15.3 Carbon capture and storage technologies

Carbon Capture and Storage (CCS) Technologies

Carbon capture and storage (CCS) is a set of technologies that capture $CO_2$ emissions from industrial sources, transport them, and store them underground to keep them out of the atmosphere. It's especially relevant for industries that are hard to decarbonize through renewable energy alone, like cement production, steel manufacturing, and fossil fuel power generation.

Carbon capture and storage basics

The core idea behind CCS is straightforward: instead of letting $CO_2$ escape into the atmosphere from smokestacks, you capture it at the source, compress it, and lock it away in geological formations deep underground.

• CCS targets point sources of emissions, meaning large facilities like power plants and factories where $CO_2$ is concentrated in exhaust streams
• Captured $CO_2$ is compressed into a dense, liquid-like state for transport (usually via pipeline) to a storage site
• One commercial application is enhanced oil recovery (EOR), where captured $CO_2$ is injected into aging oil reservoirs to push out remaining oil while simultaneously storing the $CO_2$ underground. This helps offset costs, though it also extends fossil fuel production.

Methods of carbon capture

There are three main approaches to capturing $CO_2$, and they differ based on when in the process the carbon is separated.

Post-combustion capture separates $CO_2$ from flue gases after a fossil fuel or biomass has been burned. This is the most widely used method because it can be retrofitted onto existing power plants and factories.

1. Flue gas passes through a chemical solvent (most commonly an amine solution) that selectively absorbs $CO_2$
2. The $CO_2$-rich solvent is heated in a separate chamber, releasing concentrated $CO_2$
3. The solvent is recycled back to absorb more $CO_2$

Pre-combustion capture removes carbon before combustion by first converting the fuel into a mixture of hydrogen and carbon monoxide (called syngas).

1. Fossil fuel is gasified to produce syngas ($H_2$ + $CO$)
2. A water-gas shift reaction converts the $CO$ into $CO_2$ and additional $H_2$
3. The $CO_2$ is separated using physical or chemical solvents
4. The remaining hydrogen is used as a clean-burning fuel for power generation

Oxy-fuel combustion burns fossil fuels in nearly pure oxygen rather than regular air. Because nitrogen is removed from the process, the resulting flue gas is mostly $CO_2$ and water vapor, making separation much simpler. The tradeoff is that producing pure oxygen requires an energy-intensive cryogenic air separation process.

Carbon Storage and Challenges

Process of carbon storage

Once captured, $CO_2$ needs to be stored permanently. The two main options are geological sequestration and ocean storage, though geological storage is far more developed and favored.

Geological sequestration injects compressed $CO_2$ into deep underground rock formations, typically at depths greater than 800 meters. Suitable formations include depleted oil and gas reservoirs, deep saline aquifers, and unminable coal seams. The $CO_2$ is trapped by three mechanisms that work on different timescales:

1. Physical (structural) trapping holds $CO_2$ in the pore spaces of rock beneath an impermeable cap rock, similar to how natural gas is trapped underground
2. Solubility trapping dissolves $CO_2$ into the salty water (brine) already present in the formation, which happens over decades to centuries
3. Mineral trapping occurs when dissolved $CO_2$ reacts with minerals in the rock to form stable carbonate compounds, effectively locking the carbon away permanently (over centuries to millennia)

Careful site selection and long-term monitoring are essential to ensure the stored $CO_2$ doesn't migrate or leak back to the surface.

Ocean storage involves injecting $CO_2$ directly into the deep ocean (below ~1,000 m), where high pressure keeps it dissolved. However, this approach is largely disfavored because dissolved $CO_2$ increases ocean acidity, which can harm marine ecosystems. Monitoring stored $CO_2$ in the open ocean is also far more difficult than monitoring underground sites.

Challenges of CCS implementation

CCS technology works, but several barriers have slowed its widespread adoption:

• High costs: Building capture infrastructure, pipelines, and storage facilities requires massive capital investment. This is a particular barrier for developing countries. As of recent estimates, CCS can add $50–100+ per ton of $CO_2$ captured, depending on the source.
• Energy penalty: The capture and compression process itself consumes significant energy, reducing the overall efficiency of a power plant by roughly 15–30%. This means more fuel is burned to produce the same amount of electricity.
• Leakage risks: If stored $CO_2$ escapes back to the surface, it undermines the entire purpose of CCS and could pose localized health and environmental risks. This demands rigorous site selection, continuous monitoring, and strong regulatory frameworks.
• Public acceptance: Communities near proposed storage sites may oppose projects due to concerns about safety, groundwater contamination, or long-term effectiveness. Transparent communication about risks and benefits is critical.
• Scalability: To make a meaningful dent in global emissions, CCS would need to scale from the current handful of large projects to hundreds or thousands worldwide. That requires coordinated international investment, policy support, and infrastructure development that doesn't yet exist.