Effect of contact resistance on bulk resistivity of dry coke beds

Measurements show that bulk resistivity of dry coke beds decreases with increasing particle size. A further development of a coke bed model is proposed to explain this correlation. By image analysis, it has been determined that the total porosity increases with increasing particle size. An increased total porosity of the particles decreases the mechanical strength of the particles. In the modeling work, the strength of the coke particles is introduced through Young's modulus. By the use of discrete element method (DEM) modeling of a dry coke bed, the particle-to-particle contact area variation with varying particle size and particle strength has been introduced into a model of the dry coke bed. This was done by the introduction of the concept of the Holm's radius, known from metal contact theory for describing how the contact resistance is affected by the material resistivity and the contact area. By assuming a decrease in the particle strength due to increased porosity of the coke particles with increasing particle size, the calculated bulk resistivity for 7.3-mm particles with a Young's modulus of 1.0 GPa is 5.24•10^-3 Ωm and 3.44•10^-3 Ωm for the 20-mm particles with a Young's modulus of 0.1 GPa. By comparison, the measured bulk resistivity of the Corus coke is 4.67 ± 0.30•10^-3 Ωm for the 5- to 10-mm fraction and 3.71 ± 0.45•10^-3 Ωm for the 15- to 20-mm fraction. The measured contact resistance of Swedish Steel AB (SSAB) coke decreases with increasing contact area size from a contact diameter of 5 mm to a contact diameter of 30 mm. This is probably due to an increasing number of electrical contact spots. When two spheres are in contact, the measured contact resistance is lower compared to the 5-mm-diameter contact, which indicates that the increased contact pressure has lowered the contact resistance. This supports the modeling results.