Ash Fusion Behavior Of Zhundong Coal And Effect Of Co-burning Coal Gangue

Zhundong coal is a high-quality power coal with abundant reserves.However,relative high content of sodium existed in Zhundong coal has led to severe ash-related problems including fouling and slagging during combustion,which seriously impacts the operation safety and stability of power plant.In order to mitigate ash deposition formed in boiler associated with Zhundong coal,ash fusion behavior of high-alkali coal and the effect of coal gangue ash and coal gangue addition on combustion behavior,ash fusibility,and sodium transformation characteristic of Zhundong coal were studied in this thesis.The basic physical and chemical characteristics of Zhundong coal(ZDA and ZDB)were investigated,and the results showed that,Zhundong coal was rich in fixed carbon but absent in ash content,suggesting that Zhundong coal was prone to ignite and burn easily.The chemical composition of Zhundong coal was rich in basic oxides,such as Na2O,CaO,MgO and Fe2O3.The ash fusion temperature(AFTs)of ZDA was much lower than that of ZDB sample,which was closely related to its higher SiO2 content and lower CaO content.XRD results showed that the main mineral phases in ZDA at high temperature were fluxing minerals such as augite,while the main minerals of ZDB were refractory minerals such as yeelinmite and merwinite.The influence of alkali components,acid components and acid to base ratio(A/B)on fusion characteristics of alkaline coal ash was studied with synthetic ash as the object of the research.Result shows that as the content of Na2O increased from 4%to 12%,refractory minerals such as SiO2,Fe2O3,CaO,Ca2Fe2O5,CaSiO3,Ca3MgSi2O8,and CaAl2Si2O8 that originally presented in the sample were replaced by fluxing minerals such as Na-bearing minerals as well as CaAl2Si2O8 and Ca2Al2SiO7.The degree of low-temperature eutectic reaction between Ca-bearing minerals and sodium-containing minerals was enhanced,and the deformation temperature(DT),softening temperature(ST),hemispheric temperature(HT)and flow temperature(FT)decrease from 1225 0C,1233 ?,1255 ? and 1297 ?,to 1162?,1174?,1181 ? and 1189?,respectively.As CaO content increased,the original fluxing minerals such as CaMgSi2O6 transformed into refractory minerals such as CaO and CaSiO3,meanwhile the degree of low-temperature eutectic reaction between calcium-containing minerals decreased,resulting in an increase in DT,ST and HT monotonously.The effect of MgO content on the ash fusion characteristic was similar to that of CaO.This was because when MgO content increased from 5%to 25%,fluxing minerals such as NaAlSiO4 and Ca3Fe2Si3O12 were replaced by refractory minerals such as Ca2MgSi2O7,Ca2SiO4,SiO2 and CaO,and the degree of low-temperature eutectic reaction between sodium-containing minerals and calcium-containing minerals was reduced.With increasing Fe2O3 content,the original high-melting minerals(CaO,SiO2,CaSiO3,and Fe2O3)in the synthetic ash were partly transformed into low-melting minerals such as Ca3Fe2Si3O12.As the content of Fe2O3 in synthetic ash further increased,the content of high melting point mineral(Fe2O3)in the sample raised,so that the ash fusion temperature started to increase.Comprehensive comparison of the AFTs of synthetic ash and the content of alkali components found that for coal ash with the same A/B,DT is mainly closely related to Na2O and alkaline earth metals(CaO+MgO)in the sample,while FT is mainly affected by Na2O and Fe2O3.As the ratio of SiO2 to Al2O3(S/A)raised,the AFTs of synthetic ash showed a monotonous downward trend.When S/A exceeded 1.25,the downward trend of DT,ST,and HT became gentle.When S/A exceeded 2.25,FT decreased gently.The degree also slowed down,which was related to the refractory minerals such as CaO,Fe2O3,Ca2MgSiO7,Ca2Fe2O5,and Al2O3 originally presented in the sample were replaced by the fluxing mineral such as CaSiO3,and the low-temperature eutectic reaction between sodium-containing minerals occurred.With the increasing of A/B,the refractory minerals(CaO,Fe2O3 and CaFe2O5)in the synthetic ash were partially replaced by fluxing minerals(CaSiO3),and a low-temperature eutectic reaction between calcium-containing minerals appeared in the samples.As the A/B raised further,the fluxing mineral(CaSiO3)began to transform into refractory minerals(SiO2,Al2Si4O10,CaFe4O6,and Al2O3),and the degree of low-temperature eutectic reaction in the sample decreased.Increasing the acid compositions of coal ash,particularly the content of Al2O3 would decrease the slagging tendency of Zhundong coal remarkably.Research on the combustion characteristics of Zhundong coal and coal gangue and their blended samples with thermogravimetric analyzer showed that,the combustion process of Zhundong coal and coal gangue can be divided into three stages(water loss,volatile release and fixation carbon burn and burnout stages),and the combustion characteristic of Zhundong coal was significantly better than that of coal gangue.Coal gangue addition had no significant effect on the ignition temperature of Zhundong Coal.However,the release of volatiles and the severity of the combustion reaction at this stage were inhibited,and this situation would be improved to some extent with more coal gangue blended.The ash fusion characteristics of ash-blended samples and coal-blended samples of Zhundong coal and coal gangue were studied.The results showed that when Zhundong coal ash was mixed with gangue rich in SiO2 and Al2O3,as mass fraction of coal gangue increased from 10%to 90%,the AFTs of the two groups of blended ash samples(ZDA/CGA and ZDB/CGA ash)AFTs of both blending ashes decreased first and then increased with the increase in coal gangue ash addition ratio,the minimum AFTs were observed with the coal gangue ash ratios of 40%and 60%,respectively.The deformation temperature and flow temperature were mainly attributed to the difference in CaO and Fe2O3 weight ratio,as the A/B of ZDA/CGA is similar to that of ZDB/CGA,consistent with the previous research results of synthetic ash.XRD characterization results found that with increasing coal gangue addition,the mineral composition of ZDA/CGA and ZDB/CGA ash samples at high temperatures had transformed from refractory minerals(quartz,yeelinmite calcium,magnesite)to fluxing minerals(calcite,calcium feldspar,nepheline and pyroxene,),and then into refractory minerals(quartz and hematite).When Zhundong coal and coal gangue were directly blended,AFTs of both blended coal samples decreased first and then increased with the increase in coal gangue addition ratio,the minimum AFTs were observed with the coal gangue ash ratios of 5%and 10%,respectively SEM-EDS and XRD characterization results showed that,as the proportion of CGA and CGB addition ratio increased,interchanging between fluxing minerals(calcite,diopside,albite and other sodium-containing minerals,)and refractory minerals(quartz and anorthite),as well as the degree of low-temperature eutectic reactions were the main reasons that affecting AFTs of the blended coal samples.The measured value of sodium in residue ash(RNa/exp)of blended samples was always greater than that of theoretical value(RNa/cal),suggesting that synergistic reactions occurred between ZDA and coal gangue samples to inhibit the release of sodium into the gas phase at high temperature.XRD results revealed that silicate and aluminosilicate content of blending samples would be increased with rising the addition ratio of coal gangue,and then chemical reaction would happen between silicate,aluminosilicate and sodium element being Na2O and NaCl to Na-bearing minerals such as albite.Comprehensivly analysis of coal gangue addition on Zhundong coal during combustion revealed that CGA was applicable additive for,which was due to CGA addition could modify the slagging and fouling tendency duing combusition of Zhundong coal,while CGB could barely improve the ash deposition tendency of ZDA.The ideal addition ratio of CGA was 30%.