Olivine-supported Nickel Catalysts For Steam Reforming Of Biomass Tar

In the technology of biomass gasification for hydrogen production, the presence of tar in the fuel gas is problematic due to blockage and corrosion to the gasification system and downstream equipments. It also causes problems through deactivation of downstream catalytic processes as well as the environmental pollution. Tar elimination is considered to be a bottleneck in any gasification commercialization effort. Among the various tar removal methods, the in situ tar catalytic reforming can convert the tar in gasifier to low molecular weight gases and increase the yield of hydrogen-rich gas. However, the catalyst deactivation caused by coke deposition is very severe in the dusty environment of the gasifiers. Applying the dual fluidized bed gasification technology, a novel process of External Circulating Counterflow Moving Bed (ECCMB) biomass catalytic gasification for hydrogen production is proposed, in which the catalyst can be regenerated continuously in a combustor, and can supply the energy for gasification reactions as solid heat carrier. The aim of this thesis is to research and develop the tar steam reforming catalyst suitable for this system in the micro-scale fixed-bed reactor.Firstly, four natural iron-bearing olivines (Mg, Fe)2SiO4 from different areas of China are evaluated for steam reforming activity with benzene and acetic acid as model compounds from biomass-derived syngas, and the influence of calcination temperature on their activities is investigated. The olivines are characterized by N2 adsorption, X-ray diffraction (XRD), H2-temperature programmed reduction (H2-TPR), scanning electron microscopy (SEM) and thermal gravimetric alnalysis (TG), to explore the changes of their physiochemical properties before and after calcination. The results show that due to the different mineral structures of the raw olivines from different quarries, their physiochemical properties and activities after calcination are quite different. The satellite mineral, serpentine, is decomposed during the calcination, resulting in the increase of the amount of iron oxides and the surface area. The olivine, which has the highest reducible iron content and the largest surface area is most active in activity. The optimized calcination temperature is 900℃. The olivines calcined at higher temperature exhibit lower activity and higher coke deposition due to the decrease of the surface area.Then Ni catalysts supported on calcined olivine are prepared by wetness impregnation method. The influences of the preparation conditions, the amount of NiO loading and the catalyst calcination temperature, on the catalyst activity are studied with benzene as model compound. The in situ reducibility and the regeneration of the NiO/olivine catalyst are also investigated. The catalysts before and after reaction are characterized by XRD,SEM,H2-TPR,TEM and TG techniques. The optimized catalyst preparation parameters are:amount of NiO loading 5 wt% and calcination temperature 1100℃. The obtained NiO/olivine catalyst has strong Ni-olivine interaction, fine dispersion of NiO and low coke deposition. The reaction temperature and the reactants have great influences on the in situ reduction of NiO/olivine. It can be reduced quickly by the product gas without pre-reduction at the reaction temperature of 800℃with benzene as the reactant. The deactivated NiO/olivine catalyst can be regenerated without significant loss of activity by burning off the coke with air and keeps stable during continuous regeneration-reaction circles. XRD and TEM analyses demonstate that no sintering of the active phase is observed due to the formation of Ni-Fe alloy to restrict the growth of the Ni particles. However, as the aromatics have a great tendency to coking on the Ni catalyst surface, and the olivine support has very low surface area (<1 m2/g) and is almost nonporous, the NiO/olivine catalyst deactivates quickly due to coke deposition on the active sites.In order to increase the coking resistance ability as well as steam reforming activity of the NiO/olivine, the olivine support is modified by molding technique through addition of calcium aluminate cement (CA) as binder followed by calcination. The modified olivine (MO) is prepared through a series of processes by milling the olivine, mixing the olivine powder with CA, molding, curing, drying and calcination. The NiO/MO catalysts are prepared by wetness impregnation. The influences of the content of CA, support calcination temperature, amount of NiO loading, catalyst calcination temperature and promoter addition on the catalyst performance are investigated. The MOs and NiO/MO catalysts are characterized by N2 adsorption, XRD, H2-TPR and SEM, etc. The changes of pore structure of the supports and the surface morphology of the supported Ni catalysts before and after modification, and the metal-support interaction of modified catalysts calcined at different temperatures were discussed. Coke combustion profiles and kinetics are analysed by TG method for the catalysts after reaction. The results show that the porosity of MO is above 30%, the BET surface area increases, and the attrition rate is comparable to the raw olivine. The content of CA addition and the catalyst calcination temperature are key factors to control the Ni-MO interaction, which consequently influences the activity, anti-coking ability and in situ reduction performance of the NiO/MO catalyst. The optimized catalyst preparation parameters are:the diameter of olivine powder, less than 0.045 mm; content of CA,20%; content of water,10%; support calcination temperature,1100℃; amount of NiO loading,8 wt% and catalyst calcination temperature,900℃. More associated Ni species on the support are formed at the calcination temperature of 900℃, which lead less coke formation than the free Ni species. The addition of both magnesia and lanthana can decrease the coke formation, and the former has an effect of drastically decreasing the activation energy of coke burning. The comparative results of NiO/MO and NiO/olivine at various benzene liquid hourly space velocities (LHSV) show that the activity and coking resistance ability of the NiO/MO catalyst are greatly improved. The better activity is derived from the decrease of Ni particle size and better dispersion of Ni particles. Due to the big pore volume and large surface area of the MO, the NiO/MO catalyst is less prone to deactivation for coking.In the last part, the nickel catalysts supported on olivine, MO and Al2O3, respectively, are tested for tar elimination with coconut shell pyrolysis oil as the reactant. The results show that the support properties are of great importance to the catalyst performance. The activities of the catalysts decrease as NiO/MO>NiO/Al2O3>NiO/olivine. The activity of NiO/MO is the highest and the deposited coke is the lowest. Although the alumina has the largest surface area, the formation of unreducible nickel alumum oxide caused by the strong Ni-support interaction of NiO/Al2O3 results in the reduction of its activity. The regeneration ability of the catalyst supported on modified olivine with addition of magnesia is investigated. The C-conversion keeps stable at 85% for the accumulative reaction time of 350 min at the conditions of reaction temperature 800℃, LHSV 2.16 h-1 and S/C 2.03, and no deactivation of the catalyst is observed. It suggests that the catalyst is potential to be used in the ECCMB reactors.