The process of burning wood

The process of burning wood

For proper combustion, it is necessary to meet the conditions of good mixing of fuel with air and an appropriate temperature in the furnace not lower than 600 ° C.. It is easiest to burn gas, because mixing gas with air is not a problem. Burning solid fuels is much more difficult, as it is more difficult to bring the air particles into direct contact with the fuel particles.

The wood burning process takes place in three stages:

gasification and combustion,
burning of charcoal.

When heating wood, the first process of water evaporation and surface gasification takes place, i.e. the decomposition of chemical compounds under the influence of a sufficiently high temperature, the so-called. pyrolysis. After the moisture evaporates, this process moves deeper into the wood. Gas burns with a flame when it comes into contact with air. The wood is characterized by a very high volatile matter content of up to 80% (coal approx. 30%). the remaining 20% burns as incandescent charcoal until completely burned out, with the exception of non-flammable compounds, which create ash.

Wood ash is considered a good agricultural fertilizer. It consists of silicon compounds (And) and potassium (K), punishment (On), phosphorus (P), calcium (That) and magnesium (Mg). An important feature of ash is its melting point, which decreases with increasing potassium and partly sodium content. Melting point too low (below 1050 ° C) may cause contamination of the internal surfaces of the boiler. this problem occurs mainly when burning straw, in which the potassium content may even be 10 times greater than in wood.

Next to the temperature in the combustion chamber, which must be higher than 700 ° C in all places, the amount of air supplied to the boiler is important for the combustion process. For different fireplaces and types of wood, different values ​​of the so-called. air excess factor L (lambda), determining how many times the amount of air is greater than the theoretical amount resulting from the stoichiometric formulas.

Too little air causes the unburning of the carbon particles and the formation of carbon monoxide, and unburned hydrocarbons getting into the exhaust gas. In turn, too much air causes the boiler to cool down (part of the air is not involved in combustion) and decreased efficiency, and also promotes the formation of harmful nitrogen oxides NOx.

in addition, large excess air increases the temperature of the combustion flame, which in turn contributes to, unfavorable for the boiler, ash melting.

Approximate values ​​of the excess air factor L are presented in the table, and the relationship between lambda and the percentages of O2 and CO2 on the graph:

air L.
fireplace, logs 2,3 ÷ 3,0 12 ÷ 14
bake for logs 2,1 ÷ 3,0 11 ÷ 12
wood chip boilers 1,4 ÷ 1,6 6,0 ÷ 8,0
pellet boilers 1,2 ÷ 1,6 4,0 ÷ 8,0


The owner of the boiler or fireplace should check the combustion parameters from time to time to determine the efficiency of the device. You can do this with your own, simple devices for exhaust gas analysis (e.g.. pump with a chemical CO content marker2 in the exhaust gas) or use the service's help.

You have to know about it, that:

to be burned 1 kg of dry wood is required 3,5-4 m³ of air,
maximum CO content2 for CO flue gas2 max = 20,2 [%],
excess air factor L = CO2/ CO2 max,
CO fungibility2 and O2 for wood it is determined according to the formula: O2 [%] = 1,04 (20,2 – CO2 [%])
outlet loss, Sfestival is read from the graph knowing the content of O2 in the flue gases and their temperature at the outlet from the boiler:

The boiler efficiency is calculated
approximately L = 100% – Sfestival

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