| OCR Text |
Show Chemical equilibrium calculations at adiabatic conditions indicate that the furnace gas temperatures in these zones are sufficiently elevated to produce higher levels of N O x , as shown in Figure 4, as well as to increase the exiting flue gas temperatures. In reality, the process is far from adiabatic due to the highly stratified, radiating flames developed through burner modifications. These increases will result in initial conditions that are more favorable for gas reburning, e.g., greater than 2200°F and N O x levels approaching 600 ppmv, as shown in the experimental data (Energy and Environmental Research Corp. [EER], 1996) presented in Figure 5. Figure 5 also shows a block diagram for implementation of the gas reburn process to be implemented in the steel reheat furnace. The soak (holding) zone would be conventionally operated as part of the primary zone, perhaps at slightly lower excess air, and therefore should be neutral with respect to fuel efficiency, scale formation, and N O . Any C O slip from the soak zone will ultimately be controlled in the G R , Over Fire Air (OFA) section of zone 3. The gas reburn (GR) injectors are located in the last reheat zone or the preheat section. The G R injection location must be substantially post combustion, near stoichiometric, and at a temperature sufficient for effective free radical formation. For a walking beam furnace with multiple ceiling burners gas reburn could conceivably be implemented by operating the last burner set fuel rich. O F A injectors are located downstream based on considerations of optimum N O x reduction by maximizing reburn zone residence times subject to constraints of complete combustion and furnace efficiency. In upcoming program tasks, C F D modelling will be used to locate these positions for both baseline and O E A conditions. The design of an advanced reheat furnace system is site and equipment specific. In order to perform an evaluation in sufficient detail to support the design and development phases of the project a proforma model furnace was first developed. This model was identified from market derived data, by matching an actual furnace for which design and operational data was available to typical furnaces, as shown in Table 1. Table 1. Typical and Model Furnace Descriptions Parameter Hearth, width/length Charged Steel Production, T PH Efficiency, MMB/hr Residence Time,hr Zones NOx, #/MMBtu Slab 32/110 9"x60"x28' 250 1.8 2.4 4 0.4 Boom/Billet 25/60 6"sqx22' 100 1.5 1.6 3 0.3 Model 30/65 5.5" sqx28" 115 1.2 1.8 3 0.3 Process Analysis The current status of the process analysis activity is described below. To date two models based on the model furnace specification have been developed and refined: • a computational fluid dynamic model of the entire furnace • a zoned system model based on heat transfer and mass balances 5 |
