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Show do indicate, however, that the furnace is sensitive to the methods of oxygen introduction and to the level of oxygen involvement. 14v)S 130" 1201'S 110'1 100" w 90:15 ...J ~ eo" <i: > '0" 0( 3 60% J: 50" 4CJ" 30" 2v~ 10)! 0'1 20.9:¥ 23" 26" 40% 100" OXYO£N l£V£l IN 'OXIDIZ£R Fig. 1 - Relative Heat Availability with Preheated Combustion Air at Different Levels of Oxygen Enrichment (with 2200 F flue gases & 800 F preheated air) Oxy-fuel burners were developed fur applications where existing hardware could not be utilized. These burner~ are usually watercooled post-combustion devices although a few have a small chamber where fuel and oxygen are partially premixed and ignited. Oxy-fuel burners have demonstrated modest productivity improvements but low reliability in applications such as the electric arc furnace which is the only significant application for the burner. A final application for oxygen is a technique which injects oxygen through watercooled lances into large lazy flame patterns. This technique was developed primarily for regenerative glass tanks to increase productivity. The above methods of oxygen Introduction were developed as inexpensive marketing tools for rapid introduction of oxygen in industrial applications, benefiting the customer through productivity increases with limited capital expense. To date, industry's view of the role of oxygen has been limited to the experience provided by the methods. This paper will review oxygen's importance for high temperature industrial heating processes based on an evaluation of its function in combustion and heat transfer, and describe a new method of combustion with oxygen. 172 OXYGEN UTILIZATION AND HEAT TRANSFER In high temperature heating applications heat transfer from the flame to the load consists of two phases: -Heat transfer by radiation from transient components of the combustion products such as microparticles of carbon, hydrocarbon radicals and H20 and CO2 which ex-ist for very short periods of time during intermediate stages of combustion in the flame envelope. -Heat transfer by convection from the final combustion products leaving the flame envelope. Combustion of natural gas in traditional air/fuel burners produces a nonluminous flame. The main reason for their poor performance in radiant applications is that these burners produce combustion products in which the majority of the energy is stored in twoatom molecules of N2 and excess 02 which are unable to transfer their heat by radiation. Intensive mass transfer of gaseous molecules inside of the natural gas flame envelope practically eliminates conditions for pyrolysis of molecules of CH 4 and the creation of carbon microparticles. This tends to diminish flame luminosity and radiative hea t flux from the f 1 am epa t t ern . The recognition that radiation is poor has created the current tendency to use high velocity natural gas-air flames to boost convective heat exchange between combustion products and the load in order to overcome the deficiency of radiation by the flame even in high temperature applications where radiative heat exchange should be dominant. Utilization of oxygen in combustion results in an increase in the H20 ana CO~ content in comb~stion products (F~gure 2) and in reducing the volume of the flame pattern. With reduced volume the amount of energy stored in each standard cubic foot of combustion product is increased accordingly. Such phenomena result in hIgher radiative heat flux from CO 2 and H20 molecules at specific wavelengths (2.8 and 4.4 microns). Unfortunately, such wavelengths are preferably absorbed by other colder H20 and CO 2 molecules located between the flame and the product being heated. In the case of a reheat furnace for example, this raises the tempel'ature of the furnace atmosphere contacting the surface of the product. Higher concentrations of H20 and CO together with higher temperatures in~ |
