| OCR Text |
Show Narrow Furnace Cross-Sections Package boiler transport requirements dictate a relatively small furnace cross-sectional area. For the January 1992 to July 1992 second generation OAF start ups, a design parameter was used to correlate heat release rate per foot of furnace width. The magnitude of this number ranged from a low of 7 MMBTU/HRFT to a high of 43 MMBTU/HR-FT (refer to Table 1). The before mentioned potential problem of sidewall flame impingement, with consequential flame q~enching and heat maldistribution, will of course be augmented if this heat release rate-to-furnace width parameter is not considered by the burner designer for a given application. In addition, for retrofit units where small gaps may have developed between adjacent tangent tubes, the problem of partially burnt flue gas bypassing can result in noncompliance of stack CO and hydrocarbon levels. As in the case of high SHRR, the firing rate per furnace width parameter will impose further restrictions on the selection and extent of what NOx reducing mechanisms may be employed for a given application. A prime example is the potential constraint which may have to be imposed on a burner's aerodynamic swirl number: Although the utilization of swirl as a tool in the reduction of NOx formation has been well established, a high swirl flame is notoriously a wide flame. Henceforth, the utilization of swirl must be compatible with a furnace's cross-sectional dimensions. Single Burner Operation Although the majority of package boilers operate with a single burner, it is not unusual for end users to require turndown requirements of 10-to-1 or greater (yielding pressure variations of over 1 00-to-1). When operating with reductions in initial fuel and air velocities of this magnitude, consideration must be given to the resultant variations in 3 turbulent fuel/air mixing, as well as the effect of these variations on flame patterns and stability. In addition to maintaining burner stability at turndown loads, it is imperative that any changes in burner swirl number, turbulence level, etc. be compatible with all design restraints (Le.-furnace dimensions) and emission requirements. Although NOx levels tend to decrease with decreasing load, control related problems may result in low NOx applications utilizing flue gas recirculation: If the proper quantity of recirculated flue gas is not adjusted as a specific function of heat load, the conflicting problems of high NOx levels on the one hand and burner stability on the other may result. GENERAL PROGRAM OBJECTIVE The development of the second generation OAF burner was motivated by an industry need for a low NOx burner which could be utilized in a package boiler environment without compromise to system performance. The general program objective was to incorporate NOx reducing mechanisms into the burner design in such a way as to have no impact on flame containment (with regards to either sidewall or rear wall flame impingement) I combustion induced rumble and vibration, radiant heat distribution, boiler turndown capabilities, burner register pressure drop, burner stability, burner flame shaping flexibility, and system reliability. |
