Sina Hoseinpoori, Energy Technology

In 2024, the mean global temperature exceeded the 1.5 °C limit for the first time, highlighting the increasing likelihood that greenhouse gas emissions will significantly overshoot the limits set by the Paris Agreement. Thus, technologies for carbon dioxide removal (CDR) are crucial for limiting global warming to well below 2°C. Among the CDR technologies, Direct Air Capture (DAC) has gained attention due to its scalability and potential for placement in proximity to storage sites. However, DAC remains a relatively costly option for CO2 removal, mainly because of its substantial energy requirements due to the low concentration of CO2 in the atmosphere. This, together with its relatively low levels of technical maturity make its deployment marginal so far.

This thesis first compares the different DAC technologies in terms of energy and exergy, to assess how material use influences overall technology performance. Thereafter, the thesis investigates three alternative deployment opportunities for the two DAC technologies that have the highest technology readiness levels: Temperature Vacuum Swing Adsorption (TVSA) and Alkaline Absorption with subsequent Calcium Looping (ALK-ABS). The studied opportunities entail: (I) using DAC for capturing CO2 from low-concentration (<4% CO2) flue gas streams; (II) using DAC as a second CO2 capture step after scrubbing using monoethanolamine (MEA) from high-concentration (>4% CO2) flue gas streams, so as to achieve net-zero direct emissions; and (III) integrating DAC into combined heat and power (CHP) plants. The work combines modeling at the process and reactor levels to close the mass and heat balances, and the use of these results in a techno-economic modeling framework for calculating the costs of carbon capture and carbon avoidance.

The exergy analysis indicates that adsorption-based DAC processes have greater overall exergy efficiency than the absorption-based process. Moreover, the findings highlight the importance of material stability and degradation for overall process performance: material consumption represents around 5%–10% of the total exergy demand for absorption-based DAC systems, and the corresponding range for the adsorption-based process is 10%–40%.

Regarding the use of DAC for capturing CO2 from dilute flue gas streams (I), the modeling results reveal that MEA scrubbing has a better economic performance than TVSA or ALK-ABS when the flue gas flowrate exceeds 200 t/h. For lower flowrates (especially those <100 t/h), TVSA is the most cost-effective option under the conditions studied, and ALK-ABS is only cost-efficient for a narrow range of flowrates of 100–200 t/h and CO2 concentrations of 0.5%–1.2%. Furthermore, for such low flowrates and CO2concentrations <2%, the carbon avoidance cost is higher than offsetting CO2 at the current market prices for CDR credits (here considered to be in the range of 400–600 $/tCO2).

The use of DAC as a second step (after MEA scrubbing) in the capture sequence for high-CO2-concentration flue gases (II) has been compared with the combination of CCS and offsetting of residual emissions through CDR credits, as well as with the operation of CCS at a capture rate that is sufficiently high to achieve net-zero emissions directly. The hybrid solution is shown to involve lower overall costs to achieve net-zero direct emissions than the other two strategies studied. Furthermore, it is found that the alternative of offsetting residual emissions externally achieves its optimal overall capture cost when MEA scrubbing is operated at a capture rate of approximately 99%, i.e., far higher than the typically used benchmark value of 90%.

Finally, it is found that integrating DAC into CHP plants (III) presents a viable business opportunity, particularly in the emerging CDR market. When taking as the credit value a future cost projection for DAC of 680 $/tCO2, CDR could contribute up to 80% of the net cash flow of CHP plants in the future, with DAC alone accounting for 12%. Further estimates suggest that integrating DAC into CHP plants across Sweden could meet approximately 33% of the country’s national CDR target.