Wood fuel pellets constitute the most widespread type of pelletized biofuel due to the abundance of the raw material and the possibility of using this type of fuel both in the industrial facilities and in private homes.
A production facility that is engaged in the process of manufacturing the wood fuel pellets can be subdivided into the following production areas:
I. The preparation area:
Wooden chips, shavings, splinters, lumps of wood, slabs, cleats, technical firewood and other types of refuse wood can all be used as a raw material for wood fuel pellets. The fractional composition of the raw material supplied determines the line-up of equipment used in the preparation area. The proper organization of the pre-processing stage of production is instrumental in lowering of the cost per unit of electrical energy that is used in milling of the raw material.
During the preparatory stage of production the mechanical impurities (rocks, sand, the inclusions of metal and all other types of inclusions) are separated from the stream of the raw material. This helps to extend the service lifespan of the wear parts and, most importantly, ensures the top quality (that meets the highest European standards) of the end product.
II. The drying area:
The moisture content of the raw material varies (depending on the type of wood and the procurement season) from 35% to 60%. Such raw material is unsuitable for pelletization since the high moisture content facilitates an intensive vaporization during the pelletization process which prevents the bonding of wood fractions. The raw material’s moisture content of 10-12% is considered optimal for the pelletization; with this in view the raw wood should be dried during the pre-processing stage until its moisture content equals the above-mentioned figures. The GRANTECH™ drying unit constitutes the most suitable piece of equipment since it is capable of lowering the moisture level of the raw material to the necessary figures. This drying unit is explosion-proof which makes it safe enough to use. Furthermore, a fundamentally new fuel delivery arrangement has been worked out which improves the drying area’s safety and efficiency levels. According to this arrangement 15-20% of the dried material is reclaimed to be burned in the furnace of the heat-generator. The dried wood then undergoes the process of ultimate grinding which reduces it into fractions of the required size (2 mm max).
III. The pelleting area:
This area is the “heart” of the pellet production; it is the final stage that determines the quality of the finished product. The dry ground wood is being pressed into the cylindrical pellets of a specified diameter. Right before the raw ground material gets into the pressing chamber it is being pre-processed with dry vapors which makes it more conformable to pressing and activates the lignine compound that the wood naturally contains; this increases the durability of the ready-made pellets. Fresh pellets contain excessive moisture; their temperature is about 80°С; they are also excessively fragile; due to all these factors they are being sent to the next processing stage, which is cooling. During the process of cooling not only the temperature of pellets drops to the environmental level but their moisture content reaches the standardized level as well. After the pellets have been hardened, the small particles which have formed during the work cycle’ transportation of pellets are being removed from their bulk; thus the pellets acquire their “trade dress”.
IV. The packaging area:
Depending on the quality grade of the produced fuel pellets, on their place of destination and on an enterprise’s transportation infrastructure the finished product can be either packed into plastic bags of 15-25 kg each which are bundled into pallets of 1000 kg each or packed into soft “big bag” containers of 750-1200 kg each. In rare cases – if an enterprise is located near a sea port or a river port or near a mainline railroad – the finished product can be dispatched in bulk. As a rule, the first grade pellets (for private consumers) are packed into small bags and, more rarely, into “big bags”; the pellets that are sent to big industrial users are packed into large containers (1000 kg min) or are dispatched in bulk.