Lignocellulosic biomass mainly consists of three components, namely, cellulose, hemi — cellulose, and lignin. Cellulose (major component) susceptibility to hydrolysis is restricted due to the rigid lignin and hemicellulose protection surrounding the cellulose micro fibrils. Therefore, an effective pretreatment is necessary to liberate the cellulose from the lignin- hemicellulose seal and also reduce cellulosic crystallinity. Some of the available pretreatment techniques include acid hydrolysis, steam explosion, ammonia fiber expansion (AFEX), alkaline wet oxidation, and hot water pretreatment. Besides reducing lignocellulosic recalci­trance, an ideal pretreatment must also minimize formation of degradation products that

inhibit subsequent hydrolysis and fermentation. Pretreatment methods are subject to ongoing and intense research worldwide. Possible pretreatment methods can be classified as follows, although not all of them have been developed yet enough to be feasible for applications in large-scale processes (Taherzadeh and Karimi, 2008):

i. Physical pretreatments: milling (ball milling, two-roll milling, hammer milling, colloid milling, vibroenergy milling), irradiation (gamma ray, electron beam, microwave), others (hydrothermal, high-pressure steaming, expansion, extrusion, pyrolysis)

ii. Chemical and physicochemical pretreatment methods: explosion (steam explosion, ammonia fiber explosion, CO2 explosion, SO2 explosion),alkali treatment (treatment with sodium hydroxide, ammonia or ammonium sulfite), acid treatment (sulfuric acid, hydrochloric acid, phosphoric acid), gas treatment (chlorine dioxide, nitrogen dioxide, sulfur dioxide), addition of oxidizing agents (hydrogen peroxide, wet oxidation, ozone), solvent extraction of lignin (ethanol-water extraction, benzene-water extraction, ethylene glycol extraction, butanol-water extraction, swelling agents)

iii. Biological pretreatments (fungi and actinomycetes)

Mechanical comminuting reduces cellulose crystallinity, but power consumption is usu­ally higher than inherent biomass energy. Steam explosion causes hemicellulose degradation and lignin transformation and is cost effective but destroys a portion of the xylan fraction, causes incomplete disruption of the lignin-carbohydrate matrix, and generates compounds inhibitory to microorganism. AFEX is an important pretreatment technology that utilizes both physical (high temperature and pressure) and chemical (ammonia) processes to achieve effective pretreatment. Besides increasing the surface accessibility for hydrolysis, AFEX promotes cellulose decrystallization and partial hemicellulose depolymerization and reduces the lignin recalcitrance in the treated biomass. This process is not efficient for biomass with high lignin content. CO2 explosion increases accessible surface area; are cost effective and do not cause formation of inhibitory compounds but does not modify lignin or hemicelluloses. Ozonolysis reduces lignin content and do not produce toxic residues, but a large requirement of ozone makes it very expensive. Acid hydrolysis hydrolyzes hemicellulose to xylose and other sugars and alters lignin structure. Its disadvantages are high cost, equipment corrosion, and formation of toxic substances. Alkaline hydrolysis removes hemicelluloses and lignin and increases accessible surface area but long residence times are required, irre­coverable salts are formed and incorporated into biomass. Organosolv hydrolyzes lignin and hemicelluloses but solvents need to be drained from the reactor, evaporated, condensed, and recycled; hence, the process cost becomes high. Pulsed electrical field process is carried out in ambient conditions which disrupts plant cells and is simple equipment, but this pro­cess needs more research. Biological process involves degradation of lignin and hemi- celluloses and has low-energy requirements, but the rate of hydrolysis is very low (Kumar et al., 2009b).

Lignocellulosic biomass has lignin, cellulose, and hemicelluloses with the complex structures with high molecular weight. The selective and effective lignocellulosic biomass conversion methods are highly desirable to produce the wide range of usable hydrocarbons as fuels, chemicals, and other products. The decomposition of complex structure can be performed by using biochemical and thermochemical methods using conventional and nonconventional energy sources.