Destruction of nitrogen oxides in diffusion flame environments.

Abstract

This research is concerned with reburning, a combustion modification for NOₓ abatement. There are problems associated with conventional application of reburning, such as the three stage requirement, and slagging and corrosion in the reburn zone due to its reducing atmosphere. It is desirable to employ a more benign environment, and simultaneously eliminate both problems. One method which may accomplish this is to exploit the local reducing environment in diffusion flames to reduce NO, rather than hold an entire furnace zone under reducing conditions. To test this hypothesis was the motivation behind this research. Both experimental and theoretical studies were performed. Experiments employed a bench scale, CH₄ - O₂/CO₂, laminar counterflow diffusion flame. Flue gas was simulated by the oxidant, in which NO was added. The system was theoretically modeled by solving the governing equations numerically, using literature (detailed) kinetics. A novel finite difference method was developed to solve the stiff boundary value problem with steep gradients. The NO destruction potential of laminar counterflow diffusion flames was investigated under overall fuel-lean conditions. The integral results in the exhaust demonstrated large NO destructions under such conditions. Axial profiles of temperature, major species, C₂ hydrocarbons and stable nitrogenous species were measured in order to investigate the flame structure, and to allow comparison with the one-dimensional model. Excellent agreement between measurements and predictions was achieved for both axial profiles and integral results, in absence of edge effects. When edge effects are present, measured profiles agreed with those predicted, but the integrated results did not. From these results, it may be concluded that overall flame type is the predominant factor in determining NO destructions. A co-flowing diffusion flame which occurs beyond the burner edge can achieve high NO reductions (in excess of 88% in one case reported here). In order to obtain maximum destruction in the fuel region contained therein, NO should travel alongside the diffusion flame, and be continuously depleted. In addition, this type of flame was capable of reducing HCN emissions. Yet, the destruction of NO in a truly flat, one-dimensional counterflow diffusion flame is not great.