Pretreatment and Thermochemical and Biological Processing of Biomass - Introduction to Chemicals from Biomass - page 54

depicts the various biomass conversion routes for the production of chemicals, mate-
rials, and fuels. In this chapter, the concepts of various pretreatment processes includ-
ing mechanical, physical, chemical, and biological methods are discussed. Different
thermochemical and biological conversion processes are also introduced.

3.2 Biomass Pretreatments
Direct conversion of biomass into products is not efficient in most cases due to the
presence of hemicellulose (20-40%), lignin (20-30%), and extractives in large
quantities [1, 2], which serve as a protective layer (Figure 3.2). For this reason, it
is necessary to pretreat the biomass before any processing. Pretreatment helps
alter the physical properties and chemical composition of biomass, and makes it
more suitable for conversion. It can be a biological, chemical, thermal, or mechan-
ical process such as ammonia fiber explosion (AFEX), torrefaction, and steam
explosion. These pretreatment processes help alter the amorphous and crystalline
regions of the biomass and bring significant changes to structural and chemical
compositions.

3.2.1 Mechanical Pretreatment of Biomass

3.2.1.1 Chipping, Grinding, and Milling

The objective of the mechanical pretreatment is a reduction of particle size
and crystallinity of lignocellulosic materials in order to increase the specific
surface area and reduce the degree of polymerization. This can be produced by
a combination of chipping, grinding, or milling depending on the desired
particle size of the final material (10-30 mm after chipping and 0.2-2 mm
after milling or grinding) [3, 4]. Different milling processes (ball milling, two-
roll milling, hammer milling, colloid milling, and vibro-energy milling) can
be used to improve the enzymatic hydrolysis of lignocelullosic materials [5].
The power requirement of this pretreatment is relatively high depending on
the final particle size and the biomass characteristics.

3.2.1.2 Densification

According to the European Commission Green Paper, European energy policy is
built on three core objectives [6]:

• sustainability : to combat climate change actively by promoting renewable
energy sources and energy efficiency;
• competitiveness : to improve the efficiency of the European energy grid by cre-
ating a truly competitive internal energy market; and
• security of supply : to better coordinate the EU's supply of and demand for
energy within an international context.