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Show INTRODUCTION Industrial steam generators and users are facing increased challenges in meeting N O x emissions compliance solely with burner equipment. As industry expands and adds new capacity, steam demands increase requiring purchase of new boiler equipment. Such new installations are required to meet the stricter emissions standards called Best Achievable Control Technology (BACT). The South Coast Air Quality Management District has implemented a N O x emissions permit level for most new industrial sources of 5 ppm, corrected to 3 % 02. Currently, this level of emissions on natural gas can be achieved with low N O x burners generating less than 30 ppm N O x and post combustion NOx control known as Selective Catalytic Reduction (SCR). S C R can reduce N O x further by up to 85%. The use of SCR, however, carries very high installation and maintenance costs. Substantial efforts in recent years were directed toward developing industrial burners firing natural gas with single digit N O x emissions and numerous devices were designed and tested. [Altpfart and Schindler, 1997; Joiner, et al, 1997]. The dominant ultra low N O x approach is to ignite all or a portion of natural gas after it is uniformly mixed with combustion air and large amount of Flue G as Recirculation (FGR). The premixed, or simulated premixed combustion substantially reduces prompt NOx, by eliminating local conditions of fuel rich combustion. Thermal NOx is controlled by reducing the adiabatic flame temperature with F G R or excess air. These ultra low N O x burners, utilizing premixed concepts, have shown the ability to achieve single digit N O x emissions; however the issue of adequate combustion controls when NOx is in single digits in most reports have been avoided. From the practicable experience it is well known that burners utilizing premixed type of combustion are prone to combustion driven pulsation on some regimes [Lifshits, 1996]. The spectrum of these regimes, defined by the burner load, amount of excess air and FGR, are dependent upon not only the burner design but also upon characteristics of the overall system (the fan, wind box, furnace, convection section and a stack). The main function of the control system is to steer parameters of the firing away from regimes that may develop pulsation. It is interesting to note that effects of the excess air and F G R on pulsation are not obvious. In some systems additional air and/or F G R to some extend can help to suppress pulsation. However, this is detrimental to the system efficiency. In other systems, the excess air and/or F G R has to be reduced. If the operating range become too narrow for the control system to maintain it dynamically during the load shifts and other disturbances, the system may generate pulsation resulting in a loss of flame, or mechanical damage. The quantities of flue gas recirculation required to meet single digit N O x and especially 5 ppm N O x raise questions about flame stability, operational performance, flame scanning and safety. These problems have become the primary stumbling block on the way to wider acceptance of ultra low N Ox technology in industry. 2 |