Energy efficient storage of biomass at Vattenfall heat and power plants
Storage of biomass is often associated with problems such as heat development, dry
matter losses and reduction of fuel quality. The rise in temperature can potentially
cause a risk of self-ignition in the fuel storage. Moreover, emissions from storage piles
can cause health problems in the surrounding. The dry matter losses and reduction of
fuel quality can have economical effects. The aim of this thesis project is to develop
guidelines on how to store large amount of biomass at Vattenfalls heat and power
plants in an optimal way. Storage trials at Idbäckens CHP were done in order to study
the effect of storage on fuel quality, dry matter losses and temperature development.
Two storage trials were performed over six weeks with waste wood chips and stem
wood chips stored in about 4.5 m high outdoor piles. A trial over four days in which
waste wood chips was placed on a heated surface was evaluated. A study to test the
possibility of using waste heat to dry waste wood chips was performed.
Small but not negligible dry matter losses were observed in both of the piles of stored
material. The largest weekly losses were found during the first week of storage and a
declining behavior could thereafter be seen. The accumulated losses during six weeks
of storage were 2.0 % and 1.7 % respectively, for waste wood and stem wood. Storage
during six weeks of waste wood and newly chipped stem wood did not cause any
major deterioration of the fuel quality as such, beside the substance losses.
No drying effect could be seen in the heated surface trial. The surface became warm,
about 50°C, but it was not sufficient to dry the chips. The conclusion is that it is not
possible to dry large amount of chips on a heated surface with the design used here
and during four days.
The overall conclusion is that in order to minimize the dry matter losses the material
should be handled according to the LIFO (last in first out) principle. Whenever
possible, try to purchase fuel that has been stored for a while since the more easily
degraded compounds has already been degraded through microbial activity. There is a
possibility that the largest losses has already occurred. Furthermore, try also to
comminute the material (reduce the particle size) at the plant and as close in time to
combustion as possible.