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:

drying,
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.

Popiół drzewny uznawany jest za dobry nawóz rolniczy. 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, która w każdym miejscu musi być wyższa od 700°C ważna dla procesu spalania jest ilość powietrza doprowadzana do kotła. Dla różnych palenisk i rodzajów drewna wymagane są różne wartości tzw. współczynnika nadmiaru powietrza 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 i CO2 na wykresie:

factor
excess
air L.
THE2
[%]
kominek, logs 2,3 ÷ 3,0 12 ÷ 14
piec na szczapy 2,1 ÷ 3,0 11 ÷ 12
kotły na zrębki 1,4 ÷ 1,6 6,0 ÷ 8,0
kotły na pelety 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 w spalinach) or use the service's help.

You have to know about it, that:

to be burned 1 kg suchego drewna potrzeba 3,5-4 m³ powietrza,
maximum CO content2 dla spalin CO2 max = 20,2 [%],
współczynnik nadmiaru powietrza L = CO2/ CO2 max,
CO fungibility2 i O2 dla drewna ustala się wg wzoru: THE2 [%] = 1,04 (20,2 – CO2 [%])
outlet loss, Sfestival odczytuje się z wykresu znając zawartość O2 w spalinach i ich temperaturę na wyjściu z kotła:

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

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