51 AJAE 3(2):51-58 (2012) ISSN:1836-9448 Evaluation of the performance of a three-dimensional vibrating test rig for cleaning stumps Erik Anerud, Raida Jirjis and Girma Gebresenbet Department of Energy and Technology, Swedish University of Agricultural Science, Box 7032, SE-75007 Uppsala, Sweden Abstract It has been shown that vibrating devices can successfully be used for separation and removal of unwanted/contaminant material, for both laboratory and full-scale industrial purposes. The presence of impurities on energy-rich softwood stumps is a major drawback when using stumps as fuel, since it leads to high ash content. Vibration methods can be an efficient approach to remove excess impurities and achieve acceptable ash content. Stumps are usually shaken in connection with harvesting and stored afterwards to facilitate the removal of contaminants. The required storage duration can lead to high dry matter losses and a faster and efficient cleaning method such as vibration is highly desirable. This study evaluated the performance of an adjustable three-dimensional vibrating test rig designed to assess the cleaning efficiency of vibration-based methods. Vibrating parameters such as acceleration amplitude, frequency and displacement of the test rig were determined for 12-24 mm distance between eccentric weight centre of mass and the rotating shaft of electric motors at rotation velocity 19.16-29.17 Hz. The acceleration amplitude ranged from 1.38 to 5.71 g. The parameters could be repeated irrespective of direction and number of vibration directions. The test rig was therefore considered to be a reliable tool for determining and evaluating the efficiency of vibration in removing contaminants from stumps. Keywords: Tree stump cleaning, Vibrating test rig. Introduction Removals of material, separation, fractionation and packing have been successfully carried out with vibration devices in a wide spectrum from laboratory to full-scale industry equipment (Wu, 2011). Vibration devices have also been used to sort and clean crops such as potatoes, carrots and sugar beet (Kakhno & Myalkovskii, 1990) and for quality evaluation of fruit and vegetable products (Chen & Sun, 1991). Full-scale devices such as screeners are used for bulk material processing in the mining industry, as well as gravel fractionation and dewatering at sandpits (Anon 2011a). Vibration can be a potential method to clean woody biomass, such as stumps, which are normally contaminated with soil and stones. Stumps contain a relatively high energy content and can provide a potential source of renewable energy. However, a major drawback with using stumps as fuel is that the presence of inorganic contaminants leads to high ash content. This results in lower heating value and large ash volumes to handle, which thereby reduces the value of stumps as a fuel. High levels of various minerals can also affect ash melting behaviour during combustion, which can lead to sintering and operating problems (van Loo & Koppejan, 2008). Methods to reduce inorganic contamination are therefore highly desirable. The technique used for stump harvesting today is based on a harvesting head, weighing approximately 1-2 tonnes, mounted on an excavator weighing around 20 to 25 tonnes. Stumps are cleaned by shaking the head during harvesting. However, this shaking is time- consuming and usually causes stress for the machine operator. It has been reported that cleaning normally occupies 40% of the total time taken for harvesting (Laitila et al., 2008; Athanassiadis et al., 2011). Ash contents ranging between 1 and 24% have been reported for crushed stumps (Korpinen et al., 2007). The harvested stumps are piled and stored outdoors for more than one year to remove as much as possible of the adhering contaminants. A storage phase in the fuel supply chain is also necessary to meet the uneven demand for heat production during late autumn and winter. Although fuel quality is positively correlated with storage duration due to the combined decrease in moisture content and ash contend, it also has drawbacks such as high dry matter losses (Anerud & Jirjis, 2011). More efficient procurement methods to produce clean stumps that can be used shortly after harvesting are therefore desirable. Cleaning stumps using a vibrating machine could be an efficient and fast method to reduce contaminants and thereby eliminate or shorten the required storage duration. Use of vibrating heads to get cleaner stumps has been considered (Nylinder, 1976; Jonsson, 1985; Hakkila, 1989) but no test studies have been reported. To study and evaluate a cleaning method based on vibration, a reliable test rig has to be designed. Test rigs for simulation of machinery vibrations, such as a six-degrees-of- freedom device, have been successfully developed (Hostens et al., 2000). With that test rig the oscillations are achieved using six hydraulic cylinders, controlled by a digital hydraulic numerical controller. The aim of the present study was to evaluate the performance of an adjustable three- dimensional vibrating test rig in terms of its amplitude and frequency. The test rig was specially designed for field tests on cleaning stumps of adhering contaminants.