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Show process. Lower excess air should result in a 10 to 20 % N O x reduction and some fuel efficiency improvement. Gas reburn fuel is then added through jets in the furnace ceiling, bottom and walls at a location about midway along the heating zones where bulk temperatures are about 2600 °F. This should result in an additional 50 to 5 5 % N O x control. Note that the area immediately adjacent to the steel can be kept relatively oxidizing downstream of the fuel injection point to avoid carburizing if required by controlling the penetration distance of the reburn fuel jet. O F A is introduced in the top and bottom zones about 0.4 seconds (about 8 ft downstream) at a point where the process gas temperature is still » 1 6 0 0 ° F to complete combustion. Thus, only a very small steel surface area in the treated zones is exposed to the reburn reducing environment. The released heat is substantially recovered (70 to 8 5 % ) in the preheat zone and the recuperator. Overall, the gas reburn process is expected to be nearly fuel efficiency neutral. Gas reburn N O control effectiveness is estimated from Figure 5 at 50 % or 0.15 lb/MMBtu. If less N O x control is required, only the top zone would be treated. Case 3 incorporates the combined GR/OEA technologies to obtain high radiant heat in the top heat zone using about 4.2 T P H of oxygen enrichment (50% O2). This amount was previously shown to achieve about a 2 0 % efficiency improvement without significantly impacting furnace refractory temperatures and does not result in local steel surface melting. The process conditions have now moved to a more favorable condition, high N O x (about 0.51 lb/MMBtu), but injection temperatures similar to the baseline reburn case (2600°F), improving the gas reburn performance. N O x reductions of about 6 0 % , as shown in Figure 5,-result in a N O x emission of about 0.18 lb/MMBtu, or 4 0 % less than baseline. In reality, actual furnace design, site and regional requirements will dictate the level of enrichment; only sufficient O 2 will be introduced to meet the new production requirements and only enough gas reburn to meet the regulatory requirements. The potential effects on furnace efficiency due to gas reburn heat input to the top heat zone is shown in Figure 12. Thus, the approach is seen as a tailorable technology for an evolving and changing set of performance requirements. For example, during periods of extended production delays due to mill problems O 2 enrichment would not be necessary or cost effective, and could result in unwanted steel oxidation. Long mill delays increase the potential for steel carburization in the reburn zone where the furnace environment is more reducing due to the injection of the reburn fuel. Under full reburn conditions at a stoichiometric ratio of about 0.93, the process gas between the G R and O FA injection points will typically be composed of about 0.025 and 0.08 mole percents of C O and CO2, respectively. The carbon potential for this mixture has been estimated and indicates that even at low stoichiometrics the potential is several orders of magnitude lower than that used for heat treating. O n extremely long delays the G R system can always be adjusted to a less severe C O level or shut down to control carbon potential. Proforma Economics and Market Potential A proforma cost-effectiveness evaluation has been performed for the two cases and compared to the baseline furnace performance. The figure of merit for the comparisons is as follows: • Gas reburn only: the annualized cost per ton of NO2 reduced, absolute and as compared to other technologies • Gas reburn with OEA: the net annualized value of incremental steel 10 |