Combustion With Oxygen Enriched Air

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Table 1 LIST OF IHDUSTRIAL P'UltMACBS AND IULHS FOIl APPLICATIONS OF OXYGRN BNRIaDmNT Priaary Industry Furnace/Kiln Benefits Iron and Steel Soaking pits 2, 1 Reheat 2, 1 Ladles 1 Forging 1, 2 Electric Arc 1, 2 Glass Regenerative .-e1ters 1, 2 unit .-elters 1, 2 Day tanks 2, 1 Copper Sitelting 1, 2, 3 &node 2 R_lting 1, 2 Coke calcining 1 Pulp &. Paper Liite 1, 2, 3 Black liquor 1, 2 C_nt 1, Petrolew. F1d. Cat. Crack Claus sulfur Clay Brick 1, 2, 3 Incinerator 1, 2, 3, 4 Benefits of olrJgen 1. Productivity :il!prov_nt 2. Energy savings 3. Quality :il!prov_nt 4. Reduction of e.ission Table 2 EFFBCTIV1!JII!SS OF OXYGKN KNRIaDmNT FOR PRODUCTIVITY IlU'ROVIDUDIT Liaitationa CAPACITY Air Blower Flue Syst_ Furnace Pressure Fuel supply Burner Air Pollution Control Syst_ DUST CARRYOVER Flue Gas Furnace Refractory Fl_ Gea.etry (T~erature unifomty) PRODUCT QUALITY • Depends on Applications Effectiveness of 02/02 Inrict.ent very Good Very Good Good • Good • • • than that from a 6 inch diameter "flame" at 34000 F for natural gas and air combustion. Thus the preferred condition to increase the heat flux uniformly within a furnace for higher productivity is to increase the bulk gas temper­ature of the furnace, not the localized flame temperature, by increasing the available heat input to the furnace either through higher fuel input or through oxygen enriched combustion. A new oxygen enriched burner concept has been developed where a high momentum and low temper­ature flame is created using high velocity oxygen jets (Ref.1,2). The burner has demon­strated high heat transfer efficiency and 156 uniform heating without the normal high tempera­ture intense flame associated with oxygen (Ref. 3-5). Although the economics of oxygen enrichment for productivity improvement have primarily been justified by the value of the additional prod­ucts rather than actual reduction in the unit utility cost of heating or melting, a signifi­cant reduction in specific energy consumption is often realized through productivity increases. This is because the furnace wall heat losses remain constant for a given furnace operating temperature. The energy requirement per unit amount of product, 1. e., specific energy con­sumpt ion, is therefore expected to be reduced with a productivity increase. On the other hand, the sensible heat loss tends to increase with productivity improvement due to the higher furnace gas temperature required to transfer greater heat flux to the furnace heat load. As discussed further on, the relative importance of these two factors determine the magnitude of the reduction in specific energy consumption. The extent of possible productivity improve­ment is highly dependent on the specific process constraints and each furnace must be individu­ally analyzed. However, the differences in energy requirement between air and oxygen enriched combustion resulting from productivity improvements can be analyzed by using a generic furnace model. Figures Sa, sb, and Sc show the results of calculations for a typical forging furnace (9'x ·9'x 9'). The furnace model is based on a well-stirred, two-sink, gray gas furnace with heat transfer effects both by radiation and by convection. Figure Sa shows that the fuel input required at different heat transfer rates for four different levels of oxygen concentration in oxidant (i.e. air, 35% 02' 50% ° and 100% 02)' Productivity of the furnace is dlrectlY propor­tional to the net heat flux to furnace heat load in this example. Fuel input increases sharply with the net heat flux due to the greater heat requirement to the furnace heat load and higher flue gas temperature and resulting greater sensible heat loss to the flue. As shown in Figure Sa this effect is particularly prominent with air for two reasons: (1) the gas emis­sivity of combustion products with air as the oxidant is lower than that with oxygen enriched air increasing the required gas temperature for a given heat flux as shown in Figure sb (1. e. , radiation heat transfer is improved with oxygen enrichment due to higher emmissivity, not due to higher flame temperature in this case). (2) The sensible heat loss of combustion products with air is much higher due to higher nitrogen content. Thus, oxygen enriched combustion becomes more advantageous when a high produc­tivity is required. Figure sc shows the fuel efficiency in percent net heat flux to furnace heat load relative to the heat input based on the higher heating value of fuel. At low net heat transfer rates, fuel efficiency increases with net heat flux (i.e., with productivity) due