Study On The Conversion Characteristics Of Char Structure And Typical Tar Components During Thermochemical Utilization Of Zhundong Coal In Molten Salt

Molten salt pyrolysis/gasification technology offers an efficient and green way for thermochemical conversion of solid fuels.However,the reaction mechanism and key factors of such technology?such as conversion characteristics of carbon structure and tar components?are not clear so far,which hinders its further utilization.Accordingly,the present study investigated the molten salt CO₂ gasification features of a typical Zhundong coal in ternary eutectic carbonates?Li₂CO₃-Na₂CO₃-K₂CO₃?,which were commonly adopted in molten salt thermochemical conversion.And the fate of carbon in molten carbonates was examined.Furthermore,the thermal cracking and CO₂ reforming behavior of typical tar components,such as toluene and furan,was researched.The main conclusions were discussed below:In molten carbonates surroudings,the carbon conversion of Zhundong coal was influenced by feeding modes,CO₂ concentration and thermochemical conversion time.Altering feeding form from tablet coal to pulverized coal would enlarge the contact area of coal,molten salt and gasifying agent,consequently promoting the devolatilization and char gasification process of coal.Raising the CO₂ partial pressure could enhance the reforming reactions of macromolecular compounds released during devolatilization stage.Besides,the degree of polycondensation/graphitization of char in carbonate salts was higher than that in conventional pyrolysis.In addition,with time increaseing,O-containing groups,such carbonyl groups,would be formed in the char with carbonates acting as oxygen supplier.But the carbonyl inside the char is difficult to decompose due to the mass transfer resistance of CO and CO₂.Moreover,the present study investigated the characteristics of pyrolysis and CO₂reforming of toluene,which was one of the typical tar components.The molten carbonate salts altered the thermochemical conversion pathway of toluene,and its thermal cracking for the gaseous product was inhibited,probably thourough CO₃^2-and CO₂ consuming the H radicals.The fracture of C—H bond on the benzyl groups might be enhanced under the catalysis of alkali metal cations.And subsequently formed dibenzyl,diphenyl and polycyclic aromatic hydrocarbons?PAHs?.Besides,alcohols and carboxylic acids in the tar could provide H and OH radicals to induce the ring-opening reaction.Hence,the tendency towards PAH formation in carbonate salts was suppressed.Similar to toluene,the gas selectivity of furan would be inhibited by both molten salt and CO₂.However,the soot formation during thermochemical conversion of furan was reduced in ternary alkali metal carbonates.At the same time,the alkali metal would catalyze CO₂ gasification of soot.And part of the soot could be directly gasified by the molten salts,resulting in more defects and larger CO production.Due to the well thermophysical properties and the catalysis of molten carbonate salts,the preliminary cracking of C—O bond of furan might be promoted.Afterwards,the aromatization process of the generated fragments to form aromatic compounds would be enhanced.