Influence of transient entropy generation distribution and combustion on the parameters of a regenerative reheating furnace
In this work,the entropy generation analysis method was applied to calculate the entropy generation rate distribution and exergy equilibrium of a typical heating unit in a regenerative reheating furnace.The entropy generation rate of the heat conduction,convective heat transfer,viscous dissipation,mass diffusion,combustion,and radiative processes were numerically determined.The transient change of exergy efficiency of the heating unit and the distribution of exergy loss and exergy efficiency along the furnace length during the billet heating process were investigated.The effects of gas jet velocity,oxygen-enriched concentration,and blast furnace gas combustion with natural gas on the furnace efficiency were investigated.The results showed that when the billet temperature is low,the radiative entropy generation accounts for 70.5％of the total entropy generation of the heating unit.Improving the billet loading temperature can effectively reduce the entropy generation rate.When the billet temperature is high,entropy generation during combustion is dominant,accounting for 59.9％,and the optimization of combustion is the main measure to reduce entropy generation.The new flame takes approximately 15-20 s to stabilize after burner reversal.Because the original flame is pushed out,the furnace temperature fluctuates,and mass diffusion intensifies,which have serious impacts on the stabilization of the furnace exergy efficiency;the regenerators need to have a sufficiently high recovery efficiency to ensure that the overall exergy efficiency of the heating unit can be improved.For the entire furnace,the first type of furnace exergy loss is approximately 33.4％,mainly in the soaking zone and caused by the high-temperature flue gas leaving the furnace,accounting for 45.6％of the input exergy in the soaking zone.The second type of exergy loss is approximately 24.4％,mainly in the preheating section and caused by radiative entropy generation,accounting for 17.4％of the input energy in the preheating zone.Keeping other parameters unchanged,when the jet velocity is 19 m/s,the heating unit has the highest exergy and thermal efficiency.When the oxygen concentration is increased to 35％,the exergy and thermal efficiency of the heating unit are increased by 4.1％and 5.5％,respectively.Blast furnace gas combustion with 10％natural gas can increase the furnace temperature and surface heat flux of billet,but the exergy and thermal efficiency are reduced by 1.2％and 4.4％,respectively,which is unfavorable to the energy saving of the reheating furnace.This study provides some new insights into the energy saving of reheating furnaces.
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