How to Make Wood Pellets
How Wood Pellets are Made
This short article will give you a brief introduction to how wood pellets are made. If you are just curious about how to make wood pellets or are interested in making your own wood pellets this is a good place to start.
Pellets can be made from many organic materials such as wood, paper, cardboard straw, grain, alfalfa, corn husks and stalks, grass clippings, yard wastes and many forest and agricultural wastes. In this article we will focus on making pellets from wood but the process for using other materials is basically the same.
To make pellets the wood or other raw material is reduced to small pieces like sawdust. This material is then pressed into a funnel shaped hole called a die. As the material is forced deeper into the die it compresses as the diameter of the die gets smaller. This compresses the material into a dense pellet.
The compression and friction from this process creates heat which melts compounds called lignins that occur naturally in fibrous materials like wood. When the pellet passes out other end of the die it is then allowed to cool. As the lignins cool they act as a glue which binds the material together and produces a hard and strong pellet.
Most pellets are made in large scale pellet mills but in recent years small scale pellet mills are becoming more popular for home owners and small scale pellet businesses. If you are considering making pellets yourself the following will help teach you how to make wood pellets and will better inform you so you will know how to select a pellet mill.
Steps Involved in Making Wood Pellets
- Size Reduction
- Material Transportation
- Pellet Production
- Pellet Transportation
- Bagging and Storage
The material you make into wood pellets must be reduced to a small size. Usually small enough so that it will fit into the dies. If the raw material is over an inch in size it is usually chipped or shredded first. If small material like sawdust is being used this process can be skipped.
The chips or other small pieces can be reduced even more with a hammer mill. If the material is small to begin with the hammer mill may be the only size reduction step needed.
Once the material is reduced it must be transported to the dryer. Depending on the mill there are several ways to transport the material. The most common are screw augers, conveyor belts and vacuum systems. Screw augers are the most common way to move the raw material through the pellet making process before it gets to the pellet mill.
Before making pellets the material must be dried. Depending on the material used the moisture content should be between 10-20%. For the pellet making process to work properly there needs to be some moisture in the material but only in the proper amounts. Drying the material to the proper moisture content is critical in making quality pellets.
Drying with heat is the most energy intense and expensive part of the process of making pellets. It is usually more cost effective to burn pellets to generate the heat. The drier also serves to heat the raw material which makes it more millable.
To create pellets with a consistent quality it is important to have a consistent batch of material. Some materials are already consistent with moisture and density and don’t need to be mixed. Other materials may come out of the dryer with inconsistent moisture size or density. If there is inconsistency in the material it should be fed through a batch mixer, usually a rolling drum or agitator.
Before the pellet is produced the raw material must have the right properties. Many materials contain enough natural lignins to bind the pellets together. Other materials may need to have additional binding agents added such as vegetable oil.
If the moisture content is too low when the material comes out of the dryer moisture can be added or material with a higher moisture content can be mixed in.
Some materials take more heat and pressure to form a pellet than others so they are pre heated. In some cases the dryer will heat the material enough. In some larger pellet mills the material is heated with dry steam.
The pellet making process consists of a die and a roller. A die is a piece of metal with holes drilled through it. A roller rolls across the die and presses the material through the holes which are the size of the finished pellets. The opening of the holes are tapered like a funnel so as material gets compressed as it gets squeezed into a smaller and smaller hole. The compression creates a dense pellet and also creates enough heat to melt the binders in the material that will act as a glue to hold the pellet together.
There are two basic pellet mill types. Flat die and round die.
With a flat die pellet mill the material falls onto the top of a flat die surface and a roller rolls across the material pressing material between the roller and die.
A round die pellet mill has a round ring shaped die. The rollers are inside the ring and press the material against the inside of the die. The pellets are then extruded out of the holes on the outside of the ring.
Not all pellets form properly. Some will fall apart into small pieces. These pieces are sifted through a screen. Depending on the quality and consistency of the pellets you are producing the pellets may be sifted for size.
When the finished pellets leave the mill they will be hot and releasing moisture vapour so they will need to cool and dry. The most common way is to spread them out and let them cool and dry naturally. Sometimes air is circulated through them with blowers or fans. Care must be taken to not dry them too quickly or they could develop stress cracks.
Once the pellets are cooled they will be transported to the bagging facility if it is a commercial pellet mill. Screw augers are not used in this process since they will damage the pellets. Bucket elevators are often used at this stage.
Bagging and Storing Pellets
At this point the pellets are finished and ready to be used. But in most cases they will need to be stored and transported to the consumers. Pellets must be stored in a way that will protect them from absorbing moisture. If the pellet absorbs moisture they will expand and become useless.
Pellets are commonly put into plastic bags and sealed. Bags for most consumers are a size that can be easily carried. Large sacks weighing up to a ton are also used for large scale equipment. Pellet tankers can be used for bulk shipment to consumers with large pellet silos.