Co-DAC: Low-energy Direct Air Capture potential when combined with a Post Combustion Capture plant (Flexible Funding 2022)

Dr Stavros Michailos, University of Hull, was awarded funding in the UKCCSRC’s Flexible Funding 2022 call to look at “Co-DAC: Low-energy Direct Air Capture potential when combined with a Post Combustion Capture plant”.

Direct Air Capture (DAC) has emerged as an important engineering tool to combat climate change as it can indirectly capture CO₂ emissions from different sectors that are hard-to-abate. As with any new technology, improving costs is a key challenge for DAC and a significant contributor to these costs is the energy penalty to capture CO₂ from an initial concentration of ~420ppm.

To tackle this, we have developed a novel design that integrates an MEA DAC absorber with a conventional post-combustion capture (PCC) plant, co-DAC, as depicted in Figure 1. Two configurations have been proposed as follows:

a) the air absorber and the flue gas (FG) absorber are placed in series and the whole amount of the lean flow goes to the air absorber to capture air CO₂ and then is sent to the top of the FG absorber; three cases have been tested in which the semi-rich solvent loading at the exit of the air absorber is either 0.3, 0.25 or 0.20 molCO₂/molMEA;

b) because it is expected that at high semi-rich loadings the FG absorber is not efficiently utilised, a split stream of the cold lean (0.12 molCO₂/molMEA) is sent to the top of the FG absorber (with the flowrate adjusted to get a loading of 0.3 molCO₂/molMEA at the bottom of the top FG absorber bed to avoid irreversibilities when mixing with the incoming semi-rich) while the rest of the lean solvent enters the air absorber and captures CO₂ until the loading reaches 0.3 molCO₂/molMEA and then enters at the top of the second bed of the FG absorber.

Figure 1. Process configurations for the proposed Co-DAC system

Simulations have showcased that the specific reboiler duty (SRD) can drop to 3.5 GJ/tCO₂, a figure that is well within the range of reported energy penalty for conventional FG capture. The main reason for this energy reduction is that, with co-DAC, the rich loading entering the desorber is high enough, >0.45 molCO₂/molMEA, compared to a standalone DAC, ~0.30 molCO₂/molMEA.  The importance of achieving a high rich loading for efficient operation has been showcased by the authors in a recent paper and we have also presented the results of the co-DAC assessment in the GHGT-16 conference.

It can be seen in Figure 2, that if solvent from an air absorber can be fed to an FG absorber, its rich loading can be increased and the overall SRD for combined DAC and PCC will remain low, as long as the lean loading is above (just above, for maximum effect) the desorber inflexion value.

Figure 2. The air absorber solvent flowrate, the average SRD for all CO2 captured, the total reboiler duty, the fossil CO2 capture, L/G ratio and rich loading as a function of the net negative CO2

After the proof-of-concept modelling studies, we have proceeded with designing the DAC absorber lab set-up. Figure 3 is the 3D schematic of the co-DAC process, with an integrated bypass for the DAC side so that the PCC unit can still run without the DAC unit if needed.

Figure 3. The 3D schematic of the lab scale Co-DAC process

The lab DAC absorption column will have a large diameter compared to PCC, as a large quantity of air will be required to pass through the column for the solvent to have a meaningful change in loading. The absorption column will have a diameter of 300 mm, and BX structured packing from Sulzer with the option to upgrade this packing to a hybrid one that specialises in ultra-low liquid fluxes. The DAC column will have a maximum height of 1.5 m, due to height constraints within the laboratory, including space for the liquid distributor and gas disengagement. However, there is potential for multiple columns to be used if more packing height is required. Sulzer have advised that the BX packing can take a maximum gas velocity of 3 m/s. The main equipment of the DAC has been delivered, see Figure 4, and will be assembled following a HAZOP study.

Figure 4. The DAC and the water wash columns (A) and the Sulzer BX packing (B)

Further, modelling of the DAC absorber as once through configuration has been carried out. Figure 5 shows that for a liquid flowrate between 10-12 kg/h, a capture efficiency of ~50% and a change in the loading of ~0.1 molCO₂/molMEA can be achieved.  The latter is suitable for a co-DAC configuration as higher differential can lead to inefficient mass transfer in the FG absorber.

Figure 5. The CO2 Capture rate (A) and the (B) loading differential as a function of liquid flowrate for a column height of 1 m

A preliminary TEA has been carried out based on the Sherman FEED study and revealed that co-DAC capture costs are in the range of 116-614 $/tCO₂, showcasing the potential of the proposed co-DAC system for cost-effective net-negative capture.

Further modelling will be done to accurately model the DAC system.  For example, a greater range of L/G ratio will be tested, through changing the gas flowrates and a wider range of liquid flowrates. Special effort should also be made to find a solution to reach a target concentration of solvent at the outlet for a target absolute loading differential, through changing the inlet concentration of MEA for different flowrates. This could be done either through inclusion of air saturator before the absorption column, or by adding water to the lean stream.

For each case investigated, the process conditions for the wash water column will be investigated to minimise emissions whilst minimising the water footprint of the DAC unit. Finally, a comprehensive techno-economic assessment of the co-DAC system will be carried out.

Overall, the study proposed a novel solution of reducing the energy costs of DAC, and future research will focus on examining the performance of the DAC absorber, including mass transfer measurements and associated fluid flow rates, and reaction rates based on realistic system scenarios. Finally, a detailed discussion about the benefits and challenges of the DAC system can be found here.

Lucas Joel (University of Sheffield), Stavros Michailos (University of Hull), Mathieu Lucquiaud (University of Sheffield), Jon Gibbins (University of Sheffield)

Read more on Stavros’s Flexible Funding 2022 project page.