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The will for replacement of fossil fuels increased the use of woody biomass as fuel in heating and power plants. An important parameter for the use woody biomass as fuel is the water content of biomass as it influences the heating value and thereby the combustion control and pricing. Currently, water content is determined using the gravimetric method by drying samples  in an oven. This method is neither fast nor representative, as the water content can vary largely in different parts of the container which raised the need to develop sensors that are able to measure water content of biomass on-line and in a representative way.This thesis presents results from studies on the measurement of water content of woody biomass with radio frequency. Included are the study on the influence of temperature in the measurements, the use of dielectric mixing models to explain the dielectric behavior of woody biomass and the application of the measurement principle to full-scale.

It was found that varying temperature between 1 and 62 °C did not influence the prediction of water content in biomass. It was possible to explain the behavior of the dielectric constant of woody biomass with a model with physical basis using empirically obtained parameters. Application of the principle to full-scale had positive results but larger prediction error than laboratory-scale measurements.

The use of bioenergy has been increasing due to efforts in fossil fuels replacement. Modern bioenergy technologies aim for high efficiency and low pollution levels, which increases the need for methods for the on-line characterization of biofuels.

Dielectric methods have been identified as useful for the sensing of solid biofuels because they allow for rapid, nonhazardous, nondestructive, and bulk determination of material properties. The dielectric properties describe the interaction between the material and the electromagnetic waves. Dielectric properties are intrinsic of the materials and can therefore be used for the development of prediction models that can be applied regardless of the measurement technique. The study of the dielectric properties is also important as it improves the understanding of the dielectric behavior of the materials.

This thesis focuses on the dielectric properties of solid biofuels and their use in the characterization of these materials. The work presented includes the development of new methods permitting the determination of the dielectric properties of solid biofuels with large particle size (waveguide method), broadband measurement of the dielectric properties (coaxial-line probe), and the use of a previously developed method for the accurate determination of the dielectric properties (free-space method). The results includes the dielectric properties of solid biofuels and their dependence on parameters such as frequency, moisture, density, and temperature.

This thesis also presents semi-theoretical models for the determination of moisture content, which obtained a RMSEP of 4% for moisture contents between 34 and 67%, and an empirical model that resulted in a RMSEC of 0.3% for moisture contents between 4 and 13%.

Finally, this thesis includes measurements of the influence of salt content on the dielectric properties and a discussion of its use for estimation of the ash content of solid biofuels.