59 research outputs found
Characterization of Grain Boundaries in Superplastically Deformed Y-TZP Ceramics
The effects of compressive deformation on the grain boundary characteristics of fine-grained Y-TZP have been investigated using surface spectroscopy, impedance analysis, and transmission electron microscopy. After sintering at low temperature (1150°C), the grain boundaries are covered by an ultrathin (1nm) yttrium-rich amorphous film. After deformation at 1200°â1300°C under low stress, some grain boundaries are no longer covered by the amorphous film. Yttrium segregation seems to occur only at wetted grain boundaries. Evidence has been found that the extent of dewetting increases with increasing applied stress
Effect of annealing in O2 or N2 on the aging of Fe0.5Mn1.84Ni0.66O4 NTC-ceramics
Fe0.5Mn1.84Ni0.66O4 NTC ceramic thermistors were annealed in nitrogen or oxygen atmosphere at 600 °C. The change of the electrical properties of the thermistors with time was reduced sharply by annealing in N2, whereas it was enhanced upon annealing in O2. N2-annealed samples exhibited a less degree of redistribution of cations in the lattice as compared with O2-annealed samples. The aging of the electrical properties is believed to result from the cation redistribution which in turn is favoured by the presence of cation vacancies. And the improved aging behaviour of the N2-annealed thermistor is explained by the reduction of the concentration of the cation vacancy upon annealing under lower oxygen partial pressure and the suppression of cation redistribution
CFD Simulation of Hydrogen Generation and Methane Combustion Inside a Water Splitting Membrane Reactor
Hydrogen production from water splitting remains difficult due to the low equilibrium constant (e.g., Kp â 2 Ă 10â8 at 900 °C). The coupling of methane combustion with water splitting in an oxygen transport membrane reactor can shift the water splitting equilibrium toward dissociation by instantaneously removing O2 from the product, enabling the continuous process of water splitting and continuous generation of hydrogen, and the heat required for water splitting can be largely compensated for by methane combustion. In this work, a CFD simulation model for the coupled membrane reactor was developed and validated. The effects of the sweep gas flow rate, methane content and inlet temperature on the reactor performance were investigated. It was found that coupling of methane combustion with water splitting could significantly improve the hydrogen generation capacity of the membrane reactor. Under certain conditions, the average hydrogen yield with methane combustion could increase threefold compared to methods that used no coupling of combustion. The methane conversion decreases while the hydrogen yield increases with the increase in sweep gas flow rate or methane content. Excessive methane is required to ensure the hydrogen yield of the reactor. Increasing the inlet temperature can increase the membrane temperature, methane conversion, oxygen permeation rate and hydrogen yield
CFD Simulation of Syngas Combustion in a Two-Pass Oxygen Transport Membrane Reactor for Fire Tube Boiler Application
The oxygen transport membrane reactor technology enables the stable combustion of syngas and reduction in NOx emission. Applying the syngas combustion membrane reactor to fire tube boiler can integrate oxygen separation, syngas combustion, and steam generation in a single apparatus. In this study, a CFD model for oxygen permeation and syngas combustion in a two-pass LSCoF-6428 tubular membrane reactor for fire tube boiler application was developed to study the effects of the inlet temperature, the sweep gas flow rate, and the syngas composition on the reactor performance. It is shown that the inlet temperature has a strong effect on the reactor performance. Increasing the inlet temperature can efficiently and significantly improve the oxygen permeability and the heat production capacity. A 34-times increase of oxygen permeation rate and a doubled thermal power output can be obtained when increasing the inlet temperature from 1073 to 1273 K. The membrane temperature, the oxygen permeation rate, and the thermal power output of the reactor all increase with the increase of sweep gas flow rate or H2/CO mass ratio in syngas. The feasibility of the syngas combustion membrane reactor for fire tube boiler application was elucidated
Dry Bond Strength and Water Resistance of Konjac Glucomannan, Chitosan, and Polyvinyl Alcohol Blend Adhesive
An environmentally-friendly wood adhesive was developed by blending konjac glucomannan (KGM), chitosan (CH), and polyvinyl alcohol (PVA) together. The viscosity of the KGM-CH-PVA (KCP) blend adhesive was determined, and the morphology of the film was observed using scanning electron microscopy (SEM). The KCP blend adhesive was applied to plywood during the manufacturing process, and the effects of the KGM, CH, and PVA contents on the bond strength was investigated. Results showed that KGM greatly increased the viscosity of the KCP blend adhesive, whereas the addition of PVA decreased the viscosity in the test range. The SEM observations showed that the KCP blend adhesive was homogeneous. The bond strength of the plywood that was treated with KCP blend adhesive increased with increasing KGM and CH concentrations, and desirable performance could be obtained with a total solids content of 4.6%. The KCP blend adhesive with 2.0% KGM, 2.0% CH, and 0.6% PVA exhibited a comparable bond strength with phenol formaldehyde. Findings suggest that the KCP blend adhesive can be used as a wood adhesive with all raw materials, having the advantage of being environmentally friendly
Effect of microstructure and catalyst coating on the oxygen permeability of a novel CO2-resistant composite membrane
Dual-phase membranes based on oxygen-ion-conducting doped ceria and electron-conducting lanthanum chromite show appreciable oxygen flux and good chemical compatibility between the two phases. In this work, Sm0.2Ce0.8O2âδâLa0.7Ca0.3CrO3âδ (volume ratio 60:40) composite membranes with different grain sizes are prepared. Membranes with large grains (1â5 Îźm) show much higher permeability than those with small grains (~ 300 nm). Applying a surface coating of a Sm0.2Ce0.8O2âδâSm0.5Sr0.5CoO3âδ catalyst reduces the running-in time from 220 to several hours and improves the steady-state permeability. The performance of the composite membrane is maintained when applying pure CO2 as sweep gas, showing promising potential in the application of these membranes in oxyfuel processes using CO2-diluted oxygen
Stability and oxygen permeation behavior of Ce0.8Sm0.2O2âδâLa0.8Sr0.2CrO3âδ composite membrane under large oxygen partial pressure gradients
The stability and oxygen permeation behavior of the Ce0.8Sm0.2O2âδâLa0.8Sr0.2CrO3âδ dual-phase composite were investigated under a large oxygen gradient with one side of it exposed to air and the other side to CO, CH4 or H2 at elevated temperatures. An oxygen permeation flux of 8.6 Ă 10â7 mol cmâ2 sâ1 was obtained with a 1.1 mm thick membrane tube under air/CO gradient at 950 °C, and no decrease in the flux was observed within a period of 110 h. The oxygen flux under air/CO gradient was found to be about twice that under air/CH4 or air/H2 gradients, which may be attributed to the higher catalytic activity of the membrane towards the oxidation of CO. The membrane tube remained intact after high temperature operation for over 1000 h, and no significant change in the phase composition and microstructure occurred. The dual-phase composite may satisfy the stability requirement under the stringent membrane reactor condition
Oxygen-selective membranes integrated with oxy-fuel combustion
The perovskite-type oxide SrCo0.8Fe0.2O3 δ (SCF), a highly oxygen-permeable material, is restricted for application in the membrane-integrated oxy-fuel combustion process by its low tolerance to CO2. In the present work, we found that the CO2 tolerance of SCF is improved by increasing the oxygen partial pressure in the CO2-containing gas. Long term oxygen permeation experiments, at 950 1C, show that mixing 5% of oxygen into the CO2 sweep gas effectively prevents degradation of the SCF membrane. X-ray photoelectron spectroscopy indicates that the increase in CO2 tolerance of SCF is caused by a decrease in basicity of the material with increasing oxygen partial pressure. Based on these experimental results, a modified oxy-fuel combustion process is proposed. Calculation of the required membrane area for operating a 50 MW coal-ired power plant showed that the modified process comprises a viable option
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