OCR Text |
Show • Soak Zone: In this zone, fuel and air are fired through the existing furnace burners at normal or reduced primary fuel stoichiometry. This zone is not significantly changed from normal operation. The level of N O x exiting the zone is likely to be fairly significant because of high furnace temperatures. The exhaust then flows into the next zone. • Heat/Rebuming Zones: Immediately upstream of the soak zone, the heating zones require highly radiant heat transfer for rapid ramp up to approximately rolling temperature. Oxygen enrichment of the combustion air in these zones promotes heat transfer and decreases furnace volumetric gas flows, all with the beneficial result of increased productivity and thermal efficiency. Reburning fuel is injected in the cold steel zone downstream of the primary zone to create a fuel-rich, N O x reduction zone. The input N O x reacts with the hydrocarbon fragments formed during oxidation of the reburning fuel, primarily C H species, to produce intermediate species such as H C N and N H 3 , which then undergo a reaction sequence whereby they reduce N O x to molecular nitrogen, N2. • Preheat/Burnout Zone: In this final zone the flow from the preceding zones provides heat, primarily by convection, to the incoming cold steel. Additional air is added in this zone to produce overall fuel lean conditions and oxidize all remaining fuel fragments. Overfire air (OFA) is added through new overfire air or existing burners at furnace gas temperature greater than 1600°F to insure good C O burnout without significantly increasing thermally generated N O x . The burn out is accomplished sufficiently upstream to ensure that almost all of the fuel heating value is recovered by heat transfer to the steel or in the recuperator. Market Drivers and Objectives The proposed technology should generally be cost effective for steel heating furnaces with firing rates >50 MMBtu/hr. These furnaces will typically be the mill bottleneck and/or are found in the Title I ozone non-attainment areas. A number of steel processing furnaces (reheat, annealing and galvanizing) are target applications as they comply with general requirements of high firing rate at high temperatures (>2,200 °F) and with high N O x (>0.2 lb/MMBtu). The total number of furnaces within this demographic range is estimated at about 250, most of which may be candidates during some part of their economic life as industrial modernization proceeds and air emission regulations stiffen. One of the major objectives is an improvement in steel throughput by up to 25 %, without major structural or combustion system modifications. Using gas reburn, N O x reductions using of order 60 to 7 0 % have been demonstrated for glass furnaces, and by analogy, are targeted for steel furnaces. Thermal efficiency improvements are an ancillary benefit; 2 0 % increases are targeted and result from a combination of higher productivity and lower stack exhaust sensible heat loss, e.g., less N2 in the flue gas. It appears feasible to control the furnace atmosphere and temperature/time profiles to match or improve those of an operating furnace. The steel industry is an ideal target for enhanced oxygen combustion because the infrastructure (oxygen separation plants and pipelines) already exists in integrated steel. These facilities are frequently underutilized due to retrenchments in the 80's and the greater use of recycled scrape, reducing the demand on basic oxygen furnaces. Small scale, energy efficient (>40 T P D ) oxygen separation plants using pressure and vacuum swing absorption are now 2 |