Abstract: The arrangement is for reducing NO.sub.x content in the flue gas of a single stage combustion boiler, with a combustion chamber defined by tubular water walls and having plural burners, fired by liquid or gaseous fuel, mounted through one wall, with the combustion chamber communicating with the flue gas duct adjacent an opposite wall. The NO.sub.x content of the flue gases is reduced by dividing the combustion chamber into sections, each respective to a single burner, by gas-tight partitions, formed of water tubes, extending axially of the burner between the burner mounting wall and transition zone to the flue gas duct. The tubes of the partition may communicate with drums to which the tubular water walls are connected, and may communicate with the tubular water walls.