Hydrothermal conversions of waste biomass: Assessment of kinetic models using liquid-phase electrical conductivity measurements

Abstract This experimental study proposes the systematic monitoring of liquid phase electrical conductivity as a new technique for evaluating kinetic models for hydrothermal conversion of biomass. The application to the hydrothermal carbonization of three different wooden materials is validated by batch experiments at 200 °C, up to 120 min of reaction time, and at a 7:1 water to solid ratio. Whatever the biomass, the time course of electrical conductivity follows a unique law, unquestionably corresponding to the evolution of solid-phase carbon content. The model tested comes from literature, and is a simple first-order pattern. The network of elementary steps satisfactorily explains the experimental data. The evidence reported demonstrates that the electrical conductivity should become a standard measurement. In fact, this lumped parameter is for the first time used for predicting the time variation of furfural, an important compound ubiquitously found in the HTC liquid phases. Ordered trends also appear from experiments at higher temperatures, up to 440 °C, but the method highlights a different behavior. The observed discrepancies give useful feedback for steering the upgrading of kinetic equations toward a more structured model, which necessarily should account for the bio-crude. Additional runs with potato peels, an entirely different kind of biomass were used here as a stress test for the method, and as expected gave different results. This new response correctly signals that another model is required for describing the process applied to starchy materials, and confirms the power of the proposed technique as a tool for build-up suitable kinetic models.