The Study On Mechanism Of Coke Deposition On Modified Nickel-based Catalyst During CO₂ Reforming Of Pyrolysis Tar

As a new conversion technology for energy use of solid feedstock,gas production via high temperature pyrolysis technology has become an important research direction in the resource utilization of municipal solid waste.However,the coking and utilization of heavy component limit the application and promotion of gas production via high temperature pyrolysis technology.The catalytic CO₂ reforming of tar technology can convert tar and CO₂into H₂ and CO,which can increase the yield of syngas and remove tar and CO₂.Nowaday,catalytic CO₂ reforming of tar technology is still in a initial stage.The design of catalysts with excellent performance has important practical significance for improving tar conversion and catalyst service life.This work investigated the effect of preparation process conditions on the physical-chemical properties of bio-char support and the active site load of nickel-based catalyst.Toluene was used as a model compound of pyrolysis tar in CO₂ reforming of tar experiments to explore the coke characteristics on catalyst surface.Based on the results,metal doping and core-shell structure were applied to modified the nickel-based catalysts.The filamentous coke resistance mechanisms of the modified nickel-based catalysts during CO₂reforming of tar were investigated.The specific research contents is as follows:（1）Sweet potato waste was applied as feedstock to perform hydrothermal carbonization experiment.The effect of reaction temperature on yield and physical-chemical characteristics was investigated.The results showed that the decomposition of organic components with low-thermal stability in sweet potato waste was promoted as the reaction temperature increased from 200℃ to 300℃,resulting in the increase of bio-char yield from 63.89%to50.68%.In additions,the increase of reaction temperature promotes the dehydration reaction,decarboxylation reaction,polymerization reaction and aromatic reaction.The decline of volatile matter promoted the thermal stability of bio-char.The decomposition of organic components led to the rough surfaces of the bio-char.Thus,the specific surface area of bio-char increased from 142.29 m²/g to 284.51 m²/g,contributing to the subsequent active metal loading in catalyst preparation process.（2）Bio-char was applied as spport of nickel-based catalyst to synthesize Ni/bio-char catalysts via the impregnation method and a one-pot hydrothermal carbonization method.The effect of synthesis method on the active site loading and the physical-chemical characteristics of Ni/bio-char was explored.The results showed that more nickel particles enabled in pore structures,resulting in the specific surface area of catalyst form one-pot hydrothermal hydrothermal carbonization method（67.79-117.29 m²/g）lower than the specific surface area of catalyst form the impregnation method（113.48-196.89m²/g）.However,the high temperature and high pressure promoted the loading of the nickel precursor into bio-char,leading to the nickel content of catalyst form one-pot hydrothermal hydrothermal carbonization method（0.07-0.45g/g）higher than the nickel content of catalyst form the impregnation method（0.04-0.26g/g）.Besides,the Ni/bio-char catalyst form one-pot hydrothermal hydrothermal carbonization method exhibited higher H₂ yield（88-232 m L/g）and CO yield（92-323 m L/g）in tar reforming process.（3）The Ni/bio-char catalysts were prepared by one-pot hydrothermal carbonization synthesis method under different synthesis temperature.The effects of hydrothermal synthesis temperatures on the active site loading and the physical-chemical characteristics of Ni/bio-char catalysts was analyzed.The results showed that appropriately improved the hydrothermal synthesis temperature can promote the formation of cracks and pores on bio-char surface.And the loading of the nickel particles was promoted,which increased the content of metal nickel and the specific surface area of Ni/bio-char.As the hydrothermal synthesis temperatures further improved,the higher reaction temperature causes more metal nickel particles to be oxidized into oxide nickel.The nickel oxide clogging on surface can cause the decrease of surface area and pore volume of Ni/bio-char catalyst,which leading to the decrease of reaction activity of Ni/bio-char catalyst.Therefore,the H₂ yield decreased from 392-502 m L/g to 125-270 m L/g,and the CO yield declined from 1366-1571 m L/g to226-794 m L/g.（4）Water steam and CO₂ were applied as the reforming agent in the tar catalytic reforming experiments.The effect of different reforming agent on tar conversion rate and product selectivity was explored.The catalyst before and after the reaction was characterized to investigate the coke evolution characteristics on catalyst surface during CO₂ reforming of tar.The results show that the addition of CO₂ introduced carbon source,which can resulted in higher CO content in gas products thatn steam reforming process.The higher content of CO can promote the Boudouard reaction,which convered CO to filamentous coke deposited on catalyst surface.Therefore,more filamentous structures occurred on catalyst surface and a higher content of filamentous coke deposited on Ni/bio-char（0.57 g/g）during CO₂ reforming of tar.The hydroge conversion decreased from 51%to 40%,and carbon conversion declined from 47%to 40%.（5）Based on the deactivation problem caused by filamentous coke deposited on catalysts during CO₂ reforming of tar,the iron doping was applied to prepare nickel-iron bimetallic catalysts.The effect of iron doping on catalytic activity and filamentous coke resistance of nickel-based catalysts was explored.The results show that the iron site in the Ni-Fe bimetallic catalyst can be converted to FeO_X under the CO₂ atmosphere,which can supply lattice oxygen to promote the oxidation of filamentous coke.Compared with monometallic Ni/bio-char catalysts,bimetallic Ni-Fe/bio-char catalysts exhibited higher lattice oxygen coverage on surface,effectively inhibiting the formation of filamentous coke.Therefore,lower content of filamentous coke deposited on bimetallic Ni-Fe/bio-char catalysts after reaction（0.21 g/g）.The hydrogen conversion rate and carbon conversion rate of bimetallic Ni-Fe/bio-char catalysts can maintain at 49-56%and 54-63%,respectively,reflecting better catalytic performance and stability.（6）Different sizes of nickel oxide particles were applied as raw material to synthesize core-shell nickel-based catalyst Ni@SiO₂.The effect of active nickel particles size on catalytic activity and filamentous coke resistance of Ni@SiO₂ catalysts was investigated.The results show that the smaller size of nickel oxide particles can well dispersed in the silica shell,resulting in the higher specific surface area,pore size,and reaction activity of Ni@SiO₂-20.Coke still deposited on Ni@SiO₂-20 during catalytic reforming of tar process,resulting in a decrease in catalytic activity of catalyst with reaction time proceeding.Thus,the hydrogen conversion decreased from 65%to 28%,and carbon conversion decreased from 53%to 33%.The formation of coke on nickel particles with small size is more difficult.Consequently,the content of filamentous coke deposited on Ni@SiO₂-20（0.19 g/g）with smaller size was lower than that on Ni@SiO₂-50（0.46 g/g）during CO₂ reforming of tar process.In summary,this paper explores the mechanism of coke deposition on nickel-based catalyst during CO₂ reforming of pyrolysis tar.Besides,the synthesis method and modification design method for nickel-based catalysts with high catalytic activity and high coke resistance are provided.