Experimental Study On Characteristics Of Zhundong Coal Ash Sintering

In China,Zhundong coal field is abundant to maintain the China coal supply nearly one century.However,the alkali metal content in ZD coal ash is extremely high,especially the elements such as calcium,sodium,which makes great contribution to the severity of the soot formation and heavy slagging contamination.The deposition height at low temperature superheater highly exceeds the corresponding natural accumulation rest angle of coal ash on the tube surface,which is hard to remove.This phenomenon is most likely the sintering,especially in which the flue gas temperature is far lower than the melting temperature of convection heating surface,and it has already been confirmed.So it can be described as a kind of enhanced contamination with "sintering" growth process.In order to ensure the stability of the power plant equipment operation and the widespread use of ZD coal,the experimental study on fouling and sintering characteristics of Zhundong coal ash is of essential significance.In according with Materials Science,this paper used a variety of common parameters to reflect the coal ash sintering process characteristics,by which we can explore the basic rules in the process of sintering from the perspective of mechanism.Two similar kinds of ZD coal were chosen,WuCaiWan(WCW)and JiangJunMiao(JJM)coal samples,with remarkable difference in characters of the corresponding National Standard ash.It retains the general characteristics of ZD coal ash,high alkali and high silicon;and highlights the widely recognized elements that affect sintering characteristics,such as Ca,S,Al and so on.In this paper,the macroscopic characteristics,such as initial sintering temperature,sample size,mass,density and mechanical strength,were studied.Furthermore the microstructure,phase migration and pore development in the sintering progress were also studied.To optimize the characteristics of coal ash sintering,qualitative and quantitative comparison was summed up,which divides the stages of sintering process.Eventually,the research gained valuable conclusions and knowledge.First of all,this paper drew up the idea that represents the degree of porosity development from the internal pressure-drop of the specimen,and designed a modified experimental device which can dynamically measure the sintering characteristics of coal ash.The ‘pressure-drop technique’ can be used to achieve accurate determination of the initial sintering temperature.As the coal ash modified a uniform ball,it can avoid the inevitable size factor on the internal heat and mass transfer process of the sample,with timely monitoring temperature and pressure changes in the geometric center.Taking into account the operation of the boiler during the process,the internal high temperature and high pollution environment on the sintering process,the experimental platform add a window,with the use of high-definition video capture device to achieve the entire process of visual monitoring.In addition,the image analysis software,Image Pro Plus,was used to analyze the size change of coal ash sintered images quantitatively,and the parameters of approximate "individual particle density" were able to calculated according to the corresponding mass change.Secondly,the sintering characteristics of WCW and JJM National Standard coal ash were carried out.The macroscopic characteristic parameters of coal ash during sintering were measured and analyzed,and the influence of forming size on the heat and mass transfer process was not neglected.It was found that the size had a slight effect on the sintered properties of the ash ball within a certain range.The presence of alkali and alkaline earth metal on the sintering process is of significant influence,so that the initial sintering phase is advanced and the later sintering phase is prolonged.The macroscopic characteristics are reflected in the internal pressure,size,mass,density and surface morphology.Once the ash sample reaches the initial sintering temperature,surface energy decreases and the internal voids develop rapidly.The pressure-drop technique is the most universal and scientific.The measurement of the central temperature of spherical ash sample can minimize the influence of the irrelevant factors,so that the initial sintering temperature and the pressure fluctuation phenomenon is more significant.The change of mass and density is suitable for characterizing the initial sintering temperature,while the change of size and density is suitable for the change of the whole structure in the middle and late sintering phases.The change in the mechanical strength of the sintering can be used as an auxiliary parameter to reflect the degree of sintering.Finally,the microscopic mechanism of coal ash sintering process was deeply studied by analyzing the both kinds of ash samples,combined with SEM,XRD,nitrogen adsorption and other instruments.It was found that the particles were thermally expanded during the sintering process.The micropores were generated at the initial sintering temperature and gradually developed into mesopores and macropores.The diffusion effect on the viscous flow in the pore structure was firstly suppressed and then strengthened.In the late sintering phase,the structure has already developed.Although the overall development of the sample is densifying,the porosity is well developed,to the extent that it can be seen with naked eye.The first shrinkage area of coal ash sintering is mainly due to the phenomenon of volatilization,in which stage Ca and S determine the degree of contraction and mechanical strength.The second shrinkage area is complex physical and chemical changes,with low temperature eutectic and polymer.Then the density of Ca,Al and Si increased significantly with the increase of temperature.At about 700℃,substances,similar to kaolin,containing hydroxyl and Si,Al oxide structure collapsed,while a certain amount and type of clay minerals and reached the scope of dihydroxylation.Before 800℃,Na mainly in the form of Na2SO4,increases the ash viscosity,and gradually occurs in the aluminosilicate with the temperature rising,such as NaAlSi3O8 and NaAlSiO4(nepheline),which mainly depends on the ratio of Si and Al.At about 900℃,reaction between CaSO4 and aluminosilicate contributes to sticky calcium aluminosilicate,promoting the development of sintering process.As the calcination temperature of Ca structure is coming,sintering continues to the late development of the densification.The sintered structure consists of a simple viscous connection and partial melting,converts to complex physical and chemical reactions,producing a large amount of eutectic and polymer.The specific forms of occurrence are closely related to the contents of Ca,Si and Al,which determine the changes in the macro parameters of the sample together.