Combined CO₂ And H₂O Reforming Of Pyrolysis Vapor Based On Biomass Staged-Gasification

With the rapid development of society,wanton exploitation and utilization of fossil fuels have led to serious environmental pollution and resource depletion.In recent years,biomass has attracted more and more attention as a renewable and clean energy resource.Based on biomass staged-gasification,biomass can be efficiently converted into syngas via catalytic CO₂ and H₂O reforming of pyrolysis vapor,which has a broad prospect of application.CO₂ used as the reforming medium can not only reduce the consumption of H₂O,but also realize the utilization of CO₂.Therefore,in this paper,in-line CO₂reforming of biomass pyrolysis gas and in-line combined CO₂ and H₂O reforming were carried out based on Ni-based catalysts modified by alkali metals(K)and alkaline earth metals(Mg).Moreever,the experiments of catalytic reforming of tar model compound benzene under CO₂ and H₂O atmosphere were explored.Firstly,for in-line catalytic CO₂ reforming of biomass pyrolysis gas,the addition of K-Ni/Al catalyst can effectively promote CO₂ reforming of pyrolysis gas.As the concentration of CO₂ increases,the gas yield increases but liquid yield decreases.When the CO₂ concentration is greater than 40%,the increase in carbon conversion rate slows down.On the other hand,the increase in reaction temperature significantly improves the gas yield and the carbon conversion rate of pyrolysis gas(Xc),especially for the increase in CO yield.K-Ni/Al catalyst shows a greater effect at low temperature than that at high temperature.Under the same concentration of CO₂ and H₂O,H₂O reforming of liquid products is slightly better than CO₂ reforming.At700^oC,Xc can reach 93.9%under 80%CO₂ atmosphere,which is close to that obtained under 40%H₂O.The characterizations of the used catalyst under CO₂ and H₂O atmosphere show that the amount of carbon deposition on the catalyst is almost same.And the amount of carbon deposition after CO₂ reforming decreases with the increase of the temperature and the thermal stability of the carbon deposition has little change.Secondly,the experiment of in-line CO₂ and H₂O reforming of the pyrolysis gas was carried out using Ni-based catalysts modified with K and Mg.The results show that the 5wt.%Mg modified Ni-based catalyst exhibits the best catalytic activity.Compared with the CO₂ atmosphere,the CO₂/H₂O mixed atmosphere promotes the reforming reaction more effectively,as a result,the gas yield and carbon conversion rate increased.When the proportion of CO₂ in the reforming medium is higher,CO₂ plays the main reforming role.Conversely,H₂O plays the major reforming role.Therefore,a high carbon conversion rate and an appropriate H₂/CO ratio of syngas can be obtained by adjusting the H₂O/CO₂ ratio and increasing the concentration of the reforming medium.At the temperature of 700°C and the high ratio of H₂O to CO₂(Y_S:Y_C=2:1),the carbon conversion rate reaches 97.6%and the H₂/CO ratio is 1.19.The characterization of the fresh and used catalysts show that under a CO₂ atmosphere,the Mg Ni catalyst is benefical for reducing the amount of carbon depositions on the catalyst and nickel can basically maintain the elemental substance Ni⁰.In a mixed atmosphere,as the amount of H₂O increases,the amount of carbon deposition on the catalyst is further reduced,however,the elemental Ni⁰ will be seriously oxidized.Lastly,the catalytic reforming of benzene under CO₂ and H₂O atmosphere was carried out based on K and Mg modified Ni-based catalysts.The results show that under the CO₂ atmosphere,the increase of the Mg Ni/Al catalyst bed effectively promotes the CO₂ reforming of benzene to improve the conversion rate of benzene and the utilization rate of CO₂.Mg Ni/Al catalyst exhibits significantly better catalytic activity of CO2 reforming of benzene than K-Ni catalyst.However,Mg Ni/Al catalyst is only slightly worse activity of H₂O reforming of benzene.Compared with K-Ni/Al catalyst,Mg Ni/Al catalyst is more suitable for catalytic reforming of benzene in mixed atmosphere.When the space velocity of benzene is 0.67ml-ben./g-cat.h using Mg Ni/Al catalyst,the conversion rate of benzene and the utilization rate of CO₂ in CO₂ atmosphere reach95.5%and 52%respectively,and the yield of combustible gas is 14.4 mol/mol-ben as well as the H2/CO ratio in the syngas is 0.19(C/B=14,700?).Under H₂O atmosphere,the conversion rate of benzene reaches91.8%,and the H₂/CO ratio is 2.41.Under the mixed atmosphere of H₂O and CO₂,the increase of molar ratio of reforming agent and benzene will enhance the conversion of benzene.The conversion rate of benzene can reach 99.2%at the molar H₂O/CO₂ ratio of 35.When the molar H₂O/CO₂ ratio is 1.5,the H₂/CO ratio of 1.71is obtained.Moreever,Mg Ni/Al catalyst keeps a high catalytic activity after a long reaction time of 1200 min,and the conversion rate of benzene remains stable at around 95%.The characterizations of the long-used catalyst show that Mg modification can depress the carbon depisition on Ni-based catalyst.Although the long-time reaction could cause the growth of particle size of the active Ni and carbon deposition,it has no significant effect on the stability of the catalyst.