(310d) Design of Adsorption-Based Direct Air Capture Units Towards Negative Emissions

The scientific and engineering communities are dedicating substantial efforts towards identifying technologies to capture carbon dioxide (CO₂), also considering its active removal from the atmosphere (Direct Air Capture, DAC) [1]. In addition to permanent subsurface storage (DACCS), the application of DAC is attractive in the context of carbon utilization (CCUS) where potential routes include synthesis of fuels [2]. Various global leaders in DAC (e.g., Climeworks, Global Thermostat) have pioneered the use of Temperature Swing Adsorption (TSA) technology [3,4]. In TSA, the separation is carried out in a sequence of steps through a cyclic variation of temperature to saturate and regenerate one or more fixed bed columns packed with a given adsorbent. The design of such units is a complex, multifactorial problem, where variables including the number and duration of each cycle step, the column temperature and pressure, and the feed flow rate need to be optimised [5]. With approximately ten small TSA-DAC plants operating worldwide (capturing almost 0.01 Mt CO₂/year) [1], the potential to develop a fully scalable process (capturing 1 Mt CO₂/year) is still high. In this effort, a key element of the design is the reduction of the energy penalty of the process. The latter is one of the key drivers to the total cost of capture and directly correlates to the climate benefits of negative emission technologies, such as DAC.

Here we present a model-based framework for the design of an adsorption-based TSA-DAC process that yields negative emissions. We propose a design methodology that enables TSA-DAC units to optimally operate under variable, local ambient conditions. We consider a TSA process in an adsorber packed with an amine-functionalised sorbent and apply a rigorous mathematical model to assist with the design and optimisation of the unit. At its core is a detailed first principles model of a transient, one-dimensional cylindrical column, consisting of both energy and material balances, previously validated against experimental data [6]. For cycle design, we consider a two-bed TSA cycle in a fixed-bed configuration to provide direct comparison against recent literature findings [7].

The identified candidate designs are assessed and compared based on three Key Performance Indicators (KPIs): quality (CO₂ purity), economic feasibility (CO₂ productivity and CO₂ recovery) and environmental sustainability (energy consumption). We demonstrate how these KPIs can be sensitive to operational uncertainties, challenging the identification of a universal operating profile for DAC. Specifically, we discuss the role of both manipulated variables (e.g., flow rate, regeneration temperature) as well as of process parameters (e.g., feed temperature and humidity) within the ranges of interest for locations where current DAC pilots operate. The outcomes of this analysis are captured in a set of look-up maps that represent feasible operating points under different design setups. These maps represent a key starting point to quantify the flexibility of the process and to identify suitable control strategies that can guarantee stable operation over time. This novel ability to consider operability aspects at the design stage is key to enable the full integration of TSA-DAC into the energy system for worldwide adoption of this important negative emission technology. Importantly, we assess how industrial symbiosis can be exploited by examining the operation of the identified designs within industrial clusters.

References

[1] IEA 2021, Direct Air Capture: https://www.iea.org/reports/direct-air-capture;

[2] Gabrielli et al. 2020 Ind Eng Chem Res 59:7033

[3] Deutz et al. 2021 Nat Energy 6:203

[4] Terlouw et al. 2021 Environ. Sci. Technol. 55:11397

[5] Idelfonso et al. 2020 Ind Eng Chem Res 59:14037

[6] Ward et al. 2022 Adsorption 28:161

[7] Stampi-Bombelli et al. 2020 Adsorption 26:1183