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Pellet firing
The burner's parts in contact with flames are wearing parts that need
replacement as required. To increase the efficiency and the service life
of the grate and inner burner pipe, please observe the following:
• The burner must be adjusted with a flue gas analysis instrument for
the pellet quality used.
• A draught damper must be fitted between the boiler and the chimney.
• A store designed for pellets must be used. This makes the pellet sup-
ply more regular.
• If the make of pellets is changed, adjustment must be repeated. Pel-
lets may have different contents that may affect combustion.
• Use only pellets complying with Swedish Standard Group 1.
• It is important for the grate to be placed correctly in the burner pipe.
Pellet quality
Use only wood pellets that meet the requirements under
standard SS 18 71 20, group 1 or equivalent. Any other pellet
quality can produce operating problems.
Some of the pellet requirements under SS 18 71 20 group 1:
Length:
≤
4 times the diameter
Bulk density:
≥
600 kg/m3
Fines content < 3 mm:
≤
0.8% by weight
Calorific value:
≥
4.7 kWh/kg
Ash content:
≤
0.7%
Total moisture content:
≤
10% by weight
The melting point of the ash should be high, >1350 °C, as molten (sin-
tered) ash is a problem. Molten ash is very difficult to remove.
Handling and storing pellets
Wood pellets must be stored dry under a roof but do not need to be in a
heated room.
Ready-made pellet stores are available on the market. These are prefer-
able to homemade stores.
Combustion
The ash sinters if the combustion temperature is too high. If this occurs,
the combustion temperature is too high or the pellets are of poor quality
with a high ash content with a low melting point. The melting point of the
ash should be >1350 °C. Check the adjustment of the burner.
Do not confuse sintered ash with the easily removable ash cakes or ash
balls that may be found in the combustion chamber.
Correct adjustment is important for reliability, heating economy, efficiency,
low emissions of environmentally hazardous substances and the service
life of the parts of the burner in contact with the flame. Optimum adjust-
ment is possible only using flue gas analysis instruments.
Flame
A few minutes after the burner has been started, the flame must have
a yellow-white colour. It is normal for the colour to vary a little between
white and yellow. The colour of the flame is an indication of the quality of
combustion:
Light yellow:
good combustion, invisible smoke at normal tem-
perature.
Reddish:
too little air or too much fuel, low efficiency, the
boiler's heat-absorbing surfaces are covered in soot.
Whitish:
short flame, depending on whether there is too much
air or too little fuel, low efficiency, high flue gas
temperature.
Flue gas temperature
A high flue gas temperature may be because the boiler has not been swept
or there is too much combustion air. This produces low efficiency and un-
necessarily high pellet consumption. An older boiler often has higher flue
gas temperatures than a modern boiler under similar conditions.
A low flue gas temperature may be because of poor consumption on ac-
count of too little air or because the boiler is overdimensioned, particularly
if the burner is set to low power. There is then a risk of condensation in the
chimney, which will cause damage.
During combustion, water is formed as steam, which accompanies the
flue gases out into the chimney. Depending on the temperature drop in
the chimney, the steam may condense as water. Condensation damage
can be avoided if the flue gas temperature is minimum 70 °C one metre
below the top of the chimney. A low flue gas temperature results in higher
efficiency but this must be weighed up against the risk of condensation.
The temperature is measured when the boiler is at its normal operating
temperature no earlier than five minutes after the burner was started and
with the draught damper closed.
Measures to increase the flue gas temperature:
• remove any turbulators or baffle plates in the boiler
• insulate the boiler's flue pipe and the chimney in cold rooms
• increase the capacity of the burner
• install flue lining tubes
Turbulators
Certain boiler types have or can be fitted with flue gas turbulators. Their
task is to make the flue gases turbulate to extract more heat and thus
increase efficiency.
At low burner power, the flue gas temperature is low and there is a risk of
condensation forming in the chimney. Try to shorten the turbulators until a
suitable flue gas temperature is obtained.
Draught damper
The chimney must be fitted with a draught damper.
The draught is affected by temperature, weather and wind. As the com-
bustion is affected by the draught, stable draught conditions should be the
objective. The hatch lets boiler room air into the flue. The advantages of
this are:
• more stable draught and flue gas temperature
• reduced stoppage losses
• ventilation of the flue
• drier flue gases, which reduces the risk of condensation
Smoke from the chimney
The colour of the smoke reveals the quality of combustion:
Greyish-brown:
smoking combustion on account of too little air.
Invisible:
heat shimmer. When the outdoor temperature is
above zero or down to a few degrees below zero, the
smoke must be invisible.
White:
when the outdoor temperature is lower, only a weak
white smoke of steam must be visible.
If the boiler was previously fired with wood, the smoke may be dark and
malodorous because tar on the boiler and chimney walls is being burned
off. This may last for a week.
