If you are guided by Wikipedia, the burning temperature of firewood is in the range of 800-1000 ° C (approximately), and the ignition and decay of a tree begins at 300 ° C. In fact, this range is even wider - from 450 to 1050 ° C. Not too accurate data, right? There is an explanation for this - the temperature in the center of the flame depends on many conditions. Those who want to achieve efficient burning of wood in the stove or solid fuel boiler will not hurt to get acquainted with these factors.
When does firewood give more heat?
To organize economical heating of a private house, you need to extract maximum heat from wood, and then transfer it to the premises with minimal loss. The second condition depends on the efficiency of the heat source, but the first depends on the temperature of burning wood in the furnace of the furnace or boiler. The higher it is, the more heat is released.
The following factors affect the amount of heat generated during wood burning (heat production):
- calorie content (the amount of thermal energy that can be obtained from a unit of fuel);
- natural moisture content in the tree;
- volume of air supplied and its temperature.
In practice, all of these parameters are interconnected. The calorific value of fuel, oversaturated with moisture, is about 2 times lower than that of a dry tree. And without the right amount of air, it will smolder and give off a minimum of heat. That is, the real burning temperature of a tree is a variable value and cannot be accurately indicated in theoretical considerations.
For reference. There is a simple way to determine what the temperature of the fire is achieved by burning wood in the open firebox of the fireplace. Use an infrared pyrometer that measures the heating of surfaces at a distance, as the master stove-maker did in his video. It demonstrates tests of the fireplace insert of the barbecue, measurements are made in the second half of the video.
Now let's look at each factor individually.
About the calorific value of fuel
Different types of wood differ in density and weight, and also contain different amounts of carbon and hydrogen - the main components that produce heat during combustion. As a rule, denser and heavier species, such as oak and ash, give more energy than poplar or alder. We present to your attention a table with data on the density, specific heat and combustion temperature of various wood species.
Note. The table shows the values of mass and volume heat of combustion of dry fuel per 1 kg and 1 m³ of storage. The maximum temperature is given for ideal combustion conditions with excess air.
If we compare the calorific value by volume, it is noticeable that oak or birch logs will release more heat than less dense conifers. And although the conditions for burning firewood in a stove, potbelly stove or boiler are far from ideal, the trend will still persist. That is, the flame temperature directly depends on the calorific value of the selected wood species.
Moisture effect
High moisture content is the first enemy of effective heat transfer during the combustion of any biomass. Freshly chopped wood with a moisture content of 50-65% will never provide the indicators given in the table above, and here's why:
- In the process of heating the furnace, the water contained in the logs absorbs part of the heat generated.
- When the firewood has flared up well and given enough heat, moisture from the liquid phase goes into steam, taking away the lion's share of the energy received. Since the hot steam leaves the firebox along with the flue gases, it is impossible to recover the lost heat.
- To ignite a raw tree, a higher temperature is needed. From a match with a piece of paper, it is quite difficult to ignite them; you need to use a gas burner.
For reference. Part of the water vapor condenses from contact with the walls of the chimney, which causes a small fraction of thermal energy to be released. It only warms the pipe, and not your home.
How much the calorific value decreases, and then the burning temperature of the tree depending on humidity, is reflected in the following table:
It is easy to see that the heat transfer of freshly sawn firewood is half that of dry fire (humidity less than 20%). Accordingly, to heat the house will have to spend twice as much solid fuel. A separate issue is the stove in the bath, which requires maximum heat for kindling. When using raw wood, it will not be possible to reach a high temperature, no matter how much logs are thrown into the furnace. The thing is their weak heat production, which will not allow you to flood the bathhouse as it should.
Combustion air consumption
To completely burn wood and achieve maximum temperature in the firebox, it is necessary to supply air with an excess of 130% of the required amount. When creating ideal conditions, the reaction of carbon oxidation (combustion) is described by a simplified equation:
In words, this means that when heated, the wood decomposes into carbon (C) and hydrogen (H), and oxygen (O) is supplied from the outside. Each carbon atom meets 2 oxygen atoms and forms carbon dioxide. In turn, hydrogen combines with oxygen residues, which causes water to be released (again, in the form of steam). Latin Q at the end of the formula means the release of heat going to heating needs.
But what happens if the ideal conditions are violated:
- Hydrogen is a very active element, therefore, it is oxidized in the first place. When there is not enough air (you covered the blown furnace or boiler), then carbon remains without oxygen molecules. As a result, unburned carbon monoxide (CO) is formed, flying out into the pipe along with the smoke.
- With a total lack of oxygen (the so-called smoldering mode), carbon turns into charcoal, ash and soot. The latter, under the influence of chimney draft, also rushes into the pipe and settles on its walls.
- Excess air occurs from strong chimney draft or improper operation of the turbocharger of the boiler. The phenomenon is fraught with heat loss through the chimney.
Note. Bonfire is an example of a heat source with uncontrolled air supply. The more it is, the lower the temperature of the fire in the fire and less heat at the exit. To warm for a long time near the hearth, logs need to be planted continuously.
An obstacle to obtaining good heat from firewood can be the temperature of the air supplied to the combustion chamber. Cold airflow just as successfully removes precious heat as does water vapor. It is no accident that manufacturers of high-quality heating units arrange a special channel for heating air from the outer walls of the firebox. How this is implemented in practice is shown in the photo of a self-made solid fuel boiler from our expert.
Useful Tips
The high temperature of burning wood is the key to effective heating of a private house. Even manufacturers of TT boilers and wood-burning stoves indicate in the instructions that the best efficiency of the heat generator is observed when operating at maximum mode.
In the end, we will give some recommendations on how to achieve high heat production, and hence energy savings:
- If you have to buy firewood, do not take low-calorie species, such as willow and poplar. The first is too saturated with moisture, and the second is low in density. Remember that you pay cubic meters, not kilograms of solid fuel.
- Dry the logs as much as possible. A freshly cut tree reaches a moisture content of 20-25% after a minimum of one and a half years of drying in an open firewood.
- Try not to operate the boiler in the vaunted mode of decay, which supposedly helps to save.As you can see from the photo, the fire temperature during normal combustion is 800 ° C, and smoldering wood will give no more than 450 ° C. With such heat dissipation, the house cannot be heated.
- Carbon monoxide CO is not only combustible, but also toxic. Trying to save fuel, we block the air supply to the furnace and provoke its release. Not only is the potential fuel thrown out to no avail, but it also pollutes the environment.
- Do not organize the air supply to the furnace from the street, so as not to lose energy on its heating. When choosing a solid fuel boiler, pay attention to models with a heating channel. They can easily be distinguished by the fan installed on the top panel, and not in the ash pan door.
Of course, it is impossible to continuously maintain high temperature in the chamber, because the house’s need for heat also changes and depends on many factors. To accumulate excess energy, there are heat accumulators, they are also buffer tanks. In extreme cases, purchase a forced-charge heat generator, where during periods of shutdown of the fan, air access is completely blocked.