Firewood - calorific value, species comparison, factors influencing burning

Speaking of furniture of all kinds, log houses or on wooden structure, wooden table tops and epoxy resin, floors, doors, windows and so on, I forgot about one of the oldest uses of wood, that of firewood. Until recently, burning wood in stoves, ovens or under firewood was the norm. My grandmother on my father's side never cooked anything but wood fire. Although most of the village had switched to cooking on the stove, Grandfather did not like the taste of food cooked this way. So Grandma used to cook her food on the pyrostat in front of the stove over a wood fire. The taste was really unique. The great joy for us grandchildren was to rummage through the pots. When we had finished eating, we were all dirty with soot from the tuciuri, but what did it matter!?

Let's get back to the combustible properties of wood. Do all species burn the same? If wood is so different from species to species and even within the same tree, is the heat the same regardless of which wood is burned? Why does the fire sometimes burn slowly and smoke a lot? At what temperature does firewood ignite? How do I get wood burning efficiently? I'm sure many of you have asked these questions and many more, as I have. I think there are much better ways to use wood than as firewood, but I still appreciate pizza made in a wood-fired oven and a fire crackling in the stove on a winter's day 🙂 .

What is the Calorific Value of Wood?

When wood burns, it produces energy. We feel this energy as heat release. The amount of heat that wood gives off when it burns is its calorific value. Calorific value is the amount of heat given off when burning 1 kg of firewood. It is measured in kcal/kg, kJ/kg or kWh/kg, with different values depending on the unit of measurement. The heating value determines how efficiently different types of wood can heat the space.

The calorific value is dependent on the moisture content of the wood and is inversely proportional to it. That is, it is lower the higher the moisture content of the wood. Freshly cut green wood gives off less heat than well-dried wood. The maximum heating value is determined at moisture content 0% (absolute heating value). As it is almost impossible to obtain such a moisture content under normal firewood storage conditions, the moisture content referred to is 15%, which is naturally determined by the wood when drying in the ambient environment. It is recommended, however, that the wood be kept indoors near the stove or fireplace for at least 24 hours before being placed on the fire. The humidity will decrease during this time and the wood will burn better and more efficiently.

Calorific value - influenced by wood species and internal wood structure

The calorific value of wood species differs from species to species. It depends on:

1. cellulose and lignin content. Lignin has a calorific value of 6000 kcal/kg and cellulose and hemicellulose 4150 kcal/kg. This means that firewood with a higher lignin content (wood from resinous) gives off more heat.
2. density - Firewood density is very important. The denser the wood, the higher the calorific value. Hence the differences between the base of a tree trunk - denser, with higher heat output - and the top - looser, burns faster and gives off less heat.
3. Water content at cutting - It is very important to know how much water the wood had at the time of felling because, after drying, the water leaves gaps, resulting in a looser wood. Wood that has 35-40% moisture content when cut (e.g. wood of oak) will be much denser at 15% moisture content than a wood that had a water content of 60-65% when cut (e.g. plop, willow).

Despite these differences, there is not much difference between the calorific value of the species when related to mass. Not so much that you would consider that you are putting one kind of wood in the fire for nothing while another kind of firewood will heat your home immediately. The noticeable difference is in the moisture content, burning rate, surface area to volume ratio, which we will deal with in a moment. But first some examples of calorific values determined at 15% humidity. Not all sources give the same values, but they are fairly close.

• Pine - 4000 kcal/kg
• Molid - 3725 kcal/kg
• Mesteacăn - 3700 kcal/kg
• Salcamis - 3575 kcal/kg
• Fag - 3550 kcal/kg
• Cherry - 3550 kcal/kg
• Oak - 3450 kcal/kg
• Plop - 3200 kcal/kg

In the above examples, the calorific value of the wood in question is related to its mass. However, if it is related to the unit volume, dense wood - oak, acacia, beech - will have the highest calorific value.

Flammability of wood. Factors influencing burning

Ratio of area to volume

Wood burns because it is a flammable material. It needs a source of fire to ignite. Wood with a medium density and moisture content of 12-15% will ignite at 300ºC. The flammability of wood increases directly proportional to ratio of surface area to volume. That's why the thin wood, the twigs, burn faster than a log. Also in this idea, the suspension of wood dust in the air can be an explosive mixture. Wood burns from the outside inwards. A thin wood will ignite faster and burn quickly, while a thicker wood will keep burning longer.

Humidity

Humidity is another factor influencing burning. Wet wood burns more slowly because the water has to be removed first. The calorific value of wet wood is lower because part of the energy is used to evaporate the water. First it is released free water and then bound water of the woody structure. The water gradually disappears as the fire moves inwards. Rapid removal creates tension and the wood cracks, spreading sparks. The denser, harder the wood, the greater the stresses, increasing the likelihood of sparks.

Air circulation

Burning wood is an oxidative process. To be maintained air is needed (especially oxygen in the air). That's why a covered fire goes out and when a source of air appears, the fire gets hotter (like when you open a door or window in a room where a fire is burning brightly). It has been determined that it takes 0.6 kg of air to burn 1 kg of wood. Due to the larger amount of air present in large pots, deciduous trees burn better than coniferous treesThe latter have very thin vessels closed at the ends (tracheids). Large-pored wood burns better than small-pored wood. However, the density of the wood also plays a part in this process. The denser the wood, the harder it will burn and the slower it will burn, giving off heat for a longer period.

Which species burn more efficiently

Softwoods burn more slowly and give off more heat (because of their higher lignin content). Another reason why softwoods give off more heat is the resin content, which is considered to be a real fuel. Young or less dense hardwoods burn faster and give off less heat. As their density increases, they will give off more heat than a similar volume of softwood.

Burning wood - for heat, protection and staining

Burning wood for heat or cooking is actually combustion, as it is total combustion. I've written about burning wood before, but then it was about protection and staining. Those interested in the Japanese Shou Sugi Ban method of protecting wood outdoors or highlighting the natural design of wood by burning, find information here.

The total combustion of firewood at maximum yield results in water, carbon dioxide and ash. Ash represents between 0.5 and 1% of the dry wood volume. If the firewood is too wet and too thick or the air is insufficient (the draft is not good, the wood has been arranged in such a way that it does not allow air to flow), combustion is incomplete, resulting in dangerous gases (carbon monoxide, nitrogen monoxide) and smoke. Smoke is carbon (charcoal) entrained by air or water vapour before it is completely burnt and converted into carbon dioxide.

The heat given off by burning wood is not dry because water - both that in the wood and the total burning of the wood mass - is removed during the process. Therefore, in rooms with a wood-burning stove or fireplace, the heat is pleasant and there is no feeling of dryness in the nostrils.

Wood waste - raw material for combustible materials

It is also possible to use wood as fuel without burning tree trunks, firewood in general. Waste from processing - sawdust, wood dust - can be processed and turned into an energy source. Processing results in briquettes or pellets which are used to power boilers and heat-producing stoves. Lately pellets have become a favourite because they are lightweight and 10 times smaller than briquettes, and can be pneumatically transported to the combustion zone.

Both briquettes and pellets are produced by pressing waste wood into special devices without the addition of glue. Gluing is done due to the moisture content of the wood. The wood must therefore have a moisture content between 12 and 15%. If it is wetter it must be dried beforehand because otherwise it will split. Also, for good gluing, there must be no bark in the mix. Technological processes and installations are different, as briquettes are produced at lower pressure than pellets.

Although it is such a common topic, I hope I have come up with some new, useful information for you. If you think it would be of interest to others, please feel free to share. If you have any comments, questions or additions, please leave them in the space below.