Pellet mill and granulator – similarities and differences
When searching the internet for information about biomass fuels, and in particular the equipment required to start their production, you will undoubtedly come across two recurring terms: pellet mill and granulator. In some cases, they will be used interchangeably – to describe the same device, while in other contexts they will refer to different types of machines. Sometimes you may even encounter them in the combined two-word form “pellet mill/granulator”.
It is easy to get lost in this complex terminology.
So what is the difference between a pellet mill and a granulator? What are their applications? Let’s find out.
Pellet mill and granulator – presses for extruding granulate
Both pellet mill and granulator are so-called presses for extruding biomass granulate.
This type of equipment is used to process biomass, i.e. natural-origin waste, into uniform pellets of consistent size and density.
The operation of a granulate extrusion press is based on the cooperation of several mechanisms. These are:
Drive motor
The motor provides the energy required for the pellet mill to operate, most commonly in an electric version, although combustion-engine models are also available.
Shredder
The shredder grinds the raw material into small pieces using rapidly rotating blades, preparing it for further processing.
REMEMBER: The shredder finely processes the inserted biomass; however, before feeding the raw material into the press, it should already be pre-chopped, e.g. using a wood chipper. The fraction of the introduced raw material should not be larger than the diameter of the die holes.
Rollers or cylinders
Rotating rollers push the biomass through the die, shaping it into granulate and ensuring proper pellet density. Depending on the mechanism driving the rollers, there are single-axis models with two, three or four tracks. The greater the number of tracks, the higher the machine’s efficiency.
Die
The die is a metal plate with holes through which the biomass is pressed, forming pellets of a specific diameter. This element determines the final shape of the granulate.
IMPORTANT: The type of die is the element that distinguishes pellet presses from granulators. Read on.
Granulator and pellet mill – differences
Since we have established that a pellet mill and a granulator operate on the same principle, what makes us divide them into two separate categories?
The main differences are:
- Application
Pellet mills are machines for producing high-quality biomass fuel – pellets.
Pellet production most often uses wood-based materials such as shavings, chips or sawdust.
A granulator is, by definition, an agricultural device used to process agro-material in order to produce granulated feed for livestock. Due to this function, a granulator is mainly intended for processing raw materials edible by animals, e.g. grain husks. Therefore, it is sometimes called a "feed pellet mill". Granulators in agriculture may also be used to produce granulated fertilizer.
It is worth noting that the raw material processed by granulators is generally softer and, due to its application, granulate from a granulator may exhibit lower quality (uniformity, density) than the specialized fuel that is pellet – the product of a pellet mill.
Therefore, granulators intended solely for agro-material processing (i.e. those models that do not perform both functions) may be less precise and durable.
The margin of error in the production of feed or fertilizers is greater.
- Die thickness
The most important structural difference between the discussed types of biomass presses is the thickness of the die. The general rule for selecting die thickness according to the type of processed biomass states that the softer the raw material, the thicker the die. This means that a pellet mill should be equipped with a THINNER die than a feed granulator, because wood-based material is harder than agro-material.
Although at first this rule may seem illogical, it has functional justification. Harder material has a greater tendency to form blockages in the die. With thicker dies, wood shavings could accumulate in deep holes and hinder the extrusion process. Softer material has a lower tendency to clog the die perforations.
Therefore, dies for feed extrusion are thicker than those dedicated to granulating wood shavings.
For example, for spelt husk it is a thickness of 50 mm, for straw 45 mm, and for pine shavings the recommended die thickness is about 33 mm.
In the case of harder wood species, the appropriate die thickness is proportionally smaller.
IMPORTANT: In many pellet mill models, the die is a replaceable element. Some manufacturers consult the thickness of the installed die with the buyer before selling a granulation press. This explains the presence on the market of multifunctional pellet mill–granulator devices.
Every pellet mill is a granulator – not every granulator is a pellet mill
On the internet, it is not difficult to come across pellet mills/granulators combining the functions of both devices. In our offer, most pellet production machines can also serve as granulators for feed or fertilizers (such as this model). From a practical point of view, this is the best solution, because when designing a device intended for the more technically demanding production of pellets, the requirements for agro-granulate extrusion are also met, as it were, incidentally.
Pellet mills designed for producing fuel from various wood species are by default equipped with the possibility of die replacement. In most cases, nothing prevents replacing it with one that enables forming feed or fertilizer pellets.
Summarizing this complex rule: Every type of pellet is granulate. However, as we have established, there are many types of granulate and definitely not every granulate is pellet. It directly follows that EVERY pellet mill is a type of granulator and can successfully perform the function of a feed granulator, but NOT EVERY feed granulator (for technical reasons) is at the same time a pellet mill.
In short:
A pellet mill is the most versatile device in the category of biomass granulation presses and can be used for various purposes, both industrial and agricultural.
