The pelleting of wood
Wood pellets are generally produced from the by-products of wood processing industries (such as wood shavings or sawdust) or from forest residues. As these potential feedstocks are tapped, much attention is paid to the possibility to pelletise agricultural residues (such as straw, kernel residues etc). This possibility is studied in detail within the Pellets for Europe project.
Pelletising technology overview
The process of pellet manufacturing was first developed for the livestock feed industry. It can be said to consist of a few basic sub-processes: drying (if raw material is wet), comminuting, pelleting, cooling and packing.
Basic steps of the densification proc
Comminution. The raw material is first freed from heavy contaminant (e.g. rocks, metals), and then comminuted, typically in a hammermill. The maximum particle size must be inferior to the thickness of the pellet to be produced.
Drying. If the raw material is wet (which is typically the case for fresh forest residues, around 50% moisture content), it has to be dried. A dryer d to a dryer (generally a rotary drum type one) where the moisture content is reduced to about 8-10 % (w.b.).
Conditioning. The material can be conditioned with dry steam and water to the required temperature and moisture content to activate the lignin as a pellet binding agent and to obtain the necessary malleability of the product. Organic binding agents can also be added according to the standards that apply.
Milling. After conditioning, the particles are then moved by conveyor to a pellet mill, where the pellets are extruded through the action of rollers acting on a perforated matrix. On the outer side of the latter, a knife cut off the pellets at the desired length. Two main types of pellet presses can be distinguished: flat die and ring die types. The flat die type has a circular perforated disk on which two or more rollers rotate and force the material through the holes. The ring die press features a rotating perforated ring on which rollers press on the material to the inner perimeter. The most common output range for pellet presses is 3-5 t/h. According to manufacturers, pellet dies have an average service life of 1,000-1,500 hours, corresponding approximately to 10,000 tons.
Flat die and ring die pellet press
Source: Amandus Kahl, Salmatec; Sprout Matador
A ring matrix with rollers
Source: UMT, Larus Impianti
Cooling. After extrusion, pellets are very hot (90-100°C) and are immediately air quenched down to 25°C. This sets up the lignin and hardens the product, and contributes to maintain its quality during storage and handling. Finally, pellets are sieved so as to separate the residual fines which are generally re-used in the process. Dust free pellets are then ready for storage or for automatic packing (in 25 kg bags or big bags – 1 to 1.5 m3).
New technological development
EUBIA pushes forward the development of a new technology, whose process is absolutely new: the pellet mill has two dies, and raw material is pressed between them from outside towards the inside cavity of each dye. According to technical specification, specific electric consumption ranges from 25 to 45 kWh/t pellets depending on the type of wood. The system operates without the need of any additives and maximum operating temperature of the dies is 55 to 60°C which avoid vapours emissions and permit direct bagging without cooling stage, which lower the investment costs. Another advantage is that material with up to 30 to 35% humidity can be handled, thereby doing away with the need for a dryer.
A new pelleting concept