PELLETS can be defined as densified by mechanical means biomass into the form of small cylinders which are addressed for energetic utilization.
Typical dimensions of pellets range from 1 – 3 cm in length and from 6 – 10 mm in diameter. Alternatively, they can be produced into the form of small solid “bricks” of various shapes, thus known as briquettes.
The net calorific value of pellets at a moisture content of less than 10% ranges between 16 – 19 MJ/kg depending on the type of biomass raw material. This roughly means that 2 kg of pellets are energetically equivalent to 1 liter of heating oil.
Raw materials for pellets can be forestry residues, by-products of saw mills and wood-processing factories, tree pruning, straw, agricultural residues, olive pits and energy crops (such as cardoon, miscanthus, reed, willow, etc.). Pellets resulting from forestry and wood-working residues are known as wood-pellets, while pellets resulting from agricultural rests are known as agro-pellets or agri-pellets.
Applications and Quality Standards of Pellets
Pellets are utilized as standardized biofuel for space heating and sanitary hot water production in residences, commercial companies and industries, as well as fuel feedstock for combined heat and power systems. Depending on final pellet uses and biomass raw material origin, there are different quality standards and specifications.
According to European Standard EN 14961-2 which is related to wood pellets for non-industrial use and which will gradually supersede all national standards (e.g. Austria’s ÖNORM M1735 or Germany’s DINplus), wood pellets can be classified into three basic categories: ENplus-Α1, ENplus-Α2 και EN-Β or shortened to Α1, Α2 and Β respectively. Indicatively, class A1 includes wood pellets originating from stemwood, without chemical additives and with low ash and chlorine content. Wood pellets with slightly higher ash and/or chlorine content are classified into category A2, while wood pellets derived from reused wood, residues or bark are included into class B. Tables 1 & 2 present the specifications of the three wood pellet classes according to EN-plus (EN 14961-2) standard in comparison to german standard DIN-plus.
In parallel, standard ΕΝ 14961-6 currently under development will cover the pellets from different raw materials other than wood (agro-pellets) for non-industrial use. In addition to biofuel certification, it is important to certify the complete pellet supply chain from raw material reception, through to in-house material management, to production and finally to end product storage and delivery to final consumer. Table 3 presents the certification scope of pellet supply chain according to ENplus.