World Journal of Environmental Biosciences
World Journal of Environmental Biosciences
2023 Volume 12 Issue 3

The Role of Physicochemical Pretreatment in Lignocellulosic Biomass Energy Valorisation – A Review

 

Erick Auma Omondi1*, Arnold Aluda Kegode2

 

1Department of Civil and Construction Engineering, University of Nairobi, Nairobi, Kenya.

2Department of Civil & Structural Engineering, Moi University, Eldoret, Kenya.


Abstract

The surging climate change resulting from carbon dioxide emissions primarily from the employment of fossil fuels poses a threat to the global economy and civilization. Additionally, fossil fuels are quickly experiencing depletion prompting the need to explore alternative, sustainable energy sources. Lignocellulosic biomass is a renewable energy source whose potential remains underexploited. Harnessing biomass energy faces challenges that limit its economic exploitation through; limited knowledge to enhance its full potential and inefficiencies experienced through the experimental stages that affect full rollout and optimal performance. An efficient valorization of lignin and cellulose components of the biomass to desired energy products remains contingent on effective depolymerization of the biomass through pretreatment intervention. Several studies have focused on pretreatment methods such as chemical, physical, and biological individually while a few are attempting to evaluate the effect of combined methods such as physicochemical which combines physical and chemical action in biomass pretreatment. This study characterizes the lignocellulosic biomass and reviews the existing common physicochemical pretreatment methods for enhanced performance in bioenergy production. The reviews examine the performance of different techniques including; steam explosion, liquid hot water, ammonia fiber explosion, CO2 explosion, soaking in aqueous ammonia, and wet oxidation methods. Further, the reviews focused on highlighting the performance criteria and comparing the derived benefits of each technique.

Keywords: Pretreatment, Lignocellulosic biomass, Physicochemical pretreatment, Bioenergy


INTRODUCTION 

 

Global efforts to minimize greenhouse gas discharges oblige the enhancement of more maintainable, green, and environmentally friendly energy substitutes. Following the depletion of fossil fuels and the attendant environmental issues, biomass research has become an appealing subject (Kurniawan et al., 2014). Biomass is an organic material that is renewable and is derived from live or recently living organisms such as plants or animals (Benti et al., 2021). During their growth, plant biomass produces a primary wall that is significant in offering structural functions such as protection, signal transduction, and interactions with neighboring cells (Alberts et al., 2002; Zeng et al., 2017). The primary wall contains a low proportion of cellulose but a greater presence of pectin that surrounds the growing and dividing plant cells (Alberts et al., 2002; Sarkar et al., 2009). Likewise, plant biomass also produces a secondary wall which provides strength and rigidity in plant tissues that have ceased growing (Sorieul et al., 2016; Avci, 2022). In the wake of the exploration of renewable energy sources, biomass can be used as part of the energy supply chain (Helal et al., 2023). Lignocellulosic biomass (LCB) as a feedstock is abundant (Dahmen et al., 2019), inexpensive, and evenly distributed in nature.

The exploitation of LCB as an alternative bioenergy source from crop residues, such as; corn straw (Aghaei et al., 2022), wheat straw (Taghizadeh-Alisaraei et al., 2023), rice straw (Al-Haj Ibrahim, 2018), water hyacinth (Gaurav et al., 2020) and other lignocellulosic biomasses from agricultural practices is on the rise due to its availability and accessibility (Amin et al., 2017). LCB is a high-potential alternative energy source to avert dangers posed by fossil fuels through second-generation biofuels from feedstock that do not compromise global food security (Zoghlami & Paes, 2019). LCB possesses desirable plant biomass characteristics for bioenergy generation including high cellulose and low lignin, biodegradability, resistance to pests and diseases coupled with its assured perennial availability (Carlini et al., 2018). However, its valorization can be limited by the recalcitrant nature associated with; rigid cell wall structure, crystalline cellular machinery, and lignin component, making it resistant to chemical and biological actions (Silveira et al., 2013).

LCB pretreatment technologies focus on modifying the biomass structure to remove hemicellulose and lignin, making the carbohydrate fraction accessible to enzymatic hydrolysis for optimal bioenergy yield (Sharma et al., 2023). Pretreatments may take the form of; physical, chemical, biological, or physicochemical techniques incorporating both physical and chemical interventions (Bensah & Mensah, 2019). Although each of the methods can be applied based on the prevailing conditions, the methods are not without unique challenges to overcome (Antunes et al., 2019). Biological pretreatment methods, for example, are green and eco-friendly, but are inherently slow and difficult to scale up (Sharma et al., 2023), whereas physical pretreatment is associated with the inability to eliminate lignin content in LCB materials, rendering the cellulose content inaccessible (Brodeur et al., 2011; Saritha et al., 2012). Similarly, chemical pretreatments are distinguished the loss of fermentable sugar due to an increase in the breakdown of complex substrates, the production of inhibitory byproducts as hydroxymethylfurfural (HMF) under very acidic conditions, and high chemical expenses, among other problems (Kucharska et al., 2018). As a result, an optimal pretreatment process must strike a compromise between three important parameters: efficiency, cost, and the formation of unwanted byproducts (Hernández-Beltrán et al., 2019).

Lignocellulose biomass and its characteristics

Lignocellulosic materials can be obtained from plant feedstock, such as purposely grown energy crops (Sluiter et al., 2010) with examples of; corn stover, rice straw, and sugar cane bagasse (Kadam & McMillan, 2003; Adewuyi, 2022). Other examples of lignocellulosic energy plant biomasses include; miscanthus and switch grass which are present in huge amount to provide biofuels, biochemicals, as well as animal feed (Saini et al., 2015). Despite the great effort invested in the improvement of lignocellulose material digestibility for positive green energy generation, the impact on efficiency, energy cost reduction, and adoption is yet to be realized (Adewuyi, 2022). The considered high cost of energy production from such biomass has been aggravated by its complexity despite its potential (Reid et al., 2020). The availability of various lignocellulosic materials with varying characteristics also calls for research to narrow down on best feedstock and capitalize on its production (Saini et al., 2015; Adewuyi, 2022). However, irrespective of the variability, the general characteristics conform to; 50-60% carbohydrates (cellulose and hemicellulose), and 20-30% lignin while the other minor components such as extractives, fatty acids, and ash comprise 10-30% (Galbe & Wellberg, 2019). Figure 1, illustrates the lignocellulose structure, an example of a bioenergy crop and its constituents.

Figure 1. Structure of lignocellulosic biomass and its biopolymers.

Lignocellulose, which is developed with cellulose, hemicellulose, and lignin, is the fundamental building block of plant cell walls. Furthermore, the plant cell wall includes less pectin, protein, ash, and extractives. The cell wall also contains soluble non-structural materials like N , non-structural sugars, chlorophyll, and waxes. The general structure comprises polymers with different chemistry, and that perform different functions in the lignocellulosic plants. the total quantity for every  oconstitunetns mentioned differs in various plant species and the part of the plant considered. For instance, hardwood and softwood stems have greater amounts of cellulose than grasses and nut shells, whereas their leaves contain more than 80% hemicellulose (Table 1).

 

Table 1. Cellulose, hemicellulose, and lignin contents in common agricultural residues and waste (Kumar et al., 2009)

Lignocellulosic Material

Cellulose

(%)

Hemicellulose

(%)

Lignin (%)

Hardwood stems

40 - 45

24 - 40

18 - 25

Softwood stems

45 - 50

25 - 35

25 - 35

Nutshells

25 - 30

25 – 30

30 - 40

Corn cobs

45

35

15

Grasses

25 - 40

35 - 50

10 - 30

Paper

85 - 99

0

0 - 15

Wheat straw

30

50

15

Sorted Refuse

60

20

20

Leaves

15 - 20

80 - 85

0

Cotton seed hairs

80 - 95

5 - 20

0

Newspaper

40 - 45

25 - 40

18 - 30

Waste papers from chemical pulps

60 - 70

10 - 20

5 - 10

Primary wastewater solids

8 - 15

 

 

Solid cattle manure

1.6 - 4.7

1.4 – 3.3

2.7 – 5.7

Coastal Bermuda grass

25

35.7

6.4

Switchgrass

45

31.4

12

Swine waste

6

28

na

Cellulose

Cellulose is the most abundant component of plant cells, with an unbranched polymer chain made up of glucose units linked by -1, 4-glycosidic linkages that give the cell wall structure rigidity and stability (Rongpipi et al., 2019). Polymerization can reach up to 14,000 glucose units, with each glucose unit rotated 180° relative to the next unit (Gautam et al., 2010). The orientation and direction of the glucose units play an important role in determining their functionality (Van Schaftingen & Gerin, 2002). The connectivity between the glucose units is aided by two intra-chain hydrogen bonds and two to three inter-chain bonds which tightly pack and stabilize the structure cellulose structure (Khazraji & Robert, 2013). As a result, the cellulose chains form compact aggregates of three-dimensional microfibrils that are further stabilized by hydrogen and van der Waals links (Heise et al., 2021). Every microfibril is made up of 30-36 parallel cellulose chains. Figure 2 illustrates the chemical structure of cellulose.

 

Figure 2. Cellulose Structure

 

Hemicellulose

Hemicellulose is a natural polymer found in lignocellulose biomass like cellulose, consisting of a variety of carbohydrate monomers (Lu et al., 2021). Hemicellulose is present as the plant cell wall matrix that surrounds the cellulose skeleton (Zoghlami & Paes, 2019). Pentoses (such as xylose and arabinose) and hexoses (such as glucose, mannose, and galactose) make up the carbohydrate monomers (Navarro et al., 2019). The contribution and composition of hemicelluloses change between plants and cells. Most hardwoods and agricultural plants, such as grasses and straw, have xylan as the dominating hemicellulose, whereas glucomannan and mannose are the predominant monomers in softwoods (Lu et al., 2021). Xylan is the main component of heterogeneous polysaccharides in hemicellulose, which contains C5 and C6 sugars (Huang et al., 2021). In hemicelluloses, the degree of polymerization of glucose units is between 100 and 200 units, which is substantially lower than in cellulose (Zoghlami & Paes, 2019). Unlike cellulose, the hemicellulose structure is more complex and characterized by many branches, predominantly of acetyl groups responsible for its noncrystalline nature, whereas cellulose is a linear polymer (Wohlert et al., 2022). Hemicelluloses are highly hydrophilic, soluble in alkali, and easily hydrolyzed in acids (Huang et al., 2021; Lu et al., 2021). The hydrophilic nature of hemicelluloses relates to its acid groups that increase its water uptake in the fibers hampering the microbiological fiber degradation (Xu et al., 2023). Figure 3, illustrates the chemical structure of hemicellulose (C5H8O4)n.

 

Structure

Figure 3. Hemicellulose Structure

Lignin

After cellulose and hemicellulose, lignin is the third most abundant building element of lignocellulose materials (Yang et al., 2020). Lignin is an encrusting substance that acts as a protective layer on the plant cell wall and is known to inhibit anaerobic breakdown (Alberts et al., 2002). The oxidation of phydroxycinnamyl alcohols: p-coumaryl, coniferyl, and sinap is required for the production of lignin. The lignin formation process can occur in three-fold biosynthesis dimensions involving; the shikimate pathway, phenylpropanoid pathway, and the synthesis of monolignols (Barros et al., 2022). As a polymer binding the plant cell wall, lignin is considered a complex, amorphous, branched polymer constructed of different phenylpropane units (Aro et al., 2005; Ganewatta et al., 2019). Lignin’s structure can exist as a three-dimensional mixture of p-coumaryl, coniferyl, and synapyl, based on their aromatic ring substitution pattern (Ralph et al., 2019). The adaptability of lignin in the plant cell wall, caused by its three-dimensional composition and amorphous heteropolymers, protects the cell against structural stress, metabolic damage, and pathogenic attack (Gierlinger, 2014; Zeng et al., 2017). Lignin can extremely be resistant to biodegradation as a result of the strong linkage bonds (Alberts et al., 2002). Its component in lignocellulose biomass presents a challenge to efficient anaerobic digestion of the feedstock for biogas generation (Zheng et al., 2014). Thus, lignin is an agent of low biomethanation of lignocellulose biomass and is known as a nuisance material for ethanol makers as it retards the enzymatic hydrolysis procedure. In addition, the native crystalline structure of cellulose can considerably limit its potential in cost competition as a precursor to biofuel production (Sasmal & Mohanty, 2018). Figure 4 presents the chemical structure of three monomers of lignin: (1) p-coumaryl alcohol; (2) coniferyl alcohol; and (3). synapyl alcohol.

  1. p-coumaryl alcohol

  1. coniferyl alcohol

  1. synapyl alcohol

Figure 4. Lignin Chemical Structure

Pretreatment of lignocellulosic biomass

Pretreatment is an intervention process that rapidly disintegrates the lignocelluloses to its primary constituents such as; lignin, cellulose, and hemicellulose (Sasmal & Mohanty, 2018). The intricate structure of lignocellulose can limit microbial degradation and result in slow digestion and reduced biogas yield (Omondi et al., 2019). Lignocellulosic and starch-based feedstocks require various forms of pretreatment to enhance biofuel and bioenergy production (Saritha et al., 2012). Pretreatment is an important tool for cellulose conversion processes and is essential to modify the structure of cellulosic biomass making cellulose more available to the enzymes responsible for the conversion of carbohydrate polymers into fermentable sugars (Mosier et al., 2005). The high cost and low efficiency of enzymatic hydrolysis of lignocellulosic feedstock are considered major impediments to ethanol production (Vasic et al., 2021). Pretreatment focuses on the disintegration and disruption of the crystalline and amorphous regions in the structure of cellulose and starch (Kumar et al., 2009), thus improving acid or enzyme access to carry out hydrolysis of the substrate (Maurya et al., 2015). Although it is difficult to single out the best pretreatment applicable in all situations, factors such as the high recovery of individual polymers and other compounds in the lignocellulosic material remain outstanding (Galbe & Wellberg, 2019). Additionally, reduction of secondary effect of toxic inhibitors must be minimized to avoid low enzymatic hydrolysis and fermentation (Kim et al., 2009; Brodeur et al., 2011). Figure 5, illustrates the action of pretreatment on lignocellulosic material and the resultant disintegration of the biomass structure.