Study On Migration And Transformation Characteristics Of Heavy Metals During Coal Pyrolysis Upgrading Process

Coal contains a variety of heavy metals.In the process of coal thermal conversion,they will not only reduce the quality of liquid fuel,but also endanger human health and ecological environment.As the initial stage of thermal conversion process,pyrolysis is very important for combustion and gasification process.Pyrolysis upgrading is not only an important direction of coal utilization in the future,but also one of the important ways of clean and efficient utilization of coal.Therefore,the study on the migration and transformation characteristics of heavy metals in the pyrolysis upgrading process of coal is of great significance to accelerate the research on the preparation of high-quality fuel from coal and promote the clean and efficient utilization of coal in China.Great progress has been made in the research on the modes of occurrence of heavy metals in coal and their migration and transformation characteristics in the process of coal pyrolysis upgrading,but the method used to study the modes of occurrence of heavy metals in coal is not accurate enough and the research on the migration and transformation characteristics of heavy metals in the process of coal pyrolysis upgrading is not comprehensive.In view of the above shortcomings,the modes of occurrence of heavy metals in coal were studied more accurately by sequential chemical extraction method,and the organic-enriched and mineral-enriched components were prepared;The migration and transformation characteristics of heavy metals in the pyrolysis of raw coal,organic-enriched components and mineral-enriched components were studied.In this thesis,Ulanqab low rank coal（U-coal）in Inner Mongolia and Shanxi high rank coal（S-coal）were selected as experimental raw materials,and the heavy metals arsenic（As）,lead（Pb）,cadmium（Cd）,chromium（Cr）and mercury（Hg）were studied.The modes of occurrence of heavy metals in raw coal and chars were studied by sequential chemical extraction method;The migration characteristics of heavy metals in raw coal at 300～900℃were studied by using a fixed bed pyrolysis device,at the same time,the migration and transformation characteristics of heavy metals were analyzed by using the modes transformation,the minerals decomposition and the functional groups cracking behavior;the organic-enriched component（E1）,aluminosilicate-enriched component（E2）and disulfide-enriched component（E3）were prepared by sequential chemical extraction method,and the migration characteristics of heavy metals in E1,E2 and E3 pyrolysis at 300～900℃were studied,at the same time,the migration and transformation characteristics of heavy metals were analyzed by using the modes transformation behavior of heavy metals;Finally,the migration and transformation characteristics of heavy metals were studied by thermodynamic software.Based on the above experiments and simulations,the main conclusions are as follows:（1）Heavy metals were divided into water-soluble and ion-exchange mode（M1）,carbonate-bound mode（M2）,aluminosilicate-bound mode（M3）,disulfide-bound mode（M4）,organic-bound mode（M5）and insoluble mode（M6）by sequential chemical extraction.It is found that As was mainly M3 and M4,Pb was mainly M2,M3 and M5,Cd was mainly M2 and M5,Cr was mainly M5 and M4,and Hg was mainly M5,M4 and M3 in U-coal;In S-coal,As was mainly M3,Pb was mainly M3and M2,Cd was mainly M2,Cr was mainly M5 and M4,and Hg was mainly M4 and M5;The content of M5 in U-coal was higher than that in S-coal.（2）The migration and transformation characteristics of heavy metals during the pyrolysis upgrading of raw coal were analyzed by studying the modes transformation,mineral decomposition and functional group cracking behavior of heavy metals.It is found that the modes transformation ratio（Tr）and migration ratio（Mr）of heavy metals in the two coals were in good agreement at 300～900℃.The migration of As in U-coal was mainly produced by M4,M3 and M5 in the transformation process,and the migration of As in S-coal was mainly produced by M4 and M5 in the transformation process;the migration of Pb in U-coal was mainly produced by M2,M3 and M5 in the transformation process,and the migration of Pb in S-coal was mainly produced by M3,M2 and M5 in the transformation process;the migration of Cd in U-coal was mainly produced by M5,M2,M4 and M3 in the transformation process,and the migration of Cd in S-coal was mainly produced by M2,M4,M5 and M3 in the transformation process;the migration of Cr in the two coals was mainly produced by M5,M4 and M1in the transformation process;The migration of Hg in U-coal was mainly produced by M5,M4 and M3 in the transformation process,and the migration of Hg in S-coal was mainly produced by M4 and M5 in the transformation process.（3）E1,E2 and E3 were prepared by sequential chemical extraction method,and the migration characteristics of heavy metals were studied by pyrolysis of E1,E2 and E3;The migration and transformation characteristics of heavy metals were analyzed by using the transformation behavior of M5,M3 and M4 of heavy metals in chars.It is found that 300～600℃was the main migration temperature range of heavy metals in E1 pyrolysis process,and the migration behavior of heavy metals was in good agreement with its M5 transformation behavior;600～900℃was the main migration temperature range of heavy metals during E2 pyrolysis.The melting of aluminosilicate minerals inhibited the migration of heavy metals;500～700℃was the main migration temperature range of heavy metals in E3 pyrolysis process.The migration behavior of heavy metals was in good agreement with its M4 transformation behavior.（4）Fact Sage was used to simulate the migration and transformation behavior of heavy metals at 300～900℃.It is found that solid phase As₂S₂ decomposed at300～400℃,As in U-coal mainly migrated in the form of simple substance,and As in S-coal mainly migrated in the form of simple substance and its sulfide;Solid phase（Pb O）（Al₂O₃）₆ decomposed at 400～600℃,and Pb mainly migrated in the form of simple substance and its sulfide;Solid phase Cd S decomposed at 300～500℃,and Cd mainly migrated in the form of simple substance;Cr mainly migrated in the form of simple substance and its sulfide;Hg mainly migrated in the form of simple substance;As,Pb and Cr appeared molten oxide and sulfide at high temperature.