Machinery Used in Physical Pretreatment
The breaking and grinding of biomass material to form smaller particles is also known as comminution. Efficient mechanical size reduction can help retain biomass quality, reduce biomass losses and minimize the economic and environmental costs of cellulosic ethanol production. There are a number of parameters useful in evaluating the efficiency of size reduction of biomass in this pretreatment, and the two most important ones are [19, 20]:
1. Energy efficiency of the equipment.
2. Bulk density of the resulting product, and physical properties such as particle size, shape, shape distribution, and particle surface area.
Biomass size reduction is usually achieved by using a number of well-established machinery such as hammer mills, knife mills, ball
Figure 5.4 Knife mill.
Figure 5.5 Hammer mill.
mills, needle mills, shredders, linear knife grids, and disk attrition mills [19, 20]. Two commonly used machines in biomass comminution are knife mill and hammer mill; these machines are shown in Figures 5.4 and 5.5, respectively.
A knife mill is a machine similar to a household blender, where high-speed rotating knifes shred the biomass into smaller pieces. Rotating hammer heads with heavy metal surfaces crush the biomass in hammer mills, whereas a ball mill uses steel balls inside a rotating cylinder as the size-reduction device.
Energy consumption is a very important factor in physical pretreatment more so than in other pretreatments and the total energy requirement depends on a number of factors:
1. Machine parameters such as motor speed, material storage capacity of the mill chamber, as well as cutting mechanism such as knife, hammer, ball or disk.
2. Material throughput characteristics.
3. Initial biomass form and properties.
4. Desired particle size and shape.