Study On Gasification Reaction Characteristic Of CO₂ Gasification Of "Tri High" Coal And Its Relationship With The Char Structure Evolution

Clean utilization of coal resources is an inevitable choice for the development of national economy and society.Coal gasification is a restrictive link of coal clean utilization technology such as IGCC,chemical looping combustion and coal gasification technology.Coal resources in southwestern China have the characteristics of high ash,high ash melting point and high sulfur content?commonly known as"tri-high"?,which lead to poor reactivity of coal,low coal conversion rate and less effective components of coal gas.In view of the above problems,this topic chooses three typical"tri-high"coals,and studies the coal pyrolysis and coke gasification processes in the coal gasification process respectively.The influence of temperature,particle size and ash content on the reaction characteristics of char-CO₂ gasification was investigated.Then the evolution of micro-structure in the process of coal gasification was studied.The research results can provide theoretical basis and technical support for the efficient and clean utilization of"tri-high"coal.Firstly,homogeneous model,unreacted shrinkage reaction core model and random pore model were used to describe the gasification reaction of#1 coal char-CO₂.The results show that the random pore model has the best fitting effect on the gasification reaction of 1 coal char-CO₂.Then the random pore model is used to describe the gasification reaction of#2 and#3 high ash coals.The results show that the random pore model can not describe the trend of#2 and#3 coal gasification reaction rate increasing first and then decreasing later.It can describe the first increase of#2and#3 deashing coal gasification reaction rate.After the trend of decline.Therefore,based on the random pore model,ash is introduced as a limiting parameter for char-CO₂ gasification reaction.Experiments show that the modified random pore model can well describe the char-CO₂ gasification process of#2 and#3 coal at various temperatures.Then,the evolution of physicochemical structure of coal char at different temperatures is studied.The microstructure of coal char after pyrolysis is characterized by SEM,Raman,FTIR and XRD.The results show that the degree of coal coalification decreases slightly after coal pyrolysis at 950?,but with the increase of pyrolysis temperature,the degree of coal coalification decreases continuously,the microcrystalline structure develops into disorder and the degree of defects increases;after pyrolysis temperature reaches 1000?,the change of coal microcrystalline structure is small with the increase of pyrolysis temperature.At the same time,the influence of coal pyrolysis particle size on its specific surface area and pore size distribution is studied.The results show that with the increase of coke particle size,the specific surface area and pore volume decrease,and the average pore size increases,which indicates that the pyrolysis of coal with smaller particle size is beneficial to the formation of smaller mesoporous and the increase of specific surface area.This may be due to the faster volatile overflow of smaller coal particles during pyrolysis,resulting in more pore structure and smaller pore size of coke particles.It can be inferred from nitrogen adsorption desorption curve and differential integral pore volume pore size distribution that smaller coke pore size is conducive to the formation of micropore and the growth of smaller mesopore.Finally,the influence of ash on the reaction characteristics of coal gasification is studied.The microstructure of raw coal,raw char,ash free coal and ash free char were characterized by SEM,BET,Raman,FTIR and XRD.The results show that the content of graphite structure in#3 coal is relatively low.Deashing treatment can promote the formation of aromatic functional groups with macromolecule defect structure and the growth of graphite lamellar structure,which is not conducive to the development of pore structure of coal char.At this time,chemical reaction is the limiting link in the gasification process.More active components of coal char after deashing are exposed to the surface of coke particles,which is conducive to the gasification reaction.The content of graphite structure in#2 coal is higher,and more carbon structure are exposed to the surface after deashing treatment,but the reactivity of graphite itself is very low,and ash has a certain promoting effect on gasification reaction,so ash removal has little influence on#2 coal gasification reaction.