Study On Flow And Heat Transfer Characteristics In The Sinter Vertical Cooling Process

In recent years,the pollutant emission and energy consumption of iron and steel enterprises have been reduced.However,the latest statistic indicates that the energy consumption and pollutant emission of the iron and steel industry still occupy a very high proportion in industry.The 13th five-year plan for the ecological environment protection puts forward higher requirements and standards for the energy saving and emission reduction of the iron and steel industry,especially in the sintering process.At present,most of the sintering waste heat recovered in China and even in the world comes from the high-temperature sinter,which is mainly realized by the annular cooling process.However,the horizontal structure of the annular cooler makes it difficult to overcome the disadvantages of the high air leakage rate,low waste heat recovery rate,low quality of the waste heat,and high emission of particulate pollutants.Therefore,a new vertical cooling process of the sinter is proposed based on the CDQ process.Its vertical structure avoids the above problems from the root.In recent years,many scholars have done a lot of research on the gas resistance and gas-solid heat transfer characteristics in the sinter vertical cooling process.However,most of them focused on the sinter with the mono-size or the uniform particle size distribution,which makes it difficult to apply the research results to the non-uniform sinter with the multi-size distribution in the actual productionTo further analyze characteristics of the flow and heat transfer in the sinter vertical cooling process of the non-uniform sinter with the multi-size,the differences of the gas resistance and gas-solid heat transfer characteristics between the multi-size and mono-size sinter are studied by experimental means based on the detailed characterization of the characteristics of sinter particles.In addition,according to the production parameters of a certain sintering machine of 360 m2 in China and the measured results of the multi-size sinter,the ratio of air to material of the operation parameter was numerically optimized based on the criterion of the income of the gas exergy.The work and innovative achievements of this paper mainly include the following aspects:(1)Based on national standards and common measurement methods,the geometric and thermophysical properties of the sinter are characterized in detail,especially for the multi-size sinter.Thus,the disadvantages of experiential and inaccurate values in the past are avoided.The results show that the bed voidage of the sinter increases with the increase of the particle size,and the increasing extent decreases gradually.However,the particle size distribution of the multi-size sinter is wider,which makes its voidage lower than that of the mono-size sinter under the same equivalent particle diameter(dp).With the increase of dp from 19.25 mm to 30.95 mm,the difference between them decreases.To meet the engineering practice,the temperature measurement range of thermophysical properties is broadened to the room temperature?800? by the high-temperature differential scanning calorimeter and the laser conductometer.The above characterization of particle characteristics provides accurate parameters for the follow-up research.(2)To reveal the difference of gas resistance characteristics between the mono-size and the multi-size sinter and obtain the resistance correlation suitable for the engineering,an experimental study on gas resistance characteristics of the non-uniform sinter with the multi-size distribution was carried out.Then the effect of the layered distribution pattern on the gas resistance characteristic is clarified to improve the feasibility of the application of the vertical cooling process.The results show that the stacking structure of the multi-size is more disordered than that of the mono-size,which leads to a significant increase in the gas flow disorder.This results in a higher gas resistance than that of the mono-size sinter under the same equivalent particle diameter.However,with dp increasing from 19.25 mm to 30.95 mm,the difference of the gas resistance between them decreases from 46.82%to 12.76%,which is attributed to the improvement of the non-uniformity degree of the structure with the multi-size.The non-uniformity degree varies with the particle size composition of different multi-size sinters,which makes the decreasing extent of the gas resistance first increase and then decrease with the increase of dp.When the mass ratio of each mono-size sinter is close to each other(dp=19.25 mm),the non-uniformity degree of the the structure increases abruptly,leading to the peak of the decreasing extent.The predicted deviations of the resistance correlations for the mono-size and regular particles are 28.12%?56.15%and more than 70%,respectively,indicating that the influence of the particle shape and particle size distribution on the gas resistance can not be ignored.Compared with the random distribution mode,most of the layered distribution modes can reduce the gas resistance.Among them,the layered mode with the particle size increasing from the bottom-up has the smallest resistance,followed by the mode with the particle size decreasing from the bottom-up.These two modes ensure the the possibility of the application of the layered distribution pattern in the vertical furnace.The increase of the layer number is beneficial to further reducing the gas resistance.The gas resistance of the multi-size sinter with dp of 11.45 mm can be reduced by 13.99%and 25.7%at most under the three-layer and five-layer mode,respectively.The increase of the equivalent particle diameter weakens progressively the influence of the layered distribution pattern on the reduction of the gas resistance.Under the five-layer layered mode,the gas resistance of the multi-size sinter with dp of 11.45 mm,15.45 mm and 19.45 mm can be reduced by 25.7%,22.9%and 18.37%at most,respectively.(3)To calrify the effect of the gas velocity and particle size on gas-solid heat transfer characteristics and reveal the difference of gas-solid heat transfer characteristics between the mono-size and multi-size,the hot state study was carried out for the vertical cooling process of the non-uniform multi-size sinter.The improved inverse problem method of the heat transfer is used to obtain the gas-solid heat-transfer coefficient and its correlation,which provides a premise for the accurate prediction of the temperature field in the eggineering.The results show that the increase of the gas velocity is conducive to improving the gas-solid convective heat transfer,but it weakens with the increase of the particle size.The uneven accumulation structure of the multi-size makes the effective area of the gas-solid contact much smaller than that of the mono-size,which becomes the limiting factor of the gas-solid heat transfer.This makes the the heat transfer efficiency lower than that of the mono-size sinter under the same equivaqlent particle diameter.However,when dp increases from 19.25 mm to 30.95 mm,the difference of the heat-transfer coefficient between them decreases from 28.99%to 14.25%,which is attributed to the improvement of the non-uniformity degree of the multi-size.The prediction deviation of the heat-transfer correlation with the mono-size applied to the multi-size is as high as 72.37%,indicating that the effect of the particle size distribution on the gas-solid heat transfer can not be ignored.(4)A numerical model for the sinter vertical cooling process was established based on the porous medium theory and the local non-thermal equilibrium.The particle characteristics,gas resistance characteristics and gas-solid heat transfer characteristics of the multi-size sinter obtained from the experiment is used for the numerical simulation.Also,the ratio of air to material of the operational parameter is optimized based on the evaluation index of the income of the gas exergy considering the combined effects of the temperature and resisitance,which provides a reliable reference for the engineering application.The results show that the income of the gas exergy first increases and then decreases with the increase of the ratio of air to material.Moreover,the optimal value of the ratio of air to material and the corresponding income of the gas exregy increase with the increase of the equivalent particle diameter.The optimal value of the ratio of air to material is 850.22?1209.93 m3·t-1 in the scope of this study.To sum up,the experimental and numerical study on the non-uniform multi-size sinter from the actual production in this paper is helpful to reveal the resistance and heat transfer characteristics in the sinter vertical cooling process,and lay a solid foundation for the practical application of the process.