The Effect of Air/Fuel Ratio Control Strategies on Nitrogen Compound Formation in NSCR Catalysts
The ability of NSCR (Non-Selective Catalytic Reduction) catalysts to effectively remove CO and NOx from exhaust is directly connected to the manner in which the engine is operated, namely the air/fuel ratio. In a stoichiometric engine, small variations in air fuel ratio have large effects on the catalyst performance. These effects include wide variations in removal efficiencies and catalytic production of ammonia. In this paper, a detailed look at the effect air/fuel ratio has on the catalyst efficiency and ammonia formation is examined. Ammonia is measured with an FTIR. Criteria pollutants are measured with an FTIR and a standard 5-gas emission rack. In addition to steady state scans of the air/fuel ratio near the stoichiometric air/fuel ratio, the effects of oscillating the air/fuel ration (“dithering”) as a control method is evaluated. Dithering is a technique that is used to control air/fuel ratio in automotive catalysts. In this work it is shown to be very effective on industrial engine catalysts at “broadening” the control requirements on A/F ratio required to achieve acceptable emissions control and minimize ammonia production.
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