Study On The Pyrolysis Characteristic Of Low Rank Coals With Pretreatments

Low temperature pyrolysis is one of the main ways for clean and efficient conversion and grading utilization of low rank coals.At present,by applying the existing low-temperature moving bed pyrolysis technology,only lump coal can be pyrolyzed in small-scale,while pulverized coal accounting for 70%of the mining volume,can hardly be pyrolyzed.In fact,the key problems in the production process are often the macroscopic manifestations of micro scientific problems in engineering and technology.Therefore,it is of great significance to explore the pyrolysis mechanism of low rank coals.The pyrolysis characteristics of low rank coals are closely related to their compositions and structures.In this paper,the hydrothermal pretreatment,atmospheric thermal pretreatment and demineralization pretreatment were applied to artificially change the composition and structure of Shendong coal.Then,the influence of the changes on pyrolysis characteristics and product compositions and distributions of Shendong coal were investigated.The results aim to provide a theoretical basis for the study of the structure-activity relationship between the changes in low rank coals compositions and structures and the pyrolysis characteristics.Firstly,the effects of coal types,heating rates,reactor types and final pyrolysis temperatures on the pyrolysis characteristics and product distributions of low rank coals were investigated to optimize the pyrolysis process parameters.The results showed that with the increase of heating rates(10~900 ~oC/min),the final weight loss of low rank coals first increased and then decreased,up to the highest at the heating rate of 500 ~oC/min.Compared with the slow pyrolysis(at fixed bed reactor)of Shendong coal,the tar and gas yields increased,but pyrolysis water and char yields decreased during coal rapid pyrolysis(at fluidized bed reactor).With final pyrolysis temperatures(500~700 ~oC)during rapid pyrolysis of Shendong coal increased,the gas and water yields increased,but the char yield decreased;the tar yield increased first and then decreased,up to the highest at the final pyrolysis temperature of 600 ~oC.Secondly,the effects of different pretreatment methods on the composition and structure of Shendong coal and the distributions of pyrolysis products were investigated.The results indicated that the hydrothermal pretreatment was effective in the removal of the oxygen-containing functional groups and water-soluble inorganic metals for Shendong coal.Atmospheric(methane)thermal pretreatment presented higher removal rate of oxygen-containing functional groups for Shendong coal,and inhibited the fracture of aliphatic hydrocarbon side chains.Thermal pretreatments decreased the cross-linking degree of Shendong coal and increased the mesopores.Demineralization pretreatment was effective in the removal of inorganic minerals for Shendong coal,has little effect on coal organic matters,and lead to the collapse of pore structure in coal.During pyrolysis of treated coals,the pyrolysis tar and char yields increased,and the pyrolysis water and gas yields decreased.Additionally,the pyrolysis yield variation of each product was different for treated coals with different pretreatment methods.Then,the solvent extraction and column chromatography were applied to the separation of pyrolysis tars from Shendong coal and thermal treated coals,and the composition of the separated components was analyzed.The results showed that the yield,heavy component content,oxygen content of pyrolysis tar for thermal treated coals increased,and more oxygen was enriched in asphaltene components.Additionally,the compositions and contents of separation components for each tar sample were different.In addition,the structure and gasification reactivity of pyrolysis chars from Shendong coal and treated coals were investigated.The results showed that the microporous structure of pyrolysis chars from thermal treated coals was enriched,the graphitization degree of aromatic microcrystalline structure was lower,and the yield and reactivity of char gasification was enhanced.The char gasification yield and reactivity of hydrothermal treated coals presented the highest.In contrast,the micropores specific surface area of pyrolysis chars from demineralization treated coals slightly increased,but graphitization degree of aromatic microcrystalline structure was higher,and the inherent mineral catalytic effect disappeared,resulting in the decrease of char gasification yield and reactivity.Finally,the Shendong coal pyrolysis process parameters,the structures and compositions of Shendong and treated coals,the pyrolysis characteristics and product distributions,the composition and distribution of the gases product,the separation and composition of the liquid product,and the structure and gasification reactivity of the char were comprehensively evaluated and correlated to reveal the structure-activity relationship between the treated coals and their pyrolysis characteristics.