OCR Text |
Show lower flue gas volume and/or a lower flue gas temperature. When oxygen is used to replace air in batch type furnaces, the fuel savings is a result of the lower flue gas volume due to the reduction in nitrogen. The flue gas temperature does not change significantly going from air to oxygen. For continuous furnaces the situation is different. Typically, continuous furnaces are designed with an unfired preheat section near the charqe end. This allows for heat transfer to take place between the cold incoming steel and the hot combustion gases from the upstream zones. The temperature of the flue gases is thereby reduced with the addition of an unfired preheat zone. This improves the overall fuel efficiency of the furnace since lowering the flue gas temperature causes an increase in available heat. When zones are converted to oxygen there is an additional benefit besides the elimination of ni trogen. Since the volume of combus tion gases is reduced in the unfired preheat zone, the flue gas temperature decreases substantially by conversion to oxygen. This lowering of the flue gas temperature further increases the overall fuel efficiency due to the increase in available heat. Thus, for counter-current continuous furnaces, fuel savings is achieved by both a reduction in the flue gas volume and a lower flue gas temperature. As mentioned previously, in batch reheat furnaces and soaking pits, the flue gas temperature does not change significantly by going from air to oxygen. Therefore, both fuel savings and total dollars saved are maximized at 100% conversion to oxygen. The analysis is a little more complicated for continuous furnaces. When a zone is converted to oxygen, the volume of flue gas products will be reduced and the flue gas temperature will drop. As other zones are converted the incremental fuel saved per volume of oxygen added will decline rather than remain constant because the base flue gas temperature will be lower each time. However, there is still the savings associated with the reduction of nitrogen even at lower flue gas temperatures. Therefore, fuel savings as a percent of the total will increase as the number of zones are converted to oxygen but at a lower incremental rate each time. In summary then, as the percentage of furnace conversion to oxygen increases, the fuel savings as a percent of the total will increase but the fuel saved per unit of oxygen consumed will decrease. Therefore, the relationship between total dollars saved and extent of conversion to oxygen is dependent on the relative prices of fuel and oxygen. Each furnace must be analyzed carefully to determine the appropriate degree of furnace conversion. The parameters affecting the fuel savings such as flue gas temperature , flue gas 260 temperature drop, oxygen concentration in the flue gases, and extent of furnace conversion are very much interrelated. In addition, the prices of fuel and oxygen must be taken into account. Due to the complexities involved, Linde has developed specialized computer models for continuous furnaces as well as batch and soaking pits in order to analyze the fuel savings potential of each individual application. FURNACE DESCRIPTION As a first step in any fuel savings analysis, Linde generally conducts a field measurement of the furnace and operating practice. For the 18-2 Bar Mill continuous furnace this measurement of the air base conditions took place in the fall of 1983. A schematic of the furnace is given in Figure 1. There were four top-fired zones and one bottom- fired zone. The furnace was about 100 ft. long and 34 ft. wide. Blooms ranging in cross-section from 6" x 6" to 10" X 12" were hea ted in the furnace. The furnace was designed for a maximum production rate of 200 tons per hour for 8" x 8" x 30' long blooms with a maximum rated total firing rate of 500 MMBTU/hr. The blooms were rolled into bar mill products such as rounds, flats, round-cornered squares and square-cornered squares. STEEL CHARGE END STACK Fig. 1 - Furnace schematic The furnace had no heat recovery system so fuel efficiency was relatively low. In addition, this particular furnace had a rather short unfired preheat section. The distance between the last row of burners in the Preheat zone and the start of the flue uptake was only about 9 feet. The result was a relatively high flue gas temperature with correspondingly high flue gas heat losses which further contributed to the low overall fuel efficiency of the furnace. The flue gas temperature which was measured varied considerably, anywhere from 1700 to 2400 OF depending on firing rate and push rate of the furnace. For the analysis an average range of 1850 to 2100 OF was used to be more typical. This range was slightly lower than what was actually measured to take into |