First Principles of Pretreatment and Cracking Biomass to Fundamental Building Blocks 195
- preserving the pentose (hemicelluloses) fractions;
- preventing sugar degradation during the pretreatment process;
- limiting the formation of degradation compounds that inhibit the growth of ferment-
ing microorganisms;
- minimizing energy demands and reducing fermentation costs.
6.4 Steps of Production of Cellulosic Ethanol
The process of bioethanol production from biomass consists of
1) pretreatment of biomass
2) hydrolysis of biomass
3) fermentation of released sugars
4) separation of produced ethanol.
The generation of ethanol from biomass is a complicated procedure.
For efficient ethanol production, these problems must be answered.
6.4.1 Pretreatment
To hydrolyze biomass, it must be attended to inhibitors, especially in pretreatment
phase. At this stage, the crystalline structure of cellulose changes, lignin is distilled, and
more surface area is formed for enzyme action. The pretreatment stage consists of the
softening of biomass, breaking down of cellular structure, and increasing enzymatic
digestion of biomass. Pretreatment is effective to produce sugar from biomass and help
with the optimization of this goal (Mosieret al., 2005).
6.4.2 Hydrolysis
Fermentation of cellulose to produce ethanol is not possible directly because it is a
polymer.
To produce glucose, long chains of cellulose must be a broken down. This process is
named hydrolysis or conversion to sugar.
The cellulose fraction of biomass is converted to glucose by hydrolysis. Hydrolysis is
carried out by two methods:
- Chemical (acid) hydrolysis
- Enzymatic hydrolysis.
Hydrolysis by concentrated acids like sulfuric acid has been done since many years
ago. The time of this procedure is short and its yield is high. Enzymatic hydrolysis is a
new method. In this method, enzymes such as cellulase is used to break down cellulose.
In order to make bioethanol production more economic, this process must be done fast.
6.4.3 What Are the Inhibitors for Biomass Carbohydrate Hydrolysis?
- Crystalline structure of cellulose with difficult hydrolysis.
- The existence of lignin and hemicellulose complex around cellulose component.
- Lignin links cells to each other and reduces the surface area.
- Hydrolysis of pentoses is more complicated than hexoses (Laureano-perezet al., 2005;
Mosieret al., 2005).
