Sensing Moisture Content and Bulk Density of Pelleted Biofuels by Microwave Permittivity Measurements

Authors: Trabelsi, Samir; Nelson, Stuart

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 4/15/2009
Publication Date: 6/1/2009
Citation: Proceedings of the 8th International Conference on Electromagnetic Wave Interaction with Water and Moist Substances (ISEMA 2009), June 1-5, 2009, Epoo, Finland. Sensing Moisture Content and Bulk Density of Pelleted Biofuels by Microwave Permittivity Measurements. Meeting Proceedings. Trabelsi, S., Nelson, S.O. 2009. pp. 256-262.

Interpretive Summary: Electrical permittivities, or dielectric properties, are electrical properties that determine how materials interact with electric fields such as those of high-frequency and microwave electromagnetic energy. Therefore, the dielectric properties of materials determine how rapidly they will heat in microwave ovens and lower radio-frequency dielectric heating equipment. Dielectric properties are also important in low power applications, such as the rapid measurement of moisture content in grain and other commodities. Biofuels represent an attractive alternative source of energy to fossil fuels. Moisture content of solid biofuels is a determinant factor in pricing, optimum combustion conditions, and storage management. This paper discusses the measurement of dielectric properties at microwave frequencies between 2 and 18 GHz for pine pellets prepared as a biofuel. The moisture contents of the pine pellets are determined from the dielectric properties at a frequency of 10 GHz and compared with moisture contents determined by standard methods. Moisture contents by this rapid, nondestructive measurement were determined with standard error of 0.35% moisture content. Bulk densities of the pine pellets were also determined simultaneously with a standard error of 0.026 grams per cubic centimeter. The dielectric method presented in this paper is nondestructive, does not require contact with the material and can be used for real-time characterization of solid biofuels. The data are of interest to engineers and scientists working on biofuels applications. The new method offers promise for the monitoring of moisture and bulk density in biofuels processing and utilization and can improve the energy efficiency in such applications.

Technical Abstract: A method is described for nondestructive, instantaneous determination of bulk density and moisture content of pine pellets based on measurement of dielectric constant and loss factor at a single microwave frequency. Bulk density of pine pellets was determined from a complex-plane representation of the dielectric properties independent of moisture content or temperature. Moisture content was determined from measured dielectric properties independent of bulk density with a density-independent calibration function. Results of bulk density and moisture content prediction from dielectric properties are reported at 10 GHz. The standard error of calibration for bulk density was 0.026 g/cm3 and that for moisture-content prediction was 0.35%.