Development of CO2 stable (La0.6Ca0.4)(Co0.8Fe0.2)O3-δ hollow fiber membranes for the plasma induced CO2 conversion

Abstract

The present work deals with the perovskite (La0.6Ca0.4)(Co0.8Fe0.2)O3-δ (LCCF) in the form of dense hollow fiber membranes for the oxygen separation from CO2 containing gas mixtures. A potential application is the in situ extraction of oxygen from a CO2 plasma. In such a plasma, the CO2 can be split into CO and oxygen by microwave energy. The goal of this work was the development of LCCF hollow fiber membranes, the optimization of the oxygen transport through these membranes and to confirm the applicability of the extraction of oxygen from a CO2 plasma. Prerequisite for the material and the membrane was a good CO2 stability and a good thermal shock resistance. In a long-term test (>200 h) at 900 °C a good CO2 stability could be verified. To optimize the oxygen permeation flux through the membrane, the bulk diffusion and the surface exchange reactions were affected. To optimize the bulk diffusion, hollow fiber membranes with different wall thicknesses and different sintering temperatures were manufactured. With a wall thickness of 81 µm and a sintering temperature of 1220 °C the highest oxygen permeation flux (6.2 ml min-1 cm-2 at 1000 °C) in a CO2 containing atmosphere could be achieved. The surface exchange reactions were affected by the surface etching method. The goal was to increase the roughness of the surfaces, what could be obtained, by H2SO4 treatment. The best results could be achieved by treating the inner and outer surface with H2SO4 for 180 min. The permeation flux could be increased by 86 % compared to the pristine hollow fiber. Furthermore, both permeation limiting steps were addressed by developing asymmetric hollow fibers with a porous support of LCCF and a thin dense layer of LCCF. A gastight dense layer of 22 µm could be achieved. The oxygen permeation flux compared to a dense hollow fiber membrane (wall thickness 179 µm) could be improved by 68.6 %. Finally, the applicability of the plasma induced CO2 conversion was analysed. The oxygen permeation flux of LCCF hollow fiber membranes in different plasmas (air and CO2) were compared with the permeation in an oven heated system. The permeation in an air plasma was 60.6 % higher at a similar temperature, which is caused by the special plasma atmosphere. In the CO2 plasma, the feasibility of the conversion of CO2 in oxygen and CO could be proven and the generated oxygen can be extracted through the LCCF hollow fiber membranes. At a microwave power of 1 kW an oxygen permeation flux of 4.96 ml min-1 cm-2 in a CO2 plasma could be achieved.