(150f) Modeling of NOx reduction By Urea-Based Selective Catalytic Reduction in a Monolith Reactor

Catalytic control of exhaust emissions is essential for decreasing the amount of harmful pollutants. As NOx emission standards have turned stringent in the past few decades, selective catalytic reduction (SCR) has become a key technology for diesel exhaust aftertreatment systems. SCR is an emerging technology for the control of NOx emission from automobiles, industries and marine applications. Ammonia required for NOx reduction in SCR is provided via hydrolysis of urea. In a urea-SCR converter, urea-water solution is injected into the hot gas mixture where urea decomposes to form ammonia. The two reactions assisting urea decomposition, viz., thermolysis and hydrolysis, occur in the mixing chamber. Thereafter, ammonia along with the gaseous mixture enters the converter and NOx is reduced through a series of catalytic reactions occurring in the monolith.

Several models for urea injection, as well as reactor-level and kinetic models have been developed by researchers to study the performance of urea-SCRs. These include models to analyze droplet behavior of injected urea solution upstream of the SCR or models for SCR catalyst itself. However, the former models are often cold-flow simulations, whereas the latter are based on single channel approximation that do not account for non-uniformity in ammonia distribution. A combined study accounting for urea injection and hydrolysis on one hand, and catalytic convertor performance incorporating the effect of incomplete mixing of ammonia with the gas mixture, is missing in the literature. The ammonia distribution at the inlet has been known to significantly affect SCR performance (McKinley et al., 2010). Although efforts were made in the past decade to analyze and improve the design and performance of NH₃-SCRs, the issue still lacks complete understanding.

In this work, we plan to examine the performance of urea-SCR through realistic CFD simulations using full SCR chemistry, considering the effect of radial variations in ammonia concentration, inlet flow mal-distribution and thermal variations across the monolith. Ammonia concentration distribution is obtained by modeling a urea-water injection spray as a discrete phase in a Lagrangian frame of reference. A multi-component droplet model with two-way coupling between the phases is enabled using ANSYS Fluent CFD software. The spray model and kinetics used for urea decomposition is validated against the experimental results of Kim et al. (2008). The spray-induced mixing characteristics are further analyzed under different exhaust conditions and injector locations. The ammonia distribution profile obtained through subsequent urea decomposition reactions is incorporated in SCR convertor simulations as inlet boundary condition for the SCR. The porous medium approximation is used for modeling the monolith, while a 7-step kinetic model for SCR developed by Olsson et al. (2008)for a single adsorption site is modeled using CHEMKIN-PRO incorporated into Fluent. Three-dimensional catalytic converter simulations will be performed to analyze the radial and axial variations in flow, temperature, species conversion and coverage fraction across the monolith. The performance of the SCR will be assessed in terms of its NOx conversion efficiency and total unreacted ammonia leaving the converter.

REFERENCES

Kim, J.Y., Ryu, S.H., Ha, J.S., 2008. Numerical Prediction on the Characteristics of Spray-Induced Mixing and Thermal Decomposition of Urea Solution in SCR System. https://doi.org/10.1115/icef2004-0889

McKinley, T.L., Alleyne, A.G., Lee, C.-F., 2010. Mixture Non-Uniformity in SCR Systems: Modeling and Uniformity Index Requirements for Steady-State and Transient Operation. SAE Int. J. Fuels Lubr. https://doi.org/10.4271/2010-01-0883

Olsson, L., Sjövall, H., Blint, R.J., 2008. A kinetic model for ammonia selective catalytic reduction over Cu-ZSM-5. Appl. Catal. B Environ. https://doi.org/10.1016/j.apcatb.2007.12.011