The particle size of LC biomass is an important parameter that impact the mixing, fluidization, contact area for mass and heat transfer and the flowability during pretreatment. Therefore, LC biomass with varying particle size could possess various conversion ability and consumption of energy. The LC biomass are normally pre-treated prior to thermal conversion processes (Vidal et al., 2011). In the selection of thermochemical conversion processes, the particle size plays a major role. During enzymatic hydrolysis, the enhancement of surface area contact between cellulosic fibers and enzymes, deconstruction of compact LC biomass structure, increment of hydrolysis rate can be achieved due to particle size reduction through milling, grinding, and extrusion (Silva et al., 2012; Pang et al., 2019; Yu et al., 2019). Many reports have proved that disintegration of LC biomass such as woody chips (Jiang et al., 2017), corn stover (Yu et al., 2019), miscanthus and wheat straw (Kim et al., 2018) via mechanical pretreatment reduces the particle size and this facilitates the subsequent enzymatic hydrolysis (Kavitha et al., 2017a; Banu et al., 2018b) On the otherhand, few literature have reported that depending on the LC biomass, there is a threshold limit for particle size. Chang and Holtzapple. (2000) have reported that reduction in particle size less than 400 μm does not impacts rate of hydrolysis in poplar biomass. Silva et al. (2012) reported particle size threshold limit for wheat straw was 270 μm.

Grindability is another physical property that impacts the LC biomass pretreatment which involves particle size reduction. The measurement of the resistance of any substance to grind is called as grindability. The components of LC biomass which are fibrous and hard to grind are cellulose and lignin. Presently, no typical grindability analysis tests for LC biomass there is available. Various literature have reported that Hardgrove Grindability Index (HGI) test has been used for analysis of coal in LC biomass (Capareda, 2013). The HGI analysis is insufficient for grindability characterization of LC biomass as it encompasses pregrinding to get biomass of particle size ranging from 0.6 to 12 mm before HGI analysis. In HGI analysis, the energy spent for grinding is not taken into account. Therefore, another alternate analysis such as Bond Work Index (BWI) was suggested for grindability analysis of LC biomass (Williams et al., 2015). In a ball milling pretreatment, in order to grind a substance, more energy is needed.

Another important factor that critically affect the pretreatment of LC biomass is the Accessible surface area. Accessible surface area is linked to structural characteristics which includes specific surface area and pore volume (Liu et al., 2015). Particle size reduction and pore volume increment can cause increase in accessible surface area. Reduction in the particle size or increase in pore volumecan leads to increment in accessible surface area. Torr et al. (2016) have suggested that accessible surface area can be increased in enzymatic hydrolysis of disintegrated pine wood. In addition, another researcher, Goshadrou et al. (2013) have suggested that disintegration of aspen wood increases the accessible surface area which in turn increases the accessibility of fibers to subsequent to enzymatic hydrolysis. On the otherhand, analysis of accessible surface is very hard. In that case, specific surface area is SSA is employed to estimate the actual availability of surface to enzymatic hydrolysis (Octavia et al., 2017). Reducing the particle size will increase the specific surface area (Octavia et al., 2017). It has been reported in literature that hydrothermal pretreatment of corn stover leads to two fold increase in specific surface area and resulted in 138% improvement in subsequent enzymatic hydrolysis Zhang et al. (2018). Lu et al. (2019) reported that pretreatment of LC biomass with ball milling pretreatment causes increase in specific surface are of cellulosic component because of particle size reduction. This causes increment in accessibility of cellulose and enhanced yield of glucose.

In LC biomass, accessible volume of cellulose is regarded as an essential parameter impacting physical, chemical and enzymatic pretreatments. Based on the particle sizes and shapes, pore volume accessibility was estimated. It has been reported in literature that pore size and the product yield potential of enzymatic conversion for dilute acid pretreated poplar biomass and cellulosic substrates have close association (Meng et al., 2013; Peciulyte et al., 2015). Herbaut et al. (2018) reported that the ranges of porosity are definite to LC biomass and they reliant on pre-treatment. For instance, yield of hydrolysis linked sturdily to porosity range of 15–30 nm for wheat straw, while it linked to porosity range of 10–15 nm for poplar plants. Herbaut et al. (2018) reported that porosity range limited to 10 nm linked with pretreatment for miscanthus biomass. The authors added that no specific porosity range for improvement of enzymatic hydrolysis was reported so far and proved that enzyme diffusion within the cellwalls of plants is specific to LC biomass. In contrast, few reports suggested that no specific relation among porosity range and yield of hydrolysis was observed in case of disintegrated pine (Kruyeniski et al., 2019) and delignified and dilute acid disintegrated sugarcane (Santos et al., 2018). Besides, few reports suggested that in case of biomass with lignin content below 15%, increment in pore volume has no impact on enzymatic hydrolysis of disintegrated pine (Stoffel et al., 2014; Vaidya et al., 2016).

The thermal chemical conversion efficiency of LC biomass is strongly influenced by its thermal characteristics. The significant thermal characteristics of LC biomass are thermal conductivity and specific heat.

In addition to physical characteristics, chemical factors such as interaction between polymers also plays a major role in recalcitrant nature of the cell walls to pretreatment. Du et al. (2014) reported that interactions among microfibers of celluloses and hemicelluloses and lignin carbohydrate complex association imparts an important part in woody biomass structure and considerably impacts the biological pretreatment thereby minimizing the accessibility of cellulose for enzymes. Literature reports on LCC biomass is a debated theme so far owing to the complications in the LC biomass properties and due to lignin carbohydrate complex. Therefore, it is essential to invent effective techniques to investigate lignin carbohydrate complexes which includes chemical or enzymatic pretreatment to analyze lignin carbohydrate complex qualitatively (Giummarella et al., 2019).

During AD process, accessibility of substrate (cellulose) to microbial enzyme (cellulase) can be limited due to reduced accessibility of surface area between the biomass and the microbes. This would restrict the biodegradation of lignocelluloses and will ends in inadequate fermentable sugars for succeeding biological process. Surface area accessibility can be influenced through various parameters such as layer of epidermis, particle size of feedstock, physiochemical composition of the LC biomass cell wall. Surface area accessibility are of two types such as inner surface area that is associated with porosity of raw material and the external surface area that is associated with size of particles (Xu et al., 2019). Normally, LC biomass possess tiny inner surfaces particularly when it is in dry form. It has been reported in literature that accessibility of cellulose to enzymatic microbial attack is chiefly via the inner pores of the LC biomass instead of exterior surface signifying that the outer surface does not contribute much to hydrolysis process (Arantes and Saddler, 2011). The hydrolytic microbes hydrolyze the LC biomass to discharge the monosaccharides for AD process. These microbes will attach to surface layer of LC biomass via physical mode of attachment and initiate the secretion of exoenymes to hydrolyze the biomass. Normally, the pore dia of inner surface extended from 0.2–2 μm and this is alike of the size of microbes. In AD process, the surface area contact will show increment with increase in fractional exclusion of cell wall compounds and this result in greater surface area accessibility. But the hydrolysis process mediated by enzymes is generally rapid at the initial phase and stumpy at the final phases (Vivekanand et al., 2014), representing that accessible surface are is not only the important regulating parameter in fermentative process. During start phase, the increased surface area permits adequate relationship among enzymes and biodegradable amorphous cellulose and will results in rapid fermentation. However, during the final phase of AD, although the surface area is sufficiently accessible the greater existence of residual crystal cellulose and its complicated nature are the critical parameters that ultimately restrict the hydrolytic potential.

The polymerization extent of celluloses which is associated with mol wt of cellulose linkages is regarded as an essential parameter influencing the hydrolytic potential of cellulose by enzymes. Depolymerization of cellulose is defined as the hydrolysis of cellulose by cellulase enzyme. Usually, hydrogen linkages in longer cellulosic chains will hamper the bioconversion of cellulose than shorter cellulosic chains (Xu et al., 2019). Huang et al. (2015) have reported that steam explosion pretreatment reduces the extent of cellulose polymerization and results in 5–6 times increment in enzyme mediated saccharification. The crystalline portion of cellulose contributes 30–80 percent and this region is responsible for crystallinity of cellulose. The two forces or bonds responsible for crystallinity of cellulose are hydrogen bonds and vander waals interactions. When cellulase enzyme comes in contact with cellulose, initially it may show greater endo-glucanase action with amorphous region of cellulose and in such cases, crystallinity of cellulose imparts obvious role in impacting primary hydrolysis of cellulose. The yield of reducing sugars are reported to decrease with increase in cellulose crystallinity designating that amorphous phase of cellulose are hydrolysed initially prior to the hydrolysis of crystalline phase of cellulose (Ling et al., 2017). It has been reported by Mirahmadi et al. (2010) that pretreatment of birch and spruce with sodium hydroxide shows considerable drop in crystallinity of cellulose and this in turn enhances the enzyme mediated hydrolysis with 83 and 74 percent enhancement in biomethane yield.

Hemicellulose is a cleft saccharide complex comprising of different units of sugar molecules. Xylan is considered as the major copious compound of hemicellulose. The structure of hemicellulose is complex in nature and considerably impacted via cross linkers. for example ferulate; composition of saccharides and existence of branching chains. Usually, it has been supposed that hemicellulose could enhance the physical integrity of LC biomass and restricts the increase in surface area ensuing in decrease of hydrolytic potential. The availability of cellulose to microbial or enzymatic attack can be achieved via pretreatments which can efficiently solubilize or remove lignin and hemicellulose. During AD, with cautious regulation of biomass retention time, yield of methane can be increased from hemicellulose wholly, whereas cellulose and lignin are remained as residuals. In anaerobic biodegradation of LC biomass, hemicellulose can be normally disintegrated priorly which could reduce the hindrances extent of enzyme mediated hydrolysis. Due to the complex nature of hemicellulose and existence of cross linkages among cell wall compounds of LC biomass, further investigation is yet required to cautiously infer the hemicellulose characteristics and its impact on biomethane yield.

Lignin is the polymer of complicated structure with units of phenylpropane which are arranged in 3-dimensionalmesh like structure within the cell wall of LC biomass. Lignin is usually regarded as the major parameter restricting the biodegradation of LC biomass. It has been reported in literature that lignin content in excess of 1 g/L will reduce the methane yield of LC biomass (Kavitha et al., 2020a). Lignin hampers the hydrolysis of polysaccharides of LC biomass mediated by lytic enzymes thus hindering the anaerobic conversion of lignocelluloses (Banu et al., 2018a). The major mechanism of lignin which imparts predominant role in LC biomass recalcitrance is the covalent cross linking of lignin with other cell wall compounds. This results in reduced surface area contact and hinders the enzyme mediated hydrolysis of sugars. The structural characteristics of lignin in addition may have impact on LC biomass biodegradation potential. The ratio of monolignols and interlinking phenols could impact enzymatic biodegradation although after alkali and acidic disintegration of LC biomass.

The thermal pretreatment process increases the solubilization of the LC biomass. Increasing the temperature decreases the viscosity, and the heat treatment also causes the solubilization and the release of the organic matter from the biomass (Banu et al., 2020b). The hydrogen links in the molecule are broken down by the thermal process (Carrère et al., 2010). The thermal process promotes the methanogenic enzyme in the anaerobic biodegradation process. For the thermal treatment, the temperature varies from 50°C to 160°C. increasing the temperature increases the biomass’s solubilization. In a study conducted by Chandra et al. the rice straw was hydrothermally pretreated prior to anaerobic digestion and the increase in methane production in pretreated sample (132.7 L/kg VS) was two times higher than the untreated substrate (59.8 L/kg VS). In another study by Rodriguez, grass (Eleusine indica) was pretreated thermally in a oil bath at 80°C for 3 days to increase the biogas production. There was 46% increase in the methane yield. When Two-phase olive mill solid waste or alperujo was pretreated thermally when exposed to steam at 200°C for 5 min at a pressure of 1.57 MPa, there was 61% increase in methane yield (Rincón et al., 2016). Many such studies revealed that the thermal process from 70 to 90°C enhances the soluble organic release, which favors the rate of biodegradability (Jard et al., 2013). Apart from the high energy requirements and high-pressure operation, the main disadvantage of these procedures is the potential creation of inhibitors, such as furfural and soluble phenolic compounds, which impede methane synthesis (Hendriks and Zeeman, 2009). To avoid the generation of these inhibitory compounds, pH should be maintained in the range of 4-7.

The microwave treatment process is a powerful technique, it has become more trending because of its reduction in the treatment time (Eskicioglu et al., 2007). The microwave fields disrupt the molecule’s links, and it was regularized by an electric field, which causes cleavage of hydrogen bond and changes in the liquid phase. The main parameters of microwave irradiation that disturbs the dielectric system are the temperature, intensity, and reaction period (Park et al., 2010). The thermal and non-thermal effect are the major mode of action of this treatment (Banu et al., 2019b). The thermal process is initiated with the bipolar components containing the electric field. The rotating motion obtained by the dipoles causes the heating effect (Doǧan and Sanin, 2009). The study of Passos et al. (2014) explains the non-thermal effect that changes the dipole direction in the biomass cell wall layer containing side chains, leading to the breakage of the bonds causes protein deconformation. This leads to the increase in the biogas yield. In a research conducted by Siddique et al., microwave pre-treatments on waste sludge resulted in further biomethane enhancements of 53%. When microwave radiation of frequency 2.45 GHz, power 680 W, time 24 min was applied to the lignocellulosic biomass prior to anaerobic digestion (Liu et al., 2012), the biogas generation speed was increased rather than the enhancement.

In the acid pretreatment, both, dilute acid or concentrated acid are usually used. The nature of concentrated sulfuric acid and then concentrated hydrochloric acid are toxic, corrosive, and hazardous, and so it is not preferred (Singh et al., 2014). While pretreating with dilute acid, high sugar yield can be obtained from hemicellulose at higher temperature. If the temperature is high, the yield can be obtained at a short interval of time, and for the lower temperature, it involves a longer time period. While treating wheat straw with sulfuric acid, the saccharification yield of 74% is obtained at 121°C for 1 h (Rezania et al., 2017). When the cashew apple bagasse was processed with dilute sulfuric acid, 0.47 g/g glucose of ethanol yield was obtained at 121°C for 15 min. Similiarly when rice straw was pretreated with propionic and acetic acid, there was 36% increase in methane when compared with the untreated rice straw (Zhao et al., 2010).

But due to the usage of concentrated acids which may be corrosive and toxic, they may incur cost for maintenance. Furthermore, they cause cellulose degradation that results in the formation of inhibitory chemicals such as furfurals, 5-hydroxymethyl furfural, acidic phenolic compounds, and aldehydes. The effect of these inhibitors has to be neutralized further using chemical (peroxide and ozone) and biological (enzymes or live microbes) detoxication methods by converting them into inert substances.

The alkaline treatment is considered as the supporting step for the anaerobic digestion process (Hendriks and Zeeman, 2009). The alkaline pretreatment is mainly preferred for the LC biomass as it highly disintegrated the phenolic nature and lignin content. The alkaline pretreatment efficiently enhances the methane generation (Cheng et al., 2010). Sodium hydroxide is the mainly used alkali for many years in lignocellulosic biomass since it interrupts the lignin structure (Brodeur et al., 2011). The corn Stover, switch grass, bagasse, wheat, rice straw are the reported biomass pretreated with alkali (Zhu et al., 2010). The methane yield of NaOH-pretreated corn straw was found to be approximately 220 ml/gVS, which was 73.4 percent greater than that of untreated corn straw, according to the study by Zheng et al. (Zheng et al., 2009). When calcium hydroxide was used to pretreat municipal solid waste, the methane yield was increased by 172% which was considered as the highest increase of biogas production (López Torres and Espinosa Llorénsdel, 2008). Also, the alkaline pretreatment helps the enzymes to reach the cellulose and hemicellulose easily. The lime is also one of the cheaper alkali used for pretreatment process (Ibrahim et al., 2011).

Oxygen and water take part in wet oxidation process. The oxygen is used as an oxidizing agent. The hydrolyte reaction takes place at low temperatures and the oxidative reaction happens at elevated temperatures in the wet oxidation procedure (Martín et al., 2007). The samples are well dried and milled into small crumps of 2 mm in length. The biomass and water are taken as 6 g/L, respectively. The sodium carbonate is used in the wet oxidation method to minimize the byproducts formed in the process. The air is pumped at a particular temperature above 120°C (Pedersen and Meyer, 2009). The wet oxidation process splits out the LC biomass by disintegrating hemicellulose and eliminates the lignin (Martín et al., 2007; Chaturvedi and Verma, 2013).

Green solvents are ionic liquids (IL) used for the pretreatment and it do not produce any toxic products during disintegration of LC biomass and 1 L is recoverable. The ionic liquids with very low vapor pressure consist of the small amount of anion and a large amount of organic cation (Fort et al., 2007). These liquids dissolve various types of LC biomass, and it involves the hydrolysis of lignin and hemicellulose. The adverse effect found in this pretreatment involves high solvent cost, and the solvent recovery and the recycling are also needed. However, a main issue linked to ionic liquid usage at pilot scale extent is its extreme expensiveness when compared to conventional solvents. But this cost can be counterbalanced to somewhat when the ionic liquids are recycled. Thus, effective separation and recovery of ionic liquids are important on the aspect of environmental applications. In recent years, more attention have been focused on recovery and recycling methods of ionic liquids such as distillation, extraction, adsorption, crystallization and aqueous dual phase separation etc (Zhou et al., 2018).

Surfactant plays a very important role in disrupting the secondary layer of the LC biomass. It also helps in removing the inorganic material from the biomass. Surfactant is amphiphilic molecule which has both hydrophilic and lipophilic group. The surfactant is classified into ionic and non-ionic surfactant. The ionic surfactants are cationic and anionic surfactants. The non-ionic surfactants are more effective than the ionic surfactant (Agrawal et al., 2017). The surface tension between two liquids or the surface tension between the solid and the liquid is reduced by the surfactant addition (Banks et al., 2014). The surfactant is mainly used in the LC biomass case since it helps to inhibit the binding of cellulase with lignin due to the hydrophobic interactions (Okino et al., 2013). Adding surfactant also helps to prevent aggregation of the biomass (Kumar et al., 2018). The surfactant acts as the barrier and does not allow the lignin to bind with the methanogenic enzyme, and hence the methane generation is also enhanced by the addition of the surfactant.

Pretreatment of lignocellulosic biomasses with organic solvents (eg: methanol, ethanol, acetone, etc.) weakens the internal bond between lignin and hemicelluloses and thereby lysing it, resulting in a reasonably pure cellulose residue. The temperature of about 200°C is used for the process, and it is lowered in some processes based on biomass nature and the addition of catalyst. The mainly used catalysts are the organic and inorganic acids (HCl or H₂SO₄). Solvents used in the process need to be evacuated from the reactor, evaporated, condensed and recycled. The solvent shows a negative effect in the hydrolysis process, and hence the solvent must be removed before the fermentation process. The solvent recovery helps in the reduction of cost and has the environmental safety (Sun and Cheng, 2002). However, cost effective recycling of solvents is needed for large-scale application.

The size of the particle plays a significant task in the methane generation. If the size of the particles is small, it is easier for the microbes to degrade, so smaller size particle provide greater solubilization. The smaller sized particle has the greater hydrolysis rate breaking the long chains into the smaller chains and increases the hydrolysis rate, which promotes easy biodegradation. To reduce the particle size, milling or grinding is required, but it is not suitable for the hardwoods, so chipping is done to reduce the particle size. The study of Sharma et al. (1988) explains about agricultural residue treated in five different particle sizes. It is found out that the smaller particle size provides greater biogas production due to the reason of more comfortable handling and the increased surface area. In the case of the mechanical pretreatment, the parameters like the capital cost, operational cost, scale-up possibilities, and reduction of the equipment are also considered. However, in this process, there is absence of generation of toxic compounds or inhibitors and thus this treatment is preferred widely.

Ultrasonication is one of the effectual mechanical pretreatment for the LC biomass. The use of the high-intensity ultrasound changes the biomass structure and thus, it favors the methane generation. Compared with the untreated sample, the ultrasound treated sample gave more significant methane generation. The study of Kim et al. (2003), Wang et al. (1999), and Dewil et al. (2006) revealed that the ultrasonic pretreatment increases the biogas production and decreases the volatile solids. In a study by Zeynali et al. (2017), it was reported that the biogas yield increased from 139 to 396 ml biogas g⁻¹ VS_in when exposed to ultrasonication for 18 min. The drawback of this pretreatment on LC biomass is not noticed in these studies.

Extrusion is the process of combining different operations in one unit. It is the mechanical process in which the biomass is taken into the extruder in one end and travels along the length (Haghighi et al., 2013). The barrel is placed along the length, and the screw drives it. The barrel is shaped in a way with a compression zone at the center and the expanded part at the end. The pressure release taking place at the end breaks down the cell wall of LC biomass. The cellulose, hemicellulose, and lignin are depolymerized. The main operating factors are the time, pressure, and dry matter of the biomass (Zheng et al., 2014). The study of Hjorth et al. (2011) revealed that the extrusion process improves the biogas production of LC biomass. It reduces the particle size, surface area increment, promotes the hydrolysis rate, and improves the anaerobic digestion process. The research conducted by Novarino and Zanetti. (2012) reported that when the organic fraction of the municipal solid waste is treated by extrusion, it resulted in a biogas yield of 800 L/kg VS, containing about 60% of methane content.

The disperser or homogenizer is the advanced mechanical pretreatment process that replaces all the other mechanical treatment since it overcomes the other process by reducing the treatment time. Mainly all the type of the biomass is treated by this process (Tamilarasan et al., 2017). The disperser consists of the rotor and the stator, which breakdown the complex matter into a simpler one. The rpm plays a vital role in the disperser (Kumar et al., 2018).

The biological pretreatment of the lignocellulosic biomass is categorized into fungal pretreatment (oxidative ligninolytic system and hydrolytic system), microbial consortium pretreatment, and the enzymatic pretreatment. The oxidative ligninolytic and hydrolytic system present in the fungus distract the lignin phenyl bond and disintegrates cellulose and hemicellulose (Kudanga and Le Roes-Hill, 2014). The microbes are involved in the microbial consortium pretreatment. The enzymes are employed in the enzymatic pretreatment process to degrade the lignocelluloses. The pretreatment is mainly done to increase the methane generation. The main role of the biological treatment (using delignifying and cellulose degrading enzymes) is to reduce carbohydrate use and maximize the removal of lignin for the lignocellulosic biomass.

The fungal pretreatment consists of two systems the oxidative ligninolytic system and the hydrolytic system. The oxidative ligninolytic system present in the fungus distracts the phenyl bond in the lignin. The hydrolytic system in the fungus degrades the cellulose and hemicellulose in the lignocellulosic biomass (Ma et al., 2017). The rotten LC biomass is used as a substrate for the live bacteria. Compared to the fungal treatment process, the bacterial consortium process degrades the cellulose and hemicellulose effectively. Though these biological methods involve low energy and low chemical demand, it involves longer treatment time which limits its potential towards commercial purposes. Another limitation of the bacterial consortium process is the requirement of carbon sources, which reduces the methane generation (Zhang et al., 2014).

The commercial enzymes are employed in the enzymatic pretreatment process to degrade the lignocelluloses. The mainly used enzymes are lactases and manganese peroxidase, commonly known as degrading enzymes for cellulose, hemicellulose, and lignin. The enzyme lactase and the enzyme manganese peroxidase help remove the fermentable sugars in the LC biomass, which improves the methane generation. When the paper and pulp sludge is pretreated with the enzyme endoglucanase laccase, there was an increase in 34% of the methane yield (Yunqin et al., 2010). The drawback of the enzymatic pretreatment is that enzymes are available at a high cost (Amin et al., 2017).

In the thermochemical pretreatment, both the thermal, and the chemical treatment, including the acid or the alkali, are used. For the thermochemical pretreatment, the temperature is maintained in the water bath to adjust the temperature, and the acid or the alkali is added to adjust the pH. The addition of the chemical to adjust the pH disrupts the nature of the lignin, which is a recalcitrant in the LC biomass (Koyama et al., 2015).

The thermochemical treatment of LC biomass was performed to improve methane generation (Patel et al., 1993). In this study, thermochemical pretreatment of water hyacinth was done to improve the anaerobic biodegradation and methane generation. From the result, it is clear that the solubilization is increased by the pretreatment and improves the methane production. The efficient methane generation was obtained at pH 11 and 121°C. Above this optimized condition, the methane generation was decreased due to the toxic compounds produced due to harsh disintegration of LC biomass.

In another study by Monlau et al. (2013), when the sunflower oil cake is treated at 170°C along with the addition of 1% (weight) concentrated sulphuric acid, the biogas yield was 302 ml CH₄/g VS which is 50% greater than the untreated waste.

In steam explosion pretreatment, both the physical and the chemical methods are involved to disrupt the structure of the LC biomass. This process is mostly applied because of its low chemical usage and minimal energy consumption. It is the hydrothermal method of treatment in which the high pressure and temperature are given for a short period and when it suddenly depressurizes, it collapses the fibril structure. This action helps the cellulose to be easily accessible for the enzymes reported by Duff and Murray (1996). While injecting the steam the temperature was increased from 160 to 260°C and at the same time pressure was suddenly decreased and the biomass undergoes disintegration. The particle size, residence time and the temperature have a major role in the effectiveness of the treatment (Ballesteros et al., 2002). Viola et al. (2008) explains that the addition of the some chemicals such as the acid or the alkali improve the outcome of the steam explosion treatment. Furthermore, harsh conditions favours the generation of inhibitors such as aromatic compounds and furan derivatives which greatly impacts the subsequent hydrolysis process which is considered as the major drawback of this process (Verardi et al., 2018); (Ballestero et al., 2006).

In the AFEX process, both the physical and the chemical methods are involved. In this the biomass is treated with the liquid ammonia at high pressure and temperature for a certain time period and the pressure is reduced suddenly (Lansing, 2005). Taherzadeh and Karimi (2008) explain that the AFEX process carried out at temperature 90°C- 120°C for a time period of 30 min. This method is suitable for the biomass with less lignin. The AFEX pretreatment cost includes the cost of ammonia and the ammonia recovery cost. Due to the volatile nature of the ammonia, it can be easily recovered and it must be recovered to manage the cost and to prevent the environmental hazard. The main advantage of this pretreatment is that only trace amount of inhibitor compounds are formed.

In this method, water is used at high temperature and pressure for disintegrating LC biomass. It is the hydrothermal process in which the water under high pressure penetrates the biomass and disrupts the layer of the lignin and degrades the hemicellulose. The major benefit of this process is the devoid of chemicals usage for treating the biomass in liquid hot water process (Taherzadeh and Karimi, 2008). The liquid hot water is a type of thermal pretreatment here in the place of the steam the liquid hot water is used. The main aim of this treatment is to degrade the hemicellulose and make the cellulose easily accessible for the AD process. The pH is maintained in the range of 4-7 to avoid the formation of the inhibitors (Hendriks and Zeeman, 2009).

CO₂ explosion system is used for disintegrating LC biomass similar to the steam explosion system and ammonia fiber explosion system. In this method, the CO₂ is injected with high pressure and it forms the carbonic acid and increases the hydrolysis rate. The outcome obtained from this pretreatment is found to be lower than the steam explosion or the ammonia explosion pretreatment (Sun and Cheng, 2002). The benefits of this pretreatment is that it can obtain acid catalyst by the action of the carbonic acid without the addition of the sulphuric acid. The pH of the carbonic acid is noted by determining the partial pressure of the CO₂ in water. The water dissolved carbon dioxide induces the dissolution of LC biomass and results in carbonic acid production. During carbonic acid production, an increment in hydronium ion content is noted owing to the solubilization of unstable acid. The dissolution of unstable acid causes a decrement in pH which is adequate to enhance solubilization and hemicellulose degradation into sugars. In addition, the acidic condition of the medium would not lead to ecological issue as the depressurization removes carbon dioxide leading to increment in pH of the medium. Under supercritical conditions, carbondioxide acts as a catalyst in the presence of liquid hot water leading to greater diffusion and biomass swelling.

The study of Zheng et al. (1998) explains about the pretreatment of recycled paper mix and the sugarcane bagasse with CO₂ explosion, ammonia explosion and steam explosion and it found out that CO₂ explosion is found to be more cost effective than the ammonia explosion and the CO₂ explosion does not produce any inhibitory compounds as formed by the steam explosion.

The supercritical fluid is an element which can be a liquid or a gas which is used in the condition above the critical pressure and the critical temperature where the supercritical fluid can coexist. Though it is a type of the liquid or the gas it has some unique properties like liquid density and the transport properties like the gas viscosity. This is the reason and the benefit of the supercritical fluid pretreatment. This fluid can easily penetrate into the LC biomass structure where other pretreatments fails in this case (Brodeur et al., 2011). Brand et al. investigated the conversion of red pinewood at temperatures ranging from 280 to 400°C, nitrogen pressures ranging from 0.4 to 7.5 MPa, residence times of up to 240 min, and ethanol as the solvent. The highest biofuel yield and biomass conversion rates were 59.9% and 98.1 percent, respectively (Brand et al., 2013).