| As the main energy source of China,coal plays a vital role in the development of the national economy.Xinjiang Zhundong coal have therefore been receiving increasing attention due to its huge reserves and high-quality.However,due to its special coal-forming environment and geological characteristics,the total content of alkali and alkaline earth metallic species(AAEMs)is extremely high,especially the Na and K in Zhundong coal(in terms of oxide content in ash)is usually higher than 4wt%or even more than 10wt%,which was much higher than that in coals mined from other area(usually below 1wt%).As a result,during the utilization of Zhundong coals,such as combustion or gasification,will inevitably give rise to severe fouling,slagging and corrosion in equipments,which seriously affected the stable and safe operation of the equipment and significantly increased production costs.Thus,seeking the suppression technology of alkali metal ash-related fouling and slagging problems has become the most urgent industrial demand.Accordingly,the aims of this paper are to elaborate the initial physicochemical properties,high temperature release and deposition behavior and slagging characteristics of alkali and alkaline earth metals during high alkali coal thermal conversion.In this paper,the occurrence of alkali and alkaline earth metallic species in the typical Xinjiang high alkali coal was quantitatively analyzed using chemical solvent extraction.Besides,some advanced in-situ analysis technology,such as X-ray and nuclear magnetic resonance,were employed to reveal the initial physicochemical properties of AAEMs in coal.In addition,a laboratory-scale high temperature pressurized fixed bed pyrolysis/gasification reactor was used to simulate the thermal conversion characteristics of coal in the Lurgi gasifier.Then the transformation,release and interfacial deposition behavior of AAEMs during high-alkali coal thermal conversion was systematically investigated.The following results can be drawn based on the experimental data.1)The occurrences and transformation mechanism of corrosive materials in raw coal and during pyrolysis of Zhongdong coal by combining solid-state nuclear magnetic resonance(SSNMR)and in situ high-temperature X-ray diffraction(HT-XRD).The results show that the soluble Na in Xinjing coal mainly exist either as ions in moisture in the micropores of coal,or as organic ion-exchanged and covalent bonding forms;K is mainly in the form of aluminosilicate;while the organic bonding Ca or calcite is predominant.For the first time,the experimental evidence in this study shows that the corrosive elements demonstrated distinct forms in coal and could be significantly varied by thermal treatment.Specifically,homogeneously distributed Na ions could mutually transform between inorganic and organic-bounded form under the influence of ionic force,resulting in its elutable feature.During pyrolysis,Na was successively transformed to be inorganic form and most of the Na would volatilize above 800?,thus diversifying Na-related fouling propensity in various pyrolysis stages.The Cl was unlikely to entirely exist as inorganic form;nevertheless,it was strongly restrained by functional groups of coal matrix.The organic Cl-containing functional groups was gradually decomposed to volatile Cl at pyrolysis temperature higher than 500?,whereas the inorganic Cl was more stable and possibly exposed on the surface of char particle.During the thermal conversion of high alkali coal ash,the Na-bearing active mineral(K0.2Na0.8Cl)will decompose and release Na above 700 0C,and the inorganic Cl can combine with the metastable Ca produced by the decomposition of Ca-containing minerals to form Ca2ClP.In addition,the presence of acidic minerals(such as quartz,clay)can significantly change the conversion behavior of alkali elements.The Na would be captured to form stable NaAlSi3O8 at the presence of high content of SiO2.2)The diffusion and initial deposition behavior of volatilizable elements were clarified during pressurized pyrolysis of a typical Zhundong high-basic coal.The concentrations of AAEMs in raw coal and its derived char as a function of pyrolysis temperature and pressure were duly measured;besides,the features of innermost deposit in specific areas of pyrolysis reactor were particularly investigated.The release of AAEMs was strongly related with their occurrences in coal.Specifically,Na,mostly in a water-soluble form in raw coal,still existed as solid and/or liquid phase below 1000? and thus could be partially adsorbed and retained in the porous structure of char,which could be enhanced by elevated pyrolysis pressure with the average weight loss of Na in the range of 16.0-29.1 wt%.Nevertheless,the diffusions of K(6.8-17.3 wt%),Ca(16.4-29.7 wt%),and Mg(10.7-21.2 wt%)were substantially dependent on the decomposition of minerals and the diffusive force of volatiles.The deposits were shaped at lower temperature(<423?)largely by fine carbonaceous substances,inherent minerals,and trace amounts of Ca-bearing matter.Abundant crystals,such as NaCl in the aerosol,were deposited on the probe at 514-932?;however,the elements in deposition tended to form eutectics,e.g.,NaSiO3,Na-Si-Al-O,Na-S-O,NaCl and so on,at higher temperature(932-1117?).Subsequently,the diffusion of AAEMs in different forms resulted in the initial deposition on the surface of the probe after experiencing a series of physical and chemical processes and gave an appreciable ash-fouling propensity during pressurized pyrolysis.Moreover,the remaining minerals in char with high proportion of basic compounds were apparently dissociated with char;such minerals demonstrated sintering and agglomerated morphologies,posing a latent threaten to the subsequent utilization of char.3)The effects of different forms of sodium,chlorine and sulphur and various pretreatment methods on the evolutionary behaviors and deposition characteristics of Na species were systematically investigated.The pyrolysis experiments of Na-loaded coal samples(in which Na was loaded by NaOH and CH3COONa)blended with various Cl-and S-bearing additives were conducted in a lab-scale pressurized fixed bed reactor at 1000?,and the transformation of Na was subsequently analyzed.Experimental results show that acid elution treatment could significantly elute corrosive elements from the coal and the absence of anions in gas phase could significantly reduce Na deposition even adequate Na was loaded into the acid washed coal.However,when Cl-and S-bearing substances were added to the Na-loaded coal,abundant NaCl and Na-S-0 crystals were deposited on the probes and Cl is more competitive in enhancing Na-bearing compounds deposition,compared with S.During Zhundong coal pyrolysis,the crystalline process of Na species,morphologies of the deposits and Na retention in char are significantly affected by the interactions between Na-bearing compounds and coal matrix or other inorganic species.More importantly,the deposition of crystal NaCl was decreased with rising pretreatment temperature.While the addition of CaO in coal or char was effective in capturing volatile Cl atoms and/or HC1 to generate CaC12,resulting in the absence of NaCl in the deposits during coal or char pyrolysis.Furthermore,the addition of CaO in char could also enhance the interactions between active Na species and coal matrix or minerals,resulting in the increased Na content in CaO-loaded char.4)The characteristics of deposits formed on stainless steel substrate(boiler tube)during pressurized gasification of Zhundong coal char with excessively high Na content were systematically investigated.Results show that water-soluble and ammonium acetate solution soluble sodium and potassium in raw coal tended to be converted to hydrochloric acid solution soluble form during Zhundong coal pyrolysis.Rising pyrolysis temperature increased the content of water-soluble calcium by enhancing the decomposition of CaCO3 and organically bonded calcium.During the gasification of Zhundong coal char,the released inorganic species,especially Na,Ca,Cl and S,could condense and/or agglomerate as sticky fine particles,resulting in fouling and slagging on the probes.Specifically,after Zhundong char gasification at 800?,alkali and alkaline earth metallic species-bearing minerals loosely clung on the probes as silicate or aluminosilicate.During char gasification at 1000?,a Na-S-0 and/or Na-S deposit layer with a thickness of ca.2?m was formed on the probe No.1 at 514?,while abundant Ca-Mg-Si-O and few Na2SO4 crystals were accumulated on the probe No.2 at 771?.During Zhundong char gasification at 1200?,the minerals deposited at low temperature region(599?)were mainly composed of agglomerated and sintered alkali and alkaline earth metallic species-bearing compounds(mainly NaCl,KCl and Ca-Mg-Si-O).However,only few isolated particles were observed on the probe with temperature higher than 924?.Residual alkali and alkaline earth metallic species in bottom ash could form low-melting eutectics,such as NaAlSiO4 and Ca2Al2SiO7.5)The release and formation characteristics of ultrafine particle matters(PM)from Na-rich char gasification are systematically investigated.And different treatment methods,i.e.,water washing and mineral loading,were employed to get deep insight into the effects of various minerals on the evolutionary behaviors of PM,as well as its suppression measures.Further,the mineralogical characteristics of mineral loaded char ash were also discussed.Results showed that a large amount of fine NaCl particles generated during Na-rich char gasification.And varying gasification temperature and pressure essentially diversified the concentration of Na and Cl species in gas phase,causing the variation of quantity and appearance of PM.Specifically,the particle size of PM obtained at 1000 and 1050? is mainly distributed in the range of 0.2-0.4?m;when the W-char gasified at 1100 and 1150?,the PM with size between 0.6 and 1.2?m is predominant.While the particle size of PM decreased remarkably with the gasification pressure increase from 0.1 to 3 MPa.Additionally,due to the low carbon conversion rate of W-char during gasification in CO2 at 1100 0C,the number of PM with particle size 0.2-0.6?m accounts for more than 75%of the total PM.After gasification the kaolin loaded W-char,abundant spherical particles,mainly as alkali and alkaline earth metal aluminosilicates,were generated rather than NaCl particles.Notably,in the case of adding diatomite into char,only a few spherical particles,mainly as aluminosilicates,were detected,indicating the good performance of diatomite on restraining PM release.Basically,the addition of diatomite into W-char could reduce ash fusion temperature,and the agglomerated coarse particles in bottom ash could capture the PM,which,to some extent,inhibited the release of PM.Nevertheless,increasing the content of CaO in char will compete to react with clay and quartz,thus causing more Na-and Ca-bearing PM release.6)The characteristics of slagging generated in the coal-water slurry gasifier during the Xinjiang high alkali coal gasification were clarified.Besiedes,the evolutionary behavior of mineral matters during thermal conversion process was investigated in a lab-scale high temperature tube furnace.The results show that the content of Na in different part of slag was significantly different.And the volatilization,heterogeneity and complexity of minerals in coal will diversfy the composition and morphology of minerals.Specifically,the content of Na in the S1 and S2 ash is quite difference.The S1 ash is a siliceous glassy ash with the composition of homogeneous Ca,Mg and Fe;while the S2 ash featured with obvious ash component separation,and the XRD and SEM-EDX analysis also confirmed the coexist of Ca,Mg aluminosilicate and NaAlSiO4.In addition,due to the high volatility of Na,the sodium content in the ash residue is different,resulting in the formation of solid-liquid mineral phase system with large difference in melting point.Therefore,the diversification of high-temperature melting,viscosity-temperature characteristics and curing temperature of the mineral ash may cause slagging. |
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