Mitigating Thermal NOx by Changing the Secondary Air Injection Channel: A Case Study in the Cement Industry

Abstract

This work studies how non-premixed turbulent combustion in a rotary kiln depends on the geometry of the secondary air inlet channel. We target a kiln in which temperatures can reach values above 1800 degrees Kelvin. Monitoring and possible mitigation of the thermal nitric-oxide (NOx) formation is of utmost importance. The performed reactive flow simulations result in detailed maps of the spatial distribution of the flow, thermodynamics and chemical conditions of the kiln. These maps provide valuable information to the operator of the kiln. The simulations show the difference between the existing and the newly proposed geometry of the secondary air inlet. In the existing configuration, the secondary air inlet is rectangular and located above the base of the burner pipe. The secondary air flows into the furnace from the top of the flame. The heat release by combustion is unevenly distributed throughout the flame. In the new geometry, the secondary air inlet is an annular ring placed around the burner pipe. The secondary air flows circumferentially around the burner pipe. The new secondary air inlet geometry is shown to result in a more homogeneous spatial distribution of the heat release throughout the flame. The peak temperatures of the flame and the production of thermal NOx are significantly reduced. Further research is required to resolve limitations of various choices in our modeling approach.

Turbulent Non-Stationary Reactive Flow in a Cement Kiln

“…We studied the kiln previously described in [7]. It is used by our industrial partner for the production of specialty cement used in niche applications.…”

“…In this way, we stay away from confidential information from our industrial partner. Like in [7], we employ the open-source software environment OpenFoam [8]. We refrain from comparison with experimental data.…”

The Effect of Variable Air–Fuel Ratio on Thermal NOx Emissions and Numerical Flow Stability in Rotary Kilns Using Non-Premixed Combustion

“…Figure 14 helps to understand the complexity of thermal NOx formation. Not only does it depend on the gas temperature and oxygen concentration, but on the flow as well, as was proven in [17]. An example in Figure 14 is that the temperature profile of AFR's 8 and 10 are similar but the oxygen concentration is less in the hot region of the gas in the case for AFR of 8.…”

“…Our aim is to reduce the thermal NO emissions of an actually operating kiln burning on natural gas. As shown in our latest work [17], changing the air inlet geometry shows promising results in reducing both the peak temperatures as well as thermal NOx emissions. However it does require rebuilding the inlet side of the kiln and reinstalling the combustion air infrastructure.…”

“…For this, we continue our work in [17] by keeping the mass flow rates of the fuel inlets and primary air inlet constant and increasing the secondary air inlet mass flow rate further (see Figure 3). Some important simplifications are applied to the simulations.…”