The calorific value of wood is one of the most important information the lover of burning in the fireplace need to know.. Unfortunately in this topic there are a lot of misapprehensions., The news are often divergent. Some web sites provide an overview of caloric value of species, which shows, that pine and spruce have the highest caloric content of the deciduous species, and among the deciduous trees, the birch has higher caloric value than beech.. Data on caloric value of the wood from different sources differ strongly among each other.. The problem is that, the calorific value of wood species is only the one component of the fuel value subject matter. Equally important matter as the energy value are the incineration and other factors. For instance, species of conifer due to the high level of resin content burn too rapidly and too fastly, and the user of the fireplace wants the long-burning combined with the wood change for glow.
Calorific value of selected fuels.
The calorific value of wood on the background of other wood fuels, presents quite interesting, wood is not much weaker fuel than coal. Of course, no longer and in any way it can compete with coal, which has the calorific value half as higher. Taking into account the price of both raw materials we come to the conclusion, that the prize in the ratio of calorific value, wood beats the coal. Not to mention that, the wood is much more graceful and pleasing fuel. There is another one problem with the calorific value of the coal – It is very theoretical, in each batch it may have different value, Some tabs of coal do not want to burn at all. In contrary, the wood after appropriate seasoning always gives full power.
Fuel Calorific value [MJ / kg]
Air-dry wood 15,5
Completely dry wood 18,5
Brown coal 19,5
The topic of the calorific value of the wood is quite complex and before we move on to the analysis of species, we will introduce the factors, which have an influence on the calorific value of wood. In principle, a better term than the fuel value and calorific value (is determined in units of energy per unit volume) is an expression 'fireplace value or usefulness of the wood'. Why? We will try to explain it later.
The calorific value depends primarily on wood and wood moisture. Beech firewood is definitely more worth than the poplar fuel . However, the same beech fuel if it is not seasoned and it is wet, ,its calorific value will be practically none, because fresh wood has a very poor, close to zero combustibility rate. To sum up,, dry wood, poplar or willow will have a much higher calorific value than wet beech and hornbeam.
The speed of wood drying.
There are other characteristics of wood, which affect broadly understood fireplace usefulness . We can rank as them, the speed of drying wood. Unfortunately,, the variety is quite large. Birch is an example of wood, which dries up very quickly, even in a few months time. On the other hand, the oak is heavy species and with high calorific value, However, the time of drying is very long, some say that oak should be seasoned at least 3 for years. In many species, a greater impact on drying up has a specific design and structure of the wood rather than the water content in fresh state. For instance, beech is a species with high water content , the grab has relatively few times less water in fresh state (2 than the beech), although the time of drying of both species is similar.
Next feature, which is said to be too low is a percentage of the bark. A purchaser of the firewood usually buys it together with bark, which is also some fuel, but unfortunately a lot worse than the wood itself. And here the gaps between the species are also very large. Optimal species such as beech and hornbeam have only about. 7% of bark, whilst another species such as oak and robinia (acacia) have a share of bark more than 22% (!). It means, for example, that the purchaser. of oak or acacia gets in 1 meter 3 times more than in the beech bark or on the other hand, it is 16% less pure wood for oak than for beech in a similar volume unit. An additional disadvantage of coarse and cracked bark depends on that, it is a natural habitat for insects, sawdust and sand.
Another feature of the wood having an impact on its usefulness is fireplace is a cleaveness, which determines the level of ease of chopping wood. For example, beech and birch are cleave species and friendly for a man who splits them, unfortunately, the hornbeam is a hard-cleave species, the oak too.
Other characteristics affecting the suitability of the firewood is an acid content, of, resins and other substances tannins. On the grid, the resin content eliminates the softwood. Among the deciduous species we have to point out the oak with a high content of acids and tannins. Are these qualities important ? Unfortunately yes, during combustion, smoke, soot, acids, tar and other substances are emitted, which pollute the fireplace and the flue and may cause corrosion of steel elements.
The most important physical feature of the natural wood having an impact on its calorific value and energy value is density expressed in kg per volume unit. Wood density is a variable value depending on water content and that is why the comparison of the density makes sense only in the case of air-dry wood or completely dry wood. Some elaborations claim detailed density values for each species separately, However, in such statements a considerable mistake may appear. Wood density is a variable for a particular species and depends on the content of alburnum and heartwood, the age of the tree, the habitat conditions in which the tree grew, from which part of the tree has been obtained (butt, central part of the shots, corolla) and other factors. Therefore, this study adopted the classification of the known air-dried wood density in the intervals according to F. Krzysik, modified and supplemented by the web site author's own information.
Density [kg/m3] Species
Very heavy wood > 800 plum-tree, hornbeam, thawthorn
Heavy wood 710-800
acacia, rowan tree, beech, ash, oak, apple-tree, pear-tree, nut, elm tree
Moderately heavy wood 610-700 clone, birch, sweet-cherry tree, black-cherry tree
Moderately light wood
510-600 cherry-tree, alder, chestnut
Light wood 410-500 linden, poplar, aspen, pine, spruce, fir
Very light wood <400 willow, gray alder
A comparison of the density list shows, that the difference between the lightest and heaviest species is 100%, and that is why the voulume unit of hornbeam will have 2 times more “'wood content in wood'” if compared to willow and poplar. It is a valuable indication of the possible. purchase of the entry-level wood. Theoretically 1 meter of willow should be at a half proce of beech and hornbeam.
Summary of species is purely theoretical for the air-dried wood. In fact, it may be so, that a well-seasoned willow or poplar firewood will have a much higher calorific value than moist beech. In addition to wood density, there are other physical and chemical characteristics affecting the suitability of wood for fireplace.
Ranking of the usefullness for fireplace.
Based on his own observations and subjective opinion, the author of the text has created a ranking of the usefullness of wood for the fireplace. Species were divided into 3 categories: very good, good and acceptable. Species were placed in order of the highest value.
Very good species hornbeam, beech, ash, thawthorn, rowan tree, nut, plum-tree, apple-tree, pear-tree, acacia, black-cherry tree
Good species birch, clone, elm tree, sweet-cherry tree, oak, hazel, elderberry, cherry-tree, alder
Acceptable species aspen, linden, chestnut, willow, gray alder
Ranking may be a surprise with regard to some hard and heavy species, which have been located on the latest positions. These include the oak, which has a high density and hardness, and in popular opinion is desirable firewood. Unfortunately, the above-mentioned features are disqualifing it: very long period of drying, very high proportion of bark, difficulties in chopping and high content of undesirable substances such as acids and tannins. The same is true about robinia (acacia), species, which is very hard and caloric, However, having perhaps the high-bark content of all compared species. In extreme case, the purchaser of an acacia firewood may have in the total mass, even 30% bark of low calorific value.
On the other hand, we can be suprised by the high position of fruit species in ranking. This is dictated to the incredible density of fruit trees wood, and other characteristic features. Fruit species have relatively thin bark and emit a specific pleasant fragrance during combustion. In some Western countries the fruit trees fuel is called an aromatic wood. It often happens, that someone in the garden, cut the old apple-tree and plum-tree and gives the neighbor wood for free, recognizing that it has no value. To valuable fruit species we rank: plum-tree, apple-tree, pear-tree, sweet-cherry tree. Cherry deviates slightly from the previous ones. Also extremely valuable are fruit species but the forest ones are only a hawthorn like for example, mountain ash and bird cherry.
In the Species Menu there are more detailed information on the advantages and disadvantages of each wood species.