OCR Text |
Show Heat losses in these ducts and the recuperator are neglected but are expected to be minor. The input assumptions were kept consistent with the Fluent modeling approach as much as practical. The wall area and thermal resistance was updated to include side walls (not possible in the 2 D Fluent model), but on the other hand did not allow for heat losses underneath the steel in the region without a bottom heat zone. T o match steel energy absorption in the baseline case, wall thermal resistances and steel emissivity were modified from the Fluent model parameters, but to values reflective of available information and experience. Recuperator performance is incorporated into the model. The calcuation is based on the effectiveness method, assuming that the value of N T U (Number of Transfer Units), established initially at baseline conditions, is constant at all other conditions. Heat removal from the steel skids (by cooling water flow) is included in the furnace model, and is assumed to be the same (15 M M B T U / h r ) in all cases presented here. Figure 11 shows baseline mass and energy parameters of interest. Reburning in the top heat zone is then applied as a perturbation to the baseline model, with a given percentage of the total fuel heat input diverted from the first to the second upper heat zone. This effectively moves the heat release in this zone downstream. Figure 12 shows the impact of a variation of 0 to 15% total fuel in the reburn zone on the energy demand, measured in M M B T U per ton of steel produced. The impact of 1 5 % reburning is a reduction in overall furnace efficiency of about 2.3%. One way of viewing this is that only about (2.3/15) or about 1 6 % of the reburn fuel heat input is lost, implying 8 4 % heat recovery of the reburn fuel. Some of the heat recovery is direct heat transfer to the steel within the heat zone. Some of the heat initially escapes the furnace in the form of a higher furnace exit temperature, but is partially recaptured in the recuperator in the form of higher preheat temperatures. This temperature impact is shown as a function of the amount of reburn fuel in Figure 13. At 1 5 % reburning, the additional air preheat energy accounts for about 1.2% of the 140 M M B T U / h r fuel heat input. This implies that most of the reburn fuel heat input is recovered directly in the furnace, and is supplemented by air preheat recovery. Indirect heat recovery in the recuperator is less efficient and thus less desirable than direct heat transfer to steel, but does represent an additional mitigation of the heat loss due to delayed combustion of reburn fuel. Flow Sheet Performance The above modelling results have been used to develop flow sheets with which to establish performance and economics. The previously discussed C F D results were used to locate furnace cross sections for agent injection (reburn fuel and overfire air). The temperature and C O profiles establish the fuel injection location, e.g., just downstream of the C O < 2 0 0 ppmv contour (complete combustion) and at the highest bulk furnace temperature, T>2400 °F. The zoned system model was then used to determine the impact of these locations on the amount of the reburn fuel heat that is usefully recovered, i.e., in the steel and recuperator. For purposes of the proforma economic analysis the furnace baseline N O x emission is assumed to be 0.3 lb/MMBtu. The mass balance for Case 2 with gas reburn only, differs only slightly from normal operation. Since the gas reburn incorporates a final afterburning zone with the injection of O F A, all high temperature heating zones will be operated close to stoichiometric subject to current practice in scale control e.g., any low levels of C O will be burned out in the last stage of the 9 |