OCR Text |
Show 5 1 __ Adjustable Spin Vanes Inner Zone Outer Zone Figure 3 Dual Zone NOx Port For most opposed wall-fired (OWF) boilers, the parameter with the most significant influence is the arrangement of ports. Like the SWF boiler, the ports must be positioned properly to achieve good mixing. An increase in port momentum or MVR should not be expected to improve mixing if the ports have not been carefully placed. The mixing is less sensitive to burner swirl pattern than in SWF boilers. However, the swirl pattern still affects distribution of mass flow across the furnace plan section. The influences of the furnace geometry indicate that the best mixing is achieved by efficiently agitating the combustion products with air at the injection plane. Existing design methodology has been enhanced through the inclusion of results like these and others, as well. Specific design guidelines have been developed that may be applied with confidence across the expanding range of boiler designs addressed by these studies. Due to the site-specific characteristics of some boilers and retrofit systems, additional analyses are needed. These characteristics may include division and/or wing walls in the furnace, significant variations in furnace plan section, physical constraints for locating burners and ports, fan capacity, and customer-specified requirements. In these cases, modeling is used to extrapolate existing design methodology to facilitate modifications to the system design. SINGLE WALL-FIRED BOILERS NOx Port Placement The air staging port arrangements that often produce the best mixing in conventional SWF boilers have ports on the rear furnace wall in addition to the front wall. However, arrangements with ports on the rear wall are often precluded because the convection pass is adjacent to the rear wall of the furnace. Flow and combustion modeling were conducted on the 215-MWe boiler shown in |