Build and Experimental Characterization of a novel continuous Direct Air Capture system

Abstract

The unprecedented rise in CO2 concentration in the atmosphere, and the accompanying rise in global temperature, has created the need for the development of environmentally friendly sources of energy. A very promising line of research focuses in the development of Carbon Capture and Utilisation (CCU) technologies; through which carbon-neutral liquid fuels could be synthesized. A great example of such technologies can be found in Zero Emission Fuels (ZEF), a dutch start-up currently developing a microplant capable of producing methanol from solar energy and ambient air. ZEF’s process involves the capture of CO2 and H2O directly from ambient air, the splitting of the captured H2O into its constituent
elements through electrolysis, and the synthesis of methanol from H2 and CO2. The entire process is powered by solar energy, which makes ZEF’s methanol a carbon-neutral liquid fuel.

This thesis focuses in the capture of CO2 and H2O from ambient air, a process known as Direct Air Capture (DAC). ZEF’s DAC unit differs from industry standards by using a novel continuous chemical absorption process through the flow of an amine based sorbent between its absorption and regeneration systems, instead of using a conventional batch process through a supported sorbent. The work of this thesis pertains the build and experimental characterization of ZEF’s first full scale DAC prototype, referred to as the 10X DAC prototype.

The built prototype, a fully functional DAC unit working in a continuous process, was experimentally characterized in terms of its performance and energy efficiency for the environmental conditions of the Dutch summer (i.e. hot and wet conditions). The absorption process was found to be limited by the capture of CO2 when compared to the H2O capture process, and to be liquid side limited; which aligned with literature sources where CO2 capture is defined as diffusion limited in the liquid side. Overall, the 10X DAC absorption column has the capacity of capturing 0.52 kgCO2/m3·hr; which is higher than the reported capture capacity of a very well established company like Carbon Engineering. In the
regeneration process, residence times in the stripper column as low as six minutes were enough for the sorbent to reach vapour liquid equilibrium (VLE) loadings for a regeneration temperature of 120◦C. The overall 10X DAC process is capable of producing 0.21 kgCO2/hr with an energy demand of up to 33MJ/kgCO2; which is up to 6 times higher than the reported most energy efficient DAC unit in industry. Furthermore, the H2O:CO2 product ratio was found to vary significantly, with a dependence to absolute humidity in the environment.

Through the experimental results of the characterization process, a chemical absorption model was developed which, assuming a liquid side diffusion limited process, considers the effects of the selected sorbent’s characteristics and the environmental conditions in the absorption process. Through this model, and the use of previous developed regeneration models, a DAC cycle is presented through which the effects of varying dynamic conditions (i.e. absolute humidity and ambient air temperature) were determined. Overall, it was noticed that the performance of the absorption process was negatively affected by higher temperatures, while the regeneration process remained unaffected by temperature
changes. Absolute humidity variations affected the H2O loading of the sorbent, which then affected the performance of the regeneration process; lower H2O loadings in the rich sorbent translated to higher CO2 lean loadings after regeneration. Furthermore, it was determined that the current sorbent in ZEF’s process favors cold and humid environments.

Finally, through the learnings and findings from both experiments and the developed absorption model, an optimized 10X DAC design is presented which focuses in meeting the CO2 production and energy demand requirements established by ZEF for its process.