the main flue gas duct 13 to an inoperative position outside of the main flue gas duct 13 in their respective operative positions within the duct 13 first and second carriage frames 26 are located adjacent one another and span substantially across the entire interior of the duct 13 as shown in fig3 so that substantially all of the flue gas flow 12 passes through the catalytic material 24 carried by the carriage frames 26 as shown in fig5 a and 7b one preferred means for moving the carriage frames 26 between their operative and inoperative positions comprises motor driven rack and pinion assemblies 30 including a drive gear 31 preferably a 4 inch spur gear and a rack 33 preferably a no 5 pitch rack gear 31 is preferably driven by a conventional electric motor not shown controlled by controller 16 to move in reverse directions as indicated by reference arrow 34 a 34 so that a plurality of teeth 31a provided about the perimeter of gear 31 engage a flat rack 33 provided with a plurality of corresponding teeth 33a rack 33 is securely affixed to carriage frame 26 so that as gear 31 rotates in engagement with rack 33 carriage frame 26 is moved back and forth.