If you own a pellet stove, boiler or BBQ or you’re looking to purchase one there is a very important lesson to learn. You must keep your wood pellets away from moisture and dry. Otherwise, you will end up with a wet mush that can only be used as mulch on your garden.
To emphasise this point woodpellets.com have produced the video below. It shows what happens when wood pellets come into contact with water. Now, granted the video below is an extreme example. I’ll talk about more realistic wood pellet moisture issues below. However, first please watch the video to see what will ultimately happen if your wood pellets are exposed to moisture.
As you can see in the video, after pouring water onto the wood pellets after just two minutes they start to puff up and expand. After three minutes the wood pellets in the base of the bag (which have absorbed the most moisture) are now just sawdust. Four minutes later due to the expansion of the wood pellets, the bag is becoming ‘like a brick’ and taut. After just five minutes the wood pellets are mush. With a cut in the bottom of the plastic bag, the wet expanding wood pellets pour out.
Why do Wood Pellets Expand when in contact with Water?
The reason wood pellets expand when they come into contact with water is due to how wood pellets are made. First, the raw material is dried to a moisture content suitable to make wood pellets, typically between 12-15%, and then compressed in the pellet mill. Typically, with a suitable compression ratio to produce premium grade wood pellets, the sawdust will be reduced in volume roughly 6:1.
Due to the low moisture content of wood pellets (below 10%) and their high density they now have hydrophilic properties. What that essentially means is that wood pellets are highly absorbent and are searching for moisture. This is true for other biomass fuel pellets, such as grass pellets and hemp pellets. The hydrophilic properties of wood pellets mean they can also be used for other purposes besides fuel. However not all wood pellets absorb moisture (more details below).
Liquid Water is not the real Problem, its Humidity
So as I mentioned previously, the example in the video of pouring water on a bag of open wood pellets is an extreme example. The real problem with wood pellets is humidity. Many people store their 40lb bags of wood pellets in their basement or a shed. While these environments are protected from the rain they tend to be quite damp and humid. Therefore if bags of wood pellets are left open or there are tears in the bags the wood pellets will pull the moisture out of the air. Eventually, the wood pellets will expand back into sawdust. While it won’t happen in 5 minutes as with liquid water, the wood pellets will eventually expand and break apart.
What happens if you use Damp Wood Pellets in a Stove, Boiler or BBQ?
There are some pellet burners manufacturers who will tell you to never use open bags of wood pellets in their equipment. For instance, Clarry produces a portable pellet stove, and their advice is to discard open bags of wood pellets. However, if wood pellets are stored in a low humidity enviroment they will maintain their density. For instance, my wood pellets are just stored in my custom silo room loose, however, its a warm environment with low humidity.
There are a couple of reasons manufacturers are concerned about users putting damp wood pellets in their equipment. First, damp wood pellets will struggle to ignite and will also create more smoke, ash and potentially even creosote. Also, while damp wood pellets are not sawdust mush (yet) they do break apart more easily. Damp wood pellets have a reduced density, and they produce more fines (dust). This dust can be an issue for some automated feed augers depending on their design and quality. Cheap augers with underpowered motors can block up with the fines.
Which Wood Pellets Don’t Expand in Wet and Humid Environments?
So both soft and hard standard wood pellets will expand when the humidity of the environment in which they are stored is too high. However, there is a type of wood pellet which won’t expand and turn to mush. Amazingly, you could even leave these wood pellets out in the rain and they would be fine! They are called torrefied wood pellets, and through a process called torrefaction, the wood becomes hydrophobic.
A stated above standard wood pellets are hydrophilic, which means they attract and absorb moisture. Torrefied wood pellets are hydrophobic, so they repel moisture. The process of how torrefied wood pellets are made is very interesting, and I encourage you to read my post on them. Unfortunately, currently, there are very few manufacturers of torrefied wood pellets. So for the moment leaving your wood pellets out in the rain and then putting them in your pellet stove is not an option.
How can you Test if Wood Pellets are Damp?
So how can you test if your wood pellets have started to absorb moisture from humid air? Well, first the appearance of the wood pellets will change. A quality wood pellet will have a smooth and shiny outer surface. If you can see lots of cracks on the surface of the pellets that’s a good indication they have started to absorb moisture.
The second test I like to call the Snap Test. Essentially you put the wood pellets under horizontal force. A good quality wood pellet will break cleanly into two pieces and with an audible ‘snap’ sound. Wood pellets which have started to absorb moisture will break apart into lots of smaller pieces and create lots of fines (dust).
However, not all wood pellets that don’t pass the snap test are damp. During the wood pellet production process if insufficient moisture is present the natural lignin within the wood will fail to bind the pellets together. However, if we presume the wood pellets you have purchased had passed a premium grade test they should also pass the snap test if they are not damp.
The Absorbency of Wood Pellets does have its Benefits
While the moisture absorbency of wood pellets is not a positive feature when it comes to combustion, there is an application where it is a benefit. Using wood pellets for horse bedding is growing in popularity. Typically wood shavings and straw would be used as bedding materials. However, they can contain quite a large amount of dust which is bad for the health of the horses and can cause respiratory issues.
Wood pellets (made to a certain quality) have a very low percentage of fines (dust). And the high absorbency characteristics of wood pellets means the bedding needs to be changed less frequently. However, wood pellets are very hard to start with. Therefore, best practice is to actually add some water to the bedding to start with. However, these damp softer wood pellets can still absorb a lot more moisture.
Conclusions on Keeping Wood Pellets Dry
When it comes to storing bags of wood pellets do make sure to tape up any tears in the bags to avoid moisture getting in. Also, try to avoid keeping bags in a damp basement or shed where possible.
When it comes to combustion performance of wood pellets, just remember water doesn’t burn very well! Moisture will mean its harder to ignite the wood pellets, more smoke will be produced and therefore potential issues with creosote.
I hope you found this post useful. Please check out my other posts on the home page about making, burning and the other uses for wood pellets. 🙂