Study On Multi-component Cobalt Based Oxygen Carrier In Low Temperature Chemical Looping VOCs Removal

Volatile organic compounds?VOCs?pollution involves a variety of pollutants that threaten human health and environment.The components are complex and the hazards are enormous.Therefore,it is very necessary to control the emission of VOCs.As a new combustion technology,chemical looping combustion?CLC?is different from the traditional combustion technology,and also is a very clean combustion method.The core of the CLC is a high performance oxygen carrier.In this thesis,first,a series of Ni-Co oxygen carriers were synthesized by sol-gel method.The crystal structure and surface morphology and reducing ability were investigated by XRD,BET and SEM;Secondly,VOCs removal test was conducted on a self-made fixed bed to examine the properties of the material,the preparation conditions and reaction conditions were used to study the effect of the removal efficiency,and the best preparation conditions and optimal reaction conditions were obtained.Finally,the stability of oxygen carrier was analyzed and the reaction mechanism of oxygen carrier was explored by an in-situ infrared diffuse reflectometer?in-situ DRIFTS?.The main conclusions are as follows:A series of characterization analysis of the synthesized Ni-Co oxygen carrier shows that the physicochemical properties of the Ni-doped oxygen carrier have a great change compared to Co₃O₄.When the ratio of the Ni-doped is small,Ni enters into the crystal of Co₃O₄ to form a solid solution.With the increase of Ni doping,part of Ni precipitates on the surface as NiO.The Ni,Co,and O elements are uniformly distributed on the oxygen carrier,and it can also be confirmed that Ni enters the interior of the Co₃O₄ crystal.Proper doping will make cobalt-based oxygen carriers have larger specific surface area and more pores,but too much Ni doping will make the pores block and the specific surface area will decrease.The reduction temperature of the doped oxygen carrier is shifted to low temperature,in which the Ni_0.5Co_2.5O₄ oxygen carrier moves the most.The oxygen species on the surface of Ni-doped oxygen carriers are mainly lattice oxygen and chemically adsorbed oxygen,among which lattice oxygen accounts for the main part.Co_2p indicates that Co is mainly composed of Co²⁺and Co³⁺at the surface.As the amount of Ni doping increases,the proportion of lattice oxygen gradually increases,and the proportion of Co²⁺also gradually increases.The performance test of synthesized Ni-Co oxygen carrier was performed in a self-made fixed bed reactor.The results show that compared with pure Co₃O₄,the benzene and ethylene removal efficiency of Ni-doped oxygen carrier is increased,Ni_0.5Co_2.5O₄ is the best doping amount of oxygen carrier.The effect of calcination temperature and reaction conditions on the removal efficiency was studied.The results showed that the optimum calcination temperature was 500°C,the optimum reaction conditions were 1000ppm benzene concentration?1500ppm ethylene?and 24000 h^-1 gaseous hourly space velocity.Finally,the stability of selected oxygen carriers was evaluated.In the fixed bed cycle reaction test conducted at 275°C,three Ni-doped cobalt-based oxygen carriers can maintain high stability,and Ni_0.5Co_2.5O₄ oxygen carrier removal efficiency is about 90%in 24h reaction.XRD pattern before and after the reaction shows that the crystal structure of the oxygen carrier is basically unchanged,indicating that the synthesized Ni-Co oxygen carrier has high stability.Using in-situ infrared diffuse reflectometer to explore the reaction mechanism of oxygen carrier exploring the reaction mechanism of oxygen carriers,the results show that during the reaction of Ni_0.5Co_2.5O₄ oxygen carriers with CH₄ and O₂,CH₄ first reacts with the lattice oxygen on Ni-Co oxygen carriers to form intermediate products with C-O functional groups,and consumes the lattice oxygen in the cobalt-based oxygen carrier to generate CO₂.Oxygen carriers lose lattice oxygen to form oxygen vacancies,which act as adsorption and activation sites to decompose free oxygen molecules and regenerate.It can be concluded that the reaction of oxygen carriers with CH₄ and O₂ follows redox mechanism.