Modeling and Optimization of Selective Catalytic reduction (SCR) Systems for Post Combustion NOx Control

Abstract

Due to their hazardous effects on the environment and living beings, emission of NOx (a mixture of NO and NO2) from power/chemical plants has been a major concern in our society and are therefore subjected to strict regulations. These effects include the formation of acid rain, photochemical smog and ground layer ozone. One of these methods is Selective Catalytic Reduction (SCR) of NOx which was found to be effective in controlling NOx emission. SCR approach requires a reducing agent such as ammonia to reduce NOx selectively in the presence of excess oxygen and a suitable catalyst. The ammonia is injected into the exhaust gas stream and is mixed with NOx. This mixture of ammonia and NOx passes through the catalyst bed, where a high percentage of NOx reacts on the catalyst surface, with adsorbed ammonia decomposing into diatomic nitrogen and water molecules, thereby reducing the NOx level in the exhaust gas. Though, SCR for NOx removal is a relatively established technique, it needs to undergo continuous improvements for it to meet the progressively stricter NOx emission regulations. This in turn requires a better understanding of the physico-chemical processes that takes place within the system. In tandem with experimental design and evaluation, Computational Fluid Dynamics (CFD) models provide a cost effective means of achieving this. In this study, a 3-dimensional CFD model of a single monolith honeycomb structure of the SCR system is developed. The steady state model was developed taking into account the conservation of mass, momentum and energy of the reacting gases. The heterogeneous surface chemistry was modeled by considering the standard and fast SCR kinetic mechanism. A novel study investigating the interactions within the dual-layer SCR system was also undertaken. The model equations were solved using a pressure based solver by the SIMPLE algorithm in ANSYS Fluent software. The model results were validated by comparing the model results obtained with an experimental work. The model was employed to investigate the effect of various important parameters including the NO2/NOx ratio, space velocity, shapes and NH3/NOx ratio for stationary SCR applications. The results indicated that NO2/NOx ratio and channel cross-section profile have significant effect on the NOx reduction. In conclusion, the dual-layer interaction showed that the second layer plays a significant role in the NOx and ammonia reduction and it was found that a dual-layer SCR system is better than a single long catalyzed layer for the SCR system.

Date of Award	2014
Original language	American English
Supervisor	Tariq Shamim (Supervisor)