Preparation Of Nickel-based Catalyst And Its Application In The Purification And Upgrading Of Straw Pyrolysis Gas

Biomass is an important renewable resource,and rational development of biomass resources is of far-reaching significance to alleviate the energy tensions.Catalytic pyrolysis can convert biomass into high-quality combustible gas,and at the same time can remove tar in the gas,increase gas yield and improve gas quality.Nickel-based catalysts are the most widely studied and used catalysts in biomass tar cracking.In this paper,a series of nickel-based catalysts were prepared by homogeneous precipitation method and used for the pyrolysis of rice straw to produce syngas.The research contents as follows:（1）Rice straw was selected as the research objects.At the same time,element analysis,industrial analysis,thermogravimetric analysis and low calorific value calculation were performed on it.The analysis results showed that the rice straw had high volatile content,and the second stage of the pyrolysis process was the main stage of volatile precipitation.The low calorific value of rice straw was 14.97 MJ/kg.（2）The NiO/EVM catalysts were prepared by homogeneous precipitation method with expanded vermiculite（EVM）as support.The as-prepared samples were characterized by XRF,XRD,SEM and BET.The results showed that NiO were successfully loaded on the EVM support.The specific surface area and the pore volume increased after loading.At the same time,the effects of pyrolysis temperature,calcination temperature,holding time and nickel loading on the gas content were studied.The optimal reaction conditions of the catalyst were obtained,that is,When the pyrolysis temperature at 800℃,the calcination temperature of NiO/EVM at 400℃,the holding time at 20 min,and the nickel loading was20 wt%,the content of H₂ reached 36.76 vol%,and the content of CO reached 35.93 vol%.The stability of NiO/EVM catalyst was tested and compared with no catalyst,after the NiO/EVM catalyst was used for 6 regeneration cycles,the total content of syngas was decreased by about 16 vol%,but it was still increased by 36.5%.（3）Four nickel-based catalysts（Ni/SEP、Ni-Ce/SEP、Ni-Pr/SEP、Ni-La/SEP）were prepared by homogeneous precipitation method with sepiolite（SEP）as support.Characterization methods such as XRF,XRD,SEM-EDS and BET were used to analyze the four nickel-based catalysts.The effects of the loading of Ce,Pr and La on the performance of catalytic pyrolysis of rice straw to syngas production were investigated at reaction temperature of 700°C and holding time of 20 min in the tubular pyrolysis furnace.The results showed that the active components of four nickel-based catalysts were successfully supported on the SEP support.The Ni-Ce/SEP catalyst had the largest specific surface area and the relatively poor dispersion,while the Ni-La/SEP catalyst had the smallest specific surface area and the best dispersion,and the La₂O₃ was highly dispersed in the catalyst.Compared with non-catalyst pyrolysis,the content of H₂ and CO in syngas increased from14.96 vol%to 31.96 vol%and from 17.06 vol%to 25.22 vol%with Ni/SEP catalyst.With the addition of Ce,Pr and La,the content of H₂ and CO in syngas were further increased,especially under the Ni-La/SEP catalyst,the content of syngas reached the maximum value of 73.4 vol%,which was 41.38 vol%higher than that without catalyst.With the increase of Ce,Pr and La loading from 0 to 12 wt%,the content of H₂ in syngas increased from 31.96vol%to 38.76 vol%,39.44 vol%and 41.17 vol%respectively.The content of CO increased from 25.22 vol%to 29.7 vol%,30.65 vol%and 32.23 vol%respectively.The syngas（H₂+CO）contents of the Ni-La/SEP,Ni-Pr/SEP,Ni-Ce/SEP and Ni/SEP catalysts are 54.45 vol%,52.66 vol%,51.4 vol%and 41.5 vol%respectively.The catalytic performance of the four catalysts was Ni-La/SEP>Ni-Pr/SEP>Ni-Ce/SEP>Ni/SEP.（4）The Al₂O₃ supports and the pyrochlore structure La₂Zr₂O₇ were combined to prepare the LZA（La₂Zr₂O₇-Al₂O₃）supports by sol-gel method,and then the Ni/LZA catalysts were prepared by homogeneous precipitation method with the above support.EDX,XRD,SEM,BET and TG-DTG analysis were employed to study the physiochemical properties of the catalysts.The effects of the LZA supports composite ratio and pyrolysis temperature on the performance of catalytic pyrolysis of rice straw to syngas production were investigated at holding time of 20 min in the tubular pyrolysis furnace.The results showed that the supports prepared by sol-gel method all showed La₂Zr₂O₇ with the pyrochlore structure,and the active components of the five catalysts were well loaded.With the increase of the mass fraction of La₂Zr₂O₇ in the composite supports,the particle size of NiO in the catalysts increased gradually,and the dispersion of NiO in the Ni/LZA catalysts were best.The thermal stability of NiO in the Ni/LZA catalysts was excellent.Compared with the pyrolysis without catalyst,the content of H₂ in syngas increased from 20.57 vol%to 40.4 vol%,with a relative increased of 96.4%,and the content of CO increased from 26.2 vol%to 40.25 vol%,with a relative increased of 53.6%.When the pyrolysis temperature rised from 600°C to 800°C,the H₂content of Ni/LZA catalysts increased from 21.69 vol%to 40.4 vol%,and the CO content increased from 19.29 vol%to 40.25 vol%.When the mass fraction of La₂Zr₂O₇ in the composite supports was 50 wt%,that was,under the Ni/LZA catalysts,the content of syngas reached the maximum value of 80.65 vol%.After 6 times of continuous used,the content of syngas was still up to 61.7 vol%,in which the content of H₂and CO were 30.95 vol%and30.75 vol%respectively,which was 14.93 vol%higher than that of pyrolysis without catalyst.