Transient heat transfer during startup of a thermal plasma chamber: Numerical insights

Abstract

Thermal plasma pyrolysis of radioactive waste is a recommended technique to manage large volume of combustible radioactive waste comprising of rubber and plastics. A lumped parameter based numerical model, to predict the thermal behavior of a plasma pyrolysis chamber during startup, is reported in this work. The model takes into account the non-ideal flow conditions inside the plasma chamber which are specific to the geometry of the chamber. Three different non-ideal models – a Continuous Stirred Tank Reactor (CSTR) having exchange with a separate perfectly mixed CSTR like geometry, two parallel CSTRs, and two parallel CSTRs with exchange, as well as an ideal single CSTR model have been compared. The models consider heat transport from plasma chamber interior to the walls through convection as well as radiation. Resistance offered by five successive layers of insulation of different materials has been considered. The predictions from the models are compared against experimentally determined evolution of chamber temperature during startup for three different power ratings of the plasma torch. The model formulation of two parallel CSTRs with exchange (a two-parameter model) is observed to fit the experimental data the best. A detailed sensitivity analysis has been carried out and the optimum model parameters are determined. Temperature evolution of different insulation layers has also been obtained. The validated model has been used to study the temperature response of the chamber during two different types of operating transients, viz. step changes in torch power and sinusoidal variation in torch power.