Occurrence Modes,Removal And Fate In Pyrolysis Of Na And Ca In High Alkali Coals

High-alkali coals from Xinjiang in China,Victoria in Australia and other place contain a large number of alkali and alkaline earth metals（AAEMs）,such as Na and Ca,which could cause serious slagging and fouling of the boiler under high temperature condition during the combustion/gasification process.Low temperature pyrolysis utilizes the original part structure of coal directly without the condition of slagging and fouling,which would provide a new way for the utilization of high-alkali coal.However,the catalytic effect of AAEMs on the secondary reaction of coal pyrolysis would reduce the yield of high value-added tar,and the release of AAEMs will also corrode the equipment of oil and gas extraction and utilization.To effectively eliminate the negative impact of AAEMs on high-alkali coal pyrolysis,the theory of leaching and pyrolysis process kinetics is used as the instruction in this thesis.And occurrence forms of Na and Ca in high alkali coals are firstly studied by the chemical extraction and dynamic leaching combined with advanced instruments,such as optical microscope,SEM,EDX and XRD.Then pyrolytic characteristics and fate in pyrolysis of Na and Ca were studied by fixed bed before and after component separation,pretreatment and thermal treatment of the typical high alkali coals（inertinite-rich ZhunDong coal and vitrinite-rich Baishihu coal）.The research results provide theoretical basis for the process operation and product upgrading of high alkali coal pyrolysis.The main conclusions are as follows:1.Occurrence modes of Na and Ca in high alkali coal.（1）High contents of Na₂O and CaO in ZD and BS coal ash are the main characteristics of high alkali coal.The difference of occurrence of ash content resulted in a positive correlation between Na and ash content in the scale of low ash.The mole content of Na in ZD coal is an order of magnitude higher than that of chlorine,while is just slightly higher in BS coal,indicating that NaCl is the Na in BS coal,but not the main form of Na in ZD coal.The content of Na and Ca in high alkali coal decreases with the increase of the depth of coal seam,and the monovalent sodium ions are more likely to be enriched in the upper part of coal seam than the bivalent calcium and magnesium ions.（2）H₂O-Na（including forms of hydrated ion Na,NaCl and Na₂SO₄ and Na₂CO₃,NaHCO₃,etc.）is the main form of sodium in high alkali coal,while there are slight differences for different depth of coal seam and mining area of the sample.For example,the percentage of H₂O-Na is 82.49%in Parting of Wucaiwan mining,greater than the US coal and LS coal which are 65.50 and 64.35%,respectively.H₂O-Na in BS coal is significantly higher than in ZD coal.SEM-EDX and XRD results show that the main sodium-rich minerals are thenardite,glauberite and so on.AcNH₄-Ca and HCl-Ca accounted for more than 80%in high alkali coal,the main form of which is calcium carbonate.The optical microscope and SEM-EDX show that calcium carbonates in high alkali coal include single particle in coal seam,filling in vein fissure of coal or in the cell lumen.Comparing the dissolution capacity of NH₄Cl and AcNH₄ to Ca,there is organic Cajust like ion-exchangeable（-COO）₂Ca in ZD and BS coal,about 5-10%in ZD coal and 15-20%in BS coal.（3）Na₂O and CaO content in high alkali coal determined by the national standard method of high is lower than the level of chemical extraction method,which indicates Na,Cain high alkali coal could be released under high temperature.The sodium content by chemical extraction method is more than 50%of the value from the national standard method.2.Dynamic leaching behavior and pre-removal rule of Na and Ca in high alkali coal.（1）The concentrations of Na⁺,Cl^-and SO₄^2-in the leaching solution of ZD coal seam decline sharply at first,and then tend to be gentle.The initial leaching rate of Na⁺,Cl^-and SO₄^2-decreased as follows:US coal>Parting>LS coal;the leaching rate and the final leachability was determined by water soluble Na content,specific surface area and micropore volume of the samples.The leaching kinetic rate constants of US coal,Parting and LS coal obtained using the shrinking core model were 0.001h^-1,0.005 h^-1and 0.002 h^-1.Due to differences in specific surface area and pore volume,the leachability of total sodium and water soluble sodium decreased as follows:Parting>LS coal>US coal,and the leachability of Parting is about 99.62%.（2）With the increase of density level,the proportion of H/C atoms first decreased and then increased,while O/C atoms gradually increased in the separation components.–OH functional groups show a relative enrichment in separation components derived from middle density of BS coal and low density level of ZD coal.In the separation component,Na was relatively concentrated with vitrinite,and Ca was concentrated in both of the low density and high density.After pretreated by H₂O,CH₃COONH₄ and HCl,the different occurrence modes of Na and Cawas removed.And the structure and properties would be slight changed by the pretreated process,such as with HCl pretreated,the absorption peak of-OH and-COOH functional groups intensity significantly enhanced,fat side chain shortened,and the substituent aromatic ringreduced.With the increase of hydrothermal treatment temperature,the solid yield,volatilization,oxygen content and O/C atomic ratio decreased gradually,while the H/C atom increase first and then decreased.After partial water soluble and organic form Na and Ca was removed by hydrothermal treatment,the contents of Ca O and Na₂O in coal ash decreased,and Na₂O and CaO contents of BS260 and BS300 were higher than BS260w and BS300w.Ca content decreased with high temperature hydrothermal treatment,and in BS300 and BS300w was 19.01 and 16.26mg/g,respectively.3.Effects of component separation,pretreatment and hydrothermal treatment on pyrolysis characteristics of high alkali coal.（1）Tar yields of BS1 and BS2 coal by external heated fixed-bedpyrolysis were13.98 and 13.75%,which was far higher than ZD coal with 1.61%.The main composition of pyrolysis gas is H₂,CH₄ and CO.Tar of BS coal has lower density and higher H/C atomic ratio,combined saturates and aromatics accounted for 58.28%,and the fraction of<360℃is 65.60%.（2）With the increase of density level,the maximum pyrolysis rate and the weight loss of the separation components derived from both of BS and ZD coal reduced gradually;while the activation energy and predetermined factors showed a trend of decreasing first and then rising;the tar yield with dry ash free by Gray-king assay decreased first and then increased trend.After pretreated by H₂O,CH₃COONH₄ and HCl gradually,the initial pyrolysis temperature,maximum pyrolysis rate and weightloss of BS coal were slightly increased.The pyrolysis activation energy of ZDW and ZDAC decreased from 15.64 kJ.mol^-1 to 14.83 and 13.78 kJ.mol^-1;of BSW and BSAC reduced from 38.37 kJ.mol^-1 to 37.60 and 37.98 kJ.mol^-1.With hydrothermal treatment temperature increased the initial pyrolysis temperature and the maximum pyrolysis rate of BS increased,and the pyrolysis activation energy and the predetermined factors showed a gradual increased trend.The tar yield of BS coal by Gray-king assay decreased first and then increased trend.Compared with BS260w and BS300w,BS260w and BS300w with lower pyrolysis activation energy have higher pyrolysis gas yield and lower tar yield.（3）After pretreatment and hydrothermal treatment,the concentration of H₂ in pyrolysis gas increased and CO₂ decreased;CH₄ and C₂-C₃ derive from light separation component is higher than that from raw coal.The average pore diameter and specific surface area of the samples were significantly changed after removal of the different forms of Na and Ca with different methods.For example,with hydrothermal treatment,the pore diameter increased and surface area decreased significantly.The char derived from BS260 and BS300 have lower average hole diameter and higher surface area than BS260w and BS300w pyrolysis.4.Migration rules of Na and Ca in high alkali coal during pyrolysis process.（1）The tar yield of ZDR and ZD1.4-1.5 was 1.49 and 3.62%;of BSR and BS1.3-1.4 was 11.21 and 14.61%,respectively.And tar yield of BSW,BSAC and BSH increased from 11.21%of BSR to 11.81,14.06 and 13.56%,respectively.The tar yield of BS260w and BS300w was 12.75 and 11.94%,and the pyrolysis gas yield was 18.67and 15.13%,lower than that of BS260 and BS300 with 24.43 and 21.18%,respectively.Na content and distribution in pyrolysis products of BS coal samples by separation component,pretreatment or hydrothermal treatment ordered as Char>Water>Tar>Gas,while Ca content and distribution order is Char>Tar>Water>Gas.（2）The release ratio of Na was about 10%in the pyrolysis process of raw coal and selected light components of high alkali coal,and the former is higher than the latter;Ca was just about 1%,and the former is lower than the latter.The contents of Na and Ca in the pyrolysis products after pretreatment or hydrothermal treatment were obviously lower than that of raw coal.Na release ratios of BSR,BSW,BSAC and BSH by fixed bed pyrolysis were 9.31,12.32,14.39 and 9.7 1%,respectively;Ca release ratios were just about 0.94,1.77,0.53 and 4.60%.The content of Na and Ca in water treatment pyrolysis products is obviously lower than that of raw coal.Na release ratios of BS260,BS260w,BS300 and BS300w during fixed bed pyrolysis process were 4.43,8.13,5.57 and 12.10%,respectively;Ca release ratios were 0.65,1.49,0.79 and1.23%.Na and Ca release ratios of BS260w and BS300w were higher than BS260 and BS300.And with higher temperature of hydrothermal treatment,the content and distribution of Na and Ca in tar or pyrolysis gas would be lower.