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Show burners for the field erected boiler have two sets of louvers for air swirl and flow adjustments for flame shaping. A low N O x staged spray pattern is used to control F B N conversion to NOx. The package boiler, on the other hand, requires a complex and sophisticated combustion system approach. Both fuel and air staging are required to control N O x formation. Front wall N O x ports are employed and staged sprays are used. The installation also requires the use of selective flue gas recirculation for N O x control where the F G R is separated from the non-preheated combustion air to prevent sulfur condensation and corrosion. A sophisticated combustion control system is also required to meter the flow rates of combustion air, fuel, and F G R for N O x compliance and safety. As mentioned earlier, two burners are required to control the flame shape and maximize the N O x reduction potential of the system. Table 3: Comparison of Low NOx Approaches for the Two Boilers PACKAGE BOILER • Deep Staging of Fuel & Air Required • NOx Ports • Flue Gas Recirculation • Staged Spray • Advanced Metering Controls of Air, Fuel and FGR FIELD ERECTED BOILER • No Staging of Air Required • No NOx or Overfire Air Ports • No Flue Gas Recirculation • Staged Spray • Simplistic Controls • No Need For B O OS Performance Results Summary: The commissioning tests of each of the case studies have been completed and each has achieved both contractual and emissions compliance. However, the start up process for each of the cases are very different. The field erected boiler started up easily only requiring a single modification to the atomizer caps for reduced particulate and reduced N O x emissions. The primary differences in the two boilers as it impacts low N O x techniques are given in Table 4. The package boiler start up proved challenging requiring many adjustments to combustion controls and the burner system for optimum operational and emissions performance. The greatest challenge faced by the burner system is to provide sufficient flame stability at reduced excess air levels in the base of the flame for N O x reductions. Once an atomizer and burner adjustment proves to meet the strict 0.25 lb/MBtu N O x performance at full load, the performance at turn down and light off would prove problematic. CEM Issues: The presence of a CEM on residual oil fired systems poses many challenges for the boiler owner and burner supplier. The firing of residual oil requires that provisions be made for routine maintenance procedures such as soot blowing and atomizer cap cleaning. During these procedures, it is customary to increase excess air to prevent excessive opacity excursions. This forces the N O x to go out of compliance when the excess air is increased for opacity'control. |