Pelletization

Pellets are a carbon neutral fuel source made from compressed woody or herbaceous material, general measuring 6-8 mm in diameter and 2 cm in length. Wood pellets are commonly made from sawdust and other wood waste from processing mills, however, additional feedstocks can include forest origin wood (tops and branches, unmarketable species, undersized material) and short rotation woody crops (willow, hybrid poplar). Non-woody feedstocks include agricultural residues (oak husks, corn stover) and herbaceous crops (miscanthus, switchgrass).

Pellets are classified different grades based on factors such as bulk density, ash content, fines, chlorides, etc. In North America, pellet specifications are set by the Pellet Fuels Institute

Click here to see the differences between the Premium/Residential (PR) grade and the Commercial/Standard/Industrial (CSI) grade. There are also sub-categories, which are used for BBQ pellets and animal bedding.

The pelletizing process generally involves:

• Initial grinding of the biomass material
• Drying the material to approximately 10% moisture content
• Secondary hammering and grinding of the dry material
• Sending the ground material through a high pressure, high-temperature press. The high temperatures cause the release of natural lignin in the wood, which acts as a glue to hold the pellets together.
• Cooling and packaging of pellets

Click here for a fact sheet that explains the pelletizing process.

Since there are no artificial binders used in the pelletizing process, pellets need to be stored in a cool, dry place. Moisture will cause the pellets to expand and lose their burning capabilities.

Additional Resources:

Pellet Tool Kit: A basic how-to guide prior to starting your pellet project
Go Pellets Canada
Manufacturing Fuel Pellets from Biomass
Wood Pellets: An Introduction to their Production and Use
Densified Wood Fuels

Torrefaction

Torrefaction is a thermochemical treatment process, similar to roasting or mild pyrolysis. Originally used in the roasting of coffee beans to bring out more robust flavours, this process has shown promise in the biomass energy/pellet industry. This is because torrefaction actually boosts the energy density of woody biomass based fuels like pellets and chips.

Torrefaction is a thermochemical treatment wherein biomass is heated at around 200 to 320°C. It is carried out in an oxygen stared environment. The heat burns off water and other volatiles, and the biomass partly decomposes. This remaining material is torrefied biomass. The reason that the energy content in torrefied matter is boosted is that torrefaction reduces the mass of material by 20%, yet only 10% of the energy content is lost. This means that a lower volume of material is left with a higher potential energy concentration. Hydrophobic (water repellant) properties of the fuel are also heightened.

The energy burned off in production can also be used to fuel the process itself, making the procedure even more energy efficient. Torrefied biomass may then be pelletized to further increase the energy and bulk density of the material.

Click here for a fact sheet that explains torrefaction of biomass.

Additional Resources:

Torrefaction? What’s that?
Biomass Densification for Energy Production