No abstract

Chemical looping gasification (CLG) is considered as a novel gasification technology because gas-phase oxygen of the gasifying medium can be replaced by lattice oxygen of the oxygen carrier. The gasifying mediums (e.g., pure O 2 and steam) used as the oxygen source can apparently improve the char conversion in traditionally biomass gasification. Similarly, the objective of this study is to investigate char CLG with the oxygen carrier as an individual oxygen source. A NiO-modified iron ore oxygen carrier was prepared by the impregnation method coupled with ultrasonic treatment. The characteristics of the oxygen carrier were analyzed by an X-ray diffractometer (XRD) and H 2 temperature-programmed reduction (H 2 -TPR). The formation of spinel-type nickel iron oxide NiFe 2 O 4 can evidently enhance the reactivity of the oxygen carrier. The reduction of the oxygen carrier by biomass char was investigated using thermogravimetric analysis (TGA) together with a fixed-bed reactor under an inert atmosphere. TGA tests show that the reactivity of the oxygen carrier increased with the increase of NiO loading. An optimal mass ratio of char/oxygen carrier is determined at 4:6 with the aim of obtaining a maximum reaction rate. The presence of spinel-type nickel iron oxide NiFe 2 O 4 apparently improved the reaction rate of char gasification. The fixed-bed gasification results show that CO was generated faster than other components because carbon was partially oxidized and H 2 was quickly consumed by the lattice oxygen [O] of the oxygen carrier. A relatively high carbon conversion of 55.56% was obtained in the char CLG, in comparison to that of char pyrolysis (5.52%). The lattice oxygen [O] of the oxygen carrier was fully consumed by biomass char. Moreover, biomass char was catalytically pyrolyzed becausee the deep reduction products (metallic iron and nickel) can act as catalysts for char pyrolysis. XRD analysis shows that the oxygen carrier was deeply reduced into Fe (Ni) alloy and Fe 3 C species during the reduction stage of char CLG. However, the regenerated oxygen carrier after oxidation can be recycled for char CLG on the basis of XRD and scanning electron microscopy (SEM) analyses.

The objective of this paper is to systematically investigate the influences of different preparation methods on the properties of NiFe2O4 nanoparticles as oxygen carrier in chemical looping hydrogen production (CLH). The solid state (SS), coprecipitation (CP), hydrothermal (HT), and sol–gel (SG) methods were used to prepare NiFe2O4 oxygen carriers. Samples were characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) surface area measurement, as well as Barrett–Joyner–Halenda (BJH) porosity test. The performance of the prepared materials was first evaluated in a TGA reactor through a CO reduction and subsequent steam oxidation process. Then a complete redox process was conducted in a fixed-bed reactor, where the NiFe2O4 oxygen carrier was first reduced by simulated biomass pyrolysis gas (24% H2 + 24% CO + 12% CO2 + N2 balance), then reacted with steam to produce H2, and finally fully oxidized by air. The NiFe2O4 oxygen carrier prepared by the sol–gel method showed the best capacity for hydrogen production and the highest recovery degree of lattice oxygen, in agreement with the characterization results. Furthermore, compared to individual nickel ferrite particles, the mixture of NiFe2O4 and SiO2 presented remarkably higher stability during 20 cycles in the fixed-bed reactor. The structural and morphological stability of samples after reactions was also examined by XRD, XPS, and SEM analyses.