High efficiency selective catalytic reduction (SCR) systems are designed as retrofits for heat recovery steam generators (HRSG) to comply with increasingly more stringent environmental standards. The retrofits are designed for continuous combined cycle plant operation with ultra low NOx emissions. These retrofits require special design considerations. Maldistribution (non uniformity) in the velocity profile within a high efficiency SCR system is one consideration which is the focal point of this paper. With a specified velocity profile (having a slight maldistribution), high and low efficiency retrofits are compared. It is demonstrated that, while the slight maldistribution in the velocity profile may significantly impact the performance of a high efficiency retrofit, it may be unnoticeable for a low efficiency retrofit.

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