How does a Flat Die Pellet Mill make Wood Pellets?

If you have started to look into the process of how to make wood pellets, you will have started to look in how the pellet mill works. The image below is of a typical flat die pellet mill. Most flat die pellet mills on the market today are imported from China. Many of them are electrical single phase flat die pellet mills with some being PTO pellet presses. Huge volumes of these flat die pellet mills are being produced for farmers to make their own animal feed. However, there was a growing demand for wood pellets around the world for stoves and boilers. There is now a significant demand for these same small flat die pellet mills to produce wood pellets from home.

Flat Die Pellet Mill
Typical flat die pellet mill from China

How does a Flat Die Pellet Mill work?

Flat Die Pellet Mill Cross Section
Flat Die Pellet Mill Cross Section

The image to the right is a cross section of a typical flat die pellet mill. Material enters from above and is compressed between the rollers and die. The process at first would appear very simple. And when the pellet mill and raw material are prepared properly the description of the process is simple. However, true success at making pellets is in the detail. For instance, you need a raw material at the perfect moisture content.

Die or Roller Rotation?

Flat Die Pellet Mill Roller Driven
Roller Drive Flat Die Pellet Mill

So the first thing to understand is the different designs of flat die pellet mill. Either the die will be driven and the roller shaft is stationary (as in the image above), or the die is fixed the roller shaft rotates around the die. An example of a roller drive flat die pellet mill is shown to the right. In terms of which is a better design, there are advantages to each. The roller drive flat die pellet mills are generally better built and able to cope better with the forces that wood pellet production places on the equipment. However, they are also generally more expensive than the die drive flat die pellet mills. If you do want to make wood pellets I would probably recommend you focus on a roller driven flat die pellet mill design.

Inside the Flat Die Pellet Mill

Flat die pellet mills always work on a vertical process, the material falls from above and pellets emerge from below. This is different to ring die pellet press designs, which can be based on a vertical or horizontal design. There is a lot going on inside the pellet mill, and a lot can go wrong which we will look at below.

Flat Die Pellet Mill
The general principle of a flat die pellet mill is a vertical material flow. Material enters from above and wood pellets emerge from the bottom.

Wood which is Too Wet

Generally, when it comes to making wood pellets with a flat die pellet mill, you want a raw material with a moisture content between 12-15%. If the moisture content is more than this it will probably provide results similar to that shown below:

Flat Die Pellet Mill Too Moist
(A) wood before the roller, (B) the die of the pellet mill, (C) material carpet, (D) compression within the die, (E) the finished wood pellet.

Once the flat die pellet mill is up to temperature a lot of steam will be generated. As it’s not possible to compress water, the wood pellets will not be produced to a suitable density. Hence, soft crumbly wood pellets will be produced (E). However, another result could be a blocked flat pellet mill die.

Wood which is Too Dry?

While it’s not possible to produce quality wood pellets with wet raw material, it’s also not possible with very dry material. You need at least some moisture to melt the natural lignin within the wood to act as a binder to hold the wood pellets together. While using a supplementary pellet binder can aid the process, you still need at least some moisture for the process to work.

Flat Die Pellet Mill Too Dry
If the material is too dry it will not be possible to generate the required heat and pressure to produce wood pellets.

Trying to process very dry woody biomass in a flat die pellet mill the result will just be dust (E). This problem can be addressed if a material conditioner is mounted above the pellet mill. You can then use either drip tanks or peristaltic pumps to add small amounts of water to the raw material before it reaches the pellet press.

Problems with the Flat Die Pellet Mill Design

What flat die pellet mills do suffer from is something called ‘roller slip’ or ‘skip’. On a flat die pellet mill, the inner edge of the die and the outer edge of the die are rotating at different speeds. If you think about it, the outer edge has a large circumference than the inner edge of the die. Therefore under rotation, the outer edge has to cover more distance under one rotation than the inner edge. The problem is the rollers touch both the outer edge and the inner edge of the die at the same time.

flat die pellet mill roller skip
Flat die pellet mill roller skip, the rollers are trying to run more slowly at the blue dots compared to the red dots.

The action of roller skip causes flat die pellet mill generally produce wood pellets to a lower quality when compared to ring die pellet mills. It also results in a more rapid parts wear along with increased energy production. Another issue with flat die pellet mills is that it’s harder to set the proper roller and die gap to make quality wood pellets.

Conclusion on Flat Die Pellet Mills

If you want to produce the best quality wood pellets as part of a commercial pellet plant, personally I would only consider a ring die pellet mill. However, if you are a beginner and looking just to get to grips with how wood pellets are made then small flat die pellet mills can serve that purpose.

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