Front Matter

 

First Principles of Pretreatment and Cracking Biomass to Fundamental Building Blocks 199

Suitable pretreatment must

• produce maximum fermentable sugar yield;


• produce minimum carbohydrate degradation;


• minimize the formation of microorganism growth inhibitors;


• lead to low energy usage and have economical performance;


• be less expensive and economical in practice.
  Simply the goal of pretreatment is the breakdown of lignocellulosic structure and
  its conversion to monosaccharide components in order to be used as fermentation
  substrate. To break down lignocellulosic materials, three main factors, including size of
  pores (Grouset al., 1986), cellulose crystallinity (Goldstein, 1983), and delignification
  (Dekker, 1988) must be considered. Accessibility to cellulose can be enhanced by
  hemicellulose separation. Hydrolysis of hemicellulose is simpler than cellulose and
  results in the formation of bigger pores in microfibrils (McMillan, 1994). This researcher
  showed that the increase of enzymatic digestibility of cellulose depends directly on the
  hemicelluloses separation. Grouset al. (1986) showed that there is a positive correlation
  between accessible surface (pores volume) and produced glucose from enzymatic
  hydrolysis. Cellulose crystallinity is the second factor to determine the produced
  glucose efficiency. Higher degree of crystallinity leads to decrease of the hydrolysis rate
  (Goldstein, 1983). Weimeret al. (1995) showed that thermal and chemical treatments
  increase the relative crystallinity index (RCI) of amorphous portions. Similar research
  showed that no significant increase was seen in the rate of RCI in the crystalline
  portion of cellulose. Delignification can increase polysaccharides’ accessibility. Several
  pretreatment methods including dilute acids and alkali pretreatment, acid hydrolysis,
  ammonia fiber explosive, steam explosion, and enzymatic hydrolysis can be used for
  lignocellulosic biomass conversion to fermentable sugars.
  The principal goals of these methods are delignification and breakdown of cellulose
  crystallinity structure as shown in Figure 6.11 (Harmsenet al., 2010).

6.5.1 Effect of Pretreatment on Hydrolysis Process

Pretreatments improve enzymatic hydrolysis by

• increasing the porosity and surface area;


• modifying the lignin structure;


• delignifying and removing lignin from biomass;


• partially depolymerizing of hemicelluloses;


• separating and removing hemicelluloses from biomass.


• reducing cellulose crystallinity.

6.6 What Are the General Methods Used in Pretreatment?

The following are a few pretreatment methods used:

• mechanical pretreatments like particle size reduction (chipping, grinding, and milling
  and using high pressure (extrusion process);


• physical pretreatments (using irradiation and microwaves, ultrasound and infrared,
  thermal pretreatment like as steam explosion and compressed hot water);