Firewood - calorific value, species comparison, factors influencing burning

Speaking of furniture of all kinds, houses in bușteni or on wooden structure, wooden table tops and epoxy resin, floors, floors, doors, windows and so on and so forth, we have forgotten one of the oldest uses of wood, that of firewood. Until a while ago, burning wood in stoves, in ovens or under pyrotechnics was the norm. My grandmother on my father's side of the family never cooked with anything other than wood. Although most people in the village had switched to cooking on the stove, grandfather didn't like the taste of food cooked that way. So grandmother cooked the food on a pyrostove in front of the stove over a wood fire. The taste was really unique. Our great joy, the grandchildren's great joy, was to go through the pots. By the time we finished eating, we were all dirty with soot from the pots, but who cares!

Back to the combustible properties of wood. Do all species burn the same? If the wood is so different from species to species and even within the same tree, is the heat the same regardless of which wood is burning? Why does the fire sometimes take a long time to start and smoke comes out? At what temperature does the firewood ignite? How do I burn wood efficiently? I'm sure many of you have asked yourself these questions and many more, as have I. I think there are much better ways to use wood than as firewood, but I still appreciate pizza made in a wood-burning 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 the release of heat. The amount of heat that wood gives off through combustion is its calorific value. Calorific value is the amount of heat released 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. Calorific value determines how efficiently different types of wood can heat the desired space.

The calorific value is dependent on the moisture content of the wood and is inversely proportional to it. That is, the higher the moisture content of the wood, the lower it is. Green, freshly cut wood gives off less heat than well-dried wood. The maximum calorific value is determined at moisture content 0% (absolute calorific value). As it is almost impossible to achieve this moisture content under normal firewood storage conditions, the moisture content referred to is 15%, which is naturally set by the wood when dried in the 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 resinous) gives off more heat.
2. density - The density of firewood is very important. The denser the wood, the higher the calorific value. Hence the differences between the base of the trunk of a tree - denser, higher calorific value - and the top - more loose, burns faster and gives off less heat.
3. Cut water content - It is very important to know how much water the wood had at the time of cutting because, after drying, the water leaves voids, resulting in a more loose wood. Wood that has 35-40% moisture when cut (e.g. oak) will be much denser at 15% moisture content than wood that has had a moisture content of 60-65% (e.g. plop, willow).

Despite these differences, there isn't a huge difference in the calorific value of the species when related to mass. Not so much that you would consider one kind of firewood to be useless, whereas another kind of firewood will warm your house immediately. The noticeable difference is in the moisture content, burning rate, surface to volume ratio, which we'll come to in a moment. But first a few 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
• Chickpeas - 3700 kcal/kg
• Acacia - 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. If, however, it is related to the unit volume, dense wood - oak, acacia, beech - will have the higher calorific value.

Wood flammability. Factors influencing burning

Ratio between surface area and volume

Wood burns because it is flammable. To ignite, it needs a fire source. A wood with a medium density and a moisture content of 12-15% ignites at 300ºC. The flammability of wood increases in direct proportion to ratio between surface area and volume. That's why thin wood, sawdust ignites 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 in. A thin wood will ignite faster and burn quickly, while a thicker wood will burn longer.

Humidity

Humidity is another factor that influences burning. Wet wood burns more slowly because water has to be removed first. The calorific value of wet wood is lower because some of the energy is used to evaporate the water. It is first released free water and then bound water of the woody structure. The water gradually disappears as the fire moves inward. Removing it quickly creates stresses and the wood cracks, spreading sparks. The denser, harder the wood, the greater the stresses, increasing the likelihood of sparks.

Air circulation

Wood burning is an oxidative process. To be maintained air is needed (especially oxygen in the air). This is why a covered fire goes out and when a source of air comes in, the fire gets hotter (like when you open a door or window in a room where a fire is burning hot). 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, hardwoods burn better than softwoods, the latter having very thin vessels closed at the ends (tracheids). Wood with large pores burns better than wood with small pores. But the density of the wood also plays a role. The denser the wood, the harder it is to ignite and the slower it burns, giving off heat for longer.

Which species burn more efficiently

Softwoods burn more slowly and release more heat (due to their higher lignin content). Another reason why softwoods give off more heat is their 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 resinous wood.

Burning wood - for warmth, protection and coloring

Burning wood for heat or cooking is actually combustion, since it is total. I've written about burning wood before, but that was about protection and coloring. For those interested in the Japanese Shou Sugi Ban method of protecting outdoor wood or emphasizing the natural design of wood by burning, find information here.

Total combustion of firewood at maximum efficiency results in water, carbon dioxide and ash. Ash represents between 0.5 and 1% of dry wood volume. If the firewood is too wet and too thick or the air is insufficient (poor draught, the wood has been arranged in such a way that it does not allow air to flow), the combustion is incomplete and dangerous gases (carbon monoxide, nitrogen monoxide) and smoke are produced. Smoke is carbon (charcoal) entrained by air or water vapor before it is completely burned and converted to carbon dioxide.

The heat from burning wood is not dry because water is removed during the process - both the water in the wood and the water from the total burning of the wood mass. Therefore, in rooms with a wood-burning stove or fireplace, the warmth is pleasant and the nostrils do not feel dry.

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. The result is briquettes or pellets used to fuel boilers and heat-producing stoves. Lately, pellets have become a favorite because they are lightweight and 10 times smaller than briquettes and can be pneumatically transported to the burning area.

Both briquettes and pellets are made by pressing waste wood in special devices without the addition of glue. Gluing is due to the moisture content of the wood. Therefore the wood should have a moisture content between 12 and 15%. If it is wetter, it must be dried beforehand, otherwise it will fall apart. Also, for good bonding, there must be no bark in the mixture. Technological processes and installations are different, briquettes being obtained at lower pressure than pellets.

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