| OCR Text |
Show to a reduction in wall heat loss per unit amount of net heat transferred. As the net heat flux to the furnace heat load increases, the flue gas temperature increases resulting in an increase in the sensible heat loss to flue. Thus, the optimum point is determined by the relative magnitude of furnace wall heat loss and sensible heat loss. For air the peak fuel efficiency o~ about 34% is obtained at about 11,000 Btu/hr/ft in this example. For oxygen fuel efficiency is S' I FIRING RATES VS. NET HEAT FLUX FUEL: NATURAL CAS I 10 OXIDANT: 100"; 02 10 20 30 40 50 NET HEAT FLUX TO FURNACE HEAT LOAD, M8TU/HR/FT2 Figure 5a TEMPERATURES VS. NET HEAT FLUX FUEL: NATURAL CAS 2500~1F=======,~--~r---~--'-__ -r __ -. __ -. __ ~ 2400 - TEMP: 10 20 30 40 50 NET HEAT FLUX TO FURNACE HEAT LOAD, M8TU/HR/FT2 Figure 5b FURNACE EFFICIENCY FUEL- NATURAL CAS 70 r--- - . - --T=~===-~--.:::=t=::::'~;';;::-;-';;::Fr·:;:::::'fr:-:::':-~- ~~I""" I :: +---j-+~--'/"'-!-!:-=- - - - -= -~ ~:...--='=-=1'--=---+--~=-=--=-:=1--,--------.1- / "v-= - .r, 40 " / -- f- -- --4----t---+----+----j ~ 30 t7 "-'" - ~= ="~ -~-7_ ~ "=-:- .:::.::-~ lL. o Q 20 OXIDANT: _ . __ . --+-.---+----+-----j.----I-----i G: ~ lL. W ~5 ~ 02 21% 02 ~ -- o 10 20 30 40 50 NET HEAT FLUX TO FURNACE HEAT LOAD, M8TU/HR/FT2 Figure 5 c 157 much greater and remains at almost a constant level for a broad range of heat flux. I t is important to note that the optimum point shifts toward higher net heat flux (i.e. higher productivity) with oxygen enrichment. OTHER PROCESS CONSIDERATIONS In many direct fired industrial furnaces, increases in partial pressures of water vapor and carbon dioxide in the furnace atmosphere with oxygen enrichment will influence the product quality. For steel heating applications the effects of oxygen enrichment on scaling and decarburization have to be considered. Our experiences to date have shown no negative effects of oxygen enrichment and in some cases improvements were observed in surface quality due to the formation of less tenacious scales (Ref. 3-5). For glass melting, the refining zone chemistry is generally sensitive to the atmosphere. Commercial experiences with 100% oxygen burners, however, did not create any observable differences in quality (Ref. 9). For aluminum remelting the cost of dross loss often exceeds the cost of fuel for melting. Therefore, any impact of oxygen enriched combustion on dross loss is very important on the economics of melting. Corrosive gases and particulates generated in industrial process furnaces are often technical and economic barriers in using recuperators for air preheating. Severe problems have been reported on the recuperator used in aluminum remelt furnaces with chlorine fluxing or in certain glass melters. Oxygen enrichment offers unique advantage in energy conservation for these applications. Requirements for flue gas cleaning is another area to be evaluated. The reduction in the volume of combustion products with oxygen enrichment would reduce the size and operating costs of the flue gas cleaning system. These process related factors must be evaluated carefully for each application in order to properly assess the overall benefits of oxygen enrichment. OXYGEN ENRICHMENT AND HEAT RECOVERY The potential fuel savings by the combined oxygen enrichment and heat recovery have been calculated and shown in Figure 6 for natural gas at 24000 F flue gas temperature and 2% excess oxygen in the flue gas. At high levels of oxygen enrichment the flue gas volume becomes so small that preheating of oxygen enriched air contributes only a few percentage points in the overall fuel savings. At low level enrichment, on the other hand, additional savings with preheating are very significant. For example, 10000 F preheat of 35% oxygen enriched air increases fuel savings from 41% to 52%, which is comparable to the fuel savings obtainable at 100% oxygen of 57% . Since the size of the recuperator required to preheat enriched air is much smaller than that for air and the amount of equivalent pure oxygen required i s less than |
