(126c) Kinetic Modeling of Selective Non-Catalytic Reduction (Sncr) of Nitric Oxide Using Urea-Water Solution

Selective Non-Catalytic Reduction (SNCR) is a proven post combustion technology for controlling nitric oxide from stationary sources such as power plants, incinerators, and boilers. The process operates by injecting reducing agents such as ammonia or urea into the flue gas. Use of urea (NH2CONH2) as reducing agent has been widely accepted as an alternative to ammonia due to its easier and safer handling (Muzio et al., 1997; Sun et al., 1993). In the SNCR process, injection of urea into the flue gas is often carried out as a urea-water solution because of the problems associated in delivering and storing of dry urea. Thus decomposition and oxidation of urea-water solution can play a major role in SNCR process.

Previous experimental investigations on the decomposition of urea have identified different reaction paths depending on the form of urea (Koebel and Elsener, 1992; Caton and Siebers, 1989; Itaya et al., 1997; Lentz and Wright, 1994). Gentemann and Caton (2001) extended the experimental data on the urea-water solution decomposition at temperatures from 800 to 1300 K and found that thermal decomposition dominated for temperatures below 1000 K, whereas for temperatures above 1000 K, the oxidation became important.

A detailed investigation on SNCR of nitric oxide process using urea-water solution for various inlet O2 and CO concentrations is reported by Gentemenn and Caton (2001) and for various urea/NO ratio ( -ratio), and residence time by Park and Caton (2003). But the reported results on kinetic modeling of the urea decomposition showed uncertainties especially at higher temperatures (Aoki et al., 1999; Alzueta et al., 2000). A kinetic model for the decomposition of urea solution used in selective non-catalytic reduction (SNCR) is developed in our earlier work (Rajasree et al., 2007). The above model could predict a number of gases such as HNCO, NO and N2O in addition to NH3 and CO2. The mechanism has been validated using experimental data reported by Gentemann and Caton (2001).

In the present study, a reduced kinetic model has been developed for the SNCR of nitric oxide using urea-water solution. The focus was on the impact of the results by Rajasree et al. (2007) in the kinetic modeling of NOx reduction using urea-water solution. We have included those reactions in the reduced model proposed by Brower et al. (1996) for prediction of SNCR of Nitric Oxide. The mechanism has been validated using the experimental data reported by Gentemann and Caton (2001) and by Park and Caton (2003) for various inlet O2 concentration, urea/NO ratio ( -ratio), and residence time at temperatures from 800 to 1300 K. There is a good agreement between the model prediction and measurements and thus confirming the accuracy and potential of the developed model. The individual rate constants are provided so that mechanism could be coupled with computational fluid dynamics (CFD) based on a turbulent reacting flow model and would play a major role for accurate modeling of SNCR process.

Acknowledgements

This study is supported by Korea Ministry of Environment as ?The Eco-technopia 21 Project?.

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