Wood mainly consists of cellulose and lignite. It also contains other substances such as resins and water. One of the most important characteristic of the wood is the correct seasoning or drying; in other words, wood must have the right level of humidity (around 10-15%), for this reason it is also important the period of the year when it is cut (which should be the winter period).
The correct seasoning allows you to have low-polluting fuel with an excellent performance. The wood (already cut) should be kept in a well-sheltered and ventilated place. It is important to remark that the size of the cut is important according to the stage of combustion, the type of stove (stove, fireplace, boiler) and the economic benefits (the small pieces are more expensive than the big ones as you need more time to cut them and the waste is higher).
Wood can be divided into softwood and hard wood according to the weight in kg of one cubic meter of material.
Softwood (whose weigh is about 300-350 kg/m3) is wood from spruce, pine, poplar, alder, chestnut, willow trees, while the hard wood which weighs about 350-400 kg/m3 is that of elm, oak, holm oak, beech and ash trees.
Softwood burns out quickly and develops a long flame. It is used in ovens that require blaze.
Hardwood has a more complex structure than softwood. It is more suitable for domestic heating as the combustion is slow and tend to burn hotter and longer than softwood
Firewood for heating purposes has different characteristics depending on the variety of plant from which it is derived.
The features about the drying time and the calorific value can vary according to the type of plant.
The calorific value depends on the moisture content and its density.
Excellent quality wood is: oak, ash, beech, maple, fruit trees.
Decent quality wood is: linden, poplar and willow wood.
To avoid pitchy woods.
The calorific value of different types of wood depends on its moisture and for this reason the power of the boilers or stoves is directly influenced by the type of wood used; on the average, a well seasoned wood has a calorific value of 3200 kcal / kg.
To be more detailed, in the specially provided tables (pdf) you can see the changes in calorific value (kcal / kg) depending on the humidity of the wood (expressed in% of the weight of the wood)
It is advisable to buy wood according to the calorific power and not only according to its weight. Buying "green" wood at a low price means not only to buy a lot of water but also to face different problems such as poor combustion, formation of sooth, low outputs and high fuel consumption.
Below an example:
100 Kg of wood with 50% of humidity, corrispond to
50 kg of “anhydrous” wood and 50 kg of water 50 kg anhydrus beechwood = 230000 Kcal (calorific value)
To bring to the boil and evaporate 50 Kg of water we need 27183 Kcal (about 12% of the total Kcal, the equivalent of about 7 kg of wood at 10% moisture).
Much more relevant than the previous calculation is the presence of vapour particles in the fumes which determine the lowering of their temperature (of about 200 ° C) and that involves the impossibility of burning the exhalation gas of wood.
For more details, please refer to the Italian BUYING GUIDE.
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