OCR Text |
Show crease oxidation and decarburization. The additional oxidation of the load which results from oxygen enrichment is also the result of the inability of air / fuel burners to mix fuel and oxygen enriched air adequately so as to minimize the amount of excess oxidizer released in t o the furnace environment. 100 I I vi 80 ~ Vl ~ l!) 60 z 0 H ~ dry basis Vl 40 ~ co l: actual 0 u 20 o 10 21 30 50 70 90 100 OXYGEN CONTENT,' Fig. 2 - Chemical Content of Combustion Gases with Different Levels of Oxygen Enrichment (Calculated) Existing ~ir / fuel burners have reduced life when used with enrichment because they were not designed to withstand the temperatures and velocities which would result from the mixing and burning of oxygen enriched air within the burner tunnel. The typical burner designer's response to this problem is shortening of the burner tunnel to limit contact with the flame, thereby reducing the ability of the burner to shape the flame pattern properly. The net result is usually both a poor flame structure and excessive oxidation. Attempts to use 100% oxygen, i . e., oxy-fuel burners, for high temperature heating processes revealed the inability to provide adequate heat transfer from the reduced vulume, non- luminous flame which is inherent to the conventional use of oxygen. This def iciency of heat transfer capability, in addition to the smaller size flame, is the result of the disassociation of molecules of H20 and CO 2 as illustrated in Figure 3. This disassociation diminishes the radiative heat flux and makes the oxy-fuel flame transparent for heat waves. The oxyfuel flame is therefore only capable of transferring its heat by contacting the load directly (as in a cutting torch) or by discharging its heat by mixing with the furnace atmosphere. But increasing the temperature of the furnace atmosphere will cause furnace refractories to overheat, will increase the temperature of oxidizing gases which contact t he load and will increase the temperature of the flue gases leaving the furnace . 173 100 H 0 .. 100 vi ~ g 80 CI ~ 80 r'\ \ \.('02 Po U 60 H ~ 0H 40 ex: >< Po 20 1\ ----- ./" c~ 1\ yl 0 ~~ Cf\ ~ 60 40 HXXl 2000 300J 4000 TEMPERATURE, ·C Fig. 3 - Thermal Disassociation of 'I1lree Atom ~blecules of Canbust ion Product This limitation of the oxy-fuel burner is one reason for such developments as the aspirative type oxy-fuel burner. This burner approach attempts to reduce the level of CO2 and H20 disassociation by aspirating the lower temperature furnace atmosphere into the oxy-fuel flame so as to chill the flame and cause recombination of the molecules so that they can be used for radiation. These burners also try to enhance convective heat exchange by increasing turbulence. Such an approach promises to improve the temperature distribution and reduce the problem of oxy-fuel operation in the furnace, but it also highly concentrates the H20 and CO 2 molecules and raises their temperature. This approach can t herefore be expected to result in intensive oxidation and/or decaburization in cases of metal heating and melting furnaces when these combustion products come in contact with the load. From a practical standpoint the question is: why use highly oxidizing combustion products to chill a flame created with purchased oxygen when free air dilution with non-oxidizing nitrogen could be used? In electric arc furnaces (EAF) the use of oxy-fuel burners as an energy source is limited by the rapid deterioration of the burner~s capability to transfer heat to t he load. As in any heat transfer by flame, surface area and temperature differential are major determinants of effectiveness. The oxyfuel burner can only transfer heat by direct contact of the load and the area of contact is small due to the small size of the flame. The load, steel s c rap in this case , cannot transfer so much heat away from the point of impingmen t and sot he f I am e q'u i c k I y mel t s a hole in the scrap, des t roying both surface area and temperat ure differential, thereby losing its ability to transfer |