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Show compromised by the moderate pote~tial for combustion air preheat (about 800°F) resulting from the moderate furnace gas exhaust tem~eratures .. Preheatmg the feed by charging through the stack or a separate heat recovery unit is, therefore, sometimes practIced . . Heat treating fll!flaces of the direct and indirect (muffie) type are frequently used to prepare work for further processmg or to chan~e Its metallurgy. The work requires extremely even heating, leading to the use of highly radiant furnace walls ~d/or Improved convective heat transfer with high-velocity burners. These furnaces are moderately thermal.Iy e~clent, 30 to 500/0, depending on furnace temperatures. Natural gas is almost exclusively fIred to avoid contanunabon of the product. Direct firing is not used for annealing but may be used for other types or work heating. Furnace surface heat losses are more significant for these furnaces and stack sensible heat losses are less significant, compared to the reverberatory furnace. 4.6.2 Combustion Research Survey Results . ~ere are a n~ber of research needs identified in this evaluation. The greatest single focus was on research ~or l1nprovmg productivity of the combustion equipment. Short-ternl (within 3 years) research was considered more In1~o~ant than longer~ternl research because of the belief (especially relating to environmental concerns such as air ~nusslOns) that changmg regulatory requirements make the longer term too Wlcertain. The highest priority combustion Issues are clustered in the following categories, in order of priority: • • • • • • Furnace efficiency and productivity improvements through flame radiation for secondary aluminum melters and heat treating furnaces , enhancement and O2 enrichment for reverberatory furnaces Improved refractories for hot burner parts such as burner blocks, radiant tubes, and burner tips NOx reduction teclmologies for oxygen-enhanced combustion Multipoint, low-cost sensors for monitoring composition (gas and metals), and temperature (gas and alwninum product) NOx control and enhanced production with acoustic (pulsed) or catalytically assisted combustion teclmologies Waste control technologies, especially advanced thermal treatment (vitrification) of dross and spent pot liner. Significant cost savings may be achieved through improved efficiency of combustion equipment. Expected energy saving~ (including electrical energy) by the year 2000 is estimated to be 2-3% through improvements in monitoring and control, 10-150/0 through retrofit improvements on current equipment, and 15-300/0 through installation of new technologies (Hulgen and Kvandc, 1994). While the large portion of these savings may be from smelter improvements, commensurate improvements in combustion equipment are also desired. The research areas of improved refractories, NOx reduction technologies for oxygen-enhanced combustion and enhanced production, and multipoint low cost sensors are also of interest to reduce capital and operating costs and improve alunlinum product quality and amounts while meeting current and impending regulations. Aluminum product quality is a growing concern for secondary production, with increasing amounts of recycled aluminum. The recycled aluminum contains variable, unassayed alloys, and it also contains paint, solvents, and other contaminants that increase pollutant off-gas emissions and can aITect the quality of the alwllinunl product. New and emerging hazardous waste control technologies are important, owing to the need to meet hazardous waste treatment and disposal requirements at the lowest cost. The most promising technologies are thermal treatment of the dross to recover and recycle the alunlinwll, and vitrification of (a) the secondary wastes from the dross treatment and (b) spent pot liner. 4.7 Forest Products Industry The forest wood and paper industry is a major contributor to the nation's economy. It employs 1.4 million people directly and' produ~es products valued at more than $200 billi~n. ~er year. It ra~s among the nati.on's top 10 manufacturing industries. Mills range from large, statc-of-the-art faCIlItIes to small family-owned sawIlulls. 20 |