How do you Avoid a Blocked Wood Pellet Mill Die?

When it comes to making wood pellets the biggest problem that you will come across is a blocked pellet mill die. In this post I wish to explain some of the most common reasons a pellet mill can become blocked and how to go about fixing the issue. First I’ll provide some highlights on how to avoid a blocked wood pellet mill die.

How To Avoid a Blocked Pellet Mill Die (Highlights)

  1. Prepar the Pellet Mill Die

    A polishing material needs to be processed through the pellet mill to prepare a new die or to clean up the holes within an older/used die.

  2. Prepare the Raw Material

    If you wood is too moist (above 15% moisture content) or the particles are too large (above 6mm) this could also lead to a pellet mill die blockage.

  3. Start by Feeding the Pellet Mill Slowly

    Until a pellet mill die is up to its optimum operating tempreture (80 degrees celcius +) its very easy to feed in too much material which will lead to a die blockage.

So the above is the ‘highlights’ of what leads to a blocked wood pellet mill die, now let’s discuss the above points in more detail.

1. Preparing the Pellet Mill Die to Avoid Blockages

Wet Wood will lead to a Pellet Mill Die Blockage
Using wet wood in the pellet mill is likely to lead to a die blockage

So something worth noting is that it’s just as easy to block a new die as it is to block an older used die, but for slightly different reasons. A new die (particularly if purchased from China) will have imperfections within the die hole from the drilling process. The hole may appear smooth on its inner surface, however commonly the drilling process will also create small scratches. Even these small scratches can create enough resistance during making pellets to lead to a die blockage. Remember, if the pellet mill rollers cannot generate enough force to overcome the resistance of the die holes, that leads to a blockage.

An older die which was previously used to make wood pellets but has been left idle for more than a few days can create a different issue. Moisture within the air or from a previous production run will lead the die forming rust/corrosion on the inside of the die holes. This again makes the inner surface of the hole rough, which means the roller cannot generate enough force to push the material through the die, leading to a blockage.

How do you Polish a Pellet Mill Die?

How To Avoid a Blocked Pellet Mill Die
Even with a new pellet mill die, you want to run a polishing material through the machine to avoid blockages.

So to resolve the issues noted above of a rough surface within the pellet mill die holes, we need a polishing material to pass through the machine. This will typically be a fairly loose biomass material (I prefer using wheat bran), about 10-15% vegetable oil and about 1-2% fine sand. You mix this material thoroughly and slowly process it repeatably through the pellet mill several times. This polishing material will help to clean up the surface of the die holes and remove corrosion.

Pro Tip: Use a polishing material sparingly as if used too often reduce the life of the die and rollers by wearing away their surface. Once the inlet tape on a die has warn away the compression ratio on the die will be unsuitable to make good quality pellets.

2. Preparing the Raw Material

So we have addressed the first issue which can lead to a block pellet mill die by polishing the die holes so they are nice and smooth. Now we need to examine the raw material to make wood pellets.

What is the Particle Size of your Wood?

Hammer Mill Screens
Use a 5mm hammer mill screen to produce 6mm wood pellets.

You need to make sure your woody biomass has been properly processed through a hammer mill with a 5mm screen. This is to ensure that the particles you are placing in the pellet mill (with a 6mm die) are smaller than the diameter of the die hole.

Pro Tip: You also need to make sure there is no contamination within your raw material (stones/metal). These cannot only lead to a die blockage they can also cause some serious damage to the pellet mill itsself. This could include a damaged gearbox or motor.

What is the Moisture Content of your Raw Material?

Pellet Mill Die Blockage Due To Moisture
If steam (C) cannot leave the pellet mill, it will be absorbed by the raw material. This will likely lead to a die blockage.

As you may know from reading the main how to make wood pellets page, knowing and controlling the moisture content of your raw material is vital. The general rule to make wood pellets is you want the raw biomass below 15% moisture content. However, the material also needs to have a consistent moisture content throughout the batch. Taking a sample of raw material which is not consistent may produce a reading between 12-15%, which will produce good pellets. However, if there is a section of material above this, say 20%+, this could easily lead to a pellet mill die blockage when it enters the pellet mill. Therefore preparing the raw material to have a consistent moisture content throughout the batch is essential. No only to avoid blocking the pellet mill die, but to produce consistently good quality wood pellets.

Pro Tip: Even if you have a raw material prepared with the ideal particle size and moisture content, a pellet binder should be considered. A pellet binder can aid the process and reduce the chances of a blocked pellet mill die.

3. Feeding the Pellet Mill Slowly to Avoid a Die Blockage

Blocked Pellet Mill Die
Feeding a cold pellet mill too quickly will also likely lead to a die blockage.

Good quality wood pellets are only produced once the pellet mill and die get up to operating temperature, which is over 80 degrees Celsius. At this point, the natural lignin within the wood will melt and bind the pellets together. However, before the pellet mill reaches this operating temperature it is very easy to block the die if the raw material is fed into the die more quickly than it can process the material. You want to make sure there is sufficient material in front of the pellet mill rollers, but not to completely cover them.

Pro Tip: The pellet making process creates heat, which releases moisture from the raw material. Filling the pellet mill processing chamber with raw material stops this moisture escaping (steam). This steam is then absorbed by the incoming raw material, which means a raw material above 15% moisture, leading to a die blockage. Hence, only feed the pellet mill at a rate which it can process.

As the temperature of the pellet mill rollers and die increase the productivity of the pellet mill will also increase. At this point, you can increase the rate of raw material feed into the pellet mill. However, you still don’t want to over feed the pellet mill to stop steam escaping. Even at operating tempreture this will still likely lead to a die blockage.

What do you do once you have Finished Making Wood Pellets?

Ok, let’s presume you have finished making wood pellets. You obviously want to leave the pellet mill in a condition that means the die is not blocked for the next production run. Therefore for the last few minutes of operation add in quite a bit of vegetable oil into the mix. This will leave a soft oily material in the die, and make it much easier to start making pellets again next time.

Pro Tip: If you are leaving the pellet mill for more than a few days you will need to run the polishing material through the die again. This will clean off any corrosion/rust which has formed in the die holes.

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