the term rectifier section refers to a section of the scr reactor upstream of the catalyst having a grating or other means to straighten the flue gas flow before it reaches the scr catalyst many flue gas streams that include scr reactors position the reactor so that the flow of the flue gas is in a vertical direction often generally downwards as such for the sake of simplicity the following discussion will often use the terms above the scr reactor and will proceed with a general arrangement of lpa mitigation screens positioned near the top of a scr reactor above an optional rectifier which is in turn positioned above the scr top catalyst layer flue gas velocities in a scr reactor are generally lower than in flue gas ductwork in many parts of a flue gas pathway in ductwork flue gas velocities are in the range of 40 60 ft sec however due to the larger cross sectional area of the scr reactor relative to the ductwork typical velocities in a scr reactor are anywhere from one third to one quarter of those elsewhere placing lpa mitigation screens in the rectifier section of the scr reactor allows for less pressure drop and longer life because of lower erosion and pluggage of.