Determination of Electrical Resistivity of Dry Coke Beds
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INTRODUCTION AND PREVIOUS INVESTIGATIONS
IN several submerged arc furnace processes, e.g., FeMn, SiMn, and FeCr, a coke bed is present and acts as a resistance element where energy is developed. Although the definition of the coke bed varies, a coke enriched volume of various geometrical shapes has been observed in dig-outs of the three processes mentioned above.[1–4] A dry coke bed, which is discussed in this article, is a packed bed of randomly distributed carbon particles. The goal was to build an apparatus able to measure the electrical resistivity of various industrial reducing agents, at the characteristic temperatures and current densities of an industrial coke bed. The particle size dependency is also of crucial interest. The main differences between the previously reported methods for measuring the resistivity are the means of heating the coke sample and the particle sizes studied. The heating was either done indirectly[5–9] by means of an external heat source or by running a sufficiently high current to accomplish ohmic heating of the coke bed sample.[10–14] The particle sizes range from industrial sized materials with a particle diameter between 6 and 30 mm[7,10–14] and smaller sized, often crushed, material.[5,6,8,9] The experiments mentioned were all performed at elevated temperatures, with a maximum P.A. EIDEM, PhD Student, and M. TANGSTAD and J.A. BAKKEN, Professors, are with the Norwegian University of Science and Technology (NTNU), Trondheim N-7491, Norway. Contact e-mail: [email protected] Manuscript submitted December 1, 2006. Article published online December 18, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
temperature between 1400 C to 1600 C. Extensive work has also been done at room temperature, e.g., Dijs et al.[6] and Willand.[15] The external mechanical pressure applied on top of the coke bed has also varied, from no added external force,[5] i.e., only the weight of the sample material, to about 250 kg added on to a 0.07 m2 surface.[13] Olsen[13] varied the mechanical pressure on the coke bed from 2830 to 3540 kg/m2 without being able to see any correlation between pressure and electrical resistivity. Many authors[5–14] have reported that resistivity decreases with increasing temperature as a general trend for different carbon materials. In the matter of particle size dependency for packed beds containing solely coke, Bakken and Wærnes[7] report an increasing resistivity with increasing particle size, while others[6,15] report the opposite. Dijs et al.[6] report that increasing the amount of volatile matter increases the resistivity.
II.
EXPERIMENTAL
The apparatus shown in Figure 1 consists of a castable high-alumina refractory cylinder, built around an upper and a lower 304.8 mm standard graphite electrode. The layer of Kaowool (Thermal Ceramics LTD, England) fiber insulation reduces the heat loss and minimizes temperature gradients inside the coke sample. Water-cooled copper bus bars connect the power supply to the top and bottom graphite electrodes. Weights are added to the to
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