PELLETIZING BIOMASS – ADVANTAGES & DISADVANTAGES
As a cultured and preliminarily treated wood fuel, pellets have weight advantages over wet wood, which allow them not only to successfully compete with wet wood and, in many cases, with fossil coal but also to displace them from the fuel balance.
The main advantages of wood pellets are as follows:
(1) significantly higher combustion heat than that from original wood (by two or more times in most cases);
(2) a high level of unification of the fuel composition, which completely excludes the necessity in preliminary treatment for the final consumer;
(3) quite a simple arrangement of the automatic fuel feed to the boiler furnace;
(4) the evenness and completeness of pellet combustion and the almost complete absence of smoke during their burning, which will significantly facilitate the utilization of ash and other wastes.
These advantages prioritize wood pellets among other fuel types that are produced from wood or other vegetable sources. However, significant disadvantages of wood pellets are also observed:
(1) high hydrophilicity and the trend towards catching atmospheric moisture cause not only the excessive moistening of wood pellets but also the complete destruction of their mechanical structure, which makes their transportation and storage difficult under generally accepted conditions for solid fuel;
(2) the susceptibility to biodegradation in the atmospheric air medium that is caused by the activity of different fungi and microorganisms excludes the long-term storage of pellets;
(3) the necessity to maintain a high level of the dryness of pellets and of the surrounding air under other conditions being equal increases the fire and explosion hazard of storages, which is particularly important for large energy facilities;
(4) the use of pellets as a fuel in the existing coal boilers that are equipped with a standard pulverized coal system is fundamentally possible; however, it is limited by a higher (by several times) power consumption for grinding compared to conventional coal.
For small lots, the above-mentioned problems are solved using a hermetically sealed container during transportation and storage; however, this solution is unacceptable for large energy enterprises due to the difficulty of incorporating automatic preparation into the existing system, as well as to fuel supply.