Dielectric spectroscopy of artificial faeces for smouldering applications

Our objective is to develop an analysis concept in order to measure the in situ estimation of state parameter, such as water content, during smouldering of waste and sand mixtures. High frequency electromagnetic (HF EM) method presents a high potential for the quantitative estimation of state parameter in porous media. However, it provides indirect measurement: the major challenge is to derive robust relationship between the performed measured permittivity and the parameter under interest. Thus, laboratory measurement of dielectric properties of waste and sand mixtures under controlled boundary are urgently needed. In this preliminary study, the relative effective complex permittivity of artificial faeces was studied over the 50°MHz–3°GHz frequency range with network analyzer technique in combination with homemade open ended coaxial method. In a first step, the effect of water content on dielectric properties was investigated. The results have shown an important dispersion for the imaginary part which can be related to interface process and a systematic increase of the permittivity with water content. In a second step, a shrinking test was monitored with the homemade probe. The relative complex permittivity shows a nonlinear evolution with gravimetric water content and show marked transitions during the decrease of water content. The results of combined investigations have shown the potential of HF EM techniques for quantitative monitoring of the hydraulic state of waste and sand mixtures during smouldering combustion.