Sulfur behavior in chemical looping combustion with NiO/Al 2O 3 oxygen carrier

Abstract

Chemical looping combustion (CLC) is a novel technology where CO 2 is inherently separated during combustion. Due to the existence of sulfur contaminants in the fossil fuels, the gaseous products of sulfur species and the interaction of sulfur contaminants with oxygen carrier are a big concern in the CLC practice. The reactivity of NiO/Al 2O 3 oxygen carrier reduction with a gas mixture of CO/H 2 and H 2S is investigated by means of a thermogravimetric analyzer (TGA) and Fourier Transform Infrared spectrum analyzer in this study. An X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscope (SEM) are used to evaluate the phase characterization of reacted oxygen carrier, and the formation mechanisms of the gaseous products of sulfur species are elucidated in the process of chemical looping combustion with a gaseous fuel containing hydrogen sulfide. The results show that the rate of NiO reduction with H 2S is higher than the one with CO. There are only Ni and Ni 3S 2 phases of nickel species in the fully reduced oxygen carrier, and no evidence for the existence of NiS or NiS 2. The formation of Ni 3S 2 is completely reversible during the process of oxygen carrier redox. A liquid phase sintering on the external surface of reduced oxygen carriers is mainly attributed to the production of the low melting of Ni 3S 2 in the nickel-based oxygen carrier reduction with a gaseous fuel containing H 2S. Due to the sintering of metallic nickel grains on the external surface of the reduced oxygen carrier, further reaction of the oxygen carrier with H 2S is constrained, and there is no increase of the sulfidation index of the reduced oxygen carrier with the cyclical reduction number. Also, a continuous operation with a syngas of carbon monoxide and hydrogen containing H 2S is carried out in a 1 kW th CLC prototype based on the nickel-based oxygen carrier, and the effect of the fuel reactor temperature on the release of gaseous products of sulfur species is investigated.