Table Of Content
Why Quality Matters
Just as you can’t run a car designed for gasoline with diesel fuel, many energy-generating devices rely on specific fuel properties. Deviating from these design specifications can lead to malfunctions or inefficiency. The extent to which different appliances tolerate variations in fuel properties can vary.
This principle applies to pellets as well. To succeed in the market fuel pellets must adhere to well-defined specifications. This is why ISO standards have been introduced and adopted globally to specify pellet properties. Particularly in international pellet trade, adherence to quality standards is imperative.
Currently, the majority of traded pellets are derived from wood. Consequently, quality classes are only specified for wood pellets. These quality classes are outlined in ISO standard 17225. It defines three quality classes for wood pellets used in domestic applications (A1, A2, B) and three pellet quality classes primarily intended for power plants (I1, I2, I3). These classes differ concerning the type of woody raw material used, physical pellet properties, and chemical characteristics.
As pellet production in Africa will rely more on agricultural biomass suitable standards still need to be developed. The World Bioenergy Association will prepare proposals for such standards as soon as some experiences exist with production and use of pellets from different biomass types. These standards will not only need to accommodate the different raw materials used but also the different quality requirements based on typical use patterns. In principle they will be structured in a similar way as the current standard for wood pellets and include both requirements regarding physical properties and to some extent also requirements for chemical properties.
Physical Properties
Physical properties of pellets encompass their size, moisture content, mechanical durability, fines content, and density.
Regarding size, the ISO standard 17225 for wood pellets used in domestic appliances prescribes a diameter of either 6 mm or 8 mm, with a length ranging from 3.50 mm to 40 mm. Humidity levels should be maintained below 10% across all quality classes. Ash content varies among quality classes: A1 pellets require ash content below 0.7%, A2 pellets below 1.2%, and B pellets below 2%.
Mechanical durability is a critical characteristic, especially for pellets transported over long distances and subject to extensive mechanical handling. Pellets must remain intact during transportation and handling to avoid disintegration into dust particles. Dust particles have entirely different combustion behavior from intact pellets, leading to logistical and operational problems. It’s essential for pellets to possess sufficient hardness to withstand mechanical stress. For this reason durability needs to be tested regularly during production.
Due to the issues associated with fines and dust particles, ISO standards also dictate that the presence of fine particles (those smaller than 3.15 mm) must be less than 1% at the point of loading or bagging. Therefore, sieving pellets after production but before bagging or shipping is an essential step.
The net calorific value of pellets intended for domestic use should not fall below 4.6 kWh/kg. Additionally, the bulk density should be higher than 600 kg/m3 upon delivery. The use of organic additives (typically starch) to enhance pellet properties and increase pellet mill productivity is permissible. These additives must not exceed 2%, but in Europe, it’s common for less than 1% of additives to be used.
Wood pellets must also meet specified requirements regarding ash melting behaviour which is an important property to determine their suitability as fuel.
Chemical Properties
ISO standards 17225 also establish guidelines for the net calorific value and chemical composition of wood pellets, setting specific limits for certain elements to prevent the use of recycled wood contaminated with substances like paint and glue.
Net Calorific Value: ≥ 4.6 kWh/kg
The pelletization process effectively addresses these concerns by requiring precise control over the moisture content of the biomass used. Excessively damp materials must be dried before pelletization to prevent pellet disintegration. Pellets typically maintain very low moisture levels. The chemical composition is as follows:
Nitrogen: ≥ 0,3%
Sulfur: ≥ 0,04%
Chlorine: ≥ 0,02%
Arsenic: ≥ 1 mg/kg
Cadmium: ≥ 0,5 mg/kg
Chromium: ≥ 10 mg/kg
Copper: ≥ 10 mg/kg
Quicksilver: ≥ 0,1 mg/kg
Nickel: ≥ 10 mg/kg
Zinc: ≥ 100 mg/kg
