Thermoelectric properties of carbonaceous solids

Abstract

The application of thermoelectric measurements to characterize carbonaceous materials, such as graphite and chars, can be helpful for monitoring the chemical/physical evolution of the material during gasification, which occurs upon steps of gas diffusion and heterogeneous chemical reactions at moderate and high temperature. The present article reports on an experimental study on the application of thermoelectric measurements for qualitative and semi-quantitative characterization of carbonaceous materials, such as graphite and chars, also in course of high temperature chemical conversion.Among carbonaceous fuels, graphite and cokes are poorly reactive, whereas the reactivity increases moving from coal to biomass derived chars (e.g., carbonized wood) [1]. To date, investigations have normally been carried out in thermo-balance, providing information on the weight loss of the sample during the test interval as well as on the evolution of gaseous products, when coupled with mass-spectrometry. In addition to the morphology and the degree of order at molecular level being relevant in determining the gasification reactivity of carbons, the presence of some elements in the ashes (e.g., alkalis) can accelerate the carbon conversion [2]. Conversely, high ash content has a negative impact because of the increased diffusional resistances and heat dispersion [3]. The enlargement of the porosity and the possible melting and coalescence of ash inclusions [4] are also correlated to some degree to the loss of reactivity. During carbon gasification tests in a thermo-gravimetric (TG) apparatus, the weight loss enables quantitative characterization of the fuel reactivity, but the internal structural evolution of the particles cannot be detected. Notably, it is difficult to perform on line measurements in the core of the oxidizing particle (for example, X-ray tomography to obtain a 3-D map of pores and materials) during conversion [5].The thermoelectric or Seebeck effect is the generation of an electromotive force (EMF) in a solid, when a gradient of temperature is established for any reason [6]. The thermoelectric effect is induced by the interactions between heat carriers in the solid and thermally excited electrons, leading to the accumulation of electrons and holes at the opposite sides of the body [7]. The ratio between the EMF, measured in μV, and the temperature difference is defined as the Seebeck coefficient [8], which in most cases is referred to as a temperature gap of 100 K. The Seebeck coefficient is an absolute property of a material; however, for practical reasons, it is usually referred to the property of a standard element, in most cases Pt. Technological applications of the Seebeck effect can be found in temperature measurements by thermocouples, in sensors for measuring gas concentration, and in micro-generation of electricity.The generated EMF depends on the temperature difference between the body ends as well as on the properties of the material. The thermoelectric effect of a pure material is the combination of...