Environmental Considerations For Large-scale Direct Air Capture With Fuel Cell (Flexible Funding 2024)

Dr Ruiqi Wang, Durham University, was awarded funding in the UKCCSRC’s Flexible Funding 2024 call to look at “Environmental Considerations For Large-scale Direct Air Capture With Fuel Cell”.

Our objectives

Direct air capture (DAC) system aims to extract CO₂ directly from the ambient air, and it is generally coupled with permanent geological storage or utilised in the production of fuels, chemicals, and other materials. With the rapidly evolving potential for scaling up engineered removals, DAC has been proposed to be developed and scaled up over next decades, contributing significantly towards achieving the net-zero emissions targets.

Liquid solvent-based DAC (L-DAC) as one of the most advanced and commercial DAC technologies faces its main challenges in high-temperature heat and energy demand for regeneration process. To avoid compromised carbon reduction efficiency of DAC due to use of fossil fuels for generating requisite heat and electricity, solid oxide fuel cell (SOFC) fed by green hydrogen has emerged as a viable option to simultaneously supply high-grade heat and renewable electricity to DAC.

The longer supply chain for L-DAC integrated with green hydrogen fed SOFC introduces indirect environmental impacts from installation of renewable energy sites, and presents additional risks and environmental considerations for energy generation and storage. This research therefore aims to use life cycle assessment (LCA) to thoroughly evaluate the life cycle impacts of an L-DAC system integrated with green hydrogen-fed SOFC, which will highlight key environmental considerations for the large-scale deployment of the proposed system.

Our system

The L-DAC process adopts the potassium-carbonate loop to capture CO₂ from air. The ambient air is first drawn through an air contactor where CO₂ reacts with aqueous potassium hydroxide (KOH) to form potassium carbonate (K₂CO₃). Then the K₂CO₃ is pumped into a pellet reactor where the solution reacts with calcium hydroxide (Ca(OH)₂), further precipitating calcium carbonate (CaCO₃) pellets. The pellets are then calcined at approximately 900 ℃, liberating CO₂ for compression and subsequent sequestration and utilisation.

The SOFC operates on green hydrogen and supplies both power and high-temperature heat to L-DAC unit. At the anode of SOFC, hydrogen is oxidised to water molecule whilst releasing electrons. Oxygen ions (O^2-) formed at the cathode migrate through the electrolyte to complete the electrochemical reaction. The electrons flow between the anode and cathode via an external electrical circuit to generate DC electricity which drives the air contactor fans, pellet reactor, slaker, air separation units, and the CO₂ compression unit. The high-temperature anode exhaust delivers steam and residual fuel to the calciner, and high-temperature vitiated air from cathode exhaust is fed to heat jacker surrounding the calciner, meeting the thermal demand of L-DAC.

The green hydrogen is produced on-site via a high-temperature steam electrolysis (HTSE) unit. It also has a thermal linkage with SOFC by using the exhaust from stacks of fuel cell for the required high-temperature heat, while the electrolyser’s electrical load is met with external power such as grid electricity and renewable electricity.

Figure 1. Schematic view of L-DAC integrated green hydrogen-fed SOFC system

Our findings

Relative to the L-DAC with natural gas-based combined heat and power supplied baseline (0.305 kg CO₂ eq./kg CO₂ captured), coupling the L‑DAC unit to an SOFC fed by hydrogen produced from wind-powered HTSE cuts self‑emitted GHGs by up to 86 %. The results from LCA show that the deployment of SOFC with L-DAC has significantly decreased the impacts in climate change, marine and terrestrial eutrophication, ozone depletion, photochemical ozone formation, and fossil resource use. Producing hydrogen through HTSE is not shown as the best option in several impact categories compared to hydrogen production from steam methane reforming, namely acidification, freshwater ecotoxicity and eutrophication, human toxicity, land use, particulate matter, mineral and metals resource use.

Read more on Ruiqi’s Flexible Funding 2024 project page.