Simulation of stump crushing and truck transport of chips

The use of stumps for energy production has grown rapidly in Finland and is also commencing in Sweden, the UK and a number of other countries. Practically all stumps are comminuted either at the plant or at terminals, whereas a major part of small-diameter trees and logging residues is chipped at roadside landing. Until now, crushing of stumps has been done with heavy, often stationary, crushers. In smaller plants, construction of a stationary crusher is not economically feasible. In addition, transportation of stumps calls for special trucks, while economical transport distances are short owing to the small payload. Recently, effective mobile crushers suitable for the comminution of stumps, that can also operate as mobile chippers for logging residue and small-diameter trees, have been introduced. They move from landing to landing with the crushed material transported to the end-user by trucks. In this study a discrete-event simulation model was programmed to find optimal set-ups for the supply chain of crushed material made from stumps at different road transport distances. The simulation model was based on the continuous supply of crushed material from landings to a district heating plant. Transportation distances varied from 20 to 120 km, while the number of trucks varied from one to four. It was found that already at 20 km the use of two chip trucks is competitive and after 40 km a third truck should be introduced into the system. Beyond 100 km four trucks would be needed. The results were compared with those obtained from a static spreadsheet model. The static model underestimated the waiting cost in the situation where the productivity of crushing and truck transport was almost balanced.