Abstract: The pressure pulsations caused by the flow patterns of flue gases entering a flue gas stack from a horizontal duct are eliminated or reduced by the installation of a partition plate in the stack opposite the inlet duct. This negates the adverse interaction of the two vortices in the stack formed by the entering flue gases and allows the vortices to travel independently up the stack.

Abstract: A combined cycle system has a gas turbine connected to a heat recovery steam generator and the heat recovery steam generator contains a selective catalytic reactor section wherein nitrogen oxides (NO.sub.x) are reacted with reactant gas such as ammonia or urea to produce molecular nitrogen and water vapor. The expanding transition section contains an array of flow profile control pipes extending across the section in a pattern which distribute the flue gas uniformly over the large cross-section of the heat transfer section. The reactant gas is injected into the flue gas stream through a series of nozzles spaced across the flow control pipes so as to uniformly mix and distribute the reactant gas in the flue gas stream before it enters the selective catalytic reactor section.