OCR Text |
Show account the heat transfer and associated qas temperature drop between the area where the measurement was made and the actual entrance to the flue. An overall furnace energy balance was used to estimate this true flue gas temperature. Previous analysis had shown that conversion of the Preheat and No.1 Heat zones would generate maximum fuel savings. During the measurement period the firing rate of these zones as a percentage of the total firing rate was monitored. This percentage would vary, of course, as production rate and furnace operating practices changed. Based on these measurements, conversion of the Preheat zone and No.1 Heat zone to oxygen was estimated to be equivalent to a conversion ranging from 55% to 42%. In addition to flue gas temperature and extent of furnace conversion, the other important parameter influencing the fuel savings is the expected drop of the flue gas temperature after conversion to oxygen. This parameter could not be determined directly from the field measurement. However, based on the information gained during the measurement, the computer model was used with more confidence and based on results from computer studies, it was estimated that after conversion to oxygen the flue gas temperature would drop in the range of 100 to 300 of. The prediction for the fuel savings based on the ranges of the three key variables can be illustrated with Figure 2. This shows an overall range for savings to be quite large. However, since the parameters are not completely independent of one another, the most likely range was expected to be 12 to 15 MMBTU saved/ton oxygen. ~.----------------------, 28 28 10 • • FUJE TatP DROP lIT-IOO'f'-o. T-3CIO"'--- 4~ __ ~ __ ~ ____ ~ __ ~ __ _4 1400 1800 1800 2000 2200 2AOO !'We TatPERAT1JRE iF) Fig. 2 - Effect of key variables on fuel savings 261 CONVERSION TO OXYGEN Conversion of the Preheat zone and No. 1 Heat zone took place in the beginning of 1985. The 12 flat-flame roof air burners in the Preheat zone were replaced by 6 "A" Burners installed in the side walls of the furnace. An identical "A" Burner system consisting of 6 burners was used in the No.1 Heat zone to replace the 24 flat flame roof air burners. The conversion was accomplished with a simple retrofit. Most preliminary installation work was performed with the furnace in operation. Final installation and checkout of the system was completed during a normally scheduled down week. No production was lost during the furnace conversion period or system start-up. The furnace operated with the II A" Burner Systems for 15 weeks. During these weeks of operation, significant improvements were made in the operation of the system. In the first few weeks of operation, our main concern was the smooth and reliable operation of the equipment. This included achieving a relatively uniform temperature distribution within the oxygen zones so that their normal production rates as well as the surface quality of the steel could be maintained. Once these goals were accomplished, steps were taken to improve the operation of the oxygen systems. These steps included elimination of the cooling air in the air burners and changing the firing location of some of the oxygen burners. Understanding the effect of the burner placements and firing pattern of the furnace was important in optimizing the operation of the systems. These factors will be discussed in more detail. Initially the roof air burners in the Preheat and No. 1 Heat zones were kept available so that in case of problems with the oxygen burners, the zones could be switched to the air burners immediately. A small amount of air flow was used as a coolant for these burners. This resulted in an additional heat loss which reduced the fuel efficiency of the furnace. After the first three weeks of operation during which no problems were encountered with the oxygen burners, the mill people had enough confidence in the systems to plug the air burners with refractory thereby eliminating the additional heat loss due to the cooling air and improving the overall fuel efficiency. Once the reliability of the II A" Burner systems had been established, efforts were made to optimize the operation of the systems . Since identical "A" Burner systems were used in both zones, yet the required firing rate in the No. 1 Heat zone was much greater than in the Preheat zone, it was possible to reduce the firing rate of the Preheat zone and still maintain the production rate. This was accomplished by shutting off the first two |