Why is this research needed?
This research focuses on two types of CO2 sorbents with high degree of tunability that are at different levels of maturity in terms of testing and manufacturing. These materials include: metal-organic frameworks (MOFs) materials for post-combustion and industrial capture, and also inorganic porous materials for chemical looping combustion (CLC).
Despite intense research efforts in the area of low temperature adsorbents for CO2 capture, e.g. MOFs, and their potential benefits compared to liquid-based systems, amine-based solvents remain the benchmark in the field. Most CO2 adsorption studies stagger at the lab-scale. The reason for such stagnation is simple: we do not know the exact features an adsorbent should display. Scientific works have placed a large emphasis on enhancing sorption capacity and CO2/N2 selectivity and identifying the sorption mechanisms. While these are necessary features to investigate, they are in no way sufficient to deliver material decreases in the $/tCO2 of CCS. For instance, a sorbent should exhibit a “fast” rate of mass transfer and be “easily” regenerated under moderate conditions. Identifying quantitative targets for these materials screening parameters and linking them to process performance metrics (e.g. CO2% removal, CO2 purity) and cost is needed to accelerate the transition of this technology to higher TRLs and engage with relevant stakeholders. This has yet to be achieved and is precisely an area of focus of this core research project.
Chemical looping combustion (CLC) is a method of capturing and then separating a high purity CO2 often from coal or natural gas combustion using an oxygen carrier, without the fuel and air touching each other. The CLC process can be used for many different types of fuel, though. Being able to separate flue gases into two separate streams makes this method one of the more energy efficient ways to capture a greater proportion of the CO2 emissions. This research seeks to investigate the potential for scale-up and sustained usage of promising metal oxides and mixed metal oxide sorbents.
What is this research investigating?
On the MOF side of the project, our work merges aspects of process engineering, chemical engineering and materials engineering. Our objective is to develop approaches that enable to identify quickly ‘the best’/most promising adsorbents or the properties of the ‘ideal’ adsorbent for a given gas separation, here CO2 capture. Towards this goal, we have established a screening methodology to evaluate the performance and cost of MOFs for CO2 capture in different scenarios. In our first study, we have used an equilibrium-based pressure vacuum swing adsorption (PVSA) model. Here we screened 22 MOFs along with 3 reference materials (i.e. 2 zeolites and an activated carbon) in the context of post combustion CO2 capture from energy and industrial streams: NGCC, coal, cement and steel. We are now expanding this assessment to consider temperature vacuum swing adsorption.
In regard to the inorganic porous materials for chemical looping combustion (CLC), these will be developed for both H2/O2 production and standard “unmixed” combustion. Preliminary studies at Imperial have produced highly porous Fe2O3 and CuO carriers retaining strength and reactivity over multiple cycles. Besides O2 carriers containing only one active metal (and the support material), mixed-metal oxides (MMO) will be produced. Those contain, for example, oxides of Cu, Mn, Al and/or combined phases. Kinetics and thermodynamics (such as the position and slope of the phase equilibrium curve between the two relevant oxidation states) will be optimised. The work will be used in pilot demonstrations at Cranfield University, where the low-cost CLC materials can be tested.
What does the research hope to achieve?
Ultimately, this work aims to provide clarity on the applicability of solid CO2 capture sorbents for post combustion and industrial CO2 capture.
In the context of MOFs, the low and medium term impact are two-fold:
- to develop an adsorbent screening and evaluation methodology that can be used by academics and industrialists to identify suitable adsorbents for CO2 capture; and
- to provide the basis required to develop business models for the commercialisation of low temperature CO2 adsorption technologies.
This work, along with that from our work packages A2 (a and b), AC1 and AC2, feeds into the modelling work in AC3 and AC5. The modelling work in AC3 and AC5 then links closely to the modelling work in our Systems and Policy theme.