Wood and other types of biomass containof dry matter. This means that the component part of wood will give up 80% of its weight in the form of gases, while the remaining part will be turned into charcoal.

This is one reason why a sack of charcoal seems light compared to the visual volume. The charcoal has more or less kept the original volume of the green wood, but has lost 80% of its weight.

The high content of volatiles means that the combustion air should generally be introduced above the fuel bed (secondary air), where the gases are burnt,
and not under the fuel bed (primary air).

Excess Air
A given fuel requires a given amount of air (oxygen) in order to be converted stoichiometrically, i.e. the amount of excess air (lambda) should be equal to 1.

The fuel is converted stoichiometrically when the exact amount of oxygen that is required for the conversion of all of the fuel under ideal conditions is present. If more oxygen is introduced than an amount corresponding to is equal to 1, oxygen will be present in the flue gas.
At, e.g. is equal to 2, twice as much air is introduced as necessary for the combustion of the fuel.

In practice, combustion will always take place at an excess air figure higher than 1, since it is not possible to achieve complete combustion at a stoichiometric amount of air.

In Table 12, the typical excess air figures are shown together with the corresponding, resulting oxygen percentage
in the flue gas. As shown in Table 12, the excess air figure depends to a high extent on the heating technology and to some extent on the fuel.



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