The Role Of Initial Moisture In Sewage Sludge During Catalytic Pyrolysis Via Reactions Decoupling

Sewage sludge,the by-product of sewage treatment,containing much water and other toxic substances,needs to be proposed properly.Catalytic pyrolysis of sewage sludge,with many advantages such as energy recovery and solidification of heavy metals,is a highly potential disposal method.Before pyrolysis,the sewage sludge always undergoes drying process,with consuming much energy and losing a lot of volatile in sludge.However,to keep a certain moisture content of sewage sludge is beneficial to catalytic pyrolysis,since the steam has positive effects on promoting the cracking of tars and energy can be saved for unnecessary drying.Thus,elucidating of the roles of initial moisture content in sewage sludge during its catalytic pyrolysis is a key consideration in optimization of the process.In this study,based on reactions decoulping,pyrolysis of sewage sludge?with or without Ni/?-Al₂O₃ catalysts?with different initial moisture content were conducted in a two-stage quartz reactor.The char,tar and gas produced were collected and the yields were calculated.The tar was characterized by gas chromatography-mass spectrography?GC-MS?,electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry?ESI FT ICR-MS?and UV-fluorescence spectroscopy?UV?to analyze the light components,heavy components and aromatics.The gas,char and catalysts were also characterized.The effect of moisture content on pyrolysis,reforming and nitrogen migration and evolution were researched,to throughly elucidate the roles of initial moisture content in sewage sludge during its catalytic pyrolysis.The results indicate that,at the pyrolysis stage,the increase of moisture slightly promote the release of organics in sludge.With moisture content increased from 1.2%to 31.5%,the average molecular weight in heavy components in tar increased while the contents of light components decreased,and the ratio of aliphatics in light components decreased while the aromatics increased,which might be because the increase of moisture promote the formation of highly reactive radicals,and the polymerization reaction and aromatization reaction were strengthened.With the moisture content continuedly increased to 50.6%,the average molecular weight in heavy components in tar decreased while the contents of light components increased,and the ratio of aliphatics in light components increased while the aromatics decreased.This may be because both the polymerization and the aromatization are usually achieved by a dehydration,and excessive moisture can suppress the dehydration reaction.At the reforming stage,the sludge with different moisture content was pyrolyzed,and the volatiles produced are reformed over 10%loaded Ni/?-Al₂O₃.With moisture content increased from 1.2%to 31.5%,the size of Ni lattice increased,and the agglomeration was obvious.But the aromatics in tar increased,and the content of light components increased,and the ratio of aliphatics in light components decreased while the aromatics increased.The hydrogen production significantly increased.Therefore,it can be inferred that moisture can directly react with volatiles,consuming heavy components into light components,and promoting aromatization of aliphatics.With the moisture content continuedly increased to50.6%,the size of Ni lattice decreased,and the agglomeration was suppressed.Catalytic reforming is significantly enhanced.The aromatics in tar decreased,and the ratio of aliphatics in light components increased while the aromatics decreased,and the content of light components increased,and the average molecular weight in heavy components in tar decreased.The content of N in sludge is normally high.During the thermochemical process,the N may be converted into harmful gases such as NO_x and N₂O.Therefore,the effect of moisture content on the migration and transformation of nitrogen during catalytic pyrolysis was studied.N was mainly present in char in the form of pyridine-N,pyrrole-N,protein-N and quaternary nitrogen.The increase of moisture content mainly promoted the release of pyridine-N.The N in the gas mainly existed in the form of N₂,which was derived from the catalytic conversion of NH₃ and HCN,and its content was closely related to the activity of the catalyst.N in tar was mainly in the form of nitrogen-containing heterocycles,nitriles and amides.With relatively low moisture content?1.2%to 31.5%?,the moisture was mainly involved in the reforming reaction.With relatively high moisture content?31.5%to 50.6%?,the catalytic cracking was dominant.And the two together worked to gradually migrate the nitrogen into the small molecules.