Study On Mineralogy And Environmental Geochemistry Of Coal Gasification Products

Coal occupies the dominant position in China’s energy structure.For the purpose of economic transformation and environmental protection,gasification technology which is a core means of clean and efficient use of coal has been rapidly developed in the past few years.Minerals and potentially hazardous trace elements in coal are the main impurities in coal.The studies on their transformations and emissions in the coal utilization processes are mainly conducted in the field of coal combustion,and there are relatively few studies in the gasification system.The rapid development of commercial coal gasification has caused a large amount of coal gasification residues.Coal gasification residues are mainly disposed of in landfill because of the high yield and low utilization,which has certain potential environmental pollution and health risks.The feed coal and coal gasification products from commercial coal-to-methanol plant(GE gasification)and coal-to-olefin plant(GSP gasification)at the Ningdong Energy Chemical Industry Base in Ningxia Hui Autonomous Region,China were selected as the main research objects.Combining with the coal gasification experiments using highly-elevated-hazardous-trace-elements coal from Ganhe,Yunnan Province,China and based on the theory of mineralogy and environmental geochemistry,the thermal transformations of minerals and the migrations of potentially hazardous trace elements(Be,F,V,Cr,Co,Ni,Cu,Zn,As,Se,Sr,Mo,Cd,Sb,Cs,Ba,Tl,Pb,Hg,Th,and U)during coal gasification were studied by coal petrology and coal quality methods,using proximate and ultimate analyses,X-ray fluorescence spectroscopy(XRF),X-ray diffraction(XRD),scanning electron microscopy combined with X-ray energy spectrum(SEM-EDX),field emission computer-controlled scanning electron microscopy combined with Feature single particle analysis module combined with X-ray energy spectrum(FE-CCSEM-Feature-EDX),and inductively coupled plasma mass spectrometry(ICP-MS).The leachability of the potentially hazardous trace elements in coal gasification residues and their potential environmental pollution were studied by using the column leaching procedure proposed by the US Environmental Protection Agency(EPA).The chemical properties of the GE gasification and GSP gasification feed coals were similar,but the distribution characteristics of the chemical compositions and the relationship between elements in the gasification residues showed that the properties of GSP gasification residues are more heterogeneous than that of the GE gasification residues.The ash compositions of the GE and GSP gasification residues were mainly controlled by the properties of the feed coals and was not affected by the gasification technologies.The micro images indicated that the coarse coal gasification residues were mainly made of irregular angular glass,containing spherical particles;while the fine coal gasification residues contained abundant spherical glass.Most of the mineral particles are amorphous glassy materials in coal ash,but to better discuss the regularity of mineral transformations during coal gasification,the CCSEM criteria for mineral definitions were updated based on the mineralogical theory and the parameters of the device.The major purpose was to reduce the fractions of unclassified phase in the feed coal and coal gasification residues.Generally,the GE gasification made the fractions of Na-Fe silicate and Mg-Fe silicate increase significantly and the fraction of Fe oxide decrease.The GSP gasification resulted in the significant increase of Mg-Fe silicate and Fe aluminosilicate,while the fraction of silica decreased significantly.Some quartz and clay minerals did not change in the high temperature and other minerals melted and decomposed.The glass formed when the molten slags were cooled rapidly and some new minerals formed at the same time.The analysis of mineral phase indicated that the mineral species forming in the GE gasification were more abundant than the GSP gasification.Fluorine,Zn,As,Se,Cd,Sb,Tl,and Pb were enriched in the fine residues.Except for lithophile element F which has low boiling point,other elements were chalcophile in coal,which made them be volatilized easily in the gasifiers and then captured by the fine residues in the syngas purification system.Hg was the most volatile element during the gasification and Cu and Zn were relatively volatile according to the calculation of relative enrichment facts of elements in the coal gasification residues.GE gasification seems to be prior to GSP gasification in capturing more potentially hazardous trace elements in the residues instead of releasing them into the gaseous phase.The CCSEM-Feature analysis indicated that the detected potentially hazardous trace elements in the coal gasification residues tend to be associated with Fe-bearing silicates or aluminosilicates.The concentrations of Sr in black waters were the highest among all the elements and its concentrations in the GE black water and GSP black water were 42,549 μg/L and26,984 μg/L,respectively.Also,the concentrations of other potentially hazardous trace elements in the black waters were higher than that in other coal gasification wastewaters.Most of the selected potentially hazardous trace elements in the coal gasification wastewaters can be reduced effectively by removing the particulate matter,but it did not work for Sr,Mo,Sb,and Cs because of their solubility.The leaching patterns of the potentially hazardous trace elements in the coal gasification residues can be divided into five types.In most cases,the leached concentrations of the potentially hazardous trace elements decreased continuously or increased first and then decreased with the increase of liquid-to-solid ratio in the column leaching procedure.However,the leached concentrations of V in the GSP coal gasification residues kept increasing in the entire leaching experiment.The CCSEM-Feature-EDX analysis of the coal gasification residues indicated that some of the studied potentially hazardous trace elements were associated with the detected mineral particles and there distributed in the particles of <2.5 μm in equivalent diameter.The potential health risks of the nanoscale acicular Si-rich particle should be paid attention.The environmental pollution of the coal gasification wastewaters and waste residues was evaluated by comparing the concentrations of the potentially hazardous trace elements to the related environmental quality standards.The potential environmental and health threatens of As,Se,Cd,and Sb in the coal gasification wastewaters deserve more attention than other elements.Arsenic,Se,Mo,Sb,and Tl in the coal gasification residues were mobile than other studied elements and easier to exceed their thresholds in the water quality standards.