| Coal is the main primary energy,and there are abundant coal reserves in China.Coking for the purpose of producing coke is one of the important ways to utilize coal resources,which can produce a considerable amount of coal tar.The light aromatic hydrocarbons,such as benzene,toluene,ethylbenzene,xylene and naphthalene?BTEXN?are important chemical raw materials,but the content of BTEXN in coal tar is relatively low.To improve the BTEXN content in coal tar,the effect of HZSM-5 zeolites with SiO2/Al2O3 ratio of 50 on coal tar catalytic upgrading was studied in our previous work.Results showed that HZSM-5 zeolites can improve the yield of BTEXN.However,the catalytic upgrading effects of HZSM-5 zeolites with varying SiO2/Al2O3 ratios on different coal samples would be different.Therefore,it is meaningful to study the release behavior of BTEXN during coal pyrolysis over HZSM-5 zeolites with different SiO2/Al2O3 ratios and obtain the corresponding relationship between coal structure and HZSM-5 zeolites,which can provide a theoretical basis for the industrial application of uncondensed coal tar catalytic upgrading.In this study,lignite?Coal A?,gas-fat coal?Coal B?,coking coal?Coal C?and lean coal?Coal D?were used,and the pyrolysis volatiles were catalyzed by HZSM-5 zeolites with SiO2/Al2O3 ratios of 18,25,50,80 and 150.First,the release behavior of BTEXN during 4 coal pyrolysis under different temperatures and the effect of SiO2/Al2O3 ratio of HZSM-5 zeolites on the yield and components of BTEXN and phenols during coal pyrolysis were analyzed by Py-GC/MS online.Then,the effect of different HZSM-5 zeolites on the components and distribution of products generated from different coal-related model compounds were analyzed,and the formation pathways of BTEXN over HZSM-5 zeolite were further discussed.The main conclusions are listed as follows:?1?The total amount of BTEXN during Coal A and coal B pyrolysis at 600 °C is highest,however,the highest amount of BTEXN is produced by Coal C and Coal D pyrolysis at 700 °C.This is related to the degree of coalification.The bond energy of methylene and methyl enriched in lignite and gas-fat coal are low,thus BTEXN are formed at 600 °C.However,the structure of coking coal and lean coal are decomposed rapidly until at 700 °C as a result of their higher aromaticity.When pyrolysis temperature is higher than 800 °C,no BTEXN is detected.?2?The effect of HZSM-5 zeolites on the catalytic upgrading of uncondensed coal tar from coal pyrolysis is related to the acid sites and pore structure of zeolite and coal properties.HZ5-25 zeolite?SiO2/Al2O3 ratio of 25?with the higher Br?nsted and Lewis acid sites shows the best effect of catalytic upgrading of coal tar during Coal A and C pyrolysis.HZSM-5 zeolites with SiO2/Al2O3 ratio of 50 is suitbale for Coal B catalytic pyrolysis.And HZ5-18 zeolite?SiO2/Al2O3 ratio of 18?with the highest content of Br?nsted and Lewis acid sites is more suitable for Coal D pyrolysis.?3?HZSM-5 zeolites with different SiO2/Al2O3 ratios show different effects on the release characteristics of phenol compounds.HZSM-5 zeolites with lower content of acid sites promote the formation of phenols during Coal A and Coal B pyrolysis.However,HZSM-5 zeolites facilitate the decomposition of phenols during Coal C pyrolysis.?4?Anthracene,benzyl phenyl ether and petroleum ether represent the condensed aromatic ring,C-O bridge bond structure and aliphatic structure in coal,respectively,and BTEXN could be generated under the catalytic of HZSM-5.HZ5-25 zeolites with higher content of acid sites and higher specific surface area are prone to facilitate the formation of BTEXN during anthracene and benzyl phenyl ether pyrolysis.HZSM-5 with SiO2/Al2O3 ratio of 50 shows the best effect of petroleum ether pyrolysis,the yield of light aromatics increase by 41 times compared with that of petroleum ether pyrolysis.Results show that HZSM-5 zeolites have the effect of aromazation of aliphatic hydrocarbons and the effect of catalytic cracking of macromolecular fragments during coal pyrolysis. |
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