Before pellet compression in the mill can take place the wood, straw, grass or other biomass must be reduced in size. One of the most important facts in pellet production is that “only raw material of consistent quality can produce pellets of consistent quality.” Part of this consistency is reliant on the particle size of raw material used in the pellet mill. Particles either too small or too large can significantly affect pellet quality.
To reduce the size of the raw material there are machines available such as the hammer mill, chipper and wood waste shredder. Pellets produced from raw materials that are first crushed are of better, more consistent quality. If your raw material is sawdust, or a similar wood and biomass material, this section can be skipped.
The general rule of thumb is that the milled material going into the pellet mill must be smaller than the holes of the die. For example, to produce pellets 6 millimeters in diameter, the milled material must also be smaller than 6 millimeters. The extent of the necessary reduction of the raw material determines which machine, or machines, are needed for the process.
Pre-densification of agricultural biomass
Grasses and other fibrous herbaceous feedstock are known to have a relatively low bulk density in a chopped form. This density can be improved somewhat through the use of fine grinding. In the case of switchgrass, bulk densities can be reduced to 200 kg/m3 by using a 2 millimeter grinding screen. The main benefits of this process are:
- Reduced problems, leading to more stable pellet production, which often occur with herbaceous feedstock such as
- Minimized frictional energy in pellet production which is largely responsible for high energy consumption during the pelleting process.
- Reduced cleavage production within pellets.
- Reduced wear on dies.
The optimization of pellet bulk density and durability of reed canary grass pellets occurs with intermediate pre-compaction densities of 310 kg per m3 when steam and a high die temperature (minimum 80°C) are used. Pre-compaction provides a way of sourcing distant fiber sources for more economical transport to pellet mills. It is a viable, if not often used, alternative means for the wood pellet industry to source new supply from forest residues without going into whole tree pelleting, which has been initiated in some areas due to lack of wood residuals.
A new concept of pre-compaction of biomass exists through the use of a series of roller compactors to create a staged compaction process with primary and secondary compaction prior to pelletization as a means to create an improved pelleting process. This new pelleting process eliminates frictional forces by producing pellets externally through the use of tooth rollers. The conceptual design was cited as having the following features:
- Pellets (or “compacts”) are produced externally; there is no plugging the die holes every time the feed material
- Frictional forces are eliminated and only compressive forces are used to form
- The system can be heated to a desired temperature to improve compact
- A wide range of particle sizes can be
- Compact density can be positively
- Access to the machine’s working parts is easier for cleaning and
Raw materials: logs, branches and material with a diameter over 1 inch (2.54 centimeters).
Chippers and shredders are used to reduce the size of large-diameter raw materials. For example logs and branches. Chippers generally reduce material to the size of chips; shredders can produce even smaller particles.
Chippers: The conventional chipper is a rotating disc with several blades attached to the surface. As the material comes into contact with the disc, the blades shave off small sections (chips) until all the material is processed.
Shredders: Shredders push the material against a rotating roller with teeth. The teeth eat into the material and drag it against a screen. The size of the holes in the screen dictates the particle size.
If the raw material has a diameter of over 1 inch (2.54 centimeters) a chipper/shredder is recommended for initial size reduction. Chippers and shredders in many cases can be adjusted to produce particle of different sizes, however they generally cannot reduce particle size to an extent sufficient for pelletization.
Therefore in many cases, after the chipper/shredder, the material is then sent to a hammer mill for final size reduction.
Hammer mill
Raw materials: wood chips, straw, grasses and material with a diameter less than 1 inch (2.54 centimeters).
If the diameter of the raw material is less than 1 inch the material may be sent directly to a hammer mill. The hammer mill is often used for further processing after material has exited a chipper/shredder. A hammer mill is comprised of many small flailing hammer blades that, through attrition, reduce particle size. Hammer mills run at speeds ranging from 3,000 to over 8,000 rotations per minute. Particle size is dictated through the size of holes in the screen, which can range from as small as 1 millimeter up to 10 millimeters or more. Hammer mills can be fitted with screens of various sizes and are commonly found in pellet production plants.
