Density and weight are crucial factors to consider when buying wood logs and wood briquettes.
Different species of wood have different densities
In principle, there is very little difference in the composition of wood across different tree species; it consists of carbon, hydrogen, oxygen and ash. Therefore, the energy content of dry firewood varies very little, at around 5 kWh per kilogram.
The energy content of logs per kilogram primarily depends on their moisture content. Wood is a porous material, and if you were to remove the pore volume, the density would be approximately 1,500 kg/m3. Different tree species have different densities, which mean that the energy content will vary on a volume basis. This is important to know when purchasing and burning firewood.
Weight is more important than volume when buying wood
However, the density can also vary within the same tree species – or even the same tree. For example, heartwood and sapwood have different density and moisture content once the tree has been felled. Branches and twigs have a higher density than the trunk, and the top of the tree differs from the bottom. Bark also has a different composition and density compared to the trunk, and contains more ash.
All of this means that even if you’re burning firewood from specific tree species, the properties of individual logs can still vary. This is why weight is more important than volume when buying firewood.
Different wood species behave slightly differently in wood stoves
Different wood species behave slightly differently in a wood stove, partly due to variations in density, and partly due to the differences in chemical composition. For example, birch has a much higher density (approximately 500 kg/m3) than spruce (approximately 380 kg/m3). While it will take longer to heat up, it has a much higher content of volatile components, and these two parameters balance each other out.
In principle, there are no wood species that cannot be used in your wood stove.
You just need to adjust your burning technique to match the moisture content, log size and amount of bark of the wood you’re using. In other words, you need to be a conscious wood burner.
Norwegian standard use spruce
It’s worth noting that the type approval of wood stoves according to Norwegian Standard NS 3058/59 is based on their performance of burning spruce, in the form of 2×2 inch planks without bark. Despite this, the most common wood used for burning in Norway is birch. In addition, beech can also be used as a basis for approval in other standards.
This demonstrates that it’s not the wood species that matter most but rather the combustion technology, the person operating the stove and other fuel parameters than just density. This is also evident in the research conducted through the SusWoodStoves project.
Burning slightly differently
The Norwegian Institute of Bioeconomy Research (NIBIO) provides a good overview of the density and energy content of various wood species. They also suggest that wood species with a higher calorific value burn better, but in practice, it’s not that simple.
Different wood species will burn slightly differently, and you can even hear and see it from the difference in crackles and sparks. This happens because of pressure build-up inside the wood and the sudden release of gases. Spark arresters are commonly used in open fireplaces to prevent sparks from flying into the room. The extent to which this occurs depends on the wood species, and typically, wood species that contain more resin or sap (conifers) produce more noise.
Wood briquettes are not logs
Other types of biofuels, such as wood briquettes and pellets, are not logs. They are made from sawdust, which consists of small, tightly compressed particles. This results in more mass per unit volume, and they become less porous, with a typical density that is double that of birch logs.
Sawdust comes from trunk wood and has the same composition, so as far as composition is concerned, wood briquettes are analogous to logs. Wood briquettes are also drier than normally seasoned logs. However, be careful, as if wood briquettes come into contact with water, they can absorb it and literally dissolve.
Burning wood briquettes can go wrong
However, wood stoves have been approved for burning logs, not wood briquettes. Of course, you can burn wood briquettes in a wood stove, and it can work well, but it can also go very wrong.
In the worst-case scenario, it can lead to explosive burning, which results in a red-hot stove, or it can cause excessive off-gassing when the air supply becomes insufficient when the wood briquettes dissolve and swell, releasing a large number of small particles with a significant surface area simultaneously. A log will release gases in a much more controlled manner, as it does not dissolve but emits gases gradually before the charcoal finally burns.
Explosive burning can lead to chimney fires when particles at the bottom of the flue ignite. Off-gassing without adequate combustion can result in high emissions, including particles that accumulate in the flue, which can also lead to chimney fires.
Therefore, you should think carefully before choosing to burn wood briquettes, and by all means, do not load more than one wood briquette at a time. Two wood briquettes (one typically weighs 1 kg) that off-gas rapidly can be too demanding for your wood stove.
If you have a stove with a grate at the bottom and an ash pan underneath, small particles can fall into the ash pan while still off-gassing. They can also block the airflow coming from below. It’s worth noting that wood stove manufacturers are unlikely to accept warranty claims based on damage caused by the use of wood briquettes.
Some wood briquettes have Swan labelling, which means they meet requirements for purity and emissions after testing according to a standard. However, Swan labelling does not have any requirements for the mechanical durability of wood briquettes. This means that when using such wood briquettes in your wood stove, unwanted incidents can still occur.
Efficiency in wood and fuel
When you burn wood, you cannot utilise all the energy it contains. The more moisture there is, the less energy is available to reach a sufficiently high combustion temperature. How well combustible gases burn depends on temperature, residence time, and the mixing of combustible gases with added air.
If you add too much air, the energy efficiency decreases because you need to heat up a larger volume of flue gas containing a lot of unused air.
The energy efficiency also decreases if the fire is roaring and the temperature inside the flue is high. If you have high emissions of combustible components, you lose the chemical energy they contain.
We are talking about two different types of efficiency: thermal efficiency and combustion efficiency, which together provide total efficiency.
In modern, clean-burning stoves, combustible component emissions are generally low, so the combustion efficiency is high. The total efficiency typically hovers around 80% for these stoves, meaning that 20% of the energy content in the wood is lost through the chimney.
When you are finished firing, you should close the air damper to prevent both the heat in the room and the heat in the stove materials from being drawn out through the chimney. Do not close the damper completely if there are still glowing embers; leave it slightly open until the embers have completely burned out.
The wood-burning process, from water evaporation to charcoal
Burning wood is a highly transient process, meaning the combustion process continuously changes throughout the burning period. At the start, a lot of energy is used in water evaporation and releasing volatile compounds. Eventually, charcoal is formed, which burns entirely differently from gases and at a much slower rate. The energy content of the volatiles burning at the beginning can be one-third of the energy content of charcoal, but since charcoal burns much slower, the heat output is still greater in the first half of the combustion process.
Confused? Wood burning is simply an extremely complex combustion process, both in terms of understanding it in detail and controlling it. However, a lot of research and development activities over the years have revolutionised combustion technology through a new combustion principle (air supply in multiple stages, in stoves produced from 1998 onward), and continuous development of this principle. Therefore, modern wood stoves burn cleanly when operated as intended.
This brings us back to your role as a conscious and responsible wood burner. Burning wood in your stove correctly will not only give you the best performance from your stove, but it will also contribute to a less negative impact on the environment, climate and your health. Managing to utilise more of the energy content in wood will also improve the cost-efficiency of your heating.
It doesn’t take much to achieve this!