Turbulent Non-Stationary Reactive Flow in a Cement Kiln

Abstract

The reduction of emissions from large industrial furnaces critically relies on insights gained from numerical models of turbulent non-premixed combustion. In the article Mitigating Thermal NOx by Changing the Secondary Air Injection Channel: A Case Study in the Cement Industry, the authors present the use of the open-source OpenFoam software environment for the modeling of the combustion of Dutch natural gas in a cement kiln operated by our industrial partner. In this paper, various model enhancements are discussed. The steady-state Reynolds-Averaged Navier-Stokes formulation is replaced by an unsteady variant to capture the time variation of the averaged quantities. The infinitely fast eddy-dissipation combustion model is exchanged with the eddy-dissipation concept for combustion to account for the finite-rate chemistry of the combustion reactions. The injection of the gaseous fuel through the nozzles occurs at such a high velocity that a comprehensive flow formulation is required. Unlike in Mitigating Thermal NOx by Changing the Secondary Air Injection Channel: A Case Study in the Cement Industry, wave transmissive boundary conditions are imposed to avoid spurious reflections from the outlet patch. These model enhancements result in stable convergence of the time-stepping iteration. This in turn increases the resolution of the flow, combustion, and radiative heat transfer in the kiln. This resolution allows for a more accurate assessment of the thermal NO-formation in the kiln. Results of a test case of academic interest are presented. In this test case, the combustion air is injected at a low-mass flow rate. Numerical results show that the flow in the vicinity of the hot end of the kiln is unsteady. A vortex intermittently transports a fraction of methane into the air stream and a spurious reaction front is formed. This front causes a transient peak in the top wall temperature. The simulated combustion process is fuel-rich. All the oxygen is depleted after traveling a few diameters into the kiln. The thermal nitric oxide is formed near the burner and diluted before reaching the outlet. At the outlet, the simulated thermal NO concentration is equal to 1 ppm. The model is shown to be sufficiently mature to capture a more realistic mass inflow rate in the next stage of the work.

Modelling a Turbulent Non-Premixed Combustion in a Full-Scale Rotary Cement Kiln Using reactingFoam

“…The kiln is represented on the true scale. The same geometry of the freeboard as published in [18,19] is employed. The insulating lining, however, is omitted.…”

“…This paper is a direct extension of the two previous papers [18,19]. In [18], the geometry of the kiln, the mesh generation using cfMesh, and the case setup are described.…”

“…Simulation results for the non-reactive and reactive flow are established. In [19], case-specific guidelines to obtain stable convergence of the transonic non-stationary reactive flow simulations using reactingFoam are outlined. In [19], however, inflow conditions of preheated air that are of merely academic interest are used.…”

“…In [19], case-specific guidelines to obtain stable convergence of the transonic non-stationary reactive flow simulations using reactingFoam are outlined. In [19], however, inflow conditions of preheated air that are of merely academic interest are used. The wall of the furnace was furthermore thermally insulated.…”

“…The wall of the furnace was furthermore thermally insulated. The goal of this paper is to discuss how simulation results published in [19] change in case the more realistic inflow conditions for the oxidizer (and, thus, a more realistic air-to-gas ratio) are employed. The authors also wish to alleviate the restrictive condition of the insulation of the walls.…”