Preparation Of A Ni-Based Oxygen Carrier And The Study Of Its Sulfur-Resistance Properties In Chemical Looping Reforming Of Biogas

Hydrogen energy is a clean energy carrier that has received widespread attention in recent years.Chemical looping reforming（CLR）for hydrogen production is a very promising technology,that not only separates CO₂internally,but also produces high purity H₂.In addition,CLR can utilize energy in cascade way,which achieves efficient energy utilization in the reaction process.In CLR,development of oxygen carriers is a research priority.Among them,Ni-based oxygen carriers with high selectivity for H₂and low cost are expected to be used in CLR.Biogas contains a large amount of CH₄and CO₂,widely available and renewable,which can be very well used for CLR technology for hydrogen generation.However,biogas contains some sulfur,which can reduce the catalytic activity and reforming performance of the oxygen carrier,especially for Ni-based oxygen carriers.Therefore,it is important to investigate the toxic effects of sulfur on Ni-based oxygen carriers of CLR.It makes sense to enhance the catalytic activity and the resistance against sulfur poisoning for Ni-based oxygen carriers.In this thesis,we use high alumina refractory brick as support for the Ni oxygen carrier.A mixture of CH₄and CO₂was used to simulate biogas during the CLR process.H₂S was added to the reacting environment to study the effect of sulfur presence.All the CLR tests were carried out in a self-built batch fluidized bed reactor.To increase the resistance against sulfur,the Ni-based oxygen carriers were modified with foreign metals.The main research contents and conclusions were as follows:（1）The 10Ni oxygen carrier prepared by impregnating Ni O on high alumina refractory bricks was gradually activated during cycles with biogas.It was found that the fresh 10Ni oxygen carriers required several cycles fully activate the activity.The activated 10Ni oxygen carrier has excellent catalytic activity and has high H₂and CO yields under sulfur-free biogas experiments.In the experiments with sulfur biogas,most of CH₄and CO₂could not be converted,the H₂yield and CO yield were low.XRD and SEM characterizations were performed on the fresh and reduced samples after the sulfur tests.Ni S₂was found in the phase composition of the oxygen carrier,and it was the main reason for the significant decrease in the catalytic activity of the oxygen carrier.The 10Ni oxygen carrier did not react with the refractory bricks during the preparation and experiments,and no agglomeration or sintering occurred.（2）Bimetallic oxygen carriers（10Ni10Ca、10Ni10Cu、10Ni10Fe、10Ni10W、10Ni10Ce and 10Ni10Mn）were prepared by doping other metals in 10Ni oxygen carrier using co-impregnation method to enhance the resistance of oxygen carriers to sulfur poisoning.The bimetallic oxygen carriers were subjected to sulfur biogas experiments and sulfur-free biogas experiments.Typical cycling performance,gas yields,carbon deposition and relative sulfur-resistance of each oxygen carrier were compared.It was found that the doping of metals improved the H₂yield and carbon resistance of Ni-based oxygen carriers in sulfur-free biogas.The doping of Mn（10Ni10Mn）improved the sulfur-resistance substantially.（3）The 10Ni10Mn oxygen carriers were tested for cycling performance,and combined with H₂-TPR,H₂S-TPS,XRD,XPS,ICP-OES and morphological characterization techniques to investigate the mechanism of sulfur-resistance.It is shown that the 10Ni10Mn oxygen carrier has a more developed pore structure,which can be activated more quickly and has a stable sulfur-resistance poisoning.The interaction of Ni and Mn enhances the oxygen storage capacity and oxygen mobility of the oxygen carrier.Therefore,10Ni10Mn oxygen carrier inhibits the production of metal sulfide on the surface and reduces the adsorption of H₂S.（4）The reactivity regeneration of 10Ni10Mn was explored under sulfur-free conditions.The used oxygen carriers were subjected to sulfur-free biogas experiments and sulfur biogas experiments.The catalytic properties of the regenerated oxygen carriers were analyzed by XRD,XPS and morphological characterization techniques.It was found that the 10Ni10Mn oxygenates could be regenerated after sulfur poisoning.However,the catalytic activity decreases after regeneration as compared to the fresh oxygen carrier and the sulfur-resistance poisoning is completely lost.