We have constructed a quasi-2 dimensional NH 3-SCR model with detailed surface reactions to analyze the NOx conversion 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 phase-catalyst 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 (NH 3) feed tests at various catalyst temperatures, and results show that ammonium nitrate (NH 4NO 3) is the inhibitor in NH 3-SCR reactions at low temperatures. In addition, we found that cutting the supply of NH 3 causes decomposition of NH 4NO 3, providing surface ammonia (NH 4 +), which rapidly reacts with adjacent NOx, leading to an instantaneous rise in nitrogen (N 2) formation. However, the decomposition rate of NH 4NO 3 depends on the catalyst temperature, hence an optimum addition of reductants, depending on the NH 4NO 3 formation/decomposition rate (and thus catalyst temperature) is required to maximize the NOx conversion efficiency of SCR catalysts.
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