Fuel granules (also referred to as wood pellets) — comprise a type of bio fuel produced from waste wood and agricultural waste. The pellets are like cylinder-shaped granules of standard size.
The raw material for wood pellets production may be wood scrap including bark, sawdust, wood chips – and other wood cutting and clearing waste – as well as miscellaneous agricultural waste including corn by-products, straw, fines of grain manufacturing, sunflower peelings, etc.
The feedstock (wood chips, bark, etc.) is fed to a breaking machine (crusher) where it is grinded up into powder. The pulp enters the dryer and, upon drying, it is transported to cubing and pelleting machine (pelleting press) where the wood powder is compacted to form pellets. The temperature of material rises during compaction due to pressing and lignin is squeezed from wood particles to glue them into solid cylinders. Approximately 6 to 8 cubic meters of sawdust are needed to produce a ton of pellets. The finished pellets are cooled down, and packed into regular-size packages (12 to 30 kg), 1 ton big bags, or delivered to the consumer in bulk as required.
Fuel granules are considered as a sort of “clean” i.e. pollution-free fuel, with ash content below 3%. Upon combustion, the pellets release to atmosphere a quantity of СО2equivalent to the amount absorbed by the plant in its growth phase. Pellets are relatively less exposed to self-ignition due to the fact that they are free of dust and spores that can provoke allergic reactions in humans. The pellets differ from common wood in their high dryness (i.e. 8 to 12% of moisture in pellets against 18 to 30% of moisture in firewood) and in their higher specific density (about 2 to 3 times as great as in firewood). These characteristics provide high caloric value for pellets compared to wood chips or firewood: about 5.000 kWh of heat is released by a ton of pellets upon combustion, half as much as common firewood releases. Low humidity, however, is not just a benefit from use of pellets as a fuel – it introduces a technological problem as well. Drying operation may be a principal cost item in producing fuel from woodworking waste. Besides, process-dependent operations like raw material gathering, sorting and purification may also incur additional charges. The drying process is to be thoroughly planned in order to minimize quality-related risks related to quality of finished product, prime cost and fire hazards of production cycle. The best option is apparently to produce bio fuel from dry chips. One of the major advantages of pellets is their high and stable bulk specific density which facilitates long-range transportation of this bulk material. The pellets, due to their regular shape, small size and homogeneity of finished product, could successfully “flow” through special hoses, thus enabling us to make handling operations and combustion process essentially automatic in this case.
Wood pellets of high quality (colored white and black) are used to heat households through burning them in small boilers (referred to as pellet boilers), furnaces and chimneys. They typically have the size of 6 to 8 mm in diameter and 50 mm long as a maximum.
Demand for wood powder briquettes and pellets as well as for devices to produce and burn those fuels increases in proportion to prices for conventional fuels like oil and gas. In some European countries where alternative energy sources contribute notably to the market, up to two-thirds of premises are heated with pellets. Such widespread use is the result of the fact that this sort of fuel is considered as eco-friendly as well – indeed, atmospheric СО2emissions match amount of gas absorbed by the tree in its growth phase while atmospheric emissions of both NOx and volatile organic compounds are dramatically reduced due to implementation of modern burning technologies.