Wood pellets

Wood pellets constitute a biological fuel material, which, in essence, is sun-energy accumulated over longer time periods. The bio-mass, or grass, plants, bushes and trees, attract and concentrate a large amount of light and heat energy that we can later conveniently use to generate warmth for our home.  Bio-mass is unique, because it consumes a large amount of the CO2 carbon dioxide gas during its growth, not to mention the tremendous amount of  O2 , or oxygen that is produced during the photosynthesis process.
Wood pellets are a renewable and nature-friendly biological fuel material, whose energy is turned into heat energy as a result of a chemical transformation: burning. The biological fuel material is CO2 neutral, because the biomass raw material that is used for its production binds the same amount of  CO2 when it grows, as it releases during the burning process.
Wood pellets are also environmentally friendly as a biological fuel material due to the fact that the amount of sulphur and nitrogen in it is so minuscule, that, when maintaining an appropriate burning process, the flue gases contain very minor, difficult to detect quantities of nitrogen oxide and sulphur dioxide. Yet fossil fuels, like coal, oil products and gas, release considerable amounts of these gases as they burn.

Wood pellets have to correspond to certain production standards, which are summarized in this table:

Technical data

Characteristics

Geometric size

Diameter of 6; 8; 10 mm and length of up to 3 diameters (20; 25; 30 mm)

Compacted material's density 

650 – 750 kg per cubic meter

Heat capacity

4100 - 4300 KKal/kg

Content of ash and sand

0,5% - 1,5% of burned volume

Relative humidity

8 – 12%

In Latvia, the Baltic countries and the CIS countries, as well as in Scandinavia, the 8 mm pellets are used most widely. This is because these pellets can be used both in small-power and industrial, and heavy duty wood pellet burners. Their production is less energy-demanding and pellets of this size can be easily forwarded by pneumatic transportation.

Wood pellets have to be dry, free from chemical admixtures, solid and mechanically durable, crumble-proof, without foreign bodies and sawdust. They must be stored in a dry room and must not be exposed to direct humidity in form of condensate water from the floor, the walls or the ceiling. In an open state, wood pellets can be stored for several months: the air humidity affects them slowly. The storage time of wood pellets in polyethylene packaging is unlimited.

Wood pellets are offered in various packagings for sale to customers, for instance: 15 kg polyethylene bags, 30 kg glass fibre bags, 1 ton “Big – Bag” glass fibre bags, as well as in open, loose state, being delivered with specially fitted trucks, which use pneumatic transporters to blow the pellets into a container that has been installed next to the boiler house: a silo, whose volume is unlimited; it can be from 3m3 to 30m3 and more.

What is the difference between light and dark wood pellets?
Without doubt, the light colour wood pellets are better, mainly because the smallest amount of ash (minerals) remains after they are burned, since the light colour pellets are produced from pure sawdust that remains after processing dried wood, or from pulped cuttings, without wood bark.
Pellets with a light beige-brown tone can be as good, too, the difference is that pellets of such colour are produced from humid sawdust, which is dried prior to granulation, therefore the wood slightly oxidizes and gains a darker tone.
However, as a rule, the more gray or dark the pellets are, the larger the risk that they contain a larger proportion of nonflammable ash (minerals, sand), which will remain in the burner after the wood mass has been burned; as a result, the burner will have to be cleaned more often. Such pellets are usually produced from wood that contains a lot of wood bark, because the bark accumulates a lot of sand in it as the tree grows, and sand also sticks to it during forestry processes.

Is the quality of pellets affected by the type of wood-tree?
In essence, no. The difference between the heat capacity of different types of wood is minor, as can be seen in the biological fuel materials comparison section. Rather, the storage conditions of the raw wood material are of big importance! If the sawdust has been stored for a longer time (more than a half a year) in humidity under open sky, then decomposition reactions have already begun in such a heap of sawdust, including putrefaction and release of heat energy, which can be observed externally as steam coming from the sawdust heap. As a result, the sawdust does not considerably change its appearance (remains gray), but loses a large proportion of its energetic value and becomes “hay”. When heating with pellets that are produced from foul sawdust, they could be consumed even twice as fast as wood pellets made from fresh raw material. If kept dry, wood will not lose its energetic value even after prolonged storage.

Can the wood pellets be humid?
In no case! Because, as pellets are exposed to even minor amounts of humidity,  they immediately absorb it, swell up and burst into sawdust. As sawdust is compacted to pellets, its humidity varies from 12 to 14%. During the granulation process, due to the large pressure and temperature, the humidity evaporates: as a result, the produced wood pellets contain 8 – 10 % of humidity. If the humidity of the wood mass in pellets exceeds 12 – 14%, the pellets burst back into sawdust.


Based on what can one judge about the quality of wood pellets?

Another indicator of quality for wood pellets is their mechanical steadfastness. It can be easily checked by looking at the pellet mass: it has to be free of sawdust bits; otherwise, sawdust remains within pellets indicate that they either have not been screened sufficiently after granulation, or that they have not been  compacted well enough, which makes them burst when exposed to mechanical impact. This can be checked physically by taking a pellet with fingers and firmly rolling and squeezing it: it must not burst. When breaking a pellet into half, it should break with a knick; if so, it is compacted in good quality. The density ratio of compacted pellets relative to the density of loose sawdust mass is 7:1. The density of compacted pellet mass has to be 650kg/m3 on average. It should be easy to determine in domestic conditions as well, by taking a 1 liter container, filling it up with pellets, and checking for their net weight: it should be between 600 – 700 grams.


Is there any available documentation, which ascertains the quality of wood pellets?

Yes, each producer of wood pellets checks the quality indicators of their production in a testing laboratory. A certificate issued by this laboratory for each produced volume of wood pellets is
an official document, which is to be shown to the buyer. However, this case is one of those when you have to rely on the decency of the producer, and that he shows you the document that refers to exactly the volume of pellets in question. If this is not the case and the buyer suspects that the pellet fuel material does not correspond to the displayed document, the buyer can on his own take samples from the selected pellets and bring them for testing to a laboratory, where a partial or full physical and chemical analysis will be made for these and a corresponding quality certificate will be issued. Of course, this is a paid service.

You will find the independent waste products and fuel material examination and testing laboratory SIA „VIRSMA“ at the following address: Aizkraukles street 21, Riga, LV 1006;  Tel.: (+371)67558782;  Fax: (+371)67558781

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