OCR Text |
Show own version of regenerative burners which give similar performance and with some designs there is a more close coupling of the regenerator bed into the burner housing. Flameless Oxidation:- The use of flue gas recirculation (FGR) as a NOx reduction technique has been recognised for several years. The introduction of the flue gas increases the mass flow rate through the combustion zone, and depresses the peak flame temperature, and hence thermal NOx. However, at high levels of FGR. burner stability becomes a significant problem. Recently there have been a number of developments which can be viewed as an extension of the concept of F G R to extreme levels. This is achieved by injecting the gas into a furnace so that it can mix with the furnace gases, before mixing with the combustion air and reacting. One such variant is the low-NOx C G R I burner: Figure 7, although there are others, e.g. FLOX®. In all of the systems utilising flameless oxidation, N O x emissions are typically less than lOppm (at 3 % 02), even at quite high levels of combustion air preheat. Air Front Section Figure 7 Schematic of low-NOx CGRI Burner Although the emission performance of these burners is beyond doubt, it is questioned as to whether such burners can operate approaching or below auto-ignition temperatures, and whether the heat transfer from such a system is comparable to conventional burners. Other Low-NOx technologies:- As an alternative to preventing NOx formation in the combustion process, it is also possible to destroy the N O x once it is formed. This will still result in low N O x emissions from the flue. Unlike other "deNOx" methods, such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR), rebum utilises the addition of a hydrocarbon fuel to destroy the N O x through a series of complex reaction in a fuel rich zone. Additional air is then introduced to re-establish fuel lean conditions and complete the combustion process. |