OCR Text |
Show Apart from group H natural gas, group L natural gas with a nitrogen content of about 14 % (by voL) was burnt for the tests. The nitrogen in the fuel gas had no influence on N O x emissions, though. The nitrogen content of the fuel gas has a major impact only in cases where oxygen is used as an oxidant [6]. To investigate the effect of heat input on NOx formation, NOx concentrations were measured at 50 and 120 % of rated heat input. The data showed that the impact of heat input on N O x output is negligible. The reference line tests were carried out using an air ratio of 1.05. Basic research has shown, though, that the air ratio and thus the oxygen partial pressure in the high temperature zone of the name has a substantial influence on N O x emissions [7, 8]. T o examine the effect of the fuel/air ratio, burner A was tested at air ratios between 0.5 and 1.05. Fig. 3 shows a practically linear decrease in N O x emissions for near-stoichometric fuel/air ratios. N o N O x was measured for an air ratio of 0.8, although the furnace wall temperature was about 1,570 °C. A s the decrease in the air ratio reduces the mean furnace wall temperature, Fig. 3 plots these temperatures. The test data confirm that glass furnace burners should be operated at near-stoichiometric fuel/air ratios to minimize N O x output. This operating mode has been adopted for many glass tanks in recent years. To collect information on the flame length and temperature distribution in the furnace chamber, suction pyrometer measurements were carried out in the burner level The distribution of the carbon monoxide concentrations in the furnace chamber is shown in Fig. 4. If burnout is equated with a C O concentration of 2,000 ppmv, the length of the flame from burner A is approx. 1.3 m To determine radiative heat flux density as a function of furnace length, total radiation was measured at different points in the burner leveL Fig. 5 shows that radiative heat flux density was highest in the center of the furnace chamber. Total heat flux density (radiation and convection) in the burner level was also measured at different locations in the furnace chamber. A s the probe was not calibrated for a temperature of 1,600 °C, the m V signal was not converted. The m V data were only used for comparing the different burners. To account for the changes in mean furnace wall temperature, these temperatures were also plotted. The maximum mean furnace wall temperature difference in the two test series was 17 K. A s Fig. 6 confirms, total heat flux density peaked in the center of the furnace chamber at a distance of about 2 to 2.5 m from the burner wall The burner A test data served as a reference line for evaluating the performance of the modified burners. Modified Standard Burner Basic research [7, 8] demonstrates that NOx emissions can be reduced substantially by moving the zone of combustion into the furnace chamber. Reasons for the reduced N O x level include flue gas recirculation into the combustion zone and improve heat transfer from the high temperature area of the flame to the charge. The resulting decrease in flame temperature and in the oxygen partial pressure in the high temperature part of the flame will normally lower N O x output considerably. T o apply this approach to the standard burner, the burner brick was modified as depicted in Fig. 7 (burner B). This modification moves the combustion zone into the furnace chamber. 4 |