Carbon capture and storage

Carbon capture and storage is the process of capturing carbon dioxide (CO2) formed during power generation and industrial processes and storing it so that it is not emitted into the atmosphere. Facilities with CCS can capture almost all of the CO2 they produce. Carbon capture and storage systems applied to a modern conventional power plants could reduce CO2 emissions to the atmosphere by approximately 80-90% compared to a plant without a carbon capture and storage system. As of 2019 there are 17 operating CCS projects in the world, capturing 31.5Mt of CO2 per year, of which 3.7 is stored geologically.

The carbon capture and storage chain consists of three parts: capturing the carbon dioxide, transporting the carbon dioxide, and storing the carbon dioxide. Carbon dioxide is often stored underground in depleted oil and gas fields or in deep saline aquifer formations.

Capture technologies allow the separation of carbon dioxide from gases by one of three methods: pre-combustion capture, post-combustion capture and oxyfuel com­bustion.

Pre-combustion capture systems convert solid, liquid, or gaseous fuel into a blend of hydrogen and CO2. This is achieved through processes of gasification or reforming. The result can be used to fuel electricity production and has been suggested as a possible fuel source for vehicles or home heating with lower emissions than current fuel sources.

A pre-combustion capture system (credit: Costain)

Post-combustion capture involves capturing the carbon dioxide from the exhaust of a combustion system and absorbing it into a solvent before the pollutant elements are removed and compressed. These processes can include using high-pressure membrane filtration and cryogenic separation processes. This is the most common used process of carbon capture, especially for power plants.

Using oxy-fuel combustion, oxygen is separated from the air before combustion. The fuel is then combusted in oxygen using recycled flue-gas. This creates an atmosphere full of oxygen and nitrogen with flue-gasses (comprised of CO2 and water) and allows for easier purification.

In some cases, captured CO2 can be used to produce manufactured goods and in industrial and other processes, rather than being stored underground. One of the primary uses of CO2 is for enhanced oil recovery, a method of oil extraction that uses CO2 and water to drive oil up the well, improving oil recovery and sequestering the CO2 underground. A general problem is that long term predictions about submarine or underground storage security are very difficult and uncertain, and there is still the risk that some CO2 might leak into the atmosphere.

Captured carbon dioxide that is being stored is transported by pipeline, truck or by ship for safe storage. Millions of tons of carbon dioxide are already transported annually for commercial purposes by road tanker, ship and pipelines. And as many tons of carbon dioxide are transported to be stored underground. It is also possible for carbon capture and storage systems to be combined with biomass which can result in net negative emissions.

Example of underground carbon dioxide storage (credit: European Commission)