Abstract
One of the quickest ways to influence both the wall temperature and thermal NOx emissions in rotary kilns is to change the air–fuel ratio (AFR). The normalized counterpart of the AFR, the equivalence ratio, is usually associated with premixed flames and studies of its influence on diffusion flames are inconsistent, depending on the application. In this paper, the influence of the AFR is investigated numerically for rotary kilns by conducting steady-state simulations. We first conduct three-dimensional simulations where we encounter statistically unstable flow at high inflow conditions, which may be caused by vortex stretching. As vortex stretching vanishes in two-dimensional flow, the 2D simulations no longer encounter convergence problems. The impact of this simplification is shown to be acceptable for the thermal behaviour. It is shown that both the wall temperature and thermal NOx emissions peak at the fuel-rich and fuel-lean side of the stoichiometric AFR, respectively. If the AFR continues to increase, the wall temperature decreases significantly and thermal NOx emissions drop dramatically. The NOx validation, however, shows different results and indicates that the simulation model is simplified too much, as the measured NOx formation peaks at significantly fuel-lean conditions.
Highlights
Rotary kilns are long cylindrical furnaces that are horizontally positioned and slightly inclined
The focus of our work is to find out how the temperatures and thermal NOx emissions are affected by the air–fuel ratio (AFR), as this is a relatively simple way to influence both outcomes
While the flow stability is a major difference between the simulations in 2D and 3D, when we look at the wall temperature we see that the difference is very small
Summary
Rotary kilns are long cylindrical furnaces that are horizontally positioned and slightly inclined. An easier solution would be to modify the operating condition, which is a matter of turning the valves It was shown in [18] that increasing the volumetric air–fuel ratio (AFR) leads to both lower flame temperature and wall temperature, preventing ring formation. Processes 2021, 9, 1723 experimental work is conducted on the precalciner whereas the kiln is experimentally simulated In both mentioned studies, the NOx formation increases with the AFR on the lean side (excess air). The focus of our work is to find out how the temperatures and thermal NOx emissions are affected by the AFR, as this is a relatively simple way to influence both outcomes This is investigated numerically, and experimentally in this paper.
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