Theoretical And Process Studies Of The Abatement Of Flue Gas Emissions During Iron Ore Sintering

The iron and steel industry consumes more energy and brings about heavier pollutions than other ones,especially sinter mills are one of the main emissions of air pollutants in integrated steelworks, with SO₂ emissions accounting for 40%-60% of the total.At the same time the unit energy consumption amounts to about 10%-15% of total energy consumption of steelworks. At present, the techniques reducing the emissions of SO₂ and NO_x from iron and steel industry has been put into operation commercially in some developed country, even the efforts to minimize the emissions of CO₂ and polychlorinated dibenzo-p-dioxins and -furans (PCDD/F) have also been made. However, the control techniques of air pollutants such as SO₂ and NO_x are still on the prophase of industrialization in iron and steel industry of China because of the deficiency of techniques and economy.For sinter plants,the desulphuriza-tion and denitrification of flue gas has not been applied except for few small sinter strands, the minimisation of CO₂ emissions were even not brought into schedule.Theoretical and process studies of the abatement of CO₂,NO_x and SO₂ during iron ore sintering was conducted,which was supported by National Natural Science Foundation of China(NSFC50274072). Based on the viewpoint of clean production, the formation mechanism, the emissions and the factors affecting the formation and emissions of SO₂, NO_x, CO₂ during iron ore sintering were studied systematically via different measures for the first time,such as TGA, infrared spectral analysis,XRD analysis, SEM analysis, gas chromatography and online analysis.The formation mechanism of SO₂ during iron ore sintering was fully studied, the results show that sulphur oxides (mainly SO₂) in the flue gas originate from the pyrogenation of sulphur compounds in the iron ore and coke breeze at high temperatures.The pyrogenation of sulphur and the rate of desulfurization were mainly affected by these factors, such as temperature, time, oxygen concentration and coke particle size. With an increase in desulfurization,the remnants sulfur contents of sinter are further reduced improving the quality of pig iron. However, the emissions and concentration of SO₂ in the flue gas increase significantly. And the desulfurization augments rapidly along with the temperature, calefaction time,oxygen concentration and a decrease in the coke particle size. However,too higher temperature would deteriorate the desulfurization and lead to a lower emission of SO₂. In this paper, it is put forward means that the emission of SO₂ possesses the characteristics of self-sustaining which means the emission of SO₂ rises to a peak value rapidly just before the sintering end point and declines dramatically no matter how parameters of sinter technology and the properties of raw material change. It is also revealed that the emission of SO₂ is controlled by the transfering pattern of moisture in sintering bed,and also controlled by the mechanism of pyrogenation formation-adsorption-desorption of SO₂. The adsorption mechanism and kinetics of sinter bed adsorbing SO₂ were studied by adopting dynamic methods.It is shown that the relationship betweenadsorptive rate 1/C_o dA/dt and adsorption capacity A agrees linely with the first order reaction kinetics equation at low adsorption capacity. Furthermore, at the initial stages of adsorbing, the multiphase reaction of blends absorbing SO₂ is enhanced by an increase in the basicity of blends, with CaO originating from quicklime better than limestone, the moisture of blends and the concentration of SO₂ in the flue gas, and decrease in the average particle size of blends, raising. With the adsorption capacity increasing sequentially, the relationship between adsorptive rate and adsorption capacity would obey zero order reaction kinetics equation, with the adsorptive rate at steady state.During the transfering of SO₂ in sinter bed, there occur important reactions,such as the oxidization of CaSO₃ and thermal decomposition of CaSO₄ at high temperatures, which dominates the self-sustaining characteristics of the emission of SO₂. Based on the theoretical research,an emission model describing the pyrogenation formation-adsorption-desorption reactions of SO₂ in sintering bed was founded innovatively, and a new technology of sectional desulphurization of flue gas was developed triumphantly. When the limestone-gypsum wet flue gas desulfurization technology used in a sinter plant ,compared with conventional desulphurization tecnology, the new technology of sectional desulphurization can reduce the volume of flue gas by about 40% to be treated, resulting in the lower consumption of absorbing solution, an increase in desulfurization by about 2.4%, Moreover the investment of waste gas treatment facilities and operational costs can be significantly decreased.The results of online measurement of sinter flue gas showed that NO_x emitting from sintering flue gas originates primarily from the combustion of gas fuel during ignition and solid fuel in sintering bed. Fuel NO_x is the main resource while prompt-NO_x and thermal-NO_x are inferior. Furthermore, NO dominates and NO₂ only amounts to several ppm among NO_x. Compared with the emission of SO₂ during sintering, the emission of NO_x is quite different. From the finish of ignition to the end of sintering, the concentration of NO_x in the flue gas always maintaines at a high level.The amount of NO_x generated was affected by some factors, such as the fuel-N content, the presence form of nitrogen, the particle size of fuel, the oxygen content in the air,the material composition of sinter blends.It is shown that an increaseing in basicity of sinter and sinter beds height are conducive to reducing the emission of NO_x in flue gas.It is inferred that calcium ferrite(CF) forming during sintering catalyzes the reduction of NO_x.Based on the characteristics of iron ore sintering,and using the theories of crystal structure, activation energy, the thermo-dynamic and kinetics of catalytic reduction, the mechanism of calcium ferrite catalyzing NO_x reduction was studied. It is demonstrated that calcium ferrite catalyzes the reduction of NO by CO, and the catalytic capacity of calcium ferrite depends on their molecular structure.The order of catalytic capacity from strong to weak is as following: CF> C2F>2CF. Compared with the reduction of NO by CO with absence of CF, the activation energy is declined from 246.68 kJ /mol to 138.80kJ /mol when CF occurs during the reduction of NO by CO. The mechanism of calcium ferrite catalyzing NO_x reduction is proven for the first time. During the reaction course, calcium ferrite is reduced by CO and oxidized by NO. The mechanism of calcium ferrite catalysing NO_x reduction also obey the theoretics of adsorption-activation where NO molecule adsorbs, decomposes,surfactant species restructures and product desorbs at activity site of calcium ferrite catalyst.The function of calcium ferrite catalysing NO_x reduction is defined as self-catalysis in sintering system,where CF reduces the activation energy and accelerate the reaction rate. It is the self-catalysis of calcium ferrite help to abate NO_x emission by improving the calcium ferrite distribution on the surface of sinter bed and increasing the calcium ferrite proportion in sinter.Therefore, a new technology of separate granulation and heterogeneous sintering (SGHS)was put forward, which can develop calcium ferrite firstly and modify the calcium ferrite distribution on the surface of sinter. Compared with the traditional sintering process, the emission of NO is nearly reduced by 44% ,and sinter quality is also improved by the new SGHS process.On the basis of study of the combustion of solid fuel in sintering process and using the theories of catalytic combustion and combustion chemistry, an energy-saving additive with multiple functions, such as improving combustion and reducing fusion temperature, was developed successfiilly,which improves the quality and productivity of sinter, decreases solid fuel consumption, and lower the emissions of CO, CO₂ and NO_x significantly. Due to an enhancement in the oxidizing atmosphere of sinter beds, the desulfurization of sinter blends is strengthened, with a higher peak value of SO₂ concentration occurring in the flue gas. However, the characteristics of self-sustaining is not affected. It is shown that action mechanism of the additive catalysing coke combustion is by activating the crystal structure of fixed carbon and forming reaction centers, the activation energy is decreased and the oxygen transfer is promoted in the combustion process.When coke is soaked by the solution of sintering energy-saving additive,the gasification activation energy of coke was decreased from 25.8kJ/mol to 18.9kJ/mol, combustion efficiency was increased from 80.2% to 89.8%,and combustion rate was increased from 2.25%/min to 3.15% /min. The additive also contains flux which help to induce liquid formation, such as calcium ferrite which has low melting point.Therefore, the formation rate of binding liquid was speeded at the lower sintering temperatures, and the volume of bond phase was also increased. Consequently, the yield and strength of sinter was enhanced largely. The industrial tests further demonstrated that the quality and productivity of sinter was improved dramatically after using sinter energy-saving additive.The unit productivity of sinter was increased by 15.72%, the tumble index by 1.99%, the solid fuel consumption decreased by 4.19 kg/t_sinter, the emissions of CO₂,CO,NO decreased by 10.53%, 35.29% and 25%, respectively .However, the average and peak concentration of SO₂ in the flue gas increased by 16.74% and 17.70%, respectively.The abatement of SO₂ emission is only achieved by desulphurizing the flue gas of sintering.Through the study of emissions of SO₂, NO_x, CO_x during iron ore sintering process, an integrated scheme of abatement emission of sintering flue gas was designed,which consists of the new technology of sectional desulphurization of the flue gas , sinter energy-saving additive and heat recovery from sintering and sinter cooling. Compared with the traditional FGD process, it is expected that the volume of flue gas can be reduced by 40%,and the investment of FGD equipment and operation costs can also be cut down by 40%. Recycling of part of the waste gas from the sinter strand can significantly save solid fuel consumption and bring direct economic profits of RMB9.36 million for 450m² sinter strand.