Decarbonisation Technology May 2025 Issue

us and storing them sounds almost futuristic. However, DAC is already a proven technology, operating at a small scale. The challenge is to expand the capacity to deliver a meaningful difference. Unlike traditional carbon capture methods that extract CO₂ from concentrated industrial emissions, DAC works with an immense disadvantage: the gas it seeks to capture is incredibly diluted. At just 0.04%, atmospheric CO₂ is far more dilute than that in flue gas streams, where concentrations can reach 10-15%. This makes DAC an uphill battle in terms of efficiency and energy demand, requiring highly selective materials that can take CO₂ out of the air without excessive energy use. Currently, two main approaches are used to achieve this: liquid solvent-based DAC (L-DAC) and solid sorbent-based DAC (S-DAC), each with distinct mechanisms, energy requirements, and scalability potential. “ Despite the high energy demand, L-DAC systems can operate at scale, with some commercial plants capturing 1 MtCO₂ per year ” L-DAC relies on two closed chemical loops to extract CO₂ from the atmosphere. In the first loop, air is brought into contact with an aqueous basic solution (such as potassium hydroxide), where CO₂ reacts to form a stable carbonate. In the second loop, the captured CO₂ is released through high-temperature processing in a series of units operating between 300°C and 900°C, which makes this approach highly energy- intensive. Traditionally, this heat is sourced from natural gas or concentrated solar power, increasing operational costs and, unless they are fully powered by renewables, emissions. Despite the high energy demand, L-DAC systems can operate at scale, with some commercial plants capturing 1 MtCO₂ per year. A downside is the water consumption, as a L-DAC plant may require 4.7 tonnes of water per tonne of captured CO₂, particularly in regions with low humidity and high temperatures. S-DAC uses solid adsorbents, such as amine-functionalised materials, metal-organic

frameworks (MOFs), and mixed metal oxides (MMOs), to selectively bind CO₂ molecules to their surface. These materials operate through an adsorption/desorption cycling process, where CO₂ is first captured at ambient temperature and pressure, then released through a temperature-vacuum swing process at a much lower temperature than for L-DAC, typically 80-120°C. This significantly reduces energy consumption and allows integration with waste heat and renewable electricity ( Jialiang Sun, 2023 ). While S-DAC systems are generally more energy-efficient, they still face challenges related to long-term stability, sensitivity to moisture, and degradation over multiple capture-release cycles. However, they offer a modular design, meaning plants can be scaled by adding more adsorption/desorption units. At present, a single S-DAC module has a capture capacity of up to 50 tCO₂/year and, in some cases, can simultaneously extract water from the air, with early prototypes removing 1 tonne of water per tonne of captured CO₂. The largest currently operating S-DAC plant captures 4,000 tonnes of CO₂ per year, so it is much smaller in scale compared to large L-DAC plants. Further material improvements and cost reductions will be needed for S-DAC to play a critical role in decentralised and renewable- powered DAC applications. Energy dilemma: can DAC be scaled without a carbon footprint? While DAC offers an effective means of removing atmospheric CO₂, its energy demand presents a major challenge: can it be scaled without leaving a carbon footprint? Today’s DAC systems require between 5.5 and 9.5 GJ of energy per ton of CO₂ captured, depending on the technology used. The critical question is where does this energy come from? If DAC plants rely on fossil fuels, they risk undermining their own climate benefit. The ideal scenario is to power them with waste heat, geothermal energy, nuclear, or surplus renewables, but availability and cost remain barriers to large- scale deployment. The energy demand for DAC varies significantly depending on the technology and whether the captured CO₂ is stored or used.