Disruptive CO₂ Capture

Background
Modelling adsorption processes is a challenging task [ ]. The interplay between mass and heat transfer in different phases (gas and solid) should be accounted for in a transient manner. Moreover, since one or more gases are adsorbed (equivalent to say that they are removed from the gas phase), the gas velocity changes across the units. This, together with the right scale of the process requires the use of the momentum balance, which for simplicity is normally replaced by simpler equations like Darcy or Ergun. Regeneration can be done in the same unit or in different unit which means that either the column or the adsorbent is subjected to different cyclic boundary conditions.
Since the level of complexity and the degree of maturity of the different adsorption processes is not similar, the first task to be done is to use a common mathematical framework to describe all selected adsorption processes to be studied. Such a model will certainly have different boundary conditions as well as mass and heat transfer conditions. Moreover, the packing of the units can be different and thus, the relation between velocity and pressure should be adapted.

Goal
The generic model was used for the following temperature swing adsorption (TSA) processes
1. Fixed bed TSA processes with pellets (TSA), hollow fibers (HFTSA) and monoliths (MTSA)
2. Moving bed temperature swing adsorption (MBTSA);
3. Rotating bed temperature swing adsorption (RBTSA);
4. Swing adsorption reactor cluster (SARC);
5. Fluid bed TSA process (FBTSA)

The mathematical framework should render pre-defined Key Performance Indicators (KPIs) that will link the different processes to the CAPEX and OPEX and eventually include other environmental parameters [ ].
The TSA processes were coupled with a direct contact cooler upstream and a cryogenic purification unit (Figure 1).
The overall scheme was optimized using the Bayesian optimization methodology. The objective was to minimize the levelized cost of electricity (LCOE). At the minimum levelized cost, the indicators like energy penalty, CO₂ avoidance cost and the capital and operating expenses were studied.

The following sorbents were studied: Zeolite 13X, CALF-20, Silica EB-PEI from KRICT and Diamine appended metal organic framework.

Activities
The following activities were carried out in this project
1. Process modelling and optimization of the TSA processes
2. Techno economic assessment (TEA) to calculate the levelized cost of electricity (LCOE)
3. Synthesis and characterization of novel sorbents for process modelling study.
4. Evaluating the potential of advanced sorbents to lower the cost of CO₂ capture
5. Evaluating the technical and economic uncertainties.

Results
It was seen through rigorous process that the fundamentally most attractive process configurations employing sorbent circulation and indirect heat transfer for minimizing the energy penalty, i.e., MBTSA, FBTSA, and CSAR, return the lowest cost. The RBTSA process showed considerable promise owing to the lack of valves and the fact that a single rotating bed can undergo multiple steps. Fixed bed configurations (HFTSA) suffer from low heat transfer coefficients and the need to heat and cool heat exchange surfaces and monolith support structures together with the active sorbent during each temperature swing. MTSA and RBTSA overcomes the heat transfer limitation by using direct steam heating, but the quantity of steam required to carry out the heating largely using sensible heat instead of latent heat fundamentally imposes a very high energy penalty. Moving and fluidized bed concepts, on the other hand, achieve good heat transfer coefficients to enable efficient indirect heating with the latent heat in the steam extracted from the power plant. In addition, they only need to heat and cool the sorbent transferred between the two reactors and implement recuperative heat exchange to further minimize the sensible heat penalty involved.

The primary conclusion from this work is that there exists considerable scope for cost reductions from further sorbent development. Reducing sorbent cost and increasing kinetics and adsorption capacity had the largest impact. Avoiding H2O and N2 competition and increasing the temperature sensitivity of the sorbent were the most impactful. Such advances have the potential to reduce the CO₂ avoidance cost by around 40 €/ton in the NGCC power plant retrofit scenario studied in the DCC3 project.

The uncertainties related to economic assumptions are much more influential on the headline economic indicators than uncertainties related to the modelling employed. Statistically, the 90% confidence interval for the technical uncertainties is 3x smaller than for the economic uncertainties. This is a positive finding, illustrating that the 1D model used to generate the results is reasonably robust and trustworthy.

Further Work
Temperature-swing processes are essential for efficient CO₂ capture from dilute flue gas streams such as the NGCC power plant. The sorbents chosen for this work have their own advantages There is plenty of room left for further improvement over these first-generation sorbents. These potential gains were quantified, showing how CO₂ avoidance costs could be lowered by 30-40 €/ton through the development of advanced sorbents. This is a large cost-reduction lever that deserves further R&D effort, and such gains should be taken into account when considering the long-term future of sorbent-based CO₂ capture.

The DCC3 project also evaluated the CO₂ capture process using the exhaust gas recirculation option. In the case of EGR, the CO₂ concentration is around 8%. All the aforementioned processes achieved significantly better performance owing to the increased CO₂ concentration as well as the lower quantity of flue gas.
Sorption technologies are more effective when the CO₂ concentration is greater than 20%. It is therefore recommended combined different processes like sorption and membranes to improve the process performance. The advantages of a hybrid process are that it combines the benefits of two technologies, thereby potentially reducing the footprint and the energy consumption.

Publications
1. Krishnamurthy, Shreenath; Cloete, Schalk; Mondino, Giorgia; Watson, Francesca; Klemetsdal, Øystein; Popescu, Elena-Roxana; Morud, John; Llewellyn, Philip; Pugnet, Veronique; Blom, Richard; Marrying Commercial Sorbents to the Optimum Temperature Swing Adsorption (TSA) Cycles Through Process Simulations and Techno-Economic Assessment, Proceedings of the 17th Greenhouse Gas Control Technologies. https://dx.doi.org/10.2139/ssrn.5032875
2. Krishnamurthy, Shreenath; Cloete, Schalk; Pugnet, Veronique; Simulation and Optimization of a Rotary Temperature Swing Adsorption (RTSA) Process for CO₂ Capture Industrial & Engineering Chemistry Research 64 24 12131-12144 2025 American Chemical Society https://doi.org/10.1021/acs.iecr.4c04957
3. Krishnamurthy S., Blom,R., Mondino, G., Cloete, S., Morud, J., Zaabout, A., Llewelyn, P., Pereira, C., Pugnet, V., Simulation and optimization of fixed bed and moving bed TSA processes for post-combustion CO₂ capture., Computer Aided Chemical Engineering,Elsevier,Volume 52,2023, 313-318. https://doi.org/10.1016/B978-0-443-15274-0.50050-0 .
4. Cloete, S., Krishnamuirthy, S and Mondino, G., Grande, C. A. Morud, J., Zaabout, A., Blom, R., Pugnet, V. and Llewellyn, P. Performance of Lewatit adsorbent at dry conditions in various contactor/regeneration environments in a NGCC context; a comparative study (November 25, 2022). Proceedings of the 16th Greenhouse Gas Control Technologies Conference (GHGT-16) 23-24 Oct 2022, http://dx.doi.org/10.2139/ssrn.4285980
5. https://climit.no/en/news/carbon-management-project-aims-to-transform-co%E2%82%82-adsorption-technology/

Technology Readiness Level (TRL)
N/A

Intellectual property rights (IP)
Codes developed for process modelling and optimization