Analysis and simulation of straw fuel logistics
Straw is a renewable biomass that has a considerable potential to be used as fuel in rural districts. This bulky fuel is, however, produced over large areas and must be collected during a limited amount of days and taken to the storages before being ultimately transported to heating plants. Thus, a well thought-out and cost-effective harvesting and handling system is necessary to provide a satisfactory fuel at competitive costs. Moreover, high-quality non-renewable fuels are used in these operations. To be sustainable, the energy content of these fuels should not exceed the energy extracted from the straw. The objective of this study is to analyze straw as fuel in district heating plants with respect to environmental and energy aspects, and to improve the performance and reduce the costs of straw handling. Energy, exergy and emergy analyses were used to assess straw as fuel from an energy point of view. The energy analysis showed that the energy balance is 12:1 when direct and indirect energy requirements are considered. The exergy analysis demonstrated that the conversion step is ineffective, whereas the emergy analysis indicated that large amounts of energy have been used in the past to form the straw fuel (the net emergy yield ratio is 1.1). A dynamic simulation model, called SHAM (Straw HAndling Model), has also been developed to investigate handling of straw from the fields to the plant. The primary aim is to analyze the performance of various machinery chains and management strategies in order to reduce the handling costs and energy needs. The model, which is based on discrete event simulation, takes both weather and geographical conditions into account. The model has been applied to three regions in Sweden (Svalöv, Vara and Enköping) in order to investigate the prerequisites for straw harvest at these locations. The simulations showed that straw has the best chances to become a competitive fuel in south Sweden. It was also demonstrated that costs can be reduced by adopting appropriate management strategies. Moreover, SHAM has also been used to determine the number of machines that result in the lowest total fuel costs. It was shown that straw can be delivered to a plant at Svalöv at a cost of 29.6 SEK GJ-1. Two new technologies were evaluated by using SHAM: systems based on compact rolls and systems based on chopped straw stored outdoors. The costs were about 5-20% higher with these methods compared with systems based on high-density balers, but their prospects of becoming competitive alternatives in the future are good.
Rapport - Sveriges lantbruksuniversitet, Institutionen för lantbruksteknik
Publisher: Institutionen för lantbruksteknik, Sveriges lantbruksuniversitet
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