| Resources of low-rank coal with high alkali and alkaline earth metals(AAEM)content are abundant in China,but high AAEM in coal result in serious problems,including slag-bonding,staining,erosion and agglomerating in boiler and gasifier.Meanwhile,the AAEM in coal have a confirmed catalytic effect on the coal combustion or gasification reactions.It can reduce the needed reaction conditions,increase the reaction rate,enhance the yield of the target products,and adjust the composition of the products,etc.These results were derived from the role of AAEM with different occurrence modes in the processing and utilization of Zhundong coal.In nature pyrolysis can be considered as the initial stage and paralleling part of coal combustion,gasification and direct liquefaction,and pyrolysis itself is one of the fundamental technologies in coal conversion processes such as cascade utilization technology of coal by grade and quality.Additionally,the pyrolysis process has greatly influenced the transformation of AAEM in coal.Thus,identifying the occurrence modes of AAEM in coal,revealing their roles in coal conversion processes and exploring the probability of transformation among different occurrence modes of AAEM are of great significance in investigation of approaches to alleviating or eliminating the slagging and corrosion of the apparatus and to strengthening their catalytic effects.So far researchers have developed a great deal of low-rank coal with high alkali pyrolysis work and accumulated lots of high-quality data.However,there is a significant difference between situation of practical production and lab research.Furthermore,there is no clearly understanding of the law of AAEM translation in coal pyrolysis process.In this study,two Zhundong coals,Nanlutian(NLT)coal and Wucaiwan(WCW)coal,were pretreated by an improved sequential extraction,AAEM in different occurrence modes were separated from the coal samples.The contents and compositions of AAEM in different occurrence modes in Zhundong coals were investigated by inductively coupled plasma emission spectrum(ICP-OES),X-ray diffraction(XRD)and energy dispersive X-ray sepctroscopy(EDS),then thermogravimetric analysis and a modified random pore model was used to quantify the catalytic behavior of them accurately.A series of pyrolysis experiments with gradually elevated temperature in small increment were conducted using an entrained-flow bed reactor close to the actual production.Chars from pyrolysis at different temperatures were characterized by an improved sequential extraction method,inductively coupled plasma optical emission spectrometry(ICP-OES),X-ray diffraction(XRD),and scanning electron microscopy with energy dispersive spectroscopy(SEM-EDS)with the aim of revealing the occurrence modes,chemical composition,and the distributions of AAEM on the surface of coal char with the change of pyrolysis temperature.In addition,FactSage was employed to investigate the evolution of AAEM species in the whole pyrolysis process.Meanwhile,the chars from pyrolysis at different temperatures were investigated by X-ray diffraction(XRD)and Raman spectroscopy with aim of revealing the microcrystalline parameter and the degree of crystallinity defects and ordering in coal char.Then Fourier transform infrared spectroscopy(FT-IR)and X-ray photoelectron spectroscopy(XPS)were employed to expose the changes of chemical surface property of Zhundong coal before and after pyrolysis.The main contents of the paper conclued:(1)The contents and chemical compositions of AAEM in different occurrence modes in Zhundong coals were investigated.The study found that the occurrence modes of AAEM in two Zhundong coals are similar.Water-soluble AAEM consists mainly of sodium species,and the XRD and EDS results confirmed that the main component is NaCl.The content of acid-soluble AAEM is high and composed mainly of Ca and Mg salts.The XRD results showed that these are CaSO4 and MgSO4.The insoluble AAEM consists of silicates and aluminosilicates formed by a large amount of Na and K and a small amount of Ca.The content of ion-exchangeable AAEM is lower than the contents of the other forms and dominated by sodium and calcium.AAEM have a significant catalytic effect on the gasification of coal,and the higher the temperature,the greater the catalytic performance of the AAEM.A modified random pore model was used to quantify the catalytic behavior of AAEM accurately in different occurrence modes.The results showed that when the water-soluble AAEM in the NLT and WCW raw coals were removed,the activation energies required for coal gasification of two coals increased by 7.679 kJ·mol-1 and 4.035 kJ mol-1,respectively.When the ion-exchangeable AAEM in the NLT and WCW water coals were removed,the activation energies increased by 5.117 kJ mol-1 and 1.400 kJ mol-1,respectively.Finally,when the acid-soluble AAEM were removed from the NLT and WCW NH4Cl coals,the activation energies increased by 2.002 kJ mol-1 and 0.537 kJ mol-1,respectively.Combining the results of the occurrence mode of AAEM in the coals and their catalytic effects,it is apparent that water-soluble AAEM have both high content and good catalytic behavior.Although the content of ion-exchangeable AAEM is low,its catalytic effect is remarkable.The content of insoluble AAEM is high,but its catalytic performance is weak.(2)As pyrolysis temperature increases,the decrease of water-soluble Na and exchangable Na resulte in the decline of the total content of sodium in coal char.And The insoluble K decomposes slightly at high temperature and the change of total potassium content is not obvious.Meanwhile,the transformation of HCl-soluble Mg and Ca to insoluble forms is the main reason for the enrichment of Mg and Ca in coal char.With the increase of pyrolysis temperature,some water-soluble AAEM species become gaseous and released from the coal char.Part of HCl-soluble AAEM species decomposed and then reacted with SiO2 or/and Al2O3 to form insoluble silicate or aluminosilicate.With the process of pyrolysis,the surface of the char sample became rougher,and was covered with cracks and voids.Meanwhile,when the pyrolysis temperature exceeds 700?,the contents of sodium,potassium,magnesium and calcium on the char surface increased gradually,and that of silicon and aluminum were positively proportional to the pyrolysis temperature.The water-soluble AAEM was carried by pore-water to the surface of the char sample during the initial stage of pyrolysis.Then,some of the water-soluble AAEM species were released from the surface of the char with the increase of pyrolysis temperature.Some discrete minerals reached the surface of the char through crack and voids at high temperature.The char matrix inhibited the release of AAEM species,resulting in the enrichment of AAEM species on the surface of the char.(3)XRD and Raman spectroscopy showed that with the increase in pyrolysis temperature,the transition from the small aromatic ring systems(three-to-five rings)to large aromatic ring systems(six or more fused benzene rings)during coal pyrolysis,resulting in the increase in the lateral size of the basic lattice unit of carbon.Meanwhile,the condensation and combination of aromatic rings caused the stacking height of the basic lattice units to first decline and then increase.When pyrolysis temperature was lower than 1000?,the content of graphite declined,and the inter-layer spacing of the lattice changed slightly with increase in temperature.As the pyrolysis temperature continued to rise,the graphite content increased,and the inter-layer spacing of the lattice decreased sharply,reaching a minimum of 0.3466 nm at 1500?,which represented the increase in the graphitisation degree of coal char and the transformation of the microcrystalline structure of coal char to that of graphite.FT-IR and XPS analyses indicated that exchangeable AAEM cations were mainly connected with oxygen functional groups in coal char,and the bond between them was easily broken at high temperatures.Meanwhile,the content of C-H/C-C/C=C groups increased with increasing pyrolysis temperature while that of C-O decreased.As a result,there are more hydrocarbon groups on the surface of the coal char,whereas fewer oxygen functional groups are exposed to the coal char surface. |
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