Dual Phase membranes for CO2 separation in power generation
Climit financing90 % from the Research Council, 10% self financing
Project partners• University of Oslo • SINTEF Materials and Chemistry • International collaboration: LECIME (Paris, France) and Argonne National Laboratories (ANL, US)
Project period2011 – 2014 (extended to 2015)
Goal of project:
The aim of DUALCO2 is to determine the transport mechanisms and kinetics that
facilitate selective permeation and removal of CO2 from gas streams through dual phase dense ceramic membranes. Based on this, the project investigates the possible integration of dual phase membranes in post- and pre-combustion CO2 capture.
Dense dual-phase membranes are fabricated using different processing routes. The microstructure and interfaces are characterised by scanning electron microscopy and X-ray diffraction. The transport mechanisms through the bulk of the dual-phase membranes are studied by means of electromotive force measurements, electrochemical impedance spectroscopy, and flux experiments. Surface exchange kinetics are also investigated in order to identify the rate-limiting steps of CO2 separation using dense dual-phase membranes.
Membranes offer in principle an easy and efficient way of capturing CO2. Membrane devices have the advantage that they operate continuously and that they do not have sorbent materials that need to be regenerated or replenished. Dense dual-phase membranes can capture CO2 with a higher selectivity than porous membranes. These dense dual-phase membranes are highly selective for CO2 separation from multicomponent gas streams at high temperatures (450-600°C). This has the advantage that the flue gas does not have to be cooled down prior to capturing the CO2, thus the CO2 separation process is optimised.
Dense dual-phase membranes consist of a molten carbonate and a solid oxide phase which have different properties. It is thus necessary to investigate both the bulk and surface properties of these dual-phase membranes. Furthermore, it is a challenge to understand the interfacial interactions between the molten carbonate and ceramic phase. It is also necessary to optimise the microstructure of the two phases, by taking into account the electrical, electrochemical, and thermo-mechanical characteristics of both components. For the electrochemical measurements and flux studies, dedicated tools and test rigs are required to enable systematic experiments under well-defined conditions in order to identify the charged species involved in the transport through both phases of the membrane. The theory governing these methods needs to be developed and verified.
Results to date:
Dual-phase membranes consisting of selected molten carbonates and solid oxides have been fabricated and characterised by electrochemical methods and flux studies. The total conductivity of the molten carbonate phase embedded in a ceramic matrix was measured and electromotive force (EMF) measurements were carried out under well-defined atmospheres in order to obtain the transport numbers of the charged species present in the molten carbonate phase. In dry conditions, under a gradient of CO2, transport numbers close to the theoretical value of unity were obtained after correcting for the electrode polarisation. In a gradient of water vapour, evidence for the transport of hydrated (protonated) species was detected.
Flux studies of symmetrical dual-phase membranes were performed under various operating conditions to identify the transport mechanisms. The measurement results are summarised as follows:
- Relative density of the oxide support: The CO2 flux increases with decreasing relative density of the ceramic supporting phase, attributed to a better percolation of the molten phase.
- Sealing materials influence: The electrical conductivity of the sealant has a large influence on the CO2 flux. A highly electrically conducting sealant provides a pathway for electrons and increases the oxygen flux though the membrane.
- Surface polishing: The CO2 flux measurements were done for both polished and un-polishes samples. The polished sample (removing the surface layer of residual molten carbonate) gives essentially the same CO2 flux as the unpolished sample, indicating that this surface does not have a rate limiting role.
- Steam effects: There are effects of water vapour (steam) in accordance with the transport of hydrated species as found by EMF measurements. This is under further examination.
- Long-term stability: Stability of the dual phase membrane was tested under operational conditions, the membrane works from weeks to a few months depending on the initial molten phase infiltrated into the supporting oxide matrix. The degradation may be due to evaporation of the carbonate, and is under further examination.
- Theoretical derivations: The flux equations with consideration of different charge species including hydrated ones were derived, and explain largely the observed CO2 flux data and EMF measurements. A model for polarisation of electrodes in EMF measurements when ambipolar transport of more than one ionic species is needed for more accurate corrections and is work.