Study On Single-mode Microwave Pyrolysis Of Yunnan Lignite With Binary Catalysts For The Production Of Cycloalkanes And Light Aromatics

China is rich in coal resources.Coal pyrolysis is an important way to realize the efficient utilization of coal resources and obtain a variety of high valuable raw materials.Coal tar,the main by-product of coal pyrolysis,contains a large amounts of aromatic compounds and aliphatic hydrocarbons.The light aromatic hydrocarbons have significant applications in many fields,such as pesticides,synthetic rubber,medicine and so on.The cycloalkanes in aliphatic hydrocarbons can obtain aviation fuel through refining due to the chemical heat sink caused by their dehydrogenation reaction.Therefore,increasing the content of cycloalkanes and light aromatics in tar will greatly enhance the application value and economic value of tar and fill the vacancy of oil resources.Yunnan lignite is characterized by low condensation degree of aromatic hydrocarbons in coal molecule,high volatile content,high chemical reaction activity and H/C ratio for short coal forming age.Using microwave heating technology which is of selectivity,high efficiency,cleanliness and uniformity,to conduct microwave pyrolysis of Yunnan lignite to obtain tar products with high content of cycloalkanes and light aromatics is an effective way to alleviate the shortage of oil resources and realize the efficient and clean utilization of lignite resources.The results of industrial analysis,elemental analysis,FT-IR,XRD and TG-FTIR analysis of Yunnan lignite verify that Yunnan lignite is typical young lignite.In addition,in the conventional pyrolysis of Yunnan lignite at 600℃,the yield of tar reaches 10.36%,in which the contents of cycloalkanes and light aromatics reach 8.82%and 20.38%respectively.It is proved that Yunnan lignite is an ideal raw material for obtaining tar rich in cycloalkanes and light aromatics through pyrolysis.At the same time,the transition metal Fe,Ni and Mo,which can promote the production of cyclic alkanes and light aromatics in tar,were selected.By means of sol-gel method,two dimensional catalysts which are of three dimensional network structure,with great ability to absorb electromagnetic wave and chemical catalytic activity,Ni Fe₂O₄,Ni Mo O₄ and Fe₂（Mo O₄）₃,were synthesized.Through the catalytic pyrolysis of Yunnan lignite with binary catalyst,it is found that Ni Fe₂O₄ has better debranching effect on aliphatic hydrocarbons in tar and promotes the cyclization of small molecular alkanes,and has good chemical catalytic activity for the formation of cycloalkanes and light aromatics,which increases the yields of cycloalkanes and light aromatics by 10.82%and 41.68%respectively.When the addition amount of Ni Fe₂O₄ is 7%,the catalytic effect is further strengthened,The content of cycloalkanes and light aromatics in tar reached 10.16%and 33.29%respectively,and the yield increased by 12.86%and 60.03%compared with that without catalyst.In the microwave pyrolysis of Yunnan lignite,a single-mode microwave reaction chamber and precision solid-state microwave source were used to explore the effect of microwave electric field intensity on the formation of cycloalkanes and light aromatics and the microwave pyrolysis process from the perspective of electric field distribution.The results show that microwave electric field is the fundamental factor affecting the physical conditions of microwave pyrolysis reaction,rather than microwave power.When the electric field intensity is 2.89×10⁴ V/m,it is more conducive to the formation of cycloalkanes and light aromatics,and the total yield is287%higher than that of conventional pyrolysis at the same temperature.The change of dielectric constant of coal samples has an impact on the electric field distribution and microwave pyrolysis process.By adding binary catalysts Ni Fe₂O₄,Ni Mo O₄and Fe₂（Mo O₄）₃with high dielectric constant,the ways to improve the microwave absorption capacity of coal samples are explored when the microwave electric field intensity provided by microwave source is constant.so as to improve the heating rate and final pyrolysis temperature of microwave pyrolysis of Yunnan ligniteCompared with conventional isothermal pyrolysis,it is further explored that the coupling catalytic effect between binary catalyst with high catalytic activity and microwave electric field prominently improves the content of cycloalkanes and light aromatics.5%addition of Ni Fe₂O₄ increases yield of cycloalkanes and light aromatics by 48.60%and 98.21%respectively.When the addition amount of Ni Fe₂O₄ is 5%,the yields of cycloalkanes and light aromatics are increased by 48.60%and 98.21%respectively compared with that without catalytic addition.On this basis,the interaction coupling between lignite structural parameters,dielectric constant of lignite,microwave electric field intensity and the formation of cycloalkanes and light aromatics is explored,and the synergistic catalytic effect of microwave electric field and binary metals on the formation of cycloalkanes and light aromatics is revealed.It provides a theoretical basis for microwave catalytic pyrolysis of lignite to produce tar with high content of cycloalkanes and light aromatics for industrial production and aerospace fuel.