The Partitioning And Transformation Behavior Of Hazardous Trace Elements During Coal Utilization

Coal is one of the leading energy sources in China and plays an important role in Chinese industry fields.During coal utilization,the release of toxic trace elements(e.g.As,Se,Pb,etc.)may threaten the environmental safety and human health,which have aroused the concern of many researchers.It is thus necessary to study the mobility and transformation behavior of toxic trace elements during coal utilization.In this study,a series of samples were collected from a coal washing plant,circulating fluidized power plants,stokers boiler power plant,and pulverized coal-fired power plants.The samples were analysed by optical microscope,inductively coupled plasma mass spectrometry,field emission scanning electron microscope coupled with energy dispersive analysis system of X-ray,X-ray fluorescence spectroscopy,atomic fluorescence spectroscopy,Fourier Transform Infrared Spectroscopy,X-ray photoelectron spectroscopy,and X-ray induced Auger electron spectroscopy.Combining the knowledge of trace element geochemistry,coal petrology,coal mineralogy,coal combustion and environmental chemistry,the distribustion and transformation behavior of coal macerals and hazardous trace elements during coal utilization were studied.The main results and conclusions are summarized as follows:(1)The re-distribution and modes of occurrences of minerals and trace metals during coal beneficiation by DMCs were clearly studied.The modes of occurrences of toxic elements were accurately determined;and their transformation behavior during heavy medium separation is largely controlled by clay minerals(V,Cr,Co,Sb,and Pb),carbonate minerals(Co and Pb),sulfide minerals(As,Cu,Ni,Cd,and Zn)and organic matters(V,Cr,Se,and Cu).(2)It was found that during CFB combustion,clay minerals,carbonates,and iron-sulfides phases in feed coal and limestone were mainly transformed into glassy materials,calcite,anhydrite,lime,and hematite in combustion residues.Functional groups alteration was identified during coal combustion.The gaseous ash particles occurred as irregular glass particles and as unburned carbon,where the typical irregular particles were mainly composed of Al-Si,Al-Si-Ca or Al-Si-Fe substances.Co and Ni displayed a slight change with the variation of the particle size while Cr,Cd,As,Se,and Sb showed increasing enrichment as particle size decreased.It was found that As and Sb,greatly enriched in the submicron ash particles,which was probably due to the heterogeneous condensation and adsorption mechanisms.(3).The results of XPS-XAES reveal that As was mainly presented as As(V)bonded to oxygen ligands,i.e.,the[AsO4]3-anion in the coal fly ash.Two chemical forms of Se can be found:elemental Se(62.0-83.3%)with binding energy of Se 3d5/2 at 56.5±0.2 eV and selenite(16.7-38%)with binding energy at 58.5±0.4 eV.Combining with the surface chemical analysis,the extreme enrichment of both arsenic and selenium is probably controlled by iron oxides in the coal fly ash.(4)The PC fly ash was dominated by glassy materials including cenospheres,pleiosheres,ferrospheres,solid spheres,and Fe-bearing spinel,while irregular glassy particles and unburned carbon were observed in the CFB ash.The results of SEM-EDS found that PC ash mainly consisted of<100-?m-diameter aluminosilicate spherical particles.In contrast,CFB ash was primarily comprised of fine irregular Al-Si-Ca-O agglomerates.The amount of oxidizable associations(F4)in fluidized ash was higher than in PC ash,which might be associated with the lower flame temperature in CFB boiler.(5)The environmental stability of toxic elements of solid wastes generated from coal industry was studied.Three groups were classified based on the volatile ratio(Vr)of toxic elements.Group 1 includes the highly volatile element Se;Group 2 contained elements As,Pb,Zn,Cd and Sb,and V,Cr,Co,Ni and Cu were placed into Group 3.The co-combustion expriment of coal slime and sewage slude found that the enhanced retention of the heavy metals may be attributed to the formation of stable solid non-volatile compounds via the vapour-solid reactions.Compared with fly ashes,the percentage of Cr(43.1-53.2%),Cu(79-87.8%),Zn(67.8-89.6%),and As(64.5-75.6%)as effective fraction was notably higher in FGD gypsums.The result of RACs further indicated that the environmental stability of gypsums was much weaker than that of fly ashes.