OCR Text |
Show Advanced Overf"U"e Air (AOFA) Systems. Generally, combustion NOx reduction techniques attempt to stage the introduction of oxygen into the furnace. This staging reduces NOx production by creating a delay in fuel and air mixing which lowers combustion temperatures. This staging also reduces the quantity of oxygen available to the fuel-bound nitrogen. Typical overfIre air systems accomplish this staging by diverting ten to twenty percent of the total combustion air to ports located above the primary combustion zone. Advanced overfire air improves this concept by introducing the OF A through separate ductwork in greater quantities, with more control, and at higher pressures. The resulting system is capable of providing deep staging of the combustion process with accurate measurement of the AOF A airflow. Wall-Fired Low NOx Combustion Technology. Low NOx burners attempt to stage the combustion process without the need for the additional ductwork and furnace ports required by overfire air systems. These commercially available burner systems introduce the air and coal into the furnace in a well controlled, reduced turbulence manner which is significantly different from earlier combustion designs. Low NOx burners have been developed for single wall, opposed wall and cell burner applications. FWEC was competitively selected to design, fabricate, and erect the opposed Wall, low NOx burner shown in Figure 2 and the AOF A system described above. I Ignitor Perforated Plate Air Hood ~:;;,...--,.~ III --~~ovable Sleeve o~ --~ Figure 2: Low NOx Burner Installed at Plant Hammond T-Fired Low NOx Combustion Technologies. Four different low NOx combustion technologies (Figure 3) offered by Asea Brown Boveri Combustion Engineering (ABB CE) for tangentially-frred boilers are planned for demonstration during this project. The testing of these technologies progresses in the most logical manner from an engineering and construction viewpoint. During Phase I of the project, the baseline conditions of the unit were studied. During Phase II, the Low NOx Bulk Furnace Staging (LNBFS) system and the Low NOx Concentric Firing System (LNCFS) Level II are being demonstrated. Finally, LNCFS Levels I and III will be demonstrated during Phase Ill. The concept of overfire air is demonstrated in all of these systems. In LNCFS Level I, a Close-Coupled Ovettrre Air (CCOFA) system is integrated directly into the windbox. Compared to the baseline configuration, LNCFS Level I is arranged by exchanging the highest coal nozzle with the air nozzle immediately below it. This configuration provides the NOx reducing advantages of an overfire air system without major pressure part modifications to the boiler. 5 |