Electrical capacitance as a proxy measurement of miscanthus bulk density, and the influence of moisture content and particle size

This research aimed at measuring the electrical capacitance of miscanthus biomass as a potential proxy measure of bulk density. The experimental arrangement allowed for continuous variation of bulk density through compression, whereas moisture content was varied at two levels, being air-dried (<5%) and oven dried (14.5%), and particle size was varied by using material that passed milling screen aperture sizes of 6.35, 9.53, 12.7, and 25.4 mm. A chamber was constructed containing the biomass, which was compressed using a hydraulic cylinder, causing the biomass bulk density to increase during the experiment. During the compression, the pressure applied vertically onto the biomass was inferred from the measured fluid pressure in the hydraulic cylinder. In addition, the displacement of the cylinder was measured using a linear encoder, allowing for instantaneous bulk density calculations. Two capacitors, each comprising dual parallel flat copper plates, were fitted inside the chamber, where the biomass under compression acted as the dielectric medium. The conjecture was made that the force with which the biomass is pushed against the capacitor plates would highly influence the measured capacitance value, and therefore, one capacitor was placed in a longitudinal direction to capture vertical forces, and a second in the lateral direction to capture transverse forces. The results showed a quasi-linear relationship between capacitance and bulk density. A proportional relationship was found between electrical capacitance and moisture content as well as between electrical capacitance and particle size. A comparison between vertical capacitance versus bulk density and the applied pressure versus bulk density showed that they are independent measurements. Therefore, the initial conjecture being that the force with which the biomass is pushed against the capacitance plates would have a large effect on the capacitance was deemed false; instead, the internal reorganization of biomass particles seems responsible for the variation in capacitance as observed. The results imply that electrical capacitance may serve as a proxy for the measurement of miscanthus bulk density, but since moisture content and particle size have a marked effect on the capacitance, they must be determined separately or calibrated for. Currently, to determine the instantaneous moisture content of field crops, capacitor plates are already an integral part of yield monitoring systems. However, the method as investigated, adds the potential simultaneous measurement of the instantaneous bulk density of a biomass flow.