Cooperative Optimization Of Flue Gas Recycling In Iron Ore Sintering For Energy Saving And Emissions Reduction With Process Control Of SO₂ And NO_x

The sintering process is the main process of air pollution at integrated iron and steel works.The emission reduction of SO2 and NOx is the most important issue at present.Sintering flue gas recycling is an effective way of "process control" for pollutants,because the emission volume is reduced and heat is recovered.However,the temperature and composition of flue gas from different windboxes along the sinter strand are different.The efficiencies of energy saving and emission reduction are greatly different with different recycling modes.In addition,the influences of SO2 and NOx in recycling gas on sintering process are not clear,and further study is needed to do.Therefore,it is necessary to optimize the flue gas recycling process based on all the considerations of emission reduction,process control of SO2 and NOx,influences on sinter output and quality and heat recovery.The rules of SO2 generation and interaction with sinter bed are studied with sinter pot test.The results show that SO2 absorption,release and diffusion occur in sequence during the sintering process.The SO2 absorption takes place mainly at the preheating layer and drying layer,where the reactions with fluxes generate CaSO4,CaSO3 and CaS respectively under oxidizing,neutral and reducing atmosphere.With the rising of temperature,the product changes as CaSO3+CaS+CaSO4-+CaS+CaSO4?CaSO4.When the temperature is higher than the decomposition temperature of CaSO4(about 1173?),SO2 is released,which is mainly at the combustion layer.With the combustion layer goes down,SO2 is absorbed and released repeatedly.When the combustion layer reaches the bottom,SO2 reaches its peak concentration in the flue gas.SO2 diffusion mainly at sinter ore layer.The influence factors on and generation mechanism of NOx from sinter materials are analysed through NOx generation experiments.The results show that the temperature of NOx generation from iron ore fines is low,about 300?400?.The NOx generation is in proportion to N content,temperature and O2 concentration,and in inverse proportion to particle size.The NOx generated by iron ore fines mainly is NO,and the factor of interface reaction apparent rate of NO generation is about 0.210?0.346 m/s,the activation energy is about 26322?32741 J/mol;the factor of product layer effective diffusion is about(7.05?9.52)×10-9 m2/s,the activation energy of atomic movement is about 30629?44746 J/mol.The temperature of NOx generation from coke breeze is higher,about 800?900?.The NOx generated by coke breeze mainly is NO,and the NOx concentration increases first and then decreases with the increasing of temperature,reaction time and particle rise,because NO generation is influenced by the oxidation of N,reduction by C and O2 consumption.The results of sinter pot test show that NOx generation takes place mainly at the front and middle part of the sintering duration and there is no peaks.The fluctuation of NOx concentration in flue gas is influenced by negative pressure obviously.The main sources of NOx in flue gas are iron ore fines and coke breeze,the accounts are influenced by the N content.Through the simulation test of flue gas recycling,the influences of S02 and NOx concentration in recycling gas are studied for the sintering process indicators and flue gas emission.The results show that the sinter yield and particle size are decreased with the increasing of SO2 when the recycling gas is at room temperature.Above the SO2 volume concentration of 200 × 10-6 in recycling gas,the sulphur content in sinter ore is increased obviously with increasing SO2-This is mainly because the SO2 absorption by fluxes affects the solid phase reaction of fluxes and iron ore fines,which reduces the production of liquid phase and makes the binding force of sinter ore weaken.Meanwhile,CaSO4 is generated,which could not be decomposed completely at high temperature and S content in sinter ore is increased.In order to minimize the impacts on the S content of sinter ore,the SO2 volume concentration in recirculated flue gas should be no more than 200× 10-6.The NOx in mixing gas have almost no influence on the sintering performance and metallurgical properties of sinter ore.According to above results,the design principles of optimum flue gas recycling are proposed.SO2 and NOx in final flue gas should be enriched through gas recycling.Recycling plans are studied and made for the 210m2 sintering machine in Tangshan Steel through building the model of multi-objective cooperative optimization.The calculated results of the model show that,the multi-objective cooperative effect of three sections recycling is better,as well as the distribution of SO2,NOx enrichment of pareto solution set.The effect of cooperative optimization is realized in following aspects:waste heat utilization with pollution control,energy saving and emission reduction with sinter quality,sintering flue gas with end-of-pipe treatments.Compared with non-recycling sintering process,the flue gas emission of the optimum design is decreased by 24.5%,de-SO2 flue gas is reduced by 83.1%-86.6%and SO2 concentration is 2.91?3.09 times as high as that of non-recycling sintering process,NOx concentration of de-NO,flue gas is 1.51?1.53 higher,heat recovery is about 7.3 kgce per ton sinter,resulting in the advantages of high O2 concentration in the recycling gas and more enrichment of SO2 and NOx in the final flue gas.