Interaction Between CO₂ And H₂O On Char Structure Evolution During Coal Char Gasification

Coal gasification is a process in which coal reacts with gasifying agent under high temperature to convert combustible components into syngas.In the commercial gasifiers,H₂O-Char and CO₂-Char reactions occurring simultaneously.In fixed bed gasifier,CO₂ was used to replace part of H₂O as co-gasifying agent in order to reduce the phenolic-containing wastewater.In fluidized bed and entrained-bed gasifiers,CO₂ was used to replace inert gas as the medium for transporting pulverized coal,which can increase the yield of CO and H₂ in syngas.Therefore,studying the coal char gasification with H₂O/CO₂ has important theoretical significance for the commercial gasification process.For the mechanism of coal char gasification with mixed H₂O/CO₂,the competition effect,independent effect,and synergistic effect were observed.The different effects are closely related to the catalysis of alkali and alkaline earth metals?AAEMs?in coal.The evolution of coal char structure influenced by the interactions between H₂O and CO₂ has important effect on the catalytic of AAEMs.Therefore,studying the char structural evolution during co-gasification and reveal the interaction between H₂O and CO₂ are of great significance to the understanding of the mechanisms different effects during co-gasification.In this study,a sub-bituminous coal?abbreviated as YN?and graphite were used as gasification feedstock.The alternating gasification was used to study the influence of H₂O and CO₂ on the char structure evolution during coal char gasification.The alternating gasification process is as following:taking sample CO₂-25%-H₂O-50%as an example,and it represents that CO₂ was used as a gasifying agent to react with the coal char to reach carbon conversion level of25%.Then,the pure Ar was used to purge until the mass of sample became constant.The same concentration of H₂O was switched until the carbon conversion reached 50%.The series of residual chars were characterized by Nitrogen adsorption,Scanning Electron Microscopy,Raman spectroscopy,and Infrared spectroscopy et al.The coal char and graphite structure evolution under different atmospheres and the interactions between H₂O and CO₂ were obtained.The conclusions are as following:?1?During the CO₂ gasification process:the reaction mainly occurs on the surface of coal char,and the pore structure with pore size less than 2 nm is generated in the residual char,the average pore diameter of residual char is 7%smaller than that of fresh char.The aromatic structure with rings between 3⁵were consumed and the rings more than 6 were opened,therefore,the type of oxygen-containing functional groups in coal char are more abundant.?2?During the H₂O gasification process:H₂O molecule is more prone to diffuse into the char matrix to react and the pore size is therefore enlarged.The presence of minerals has important influence on pore structure evolution during coal gasification process:for raw coal,mesopores are mainly produced in the residual char;for demineralized coal,micropores are mainly produced during gasification process.The aromatic structure with rings less than six in the char is preferentially consumed during H₂O gasification,and the aromaticity of the char is increased.?3?During the co-gasification process:the gasification reaction occurs on the outer surface of coal char and inside of pores simultaneously,the pore size is enlarged while new pores are generated.When synergistic effect occurs during co-gasification,the pores produced by CO₂ gasification provide more reaction surface area and active sites for H₂O gasification,and the pore size is further expanded in the residual char after H₂O gasification,which can facilitate CO₂molecule diffuse into the char matrix to react with char more easily.The aromatic structure with rings more than 6 was opened and the aromatic structure with rings is 3⁵ in the coal char is more abundant,which makes H₂O gasification reaction easier.When the competitive effect is observed during co-gasification process,CO₂ molecule will compete with H₂O molecule for same active site.?4?The graphite was used as a model compound to gasify with different gasification agents.The results show that during the CO₂ gasification process,C=C in the aromatic ring and–CH₂ disappear,while functional groups such as–CH₃,C–O,and–O–appear,indicating that CO₂ gasification can open the aromatic ring structure;H₂O gasification consumes the functional groups such as–O–and–CH₂,so=CH₂ and C–O appear.During the co-gasification process,the disappearing functional groups is mainly C=C in the aromatic ring,and=CH₂,–CH₃,C–O,and–O–appear,which makes the functional groups in the graphite are more abundant.