A carbon monoxide (CO) boiler or steam generator having a water cooled CO boiler floor with screen gas distribution inlet to enhance distribution of CO gas in a CO boiler. Either the front or rear wall tubes of the steam generator form an integral screen and the tubes continue, forming a membraned, gas tight enclosure. The floor has a “knee” to redirect the incoming waste CO gas up into the integral screen. The screen may be provided with tube erosion shields to prevent erosion of the screen tubes and to control the distribution of waste gas across the plan area of the furnace.