Supervisor:
Thomas Helmer Pedersen

Co-Supervisor:
Eliana Lozano Sanchez

Assessment Committee:
Thomas Condra (Chair)
Professor, Derk Villem Frederik Brilman, University of Twente, NL
Professor, Liyuan Deng, NTNU, NO

Moderator:
Jakob Hærvig

Abstract:

Direct air capture (DAC) is an essential part of climate change mitigation strategies, but current DAC processes are energy intensive and face several technical challenges that must be solved before large scale deployment can be achieved. The aim of this dissertation is thus to investigate multiple DAC technologies from both an experimental and a modeling perspective to identify the key technical elements that govern the process performance of different DAC concepts.

A particular focus of this thesis is on adsorption-based DAC in which discrepancies in adsorbent characteristics and a lack of experimental data in the literature entail uncertainty when evaluating the process performance. Using a combination of volumetric measurements, dynamic column breakthrough (DCB) experiments, and CHN element analysis, the root cause behind discrepancies in the CO2 loading capacity is uncovered, relating to the particle size variation during the amine functionalization of the adsorbent material. Additionally, the underlying mass transfer mechanisms of amine adsorbents are illuminated based on a combination of DCB experiments and modeling efforts, revealing a significant dependence on macro-porous mass transfer resistances and a potential mass transfer limitation associated with the reaction kinetics of the CO2-amine reactions. Dynamic simulations of a temperature vacuum swing adsorption (TVSA) DAC process highlight the importance of correct representation of the mass transfer kinetics to determine the CO2 productivity and optimize the process design while addressing the practical implications of having a cyclic process.

As an alternative to the cyclic TVSA-DAC process, this thesis investigates the technical potential of using a cross flow hollow fiber membrane contactor (HFMC) in a DAC context, focusing on the absorption of CO2 in a KOH solution. The work includes an experimental demonstration of the capture potential and a modeling-based parametric study to evaluate the process performance. The findings indicate the potential to match the performance of existing benchmark liquid-based DAC contactors, laying the foundation for future research that also address the solvent regeneration.

Finally, this thesis provides a technical comparison of integrating either a solid-based DAC (S-DAC) process or a liquid-based DAC (L-DAC) technology with alkaline electrolysis and methanol synthesis to produce a carbon neutral fuel. The results indicate that the S-DAC process offers a greater heat integration potential and better water management than the L-DAC system. However, the continuous production of syngas from the L-DAC process can yield a favorable compatibility with other fuel syntheses.