Study On Pressurized Pyrolysis Characteristics Of Two Lignites And Trace Element Volatility During Pyrolysis Process

China is rich in lignite resources,but the direct combustion and utilization of traditional lignite has problems such as low utilization efficiency,large pollution and high carbon emission coefficient.Under the background of "carbon peaking and carbon neutrality",it is imperative to develop new technologies for green utilization of lignite.Lignite has high volatile content and good reactivity,and is easy to transform into gas,liquid and solid products through pyrolysis.The products can be subsequently used in a better way to enhance the utilization value of lignite and reduce pollutants and carbon emissions.However,the lignite tar obtained by conventional pyrolysis has high oxygen content,many heavy components and low quality.The improvement of tar yield and quality through pyrolysis process is one of the important research directions.On the other hand,char produced by pyrolysis is often used for gasification and combustion,so the properties of char,including the content of trace elements in char,are also one of the topics that need to be concerned.Based on this,the pyrolysis characteristics of two kinds of lignite and the migration of trace elements during the pyrolysis of lignite coal were studied by means of pressurized fixed bed,focusing on the effects of pressure.The main research contents and results are as follows:(1)Slow pyrolysis experiment of lignite was carried out on the pressurized solid bed to investigate the release characteristics of the main gases(H2,CO,CO2 and CH4)and gaseous hydrocarbons(C2H6,C2H4,C3H8 and C3H6)from room temperature to 900℃)and their dependence of pressure(0.1 Mpa～1.0 Mpa).The results show that increasing the pressure is conducive to strengthening the secondary reaction of volatiles,resulting in the release peak of each gas being shifted to the low temperature direction.Under the pressure condition,the cumulative yield of CO2 and CH4 and other gaseous hydrocarbons increases,mainly because the pressure promotes the decarbonylation and decomposition reaction of tar,while the cumulative yield of CO and H2 is reduced,possibly because the pressure is conducive to the volume shrinking reaction,and the oxygen and hydrogen elements in the coal are more transferred to CO2 and gaseous hydrocarbons than to CO and H2.(2)The molecular composition of tar was identified by gas chromatography mass spectrometry(GC-MS),the typical compounds were quantitatively analyzed by flame ion detector gas chromatography(GC-FID)to investigate the effect of pressure on tar compositions of solid and liquid products.The tar deoxygenation behavior was studied through material and elemental balance.The results of the study show that the number and proportion of light hydrocarbon compounds in tar increase by pressurized pyrolysis,although the yield of BTX(benzene,toluene and xylene)decreases.The yield of phenolic compounds increases,but the proportion of oxygen transferred to tar decreases,indicating that the pressurized conditions are conducive to tar deoxygenation;pressurized pyrolysis also reduces the oxygen content in coke.(3)Trace elements in char obtained under different pyrolysis conditions(Li,Be,V,Cr,Mn,Co,Ni,Cu,Zn,Ga,As,Se,Sr,Cd and Pb)were quantitatively measured by means of microwave digestion instrument and inductively coupled plasma mass spectrometry.The study has found that in general,when the pyrolysis temperature increased from 800℃ to 900℃,the volatility of many trace elements such as Zn,Ga,As,Se and Pb are significantly improved.The volatility of Li,V,Zn,Ga,Pb increases by increasing pressure,but the volatility of Mn,Ni,Cu and Cd remains basically unchanged.