Project Cluster / 2011-05-02
Separating gases using membranes
Research approach
Membranes have the potential to be used in CO2 capture from flue gases (post-combustion), in hydrogen/CO2 separation in synthesis gases after production of combustion gas (pre-combustion) and in oxygen production in the oxyfuel process. Experts believe that the application of membranes in pre-combustion and oxyfuel plants is particularly promising due to the physical and chemical conditions present here. Applications in post-combustion plants are also the subject of intensive research and development work at the moment.
One focus of research is the improvement of the availability and efficiency of the basic process of synthesis gas formation (gasification, gas purification and conditioning, heat integration). Progress in membrane technology for the separation of CO2 and hydrogen could contribute to increasing the efficiency and economic viability of this process.
Membranes could also help in a power plant when the fuel is to be converted under the exclusion of the nitrogen fraction in the air (oxyfuel). The optimisation goal here is the reduction of the energy consumption for air separation by using membrane processes. Membranes for oxyfuel and other processes represent a very promising field of research and offer great potential for innovation. The full range of applications here will probably only open up after 2020.
Research goals
- Development and characterisation of suitable membranes (microporous membranes, polymer membranes, zeolites, MIEC, MPEC)
- Development and optimisation of power plant concepts in order to specify the requirements demanded of membranes for power plant applications
- Identification of lines of technology that have the lowest development risk (path-dependency)
- Development of membrane-production processes that take advantage of nanotechnology
- Membrane tests under real operating conditions (flue gas, synthesis gas, etc.)
- Development of high-temperature H2 membranes for conversion reactions in IGCC power plants (refer to coal gasification)
- Identification of synergy effects for other industrial applications of membranes
- Development of suitable membrane materials with sufficient long-term stability
- Development of membrane modules that are as leak-free as possible
- Engineering and energy-related limitations with regard to the integration of the membrane module into the flow path of the flue gas
Outlook
Polymer-based, metallic and ceramic membranes are already being used currently in a range of gas separation processes, e.g. with high partial pressure gradients. Fundamental R&D work on the development of membranes (dense, porous, ion-conducting and electron-conducting membranes) for capturing CO2 in power plant processes is currently being carried out worldwide.
Membranes are materials whose material structures allow for selective permeation of gases. The selectivity of membranes for the permeability of various gases depends principally on the membrane material and the transport mechanisms involved. The flow of the gas through the membrane is influenced mainly by the partial pressure differences of the various gases across the two sides of the membrane. Processes with high gas pressures are thus usually preferred for membrane separation. A wide range of industrial applications of membrane separation of gases already exist – e.g. the removal of CO2 from natural gas. However, there have not yet been any proven large-scale applications in power plants. The reliability requirements and costs associated with use in power plants have thus also not yet been verified.
The development of membrane technology for air separation is being supported by state funding in many cases – e.g. the Ion Transfer Membrane project in the USA, the Oxycoal COORETEC joint project in Germany, and the ENCAP project within the EU's 6th Framework Programme for Research. The goal of the Oxycoal project is the development of a high-temperature membrane for the oxyfuel process. The separation process is based on innovative mixed-conducting ceramic membrane materials (perovskite oxides), which are only permeable for oxygen and require high operating temperatures (around 850 °C) (see figure). In contrast with the application conditions in the oxyfuel process, the specified conditions for membrane development for the IGCC process include as high a pressure as possible (pressure above that of the gasifier), high oxygen purity (>90%) and a maximum temperature of 230 °C (due to O2 handling). This temperature limitation means that it is necessary to develop a low-temperature membrane.
Catalytic high-temperature H2 membranes are regarded as an alternative for the CO conversion reaction. The goal is to combine CO conversion and the separation of H2 or CO2 in a water-gas shift membrane reactor. The research and development projects that have been initiated in this area are still at a very early stage. They are concentrating initially on the identification of suitable membrane coatings, particularly with regard to their durability. However, the application of this catalytic high-temperature (shift) membrane only makes sense in combination with hot-gas purification.
Background
Great efforts are invested in the development of the first oxyfuel process, and research work is also already underway at RWTH Aachen University on the so-called oxycoal process, a second-generation oxyfuel process.
In the oxycoal process, the nitrogen contained in the air is removed by a ceramic membrane module instead of using an energy-intensive cryogenic air separation process. This process will use ion-transfer membranes that allow only oxygen to pass through them at temperatures above around 700 °C. If an oxygen concentration gradient is present, oxygen transport takes place across these membranes at sufficiently high temperatures. In this way, the oxygen contained in the air can be selectively fed to the process and nitrogen can be excluded.
In future oxycoal power plants, air compressed to around 20 bar will be fed to the membrane module and will transfer most of its oxygen content to the recirculated flue gas in the membrane reactor. The high temperatures required here will be achieved by recirculating flue gas at around 850 °C. Once air that has given off oxygen and has been heated up leaves the membrane reactor, it is expanded in a gas turbine. In this way, a portion of the power consumed in compressing the air is recovered.
One focus of research is the development of suitable membrane modules for large-scale use. There are also design issues to be resolved in parallel with the development of membrane materials with long-term durability and sufficiently high rates of oxygen transport. Only then can leak-free operation be guaranteed even in the case of temperature fluctuations. Investigations on this matter are currently being conducted at RWTH Aachen University. Also being investigated is feasibility of flue gas recirculation at the high temperatures necessary and of purification of the hot gas upstream of the membrane module.
15 current research projects
Nano-structured Ceramic and Metal Supported Membranes for Gas Separation in fossil-fueled power plants
Organisation carrying out research: Forschungszentrum Jülich GmbH - Institut für Energieforschung (IEF-1; IEF-2)
Design, construction and operation of a module test stand for separation of CO2 out of flue gas
Organisation carrying out research: Geesthacht Centre for Materials and Coastal Research GmbH, Institute of Polymer Research
Review of separating mechanism
Organisation carrying out research: Karlsruher Institut für Technologie (KIT), Engler-Bunte-Institut / Bereich Verbrennungstechnik
Project number: 03ET2016A, 03ET2016B, 03ET2016C
Low-pressure membrane process for energy-efficient CO2 capture
Organisations carrying out research: Universität Bremen - Center for Environmental Research and Sustainable Technology (UFT), membranotec GmbH & Co. KG, Bremen
Project numbers: 0327512A, 0327512B
Chemical stability and transport properties of uncoated and coated oxygen-separation membranes
Organisation carrying out research: Rheinisch-Westfälische Technische Hochschule Aachen- Faculty of Mathematics, Computer Science and Natural Sciences - Division of Chemistry - Institute of Physical Chemistry
Project number: 0327803B
Diffusion and exchange of oxygen in ceramic membranes and kinetic stability of membranes
Organisation carrying out research: Rheinisch-Westfälische Technische Hochschule Aachen- Faculty of Mathematics, Computer Science and Natural Sciences - Division of Chemistry - Institute of Physical Chemistry
Project number: 0327803A
Structural and analytical characterisation of membrane materials
Organisation carrying out research: Rheinisch-Westfälische Technische Hochschule Aachen - Central Facility for Electron Microscopy and Ernst Ruska-Centre (ER-C) for Microscopy and Spectroscopy with Electrons
Project number: 0327803E
Electrical and electrochemical characterisation processes
Organisation carrying out research: Karlsruhe Institute of Technology (KIT) - Department of electrical and information technology - Institute of Materials for Electrical and Electronic Engineering (IWE)
Project number: 0327803F
Investigation and optimisation of electron-conduction and ion-conduction, surface structure and kinetics of oxygen-permeation membranes
Organisation carrying out research: Westfälische Wilhelms-Universität Münster - Institut für Anorganische und Analytische Chemie (IAAC)
Project number: 0327803G
Oxygen exchange, chemical expansion behaviour and calorific reaction behaviour of membrane ceramics
Organisation carrying out research: Bauhaus-Universität Weimar - Fakultät Bauingenieurwesen - Professur Bauchemie
Project number: 0327803H
Development, characterisation and testing of nano-scale, dense membrane layers for oxygen separation
Organisation carrying out research: Fraunhofer Institute for Silicate Research ISC
Project number: 0327803J
Development of materials and processes for optimised ceramic membrane components
Organisation carrying out research: Hermsdorfer Institut für Technische Keramik e.V.
Project number: 0327803K
Membranentwicklung
Organisation carrying out research: Forschungszentrum Jülich GmbH - Institut für Energieforschung (IEK-1; IEK-2)
Project number: 0327803C
Thermochemische und Thermomechanische Membran Charakterisierung
Organisation carrying out research: Forschungszentrum Jülich GmbH - Institut für Energieforschung (IEK-1; IEK-2)
Project number: 0327803D
