Cleaning of harvested Norway spruce stumps using a vibration-based method

Abstract

With the increasing demand for renewable energy in Sweden and Finland, interest in using softwood stumps as biofuel is growing. The major drawback to using these energy-rich stumps as biofuel is the presence of contaminants, which lead to high ash content and thereby lower technical feasibility and value as fuel. To reduce contaminant levels, stumps are normally shaken during extraction, but this process is time-consuming. It is also frequently inadequate, necessitating storage for further cleaning for long periods usually exceeding one year, which can lead to high dry-matter losses. Faster and more effective cleaning methods are therefore highly desirable. A vibration-based method for cleaning, as often used in other industrial sectors, could offer a solution. The reduction in contaminants on stumps harvested from two different soil types was tested using a grid-fitted test-rig bench which could be vibrated in one or three dimensions. The vibration was performed at settings from 1.33 g to 6.15 g. Ash content was reduced by 50% of the initial value after 30 seconds, which in some cases was sufficient to achieve ash content below 4%. The cleaning process was effective regardless of vibration amplitude used or orientation. By employing vibration, clean stumps could be made available as fuel within the same harvesting season. Such rapid removal of stumps would allow prompt site regeneration processes.