| Hydrogen and syngas are important feedstocks for many bulk chemicals.At present,the production of hydrogen and syngas are costly and the hydrogen and syngas are mainly originated from the methane steam catalytic reforming process.It is necessary to develop a simple and efficient production technology since the methane steam catalytic reforming method exhibits many disadvantages.In particular,the process of the method is complicated,the energy consumption and equipment investment are huge,and the energy efficiency is low.In addition,the global warming caused by greenhouse gases have received wide attention all over the world.The effective capture and utilization of greenhouse gas CO2 has become the research hotspot in the field of H2 and syngas production.Chemical looping dry reforming of methane with hydrogen generation is a new energy conversion technology developed based on the concept of chemical looping,which is of great significance to achieve high energy conversion efficiency and zero greenhouse gas emissions during the production of H2 and syngas.In the present work,the reaction between reducing gas(CH4,CH4/CO2)and iron-based oxygen carrier was studied on a bench-scale fluidized bed reactor.The characteristics of carbon deposition on the surface of oxygen carrier during the reaction between Fe-based oxygen carriers and the reducing gases,and the effects of supports and reaction condition on the carbon deposition were investigated.Besides,the coupling mechanisms between the reduction of Fe-based oxygen carrier and the methane dry reforming were also studied.The main work and research conclusions were drawn as follows.Researchs on carbon deposition that formed in the CH4 and iron-based oxygen carrier reaction were initially carried out,and the effects of reaction temperature and CH4 concentration on the carbon deposition were analyzed accordingly.The results show that the decrease of the reaction temperature and the CH4 concentration can alleviate the rate of carbon deposition,while the carbon deposition is closely related to the reduction degree of iron-based oxygen carrier.As the reduction depth of the iron-based oxygen carrier increased,iron Fe appeared on the surface of the iron-based oxygen carrier.Iron Fe catalyzed the methane cracking reaction and accelerated the partial oxidation of methane.When the rate of partial oxidation reaction between methane and lattice oxygen was lower than that of methane cracking,soild carbon would be formed on the surface of the oxygen carrier and causing carbon deposition.Carbide Fe3C,graphitic carbon,amorphous carbon and whisker-like carbon nanotubes were observed on the reduced oxygen carriers.Combined with results from the H2O-TPO test,those carbon,such as Fe3C,amorphous carbon and whisker-like carbon nanotubes,were active which can be oxided by steam before 700?.Conversely,graphitic carbon was inactive and its oxidation temperature by steam could exceed 900?.Iron-based oxygen carrier modified by five different supports,i.e.Al2O3,MgA1204,CeO2,ZrO2 and CeZrO4,and the reaction characteristics of those oxygen carriers with CH4 were compared.The results show that supports can not only optimize the pore structure of the oxygen carrier particles,but also improve the lattice oxygen transfer capacity and carbon deposition resistance of the oxygen carrier.For the fresh iron-based oxygen carriers,Fe2O3/MgAl2O4 has the highest specific surface area and exhibits the best reactivity and carbon resistance.However,after 10 cycles,the oxygen transfer capacity and carbon resistance of the used Fe2O3/MgAl2O4 were decreased obviously.Fe2O3/CeZrO4 could maintain good oxygen transfer capacity during 10 cycles and exerts the best stability on carbon resistence among those oxygen carriers.The effects of the addition of CO2 in reducing atmosphere on carbon deposition and the reduction characteristics of iron-based oxygen carriers were investigated.The addition of CO2 in the reducing atmosphere reduced the degree of graphitization of the carbon deposit while suppressing the deep reduction of the iron oxide.When the reaction temperature was 850? and CO2 concentration was higher than 10%,iron Fe could not be detected on the surface of the iron-based oxygen carrier.However,when the CO2 concentration was lower than 10%,methane dry reforming and methane cracking reaction would be occurred at same time,while the mixed gases of CH4 and CO2 were transfored to CO,H2 and solid carbon by catalysis of iron Fe.Chemical looping dry reforming of methane with hydrogen generation process was proposed base on chemical looping hydrogen generation,which consists of four different stages:the reduction,the methane dry reforming,the steam oxidation and the air oxidation.It was found that the efficient conversion of CO2 and CH4 to syngas can be achieved on the reduced iron oxygen carrier.When the reaction temperature was 850?950?,the conversion of CH4 and CO2 during the dry reforming stage could be reached 94%and 97%,respectively.The syngas with different H2/CO ratios could be obtained by adjusting the CH4/CO2 ratio in the dry reforming stage.When the CH4/CO2 ratio was 1,the equimolar mixture of CO2 and CH4 were transformed to syngas with the H2/CO molar ratio of 1 via.the Fe-catalysed methane dry reforming reaction.However,when the CH4/CO2 ratio was greater than 1,the excess CH4 not only reacted with the lattice oxygen via.the partial oxidation of CH4,but also was dissociated via.the reaction of CH4 cracking.The former reaction benefited to the H2/CO ratio of syngas increased during the dry reforming stage.However,the products of the methane cracking reaction were H2 and solid carbon,which caused carbon deposition inevitablty.Fortunately,the partial oxidation of methane can be promoted by optimizing the reduction extent of iron oxide.The results show that,when the reaction temperature was 900? and the reduction extent of Fe2O3/Al2O3(prepared by co-precipitation method)oxygen carrier was 33%,the partial oxidation between CH4 and lattice oxygen and catalysis dry reforming of methane occurred simultaneously.When the feed ratio of CH4/CO2 in the dry reforming stage increased from 1 to 3,the H2/CO ratio of syngas could raise from 1.04 to 1.69. |
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