Steam explosion, lignocellulosic

We can illustrate fermentation processes using the process developed by Iogen to convert lignocellulosic materials such as wheat straw into ethanol (Fig. 2.10) [66]. The straw is chopped and milled prior to a steam-explosion pre-treatment to... 

Dekker, R. F. H. 1991. Steam explosion an effective pretreatment method for use in the bioconversion of lignocellulosic materials. In Focher, B., Marzetti, A., and Crescenzi, V. (Eds.), Steam Explosion Techniques Fundamental Principles and Industrial Applications (pp. 277-305). Philadlphia PA Gordon and Breach Scientific Publishers-. 

Digestion of lignocellulose, to render it susceptible to hydrolysis, can be performed in a number of ways [10, 13, 14] but few of these satisfy the green commandments. There seems to be a trend towards aqueous pretreatment, such as the steam explosion that is used by Iogen (Canada) [15, 16], followed by enzy-... 

Because the hemicellulose fraction of biomass materials can be separated from lignin and cellulose by dilute acid treatment, cellulose becomes more reactive towards cellulase. Hemicellulose hydrolysis rates vary with acid concentration, temperature, and solid-to-liquid ratio. With most lignocellulosic materials, complete hemicellulose hydrolysis can be achieved in 5-10 min at 160°C or 30-60 min at 140 °C. Dilute acid hydrolysis forms the basis of many pretreatment processes for example, autohydrolysis and steam explosion are based on high-temperature dilute acid catalyzed hydrolysis of biomass. 

In cellulosic ethanol production processes, a pretreatment procedure is needed to disrupt the recalcitrant structure of the lignocellulosic materials so that the cellulose can be more efficiently hydrolyzed by cellulase enzymes [2], These pretreatments include physical, biological, and chemical ways, such as uncatalyzed steam explosion, liquid hot water, dilute acid, flow-through acid pretreatment, lime, ammonium fiber/freeze explosion, and ammonium recycle percolation [3, 4], Most of these methods involve a high temperature requirement, which is usually achieved through convection- or conduction-based heating. 

The pretreatment of any lignocellulosic biomass is cmcial before enzymatic hydrolysis. The objective of pretreatment is to decrease the crystallinity of cellulose which enhances the hydrolysis of cellulose by cellulases (17). Various pretreatment options are available to fractionate, solubilize, hydrolyze and separate cellulose, hemicellulose and lignin components (1,18-20). These include concentrated acid (27), dilute acid (22), SOj (25), alkali (24, 25), hydrogen peroxide (26), wet-oxidation (27), steam explosion (autohydrolysis) (28), ammonia fiber explosion (AFEX) (29), CO2 explosion (30), liquid hot water (31) and organic solvent treatments (52). In each option, the biomass is reduced in size and its physical structure is opened. Some methods of pretreatment of Lignocellulose is given in Table I. 

However, not all of these methods are desired for pretreatment of the lignocellulosic materials because of technical or economic feasibility. In some cases, a mediod is used to increase the efficiency of another method. For instance, milling could be applied to create a better steam explosion by reducing the chip size (8). Furthermore, it should be noticed that the selection of pretreatment method should be compatible widi the selection of hydrolysis. For example, if acid hydrolysis is to be applied, a pretreatment with alkali may not be beneficial. The most commonly applied pretreatment methods will be discussed here. 

Carbon dioxide explosion is a pre-treatment process that uses supercritical carbon dioxide to break down the biomass structure. In aqueous solution, carbon dioxide forms carbonic acid which depolymerizes lignocellulosic materials. As a small molecule, carbon dioxide can penetrate into the pores of the biomass better than ammonia. When carbon dioxide explodes due to the change of pressure, it breaks the cellulosic structure. This process is usually operated under high pressure but low temperature to prevent monosaccharide degradation. But in comparison to steam explosion and ammonia explosion processes, the sugar recovery yield from this process is... 

Examples of some of the pretreatment methods are sulphite pretreatment to overcome recalcitrance of lignocellulose (SPORL) (Zhu et al. 2009), dilute sulphuric acid (Purwadi et al. 2004), hot water-controlled pH (Mosier et al. 2005b), ammonia fibre expansion (AFEX) (Teymouri et al. 2005), steam explosion, SEW fractionation, ammonia recycled percolation (ARP) and lime treatment (Jurgens et al. 2012). 

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