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