Advanced Biomass Pretreatment Processes for Bioconversion
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On a global scale, the primary source of energy is heavily dependent on fossil fuels, especially crude oil, petroleum, diesel and natural gas (Rana et al. 2018a; Rana et al. 2019a; Rana et al. 2019b; Rana et al. 2020). Rapid urbanization, unprecedented industrialization and a continued upsurge in population growth are the key factors behind the rapid increase in the worldwide need for energy demand and depletion of natural energy sources (Nanda et al. 2016g). With the exploiting usage of fossil fuels as a primary source of energy and petrochemical derivatives in commercial products despite their rising prices, it is presumed that the world will face extreme challenges in the form of shortages of fossil fuels and experiential environmental concerns of greenhouse gas emissions, global warming and pollution (Singh et al. 2018; Shafiqah et al. 2020; Nguyen et al. 2020a; Nguyen et al. 2020b). In such scenarios, alternative renewable resources appear to reduce the adverse impact of greenhouse gas emissions and supplement the energy security and demands (Nanda et al. 2015b; Nanda et al. 2016e; Nanda et al. 2018a; Sarangi and Nanda 2020).
The potential renewable energy sources such as solar, wind, tidal, geothermal and biomass have been explored with multifarious applications. However, biomass-derived biofuels have the tendency to be used widely in the transportation sectors as well as industries and power plants for the combined heat and power generation (Azargohar et al. 2013; Nanda et al. 2014c; Azargohar et al. 2018; Azargohar et al. 2019; Kang et al. 2019; Kang et al. 2020; Parakh et al. 2020). Due to its abundant availability.
cost-effectiveness and minimal impact on the food supply and cultivable lands, ligno- cellulosic biomass is a great candidate to produce biofuels to either partially or fully replace fossil fuel usage (Nanda et al. 2013; Nanda et al. 2017e; Okolie et al. 2020a; Okolie et al. 2020b). The exploration of sustainable fuel sources and essential chemicals from bioresources can be suitable options to mitigate the environmental problems associated with fossil fuels (Yadav et al. 2019). Fossil fuel is presumed to be substituted by biomass-based renewable energy by nearly 10-50% in 2030 (Cucchiella et al. 2014).
The priorities have been diverted towards enhancing biofuel production from renewable lignocellulosic biomass to address the issues of alternative fuels and greenhouse gas emissions. Although the first-generation feedstocks (e.g. corn, potatoes, cassava and other starch-based crops and grains) can be converted to biofuels, several factors restrict their utilization such as a threat to food security, competition to fertile lands and increased food prices (Muscat et al. 2020). Therefore, the focus has shifted towards utilizing non-food biomass like agricultural residues (Nanda et al. 2018b; Sun et al. 2020; Okolie et al. 2020c), forestry refuse (Nanda et al. 2016f; Nanda et al. 2017d), energy crops (Nanda et al. 2016c), invasive crops (Singh et al. 2020), animal manure (Nanda et al. 2016b), municipal solid wastes (Parakh et al. 2020), food waste (Nanda et al. 2016d; Nanda et al. 2019a), industrial effluents (Nanda et al. 2015c; Reddy et al. 2016), sewage sludge (Gong et al. 2017a; Gong et al. 2017b) and polymeric wastes (Nanda et al. 2019b) for producing next-generation biofuels, which have zero competition to the food chain. Among all the above-mentioned alternative resources, lignocellulosic feedstocks (i.e. non-edible plant biomass) has gained much attention for thermochemical conversion to bio-oil. biodiesel, synthesis gas and biochar (Mohanty et al. 2013; Nanda et al. 2016a) as well as biological conversion to bioethanol, biobutanol, biohydrogen and biomethane (Nanda et al. 2014a; Nanda et al. 2017b; Nanda et al. 2017a; Nanda et al. 2017c; Sarangi and Nanda 2018; Rana and Nanda 2019; Nanda et al. 2020). A wide variety of lignocellulosic feedstocks have been widely explored for alcohol-based biofuel and biochemical production after undergoing different pretreatment processes to facilitate microbial fermentation. The complexity of lignocellulosic biomass does not allow it to be directly fermented by microorganisms, thus demanding a suitable pretreatment. This chapter discusses the various strategies for biomass pretreatment processes.
Biomass Pretreatment Technologies
The natural recalcitrance of lignocellulosic biomass creates hurdles for microbes and/or their enzymes to convert cellulosic and/or hemicellulosic sugars to ethanol via fermentation (Sarangi and Nanda 2018; Sarangi and Nanda 2019b). This problem can be substantially solved by pretreating the biomass that not only accelerates the hydrolysis but also enhances the product yield (Sarangi and Nanda 2019a; Sarangi et al. 2020). The pretreatment process also removes certain physical and chemical barriers that prevent the access of enzymes and other hydrolytic agents to degrade the intricate cellulose-hemicellulose-lignin network. These barriers are responsible for rendering recalcitrance and inaccessibility towards enzymatic hydrolysis. A suitable pretreatment method removes hemicelluloses, depolymerizes lignin and hydrolyzes cellulose
FIGURE 2.1 Schematic representation of biomass pretreatment
(Jonsson and Martin 2016). The schematic representation of a pretreatment process is shown in Figure 2.1. The accessibility of cellulolytic enzymes is enhanced after pretreatment, which is an outcome of alteration in pore size of biomass and reduction of cellulose crystallinity (Nanda et al. 2015a). The crystallinity index is a suitable method to authenticate the changes in the crystallinity of biomass by a pretreatment method.
Some typical biomass pretreatment strategies involve the use of mechanical forces (e.g. crushing and pulverizing), physical agents (e.g. ozonolysis, pulsed electrical field, gamma rays, electron beam, ultrasound and microwave), chemical agents (e.g. acids, alkalis, organosolv, ionic liquids, liquid ammonia, steam and subcritical water) and biological agents (e.g. cellulase enzymes, hemicellulase enzymes, lignin-modifying enzymes and Lignin-degrading enzymes) (Nanda et al. 2014b). However, the classification of pretreatments can also be reported as physicochemical, hydrothermal, chemimechanical, etc. depending on the integration of different pretreatment agents.
Several factors are responsible for the establishment of an ideal, cost-efficient and energy-efficient pretreatment process. Some key considerations and challenges encountered during biomass pretreatment are as follows (Nanda et al. 2014b; Maurya et al. 2015; Valdivia et al. 2016):
i. Enhancing the production of sugars (e.g. pentose and hexose) during enzymatic hydrolysis and saccharification from biomass.
ii. Curtailing further degradation of sugars and the formation of undesirable and inhibitory byproducts.
iii. Facilitating lignin recovery for its conversion into various value-added products.
iv. The flexibility of using reactors of moderate size to promote heat and mass transfer, recovery of products as well as minimizing the wastage of heat and power, thereby making the process cost-effective and energy-efficient.
v. Establishing an accomplished technology serving the purpose of achieving a significant volume of pretreated biomass irrespective of its type and source.
vi. Reducing the initial investments by adopting cost-effective reactor materials (e.g. steel alloys) that can tolerate corrosive acids and alkalis.
vii. Achieving cost-effective and eco-friendly methods that can perform best under ambient conditions.
Physical (also referred to as mechanical) pretreatment initially involves size reduction of biomass through chipping, milling and grinding to increase its surface area for effective action by other pretreating agents. Crushing and pulverizing the biomass ruptures cellulose fibrils and reduces its crystallinity (Fougere et al. 2016). Mechanical comminuting of biomass is a power-intensive process, which depends on the physical properties of biomass (i.e. initial size, moisture content, density and volume) as well as the desired final particle size.
Ultrasound is also identified as an effective approach to processing the waste biomass, which aids in saccharification processes along with the improvement in the extraction of hemicellulose, cellulose and lignin (Yang and Wang 2019). The need for cellulases can be curtailed after sonicating the biomass due to the depolymerization of its natural polymers (i.e. cellulose, hemicellulose and lignin). The reaction time involved in ultrasonic pretreatment is inversely proportional to the irradiation power applied (Imai et al. 2004).
A wide spectrum of acids, alkalis and oxidizing agents are involved in chemical pretreatment of lignocellulosic biomass. The effect of pretreatment on structural components varies on the type of chemicals employed. The removal of lignin becomes effective when alkalis, peroxides, oxides and ionic liquids are involved (Rana et al. 2018b). The recovery of hemicelluloses and cellulose becomes relatively easier after the depolymerization and removal of lignin from the biomass.
Dilute Acid Pretreatment
Dilute acid pretreatment is one of the widely used biomass pretreatment techniques. The solubilization of hemicellulosic, which is most effective because of dilute acid pretreatment, results in easy accessibility of cellulases for cellulose degradation (Antunes et al. 2019). High concentrations (e.g. 72% w/w H2S04) and low concentrations (e.g. 0.4-4% w/w H2S04) of acids are employed at elevated temperatures (120-200°C) for such a pretreatment process. This approach facilitates the efficient digestion of lignocel- lulosic biomass because of which less enzyme loading is needed during the subsequent saccharification step. However, in some instances, the enzyme-assisted hydrolysis stage is also avoided as fermentable sugars are generated effectively during acid pretreatment, which depends on the biomass properties including its particle size, acid concentration, temperature, reactor type and other process conditions (Zhu et al. 2009).
The concentration of acid (i.e. pretreating agent) also determines the generation of inhibitory substances like furfural, 5-hydroxymethylfurfural. phenols, acetic acid, carboxylic acids, etc., which tend to be inhibitory to the fermenting microorganisms (Nanda et al. 2014a; Sarangi and Nanda 2018; Sarangi and Nanda 2019b). For instance, the formation of furfural is reduced significantly to about three to five times with the use of dilute H2S04 than with concentrated levels (Onoghwarite et al. 2016). The efficiency of mineral acids and organic acids like maleic acid and fumaric acid to pretreat lignocellulosic biomass has also been explored (Kootstra et al. 2009).
Alkaline pretreatment of biomass mostly involves alkaline hydroxides such as NaOH, KOH, Ca(OH)2 and NH4OH (Aswathy et al. 2010). The highly branched and cross- linked framework of lignin is damaged upon treatment with alkalis, thereby making it easier for cellulase and hemicellulases for biocatalysis (Rana et al. 2018b). The use of Ca(OH)2 is a better approach over NaOH as the former is cost-effective, easily recoverable and relatively less corrosive (Mosier et al. 2005). Moreover, Ca(OH)2 is also used for overliming the biomass hydrolyzates by neutralizing its acidity and removing the inhibitory degradation products such as phenols (Haq et al. 2018).
Alkaline pretreatment results in dramatic alterations to the lignin chemistry, distension of cellulose and solvation of hemicellulosic sugars (Sills and Gossett 2011). Cellulose is also de-crystallized to some extent during alkaline pretreatment (Goshadrou 2019). These effects are manifested due to the breakdown of ester and glycosidic chains along with the separations of structural connections between lignin and holocellulose (i.e. cellulose and hemicellulose). The internal surface area is also increased, which reduces the degree of polymerization of lignin, cellulose and hemicellulose as well as cellulose crystallinity because of alkaline pretreatment.
Certain components in lignocellulosic biomass can be oxidized when reacted with an oxidant. The dissolution of hemicellulosic material along with the elimination of lignin content occurs because of wet oxidation. Typical compounds formed because of lignin decomposition during wet oxidation are C02, water and carboxylic acids.
Wet oxidation also aids in the removal of waxes and extractives. Bjerre et al. (1996) studied the wet oxidation of wheat straw with alkalis (20 g straw per liter at 170°C for 5-10 min), w'hich resulted in 85% cellulose conversion to glucose.
The exploration of ionic liquid seems to be a novel and potential technology for biomass pretreatment to generate fermentable sugars. Ionic liquids are organic salts with enhanced thermal stability having substantial application as green solvents in biomass degradation. Ionic liquids can potentially dissolve polar and non-polar organics, inorganics and polymeric compounds. The active dissolution of cellulose and hemicelluloses becomes feasible when biomass is pretreated w'ith ionic liquids. Some advantages associated with the ionic liquids are solvent recycling, chemical stability, thermal stability (typically up to 400°C), non-flammability and non-volatility. Ionic liquids can dissolve solutes of fluctuating polarity. They are also involved in the production of novel chemicals and materials from biomass (Yoo et al. 2017).
Although ionic liquid-mediated biomass pretreatment is an appealing technology in recent years, its application at a large-scale needs to be evaluated in terms of the life cycle and techno-economic assessments. It is reported that the efficiency of ionic liquids gradually declines on their subsequent reuse (Liu et al. 2017). Another bottleneck associated with the use of ionic liquids is the requirement of higher temperatures (>100°C) and longer processing times for efficient biomass pretreatment. Fusarium oxysporum BN is an ionic liquid-tolerant fungus w'ith the potential to produce ionic liquid stable cellulase enzyme. As reported by Xu et al. (2015), F. oxysporum BN can directly convert ionic liquid-pretreated rice straw to bioethanol with up to 0.125 g of ethanol per gram of rice straw.