Numerical Simulation On Energy-saving And Oxidation-loss Of Billet In Heating Process

The change of both the thermal efficiency of reheating furnace and the formationof scale during the heating process with time under different combination of airpreheat temperature, oxygen concentration and excess air coefficient have beenstudied in this paper. The objective is to obtain theoretical basis for improving theoptimization level of design, operation and control to the heating process of billet anddecreasing the billet production cost.Firstly taking the walking beam furnace as an example, using STEEL-TEMPsoftware and coupling with billet heat-flow-density model, thermal-balance modeland linear-parabola increase model of oxidation scale, simulation calculation programhas been constructed. The temperature curve of billet, the flue gas temperature in eachzone and the temperature curve of inner surface of furnace wall, scale formation canbe obtained. The thermal efficiency and the rate of scale formation can be calculatedfurther.Secondly the change of thermal efficiency and scale formation with the changeof air temperature, oxygen concentration and air excess coefficiency has beenresearched under the different combination of air preheat temperature, the oxygenconcentration and the air excess coefficiency. The temperature of preheated air rangesfrom 200℃to 900℃, the air excess coefficient from 1.0 to 1.3 and oxygenconcentration from 21% to 54%.To compare the effects of the significant factors above on heating efficiency andscale formation, the fmal temperature of billet is set at 1235℃+2℃. The followingresults have been obtained:(1) The higher the air temperature is, the shorter the heating time is, the larger theheating velocity is, the higher the thermal efficiency is, the higher the productivity isand the less the scale formation is.(2) The oxygen concentration of flue gas has an important effect on the scaleformation. The low oxygen concentration of flue gas can restrain the ferrous oxidationreaction, increase the flue gas emissivity and shorten the heating time. Further more,the low oxygen concentration of flue gas lead to the low scale formation and the lowenergy consumption per ton steel. (3) The lower the air excess coefficient is, the larger the heating rate is, thehigher the thermal efficiency is, the higher the productivity is, the shorter the heatingtime is and the less scale formation is. The combustion conditions for the minimumrate of scale formation should be the air excess coefficient 1.0-1.1 and the airtemperature 400-500℃. The lower air excess coefficient is, the wider range of airtemperature corresponding to the minimum rate of scale formation is.(4) The higher the air oxygen concentration is, the larger the heating rate is, thehigher the thermal efficiency is, the higher the productivity is, the shorter the heatingtime is and the less the scale formation is. The combustion conditions for theminimum rate of scale formation should be the air temperature 400-600℃and the airoxygen concentration below 45%.Finally, based on the above studies, a thermo-technical operation system called"stepwise distribution" has been brought forward. According to this system, the airexcess coefficient decreases from 5% to 10% in turn Zone One, Zone Two and thesoaking zone while the total air excess coefficient remains at the value of 1.0. Thesemeasures make the soaking zone be at the weak oxidative atmosphere. The newoperation system can lead to high thermal efficiency and low scale formation.