Making wood pellets on a small scale is a challenge, this is just a fact. There are many different variables which you need to be in control of to produce good quality wood pellets. When you start to learn about how to make wood pellets you will notice the issue of raw material moisture content gets mentioned a lot. There are two main challenges. First, trying to figure out what is the perfect moisture content for the raw material to produce the best quality wood pellets. Secondly, how do I get the raw material to that perfect moisture content?
Generally, when it comes to making wood pellets I’ve found a moisture content of 12% works best most of the time. However, for different species of wood increasing the moisture content slightly up to a maximum of 15% moisture sometimes has benefits. You only really know once you start to process the raw material through the pellet mill. You’re looking for a shiny wood pellet with a smooth surface that breaks off cleanly when leaving the pellet mill. This tells us the natural lignin within the wood has been heated sufficiently to bind the wood pellets together. It should be noted that making wood pellets from logs presents a significant challenge when it comes to material moisture content.
The Challenge of Controlling Moisture Content on a Small Scale
With large scale wood pellet production plants, they have a lot of equipment and a lot of automation. For instance, they can have equipment such as infrared moisture meters constantly scanning the raw material to asses its moisture content. They then have large batch mixers which help to prepare the raw material to the ideal moisture content. Using peristaltic pumps if they need to add moisture they can precisely add in just enough moisture to produce good quality wood pellets very consistently.
On a small scale, the wood pellet making process is very different. Infrared moisture meters are not viable due to their cost. There are various handheld moisture meters on the market, however, you have to be careful. Some moisture meters do not produce accurate results for sawdust and wood chips. On a small scale often the best option is the dry weight test.
What is the Dry Weight Moisture Test?
First, you need some accurate weigh scales that can measure grams (g). I appreciate not everyone uses the metric system, using a weigh scale that can accurately measure ounces is also suitable. Measure out 100g (remember to discount the weight of your container). You then need to dry the material on a low to moderate heat source. You can use a microwave if you’re careful. Frequently measure the weight of the raw material until its weight remains constant. Once its weight stops dropping you have then reached 0% moisture. The difference in weight from start to finish is the percentage (%) of moisture the raw material contains.
So for example, if your raw material now weighs 88g the difference from 100g is 12g. Therefore the moisture percentage of the raw material is 12%. For many woody biomass materials 12% moisture content is often the perfect moisture content to make wood pellets. However, we will get onto that later. One word of caution when drying the raw material. Make sure you do not go beyond 0% moisture and start to burn the raw material. This will mess up the calculation and make it appear that the raw material contains more moisture than it really does.
Is 12% Moisture Content Perfect for Making Wood Pellets?
Quite often I’ve found that a consistently prepared woody biomass raw material at 12% moisture produces the best results. Now consistency is another important aspect of the pellet making process. For instance, let’s say the batch is fairly consistent at 12% moisture, but there is a pocket of the material above or below this. Well, when that pocket of material enters the pellet mill a couple of things could happen.
From Wood Pellets to Dust
So the pellet mill maybe happily processing the material into wood pellets at 12% moisture, then a dry pocket of material enters the machine. The lack of moisture will affect the compression within the die. It will also affect the ability of the pellet press to generate sufficient heat and pressure. This could potentially lead to the process stopping, as the rollers cannot gain traction on the material. Or, instead of wood pellets being produced only dust may emerge from the pellet mill.
Lots of Steam and Blocked Pellet Mill Die
If the pocket of raw material is considerably higher than 12% moisture content there is a possibility that this could lead to a pellet mill die blockage. The first you thing you will notice is considerably more steam emerging from the pellet mill. You will then start to hear the pellet mill motor struggling. With an internal combustion engine, you will hear the motor start to labour. If it’s an electrical motor you will see a spike in the amps the motor is using. If this continues then is likely the engine will stall or the electric motor will trip its breaker.
Lower Wood Pellet Durability
The previous two examples above of either dust or a blocked pellet mill die are at the extremes. The other possibility is just reduced wood pellet quality. You may notice that the pellets produced do not have a shiny surface, or they are emerging from the pellet mill with small cracks. These lower quality wood pellets will have lower durability. They will break more easily producing fines (dust). The consequences for the end users could be significant. For instance, with wood fuel pellets fines will produce more ash content during combustion. It will also produce more smoke. For animal bedding products it will be detrimental to the animal’s health. Wood pellets for horse bedding is a premium product. However, customers purchase them for the lower dust content compared to wood shavings. Therefore, if the wood pellets contain a lot of fines then this will lead to frustrated customers.
Drying Woody Biomass on a Small Scale
With large scale pellet plants, rotary dryers are used. However, on a small scale, rotary dryers are just not viable. Another option that is available is pipe dryers, also known as flash dryers. This option is used more frequently for making wood pellets on a small scale, however, there are some downsides. Flash dryers are often made up a solid fuel stove, large fan and a long section of snaking pipework with a cyclone separator at the end. The fan pulls the smoke from the stove through the pipework along with the raw material you are trying to dry. How successfully it does this depends on the moisture content of the raw material to start with.
Drying Retention Time
The raw material is pulled through the pipework and dried in one single pass. However, the retention time is very low, as the raw material is carried in the hot smoke for only a second or so. Now if the raw material is below for example 25% moisture content this may be sufficient time to dry the material to below 15% moisture. However, if the raw material was produced from virgin wood above 50% this would not be sufficient drying time. Therefore it would require multiple passes through the dryer to get to a moisture content below 15%.
However, repeatedly passing material through the dryer is also likely to lead to a raw material below 12% moisture. Therefore the process is not that efficient and can lead to wasting fuel overdrying the raw material. Furthermore, passing a fairly dry material through the flash dryer also carries significant fire risk.
Flash Dryers Lack of Precise Process Control
Large scale rotary dryers have the benefit from a controlled retention time as the operator can either speed up or slow down the rotation of the dryer. There are blades within the rotary dryer which moves the material from one end of the dryer to the other. This lack of precise process control trying to dry woody biomass on a small scale is often why it’s important to only source a dry material to process into wood pellets.
Conclusions on the Perfect Moisture Content to Make Wood Pellets
You will not know what the perfect moisture content is for your raw material until to have tried to process the raw material. It’s best to get the raw material to an average of 12% moisture and see what happens. On a small scale, it’s unlikely that dryer than 12% will produce good quality wood pellets. However, slightly higher than 12% up to 15% may produce better results. It comes down to trial and error. However, it is worth noting when making wood pellets on a small scale, pellet binders can provide a useful helping hand to smooth out the process.