OCR Text |
Show Fig. 8 compares the N O x emissions of the standard burner (burner A ) and the modified burner (burner B ) as a function of mean furnace wall temperature. A s Fig. 8 shows, the modification of the burner brick reduced N O x emissions to 1,250 mg/m3 for a mean furnace wall temperature of 1,600 °C. This decrease in N O x output is equivalent to 44 %. Fig. 9 reflects the effect of the design change on the flame length. The combustion area occupied nearly the entire furnace chamber. If burnout is again assumed to be equivalent to a C O concentration of 2,000 ppmv, the flame length measured was 3.5 m and thus about 2.2 m more than in the case of burner A. The diameter of the reaction zone increased from some 0.4 m for burner A to 1 m for burner B. The design changes moved the peak total heat flux density to the rear part of the furnace (see Fig. 10). The maximum heat flux density was recorded at approx. 3.5 m from the burner walL Total heat flux densities were somewhat lower than in the case of burner A partly because the furnace wall temperature was about 50 K lower. For technical reasons, it was impossible to carry out total radiation measurements for this burner. Staged-Combustion Burner Basic research [7, 8] shows that combustion air staging can reduce NOx emissions further. It is crucial though, for heat to be removed from between the primary and the secondary combustion areas. This dissipation can be achieved by moving the burnout zone into the furnace chamber. Aspiration of flue gas from the furnace chamber into the secondary air lowers N O x emissions by another percentage [9]. For staged combustion in accordance with the principles described, the burner brick was modified as shown in Fig. 11 (burner C). This burner brick geometry splits combustion air flow into equal primary and secondary air flows. The effect of the modification on NOx emissions is depicted in Fig. 12. While the mean furnace wall temperature was 1,600 °C, N O x output was only about 750 mg/m3 equivalent to a reduction of 1,500 mg/m3 (67 % ) by comparison with burner A (the standard burner). Considering that the rate of N O x formation in the test furnace is higher than in a real glass furnace, it appears feasible to achieve the State Pollution Control Committee objective of 500 mg/m3 using this burner. This optimistic prediction requires confirmation by testing the n e w burner system in a glass tank, though. As Fig. 13 shows, the flame length of burner C is about 2.6 m, while the diameter of the combustion area is approx. 0.6 m. The comparison of the flame length produced by burners B and C demonstrates that the combustion zone of a staged-combustion system can be relatively narrow if the secondary air nozzle arrangement is optimized. Fig. 14 plots total heat flux density as a function of furnace length. As in the case of burner A, two series of measurement were carried out. A s the mean furnace temperatures varied slightly, the wall temperatures are also given in the graph. The measurement reveals that this burner produces the highest heat flux density in the center of the chamber between 2 and 3 m from the burner wall For a comparison between the burner A and burner C heat flux densities, it is important to note that the mean 5 |