| Under the background of global warming and environmental deterioration, reducing energy consumption and pollutant emissions becomes a key issue in iron&steel industry. Sintering is the first procedure of iron&steel production. However, the energy consumption of sintering process ranks second, and the flue gas it emits contains diverse pollutants, so it is the large energy consumption and the main source of air pollution in iron&steel industry. Therefore, replacing fossil fuels with renewable and clean energy of biomass fuels, which is low in N and S content, and CO2participates in carbon cycle of atmosphere, can decrease the generations of COx, SOx, NOx et al. from the source, which is significant to the sustainable development of iron&steel industry and can realize low carbon economy.For such three biomass fuels as woodchar, strawchar and nutchar, their physico-chemical properties, microstructure characteristics and thermochemical behaviors have been studied systematically to illuminate the basic characteristics of biomass fuels in the dissertation. And then the fundamental theoretics of biomass affecting the flame front, the burning degree of fuels and the thermal state of sintering layer have been deeply discussed to reveal the mechanism of biomass fuels affecting sintering. Based on the research stated above, critical techniques to strengthen sintering with biomass fuels and new process of sintering with biomass energy based on flue gas recirculation have been exploited, so as to provide a theoretical basis and technical guidance for applying biomass in sintering successfully.(1) Basic characteristics of biomass fuelsBiomass fuel is low in ash content while high in volatile constituent, porosity and specific surface area, and these properties results to its good combustion and gasification, which perform as lower reaction temperatures, faster reaction speed, and lower activation energy than coke breeze does. The prior order of combustion and gasification is strawchar>woodcharl>nutchar. (2) Laws and mechanisms of using biomass as fuel affecting sinteringThe characteristics of biomass sintering indicate that, with the proportion of biomass replacing coke breeze increasing, vertical sintering speed is accelerated, while yield, tumble index and productivity are decreased. The effects of biomass types on sintering show that the order of affecting degree is strawchar>woodchar>nutchar. The appropriate proportions of three biomass fuels are20%,40%and40%respectively. Biomass replacing coke breeze realizes the pollutant emission reduction of sinter. When the replacement ratios of three biomass fuels are20%,40%and40%respectively, the emission of COx can be decreased by7.19%,18.65%and22.31%, SOx by31.79%,38.15%and42.77%, NOx by18.31%,26.76%and30.99%respectively.Mechanisms of using biomass as fuel affecting sintering were illuminated. The flame front speed is accelerated due to the excellent combustion property of biomass, which will ruin the consistency of heat front speed and flame front speed. Meanwhile, the degree of fuels combustion decreases because of the good reactivity of biomass, which will decrease the thermal utilization efficiency of fuel. In addition, the maximum temperature of sintering layer and high temperature duration time decrease as biomass replacing coke breeze. When the proportion of woodchar replacing coke breeze increases from0%to40%,100%respectively, the flame front speed rises from34.11mm/min to41.67mm/min,46.90mm/min but the heat front speed maintaining at35.71mm/min, and the combustion ratios increases from12.17%to13.08%,14.85%, the maximum temperature of sintering layer decreases from1305℃to1255℃,1178℃, and high temperature duration time decreases from2.67min to1.83min, Omin.(3) Key technologies of strengthening biomass sinteringTechnologies of strengthening biomass sintering with reinforcing the preparation of biomass fuels, modifying the properties of biomass, pregranulation technology of fuels and optimization technology of ore blending have been studied. The use of two carbonization process to strengthen the production of woodchar and nutchar, and the use of molding process combining two carbonization process to strengthen preparation of strawchar can decrease the porosity and specific surface area of biomass, and the use of liquid boric acid and silica sol, and solid powder borax and SiO2to passivate biomass can decrease the reactive suface area or take the action of physical obstruction, which result in lowering the combustion and reactivity of biomass. Taking the pregranulating technology, biomass distributes inside the balls, which can improve secondary combustion and result in increasing the combustion degree. On the base of optimization of ore blending, regulating the chemical compositions of melting zone of sinter, such as the molar ratio of CaO/Fe2O3, the contents of SiO2, Al2O3and MgO, can improve the liquid phase formation and needle-columnar SFCA generation to suit for the characteristics of sintering layer which is low in temperature and short in high temperature duration time. All the above technologies can make strawchar, woodchar and nutchar replace20%,40%and40%coke breeze respectively in the premise of not affecting the indexes of sintering yield and quality.(4)New process of sintering with biomass energy based on flue gas recirculationAccording to the complementary properties of the effect of biomass sintering and flue gas circulation sintering on combustion and heat transfer, the new process of combinating both technologies has been proposed. The results of the effects of circulating gas composition and temperature on biomass sintering show that, the re-consistent of flame front speed and heat front speed can be promoted on the conditions of15%O2content, CO26%, H2O(g) less than8%and150～250℃of circulating gas. And the maximum temperature and high temperature duration time can be improved by CO combustion and the physical heat transfer to the bed. On the appropriate cycling conditions, ie the circulating ratio of non-selective-circulation is40%and the area coverage ratio is100%, and the circulating proportion of selective-circulation is40%and the area coverage ratio is44.5%, the sintering indexes which the proportion of biomass replacing coke breeze is40%are comparative to that of using coke breeze completely. In addition, the emission of COx reduces by30.04%,31.78%, SO, by41.04%,44.51%, and NO, by42.25%,45.07%, which indicates that the combination of biomass sintering and flue gas circulating sintering may play a cooperative effect of energy saving and pollutant reduction. |
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