Direct Air Capture vs Post-Combustion : What Works Where? 

As global climate ambitions rise, so does the demand for effective carbon management strategies. While renewable energy adoption and energy efficiency are important, carbon capture has emerged as an essential third pillar, especially for industries that are difficult to decarbonize. Among the various technologies available, Direct Air Capture (DAC) and Post-Combustion Capture are two leading approaches for tackling carbon emissions. 

Though both aim to reduce CO₂ levels, they differ in application, scalability and cost. This content explores the science, advantages, limitations, and ideal use cases of each technology, that helps to understand what works where, and why. 

What is Direct Air Capture (DAC)? 

Definition: 
Direct Air Capture (DAC) is a carbon removal technology that extracts CO₂ directly from ambient air using chemical processes. Unlike traditional carbon capture methods that target emissions at industrial sites, DAC captures CO₂ from the general atmosphere—making it suitable for tackling diffuse emissions. 

Process Steps: 

1. Large fans draw ambient air into the capture unit. 

2. The air passes over a solid sorbent or through a liquid solvent (e.g., potassium hydroxide) that binds with CO₂ molecules. 

3. The sorbent or solvent is heated or placed under vacuum to release the captured CO₂. 

4. The pure CO₂ is compressed for storage underground (geologic sequestration) or reused in various industries. 

Advantages: 

• Removes CO₂ already in the atmosphere (vital for net-negative emissions). 

• Modular and location-flexible. 

• Supports long-term climate goals beyond emission reduction. 

Disadvantages: 

• Low ambient CO₂ concentration (~0.04%) makes capture energy-intensive. 

• High operational costs (~$500–600/ton of CO₂). 

• Needs renewable energy input to ensure true climate benefit. 

What is Post-Combustion Capture? 

Definition: 
Post-combustion carbon capture refers to the removal of CO₂ from the exhaust gases of fossil fuel combustion in power plants or industrial facilities. It is one of the most widely used carbon capture technologies. 

Process Steps: 

1. Exhaust gases from combustion (e.g., in a coal plant) are routed to a capture unit. 

2. The gas passes through an absorber tower where a chemical solvent (commonly Monoethanolamine – MEA) binds with CO₂. 

3. The CO₂-rich solvent is sent to a stripper tower, where heat is applied to release pure CO₂. 

4. The CO₂ is compressed and sent for storage or utilization. 

Advantages: 

• High capture efficiency at large point sources (up to 90% CO₂ removal). 

• Well-developed, proven technology with commercial-scale projects in operation. 

• Can be retrofitted to existing fossil fuel power plants. 

Disadvantages: 

• Only effective at stationary point sources—not suitable for distributed or mobile sources. 

• Significant energy penalty to regenerate the solvent (typically 3–4 GJ/ton CO₂). 

• Retrofitting older plants can be technically complex and expensive. 

What Works Where? A Contextual Comparison 

The effectiveness of Direct Air Capture and Post-Combustion Capture depends heavily on where and how they are applied. Each has unique advantages tied to the nature of the emission source: 

• Post-Combustion Capture is best suited for large, stationary emitters like coal- and gas-fired power plants, cement kilns, steel mills, and refineries. These facilities emit flue gases with relatively high CO₂ concentrations (10–15%), making capture more energy-efficient and economically viable. Additionally, many of these plants are already connected to CO₂ transport and storage infrastructure, facilitating smoother deployment. 

• Direct Air Capture, on the other hand, is ideal for situations where emissions are dispersed or mobile. This includes emissions from aviation, agriculture, shipping, etc. DAC systems are modular and can be installed almost anywhere, making them flexible tools for offsetting residual emissions and helping industries reach true net-zero or even net-negative emissions. 

• For carbon utilization purposes—such as producing synthetic fuels or materials—CO₂ captured via either method can be used, depending on the required purity, location, and economic factors. 

In essence, Post-Combustion Capture addresses current point-source emissions, while Direct Air Capture tackles legacy and distributed emissions, making them complementary technologies in a comprehensive climate strategy. 

Cost Factors and Deployment Challenges 

Cost remains one of the most significant barriers to scaling both DAC and post-combustion technologies. 

• Post-Combustion Capture is more cost-effective today, especially when retrofitted to large industrial facilities. The cost of capturing CO₂ ranges from $40 to $100 per ton, depending on the fuel type, plant design, and capture efficiency. However, energy penalties, retrofitting complexity, and solvent degradation add to operational expenses. 

• Direct Air Capture is currently more expensive, with costs ranging between $500 and $600 per ton of CO₂. This is due to the very low concentration of CO₂ in ambient air, which requires more energy to isolate. However, ongoing research, technological advances, and economies of scale are expected to drive prices down significantly over the next decade. 

• Energy source is also critical for both technologies. Using fossil energy to power capture processes can reduce or even negate their climate benefits. For optimal effectiveness, both should be powered by low-carbon or renewable energy sources. 

• Policy support, carbon pricing, and incentives (such as tax credits or carbon markets) will be crucial to make both methods more financially viable and attractive to investors and industries. 

Conclusion 

Direct Air Capture and Post-Combustion Capture are not rivals—they are strategic partners in the journey to a low-carbon future. 

Post-combustion capture is immediately deployable at large emission sources and can significantly reduce ongoing CO₂ emissions. Meanwhile, Direct Air Capture offers long-term potential for cleaning up emissions and addressing sectors that cannot be easily decarbonized. 

By aligning these technologies with the right applications, scaling up innovation, and ensuring strong policy support, we can accelerate the path to net-zero and beyond. In the carbon management toolbox, it’s not a matter of either-or, but where, when, and how to use both the strategies effectively.