| Burning coal with waste from cleaning and processing in large-scale circulation furnace boiler(CFB),such as slime and coal gangue,has been proved feasible and effective by operating CFB.It can not only enhances the energy efficiency,but also reduces the pollution emissions to the environment,meanwhile,brings significant economic and environmental benefits.However,it has been reported that ash depositions,such as adhering,sintering,slagging or fusing,is an operational obstacle for operating CFB,which located in the interior of the furnace,the heat exchangers surface,the inner surface or drop leg of cyclone separator.The ash deposition products will significantly reduce the efficiency of boiler,affect the stable operation of CFB boilers and corrode equipment.Seriously,it may cause unscheduled outages and bring security risks.Thus,it is essential to explore the mechanism and the influencing factors of ash deposition,for the purpose of alleviating the ash deposition tendency.This job would bring enormous benefits to both the reconstruction of the existing CFB and design of new boilers.Essentially,ash deposition is a complex boundary layer problem of high temperature gas-solid two-phase flow,companioned with chemical reactions.The processes of ash deposition includes adhesion,expanding,sintering,fusion and melting.In this paper,relevant experimental and theoretical researches have been carried out on ash deposition characteristics,such as the description and experimental simulation of the ash deposition process,the influence of operating parameters,structure parameters,physical parameters and chemical compositions on the ash deposition characteristics.In the first chapter,we briefly introduced the application background,significance,potential hazards,the current research situation of ash deposition.In the second chapter,we summarized the experimental method for studying the characteristics of ash depositions.The measurement of ash fusion temperature(AFT),the analysis methods of microstructure,the analysis methods of the chemical compositions and self-designed volume shrinkage ratio(VSR)method are introduced respectively.Further,we developed a drop tube furnace(DTF)for the purpose of simulating the whole ash deposition process.This DTF derived from some other platform from literatures and absorbed the operation characteristics of CFB.Then,we described in detail the design objectives,the main components,the working model and the application scope of DTF.However,the ash deposition phenomenon did not exist on the surface of deposition probe,even under limiting operational condition,suggesting that the DTF has design flaws.The fatal one is that the heating component cannot provide enough energy to heart the fine particles to semi-molten or molten state.Finally,we put forward some advises on reconstruction of DTF which contains the improvement of the hearting components,the accurate quantitative control of the feeding system,the increase of measure accuracy of temperature of deposition probe surface.In the third chapter,firstly,we compared the results of different researcher's study on ash deposition mechanisms.It is concluded that the complete deposition mechanism should consist of particle size distribution(PSD)model,particle physical model,particle collision model,particle force model,particle transport mechanism and the particle deposition criterion.Then,we systematically reviewed particle transport mechanism,force model and particle deposition criterion.Particularly,it can be found that almost all the deposition indexes are based on statistical results,and each index is applicable to a specific range,thus it is quite necessary to explore the deposition criterion which will be guided by the micro-particles.Further,we obtained the stress conditions of particles with various during each deposition process,based on the force analysis of gas-solid two-phase flow and the scale analysis of various forces.Finally,we constructed a new complete ash deposition description,which consists of the selective aspersion of fine particles,the formation of the initial layer,the formation of the porous structure,the shrinkage of the porous structure companioned by sintering and finally the formation of the compact structure companioned by fusion and melting.In the fourth chapter,we explored the effects of SiO2/Al2O3 ratios on sintering characteristics.Five synthetic coal ash(SCA)samples with different S/As were treated in a muffle furnace for 12 h at different temperatures(from 773 K to 1373 K,in 100 K intervals).The morphological and chemical results of the volume shrinkage ratio(VSR),thermal deformation analysis by dilatometer(DIL),scanning electron microscope(SEM),X-ray photoelectron spectrometer(XPS),and X-ray diffraction(XRD)were combined to describe the sintering characteristics of different samples.The results showed that the sintering procedure mainly occurred in the third sintering stage when the temperature was over 1273 K,accompanied with significant decreases in the VSR curve.Excess SiO2(S/A = 4.5)resulted in a porous structure while excess Al2O3(S/A?0.5)brought out large aggregations.The other three samples(S/A = 1.5,2.5,3.5)are made up of an amorphous compacted structure and are composed of low fusion temperature materials(e.g.,augite and wadsleysite.).Sintering temperatures first dramatically decrease to a low level and then gradually rise to a high level as S/A increases,suggesting that Al2O3-enriched additives are more effective than SiO2-enriched additives in alleviating depositions.In the fifth chapter,we explored the relationship between ash fusion temperature and chemical compositions.Data of AFT and chemical composition of 48 different coal ashes is selected.The binary regression method was used to build an empirical model of AFT,considering the combine effect of Si/Al(0)and base-acid ratio(?)simultaneously.The results of AFT which are measured by thermomechanical analyze(TMA)and X-ray diffraction(XRD)of synthetic coal ash(SCA)samples are utilized to validate the model.Result shows that,whether ?=0.4 or ?=0.8,AFT firstly shows a dramatic decline,and then a gradual raise with 0 increases.When 0 is small,the main chemical composition in these samples are Al2O3 and aluminum silicate,which are refractory minerals;As 0 increases,eutectics plays a major role on AFT;At last,the main components in SCA samples are crystal SiO2 and little eutectics,which will elevate AFT.Since the results of TMA and XRD are in good agreement with the calculation,the empirical model is deemed applicable.In the sixth chapter,we presented a summary of the thesis and some recommendations for future research on ash deposition characteristics. |
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