Financing: 90% from the Research Council of Norway, remainder is self-financing
2010 – 2013
Objective: This project studies the fundamental aspects of a new process for selective capture of CO2 from a gas stream, with controlled desorption. The process is based on the principle that CO2 reacts with a metal oxide such as CaO to CaCO3, while this carbonate decomposes and releases CO2 at a higher temperature – also called carbonate looping. The unique aspect of the project is that the process will take place in a molten salt reactor using CaCl2 or another inorganic salt.
Technical content: Three technical sub-projects have been defined: i) Construction and make up of a simple single stage laboratory reactor with gas and gravimetric analysis. ii) Identification of potential chemical systems. iii) Optimisation and testing of the selected systems.
Technical benefits: Thermal cycling at high temperatures enables lower energy loss in the form of electric output using CCS from a thermal power station. Rapid gas-liquid reactions facilitate CO2 capture plants that are small in terms of physical size. When alkali metal fluorides are added, an exchange reaction occurs which enables extremely high selectivity in the absorption.
Research challenges: Absorption of CO2 from gaseous mixtures represents a new application for molten salt reactors. A number of fundamental thermodynamic data elements, such as phase diagrams, must therefore be established from scratch. Dissolving the active substances in a liquid will mean that dilution effects may occur. This will thus pose a challenge in achieving a sufficiently high absorption capacity in the liquid in terms of weight, compared with alternative technology. The same factors may arise for the heat capacity and thus energy loss in the thermal cycling.
Results achieved: We have now tested two fundamentally different molten salt systems as carrier/solvent.
i) In a chloride-based molten salt reactor, tests have been performed with cyclical absorption/desorption of CO2 from a simulated flue gas blend (14% CO2 + N2). To date, up to 10 cycles have been performed in the same experiment with an efficiency at the start of the experiment in the area of 90% (stable concentration in gas after desorption 1-5%) in a 10-cm high column. No degradation of the absorption medium observed after 10 cycles. Rapid desorption takes place at 930C.
ii) A molten salt composition based on fluorides has proven to exhibit extremely selective absorption. A 10-cm high column is able to absorb nearly all (<100 ppm) CO2 from a simulated flue gas (14% CO2). This indicates that such a molten salt can absorb CO2 from extremely diluted gas mixtures without requiring large facilities. On the other hand, CO2 is desorbed at a somewhat higher temperature in this molten salt reactor.
The results are as expected, based on theoretical calculations and what is achieved in solid phase carbonate looping that is not based on molten salts. Degradation of the absorbing medium – which is the main challenge in solid phase processes, does not appear to occur in a molten salt-based process.