INTRODUCTION 

 

         

Steam explosion

Steam explosion (SE), also known as auto-hydrolysis, involves briefly heating biomass with high-pressure saturated steam. (Ziegler-Devin et al., 2021). The heating temperature causes a catalytic reaction that breaks down the hemicellulose and changes the lignin content, preparing the substrate for hydrolysis (Zhang et al., 2022). Organic acids like acetic acids and other acids made from acetyl or other functional groups enhance the hydrolysis of hemicellulose (Swiatek et al., 2020; Huang et al., 2021). Variables including moisture content, particle size, residence duration, and temperature can all affect how effective SE is. The typical pretreatment temperature ranges between 160-260ºC (Ziegler-Devin et al., 2021), whereas the pressure range is between 0.6-4.83MPa (Ma et al., 2022). As the pressure is progressively let go, the steam causes an expansion within the lignocellulosic matrix, upsetting the design of the cell membranes and causing the separation of individuals (Kumar et al., 2009; Agbor et al., 2011). The original biomass moisture content directly affects the length of pretreatment (Zhang et al., 2022). However, according to Li et al. (2017), 2 to 10 minutes is usually the right amount of time. Waste biomass, such as paper waste, has frequently produced superior results with steam pretreatment (Elliston et al., 2015).

To enhance the efficacy of steam explosion, acid additives such as H₂SO₄, SO₂, and NaOH can be used to catalyze the process (Tan et al., 2021; Yin et al., 2021). In this process, catalysts can lead to a more complete solubilization/recovery of hemicellulose (Zheng et al., 2014), a reduction in the generation of inhibitory chemicals (Jonsson et al., 2013), and an improvement in the biodegradability of lignocellulosic biomass (Rangel et al., 2023). However, even without a chemical catalyst, water, which is used in steam generation also possesses acidic properties at high temperatures, and contributes the catalytic effect that aids the hemicellulose hydrolysis (Lu et al., 2021). Acid addition is not mandatory and the SE process without acid addition is referred to as auto-hydrolysis (Amin et al., 2017). One of the most popular pretreatment techniques for lignocellulosic biomass is steam explosion.

To determine the extent of SE in the pretreatment of biomass, a “severity factor” (log R₀), is used to measure the severity of the process. The harshness factor incorporates the effect of temp and the pretreatment duration in the pretreatment process. Equation 1 below presents the severity factor.

 

(1)

place log The severity factor for SE is called Ro, and it ranges from 3.14 to 3.56 depending on how long the therapy is, T is the temperature (oC), t is the residence time (in minutes), and 14.75 is the activation energy under the current conditions, where the process complies with first-order kinetics and the Arrhenius law. 100 C° is the reference temperature at which no solubilization occurs (Amin et al., 2017).  

Steam explosion pretreatment has reported significant results on different biomasses. For instance, Barbanera et al. (2014) studied the impact of SE pretreatment on the generation of sugar from the enzymatic hydrolysis of olive tree pruning. The experiment includes pretreating the biomass at seven different levels of severity before enzymatic hydrolysis. The findings led to a predicted ethanol production of 14.41 g/100 g raw material at a severity level of 4.41. In another study, Pielhop et al. (2016), evaluated the effect of SE pretreatment on softwood to comprehend the influence of explosive decompression on enzymatic digestibility. The findings presented that the intensity of the pretreatment and the pressure differential of the explosion are two important parameters that influence enzymatic digestibility. According to the study's findings, pretreatment with SE improves the enzymatic digestibility of refractory biomass, such as softwood.

Liquid hot water (Hydrothermolysis)

LHW pretreatment method involves subjecting the biomass to a high temperature of heated water at high pressure (Jimenez-Gutierrez et al., 2021). The process targets breaking the biomass cell structure, hydration of cellulose, solubilizing of hemicellulose, and incomplete elimination of lignin materials (Kim et al., 2009). Further, the process also focuses on the suppression of inhibitor formations. Effectively, the action leads to enhanced cellulose accessibility and susceptibility, leading to better microbial and enzyme degradation  (Kim et al., 2009). The technique is a hydrothermal method of pretreatment that does not require the addition of chemicals (Chen et al., 2022). The method has widely been used in the pulp industry and for bioethanol production (Broda et al., 2022). The optimal pH condition for LHW is preferably between 4-7 to minimize the monosaccharides formation and catalyze hydrolysis of the cellulosic material in the pretreatment process (Taherzadeh & Karimi, 2008; Maurya et al., 2015). The usage of a high amount of water in this method enhances the solubilized products while lowering product concentration (Serna-Loaiza et al., 2022). LHW has widely been applied in pretreatment studies for biomethanation of various feedstalks such as; Municipal solid waste (Perez-Pimienta et al., 2017), microalgae, sugarcane bagasse (Gurgel et al., 2014).

Jimenez-Gutierrez et al.'s study from 2021 examined the effects of LHW before management on poplar wood chips biomass on a small scale. At Temps going from 180 to 188 °C, pretreatment times between 30 and 240 min were investigated. The liquid and solid fractions gained after pretreatment were analyzed for acetic acid, which led to a full deacetylation of the poplar biomass. Similar to this, Li et al.'s (2017) research looked into the impact of LHW pretreatment on the chemical-structural change and decreased recalcitrance in poplar. The study found that LHW can significantly reduce the lignocellulosic biomass's cell wall resistance by improving the conversion of polysaccharides, notably cellulose, into glucose at a negligibly high initial investment. The research also demonstrated that xylan solubilization, hemicellulose molecular weight reduction, cellulose degree of polymerization, and the cleavage of alkyl-aryl ether bonds in lignin as a result of LHW pretreatment are significant factors linked to decreased cell wall recalcitrance.

Ammonia fiber explosion (AFEX)

The ammonia fiber explosion (AFEX), described by Brodeur et al. (2011), is a physicochemical pretreatment method in which biomass material is subjected to liquid anhydrous ammonia at high pressures and moderate temperatures before being quickly depressurized. 60 to 120 degrees Celsius, the appropriate mild temperature is lower than that of a steam explosion (Chundawat et al., 2020). The pretreatment approach aims to increase the pretreated material's digestibility through cellulolytic enzymes and/or microorganisms (Maurya et al., 2015; Chundawat et al., 2020). AFEX is a promising method for pretreating agricultural material of lignocellulosic nature, for bioenergy production (Kucharska et al., 2018). The main process parameters that control AFEX include; the reaction temperature, substrate residence time, ammonia loading, and water loading (Bals et al., 2010; Dong et al., 2022). The process focuses on the decrystallization of the cellulose, hydrolysis of hemicellulose, depolymerization of lignin (Questell-Santiago et al., 2020), and increase of size and number of micropores in the biomass cell wall, boosting enzymatic hydrolysis rate noticeably as a result (Harun et al., 2013). The AFEX pretreatment method does not always remove lignin and other substances from the biomass; instead, it causes the cleavage of lignin-carbohydrate complexes and the deposition of lignin on the surface of the material, which may prevent cellulases from converting the biomass into cellulose.

Teymouri et al. (2004) looked at how AFEX treatment affected the activity of acidothermus cellulolyticus endoglucanase in transgenic plants. Tobacco leaves treated with AFEX and left untreated were tested for endoglucanase from Acidothermus cellulolyticus to see how the treatment affected the enzyme's activity. At 60 C°, with a 0.5:1 ammonia loading and 40% moisture content, the investigation was run. The study found that AFEX pretreatment was unsuitable for the release of cellulose enzymes from transgenic plants and estimated the maximum activity retention in AFEX at about 35%. In another study to determine the AFEX Pretreatment and Enzymatic Hydrolysis on canary grass, Bradshaw et al. (2007), Compared to their untreated controls, tests were performed on enzyme digestibility and potential improvements in sugar conversions. The most efficient AFEX treatment settings were identified through testing employing 168 h hydrolysis and 15 filter paper units of spezyme CP cellulase/g glucan. The study determined that 100°C, 60% moisture content, a dry matter to ammonia ratio of 1.2:1 kg, and canary grass growing age of seed stage were the ideal conditions for AFEX activation. Alizadeh et al. (2005) conducted a comparison study to examine the effects of AFEX on switch grass, and they determined the ideal conditions to be a reactor temp of 100°C, an ammonia loading rate of 1:1 kg (ammonia: dry matter), and an 80% moisture level. The switch grass AFEX treatment increased ethanol production by 150%.

Soaking in aqueous ammonia (SAA)

Delignification of lignocellulosic biomass using the soaking aqueous ammonia (SAA) process improves hydrolysis and biomass-to-energy conversion without having a major impact on the amount of carbohydrates present (Kim et al., 2009). The preference for ammonia-based pretreatment revolves around the advantage of retained cellulose and hemicellulose in the treated biomass (Latif et al., 2018), making it suitable for simultaneous saccharification and co-fermentation (Isci et al., 2008). Similarly, ammonia-based pretreatment has become a preferable method due to its high potential in the post-pretreatment effect and future commercial utilization (Zhu et al., 2014; Latif et al., 2018). The pretreatment method can alter the original biomass structure by inducing cellulose swelling and lignin and hemicellulose solubilization leading to cellulose exposure to enzymatic action (Akus-Szylberg & Zawadzki, 2021). Pretreatment by soaking in ammonia occurs in two forms: high severity, low contact time process (ammonia recycle percolation; ARP) and low severity, high treatment time process (soaking in aqueous ammonia; SAA) (Kim et al., 2009). During SAA pretreatment, high temperatures can result in a variety of different reactions, such as the dissolution of poorly graded polysaccharides, peeling of end groups and the formation of alkali stable end groups, alkaline hydrolysis of glycosidic bonds and acetyl groups, and the degradation and decomposition of both dissolved polysaccharides and peeled monosaccharides. (Wang et al., 2013; Subhedar & Gogate, 2014).

Norwell et al. (2018), conducted a study to ascertain how entire maize kernels should be prepped for the synthesis of cellulosic ethanol from fiber fractions. Whole maize kernels were immersed in aqueous ammonia solutions containing 2.5, 5.0, 7.5, and 10% wt of ammonia at 100°C for 24 hours as part of the investigation. According to the findings, pretreatment with ammonia at a 7.5% weight of ammonia increased ethanol production from 334g/kg to 379g/kg of maize or a 14% increase. In a similar study by Akus-Szylberg et al. (2021), SAA pretreatment of corn stover resulted in 38.7% and 68.9% delignification of biomass treated at 50ºC and 90ºC respectively. The results showed a significant impact of SAA pretreatment on biomass delignification.

CO₂ explosion (Supercritical fluid pretreatment)

This is a technique developed to improve the pretreatment of lignocellulosic biomass using supercritical CO₂ explosion which reduces temperature demand lower than what is required for steam explosion and eventually lowers the cost (Srinivasan & Ju, 2010). The supercritical CO₂ takes the form of a fluid, however, compressed in a gaseous state at temperatures higher than its critical point (Pu et al., 2022). The method is based on the knowledge that CO2 molecules, which are smaller than water and ammonia molecules, may be able to enter tiny holes in biomass (Maurya et al., 2015; Morais et al., 2015). Hemicellulose and cellulose are hydrolyzed using CO2, and the low temperature used in the procedure prevents the acid from breaking down monosaccharides (Brodeur et al., 2011; Mussatto et al., 2021). The action mechanism for supercritical CO₂ entails the penetration of biomass micro pores under pressure and the rapture of the pores upon the pressure release (Aftab et al., 2019). These findings in the contact of cellulose surfaces to cellulose enzymes during the hydrolysis stage. CO₂ is considered a green solvent with its critical temperature T_c being 31.0ºC compared with 374.2 and 243.1ºC for water and ethanol respectively (Gu et al., 2013). Unlike steam explosions characterized by high temperatures causing degradation of sugars, low-temperature conditions for CO₂ explosion prevent such sugar degradation (Brodeur et al., 2011). Supercritical CO₂ has effectively been used as a green and non-flammable solvent for chemical reactions and separations (Gu et al., 2013).

The effects of CO2-added ammonia explosion on the pretreatment of rice straw for bioethanol production were examined in a study by Cha et al. (2014). With a maximum glucose production of 93.6% for a 14.3% ammonia content, 2.2 MPa of CO2 loading level, 165.1°C of temperature, and 69.8 min of residence time, the study's pretreatment parameters were maximized. For the biomass, a scanning electron microscope revealed much more pores and surface area, enhancing enzyme accessibility for enzymatic saccharification. Finally, the study achieved a 97% ethanol yield through saccharification and fermentation. Similarly, a study by Zheng et al. (1998), investigated pretreatment for cellulose by CO₂ and realized a disruption of cellulosic structure on biomass increasing accessible surface area to enzymatic hydrolysis. The results indicated the suitability of supercritical CO₂ in cellulose pretreatment. Additionally, a rise in pressure made it possible for CO2 to enter the crystalline structure more quickly, yielding more glucose following pretreatment. In a separate study by Srinivasan and Ju (2010), supercritical CO2 pretreatment for guayule grass was examined. This method outperformed others and produced much higher overall sugar yields for guayule (77% for glucose and 86% for all reducing sugars) via both pretreatment and hydrolysis. Compared to a dilute acid pretreatment, which only produced results of (50% for glucose and 52% for total sugars), the results were significantly superior.

Wet oxidation

Wet oxidation (WO) entails treatment of lignocellulosic biomass with water and air at temperatures ranging between 170-200^oC and pressures of 0.5-2.0 MPa, for a residence time of 10-15 minutes (Refaat, 2012). According to Peral (2016), WO is used as an alternative to Steam Explosion as it aims to convert hemicellulose and lignin to oxidized compounds, e.g., CO₂, H₂O, alcohols and low molecular weight carboxylic acids. It facilitates the disintegration of cellulose after a significant amount of hemicellulose and lignin have been solubilized (de Jong & Gosselink, 2014), with the amount of lignin removed after pretreatment ranging from 50-70% depending on the type of biomass used and the prevailing conditions for the process.

In comparison to Steam Explosion and Liquid Hot Water pretreatment methods, WO produces lower amounts of furfural and 5-HMF (Refaat, 2012), which are strong inhibitors of the fermentation process (Peral, 2016). When combined with other methods such as Steam Explosion, larger particle sizes as well as higher biomass loadings can undergo energy valorization (Brodeur et al., 2011). Further, the addition of oxygen at temperatures above 170^oC results in an exothermic process resulting in a reduction in total energy demand (Tomás-Pejó et al., 2011). Chen and Wang (2017) suggested that this method of pretreatment enhances the sensitivity of the biomass to enzymatic hydrolysis thus resulting in higher enzymatic convertibility (de Jong & Gosselink, 2014). However, Tomás-Pejó et al. (2011) highlighted that WO does not catalyze hydrolysis of solubilized hemicellulose. Subsequently, it uses a significantly large amount of oxygen hence making it relatively expensive for commercialization (Merklein et al., 2016).

The method has successfully been used in the pretreatment of wheat straw, newspaper waste, rice husk, sugarcane bagasse, and maize silage, among other organic matters (Zhang et al., 2020). For wheat straw, for instance, the author highlighted that the released glucose from the smallest particles attained 90% of the theoretical maximum after a 24-hour enzymatic hydrolysis. Peral (2016) It was also mentioned that WO was employed industrially for the treatment of wastewater and soil remediation by the oxidation of suspended particulates, which might then be used for energy production. McGinnis et al. (1983) used WO on a variety of low-grade hardwoods, including loblolly pine, black oak, and hemlock, and discovered that the procedure was successful in separating the cellulose, hemicellulose, and lignin components of the wood.

Conclusion

Physicochemical pretreatment methods can overcome the recalcitrance of LCB, by separating the cellulose from the matrix polymers, expanding the surface area that is available for enzymatic hydrolysis, decrystallizing cellulose, and separating hemicellulose and lignin. Physicochemical pretreatment methods promote a reduction of inherent moisture and particle size in the biomass, thereby enhancing its downstream valorization for energy production. The techniques involved, enhance the efficiency of biological activities for maximized energy generation through enzymatic processes. Whereas most reviews in the past, have focused on pretreatment techniques independently, a synergistic approach involving a combined compatible action by different methods such as through physicochemical methods, promises better results. The diversity in properties of lignocellulosic biomass demands a balanced action of more than one technique, thus the applicability of physicochemical methods which comprise both physical and chemical intervention. The choice of the specific method grossly depends on its efficiency and cost. Additionally, the choice of a pretreatment method should focus on integrated techniques capable of minimizing the inhibitor effects often a major drawback to pretreatment success. The most common setback in most pretreatment processes involves buildup of inhibitors that reduce saccharification by soluble lignin-derived phenolics, non-productive adsorption of enzymes on insoluble lignin, or low enzyme accessibility to cellulose occasioned by insoluble lignin hindrance. Deriving maximum benefits from the physicochemical pretreatment of lignocellulosic biomass demands intense exploitation of emerging and promising combined techniques that rely on the physiochemical fractionation of the biomass. The process description, benefits, disadvantages, and innovations used to address innate technological, economic, and environmental issues should take center stage in the thorough analysis. The physicochemical techniques examined in this study, such as soaking in aqueous ammonia (SAA), ammonium fiber explosion (AFEX), and liquid hot water (LHW), reveal highly promising results.

ACKNOWLEDGMENTS: None

CONFLICT OF INTEREST: None

FINANCIAL SUPPORT: None

ETHICS STATEMENT: None

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