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Show Average FEGT values obtained from both the Avon 9 and Eastlake 5 boilers with different burner arrangements are plotted on Figure No. 16. The new register with the counter spin impeller resulted in a reduction of nearly 100°F throughout the load range compared to the earlier Avon 9 measurements and is also well below the Eastlake 5 B&W cell burner. When this arrangement in Avon 9 was changed to a 50/50 mix of parallel and counter spin impellers, the fire was raised and the FEGT improvement mostly lost. However, the temperature and fire are more uniform and the earlier high peak temperatures are no longer present. Preliminary Eastlake 5 data with all parallel spin impellers appears to be slightly better than the 50/50 mix. While the Avon 9 upper furnace condition with the mixed burners showed more signs of delayed combustion and increased slagging of the superheater, the Eastlake 5 (with 100% parallel spin impellers) produces an upper furnace appearance which is excellent with no evidence of previous heavy superheater slagging tendencies. The more complete combustion is also supported by lower carbon in the fly ash. With the old burners, Eastlake 5 had experienced unburned carbon in the 2% LOI range. This was reduced to less than 0.5%. Several other factors should be noted. The original Eastlake 5 data was obtained while the furnace was still a pressurized unit while current data is with the furnace as balanced draft. This has increased gas velocity and effectively made the furnace volume smaller. Slagging Patterns The cell burner with the conical impeller creates a lazy flame and made it difficult to maintain a good side to side 02 balance in the furnace. Frequently, the 02 difference would be greater than 1%. Delayed combustion at higher loads was evident and this resulted in heavy slag accumulation on the superheater pendants. Wall slag in the lower furnace was usually minimal. The new burner with the spinner impeller and workable air damper doors results in little delayed combustion in the upper furnace, much more manageable slag accumulations on the superheater pendants and provides the ability to balance 02. Usually the 02 balance is within 1/4 to 1/2% or better. This burner, however, tends to create more wall slag and build burner eyebrows (see photograph, Figure No. 17). The combination with the impeller spin direction opposed (counter flow) to the air register created excessive eyebrows and side wall slag in the Avon 9 furnace. At high loads, the side wall slag would become wet and run onto the bottom slope. There was danger that this slag would block the throat opening to the ash pit. The eyebrows would become large and also drip and run slag down the wall and onto the slope. The cause of this problem appears to be the skewed flame path created by the opposed spins and generally higher furnace bottom temperature (see Figure No. 18, furnace bottom temperature). Changing the impeller rotation to parallel spin has helped reduce eyebrows and wall slag. Eyebrow formations are smaller and don't pose a substantial problem. Wall slag accumulation is reduced, remains dry and is manageable. If the pulverizer air/fuel ratio is allowed to increase above recommended limits, however, the quality of the flame is significantly degraded and eyebrows and wall slag increase. |