Opti-Flow™ Overfire Air (OFA) Systems
- Description
- Features
- Arrangements and CFD Analysis
Siemens Overfire Air Systems can be used on boilers firing:
- Coal (pulverized or cyclone)
- Oil
- Gas
- Biomass
When used in conjunction with Opti-Flow™ burners, optimum combined burner/OFA performance can be obtained: NOx below 0.3 lb/MBtu on bituminous coal and 0.15 on lignite or PRB.
These systems:
- Control CO, and Unburned Carbon or Loss on Ignition (UBC or LOI), with an aerodynamically-staged OFA port arrangement
- Provide optimal mixing of overfire air with combustion gases (see figures below)
- using aerodynamics for penetration
- with excellent transverse mixing
- without complex registers or adjustable vanes - Keep furnace sidewall atmosphere oxidizing (not reducing) to eliminate or reduce lower furnace slag formation and corrosion, since burners are not deep staged with the Siemens approach.
Siemens OFA systems have the following features:
- Water-cooled waterwall openings without problematic refractory
- Low pressure drop components, therefore no booster fan required
- Single stage ports with only one moving part: the sleeve damper
- Aerodynamic OFA port uses high quality stainless steel for parts facing the fire, including the cast nozzle tip, yielding reliable, maintenance free operation.
Typical Arrangement and CFD Analysis for 700 MW Wall-Fired Lignite-Fueled Boiler with Opti-Flow™ Overfire Air System
The figure illustrates the air staging system for a 700 MW, 56 burner boiler that attains NOx of 0.15 lb/MBtu with CO typically below 100 ppm. A novel feature of the staging system is the use of “wing ports,” located vertically between the OFA elevation and top burner elevation and horizontally between the side (or wing) burners and the sidewalls.
The wing ports inject air at a lower elevation than the OFA ports in order to maximize the residence time between the injection point and the furnace exit to achieve maximum CO burnout. In general, wall-fired boilers generate significant CO concentrations along the sidewalls, even when burners are well balanced. Locating a set of
staging ports closer to the burner zone than the OFA ports
maximizes both the temperature and time for CO burnout.
Overfire air effectiveness is limited by the amount of CO and unburned coal that reaches the furnace exit and, when excessive amounts of OFA flow are used on coal-fired boilers, by furnace slag and corrosion in the lower furnace. The effectiveness of Siemens system in controlling CO is illustrated in the figure, which depicts CFD modeling results for the 700 MW boiler.
The experience based on the Opti-Flow™ equipment proves that:
Proper Analysis + Effective Equipment = Low Emissions – Fireside Problems