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Show Ladle Preheat Station, Midwestern U. S. A. As originally installed, two horizontally fired preheat stations for 70 ton ladles at a steel mini mill were heated to setpoint temperature (1340 or 1395 K) by a single natural gas burner designed to use air enriched with pure oxygen, with a maximum rate of 2.1 M W (7.5 x 106 Btu/h). The combined oxygen-air stream had 4 5 % oxygen. The oxygen enrichment allowed the ladles to reach setpoint more quickly and at a lower firing rate than would be possible with conventional burners. However the N O x emissions were very high, peaking at more than 600 g/h from a single station. The original oxygen-air burners were replaced with HiRam Oxy/LNI high velocity burners. Each burner is fitted with a single outboard fuel injector and a single outboard oxygen injector. The arrangement of the ladle cover with burner, injectors, and fuel and oxygen trains may be seen in Figure 12. As configured for this application, the burner fires on 1 0 0 % oxygen up to a maximum rate of 2.1 M W (7.0 x 106 Btu) with oxygen and fuel through the respective injectors, or with 1 0 0% ambient air and fuel through the burner gas connection up to a capacity of 1 M W (3.5 x 106 Btu/h). The new burners were commissioned in January, 1998, and emissions tests performed in February, 1998; the results from tests on one of the units are shown in Figure 13. As can be seen from Figure 13, the N O x emissions for the LNI system were much lower than for the combination oxygen air system which was replaced. All of the columns except the first represent operation on cryogenic oxygen of very high purity. Test # 1 was done using oxygen from an atmospheric generator, which was only about 9 4 % pure (balance nitrogen). Test # 2 was performed on the burner without adjusting the fuel / oxygen ratio for the pure oxygen; the excess of oxygen in the combustion products resulted in higher NOx. Test # 3 shows that reducing the fuel / oxygen ratio to compensate for the higher oxygen purity resulted in a corresponding reduction in N O x emissions. Tests # 5 and # 6 were conducted with the gap between the ladle lip and cover loosely plugged with mineral wool insulation over the bottom half of the ladle circumference. They demonstrate the low emissions levels achievable when infiltrated air can be eliminated from the base of the combustion region. Figure 12 N O x Emissions for 70 Ton Ladle Preheat Station Fired by a HiRam Oxy-LNI Burner at 2.1 M W E 0.4 N O x emissions for original oxygen enriched burner •so- Test # 2 • HiRam Oxy-LNI NOx emissions Test # 1 Test # 3 Test # 4 Test #5 Test #6 0 2 ratio = 2.0 02 ratio = 2.0 02 ratio = 1.8 burner gas on burner gas on burner gas on "atmospheric" 02 0 2 ratio =1.8 02 ratio =1.8 02 ratio = 2.0 burner gas off burner gas off; burner gas on; gap plugged gap plugged Figure 13 7 |
