What are the Challenges of Making and Burning Cardboard Pellets?

Back in 2008 when I purchased my first Chinese pellet mill and I was exploring how to make pellets I started to play about with making cardboard pellets. Something I quickly learnt was that making cardboard pellets presented several unique challenges that I’d not experienced with other biomass materials. However, I was eventually able to produce pellets after a long process of trial and error.

Cardboard Pellets
These are the first cardboard pellets I ever produced.

The Process of Making Cardboard Pellets

So the first part of the process is actually collecting together a sufficient volume of cardboard waste. Some waste is just paper-based and with light brown colouring. However, some waste has a waxy outer layer. Either type of waste can be compressed into pellets. However, you have to consider how the cardboard pellets will be used.

Cardboard waste for pellets
Various different cardboard wastes can be processed into pellets.

Potential uses for cardboard pellets include either as fuel or an animal bedding. The waxy cardboard produced more ash when burnt, more details on that below. Also, the waxy material can produce some very odd smelling combustion gases. Therefore I would avoid using waxy materials for fuel pellets. Before you can use the cardboard within a pellet mill it’s obviously got to be reduced in size somehow.

Processing Cardboard in a Hammer Mill

At the time I only had access to hammer mill for raw material size reduction. So first I ripped the cardboard into strips and fed it into the hammer mill. After just a couple of minutes of processing material, it became apprently this was not like processing any other biomass material. The cardboard that came from the hammer mill was not in small 5mm particles, it was more like candyfloss!

Hammer MIll
Processing cardboard with a hammer mill is not ideal and creates several issues : Stedman-machine.com

With other biomass such as wood and straws, the hammers hit the material and smash it into smaller particles that then pass through the 5mm screen. Cardboard is a soft material made of long thin wood fibres. So the hammers are not able to smash the material, it tears into the material and breaks the fibres apart. However, the hammer mill in doing so also generates a static charge, which means the end result is a very fluffy mass!

Separate Staples from the Raw Material

Much of the waste will contain staples. To avoid damage to the pellet mill it’s very important that you are separate out this metal contamination. On a small scale, you can sort through the raw material before it is fed into the hammer mill and rip out the staples. However, in a commercial operation thats not viable. In that case, an electromagnet should be used before the pellet mill. This will take out the staples before the material enters the pellet mill.

Alternatives to using a Hammer Mill when Making Cardboard Pellets?

Ideally, if you’re going to try and make cardboard pellets you want to get hold of a cross-cutting shredder. A shredder which can crosscut the cardboard into 5mmx5mm squares would be perfect for feeding into the pellet mill. However, there are very few crosscutting shredders of a suitable size to process cardboard sheets. Hence why I’m not able to recommend any such products. However, if you are able to find such a cross-cutting shredder, its a much better option than a hammer mill when it comes to making cardboard pellets.

Cardboard Cross Cutting
The best way to process cardboard for making pellets is to use a cross-cut shredder – Image: recycling.com

How does Cardboard Perform in the Pellet Mill?

Cardboard in the pellet mill actually performs pretty well. It’s possible to produce high-density cardboard pellets with no additional pellet binders. However, the moisture content of the material is far too low in most cases for pellets to form. Therefore enough water needs to be added to get the cardboard to between 12-15% moisture content to produce the best pellets.

Are Flat Die or Ring Die Pellet Mills best for Cardboard Pellets?

I’ve tried making cardboard in both flat die and ring die pellet mills. Milled or shredded cardboard is quite a light material, it doesn’t flow well through transfer augers. Therefore this is actually a scenario where a simple flat die pellet mill can actually perform better than a ring die pellet mill for making cardboard pellets.

Flat Die Pellet Mill
Basic flat die pellet mills can actually do a reasonable job of making cardboard pellets.

What are the various uses for Cardboard Pellets?

The two main uses are as either fuel or animal bedding. However, it’s important to note that when used as fuel pellets the ash content could be as high as 7-10%. Many pellet burners can only deal with wood pellets up to a 1% ash content. For instance, the pellets would not work in a simple portable pellet stove. So clearly, the market for fuel pellets is limited.

Really, the best market for cardboard pellets is animal bedding or for litter boxes. In this scenario, the high ash content that the pellets produce is not an issue. I hope you found this post interesting, please visit the home page for the various other posts I’ve made on making, burning and using pellets.

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