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Show The current environmental regulations for air pollution control in the United States and many other countries of the world require significant reduction of N O x emissions from all stationary sources using fossil fuels. Most specifically, this is valid for industrial and utility steam boilers where different low N O x control technologies are applied, depending on the type of boiler and type of steam boiler. In this case, only the low N O x control technologies related to different combustion process modifications, or so-called "in-furnace N O x control," is considered. All combustion modifications low NOx technologies are based on three basic concepts: 1. Flue gas recirculation in the flame 2. Flame-delayed combustion in the firebox achieved by fuel slagging, air staging, staged combustion, and/or reburning 3. Minimum excess air in the flame zone Besides the impact on operation and efficiency of the unit, all of these technologies have a negative impact on different boiler components directly related to the reduction of the remaining useful life (RUL®). This represents significant additional cost for the low N O x control application. The flue gas recirculation (FGR) is predominantly used for gas and oil fuels in industrial and utility boiler applications. In general, FGR has very little or no impact on furnace waterwall heat exchangers. The flame temperature is reduced, which, depending on the configuration of the heat exchangers in the firebox and the amount of FGR, could lead to the flue gas exit temperature being either the same as the original design or slightly increased. The FGR has a significant impact on the heat exchangers in the convection pass. For fuel gas and oil the F G R amount could be in the range of 1 0 % to 3 0 % , which represents a substantial increase of the flue gas flow rate and the coefficient of convective heat transfer from the flue gas to the heat exchanging surface. This has negative effects on the following boiler components: • Secondary Superheater and Reheater - Increased outlet steam temperature, increased tube metal temperature, and increased spray flow rate • Primary Superheater - Increased outlet steam temperature and increased tube metal temperature AFRC-TST.DOC |
