(284b) Development of a Hybrid System for Direct Air Capture

In order to meet the Paris Agreement targets to limit global warming below 2℃, an imperative reduction in carbon dioxide emissions is needed. Carbon dioxide removal (CDR) technologies must be implemented at giga-ton scale towards to the net-zero pathway [1]. Direct air capture (DAC) has emerged as one of the most efficient ways to minimize the impact of climate change by tackling hard-to abate sectors and historical emissions [2]. Currently, the estimated cost of DAC in the literature varies over one order of magnitude, ~100–1000$/tCO₂ removed [3]. Given the expensive nature of DAC, and its pivotal role in achieving negative CO₂ emissions, it is essential to identify technological options that deliver optimal techno-economic performance.

Today, the development of CO₂ capture systems has focused mainly on single-stage separation technologies. For CO₂ capture, aqueous amine scrubbing is the benchmark technology and is widely deployed in industry for natural-gas sweetening. For dilute sources containing a few per cent CO, aqueous amine absorption processes work well and can achieve up to 90% CO₂ capture and 98% purity. However, technological improvements are still required for large-scale CO₂ capture using aqueous amines in terms of the energy requirement, solvent degradation, absorption capacity and equipment corrosion. In part due to these outstanding issues, alternative CO₂ separation technologies have been developed. Adsorption-based technology for DAC has been developed commercially but is currently associated with high costs. Capturing CO₂ in high purity from a ppm-level feed is indeed extremely demanding for a single technology. Generally, high energy consumption and poor economic performance are major challenges for all DAC technologies, seriously limiting economic feasibility. In this context, opportunities exist to develop hybrid processes for DAC, which might reduce the costs and increase operational flexibility.

In this work, we assess the design and optimization of a hybrid process for capturing CO₂ directly from ambient air. Specifically, we integrate adsorption and absorption processes in the proposed hybrid system. Absorption is a proven technology at the large scale, while adsorption processes benefit from high separation factors and the versatility given by the wide range of sorbent materials available. For the first stage, a temperature vacuum swing adsorption process is considered (TVSA), using commercial sorbents, such as the resins Lewatit VP OC 1065 and Purolite A10, while for the second stage, an aqueous amine absorption process is developed. The adsorption process model includes a fixed bed model with an embedded shell and tube heat exchanger and a thermal design analysis to calculate heating and cooling steps. We evaluated the performance of different adsorbents in terms of the purity of CO₂ produced and energy required to complete the purification, for a mixture of CO₂/H₂O/N₂. With this cycle configuration, starting from 420 ppm CO₂, the desorbed gas was enriched to an intermediate CO₂ purity with varying recoveries, depending on the adsorbent and the operating conditions. For the second stage, we developed a rigorous rate-based absorption model in the Aspen Plus process simulator to simulate the purification of the CO₂ to the desired 98% target. Aqueous amine solvents including monoethanolamine (MEA) and amine blends were considered. We have used the process models to predict the purity and recovery of CO₂ recovered from both stages, and the energy consumption of the process per ton of CO₂ captured. We have employed a process design framework to optimize the performance of the hybrid system and provided insight into the techno-economic assessment of the technology performance. The developed economic model includes sorbent degradation and predicts the capture cost of the process. We have found that as the sorbent degrades, the rate of CO₂ capture decreases, since less CO₂ is captured every subsequent cycle, leading to increasing capital and operation costs per ton of captured CO₂. The results indicate there is a tradeoff between increased working capacity of the sorbent due to the higher desorption temperature and increased degradation rate during desorption. We therefore contend that the sorbent degradation can influence the economic viability of the DAC system. Finally, we are underway to examine experimentally the stability of DAC sorbents using a home-designed automated system in order to validate the degradation modelling results.

The genetic algorithm was used to optimize the operating conditions (operating pressure and desorption temperature, air velocity at the feed, sorbent lifetime, absorber and stripper sizing, CO₂ loading of the lean solvent and capture rate of the absorption model) of the two stages- hybrid system to obtain the minimum total capture cost. The technoeconomic analysis resulted in a capture cost from 240 to 450$/tCO₂, depending on the sorbent, the operation conditions and the sorbent lifetime. We have found that hybrid system is able to achieve lower capture cost than the baseline case of standalone adsorption and absorption processes for DAC, while increasing the CO₂ capture efficiency and decreasing the energy consumption.

References:

¹ National Academies of Sciences, Engineering, Medicine 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. (The National Academies Press).

² Fuss S. et al 2018 Negative emissions-part 2: costs, potentials and side effects. Environ. Res. Lett.

³Kazemifar F. A review of technologies for carbon capture, sequestration, and utilization: cost, capacity, and technology readiness. Greenhouse Gases: Sci Technol 2022;12:200–30.