9.6 Other fuels
Other fuels
Pellets Pellets have a higher fuel density and can only be burned with flue gas recirculation. Pellets are
always sold by weight. The heating value of hardwood pellets (4.60 kWh/kg) is about 6% less
than that of softwood pellets (4.9 kWh/kg).
Miscanthus The heating value is the same as for air-dried softwood; best bought by weight. Since its ash
has a very low sintering point, flue gas recirculation is required to prevent slag formation. To
reduce boiler corrosion, ensure that chlorine-free fertiliser (potassium sulphate instead of
potassium chloride) is used. Regardless of whether it is chopped or made into pellets,
miscanthus needs a very large combustion chamber, so for size 35/50 a maximum heating
output of 35 kW can be reached, for size 70/90 a maximum of 63 kW, for size 130 a maximum
of 95 kW and for size 200 a maximum of 140 kW.
Old wood Wood kept in dry storage loses only the volatile components (about 10% of the heating value).
Wood stored in moist storage decays (cold oxidation process) to the point of total loss of heating
value.
Wood shavings The heating value per cubic metre varies widely and is around 30 to 60% less than that of wood
chips (with 30% water content) from the same wood. Since wood shavings are usually air-dried
(15% to 20% water content), they are best bought by weight. Wood chips from some types of
hardwood cause rapid wear on the combustion chamber's fire clay. With wood shavings, the full
rated output of the boiler cannot be reached.
Sawdust The heating value per loose cubic metre is between 25 and 50% less than that of
wood chips (with 30% water content) from the same wood. Sawdust is best bought by weight,
after determination of its water content. Dry sawdust from joineries can be burned, but not fresh
(moist) sawdust from sawmills.
Chipboard Chipboard waste can be used if it is free from formaldehyde and wood preservatives, and is not
coated with PVC. However, this is only allowed with an official permit, which is usually only
granted on a case-by-case basis to wood-processing facilities.
Demolition wood Only wood that is free of halogens and wood preservatives may be burned. The fraction of
rotten wood is often high, and the heating value low, and it is often contaminated with dust,
metal and stones.
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