Composition and general properties of wood for energy use
Introduction: Average composition of wood
The elemental chemical composition results from the crude analysis of the felled wood under normal conditions (that is to say in winter in our countries).
It has been possible to observe, by comparing the results obtained with a large number of forest species, a certain constancy in these results and we can admit the following figures, representing an average composition by weight.
- 40% water: these are trees recently felled without bark, using not only the wood of the trunk, but also that of the branches.
- 1% mineral or metallic ash: the sawdust is completely burned in a very hot flame and the solid residue formed only from the non-combustible part is collected.
- 59% of “combustible or oxidizing” elements, ie transforming into gas during combustion: oxygen, hydrogen, nitrogen and carbon.
These figures are very close to the results provided by the analysis of beech (40% water - 0,65% ash - 59,35% elementary principles.)
Let us now examine in more detail each of 3 important points dice composition: water, ash, basic principles.
B - Water
A reduced humidity level is an essential parameter for good firewood: calorific power, thermal comfort and maintenance of your installation depend largely on it ...
Water forms most of the sap, but it also soaks the cell walls.
1 - The water content is variable from one species to another. Measurements taken on trees felled at the same time and in roughly similar climatic zones gave:
- 52% for black poplar
- 35% for oak
- 48% for poplar from Italy
- 31% for birch
- 47% for lime
- 29% for ash
- 45% for fir
- 27% for sycamore
- 41% for the yardstick
- 27% for cormier
- 39.7% for beech
- 19% for charm
2 - But it also varies, for the same species, depending on the part of the tree considered, the felling season, the duration of exposure to the open air, the flow, etc.
a) The water content is minimum in the trunk, average in the young branches and maximum in the twigs, and in each of these parts the sapwood is richer in water than the heart.
However, there is a difference between softwood and hardwood. In softwoods, branches contain proportionately less water than in hardwoods.
b) It is at the beginning of the vegetation (in April in general) that we will find the most water in the wood. The difference from January to April is estimated at around 10%.
c) Wood exposed to the open air dries out slowly without ever becoming completely dehydrated. After a period varying in duration with the species and with the general climatic conditions of the region, these “air-dried” woods retain a content of between 15 and 20%. It is the relatively soft woods that tend to retain the most water. Thus maple, which contains an average of 28% water when felling will retain 18% when air-dried (i.e. a difference of 10%) while oak will increase from 35 to 17% (i.e. a 18% difference).
This residual moisture can only be eliminated by extensive steaming. It would also be pointless to completely dehydrate the wood because, in the air, in a few days or weeks, it takes up some of this moisture.
The desiccation of the wood is accompanied, for certain species, by a considerable variation in mass! Thus the fir tree can lose up to 520 kg per cubic meter, which represents a percentage of about 105% of the weight of dry wood.
d) Wood debarked and cut will quickly lose much more water than wood stored under bark.
The loss of water can be even so rapid at the beginning and cause volumetric variations so sudden that it is advantageous to keep some species under bark (ash fruit trees) for a while.
C - The other "simple" elements
These are carbon, oxygen, hydrogen and nitrogen. We have seen above that all these bodies represent 59% of green wood. These elements are found in roughly identical quantities in the various species of wood, namely for green wood:
- for carbon 29,5% by mass
- for hydrogen 3,5% by mass
- for oxygen and nitrogen 26% by mass
Carbon is the essential element providing the calories of a wood fire.
More: woodburning file
Links to learn more
1) The choice of material:
- How to correctly choose a heater with wood? (Stove, boiler or boiler)
- List of stoves and boilers labeled "Green Flame"
- Help and advice for choosing a wood stove
- Choose the power of his wood stove
- The normalized power heaters Wood
- Choosing a wood boiler
2 Heating with wood every day: maintenance and improvements:
- The different types and prices of firewood fuel
- Heating and wood fireplace: How to avoid chimney fires. Maintenance and dimensions
- Regulation about the chimneys, the standards and the law
- Make a hot water collector on a wood stove
- The manufacture of pellets: diagram of a factory
3) The pollution of wood heating:
4) Feedback from wood heating experiences:
- Complete file on the presentation of a pellet boiler installation at a private home
- Presentation and photos of another pellet boiler installation in Alsace at a private home
- Presentation of a wooden and solar house
- Auto installation of our wood boiler Deom Turbo explanations and mounting scheme
- effective yield estimate of our stove Boiler Turbo Deom
- Forum wood heating and insulation