First Principles of Pretreatment and Cracking Biomass to Fundamental Building Blocks 201
Table 6.5The different types of pretreatments and their effects on lignocellulosic
biomass.
Mechanism Example Method
Particle size reduction
Reduction of the cellulose crystallinity
Milling Physical
Dissolving of hemicelluloses
Transformation of lignin
Steam explosion Physicochemical
Dissolving of hemicelluloses
Reduction of the cellulose crystallinity
HCL Chemical
Delignification
Reduction of the cellulose crystallinity
NaOH Chemical
Delignification Fungi Biological
This pretreatment consists of ball milling, chipping, and grinding. Ball milling is done
on dry and wet biomass. Generally, ball mill size reduction increases the accessible
surface area for enzymatic hydrolysis and reduces the crystalline structure of cellulose.
This process is time consuming and not economic (Sun and Cheng, 2002). Lignocellu-
losic materials can be comminuted by a combination of chipping, grinding, and milling
to reduce cellulose crystallinity. The size of the materials is usually 10–30 mm after
chipping and 0.2–2 mm after milling or grinding (Sun and Cheng, 2002). The vibratory
ball mill has been found to be more effective in breaking down the cellulose crystallinity
of wood chips and improving the digestibility of the biomass than ordinary ball milling
process (Milletet al., 1976). The power requirement for mechanical comminuting of
lignocellulosic materials depends on the final particle size and lignocellulosic biomass
characteristics (Cadoche and Lopez, 1989). The rate of energy consumption for material
size reduction depends on the particle size and the amount of size reduction. These
researchers showed that when the particle size is in the range of 3–6 mm, energy
consumption will be less than 30 kWh per ton of biomass, but in practice consumption
of energy was higher than this. Also the effect ofγ-irradiation on cellulose showed
thatγ-irradiation breaks downβ-1,4-glycosidic bonds, increases the surface area,
and reduces the crystalline structure of cellulose, but it is a high-cost pretreatment
and not economic. In conclusion, physical and mechanical pretreatments increase
biomass hydrolysis efficiency by size reduction, increase accessible surface area for the
pretreatment (chemical and enzymatic), and reduce the crystalline structure (Takacs
et al., 2000; Sun and Cheng, 2002).
6.7.1 Steam Explosion
The steam-explosion process was first carried out in the year 1925 to produce fiber-
board. Then, it was used to produce feed of livestock and pulp of wood. In the beginning
1980, steam explosion was used to pretreat biomass. Iotech Company did researches to
investigate the effect of steam-explosion pretreatment on poplar. This company reported
to the Department of Energy, United States, the effect of pressure, temperature, and
residence time of this pretreatment on the rate of produced xylose and glucose. They
showed the maximum of xylose and glucose generation different time and pressures
