Theory of Wood Firing
Efficient and complete combustion is a prerequisite of utilising wood as an environmentally desirable fuel. In addition
to a high rate of energy utilisation, the combustion process should therefore ensure the complete destruction
of the wood and avoid the formation of environmentally undesirable compounds.
In order for combustion to continue, there are certain basic conditions to be complied with
• An adequate mixture of fuel and oxygen (air) in a controlled ratio should be ensured.
• The fire already started in the boiler furnace should transfer some of its heat to the infeed in order to ensure a continuous
combustion process.
It is important to understand that gases burn like flames, that solid particles glow, and that during the combustion of wood, approx. 80% of the energy is released in the form of gas and the remaining part from the charcoal.
During mixing of the fuel and air, it is important to achieve good contact between the oxygen of the air and the combustible constituents of the wood.
The better the contact is, the faster and more complete is the combustion.
If the fuel is in the form of gas, such as natural gas, the mixing is optimal, since we have two gaseous substances that can be mixed to exactly the desired ratio. The combustion may then occur rapidly, and thus the control is fast too, since we can introduce more or less fuel.
In order to achieve approximately the same situation with wood, it may be necessary to pulverise the wood to very small particle size (like
that of flour). These fine particles will follow the movements of the air. A good mixture can thus be achieved with a combustion resembling a gas or oil flame.
The production of wood powder is very expensive, though, and therefore wood powder is only used to a limited extent. In practice, fuel is therefore marketed in sizes varying from wood chips to logs. Firing technology for wood and other solid fuels is thus difficult and more complicated than for example the firing technology in a natural gas or oil-fired heating system.
