OCR Text |
Show 2) Effect of furnace configuration on NOx concentration (Fig.7) (1) Chimney moved from D to D' (2) Burners brought closer to the inner case by movement from A, B to A', B' (3) Diameter of the inner case decreased from 800 to 600 mm. Basic conditions were used for the furnace temperature, excess air ratio and thermal input of burners A and B. Unless stated otherwise, these conditions also applied in later figures. In configuration (1), changing the chimney position is thought to have altered the flue gas flow in the furnace. The NOx concentration increased with each burner type in configuration (2), because the top of the flame tended to hit the outside of the inner case and the furnace wall. Owing to the design of cylindrical furnaces, it is difficult to prevent the flame from hitting the furnace wall. To reduce the NOx concentration, therefore, the burners must be kept as far from the inner case and furnace wall as possible. In configuration (3), the NOx concentration increased with MK, remained almost the same with LC-h, and decreased with LC-m. Long flame, LC-m burner performance is thought to have been due to the flame's failure to hit the outside of the inner case, which reduced the locally heated area (Fig. 8, No.6). In the MK burner, the EGR effect was weakened (firing volume, 0.70 m3 ). In the third configuration, there was a very interesting phenomenon (Fig. 9): with any burner, the NOx concentration was almost constant, regardless of the furnace temperature. Besides near elimination of the local heated area, this was a result of increased in-furnace flue gas volume at higher furnace temperatures, which raised the flow speed of the flue gas, enhancing the EGR effect. The MK and LC-h burner NOx concentrations approached unity because of structural differences between their burning sections. In LC burners, the fuel and combustion air were less well mixed than in MK burners, and the unused air surrounding the combustion gas was blown out of the combustion pipe. Then, as the combustion gas was cooled and heat transfer in the furnace cut off, the NOx concentration seemed to be reduced, especially when the excess air ratio was high. 3) Effect of excess air ratio on NOx The results are shown in Fig. 10. The NOx concentration decreased as the excess air ratio was increased with either LC burner. On the other hand, when the excess air ratio for the MK burner was increased, combustion was accelerated to raise the . flame temperature. Therefore, at an excess air ratio of 1.5, the NOx concentration was lower with slower flow LC burners, than with MK burners. This demonstrates a similar tendency to the NOx concentration test for the furnace shown in Fig. 3. Fig. 11 indicates that the NOx concentration of either LC burner was almost constant, regardless of furnace temperature. Results from - 3 - |