Investigation of Potential Benefits of Using Bricks of High Thermal Capacity and Conductivity in A Rotating Calcining Kiln

Petroleum coke is processed into calcined coke in a rotary kiln, where the temperature profiles of flue gas and coke bed are highly nonuniform due to different flow and combustion mechanisms. Motivated by saving energy costs, the effect of refractory brick's thermal properties on potential energy savings is investigated. This study focuses on investigating potential energy savings by replacing inner one third of existing bricks with higher thermal capacity (Cp) and/or higher thermal conductivity (k) bricks. This investigation is motivated by postulating that the bricks with higher thermal capacity can store more thermal energy during the period of contacting with the hot gas and release more heat to the cock bed when the bricks rotate to below and in contact with the coke bed. A rotational, transient marching conduction numerical simulation is conducted using the commercial software FLUENT. The impact of brick heat capacity and thermal conductivity on transporting thermal energy to the coke bed is analyzed. The results show: (a) Increasing the heat capacity of brick layer reduces brick temperature which helps increase the heat transfer between the hot gas and brick, in other words it does help brick store more heat from the hot gas, but, heat transfer between brick and coke is reduced, which is opposite to the original postulation. (b) Higher brick thermal conductivity decreases brick temperature thus increases heat transfer between hot gas and the brick layer. The heat transfer from brick to coke bed is also increased, but not significantly. (c) Usually a brick with a higher Cp value also has a higher k-value. Simulation of a brick layer with both four times higher Cp and k values actually show appreciable heat is transported from the brick to the coke bed for one rotation for both lower and higher Cp and k bricks. The difference is not significant. INTRODUCTION A rotary kiln is used to calcine petroleum coke, and the product, calcined coke, is an important industrial material used for manufacturing graphite electrode for aluminum smelting process [1]. Inside the kiln, the volatile matters (released from the green coke) are burned to provide the energy for calcination while extra energy (mostly from natural gas) is supplied to maintain the required temperature near the discharge end. Due to different flow and combustion mechanisms, the temperature distribution inside the kiln is highly non-uniform, including in both the flue gas field and the coke bed. As a result, the rotary kiln wall also inherits a non-uniform temperature distribution. Figure 1 illustrates the heat transfer inside the kiln [2]. A. Gas radiation & convection Heat transfer to material B. Brick radiation Heat transfer to material C. Brick conduction Heat transfer to material D. Radiation & convection Heat transfer to brick Heat loss by shell radiation & convection Figure 1 Modes of heat transfer in a rotary kiln [2]