news / 2013-10-30

MEM-OXYCOAL: Ceramics reduce energy needed to separate air

Required membrane surface considerably reduced

Researchers have achieved considerable improvements in the oxygen permeability of membranes. “Until recently, the membrane surface area required for membrane-based oxyfuel processes in typical 500-megawatt power plants would have covered 20 football fields,” says Dr Michael Schroeder, private lecturer at the Institute for Physical Chemistry at RWTH Aachen University. This is based on an oxygen flux of 1.8 m³/(m²*h). Following the latest laboratory development, the membrane achieves a flux of 40 m³/(m²*h) under optimum conditions. The membrane surface area required for a typical power plant is therefore now only the size of one football field. This would lead to considerable material and cost savings when implementing the oxyfuel technology.

The cross-section shows a thin membrane layer (approx. 20 µm) consisting of the oxide (Ba,Sr)(Co,Fe) O3 on a porous, carrier-like material. Foto: Forschungszentrum Jülich

MEM-OXYCOAL: Promising material for separating air

The research in the MEM-OXYCOAL project identified (La,Sr)(Co,Fe)O3 (LSCF) as a promising material class for CO2-stable, oxygen-conducting membranes. The perovskite material proved to be stable in a pure CO2 atmosphere and in a model gas at a temperature of 850 °C. The model gas consisted of 95 per cent carbon dioxide, five per cent oxygen and 400 ppm sulphur oxide, whereby the scientists investigated different compositions in which the La and Sr content were varied.

 

At a temperature of 850 °C, initial gas-tight, asymmetric membranes with a 30 µm layer thickness and a supporting thickness of 900 µm produced an oxygen ion flow of 1 ml cm-2 min-1. Potential for further increasing the membrane flux is particularly provided in the surface area enlargement and activation areas.

Composite membranes as alternative separating membranes

An interesting alternative to perovskite membrane materials are composite membranes consisting of a penetration structure with ion and electron conducting phases. The composite membranes developed in the MEM-OXYCOAL project are based on doped cerium oxide, an oxide ion conductor, and spinels as electron conductors. The dual-phase materials show stable oxygen permeation under exposure to CO2 and are stable in the model gas. They are also chemically stable towards magnesium oxide at the application temperatures and have a comparable coefficient of expansion. This therefore enables porous magnesium oxide to be deployed as a cost-effective support material for asymmetric composite membranes. The mechanical characteristic values, such as the expansion behaviour, the creep rate and the strength of the materials used, such as magnesium oxide, are considerably cheaper than those of the previously investigated perovskite materials.

Cryogenic air separation

With cryogenic air separation, oxygen is extracted from air by means of liquefaction. The Linde process is available for the large-scale generation of oxygen. This makes it possible to manufacture both gaseous and liquid oxygen and nitrogen as well as argon.

Project management MEM-OXYCOAL

Prof Dr Manfred Martin

Institut für Physikalische Chemie

RWTH Aachen

Landoltweg 2

52056 Aachen

Telefon: +49(0)241 8094712

Fax: +49(0)241 8092128

www.ipc.rwth-aachen.de/martin

GREEN-CC

The project partners are working on improving the long-term stability and the oxygen flux in the subsequent GREEN-CC, EU project, which started in September 2013.