OCR Text |
Show burners in the Preheat zone, nearest the flue, and operating that zone with only four burners instead of six. The placement of burners and the operation of the various zones can have a significant effect on the overall fuel and oxygen consumption. This was particularly evident based on the effect of switching to the four burner operation. With six burners firing in the Preheat zone, the amount of the furnace firing rate converted to oxygen was calculated to be between 60 - 63% based on the first four weeks of operation. This was higher than what was predicted. Theoretically, as more oxygen is used to replace air, the flue gas temperature would be expected to decrease and the fuel savings would increase. However, because the oxygen zones were located closer to the flue, increasing the firing of the oxygen zones caused the reduction in the flue gas temperature to be lower than predicted and therefore the fuel savings was initially lower than what was estimated. The impact of the higher conversion and reduced drop in flue gas temperature with oxygen can be illustrated with Figure 3. FU.LIA~NQI-CONnNUOUlFURNAC. FLUE GAS TEMP - 2100"F lM ~-----------., \ ." 12 10 • \ \ \ \ \ \ \ \ \ \ \ FUJ£ 'nMP DROP AT-tIIO"I'-- AT-arxr,-- \ \"'" ' ....".. ..'. ..... ....... .. ~--==-------- .~ __ L-__ L-__ ~ __ ~ ., 40 eo TO Fig. 3 - Effect of % conversion and flue gas temperature drop on fuel savings The lower fuel savings and higher furnace conversi'on resulted in higher than estimated oxygen consumption. In order to reduce the oxygen consumption and improve the fuel efficiency, the first two oxygen burners in the Preheat zone were shut off. Due to the location of the structural supports on the sides of the furnace, the last two burners were located closer to the flue uptake than the last row of air burners were. Consequently, the flue gas temperature dropped considerably when these burners were turned off. The oxygen system was still able to provide adequate heat so that the 262 production rate was not reduced. with these burners shut off the control thermocouple was no longer in the middle of the zone. It was therefore moved to an area approximately in the middle of the four remaining burners in order to record a truer zone temperature. The result of this was a dramatic improvement in the fuel efficiency. Figure 4 shows specific fuel consumption vs. production rate for different periods during the operation of the oxygen system. There was a slight reduction in specific fuel consumption. This was due mostly to eliminating the cooling air from the air burners. • te - 2 BAR IIILL CONnNUOUI FURNACI TOTAL SPECIFIC FUEL CONSUMPTION o 0r---.r--~---.---'---~-___ r--~--~ o 40 eo 100 120 TONS/HOUR LEGEND 0 0 0 GROUP 1 6 6 6 GROUP 2 ••• GROUP 3 ~p1~F~3n1m3n8 ~P2 ~ F~4/01 1'OIn7 CIIIOUP 3 ~ 'lI0II1130 1'0 71'01 140 Fig. 4 - Specific fuel consumption for three periods of oxygen system operation 110 However, the main effect of the changes made to the oxygen system was an improvement in terms of MMBTU saved/ton of oxygen consumed. This can be better seen from Figure 5, which shows specific oxygen consumption vs. production rate. Note the significant reduction in oxygen consumption. Though specific fuel consumption was not affected that much, the same MMBTUs were saved with a much lower oxygen consumption. This increase in efficiency was accomplished by shifting some of the heat input to the upstream air zones and thus lowering the conversion level and also by lowering the flue gas temperature. The final results are given in Table 2. The changes made to the system reduced the level of conversion from 60 - 63% to 52% - 54%. The fuel savings improved dramatically from 7.37 MMBTU saved/ton of oxygen to 13.4 MMBTU saved/ton of oxygen - an 82% increase from the first 4 weeks. This reduced the oxygen consumption from .095 ton/ton of steel to .062 ton oxygen/ton of steel - a 35% decrease. The overall specific fuel consumption with the oxygen system was 2.16 MMBTU/ton of steel which is a 28% savings from the air base specific fuel consumption of 3.00 MMBTU/ton of steel. |