Anisotropic Electrical Properties of Coal

This study has made use of a two-path interferometer at microwave frequencies (9 GHz) to measure the dielectric constant and conductivity of coal as a function of direction of propagation and polarization of the electromagnetic wave within the coal. Four types of eastern bituminous coal of various rank and one type of eastern anthracite coal were studied, and the effects of the physical properties (rank and anisotropy, pyrite concentration and distribution, moisture content and other mineral matter concentration) on the conductivity and dielectric constant were investigated. In general, moisture increases the conductivity and dielectric constant: however, pyrite layers along the bedding planes of the bituminous coals cause a general increase in conductivity. The anthracites (higher rank coals) have larger values of conductivity (about a factor of 10) and permittivity (about a factor of 2 to 3) than the bituminous (lower rank) coals. Further, models of optical anisotropy seem to correlate quite well with the anisotropy of the electrical properties of anthracites. No distinct relationship was found between the direction of propagation and the electrical properties.