Direct capture of CO2 from air using amine-functionalized resin - Effect of humidity in modelling and evaluation of process concepts

Abstract

Direct air capture (DAC) using adsorbents is a promising negative emissions technology if coupled with CO2 storage and is a viable option as a CO2 source for producing truly carbon-neutral synthetic fuels. However, if this technology is to become a serious climate change mitigation tool with the capture potential of several GtCO2/year, the cost needs to decrease significantly. To achieve this, the material and energy requirements of the process need to be minimized by adsorbent development and process design. Most of the developments in the field of DAC have focused on the synthesis of novel adsorbents, while some of the other aspects of the process have been less studied. This thesis aims to fill some of these gaps in DAC process research.

In this work, the performance of an amino resin for CO2 capture was studied experimentally in a wide range of conditions. Humidity and cold conditions were found to nearly double the experimental CO2 adsorption capacity of the studied amino resin in some cases. Based on a working capacity analysis using isotherm modelling, the typically proposed DAC process using temperature and vacuum swing adsorption (TVSA) with a closed inlet is severely limited in terms of CO2 working capacity.

Using an improved fixed-bed experimental setup with automatic operation, a detailed comparison of DAC regeneration processes was carried out. By using a purge gas such as air in TVSA, CO2 productivity was significantly increased. This method was also advantageous in terms of the specific energy requirement and adsorbent stability compared to the TVSA process with closed inlet. Therefore, in applications that do not require pure CO2 such as greenhouses or microbial cultivation, using inert purge gas or air is beneficial in adsorbent regeneration.

A novel kinetic model was developed that takes into account the effect of humidity in CO2 adsorption on amine-functionalized adsorbents. Using this model, humid CO2 isotherms and adsorption column dynamics were accurately modelled. The kinetic model can be expected to be generally useful in DAC process modelling, and ultimately in DAC process design and optimization.