The radial nitrogen oxides concentration and carbon dioxide mole fraction profiles are presented in Figure 3 along the height of the gas fired burner. As a result of the combustion process, combustion systems normally generate gaseous combustion products which include oxides of nitrogen which are vented to atmosphere as flue gas. It is desirable to limit these nitrogen oxides emissions since nitrogen oxides are considered a pollutant and combustion systems sold in certain jurisdictions must meet strict nitrogen oxides emission standards. The special device is a piece of material which is sized, configured, and positioned relative to the combustion flame produced by the burner to temper the combustion flame by absorbing thermal energy from the combustion flame. The device is designed to absorb thermal energy from the combustion flame at a rate which limits peak flame temperatures and residence times at these peak flame temperatures to levels which inhibit formation of nitrogen oxides, while allowing substantially complete combustion of fuel supplied to the burner. To ensure that the combustion process of the furnace is not adversely affected by the presence of the device, the device should not adversely interfere with the flow of products of combustion away from the combustion zone for each burner. Further, the device should be relatively easy and inexpensive to manufacture and easy to install, so that the device is practical from a cost-effectiveness viewpoint and so that the device is suitable for retrofitting furnaces having such burners. In addition, preferably, the device is made of a material, such as stainless steel, which is resistant to oxidation at the relatively high combustion flame temperatures, and which radiates thermal energy, which it absorbs from the combustion flame, to its surroundings. Also, if the burner is a two-zone combustion type burner, it is desirable to position and configure the device relative to the combustion flame to aerodynamically smooth at least a portion of the periphery of the combustion flame. This inhibits formation of eddies by near-stoichiometric mixtures of combustion substances at the periphery of the combustion flame which are capable of forming relatively large amounts of nitrogen oxides. Of course, different regions of the combustion flame have different peak flame temperatures and different residence times at these peak flame temperatures depending on their location in the combustion flame. Normally, the temperature of a given region of the combustion flame will vary within a certain temperature range as a function of time during any time period of operation of the burner and will remain at the peak flame temperature within this temperature range for a certain amount of residence time during this time period of operation of the burner. Gaseous fuels are injected along tangential air inlet slits into the combustion air stream, whereas liquid fuels, such as heating oil, are injected preferably via a central nozzle at the burner head. At the burner outlet, the resulting eddy current bursts open at a change in the cross-section, inducing a backflow zone used for stabilizing the flame while the burner is being operated. In order to prevent interference with the flow field under the conditions of use in gas turbines, even if fuels of different origin and composition are used, and in this way always achieve a safe flame position, downstream from the swirl generator a mixing section may be provided that ensures a better premixing of different types of fuels.