Migration And Transformation Characteristics Of Iron During The Fusion Process Of Circulating Fluidized Bed Gasification Fine Ash

The gasified fine ash discharged from the tail of the circulating fluidized bed gasifier has the characteristics of high carbon content,low volatile content and poor reaction activity.Reusing the gasified fine ash by the melting process can reduce pollutant emissions and improve resource utilization.During the liquid slagging in the melting process,there are many factors for slag blocking in the equipment.One of the important reasons is the migration and enrichment of iron-bearing minerals.Therefore,It is especially important for understanding the fusibility characteristics to study the mechanism of migration and transformation of iron-bearing minerals.In this paper,the two representative gasified fine ash discharged from the industrial circulating fluidized bed gasifiers were used as the research objects,and the material analysis of gasified fine ash is carried out by laser particle size analyzer,XRF and high temperature rotary viscometer.Ashification and high temperature treatment of gasified fine ash carried by muffle furnace and horizontal tubular furnace,and the minerals transformation,microscopic observation and ash melting temperatures measurement carried by X-ray diffractometer(XRD),scanning electron microscope(SEM-EDS)and automatic ash fusibility tester.Numerous studies have shown that the migration and transformation of iron closely related to the valence state of iron,the conversion of calcium-containing minerals and the influence of sulfur-containing substances.Therefore,the experiment carried out a comprehensive analysis of the migration and transformation of iron from the above three points.In order to analyze the influence of the valence state of iron on the meltability of gasified fine ash,the results of minerals transformation of ferrous iron and ferric iron with the same mass fraction were compared.The results show that ferrous oxide and iron oxide will reduce the fusion temperatures of gasified fine ash,and the trend of temperature curve decreases first and then increases with the additon content.It can be seen that the particles adhering to the ash gradually reduced and then slightly increased,and the surface is rough.The difference between the two on the ash fusibility is mainly due to the difference in minerals production:the hedenbergite and fayalite in the Hechi gasification fine ash containing ferrous oxide,the hercynite and troilite in the Hechi gasification fine ash containing iron oxide besides the above.The main minerals in the Suqian gasification fine ash containing ferrous oxide is iron spinel.Iron spinel and fayalite are formed in the Suqian gasification fine ash containing ferric iron.It can be seen from the results that the valence state has a great influence on the melting characteristics of the gasified fine ash.Calcium-containing minerals are closely related to the migration and transformation of iron-bearing minerals.Taking the migration and transformation of ferrous oxide described above as the basis of analysis,the experiment first added 10%calcium oxide in order to produce a large amount of calcium-containing minerals in the ash,and at the same time added different mass fractions of ferrous oxide to obtain the migration and transformation of ferrous oxide in the presence of a large amount of calcium-containing minerals.Compared with the previous migration and conversion content of ferrous oxide,then,the effect of calcium-containing minerals on the migration and transformation of iron is obtained.The results show that the fusion temperatures of the two gasified fine ash are monotonously decreasing with the increase of the addition amount.The main minerals in the Hechi gasification fine ash are hercynite and monticellite,and the main minerals in the Suqian gasification fine ash are the hedenbergite,fayalite and hercynite.The difference in the type and content of minerals produced in ash is the main cause of the change in ash fusibility temperatures.From the results,the calcium-containing minerals have a great influence on the migration and transformation of iron.The addition of calcium oxide has caused a huge change in the iron-bearing minerals in the Hechi gasified fine ash.The minerals in the Hechi ash have changed from hedenbergite and fayalite to hercynite with the calcium minerals(monticellite)increasing.Moreover,the minerals in the Suqian ash have changed from hercynite to hedenbergite and fayalite.As an important impurity element in coal,sulfur is closely related to the conversion of iron during the melting process.The migration and transformation process of ferrous oxide described above is still the basis of analysis.Experiments were carried out by adding different mass fractions of ferrous sulfide to compare and analyze the influence of the sulfur on the migration of iron.As a common fluxing material,the troilite also reduces the ash fusibility temperatures of the gasified fine ash.But the trend is monotonously increasing as the added content increases.The maximum temperature drop should occur at a low content addition.It can be seen from the microstructure at various contents that the massive slag is loose and contains a large number of light flaky aggregates with a rough surface.As the addition content increases,the adhesion phenomenon becomes more and more serious.The minerals containing in the Hechi ash are complex,including hedenbergite,fayalite and iron spinel.The minerals in Suqian gasification fine ash iron are relatively single,mainly containing the iron spinel and troilite.The results show that the introduction of sulfur affects the migration and transformation of iron.The main minerals in the gasified fine ash of Hechi change from iron diopside and fayalite to iron diopside,fayalite and iron spinel.The main minerals in Suqian gasification fine ash change from iron spinel to iron spinel and strontium iron,and the content of iron spinel is low.The addition of sulfur element increases the species of iron minerals.In short,there are three main factors to have influence on the ash fusion temperatures:the difference in the valence state of iron,the calcium-containing minerals and the addition of sulfur.A deep understanding of the transformation of minerals and the changes in their microscopic morphology have profound implications for the study of the melting properties of gasified fine ash.