Study On Characteristics Of Fly Ashes And Cenospheres From Low NO_x Pulverized Coal Combustion

Cenospheres is a small but very important component in fly ash from coal-fired power plants.Because of its various superior properties and potential application values as advanced materials,the development and high value-added utilization of the cenospheres have been attracting wide research attentions around the world.In order to strengthen controlling the emissions of NO_X pollutants,however,coal-fired power plants of our country are widely deploying deeper low NO_X combustion technology by retrofit.As compared with the conventional boiler combustion,the significant changes in fuel particle residence time,flame temperature and in-furnace combustion atmosphere in the low NO_X combustion process has the inevitable impact on the ash formation process during pulverized coal combustion,which may consequently affect the formation,quality and application of the cenospheres.Therefore,understanding the characteristics of the cenosperes of fly ash from low NO_X pulverized-coal combustion is very important and necessary for resource utilization and high value-added application of fly ash.In the work of this thesis,numerous fly ash samples were collected as raw materials from three low NO_X combustion units of two power plants dust collectors in different periods as raw material.From which,cenospheres were separated by using the float-sink method.The resulting cenospehere samples as well as the raw fly ashes were systematically characterized with various analysis methods.The basic physical and chemical properties and melting characteristics of the cenospheres and the combustion reactivity of the residual carbon were investigated and evaluated.The basic characteristics of the cenosphere and fly ash samples show that,although the mass of the cenospheres in fly ash is not so high,the volume content can reach nearly 2%of the total fly ash.The enospheres have the similar size range as the fly ash,but their particle size distributions are obviously different.The sizes of the cenospheres are larger and more uniform,containing relatively much more spherical particles.The proportion of the particles with a size of less than 45?m in the cenopheres is relatively smaller than that in the fly ash sample,implying that the cenospheres can be seperated by dry separation.The mineralogical composition of the fly ash mainly includes glass phase,mullite and quartz crystal phase.However,some cenospheric samples have the higher contents of calcite,indicating that CaO may also play an important role in the formation of ash melt and cenosphrees.Theoretical analysis on the melting characteristics of the cenospheres by using emprirical methods shows that the compositions of the fly ashes from the J power plant are different,but the compositions of the cenospheres are consistent,which means that the formation of the cenospheric particles requires certain ash compositions.The content of basic components in the ash is beneficial to the melting of coal ash,which is thus conducive to the formation of cenospheres.CaO in the fly ash of T power plant has an important influence on the formation of cenospheres.The participation of a certain amount of CaO can change the melting characteristics,including reducing the melting temperature,surface tension and viscosity,which may form hollow microspheres.However,the high content of CaO will lead to the crystallization during the melt cooling process.There is a positive correlation between the content of cenospheres in the fly ash and the sum of SiO₂+Al₂O contents in the cenosphere.Investigation on the relationship between the melting characteristics and composition of the cenospheres demostrates that the formation of cenospheres needs combined conditions for melting,and that is,the specific combination of the contents of the main ash components.Under low NO_X combustion conditions,the reducing atmosphere is beneficial to reducing the melting point of the ash and thus beneficial to the melting of the coal ash particles.However,the relatively lower combustion temperature is not sufficient to lead to too low viscosity and surface tension,which is conducive to the formation of cenospheres.The study on the characteristics of residual carbon in fly ash shows that the content of the unburned carbon in ash is generally lower;most residual carbon particles in the ash are individual particles;a part of residual carbon particles are large than commonly spherical cenospheres but with rough and porous surfaces;there are also many fine carbon particles,probably soot particles.The difference in the carbon content between the cenospheres from the two units of T power plant indicates that the significant changes in the combustion conditions of the deeper low NO_X combustion may lead to the formation of relatively more cenospheres,and may also lead to higher carbon content in the cenospheres of the fly ash.The Analysis on the combustion reactivity of the residual carbon in the cenospheres shows that the residual carbons have the similar combustion reactivity.Because its reactivity is within the commom range of the reactivity for the high-temperature laboratory-chars of pulverized coals,it is beneficial to remove the residual carbon from the cenospheres by combustion oxidation approach.In view of the different degree of low NO_X combustion in the two units of T power plant,one of the main manifestations is the difference of combustion flame temperature,but the similarity in the residual carbons of cenosphere implies that the temperature difference has little impact on char deactivation during char combustion in pulverized coal fired furnaces.It shows that the main factor affecting the formation and the canbon content of cenospheres during the low NOx combustion is the reducing atmosphere while the influence of the temperature is relative smaller.