Page 16
Prerequisites for effi cient combustion:
• suffi cient amount of combustible material
• oxidising agent, mostly oxygen
• enough heat to reach the ignition temperature or to maintain the minimum
temperature required for combustion
• the correct proportion of the combustible material to ambient air or to the
reactive gas
The combustion process
The combustion process takes place in three stages:
1. Drying stage
In this stage all the moisture still in the wood is evaporated. This occurs at
temperatures of approx. 100°C. During pre-ignition, the wood must be supplied
with heat (using small pieces of fi rewood). When it dries, the fi rewood shrinks
until cracks are formed which accelerate the drying process.
2. Degasifi cation stage
After the drying process, with temperatures rising from 100° to approx. 300°C,
the degasifi cation of the wood starts. High-energy, combustible gases, consis-
ting of various chemical components, are released (hydrocarbons). The burn-off
of these volatile components, which account for about 80 % of the wood, is
recognisable from the long yellow fl ames that shoot up from the wood. The real
combustion process begins with the ignition of the gases that are produced
at about 225°C (ignition temperature) and with the release of heat. For this to
happen, enough oxygen must also be supplied. The fi nal stage of the combus-
tion is reached at about 300°C. Now the greatest amount of heat is released,
with fl ame temperatures of up to 1100°C.
3. Burn-out stage
After the volatile components have been burnt-off, the charcoal begins to burn
and is degasifi ed at temperatures of about 500 to 800°C without forming soot.
This process can be recognised by the short, translucent small fl ames. All three
stages can occur at the same time from the inside to the outside of a piece of
wood.
Clean combustion
The fi rst prerequisite for keeping pollutant emissions as low as possible is that only
dry wood is used.
The heating-up phase, which produces signifi cant quantities of decomposition pro-
ducts, must be completed as fast as possible by using small pieces of kindling wood
in order to reach the high temperatures quickly.
Reducing the air supply during the degasifi cation stage has a detrimental effect as
the wood degasifi cation continues even without further supply of air and without
fl ames (smouldering). As a result large amounts of the substance of the wood can
be expelled without generating heat and are released into the environment without
having been being burnt-off or deposits such as tar and soot are formed on the walls
of the combustion chamber and in the fl ue gas pipes.
Even during the burn-off phase the air supply must not be restricted completely as
this would create the hazard of carbon monoxide fumes.
When continuing the heating process, make sure that not too much wood is added.
The amount of wood should always match the heat requirements.
JET
I
FIRE - FLAME CONVERGENCE PLATE
The JET
I
FIRE Flame convergence plate, which serves as a cover for the com-
bustion chamber (Figure 30), in conjunction with the pre-heated secondary air
supply through the fi rebox door, results in a higher combustion temperature
and thus in a less polluting and more effective combustion.
The fl ame convergence plate, like the stove itself, should be cleaned of com-
bustion residues from time to time. Make sure that the fl ame convergence plate
is replaced in the correct position.
combustible
material
heat
oxygen
correct proportion
Figure 29
Figure 30
JETFIRE Flame
convergence plate
Secondary air
Figure 31
Figure 32