What size are your wood pellets? | Forever Fuels

Wood pellets are cylinders whose size is described by two dimensions: the diameter (i.e. the width) and the length.

Forever Fuels supplies ENplus A1 wood pellets, with a diameter of 6mm, and length in the range 3.15mm to 40mm (and less than 1% upto 45mm).


Forever Fuels supplies wood pellets with the standard diameter required by most boilers in the UK: 6mm.

8mm pellets are also permitted by the ENplus A1 specification. In many ways, 8mm wood pellets are better than 6mm, but the makes of boiler that dominate the market (from Austria and Germany) are configured by default for 6mm, and their installers are reluctant to re-configure them for 8mm.

For those who remember the video format wars, 8mm pellets are Betamax to 6mm's VHS. They may be better, but network effects outweigh technical superiority in the market. The UK market is almost completely coalesced around 6mm nowadays.

ENplus permits a tolerance of 0.5mm. In other words, wood pellets are considered 6mm if their diameter is between 5.5mm and 6.5mm.


The ENplus A1 specification states that the wood pellets will be no longer than 40mm, except that upto 1% may be upto 45mm in length.

There is no minimum length spec. Fines are defined as anything under 3.15mm in length, so the effective length range is 3.15mm to 45mm.

There is no size-distribution parameter. A load is technically in spec even if it consists entirely of 3.2mm fragments. But it is out of spec if it contains a single 46mm wood pellet.

Forever Fuels supplies to the ENplus A1 specification, even though this spec is crazy and clearly written by people who do not understand standards or pellet quality.

Size distribution

There is no practical difference to the user between a 3.2mm particle and a 3.1mm particle. But the specification permits as many as you like of the former, but no more than 4% (delivered) or 1% (at loading) or 0.5% (bagged) of the latter.

The specification puts a tight limit on fines (particles shorter than 3.15mm) because they have practical impacts on your boiler. Smaller particles (a) flow less well, (b) fill the gaps between the whole pellets, (c) burn quicker, and (d) are more likely to become entrained in the combustion air.

(a) means that your fuel-feed is likely to struggle more to feed the required rate of fuel to the boiler if there are more smaller particles in the fuel.

(b) means that the bulk density will be higher. That may result in the wrong fuel-feed rate to the boiler, because the feed-rate may be based on an assumption that a given number of rotations of the auger equates to a given amount of energy, which assumes that the bulk density and calorific value will not vary too widely.

(c) will result in localised higher temperatures in the furnace, which will lead to increased slagging and clinkering, and a reduction in your boiler's efficiency. It is also likely to result in high NOx emissions.

(d) will result in deposits accumulating on the heat exchangers, reducing your boiler's efficiency, higher particulate emissions (i.e. smoke, with the associate health impacts) and increased NOx or CO emissions, because the combustion characteristics will not be controlled and are likely to occur with either too much or insufficient oxygen.

Some of these effects will occur just as much with 3.2mm particles as with 3.1mm particles. The standards should, therefore, have applied not just a black-and-white size threshold, but a more subtle size-distribution parameter. They might, for instance, have specified a minimum average length, so that not too many of the particles could lie at the small end of the spectrum. Or they might have specified a maximum proportion for particles in the range 3.15mm to 6mm in length.

In what we believe is the first of its kind in the UK, that is what we have agreed for 2016/17 with our largest supplier. Although it is not in the standards, they have voluntarily agreed (because we made it a condition of supply) that only a small proportion of the load will be shorter than 6mm. They felt confident to do this because their pellets have a very high durability and average length.

Combined with our superior screening technology and gentler deliveries from our pressurised tankers, you can be more confident with us than with other suppliers that the pellets that arrive in your store will be mostly whole pellets. 

Maximum length

There is no pellet manufacturing process that can guarantee never to produce a wood pellet longer than 45mm. Occasionally, a wood pellet will not be broken by the breaker bar, and will exceed the maximum permitted length. Indeed, the higher the quality, the more likely this is to happen, because a pellet that does not disintegrate much along the supply chain is a pellet that may also not break on the breaker bar. High durability increases the risk of an occasional long pellet in the load.

40mm pellets blocking a 50mm pipe

The length limit is based on a fallacy. It was set because equipment manufacturers wanted to standardise on a 50mm pipe for suction transport between a silo and a boiler. Using a 60mm pipe would have required a motor that was 44% more powerful to achieve the same air velocity. The thinking was that wood pellets should be no longer than 45mm so that they will not block a 50mm pipe.

But 40mm pellets can block a 50mm pipe. The illustration demonstrates how. A wood pellet will not necessarily bridge directly across the middle of the pipe. It may bridge across one side, and then others may bridge on it, and so on. And this does not allow for the internal coefficient of friction between the wood pellets, nor suction pipes with multiple bends, nor store designs that concentrate the weight of the mass of pellets on a narrow outlet. These forces will push the wood pellets together and make them less inclined to flow. Multiple bends reduce the effectiveness of the suction fan. A combination of these factors may result in blockages with wood pellets even shorter than 40mm.

Some pellet suppliers try to work round this impractical constraint by claiming that the test of whether a load is out of spec is whether an over-length pellet can be found in the reference sample taken when the truck was loaded. This sample may be as small as 500g, but let's be generous and say that these suppliers take a 1 kg sample. On a 5-tonne load, the sample represents 1/5000th of the load. If the load contained a dozen long pellets, there is only one-quarter of one percent chance that one of the long pellets will end up in the reference sample. Yet, with a dozen long pellets in the load, at least one of them is likely to bridge on a 50mm suction pipe. So you could have a significant chance of an operating failure, and yet a negligible chance of getting these suppliers to admit that the wood pellets were out of spec.

Forever Fuels accepts that even one long pellet is out of spec. If you experience a blockage, discover that a wood pellet longer than 45mm is the cause, and send us a photo of the offending article against a ruler to prove it is overlength, we will remove the remainder of the load at no cost to you, regardless of whether we can find a long pellet in the reference sample.

The rules are foolish, but we accept that they are the rules until the industry agrees more sensible rules.


Sustainable wood pellets, authorised by the Biomass Suppliers List (ID:BSL0037716-*) From sustainably-sourced virgin fibre, certified by the FSC (Cert.No. C130395) ENplus A1 accredited wood pellet trader (ID:UK302) Operating to ISO 9001 standard, audited by ACS Operating to ISO 14001 standard, audited by ACS Wood Heat Association member UK Pellet Council member REA Awards Winner 2015: Company of the Year