| With the transformation of China’s economic structure,clean coal technology is getting more and more attention.Coal gasification is not only the foundation of coal liquefaction,chemical production by syngas and multi-cogeneration system,but also the key technology of integrated gasification combined cycle(IGCC)system.Coal gasification is a total package reaction,including complex and diverse reaction processes.When coal enters a gasifier,it is first pyrolyzed to produce char and volatiles,which react with gasifier(steam and oxygen)to form gas components such as H2,CO,CO2 and hydrocarbon molecules.Understanding the reactivity of gasification process can provide ideas for further optimization of gasification technology.In oder to understand the reactivity of gasification,the intermediate products(char,gas,tar)of gasification need to be studied.Under mild conditions(low temperature,atmospheric pressure),the reaction rate of coal gasification is low,and there are many residual products in the gasification process,which is conducive to the research.And the research on the reaction of low temperature gasification can provide the foundation for the development of low temperature gasification technology.Therefore,the group of independent design of steam gasification test platform was used in this paper,under low temperature(<900℃),pulverized coal continuous feeding.To provide the necessary heat for gasification through partial/full combustion of coal,air or oxygen is needed to be pumped into the gasifier in the industry.Therefore,the effect of oxygen on the reactivity of gasification process was also studied.In this study,Shengli lignite was selected as the coal sample,which is not only rich in reserves,but also has high chemical activity and catalytic AAEM(alkali metals and alkali earth metals),suitable for mild gasification as the raw material.This paper studies the reaction mechanism of char-steam gasification,explores the formation path of tar and the influence of oxygen addition during gasification,and analyzes the mechanism of oxygen addition promoting the steam gasification of lignite,with the purpose of analyzing lignite gasification at low temperature.The product of gas(coal gas)-liquid(tar)-solid(char)were collected and characterized by TGA,BET,Raman,FT-IR,GC-MS,GC,MD-ICP-AES technologies.The material balance and the hydrogen balance were calculated.The main conclusions are as follows:(1)In 800℃ steam atmosphere,the splitting decomposition of ether group was the inherent reason of char activation,which could cut down the content of short chain or amorphous carbon between aromatic rings to prevent the graphitizing,and then improved the degree of defective aromatic structure.With the increasing of steam concentration(10%-25%),the char reactivity was reduced,because the regeneration of active sites was weakened by the increasing of ether group due to the growth of reaction"Ar,R-CO-Ar,R+2H2O→Ar,R-O-Ar,R+2H2+CO2“.With the increasing of steam concentration(25%-40%)continually,the char reactivity was slightly promoted,because the content of defective aromatic structure(3-5 rings)was increased due to the growth of reaction(Ar,R-CH=CH2+ H2O→ Ar,R-CO-CH3+H2)and reaction(Ar,R-+H-→ Ar,R-H).(2)Steam could promote char depolymerization to produce volatile aromatic molecules,involved in the formation of the gasification tar.It mainly improved the content of naphthalene structure of tar at 700℃,and mainly improved the content of phenanthrene and fluoranthene structure of tar at 800℃.The promotion of steam on the char depolymerization is related to hydrogenation of carbon structures by hydrogen free radicals.When the temperature is raised,the promotion of steam on the char depolymerization becomes stronger,and the increasement of average molecular weight of tar increases greatly,because the rate of reaction between steam and the main body of char aromatic structure(aromatic ring frame)increases.With the increasing of steam(0-25%),the char depolymerization increases steadily.With the increasing of steam(25%-40%),the effect of char reactivity on the char depolymerization is enhanced,because the reaction rate of steam and aromatic ring frame develops from steam-diffusion control to steam-adsorption control.(3)Oxygen addition could change the char structure and improve the char reactivity,thus promoting the gasification conversion and the yield of H2 and CO2.The activation of O2 on the char mainly includes two aspects.First,with the decomposition of aromatic nucleus by oxygen,the large π bond of aromatic rings was destroyed,and the new functional groups were generated,which could promote the reaction(C+H2O-H2+CO).Second,with the gasification going on,the large(≥6 rings)aromatic structure was depolymerized into the small(3-5 rings)aromatic structure,and oxygen atoms entered the aromatic nucleus,forming the defect position C-O-C,as a result,the defect degree of char microcrystalline structure was increased,the polymerization degree of char microcrystalline structure was decreased,and the reactivity of char were improved.(4)Oxygen addition could promote the char depolymerization and increase the average molecular weight of tar.At 700℃,it mainly improved the content of biphenyl,phenanthrene and fluoranthene structure of tar.At 800℃,it mainly improved the content of phenanthrene,fluoranthene and 4 rings structure of tar.The effect of char depolymerization on the composition of tar was enhanced by the increasing of oxygen concentration.The mechanism of O2 promoting the char depolymerization is to activate char,promote gasification reaction,and accelerate the dissolution of aromatic ring frame.At 800℃,with the increasing of steam(25%-40%),the control step of char depolymerization promoted by O2 is developed from from aromatic ring skeleton disintegration to aromatic molecule desorption.Oxygen addition could also promote volatile-H2O reaction,which is not conducive to the improvement of the average molecular weight of tar.With the increasing of temperature,or steam concentration,the promotion of oxygen addition on the volatile-H2O reaction was enhanced. |
