Size reduction is also known as comminution. You can’t put a whole piece of wood in the process, so comminution is a process of decreasing the particle size; it is also done with coal. However, biomass will be broken down into smaller particle shapes, different shapes than coal. Decreasing particle size of biomass improves accessibility to plant cell wall carbohydrates for chemical and biochemical depolymerization. It can also increase the bulk density for storage and transportation. There is a cost of energy when using mechanical size reduction. For example, 20-40 kWh/metric tons are needed to reduce the size of hardwood chips to coarse particles of 0.6-2.0 mm in size, and kWhs typically have a cost of anywhere from $0.04-0.10 per kWh. To reduce the size of particles to a fine size (0.15-0.30 mm), 100-200 kWh/ton is required.
There are multiple methods used to reduce the size of particles, and the method used will depend on whether the sample is dry or wet. There are hammer mills (a repetitive hammering of sample), knife mills (a rotating knife slices the sample), and ball mills (the sample is put into a container with metal balls and rolled). Sometimes the sample has to be shredded and dried before using some of these techniques.
Samples can also be “densified.” Samples can be mixed with some sort of binder (to keep the materials together, like a glue) and pushed into a shape, or pelletized. This increases the bulk density (i.e., from 80-150 kg/m3 for straw or 200 kg/m3 for sawdust to 600-700 kg/m3 after densification). This can lower transportation costs, reduce the storage volume, and make handling easier. After densification, the materials usually have lower moisture contents.