Catalytic Depolymerization Mechanism Of Yunnan Lignite Over Iron-based Catalyst

Coal is the major primary energy in China for its huge reserves.According to Chinese Academy of Engineering,China will still have the highest coal share in primary energy at 50%until 2030.Low rank coals(LRCs),accounting for 50%of the total reserves,are rich in high value volatiles.Thusly,special interest has been taken in a coal conversion technology that can convert high-value volatiles in coal to the scarce liquid and gaseous fuels.LRCs are mainly composed of 1-3ring aromatic clusters as the basic structural unit,and contain a small amount of nitrogen,sulfur and oxygen heteroatom structures,which are linked together by different types of bridge bonds.It is expected that clean and efficient utilization of lignite can be realized by breaking bridge bonds in coal under mild conditions and obtaining compounds with rich aromatic structure.Energy revolution is not to ban the use of coal,but to ban the backward coal unilization methods.Catalysis is the basis of modern chemical industry.More than 90% of the world's organic chemical industry is the catalytic process.Catalyst is the core of catalytic process,which has ability of lowering the activation energy,speeding up the reaction effect,facilitating some chemical bonds cracking selectively and making the enrichment of some special compounds in the products,so as to lay a foundation for subsequent chemical process.Our research group proposed the concept of“catalytic depolymerization”,and conducted in-depth research on the influence of different factors such as the type,amount and adding method of catalyst on the depolymerization products,but there is still a lack of systematic research on the mechanism of catalytic depolymerization.This paper will conduct a systematic study on the mechanism of catalytic depolymerization of LRCs through the following four aspects:(1)The distribution of catalyst in coal particles was analyzed,and the change of catalyst morphology was studied with the increase of temperature.The catalytic depolymerization mechanism was inferred by the change of product distribution and char structure with the catalyst addition.(2)Study on the product distribution and thermal behavior of coal,extract and extracted residue with or without FeCl₃,so as to get the effect of small molecular compounds on the catalytic depolymerization of lignite.The effects of catalysts on the different chemical bonds were also studied.(3)Char is the major product obtained from the pyrolysis of lignite at 600?,which can reflect the structure and property of raw coal partially.Compared with raw coal,the char becomes more regular and structured.Therefore,the catalytic mechanism can be more accurately concluded by studying the change of liquid product distribution and solid product structure using coal char as model compounds.(4)Based on the above experimental results,the catalytic depolymerization of aromatic ether bond and aliphatic chain in coal was studied by using diphenyl ether and ethylbenzene as simple model compounds.(5)In addition to the above research on the mechanism of catalytic depolymerization,this paper also makes an in-depth study on the method of rapid and accurate analysis of the whole component of coal tar by using the comprehensive two-dimensional gas chromatography mass spectrometry(GC×GC-MS).The major conclusions can be drawn as follows:1.The retention time and mass spectral information of the same chemical were similar in the total ion chromatogram of GC×GC-MS.Based on this feature,the computer language was edited to realize the rapid detection of aliphatic hydrocarbons,benzene series,naphthalene series,phenolic compounds and polyaromatic hydrocarbons in tar.The relative content of different groups of compounds in tar samples can be quickly and accurately compared by using the template function provided by GC image software.2.The SEM-EDS characterization indicates that the catalyst can enter into the coal particle by the method of adding the catalyst invented by our research group.In the coal structure,the catalytic components of cation and anion are dispersed,and Fe³⁺is located in the aromatic ether bonds in the coal,which promotes the aromatic ether bridge bond in the coal to break under mild conditions,thusly improving the tar yield of raw coal,which reflects the catalytic cracking characteristics of Fe catalyst.3.With the temperature increasing,the active component Fe catalyst experienced a change from Fe³⁺(initial temperature)?FeOOH(250?)?Fe₂O₃/Fe₃O₄(350?)?Fe₃O₄(450?)?Fe₂O₃/FeC/Fe(550?).After adding catalyst,the weight loss of raw coal increased obviously in the temperature range of 350-450?,indicating that Fe₃O₄ was the active component in the catalyst.4.By comparing the element analysis of semi-coke and tar obtained by with or without catalyst,it can be seen that the saturation of char increases and the saturation of tar decreases after adding catalyst,indicating that can activate and transfer H to form a chemical bond between metal and hydrogen atom,and then regulate H transfer during pyrolysis,which reflects the characteristics of catalytic hydrogenation5.By studying the thermal behavior and product distribution of raw coal,extract and extracted residue coal with/without Fe-based catalyst,it can be concluded that small molecular compounds of coal play the role of hydrogen donor in the pyrolysis process.With the catalyst addition,the content of phenolic compounds and naphthalene series in raw coal and extracted residue tar changed contrarily.The yield of phenolic compounds from raw coal tar increased,while the yield of phenolic compounds from residue decreased and the naphthalene series increased.6.The catalytic depolymerization of THF extract showed that Fe-based catalyst can promote the fracture of aliphatic hydrocarbon,and CH₄and H₂ can be generated in the temperature range of 100-150?,while the initial generation temperature of CH₄and H₂ is 300?without the catalyst addition.Fe catalyst can promote the fracture of carboxyl group in coal,so a large amount of CO₂ is produced at 100?.7.Catalytic depolymerization of char indicates that the anions in the catalyst may react with the resulting liquid product(tar),and the nitrified or halogenated phenols can be obtained depending on the anions in the catalyst during the catalytic depolymerization process.The anions in the catalyst can react with the liquid product during the catalytic depolymerization process.The nitrified or halogenated phenols can be obtained depending on the anions in the catalyst.C-Cl bond was formed in the char obtained from the catalytic depolymerization process with FeCl₃ addition.It can be concluded that the anion component in the catalyst can be combined with the liquid and solid products in the catalytic depolymerization process.