grasses currently screened has been shown to have these outstanding qualities [106]. Interestingly, grasses are typically composed of a polymer of carbohydrates (e.g., cellulose and hemicellulose), lignin (i.e., phenolic polymers), and reduced levels of acids, minerals, salts, and protein. The cellulose and hemicellulose content (constituting about two-thirds of the cell's dry matter) could be readily hydrolyzed to sugars and subsequently fermented to bioethanol. The maturity of grass is an essential factor that determines the carbohydrate content and positively correlates with bioethanol yield from biomass [106–108]. The most commonly used herbaceous biomass (C4 perennial grasses) for bioethanol production comprises Miscanthus sp. > switch grass, Napier grass > Bermuda grass [106]. Miscanthus cultivars currently reported to have excellent biomass yield suitable for bioethanol production include M. × giganteus (M × G), Miscanthus sacchariflorus, M. sinensis, and Miscanthus floridulus [109]. The cellulose, hemicellulose, and lignin contents (dry weight, w/w) of M × G are 45.5%, 29.2%, and 23.8%, respectively [110]. Turner et al. [110] also added that about 63.7%–80.2% of the theoretical glucose content was released after the pretreatment of M × G with dilute acid followed by enzymatic hydrolysis. Santos et al. [111] hinted that partitioning (Napier) elephant grass (Pennisetum purpureum) into different fractions minimized biomass heterogeneity, reduced the inherent recalcitrance, enhanced susceptibility to pretreatments, and improved fermentable sugar release during enzymatic hydrolysis. The authors demonstrated that the glucose yield from enzymatic hydrolysis of a pretreated leaf (89.20%) was greater than that from the whole plant (76.01%) or stem (43.54%).

4.4.4: Evolutionary engineering

Adaptive laboratory evolution (ALE) is a widely used protocol in numerous biotechnological operations to enhance the microbial population's fitness under laboratory-controlled conditions via phenotype upgrade, stimulation of dormant pathways, and environmental adaptation [112–114]. In bioethanol biorefinery, the ALE concept has been utilized to create a yeast-based cell factory with enhanced robustness for the tolerance of LCB hydrolysate inhibitors (e.g., organic acids, phenolic compounds, 5-HMF, and furfural) generated during pretreatment and enzymatic hydrolysis [113,115]. ALE presents a substantial framework for identifying and evaluating adaptive alleles via coupling of reverse engineering and whole-genome sequencing that illuminates the genetic mechanisms of yeast responses to inhibitory LCB hydrolysates [113]. For instance, Du et al. [115] developed an ALE-stimulated robust Kluyveromyces marxianus 1727–5 that utilized about 80% of sugars (xylose, glucose, and arabinose) when mixed with non-detoxified LCB hydrolysates (containing formic acid, furfural, vanillin, acetic acid) resulting in 14.2 g/L ethanol titer and yield of 0.46 g/g. With ALE, Yarrowia lipolytica mutant strain yl-XYL + *FA*4 with elevated tolerance of ferulic acid (1.5 g/L) evolved from the parent strain that could barely with 0.5 g/L ferulic acid [116]. Hemansi et al. [117] utilized ALE to develop a strain, Kluyveromyces marxianus JKH5 C60 with enhanced robustness to tolerate a pretreatment-generated inhibitor cocktail comprising vanillin, acetic acid, and furfural. Compared to the parent, the adapted strain when grown in a medium supplemented with the mixture of the inhibitors at 42°C exhibited a higher specific growth rate (3.3-fold), shorter lag phase (56%), and improved fermentation efficiency (80%). Taken together, the ALE-adapted microorganisms make bioethanol production more sustainable because it lowers the time, cost, and byproduct generation, eliminating the need for purification of the pretreated LCB before