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Thermal NOx Mitigation by Changing the Secondary Air Injection Channel: A Case Study in the Cement Industry
New legislation imposes ever stringent constraints on the emission of pollutants of large industrial furnaces. Numerical simulation will play an ever increasing role in bridging the gap between sustainability and productivity. In this work we study the non-premixed turbulent combustion of natural gas in a rotary kiln used for the production of special purpose cement. The cement type considered requires sintering temperatures above 1500 degrees Celcius. The monitoring and possible mitigation of the thermal nitric-oxides (NOx) production is therefore of utmost importance. The combustion is fed by a primary air stream through the burner and a secondary air inflow through a channel close to the burner pipe. We focus on the geometry of the cross-section of the secondary air inlet channel. The configuration currently in use is asymmetrical with respect to the main axis of the kiln. The alimentation of the flame with air is therefore unevenly distributed in space. Zones with locally high heat release are formed. In these local hot spots disproportional large amounts of thermal NOx is formed.
We propose instead to inject the secondary air co-axially around the burner pipe. To quantify the effect of the change in secondary air inlet patch, we numerically simulate the mixing of fuel and oxidizer, their chemical reaction and ensuing heat release. The turbulent flow is modeled using a RANS realizable k-epsilon turbulence model, the combustion by a mixed-is-burnt model and the radiative heat transfer by a discrete ordinate model. The lining is included by a conjugate heat transfer model and the thermal NOx formation is predicted by the Zeldovich model. The model is solved using an extension of chtMultiRegionSimpleFoam able to resolve the low Mach compressible flow regime near the burner. The combustion and radiation model are attached to this solver using fvOptions. We generate our three-dimensional meshes using cfMesh. The core mesher engine was extended by a module that allow to extrude the gas-solid into a mesh for the insulating lining. Results of the non-reactive simulations are in good agreement with those generated using the in-house immersed boundary solver developed by PM2Engineering.
Our reactive flow simulations result in detailed maps of the spatial distribution of the flow, thermodynamical and chemical conditions of the kiln. These maps provide valuable information to the operator of the kiln. Our simulations show the difference between the existing and the newly proposed geometry of the secondary air inlet inlet. The new geometry is shown to eliminated the occurrence of hot spots. The production of thermal NOx is significantly reduced. Further research is required to resolve limitations of various choices in our modeling approach.