Direct air capture [DAC] kinetics and thermodynamics

DAC is currently in the early stage of R and D. DAC costs 5-10 times the reforestation costs. Low reaction rate, exothermic adsorption, and entropy generation are three fundamental issues. One huge fundamental cost element in DAC is entropy. Just to pull 400 ppm CO2 which is 0.04% CO2, you draw 2500 times air that all contribute to entropy

Direct Air Capture: What Is It?

A technology called direct air capture extracts carbon dioxide from the atmosphere by means of chemical processes. When air passes over these substances, they react and trap CO2 while letting the other elements of the air pass through. Leading systems of today use either liquid solvents or solid sorbents, which are made of regular chemicals already used in other applications. The carbon dioxide has been drawn from the atmosphere, it is usually released from the solvent or sorbent using heat. By doing this, the solvent or sorbent is replenished for another capture cycle.

Over the past few years, DAC has seen an increase in interest and investment, and more businesses are entering the market. This is because it is recognized that carbon removal will become more and more necessary to achieve national and international climate goals, as well as because DAC has advantages over other carbon removal strategies, such as few practical scaling constraints, minimal land area use, and siting flexibility.

DAC is currently in the prototype phase

Climeworks, Carbon Engineering, and Global Thermostat are some of the businesses with the most cutting-edge technology today. Together, these businesses operate 18 plants, ranging in size from 1 tCO2/yr to 4,000 tCO2/yr (the largest plant currently in operation), capturing a combined total of just under 8,000 tCO2/yr. The majority of that is sold for use in various products, with the other half being permanently sequestered, comparable to the annual emissions from 870 cars.

In the Southwest of the United States, a DAC plant with a capacity of one million tonnes annually is currently being built. It is anticipated to go into operation in late 2024. Other businesses that use sorbent, solvent, and other cutting-edge types of technology have been founded more recently. 

On a scale of 1 to 9, DAC is currently rated as having a "technology readiness level" of 6, indicating that it is still in the large-scale and prototype stage and not yet prepared for full commercial deployment. However, it also means that there are lots of chances to enhance efficiency and cut costs by taking advantage of early technology iterations. 

What Is the Price of Direct Air Capture?

Direct air capture, despite its advantages and adaptability, is more expensive per tonne of CO2 removed than many mitigation strategies and non-manmade climate solutions because it requires more energy to separate carbon dioxide from ambient air. Depending on the technology chosen, the source of low-carbon energy, and the scale of their deployment, the cost range for DAC today ranges between $250 and $600; by contrast, the majority of reforestation costs less than $50/tonne.

Role of kinetics and thermodynamics of DAC

Kinetics of DAC

The rate at which CO2 is drawn from outside air is referred to as the kinetics of direct CO2 capture from air. In order to capture CO2 from the air, it must first be absorbed by a liquid or solid substance before being released as purified CO2. Several variables, including the material used for capture and the environment, have an impact on the rate of CO2 absorption and release.

The concentration of CO2 in the air is one of the most important factors affecting the kinetics of CO2 capture from the air.

It takes more time to capture a sizable amount of CO2 because the ambient air's CO2 concentration is so low (around 400 ppm). Consequently, the process' kinetics are relatively slow compared to other methods of carbon capture, such as capturing CO2 from industrial off-gases.

The kinetics of the process are also impacted by the effectiveness of the CO2 capture materials and external factors like temperature. Depending on their chemical structure and composition, materials used for CO2 capture, like amine-based sorbents, can have varying rates of CO2 absorption and release.

In general, the kinetics of direct CO2 capture from air remains difficult, and significant research is being done to create better materials and systems that can increase the process efficiency and reduce its energy needs.

Thermodynamics of DAC

The thermodynamics of direct CO2 capture from air involves assessing the process's viability and energy needs. Due to the low concentration of CO2 in the air (400 parts per million) and low heat of absorption, capturing CO2 from ambient air is a difficult thermodynamic process.

Materials with a high affinity for CO2, such as amine-based sorbents or solid adsorbents, are typically used to absorb CO2 from the air. These materials' thermodynamic performance is influenced by the system's working conditions, including temperature, pressure, and humidity.

Two important thermodynamic properties, the heat of adsorption and entropy of adsorption, can be used to assess the thermodynamic effectiveness of CO2 capture from air.

The energy released or absorbed by the substance when it adsorbs CO2 is referred to as the heat of adsorption. The degree of disorder in the system during the adsorption process is quantified by the entropy of adsorption.

Role of entropy in DAC

When CO2 is taken directly from air, the system's entropy typically decreases. The reason for this is that the adsorption process typically involves the binding of CO2 molecules to a solid or liquid substrate and that the concentration of CO2 in the air is low (only 400 parts per million). By aligning and attaching to particular binding sites on a surface, CO2 molecules that have been adsorbed onto it increase the surface's order and reduce its entropy.

However, when CO2 is captured directly from the air, the system's overall CO2 concentration is so low and the volume of air is so high that the increase in entropy caused by the adsorbent's mixing with air typically outweighs the initial decrease in entropy caused by the capture process. [It is a very important point to note]

Therefore, during direct CO2 capture from air, the overall entropy change is typically positive, or increases.

The feasibility and energy requirements of direct CO2 capture from the air can be significantly impacted by changes in entropy, so thermodynamics is a key factor in the design and optimization of the process.