A discrete-event simulation approach to improve efficiency in stump fuel supply chains

Sammanfattning: Current concerns about climate change and fossil fuel dependency have intensified interest in renewable energy and increased demand for sustainable alternatives. Softwood tree stumps could be a very interesting renewable fuel assortment. The stump-root system constitutes about 25% of stem volume. In Sweden, stump fuel extraction is not a well-established practice and large resources are currently left in the forest after final felling. The stump fuel supply chain is both challenging and complex due to distance between resource and end-user, bulkiness of the material, initially high moisture and ash content, and number of sub-processes involved. Optimisation of logistics issues within the stump fuel supply chain is crucial to ensure low delivery cost. Carefully planned stump fuel systems can reduce the supply costs and help deliver the fuel at a competitive price. In this thesis, various systems for stump transport and comminution were evaluated, particularly regarding resource use efficiency and cutting of unnecessary costs. Various factors associated with different aspects such as harvest site characteristics, fuel quality, biomass losses and machine performance were also evaluated in terms of their impact on fuel cost. A discrete-event simulation approach was applied. Models for all machines and activities included, from forest to end-user, were developed and programmed using the ExtendSim simulation language. The simulation results showed large variations in system performance and system cost. The cost of different transport and comminution alternatives differed by approximately a factor of two, irrespective of transport distance. The most cost-effective option proved to be crushing stumps on the ground and using a self-loading truck for wood fuel transport. Minimising idle machine capacity was identified as a key factor in achieving a cost-effective system. Moreover, well-planned stump storage was shown to reduce the delivery cost significantly. The most influential parameter for fuel cost was machine productivity. Enabling machines to operate efficiently throughout the whole supply chain is crucial for system economics and can be decisive for stump fuel feasibility.