Analysis of NOx Conversion using a Quasi 2-D NH3-SCR Model with Detailed Reactions

We have constructed a quasi-2 dimensionalNH3 -SCR model with detailed surface reactions to analyze theNOxconversion mechanism and reasons for its inhibition at low temperatures. The model consists of seven detailed surface reactions proposed by Grozzale et al., and calculates longitudinal gas flow, gas phasecatalyst phase mass transfer, and mass diffusion within the catalyst phase in the depth dimension. Using the model, we have analyzed the results of pulsed ammonia (NH3 ) feed tests at various catalyst temperatures, and results show that ammonium nitrate (NH4NO3 ) is the inhibitor inNH3 -SCR reactions at low temperatures. In addition, we found that cutting the supply ofNH3 causes decomposition ofNH4NO3 , providing surface ammonia (NH4 + ), which rapidly reacts with adjacent NOx, leading to an instantaneous rise in nitrogen (N2) formation. However, the decomposition rate ofNH4NO3 depends on the catalyst temperature, hence an optimum addition of reductants, depending on theNH4NO3 formation/decomposition rate (and thus catalyst temperature) is required to maximize theNOxconversion efficiency of SCR catalysts.