Many different sources of woody biomass can be made into wood pellets. A fairly obvious source of wood waste is damaged wooden pallets used for shipping purposes. Most bulky products which are transported are done so on wooden pallets. They make it easy for folk lifts to load and unload the products. Recycling wooden pallets for making wood pellets involves several stages of raw material preparation. Skipping any of these stages can either lead to poor quality wood pellets or potentially even damage to the pellet mill.
Recycling Wooden Pallets into Wood Pellets
Overtime and after multiple uses wooden pallets get too damaged to be reused for shipping purposes. Therefore its quite understandable to look at wooden pallets as a source of raw material for making into wood pellets. They can be recycled and processed into wood pellets using either a flat die or ring die pellet mill. However, various stages of material preparation are required first. An example of suitable equipment to process wooden pallets ready for making into wood pellets is the Peterson Pallet Grinder in the video below.
The wooden pallet grinder from Peterson shown in the video above does two important jobs. First, it reduces the wood pallets into a smaller particle size. Secondly, it features a cross-belt magnet to screen the material for nails. You can watch how the cross belt magnet works in the video below:
The cross-belt magnet collects the high-grade steel nails from the shredded wood pallets and deposits the nails into a bin. As you can see from the video, after half an hour of operation there is a considerable amount of metal nails which have been collected. Remember, this waste metal has a value. According to Peterson, some of their clients are able to cover 40% of the fuel costs of the running the wood pallet grinder by selling the nails as scrap metal!
Removing ALL Metal Nails is vital to avoid Pellet Mill damage
I cannot emphasise enough how important it is to remove all metal contamination from the material before it is used in a pellet mill to make wood pellets. The pellet mill rollers and die are set up with a 1mm clearance. Therefore any stones or metal which enter the pellet mill can cause serious damage. Best case scenario the pellet mill die and rollers are scratched. Worst case scenario the metal becomes trapped between the rollers and die. The result would be either tooth on the gearbox sheering or some other drive component to fail.
From the Shredder to the Hammer Mill
Once the wood pallets have been processed via a shredder and the metal nails removed, the material then needs to be processed by a hammer mill. The shredded wood is in particles several inches long (50-100mm). Before the material can be used in the pellet mill to make wood pellets it needs to be reduced consistently to 5mm particles. Therefore the shredded wood is processed via a hammer mill with a 5mm screen fitted. Essentially, whatever size of wood pellet you wish to produce you use a screen size which is slightly smaller. So, to make 6mm wood pellets, a 5mm hammer mill screen size is used.
Metal screening is also important to protect the hammer mill. Metal entering the hammer mill may puncher the screen reducing particle size screening efficiency. It may also spark and ignite dust particles which can lead to a dust explosion.
The Benefits of Wooden Pallets and Moisture Content
Going from a chipper or shredder then on to a hammer mill is similar to the process of making wood pellets from logs. However, the big difference between logs and wood pallets is moisture content. With logs from a recently cut down tree (virgin timber), they will contain a moisture content around 50%. When it comes to making wood pellets, you want a moisture content below 15%.
So with logs, the pellet making process has to involve some form of drying process. However, wooden pallets are a source of dry timber. Therefore the drying process of a pellet making set up can often be avoided. Therefore the cost per tonne when making wood pellets from pallets is less than making wood pellets from virgin timber logs. Not only from the fact you do not need to purchase a dryer, but you also don’t have to cover the costs of running that material dryer.
Final Metal Screening before the Pellet Mill
As stated above, metal contamination can cause serious damage to the pellet mill. Therefore to be extra safe it’s sensible to fit another electromagnet on the material conveyor before it enters the pellet mill. The additional cost of this electromagnet will easily be recovered by avoiding pellet mill downtime and damage to the rollers and die.
Avoid using Wooden Pallets which have been Treated
Presuming the issue of metal contamination can be resolved there is still the issue of treated wood. As the pallets are often left outside various chemicals are sometimes used to try and preserve the wood. Wood fuel pellets with these chemicals will not pass any accredited standards and you could get in a lot of trouble for selling them.
Conclusions on Making Wood Pellets from Wooden Pallets
If the issue of metal contamination can be resolved and only nontreated wooden pellets are used it is potentially a suitable material for fuel pellets. Wooden pallets are appealing as a source of dry wood waste for wood pellet production as dry/nontreated waste wood is becoming harder and harder to source.