Although almost everyone has heard of pellet, a lot of people don’t know quite what it is. We deem it necessary to deepen the matter as there are many circulating and discordant voices…
First, it must be said that the word “pellet” does not refer to an eco-friendly wood fuel, but to a small compact product with cylindrical shape resembling a cork or a bullet… this word has also been used with reference to the compaction/densification process in the animal feed and agri-food sector. As a matter of fact, the pellet is the result of this process and it was originated to meet the need for animal feed transportability and storage.
The correct term is “wood pellet” and we owe its existence to the development of such technologies in the agri-food and zoo-technical sectors. The Canadians, apparently, were among the first in the ‘60s to use the “pelleting” process in the feed sector and later (in the ‘80s) special machines were created to further densify sawdust, as a production waste of logs and timbers. Even though the presses used to compress sawdust are very similar, not to say identical, to those for animal feed, they actually have very different features that affect the entire production process. This apparent similarity often misleads those people wishing to enter this production sector.
Someone may wonder what’s the point in wasting effort when mother nature already gives us the wood in the natural state we all know with the only sacrifice of the cut. Actually, there are many undeniable advantages:
- First of all, we need to consider the problem of waste management of sawmills and related companies, which process large quantities of timber and planks. Sawdust is a natural material, but when it comes to huge quantities it becomes a real ecological problem of disposal, not to mention the transport and storage costs that companies have to bear
- The wood pellet originated from the aforementioned process helps to cut down less trees to produce firewood and it is therefore a double ecological advantage.
- The third ecological advantage is that during combustion its degree of pollution is the lowest ever; besides not damaging the ozone, and therefore not aggravating the greenhouse effect as a natural (and not fossil) wood, the devices that use it reach very high combustion temperatures under microprocessor electronic control: this always guarantees the perfect oxidation process of the gases produced by wood because a small “firmware” program constantly regulates the amount of fuel and air necessary for the combustion process. Something similar happened in the automotive sector with the advent of catalytic mufflers with lambda probe.
- On the practical and economic front, we have to consider that firewood users are probably the best candidates to give an answer. In fact, if it is true that everyone is interested in the huge economy on the cost of heating, it is also true that most people do not accept the inconvenience of a woodshed in their own home. Having a pile of wood implies a lot of occupied space, time to devote to it during the Summer season, coming and going of baskets to refuel the stove / fireplace resulting in dirt and debris … in short, it is not for everyone. Thanks to its small, cylindrical and homogeneous shape, the wood pellet is much more like a liquid than to a solid: the power supply in the devices is automatic and takes place from a tank of variable capacity that allows very long autonomy. The annual storage is simple, convenient and quick: just think that the calorific power of a good pellet for stoves also reaches 4,400 Kcal / h per kilogram (LHV) * while the firewood, normally, is around 2,500 Kcal / h per Kg. The pellet then takes up 1/5 of the firewood space and is also packaged in handy 10-15 Kg bags, clean and easily transportable.
As an example, here is a table comparing fuels with pellets:
| Fuel type | Unit of measurement | Heat output (L.H.V.)* |
|---|---|---|
| Wood pellets | Kg | 4.400 Kcal/h ca. |
| Firewood | Kg | 2.500 Kcal/h ca. |
| Methane | Lt | 8.200 Kcal/h ca. |
| Gas oil (1 Lt=0,85 Kg.) | Kg | 8.500 Kcal/h ca. |
| LPG (1 cbm=4,166 Lt.) | Cbm | 21.500 Kcal/h ca. |
(L.H.V.)* = lower heating value; that is the caloric power that does not consider the latent heat of water evaporation : practically, from the total energy released by combustion, that part necessary for the evaporation of the water contained in the fuel is removed. In the case of biomass this part of energy is relevant and heavily conditining the value of L.H.V.
It is understood that the caloric value of the wood presents important variations according to its seasoning and therefore water content. From the above scheme it is easy to calculate the percentage of savings compared to traditional fossil fuels: certainly 50% is an average more than reliable, even considering the differences in prices depending on the area. If we then consider the heating by means of hot air (stoves, centralized air generators, etc.) in homes and houses of normal size, we can reach peaks of 70%. As for the comparison with wood, and at its normal market price, we can say to be along the same lines with some cases of further small savings.
The ashes and yields:
The other relevant aspect of the pellet is the ash content: in fact, the firewood does not only present the problem of the water content (<40%), which drastically lowers the calorific value, but also that of the content of various bark and impurities. Most of the good quality pellets on the market offer ash content of less than 1% of the total weight and less than 10% of water; this means that the appliance that burns it (e.g. small stoves) will offer the user the possibility to limit the cleaning very much of the same and its refuelling, making the pellet combustion something acceptable even for those who have always used liquid fuels or gaseous.
Alternatives to pellet:
For correctness and completeness of analysis it is necessary to make a comparison with a woody biomass fuel called wood chip. Thanks to the shredding / grinding process of the timber and of its waste, a more or less uniform product is obtained, which also allows an automatic supply of boilers and various heat generators. We think that the wood chips market starts where the pellet ends and vice versa: in fact, the woodchips still require larger automatic feeding systems because of its larger size; its high water content, does not allow the use of simple and compact combustion chambers, but often requires very expensive and complex technologies. In practice, regardless of the availability of fuel, the overall investment in wood chip combustion technology is significantly higher, making this technology less convenient on medium-small scale plants; said in simple terms it is useless to pay the fuel very little if the small plant has very high amortization costs.