Effect Of Na And K In High-alkali Low Rank Coal On The Ash Fusibility/Viscosity And Removal Via Hydrothermal Process

China is rich in high-alkali low rank coal reserves,whose application are gradually expanding.However,the high-alkali coal has been increasingly concerned for higher ash fouling and slagging incidents within apparatus(e.g.,boiler,gasifier,etc.),during coal combustion or gasification.Therefore,basic research and countermeasures are warranted for clean and efficient utilization of the reserves stated above.In the dissertation,typical high-alkali low rank coal samples were selected from Zhundong,Yining coalfield in Xinjiang Uygur Autonomous Region(PRC)and Shengli,Baiyinhua coalfield in Inner Mongolia Autonomous Region(PRC),as well as slag and fly ash samples from certain industrial pulverized coal fired boiler and entrained-flow gasifier.Then the coal samples were subject to SCEE(abbrev.sequential chemical extraction experiment),pyrolysis,combustion and dealkalization via HTP(abbrev.hydrothermal process,same below),etc.The occurrence modes within coal and thermal evolution behaviors of alkali specie like Na and K,were inspected by using modern equipments(e.g.,XRD,XRF,SEM-EDS,FT-IR,TG,etc.).Furthermore,the effect of HTP on the changes in coal slurryability and coal pyrolysis/combustion characteristics and its mechanism were discussed.Meanwhile,simulations were carried out on the minerals thermal evolution and coal water slurry gasification by using computer-aided software like Fact Sage,MATLAB and Aspen Plus.Moreover,on the basis of BP neural network,a novel model was built for predicting the characteristics index of coal ash fusibility and viscosity.The main results of the dissertation are outlined as follows.The occurrence of Na and K differed obviously within the tested coal samples.Na was mainly in water-soluble form,while K existed as insoluble component like KAl Si₂O₆,KAl Si₃O₈,KAl Si O₄,etc.The content of Na was higher than that of K within dry ash.The amount of Na in Zhundong coal(ZD-1-R,ZD-2-R)was 3.257mg/g,2.915mg/g,respectively,Shengli coal(SL-6-R)of 2.072mg/g,and Yining coal(YN-1-R,YN-2-R)of 1.840mg/g,1.554mg/g,respectively.In view of K,the value in SL-6-R approached to 0.493mg/g,the others ranged from 0.15 to 0.3mg/g.Both Na and K were remarkably richer than Cl,manifesting that Na Cl or KCl was not the main form of alkali in the coal matrix.As the natural coal seam vertically deepening,Na in Zhundong coal declined,but K changed little.For Shengli coal,K decreased obviously,while Na showed as splint structure(i.e.,low in middle but high in upper and lower layer).The thermal evolution behavior of Na and K distinguished apparently.With the combustion or pyrolysis temperature rising,Both Na and K vaporized at 500 ^? or even earlier,the escaping ratio of Na₂O and K₂O within ash combusted at 815 ^? approaching35.81%,26.79%,respectively.Na was less stable than K,which enriched in slag or fly ash as(Ca,Na)(Si,Al)₂Si₂O₈,KAl Si₂O₆or(Si₃Al)O₈,etc.,consequently resulting in its lower escaping ratio.During the industrial gasification,Na mainly transferred into fly ash,while K was both captured in the slag and fly ash.As Fact Sage simulation displaying,when the gasification temperature overpass 1200^?,Na turned into Na₂SO₄(g),Na₂O(s)and Na(g),but K was transformed into K₂O(s),K(g)and KO(s).There were complex nonlinear correlations between the index of ash fusibility(FT,ST,DT)or viscosity(T_2.5,T₂₅,T_cv)and ash content and its component.The bigger of K amount,the higher of all the index stated above.By contrast,Na presented negative influence.For FT,ST and DT modeling,the training error of BP neural network method was 0.459%,0.583%and 0.409%,respectively,lower than that of linear regression method,as 1.705%,1.699%and 2.186%,respectively.For T_2.5,T₂₅ and T_cv modeling,the training error of BP neural network method was 0.813%,0.323%and 1.193%,respectively,also lower than that of linear regression method,as 2.539%,3.808%and3.258%,respectively.AI_G(abbrev.the coal adaptability index of gasification)was put forward.K element was endowed"tracing function"during the thermal conversion of high-alkali low rank coal,thenby K₂O-AI_G formula(R²=0.87?0.97)was established,facilitating evaluation of the coal adaptability of gasification.Under the treatment via HTP of 300^?,60min,the coal samples were remarkably upgraded,with Na₂O_eq reducing below 2%.The removal ratio of Na₂O and K₂O in Zhundong coal reached as higher as 82.05%,67.39%,respectively.Na element could be easily removed via HTP.However,K was less released for co-existing in stable species like KAl Si₂O₆,KAl Si₃O₈,KAl Si O₄,etc.In summary,the dealkalization mechanism of HTP was a complex physical-chemical process involving dehydration,deoxidation,desulfurization,deashing,dealkalization and carbonization.The elements in coal migrated during HTP,where inorganic components such as Na⁺,K⁺transformed into liquid phase,and a little amount of organics such as C and S formed gas phase concluding CO,CO₂ and H₂S.Treated via hydrothermal dealkalization,the characteristic index of ash fusibility raised obviously,and the tendency of ash fouling and slagging was depressed.The concentration of coal water slurry from low rank coal can be increased by 10%(appr)via HTP,but the increase was cut off 2?3%when using the industrial gasification wastewater as HTP liquid.A versatile polycarboxylic acid additives(marked as FY-AD)was prepared in the presence of HTP wastewater,rising concentration nearly 1%higher than that of commercial counterpart.A continuous pilot apparatus(0.2t/d dry coal)was built,enhancing the concentration of industrial slurry by nearly 7%.As the gasification simulation shown,with the lignite slurry concentration rising,(H₂+CO)in the syngas increased,while CO₂ declined,alongside with the specific coal consumption and specific oxygen consumption indexes dropping clearly.Treated via HTP,the pyrolysis apparent activation energy of Shengli coal increased from 9.418KJ/mol to 14.323KJ/mol,as well as that of Zhundong coal ascending from 14.206 KJ/mol to 16.985KJ/mol.The combustion apparent activation energy also changed significantly.The loss of organics in coal during HTP was elucidated as decisive factor accounting for the change of coal pyrolysis and combustion behavior.Alkali like Na mainly acted within carbon matrix during the later stage of pyrolysis or combustion.