| OCR Text |
Show combustion products in the heater, which reduces heat transfer to the boiler or process tubes, and ultimately results in a lower thermal effiCiency. Secondly, the additional power needed to move the flue gas through a larger primary fan (or a separate smaller fan) increases electrical energy costs to operate the boiler. While high excess air operation and external FGR both lower thermal efficiency and increase operating costs, external FGR is preferred over additional excess air because some of the energy lost in the stack can be recovered by reintroducing it into the burner as preheated combustion air. FGR may be particularly difficult to apply to package boilers because of the relatively large pressure drop built into package boilers to keep the footprint small. Reference 1 discusses the costs associated with FGR in more detail. Because of the operating penalty associated with an FGR solution, a problem that is most pronounced with package boilers, the external FGR solution was pursued as a contingency option only. Fuel Staging Fuel staging is a technique where fuel is introduced into two separate combustion zones. In the first zone, the burner is operated very lean (high excess air) to reduce thermal NOx formation. Once the first stage gas has cooled by transferring energy to the process, the second stage fuel is introduced. The secondary fuel is introduced downstream of the first stage to consume unreacted oxygen, and is introduced in such a way as to induce furnace gases to cool the reaction while not forming excessive CO levels. Although fuel staging had not been previously tried on surface combustion burners at an industrial scale, the RSB was well suited to this approach. The RSB had already demonstrated stable, low NOx operation (less than 10 ppm NOx ) under very lean operating conditions, and a secondary, fuel-rich combustion zone could easily be introduced over the surface of the primary burner by using carefully placed fuel nozzles. The primary advantage of combining fuel staging with the RSB is that the primary burner is a proven ultra-low NOx burner and is much more stable than conventional burners under very lean conditions. While the use of FGR to reduce NOx results in decreased thermal efficiency and increased fan power requirements, fuel staging does not have these shortcomings. 4 |
