Many issues for suppliers of wood pellets are similar to those for wood chips. With pellets however composition, calorific value, ash content, contamination and mechanical durability are much more significant than with chips.
In general moisture content of wood pellets is less of an issue than for wood chips, though should still be specified, being typically 8-10%.
Particle size also tends to be less of an issue than for wood chips, provided pellets of the right size are requested. Typically these are:
- 6 mm or 8 mm for domestic and small scale systems
- 10 mm or 12 mm for larger and industrial systems
A high proportion of bark in the sawdust used to produce the pellets will lead to higher ash content.
The presence of minerals such as heavy metals, high levels of alkali and alkaline earth metals, sulphur, etc. from the soil and air of the growth environment, or a range of potential contaminants from the use of recycled timber can potentially be an issue.
The mix of timber used, the presence of any additives such as binders, the quality of grinding and pressure used in production and hence final density, as well as the moisture content will all have an influence on the calorific value.
Wood pellets can absorb moisture from the air if stored in a damp environment, and this can lead to them becoming crumbly and breaking down to sawdust. The mechanical durability of wood pellets is important, especially if they are to be subjected to more than a few handling steps. This can particularly be the case with pellets imported from overseas which may have been subject to many handling processes during loading and unloading.
Mechanical durability of pellets is specified in EN 335, with determination by standard BS EN 15210-1:2009.
Pellets that break up into sawdust during handling or storage can cause difficulties in fuel feeds and also the much lower effective fuel energy density of sawdust may require the fuel feed to be recalibrated, or cause the combustion system to trip out.