Dual Phase membranes for CO₂ separation in power generation

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.

Technical content:

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.

Technical advantages:

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.

R&D challenges:

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 activity gradients of CO₂, O2, and/or H2O 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 one were obtained after correcting for the electrode polarisation. In a gradient of water vapour, evidence for the transport of hydrated (protonated) species was detected in the molten carbonate.
A new theoretical approach was developed for correcting the transport numbers of foreign ions (O2–, OH–) that takes into account the significant electrode polarisation of the native ions. The corrected transport numbers of the foreign oxide and hydroxide ions are of several percent.
In collaboration with our international project partners at IRCP-I2E in Paris, a new electrode holder was designed for performing conductivity measurements of molten carbonates. The conductivity of molten carbonates with different amounts of selected oxide additives indicated the possibility of achieving an enhanced ionic conductivity.
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 through the membrane.
- Surface polishing: the CO₂ flux measurements were done for both polished and un-polishes samples. The polished sample (removed 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.
- Oxide addition to the molten carbonate phase increases the concentration of oxide ion and therefore increases the ambipolar CO₂ transport. The effects were evaluated under both dry and wet conditions in combination with the effects of steam.
- 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.