Modeling and Numerical Solution of Coal and Natural Gas Co-Combustion in a Rotary Kiln

Due to the great importance of rotary kilns in the cement industry and the fact that these systems are known to be high energy consumers, a model was developed for one of the most important phenomena occurring inside of such kilns, i.e., combustion. This model and its numerical solution allow for carrying out process optimization techniques in order to reduce the use of the fuel required, emissions, and operation costs. Furthermore, this model permits geometrical modifications in the burner as well as variations in operational conditions, i.e., flow rates, temperatures, and concentrations. The process studied here is carried out inside a tubular rotary kiln, which has a burner of concentric tubes that allows the combustion of two fuels in different phases, in this case, coal and natural gas. The modeling accounts for phenomena such as the turbulence produced due the high Reynolds number in the feed streams; the convective heat loss from the shell to the environment; and a three-step and two-step eddy brake-up combustion model for natural gas and volatiles. The model is developed from Lagrangian–Eulerian multiphase flow, taking into consideration coal solid particles in the gaseous stream, devolatilization rate of coal, heterogeneous burnout of char, and NOx generation. Finally, velocity, temperature, and concentration fields were analyzed to evaluate dragging, reactions conversion, and pollutants formation in the process.