Steps in Carbon Capture and Storage (CCS)
The process of Carbon Capture and Storage (CCS) involves three essential stages:
1. Capture:
Capture of Carbon Dioxide (CO₂) in CCS
Introduction: Why Capture CO₂?
The first and most important step in Carbon Capture and Storage (CCS) is to capture CO₂ from power plants and large industries before it is released into the atmosphere. The goal is to produce a pure and pressurized stream of CO₂ that can easily be transported and stored safely.
Capturing CO₂ from the entire gas stream would be too costly and energy-intensive. So, the focus is on separating CO₂ efficiently from the gases produced during fuel combustion.
Where Is CO₂ Captured?
CO₂ is captured from:
- Power plants (e.g., coal or gas-based thermal plants)
- Industrial units (e.g., ammonia production, natural gas processing)
These plants already separate CO₂ in some processes, but most of it is still released into the atmosphere. Only a small portion is captured for commercial or experimental use.
Main CO₂ Capture Technologies
There are three major methods used to capture carbon dioxide, depending on the process and fuel used
- Post-Combustion Capture
- What it does: Removes CO₂ from the flue gases (exhaust gases) after burning the fuel.
- How it works: Uses chemical solvents like Monoethanolamine (MEA) to absorb CO₂ from flue gases.
- Where used: In coal-fired and natural gas power plants.
- CO₂ concentration: Typically 3–15% in the flue gas.
- Pre-Combustion Capture
- What it does: Converts fuel (coal, natural gas, or biomass) into a mixture of carbon monoxide (CO) and hydrogen (H₂) before
- How it works: CO reacts with steam to form CO₂ and H₂. The CO₂ is captured, and hydrogen is used to generate power.
- Where used: In Integrated Gasification Combined Cycle (IGCC)
- CO₂ concentration: High, 15–60%.
- Oxy-Fuel Combustion
- What it does: Burns fuel in pure oxygen instead of air, producing flue gas that contains mostly CO₂ and water vapour.
- How it works: Water vapour is removed by cooling. The remaining CO₂ can be captured easily.
- CO₂ concentration: Over 80% in flue gas.
- Challenge: Requires separating oxygen from air first, which is energy-intensive.
- Status: Currently in the demonstration stage.
CO₂ Separation Techniques
To isolate CO₂ from gas mixtures, the following techniques are used:
- Chemical solvents (e.g., amines)
- Physical solvents
- Membranes
- Cryogenic separation (cooling gases to separate CO₂)
- Solid sorbents
The choice depends on temperature, pressure, and gas composition.
How Efficient Are These Systems?
- Current systems can capture about 85–95% of the CO₂ produced.
- However, 10–40% more energy is needed for capture and compression.
- As a result, the net CO₂ captured is around 80–90%.
- Oxy-fuel systems can capture nearly 100%, but pollutants (like sulfur and nitrogen oxides) may lower this slightly.
2. Transport:
The captured CO₂ is then transported—typically through pipelines or specialized containers—to a designated storage site.
Transport of Carbon Dioxide (CO₂)
Why Transport CO₂?
After CO₂ is captured from power plants or industrial facilities, it needs to be moved to a storage site, unless the capture point is located directly above the storage location (such as a deep geological formation). Transporting CO₂ safely and efficiently is therefore an essential part of the Carbon Capture and Storage (CCS) process.
Main Methods of CO₂ Transport
- Pipelines – The Most Common Method
- Pipelines are the most widely used and technologically mature option for transporting CO₂.
- CO₂ is usually compressed to high pressure (above 8 MPa) to make it denser. This reduces transport costs and avoids issues like unstable flow (two-phase flow).
- In this dense state, CO₂ behaves like both a gas and a liquid—called dense-phase transport.
- The first major CO₂ pipeline began operation in the 1970s.
- In the United States alone, over 2,500 km of CO₂ pipelines move more than 40 million tonnes of CO₂ each year—mostly for Enhanced Oil Recovery (EOR) operations in Texas.
- Ships, Rail, and Road Tankers
- CO₂ can also be transported in liquid form using:
- Ships
- Rail tankers
- Road tankers
- These transport CO₂ in insulated tanks at low temperatures and reduced pressures.
- This method is useful when storage sites are far away or not connected by pipeline infrastructure.
- CO₂ can also be transported in liquid form using:
Operation and Safety Aspects
- CO₂ pipelines operate at ambient temperature and high pressure to maintain dense-phase flow.
- Most pipelines use compressor stations to push the CO₂ through the pipeline. In some long-distance systems, booster compressors are installed along the route to maintain pressure.
- The technology is well-established, with safe handling practices in place, especially in oil and gas regions.
Storage
Finally, the CO₂ is permanently stored in secure locations deep underground or in oceanic reservoirs, where it is kept away from the atmosphere to prevent its contribution to climate change.
Geological Storage of Carbon Dioxide (CO₂)
What Is Geological Storage?
Geological storage is a method of storing captured CO₂ by injecting it deep underground into natural rock formations. The CO₂ is compressed into a dense (liquid-like) state before being injected into the earth’s subsurface.
Where Is CO₂ Stored Geologically?
There are three main types of geological formations used for storing CO₂:
- Depleted Oil and Gas Reservoirs
- These are underground rock layers that once held oil or natural gas.
- Since these reservoirs have already proven their ability to trap fluids for millions of years, they are considered reliable storage sites.
- CO₂ can be injected into them to either store it permanently or to enhance oil recovery (EOR).
- Deep Saline Formations
- These are porous rock layers filled with salty (non-potable) water.
- Found in onshore and offshore sedimentary basins, they have huge storage potential.
- These formations are widely distributed and not connected to oil or gas extraction, making them ideal for long-term CO₂ storage.
- Unmineable Coal Beds
- These are coal seams that cannot be mined due to depth or quality.
- When CO₂ is injected, it gets adsorbed onto the coal surface, displacing methane.
- This method can help produce methane gas, but is still under demonstration and depends on the coal’s permeability.