Physical Pretreatments

Changing the structure of biomass, typically increasing the enzyme accessible surface area, and reducing the degrees of polymerization of biomass, are possible by physical pretreatments such as size reduction (Zhu and Pan 2010; Harun et al. 2011). Different types of milling (e. g., ball milling, hammer milling, colloid milling, two-roll milling, and vibro energy milling), irradiations (e. g., by micro­waves, gamma rays, electron beams, and ultrasonications), and extrusion (sub­jecting the biomass to heating, mixing, and shearing) are used for this propose (Taherzadeh and Karimi 2008; Zheng et al. 2009).

Modification of biomass structure with a single physical treatment is typically not enough for efficient enzymatic hydrolysis, although it can be enough for improvements in biogas production. Thus, the physical treatments are used prior to (or together with) chemical and biological treatments (Taherzadeh and Karimi 2008; Yu et al. 2009).

Size reduction is used prior to most chemical pretreatments. Although it can affect the efficiency of the process, it heavily impacts process economy. Most chemical pretreatments are not successful without size reduction. However, in explosive, organosolv, and solvent processes large particles may be used (Zhu and Pan 2010; Shafiei et al. 2012). Explosive pretreatments, such as steam explosions, need less energy than mechanical size reduction. However, the explosive pre­treatments are not easily possible in laboratory investigations and there are some limitations in their scalability. Furthermore, they are not much effective for soft­woods (Zhu and Pan 2010). In some organosolv processes, such as ethanol organosolv pretreatment, which is one of the most effective methods, size reduction is not necessary. The process is also effective for softwoods (Pan et al. 2005, 2006a, c, 2007a, b). However, the process is not yet considered as an alternative process for large-scale pretreatment of lignocelluloses.

Size reduction can also be performed after chemical pretreatment, as refereed post-chemical pretreatment size reduction. Post-chemical pretreatment size reduction has different advantages compared to that before chemical pretreat­ments. Besides more effectiveness, the main advantage is lower mechanical energy consumption. On the other hand, without pre-size reduction, it is possible to work with denser solids and consequently higher solid per liquid ratio in the process, resulting in more concentrated hemicellulose sugar liquid. Furthermore, separation of fibers from the pretreated mixture is easier after pretreatment (Zhu and Pan

2010) . However, post size reduction is not applicable in all pretreatments.

Different irradiation processes have also been shown to improve the digest­ibility of lignocelluloses (Fernandez-Cegri et al. 2012). Treatment of biomass with high energy irradiation can modify the structure (Bak et al. 2009). However, its application is limited to less recalcitrant biomass such as rice straw (Bak et al.

2009) . Ultrasonication, on the other hand, has been used at large scale for the improvement of digestibility of different organic material and sludge resulting in higher yield of biogas and lower amounts of residual sludge (Pham et al. 2009; Elbeshbishy et al. 2011).

Combined irradiation (mainly ultrasonic and microwave) and chemical pre­treatments also have been shown to improve digestibility than a single chemical pretreatment. The irradiations can work in conjunction with NaOH pretreatment (Rodrigues et al. 2011; Singh et al. 2011), ionic liquid pretreatment (Ha et al. 2011; Ninomiya et al. 2012), and ammonia pretreatment (Chen et al. 2012).

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