What is the ash content of your wood pellets?

Methodology

The amount of ash left when a wood pellet is burned will depend partly on the temperature to which the wood pellets are raised. Different methodologies and standards specify different temperatures. They may also express their results relative to the weight of the wood pellets as received, or relative to the dry content of the wood pellets. You should take care that you are comparing like with like and referring to the relevant testing methodology when considering the ash content. That is why we reference the ash content at two different temperatures in our specification.

Specification

Our top-grade, virgin-fibre 6mm wood pellets are specified to have no more than 0.7% ash content at 550°C (A0.7, using the EN 14961 classification), and 0.5% ash content at 815°C. In practice, they usually have around 0.3-0.5% ash. They are sampled and analysed to ensure that their ash content (and other aspects) complies with the specification.

What to do with the ash

The ash from our top-grade wood pellets is benign (basically minerals and some carbon). If you have a garden, you can spread it on your beds. In moderation, it will act as a fertiliser.

If you put too much ash on too small an area, it will be the same as over-fertilising and will have a negative effect on the productivity of the land.

Ash fusion behaviour

If the ash does not clinker and slag up your furnace, it is not very significant whether the ash content is higher or lower by a few tenths of a percent. What is more important is the ash fusion behaviour - the temperatures at which the ash (a) begins to deform, (b) forms a hemisphere, and (c) begins to flow.

Higher ash fusion temperatures will result in less clinker and slag, greater efficiency and lower maintenance. Although the European standard for biomass fuels (EN 14961) removed any specification for the ash fusion behaviour (requiring only that the deformation temperature be stated), we require our wood pellets to have a deformation temperature of at least 1200°C, in accordance with the A1 standards required for ENplus, the European wood-pellet accreditation scheme. Discussions with equipment suppliers suggest that this temperature should minimise slagging and clinkering for most boilers available today.

Other factors will influence the slagging and clinkering. For instance, dust or other impurities may cause local hotspots, which will cause the ash to melt even if the average temperature in the furnace is below the deformation temperature.

The make and condition of the boiler plays an important role in determining the combustion characteristics and therefore the production of clinker.

Operation also matters. For instance, if you regularly allow your ash-hopper to over-fill and the ash backs up to the burner, the ash that can't escape will affect the combustion characteristics and may start to fuse. Or conversely, it may prevent full combustion resulting in smoke and semi-burnt pellets in the ash.

No boiler will be able to prevent some clinker and slag from forming, so it is important to follow the manufacturer's instructions for occasionally cleaning the boiler (either yourself or under a maintenance contract). But we specify our wood pellets to minimise the frequency that this will be required, all other things being equal.