There is always a search for a new source of fuel, and biomass wood pellets have been growing in popularity as a new source of energy for many residential and commercial uses. The U.S. Energy Information Administration estimates that over 6 million tons of biomass pellets were produced, with over 80% of that amount exported to other countries. Biomass wood pellet production has had a steady 10% increase over the past few years, and is expected to continue to grow as more fuel systems convert to use them. The hardwood forests in the Southeastern region of the United States led to it being the largest wood pellet producing area, with the Northeast and Pacific Northwest producing the other largest amounts.
How they’re made
Wood pellets are commonly made from sawdust and wood chip scraps from the lumber industry, but can be made from any biomass including paper, cardboard, and straw.
These wood and biomass scraps are chipped and shredded to reduce the size, and is then sent through a hammer mill to get it into the fine size that is needed to form it into the pellets. It is then sent through a conveyor with magnets to remove any metal that might have been in the wood, and screens to remove any pieces that might be the incorrect size.
Drying & Mixing
Depending on what the raw material is and where it came from, the moisture content will range from very dry to very wet. The material for the pellets must have some moisture present for the process to work correctly, but too low or too much will cause the pellets to not form and hold together in a usable size.
To achieve the correct moisture levels the material can be sent through a vibrating fluid bed dryer until it has reached the correct moisture content. To ensure high quality in the final pellets it is important to mix the ground and dried material until it is uniform. Depending on the source material, it may need to have a conditioner or additive added at this point to help it form and keep the pellet shape.
Once the processed material has been dried, mixed, and conditioned as needed, the pulp is run through a pellet mill. The mill is made of a metal die with holes cut through it like a colander, and then a roller that rolls over the metal die. When the woody pulp is added to the pellet mill the roller will compress the material over the die, squeezing it through the holes. The pressure of the roller creates heat and compression that causes the lignin in the wood to fuze and bind the pellet together. A knife on the outside of the die will break the strands of newly formed pellets into smaller lengths, which are then collected.
Screening and Cooling
Sometimes the pellets will not form correctly or will break during the process and will be too small to be used efficiently by the customer. Once the pellets come off the pellet mill they are sent across a vibrating screener to remove any smaller pieces or shards, so that only the correctly formed and sized pieces reach the end of the processing line. This process also allows the pellets to cool naturally, since cooling them too quickly can cause stress fractures.
Once the pellets are cooled and screened for size they are taken to the bagging or storage facility, where they are packaged in retail bags or stored for bulk purchase. Due to the wood and cellulose makeup of the pellets they are highly water absorbent and must be stored in a way that prevents their exposure to moisture or humidity.