OCR Text |
Show Natural gas was injected radially from a center gas gun in a direction perpendicular to the combustion air flow. NOx emissions control from oil firing was limited to the use of FGR. No.2 fuel oil was used and, therefore, the attainment of emissions requirements was readily achievable. Oil is used in Southern California only as a backup fuel and, as such, there was no strong emphasis on minimizing NOx emissions from oil firing. 3.4 Specifying Burner Design Parameters Burner design parameters for gaseous fuel firing were defined based on analytical modeling. Oil ruing was based on conventional state-of-the-art techniques and did not require special treatment. Analytical modeling focused on two criteria: maintenance of combustion stability with FGR and natural gas firing, and the creation of a stratified flame structure when burning natural gas. The modeling effon was initiated by the selection of the swirler size and swirl number. A trade-off existed between intensifying the swirl to ensure satisfactory mixing between fuel and air versus enhancing NOx production in the near burner zone due to the intense mixing. Balancing between these competing requirements was achieved by documenting, through modeling, air-to-fuel ratio immediately downstream of the swirler. Upon defining the swirler geometry, an effon to optimize the design of the fuel injection nozzles followed. The challenge with natural gas firing was to maintain flame stability with very high turndown. As discussed above, the intent of burner development was to use natural gas merely as a suppon fuel. At the same time, the burner must be able to produce full rating on natural gas firing while providing acceptable flame geometry and low NOx• -9- |