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why and how are wood pellets made

By Jack Huang    Apr 16, 2014

What are wood pellets?

What are wood pellets? Wood pellet is kind of biomass densification fuel, which is made from the raw biomass materials, such as forest slash and construction wastes. The biomass densification coverts the low density and high moisture content and difficult to transport materials into solid bio-fuel wood pellets which provides consistent quality, low moisture content, high energy density and homogenous size and shape.

Wood pellets

Fine pine wood pellets

Why to make wood pellets?

Why to make wood pellets? Firstly, wood pellets are clean alternative to fossil fuels and offering environment advantages over traditional fuels. Secondly, Wood pellets production can take advantage of the biomass harvested from the unhealthy forests to restore forest health and lower the fire risk. Thirdly, urban sources of biomass can also be used as wood pellet machine feedstock and lower the cost of landfills. Fourthly, rural areas and remote areas can develop the densified biomass pellet mill industry. The self-made pellets can meet their needs for energy. Lastly, wood pellets fuel is a valuable commodity in the international market.

How are wood pellets made?

How are wood pellets made? Generally, the wood pellets are made from sawdust and planner shavings after drying, screening for stones and metal-like materials, hammer-milling, pressing by the pellet machines for pellets, screen again for saw dusts, and storage.

How are wood pellets made in a wood pellet machine specifically? It depends on the pellet maker you are using, flat die pellet mill or ring die pellet mill.

As for a small flat die wood pellet machine, the pellet are made in the following way:

After the feed stock, such as the sawdust are feeding into the pelleting chamber, the rollers of the wood pellet machine press the sawdust into the die holes with the pressure casting by the rollers. And then the pellets are formed and extruded out of the die holes.

How are wood pellets made by small flat die pellet mill

How  are wood pellets made by a small flat die pellet mill

As for the ring die pellet mills, the pelletizing procedures are as follow:

  • Feed stock enters into the feeder and flows evenly to the conditioner where steam and/ or molasses are added properly.
  • Feed stock is delivered over a permanent magnet after the conditioner and into a feed spout and then to the pelletizing die.
  • With the force of rollers, feed stocks are compressed into the die holes and the pellets are extruded out of the pellet die holes.
  • Pellets extruded out of pellet dies are cut off by the knives and discharged out through the pellet door.

How are wood pellets made by ring die pellet mill
How are wood pellets made by a ring die pellet mill

Specifically, wood pellets are made and extruded through die holes of pellet die under the interaction of three main forces, which are roll force, radial force, and tangential force.

  • Roll force refers to the force from the roller acting on the feed stocks. It acts on the feed stocks relative to the contact point with the die and acts to compress and extrude the feed stocks.
  • Radial force refers to the force from the die which resists the flowing of feed stocks through the die holes.
  • Tangential force maintains the material from sliding along the face of the die in front of the roller. It is related to the pressure exerted by the roller and the frictional characteristics of feed stocks.
Ring die pellet mill with pellets in the die holes

With the interaction of the three main forces, the wood pellets are formed and extruded through the die holes. As for the pellet die holes, there are several designs available, showed as the following picture.

Die hole designs of pellet mill dies

Among the above mentioned design, the first type is very common one which are usually used in plat die design and the seventh type, the stepped die hole design are the most popular and developed designs for flat die and ring die. Let’s take the stepped die hole design as an example to see how the wood pellets are made specifically.
 

Step-design pellet die holeThe picture on the left presents a typical stepped pellet die design. As for a pellet die,"compression ratio" is a important factor which refers to the a ration value of active length in press channel or diameter. As for the picture, the compression ratio would be L1/d1 (L1 is called the active length) at the very beginning for a new die.

Here is how the compression ratio works: at the first stage, d1 will gradually increase as the friction occurs when the raw material is pressed into the channel, which means the compression ratio (L1/d1) is decreasing. As soon as d1 reaches d2, the active length suddenly changes to L2. As the ratio of L1/d1 is equal to L2/d2, the compression ratio then consequently returns. In this way, the pellet quality is guaranteed and the lifespan of a die is extended.
Generally, the higher the compression ratio is, the, the denser the pellets are, the lower production capability is, and more energy consumed. However, the compression ratio is a dynamic parameter for production different feed stocks. Hence pellet dies with different compression ratios should be careful selected based on the species of your materials in order to produce quality pellets.

How to check the quality of wood pellets?

1. Smell. Smell the odor of the wood pellets from a plastic bag. The high quality wood pellets should smell like the freshly cut softwood. Or you can light a long sawdust pellet at the tip. The smoke should smell of burning wood. If they smell differently, attention is needed.

2. Watch. The wood pellets made from clean conifer sawdust are light brown. Dark brown particles are a sign of bark which means increased amount of ash. Break a pellet to see the color of the pellet layers. The outer layer of the sawdust pellets are darker due to friction pressed by the pellet makers which is not a problem if the inner of the sawdust pellet are light color.

3. Test. Place a small amount of sawdust pellets into a glass of water. The wood pellets will dissolve into sawdust in a moment. If not, a binder agent might have been used or it means the feedstock may be improper. When the pellet dissolved, shake the glass and observe the bottom of the glass where the heaviest particles would be which maybe is a sign that dust from a sanding machine has been added to the pellets. It means increased amount of ash and the risk of sintering.