Migration And Transformation Behaviors And Its Influencing Mechanism Of Harmful Trace Elements During Pressurized Fixed-bed Coal Gasification

With the rapid growth of the large-scale application of pressurized fixed-bed gasification in the coal-to-methane industry,the diffusion,migration,and transformation mechanism of harmful trace elements and their controlling factors under specific gasification conditions were revealed,which is considered to have important theoretical significance for establishing methods to reduce the pollution of harmful elements in coal gasification process,thereby realizing the clean conversion of coal.In this study,three typical coal samples(i.e.,WLCB,WLTG and SL lignite from Inner Mongolia)with remarkablely enriched harmful trace elements were used to carry out pressurized gasification in fixed-bed reactors under different scales.The migration and transformation behaviors of specific elements during gasification were systematically and comprehensively investigated by simulation and experimental methods.The migration and transformation of harmful trace elements in coal under the specific conditions of pressurized fixed-bed gasification and its influencing factors were eluciated,and the distribution of harmful elements in solid,liquid,and gas products was analyzed.On this basis,the condensation/deposition and adsorption characteristics of trace elements in gasifier were figured out,and their chemical conversion mechanism in the gas cooling process was discussed,which provided theoretical support for the development of coordinated purification method to control the harmful trace elements.In view of large-scale industrial gasification process,the evolutionary behavior of harmful trace elements was discussed in details.Additionally,the dissolving stability of harmful elements in industrial gasification ash/slag and its influencing factors were examined.The conclusions are summarized as follows:1.The content,distribution,and mineralogy characteristics of trace elements in three typical lignite samples were analyzed by digestion,XRD,and SEM-EDX.The initial occurrence form of typical trace elements in organic matter and inner/external minerals of coal was investigated.The possible enrichment mechanism of such trace elements in coal was analyzed according to their physical and chemical aggregation state.It was found that the contents of F,Hg,As,and Be in WLCB and WLTG lignite are significantly higher than the average value in Chinese coal,and the Pb content in SL lignite is higher than the average lead content in Chinese coal.The minerals in the three lignites are similar,mainly containing quartz,plagioclase,pyrite,kaolinite and white mica.A large number of mineral particles(<10 ?m)are randomly distributed in coal.Such fine particles are inhomogeneous and relatively discrete in coal.Trace elements are not completely combined with the main minerals and can also be dispersed in organic matter.Element F with relatively high abundance in coal is in a variety of forms:it tends to bond with alkaline elements but may also be adsorbed in layered minerals.The Pb was found to regularly co-exist with pyrite.2.The effects of operational conditions,i.e.,pyrolysis temperature,gasification temperature,and gasification pressure,on the transformation and evolutionary behaviors of hazardous species were systematically studied in this work.Furthermore,the influences of sulfur content and ash fusion characteristics on the transformation of harmful trace elements in coal gasification process was also discussed.Results show that the Hg and Se in coal feature high volatility during pressurized pyrolysis and the volatilization rate reached 92.1%and 75.3%,respectively,at the temperature of 700?.The evaporation of As and Cd would increase sharply when the pyrolysis temperature exceeds 400?,while the F,Pb,U and Be exhibit lower volatilization rates.The volatility of the hazardous elements(As,Pb,U,and Be)is medium at the gasification pressure of 3 MPa.As the gasification temperature increased from 800 to 1100?,the volatilization rate of Pb increased from 44.6%to 87.5%,and the volatility level was promoted from medium to high.Conversely,the release tendency of As,U,and Be showed the opposite trend with the volatilization rate reduced from 77.1%,53.6%,61.7%to 42.9%,30.9%,32.6%,respectively.Thus,the volatility of As decreases from high to low with increasing temperature,and U and Be become difficult to volatilized.During the heat treatment in the experiments,more than 80%of the F,Hg and Se were released.Moreover,increasing the gasification pressure would essentially suppress the volatilization of hazardous species.The volatility of As was significantly restrained under high pressure,causing the accumulation of As in the gasification ash.On the other hand,the sulfur in coal would facilitate the release and transformation of trace hazardous species.The increase in sulfur content would notably increase the formation of arsenic and lead sulfide in the gas phase.The composition and melting characteristics of minerals in coal also play an important role in the evolution behaviors of hazardous species.Specifically,the Ca in coal would restain the volatilization of As,and the molten minerals could adhere to and then stabilize trace hazardous elements,causing a lower emission of hazardous elements into gas phase.For instance,the release rate of Be decreased significantly near the ash melting point.3.The content of harmful elements in the gasification products was analyzed,and the mass balance before and after reaction was calculated.Meanwhile,the effects of reaction temperature and pressure on the distribution behavior of harmful elements,i.e.,F,Hg,As,Se,Cd,Pb,U and Be,in the three-phase products were investigated based on the molar fraction.It was found that under different gasification temperatures and pressures,the mass balance of harmful elements added up to be 74-123%,which was within the acceptable error range of ±30%.After pressurized gasification,the harmful elements As,Pb,U,and Be in coal tend to be enriched in the form of arsenate,lead oxide,beryllium aluminate,and uranium oxide in the gasification residue.Hg is easily released into the atmosphere and still exists as Hg(g)even at the temperature below 100?.A large number of harmful compounds,such as hydrogen fluoride,hydrogen selenide,elemental cadmium,lead sulfide,and a small number of compounds containing As,Be and U can be enriched in the condensate.With the increase of gasification reaction temperature,the residual harmful elements As,U and Be in ash gradually increased,while Pb was slightly increased.As the gasification reaction pressure was raised,the mass fraction of As and Pb in ash increased from 28%and 6%to 68%and 14%,respectively.The gasification temperature and pressure bring about less effect on the redistribution of harmful elements F,Hg,Se and Cd during pressurized fixed-bed gasification.4.During coal thermal conversion,trace elements in coal would undergo a series of physico-chemical changes,including volatilization,melting,crystallization,and deposition.In order to investigate the migration and transformation of harmful elements in the pressurized fixed-bed gasifier,the WLTG coal with abnormally high sulfur content and enriched harmful elements Hg and As was selected.The condensation and deposition of harmful trace elements and the adsorption behavior of char in the gasifier were investigated.The results show that the harmful element F in the high-temperature gas can be condensed and deposited at the temperature lower than 800?.The main compound of the deposition was CaF2,which also contained a small amount of other alkali metal fluoride.The Pb in the gas phase condensed and deposited on the NaCl crystal as Pbcl2 or on the surface of the silicate mineral system in the form of metallic Pb at the temperature lower than 700?.When the gas temperature was lower than 600?,gaseous Hg could be deposited on the sampling plate in the form of HgS or loaded on the mineral system of Na-Si-Al-O.The harmful element As in the gas phase was easy to condense and deposit during the cooling of syngas.The deposition temperature was in the range of 600-800?,and the deposited species were Al-O,Fe-O,Fe-Al-Si-O,As-S,etc.The char prepared at the temperature range of 300-700?displayed a significant adsorption effect on F and Be,and the adsorption efficiency was close to 1.Similarly,the low temperature char had a certain adsorption effect on the As and Cd in high temperature gas,with adsorption efficiency up to 0.31 and 0.55,respectively.The adsorption of Hg,Pb,Se and U by char was not pronounced.The volatile trace elements forme aerogels and condense with the gas to form fine particles,and the condensed trace elements are mainly attached to the ash particles.Alkali metals(such as Na)are easily combined with acidic trace elements(such as F)to form salt particles,which further promote the formation of particulate matter.5.The particle size distribution,residual carbon content,and mineral composition of slag from industrial gasifiation plant were characterized.The migration of harmful elements in an industial coal-to-methane gasification plant was studied,and the leaching characteristics of harmful elements in ash/slag was further explored.The results show that the ash from industrial gasification plant is mainly composed of fine particles smaller than 9 mm,and the ash/slag with particle size less than 3 mm accounts for more than 50%of the total fine ash particles.The maximum content of residue carbon in ash was 3.52%,and ash with smaller particle size possessed higher residue carbon content.The main mineral phases in ash are quartz,pyroxene,anorthite,hematite and acacia.In the industrial gasification ash,the harmful element As is most unstable and very easy to be leached outwith the dissolution rate beinghigher than 83%.The dissolution rate of the harmful element Cd reached 54.05%under the action of leaching solution with a pH value of 3.20.The dissolution rates of harmful elements Cr and Ba were between 6%and 15%.The influence of the pH value of leaching liquid differs for the dissolution of various harmful elements.The dissolution rates of the harmful elements As,Cd,Ni,and Ba gradually increase with the increase of the pH value of the leaching solution,while the opposite is true for Cr.The dissolution rates of the harmful elements Pb,Zn,Be,and Cu are not sensitive to the pH value of leaching liquid.The concentration of the harmful elements such as As,Cd,Be,and Cr in the leachate decreased while that of Pb increased with increasing ash particle size.