Laihong Shen

Analysis of reactivity of Fe-based oxygen carrier with coal during chemical-looping combustion

Abstract Chemical-looping combustion (CLC) has been suggested as an energy-efficient method for the capture of the greenhouse gas carbon dioxide from combustion. The reactivity of using Fe 2 O 3 as an oxygen carrier during CLC of coal has been investigated experimentally at 800–950°C. The experiments were carried out in a fluidized bed, where the steam acted as the gasification-fluidization medium. The reactivity of Fe 2 O 3 as a function of the reactor temperature, reaction time, and cyclic reduction number was discussed. The reactivity of Fe 2 O 3 oxygen carriers was enhanced as temperature increased at 800–950°C. Moreover, the time of chemical reaction control between the oxygen carrier and coal gasification products decreased with increased reaction temperature. When the reaction temperature was above 900°C, the rate of carbon to form CO 2 was higher than 90%; however, it was lower than 75% below 850°C. At 900°C, the dry basis concentration of CO 2 decreased with increased cyclic reduction period, while that of CO and CH 4 increased. Moreover, the value of the CO concentration was less than that of CH 4 . The performance of the reacted Fe 2 O 3 -based oxygen carriers was also evaluated using an X-ray diffractometer and a scanning electron microscope to characterize the solid residues of oxygen carrier. The results show that Fe 2 O 3 -based oxygen carriers are only reduced to Fe 3 O 4 . With the increase of cyclic reduction period, the oxygen carrier sinters gradually.

Reduction Kinetics of a CasO4 Based Oxygen Carrier for Chemical-Looping Combustion

The CaSO4 based oxygen carrier has been proposed as an alternative low cost oxygen carrier for Chemical-looping combustion (CLC) of coal. The reduction of CaSO4 to CaS is an important step for the cyclic process of reduction/oxidation in CLC of coal with CaSO4 based oxygen carrier. Thermodynamic analysis of CaSO4 oxygen carrier with CO based on the principle of Gibbs free energy minimization show that the essentially high purity of CO2 can be obtained, while the solid product is CaS instead of CaO. The intrinsic reduction kinetics of a CaSO4 based oxygen carrier with CO was investigated in a differential fixed bed reactor. The effects of gas partial pressure (20%–70%) and temperature (880–950°C) on the reduction were investigated. The reduction was described with shrinking unreacted core model. Experimental results of CO partial pressure on the solid conversion show that the reduction of fresh oxygen carriers is of first order with respect to the CO partial pressure. Both chemical reaction control and product layer diffusion control determine the reduction rate. The dependences of reaction rate constant and effective diffusivity with temperature were both obtained. The kinetic equation well predicted the experimental data.

Steam reforming of α -methylnaphthalene as a model compound of biomass tar over Ni-based catalyst for hydrogen-rich gas

Tar is a barrier to limit the development of biomass gasification. Catalytic steam reforming experiments using α-methylnaphthalene (MNP) as a model tar compound were carried out in the two-stage reactor system (TSR). Based on response surface methodology, the effects of TSR temperatures and the molar ratio of steam to carbon (S/C) on MNP reforming performances were analyzed using the Li-modified Ni-based catalyst (NBC). The results show that the proper introduction of H2 is able to improve significantly the MNP conversion, specially at lower temperatures. Furthermore, it is more appropriate for the modified catalysts by Li and Mg to be loaded in the first reactor due to their significant promotion to hydrocracking reactions, and it is favorable to place the Ni/Al catalyst in the second reactor for H2-rich gas. Additionally, the carbon deposition resistance of the NBC modified by Li exhibits better than that of the NBC modified by Mg.

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

Abstract Chemical looping combustion (CLC) is a novel technology where CO2 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/Al2O3 oxygen carrier reduction with a gas mixture of CO/H2 and H2S 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 H2S is higher than the one with CO. There are only Ni and Ni3S2 phases of nickel species in the fully reduced oxygen carrier, and no evidence for the existence of NiS or NiS2. The formation of Ni3S2 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 Ni3S2 in the nickel-based oxygen carrier reduction with a gaseous fuel containing H2S. Due to the sintering of metallic nickel grains on the external surface of the reduced oxygen carrier, further reaction of the oxygen carrier with H2S 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 H2S is carried out in a 1xa0kWth 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.

Reactivity deterioration of NiO/Al{sub 2}O{sub 3} oxygen carrier for chemical looping combustion of coal in a 10 kW{sub th} reactor

Abstract A relatively long-term experiment for chemical looping combustion of coal with NiO/Al2O3 oxygen carrier was carried out in a 10xa0kWth continuous reactor of interconnected fluidized beds, and 100xa0h of operation was reached with the same batch of the oxygen carrier. The reactivity deterioration of the oxygen carriers was present during the experimental period. The reactivity deterioration of reacted oxygen carriers at different experimental stages was evaluated using X-ray diffraction (XRD), scanning electron microscope (SEM), and X-ray fluorescence spectrometer. SEM analysis showed no significant change in the morphology of the nickel-based oxygen carrier at the fuel reactor temperature ⩽940xa0°C, but loss of surface area and porosity of reacted oxygen carriers was observed when the fuel reactor temperature exceeded 960xa0°C. The results show that the sintering effect have mainly contributed to the reactivity deterioration of reacted oxygen carriers in the CLC process for coal, while the effects of coal ash and sulfur can be ignored. The oxidization of reduced oxygen carrier with air was an intensive exothermic process, and the high temperature of oxygen carrier particles led to sintering on the surface of oxygen carrier particles in the air reactor. Attention must be paid to control the external circulation of oxygen carrier particles in the interconnected fluidized beds in order to efficiently transport heat from the air reactor to the fuel reactor, and reduce the temperature of oxygen carrier particles in the air reactor. Improvement of reactivity deterioration of reacted oxygen carriers was achieved by the supplement of steam into the fuel reactor. Nevertheless, NiO/Al2O3 is still one of the optimal oxygen carriers for chemical looping combustion of coal if the sintering of oxygen carrier is minimized at the suitable reactor temperature.