Synthesis Gas From CO₂ Reforming Of Biomass Tar And Optimization Of Ni@SiO₂ Core-shell Catalyst

Biomass gasification for the production of combustible syngas has received widespread attention as an efficient,clean and renewable technology.However,the tar produced during the gasification process can corrode the gasification facilities and seriously affect the commercial utilization of gasification products.Meanwhile,the gasification products contain about 10vol%～30 vol%of carbon dioxide（CO₂）,which significantly affects the calorific value of syngas.In this paper,the nickel core-shell catalyst Ni@SiO₂ was prepared for syngas production from CO₂ reforming of tar reaction,based on which cobalt metal doping and zirconium dioxide（ZrO₂）carrier modification were optimized.The optimization was carried out by introducing air and steam respectively to address the problem of reduced activity or even deactivation due to the accumulation of filamentous coke in the CO₂ reforming of tar reaction of the nickel core-shell catalyst.The main studies are as follows:（1）The Ni@SiO₂ core-shell catalysts were used for CO₂ reforming of tar reaction in a fixed-bed reactor.The effects of catalyst type,shell thickness of core-shell catalyst,weight hourly space velocity and other factors on the tar conversion and coke resistance performance of catalyst in the reforming reaction were analyzed.The results showed that metallic Ni monomer was the prerequisite for the CO₂ reforming of tar reaction.The loaded catalyst Ni/SiO₂ only achieved about 15%toluene conversion within 110 min,while the Ni@SiO₂-2core-shell catalyst maintained more than 45%toluene conversion within 120 min due to the unique core-shell structure protection.Suitable shell thickness could improve the reactivity of the core-shell catalyst by increasing the specific surface area and pore volume,and Ni@SiO₂-4 maintained more than 50%toluene conversion within 300 min with a single lifetime of 26 h.The filamentous coke was the reason affecting the catalyst activity in the reforming reaction,and suitable shell thickness significantly improved the core-shell interaction and molecular diffusion,and Ni@SiO₂-4 showed only 3%coke accumulation after the reforming reaction.（2）The Ni@SiO₂ catalysts were doped with cobalt metal to analyze the effect of their reactivity and coke resistance performance in the CO₂ reforming of tar reaction.The results showed that the bimetallic Ni-Co core-shell catalyst formed at a Ni/Co ratio of 8:1 had the smallest core-metal size,the largest specific surface area and the most active sites.The Ni₈Co@SiO₂ bimetallic core-shell catalyst showed the best tar conversion performance in the CO₂ reforming of tar reaction,maintaining more than 55%of toluene conversion during 300min.Moreover,the Ni₈Co@SiO₂ has a long catalytic life（26h）and a four-cycle regeneration capability.The Ni₈Co@SiO₂ catalyst had significant coke resistance performance due to the synergistic effect of Ni-Co alloy and the protection of silica shell,and only 1.8%of filamentous coke was detected after 300 min of reforming reaction.（3）The Ni/ZrO₂-loaded catalyst was modified with cobalt doping coupled with core-shell structure,and the effect of modification optimization on the reactivity and anti-coke deposition performance of the catalysts was verified by CO₂ reforming reactions with different tar model compounds.The results showed that the Ni/ZrO₂ has only 10%～21%and 44%～52%carbon conversion in the reforming reactions of toluene and acetone,and was deactivated due to coking and sintering in the reactions.The Ni/ZrO₂@SiO₂ significantly increased its specific surface area and pore volume due to the introduction of mesoporous silica shells,and the carbon conversion was significantly increased to 38%～52%and 48%～79%in the 300 min reforming reactions of toluene and acetone.The Ni₈Co/ZrO₂@SiO₂ catalysts modified by the core-shell structure coupled with cobalt metal doping showed carbon conversions of 40%～60%and73%～99%in the toluene and acetone reforming reactions,respectively,and the coke contents of the catalysts after the reactions were 0.05 g/g and 0.0001 g/g,respectively.After the reforming reaction,only filamentous coke and a small amount of pyrolytic coke were found on the used catalysts,and no encapsulated coke was found.It was confirmed that the mechanism of coke formation was not only related to the precursor,but also to the reaction atmosphere and temperature.（4）The effect of air introduction on the reactivity and anti-coke accumulation performance of nickel core-shell catalysts in CO₂ reforming of tar reaction was investigated,and the mechanism of the role of the introduced air in the tar reforming reaction was investigated.The results show that the Ni₈Co/La₂O₃@SiO₂ catalyst has higher reactivity and catalytic activity than the Ni/La₂O₃@SiO₂ catalyst due to the cobalt doping,and the carbon conversion rate can be maintained at 48%～75%within 300 min,and the coke content on the surface of the catalyst after the reaction is only 0.1 g/g.The introduction of air can significantly improve the reaction stability of the catalyst,and the carbon conversion of the catalyst can be maintained at 46%～75%within 600 min at an air flow rate of 2 m L/min,and the coke content of the catalyst after use is only 0.034 g/g.The mechanistic study of air with all combustible substances shows that the core-shell structure and the protective effect of reducing substances such as coke,CO,H₂,etc.on Ni-Co core metals kept them in a metallic alloy state during the reaction.（5）The Ni₈Co/La₂O₃@SiO₂ catalyst was used for the CO₂ combined steam reforming of tar to investigate the feasibility of the combined reforming to produce syngas with controlled H₂/CO ratio,and the catalysts were investigated for their anti-carbonization properties.The results showed that the introduction of different ratios of water vapor in the tar CO₂ reforming process significantly improved the yields of H₂ and CO,especially the CO yield could reach2005～2450 m L/g at 50%steam.In addition,the introduction of steam consumed the filamentous coke produced in the secondary reaction through the water-gas reaction,which improved the resistance of coke accumulation of the catalyst.The introduction of different ratios of CO₂ in the steam reforming of tar process had essentially no effect on the yield of H₂,but significantly increased the yield of CO,and the catalyst did not accumulate more filamentary coke due to the introduction of CO₂.There is an upper limit to the ability of a fixed amount of core-shell catalyst to produce syngas,and varying the ratio of CO₂ to steam reforming can control the H₂/CO ratio in the gaseous product between 0.2 and 2.