Experimental investigation of La_0.6Sr_0.4FeO_{3-<em>δ</em>} pellets as oxygen carriers in a counter-current packed-bed reactor for efficient chemical looping CO₂ splitting

The application of chemical looping for reverse water gas-shift provides an efficient way for the conversion of CO2 to CO, enabling the transformation of captured CO2 into value-added products. For example, by using the produced CO along with renewable H2 to synthesise liquid fuels. In this study, we applied the concept of a chemical ‘memory’ reactor, employing a perovskite-based oxygen carrier (La0.6Sr0.4FeO3-δ, LSF) in a counter-current packed-bed reactor for CO2 splitting. This approach overcomes the chemical equilibrium limitation and could produce high purity CO.Our work experimentally investigated the performance of LSF pellets as oxygen carriers in a large lab-scale packed-bed reactor with gas switching technology for chemical looping CO2 splitting. We evaluated the effects of changes in feed time, bed temperatures, and flow rates on CO2 to CO conversion. Optimal conditions gave over 90 % CO2 to CO conversion via counter-current flow, compared to 45 % for conventional co-current flow in the same reactor. Higher bed temperatures enhanced the CO2 to CO conversion

Proton and apparent hydride ion conduction in Al-substituted SrTiO3

Hydrogen ion conductivity in 2% and 10% Al-substituted SrTiO3 has been investigated by transport number measurements using the concentration cell/emf method in wet atmospheres as a function of pO(2) (10 (- 20) -1 atm) and temperature (350 - 1000 degreesC). Earlier indications of apparent negative charge transport by hydrogen under reducing conditions and high temperatures have been confirmed. By the present measurements, possible artefacts from the type of acceptor-dopant, gas buffer, electrode material, and porosity of the sample appear to have been ruled out. Electrochemical pumping experiments with gas chromatography were inconclusive with respect to hydride ion transport. Thermogravimetry as a function of hydrogen activity did not show evidence of hydride ion incorporation, but indicated uptake of neutral hydrogen under reducing conditions and high temperatures. Quantum molecular dynamics simulations indicate the existence of defect species or clusters that may be reminiscent of interstitial hydrogen with a tendency to associate with effective negative charge, e.g., on neighbouring titanium ions, under simulated reducing conditions. (C) 2002 Elsevier Science B.V. All rights reserved

Use of CaMn0.875Ti0.125O3 as oxygen carrier in chemical-looping with oxygen uncoupling

Chemical-looping with oxygen uncoupling (CLOU) is a novel method to burn fuels in gas-phase oxygen without the need for an energy-intensive air separation unit. The carbon dioxide from the combustion is obtained separated from the nitrogen in the combustion air. The technique is based on chemical-looping combustion (CLC) but does not involve any direct reaction between the fuel and oxygen carrier. Instead, the CLOU process uses three steps in two reactors, one air reactor where a metal oxide captures oxygen from the combustion air (step 1), and a fuel reactor where the metal oxide releases oxygen (step 2) and where this oxygen reacts with a fuel (step 3). This means that the fuel burns directly with gaseousO2. In this work CaMn 0.875Ti0.125O3 will be used as oxygen carrier. Experiments were first performed with a thermogravimetric analyzer (TGA). Here the sintering temperature, and thereby the porosity, for the produced granulates was varied and optimized. The substitution of Ti on Mn sites in CaMnO 3 was chosen since this material showed no coke formation even in dry CH4 at high temperatures. This was followed by fluidized bed experiments with both methane and petroleum coke as fuel. The CaMn 0.875Ti0.125O3 particles showed promising results both for the tests performed in TGA and in fluidized bed experiments. CaMn0.875Ti0.125O3 released O2 both in inert and reducing atmosphere, making it a possible candidate as oxygen carrier in CLOU

Structure, electrical conductivity and oxygen transport properties of perovskite-type oxides CaMn1−x−yTixFeyO3−δ

Calcium manganite-based perovskite-type oxides hold promise for application in chemical looping combustion processes and oxygen transport membranes. In this study, we have investigated the structure, electrical conductivity and oxygen transport properties of perovskite-type oxides CaMn1 x yTixFeyO3-δ. Distinct from previous work, data of high-temperature X-ray diffraction (HT-XRD) in the temperature range 600-1000 °C (with intervals of 25 °C) demonstrates that CaMnO3-δ (CM) transforms from orthorhombic to a mixture of orthorhombic and tetragonal phases between 875°C and 900 °C. The Rietveld refinements show formation of a pure tetragonal phase at 975 °C and of a pure cubic phase at 1000 °C. Partial substitution of manganese by iron and/or titanium to yield CaMn0.875Ti0.125O3-δ (CMT), CaMn0.85Fe0.15O3-δ (CMF) or CaMn0.725Ti0.125Fe0.15O3-δ (CMTF) leads to different phase behaviours. While CMT remains orthorhombic up to the highest temperature covered by the HT-XRD experiments, CMF and CMTF undergo an orthorhombic □(→) tetragonal □(→) cubic sequence of phase transitions. Electrical conductivity relaxation measurements are conducted to determine the chemical diffusion coefficient (Dchem) and the surface exchange coefficient (kchem) of the materials. The results demonstrate that oxygen transport is hindered in the tetragonal phase, when occurring, which is attributed to a possible ordering of oxygen vacancies. The small polaron electrical conductivity of CM in the cited temperature range is lowered upon partial manganese substitution, by about 10% for CMF and up to half an order of magnitude for CMT and CMTF