In the PelHeat Mobile Pelletizer we are using flat die pellet mills. These pellet mills consist of a die, which is horizontal mounted with rollers resting on the die applying vertical pressure down onto the die surface. Below is a diagram to demonstrate the process.

A = Raw Material
B = Die
C = Material Carpet
D = Pellet Compression
E = Pellet

There are two main types of pellet mill used to produce pellets, and they are ring die and flat die, the reasons we are adopting a flat die pellet mill are as follows.

Size and Weight
Flat die pellet mills are a lot more compact than ring die pellet mills. Building a mobile pelletizer on a trailer platform space to place the equipment is very limited. Therefore a flat die pellet mill is better suited for the application on this scale.

Maintenance
In designing the mobile pelletizer we are focused on making the unit as easy to maintain and clean as possible. This includes the pellet mill, for example on how easy it is to disassemble the pellet mill to change or clean the die. Ring die pellet mills are notoriously awkward to access to clean and change the die. However with the flat die pellet mill the die can be removed within a couple of minutes.

Visibility
As stated in the PelHeat Making Pellets Guide and throughout the website and blog I indicate that producing pellets from multiple biomass raw materials is a skilled process. As such not every material when placed in the pellet mill will create good pellets first time. With ring die pellet mills the pelletizing chamber is sealed and covered. With the flat die pellet mill it is possible to view directly into the pelletizing chamber. By viewing how the material performs in the chamber and what quality of pellets are produce the user can use that information to alter the material. A view of the pelletizing chamber also gives an indication if the die has become blocked, so it can be cleaned or changed as quickly as possible.

Summary
Ring die pellet mills have their place in the industry, particularly large scale pelletizing, however flat die pellet mills are more suitable for small scale applications.

Thank you for your interest

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Reed Canary Grass or RCG is another developing biomass fuel source, unlike Miscanthus, which is grown from rhizomes; RCG can be grown from seed. However the yields per acre of RCG are generally less than Miscanthus. With regards to burning RCG, its corrosion levels are slightly less than Miscanthus, and it produces roughly the same percentage of ash. Again check that your pellet burner can handle this.

We were donated a bale of RCG from a friend, the bale had a moisture content of roughly 30%, which is far too high for pelletizing so we placed the bale In the barn and spread some material out, so it could dry. When RCG is harvested the moisture content is practically ideal for pellet production, this bale however must have left outside. After a week or so we decided to process the RCG to see what would happen. As the material is not as dense as the Miscanthus it was easier to pelletize. With the first run through the pellet mill pellets were formed, but they did not have smooth surface, and were expanding slightly due to the moisture content still been too high. Once the pellet mill was up to temperature we placed the material through the mill again, and hard compact pellets were formed.

Our aim with every potential biomass fuel source it to learn the specific moisture contents required to produce a pellet first time, and design processes to make the process as simple as possible. Developing better knowledge in material preparation will result in high productivity.

Thank you for your interest

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Miscanthus is becoming very popular around the world as a purpose grown biomass fuel. Its high yields result in large quantities of fuel generated quickly, and provides another source of income for farmers. Miscanthus does contain a lot of energy, and will generate a lot of heat when burnt. However it does sometimes cause complications with high chlorine levels causing corrosion, and higher ash levels than some systems can handle.

We were donated a Miscanthus bale from a friend. We had only seen Miscanthus in pictures before, and the real thing surprised us. It is very hard, and looks and feels almost identically like bamboo. It’s strength enables the crop to grow high, producing the high yields.

If you crack the Miscanthus the centre is quite soft, and also contains a lot of energy. The crops had roughly 12% moisture content, we tried to run the material through the equipment, however it was too dry so would not compress. We added a little bit of moisture and the material could then bind together to produce pellets. Below is a picture of a Miscanthus pellet.

Our first impressions on trying to pelletize Miscanthus were that it requires more moisture than other materials, but also sometimes lacks binder. Even though Miscanthus clearly has a lot of binder, as it is so hard, releasing it during the pelletizing process does require the pellet mill to get up to temperature. In one test we actually mixed some wood with the Miscanthus to aid in pellet production. However we are confident with further experiments in changing the moisture content of the raw material we can improve Miscanthus productivity.

Thank you for your interest

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Straw is a common biomass fuel, but in its raw form is difficult to burn, as it is difficult to get the material into the burn chamber quick enough. There are large-scale applications where entire bales are placed in the burn chamber, however these systems are very expensive, and very large.

Converting straw into pellets removes the problem, due to the high density and energy in the pellets; they contain a lot more fuel in a smaller profile. This means small-scale units can take advantage of this widely available fuel. However you must make sure that your pellet burner can handle the high ash and corrosion straw pellets produce.

We were given a few bales of straw from a local farmer, and unlike the wood chippings the moisture of the straw was perfect, creating strong shiny pellets first time. This time we used the 6mm die plate; instead of the 8mm die plate we used for producing the wood pellets.

As straw is more malleable than wood, it is easier to compress into a pellet, so productivity of the pellet mill is higher. We are also very interested in the potential of rapeseed straw. Rapeseed is grown for producing bio-diesel, however the straw is not used and seen as a waste product. This straw will still contain oil, and have a very high heat value. We hope to acquire some in the future to make some pellets.

Thank you for your interest

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Wood pellets are currently the most common ‘biomass pellet’ that is produced. Below is a picture of some wood pellets we have produced; a friend had cut down a Cherry Tree in his back garden, he then gave us the wood in chipped form. At that time the wood was extremely wet, roughly with 30-40% moisture content, and for making good quality wood pellets, moisture content of roughly 15% is required.

We left the material to air dry in the barn for a few weeks; we then decided to put it through the hammer mill and then into the pellet mill. The material went through the pellet mill, but the end result was basically a woody mush. After a few minutes the pellet mill was up to temperature, so we decided to place the woody mush back into the pellet mill. The third pass produced the quality pellets you see in the picture below, which are extremely dense and hard wearing.

Because of the heat generated in the pellet mill it can be used to create pellets instantly, with a few passes of the material. However this clearly reduces productivity as the material has to be processed multiple times, but it’s a useful feature. There are material driers on the market, but we are also experimenting with low costs solutions in controlling relative humidity to control the moisture content of the raw material. We hope that using this method we will be able to control the moisture content of the material to whatever percentage we require. With the material at the correct moisture content the PelHeat Mobile Pelletizer can be used as a continuous process. With the material entering the hammer mill, through to the cyclone and into the pellet mill. Also correct moisture content first time means that maximum productivity for that material is possible. I will provide more details of the experiment shortly.

In the future we will provide more information on pelletizing different varieties of wood including waste wood and cardboard.

Thank you for your interest

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