Properties of pellets
Physical properties of pellets
These refer to pellet size, water content, mechanical durability, content of fines and density.
In terms of size the ISO standard 17225 for pellets used in domestic appliances defines that the diameter of pellets should be 6 mm or 8 mm and its length should be anything between 3.50 mm and 40 mm. Humidity should be below 10 % for all quality classes. The ash content is different from quality class to quality class. High quality pellets – the A1 category – must have an ash content below 0.7 %, quality A2 class requires an ash content below 1.2 % and quality class B requires an ash content below 2 %.
The mechanical durability of pellets is an important feature for pellets that are being transported over longer distances and that are subject to extensive mechanical handling. If pellets break apart and disintegrate into dust particles easily, this creates lots of problems both in the logistic chain and in the devices that should use the pellets as dust particles have a completely different combustion behavior than pellets. The key point is, that pellets must be hard enough, so they do not break up during mechanical stress. Specific test procedures must be used to check the mechanical durability of pellets regulary during producttion.
Due to the mentioned problems with fines with dust particles the ISO-standards also define, that the amount of fine particles (particles smaller than 3.15 mm) must be less than 1 % at the point of loading (or bagging). So sieving pellets after the production process and before bagging or shipping is indispensable.
The net calorific value of pellets for domestic use should not be lower than 4.6 kWh/kg. The bulk density should be higher than 600 kg/m3 as delivered. It is allowed to use additives of organic origin (in most cases starch is being used) to improve pellet properties and increase productivity of the pellet mill, as starch acts as a lubricant. The demand of additives may not exceed 2 %. In Europe in most cases less then 1 % of additives are being added.