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Show Effect of Turndown. Reducing the burner firing rate to one third of the design rate decreases temperatures throughout the combustion chamber. However, the exit NOx level is not significantly changed. The decrease in flame temperature, which lowers the rate of NO fonnation, is essentially offset by an increase in residence time. Summary of Kinetic ModeUing Findings. The key fmdings from the chemical kinetics modelling may be summarized as follows: • • • • • • With respect to the two primary NOx-control strategies employed in the design, flue gas recirculation has a greater impact on NOx emissions that does fuel staging. A combination of the two approaches leads to very low NOx levels. Most of the NOx -control benefit is associated with the fust 10 percent of flue gas recirculation. With no flue gas recirculation, NOx levels are highly sensitive to first stage heat loss. With 20% flue gas recirculation, NOx levels are relatively insensitive to first stage heat loss. Turndown does not significantly impact NOx levels. Instability is possible when a high degree of fuel staging (approximately 40 percent) is combined with 20 percent flue gas recirculation. Computational Fluid DynamiCS Modelling Approach: Fluen4 a commercially available, general purpose, finite-difference computational fluid dynamics code, was used for the chiller burner design analyses. The code is designed to model fluid flow, heat transfer (including radiation) and chemical reactions. While it is possible to model the three-dimensional, turbulent, reacting flow in the chiller combustion chamber, the number of nodes required to adequately define the computational domain make this an extremely time-consuming process. Given the large number of parameters to be investigated, it was decided to simplify the analyses and model the burner as a two-dimensional axisymmetric geometry. This approach compromises the modelling of the staged fuel jets, which are represented as an annulus as opposed to discrete orifices. However, the main features of the flowfield are still accurately represented. The parameters investigated using the CFD model included the burner geometry (air flow split, air velocity, use of swirl), flue gas recirculation rate, flue gas injection location, flam~ stabilization mechanism, secondary fuel injection characteristics, the combustion chamber geometry, and turndown. Summary of CFD ModeUing Findings: The key findings from the computational fluid dynamics modelling may be summarized as follows: • The burner Swirl Number (S) strongly affects the compatibility of the flame with the combustion chamber. A small amount of swirl (S = 0.5 for 75% of the burner flow) produces a small internal recirculation zone (IRZ), which is effective at stabilizing the flame. A moderate to high level of swirl (S > 0.5 or 12 |
