Evaluating Heat Transfer Conditions in a Plasma-Heated Rotary Kiln for Cement Production

The possibility to use an electrically generated thermal plasma as the heat source in a rotary kiln for cement production is evaluated in this work. In the kiln, the product bed material requires a peak temperature of 1450 °C, which means that the process demands high-temperature conditions. In the conventional process, the overall heat transfer is typically dominated by radiation, with particle radiation from the flame being a major contributor. Thus, the lack of fuel particles in the plasma-heated gas poses a challenge when switching from a flame of solid fuel combustion, resulting in less heat being transferred by radiation and, consequently, a significant change in the overall heat transfer conditions in the kiln. In this work, the heat transfer in a demonstration-scale rotary kiln, heated with an 8-MW_el thermal plasma, is modeled. Measurements on a 50 kW_el carbon dioxide plasma torch are used to estimate the temperature profile of the plasma. The heat transfer conditions in the kiln, as well as the effects on the heat transfer caused by varying the operational and dimensional parameters are examined within this work. Enhancing the convective heat transfer from the plasma-heated gas can be achieved by tilting the plasma toward the bed, bringing the gas with high temperatures and velocities closer to the bed. The radiative heat transfer can be improved by adding surface area to the gas through particle injection. A combination of tilting the plasma toward the bed and incorporating particles into the plasma-heated gas is found to be the most promising option for increasing the amount of heat transferred to the bed material.