OCR Text |
Show The variable geometry feature allowed a substantial improvement in the burner turn down to 8:1 and expanded the stability limits with respect to allowable deviations in the amounts of F G R and excess air, see Figure 3. However, there were still problems operating at reduced loads with low excess air. Addition of external spuds and a second fuel control valve allowed to further widen the operational limits of the burner without negative impact on the operation at high fire, as shown in Figure 4. With the second fuel control valve it became possible to deviate at the very low end from fully premixed combustion (when ultra low N O x performance may not be an issue) and still demonstrate single digit N O x emissions. Turndown of 12:1 was also demonstrated with manual controls. The patent application for the burner is pending. The operating boundaries of the burner at minimum NOx emissions determine the condition when the onset of high C O and unburned fuel occurs. This corresponds closely with the condition when the flame pattern visually becomes incomplete. However, there is additional margin at low and medium firing rates before the flame lifts of completely. The limitations with the experimental F G R system prevented testing flame lift off at high fire. The low excess air limit is determined typically by the onset of C O emissions and sometimes stability (depending on the burner tuning). The minimum F G R is limited by flashback at about 1 5 % load level. A series of tests were performed with the excess air as low as 1 to 2% and amounts of F G R over 3 5 % . With no significant C O emission increase (a proof of good air and fuel distribution by the burner) the N O x emissions were never lower than about 6 ppm even at the flammability limit. At these high F G R rates, the small thermal N O x can be additionally reduced proportionally to square root of oxygen concentration. So, the major source of N O x at this low excess air is likely prompt NOx. At least 1 0 % excess air is required for the optimum NOx. Operation with Preheated Air. The operation with preheated air is similar to the operation with ambient air if the difference between the adiabatic flame temperatures are compensated by additional amounts of F G R or excess air. During tests with up to 550°F combustion air, N O x emissions were reduced to the 7 ppm level and further reductions are likely possible. It was also observed that some improvements to the flame stability were obtained with air preheat at low loads. That can be explained by higher increase in flow velocity through the burner compared with the increase in the flame front propagation speed. Operation with air preheat at ultra low NOx levels creates, however, additional difficulties, related to the combustion controls, as it brings another variable - combustion air temperature. This temperature is a function of load and is affected by thermal inertia of the system components during load swings, or during the burner light off. Thus, extra margin may be required between the set point performance and the stability limit of the burner when operating with air preheat. 7 |