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Abstract The performance of selective catalyst reduction systems highly depends on the degree of mixing between NH 3 and NO x and the flow uniformity of velocity and concentration, which are sensitive to several parameters such as the installment of gate leafs, a hybrid grid, a straightener, and the ammonia injection method. In this work, computational fluid dynamics was applied to investigate these parameters in heat recovery steam generators. Performance of gate leafs and a new type of hybrid grid was considered. Analyses showed that gate leafs significantly improved the uniformity of velocity distribution and that the new type of hybrid grid improved the uniformity of concentration distribution. The velocity relative standard deviation decreased from 19.6% to 5.4%, and the concentration relative standard deviation decreased from 70% to 12.7%. Experiments also qualitatively confirmed the reliability of simulation results. Moreover, the hybrid grid was optimized with different structures. Results indicated that a 30° crossed angle could obtain a better mixing between NH 3 and NO x with a slight decrease in pressure. In addition, an optimal injection method of the ammonia injection grid was designed, and it was helpful in conducting the field operation. The relationship between the thickness of the straightener and the incident angle at the entrance of catalyst layer was discussed. Highlights • Performance of gate leafs and a new type of hybrid grid was considered in heat recovery steam generators. • Gate leafs significantly improved the uniformity of velocity distribution. • New type of hybrid grid improved the uniformity of concentration distribution. • Hybrid grid with a 30° crossed angle could obtain a better mixing between NH 3 and NO x. • An optimal injection method of the ammonia injection grid was designed. [ABSTRACT FROM AUTHOR]