Carbon Capture and Storage would capture CO2 produced by conventional power stations and heavy industry along the M62/M180 corridor, and store it in depleted gas fields and saline aquifers under the southern North Sea. The CO2 could remain there indefinitely, preventing its release into the atmosphere and its contribution to climate change.
Yorkshire makes an ideal candidate for a demonstration project – because of its easy access to a cluster of large scale emitters of CO2 (which between them emit about 60 million tonnes of CO2 a year) and potential storage sites off our coastline.
1. Capture
The first stage in the process is to capture the carbon being emitted at the source, such as power stations and other heavy industry. This involves separating the carbon dioxide from other gases that are being emitted during the combustion process. There are currently three ways to capture this CO2:
i) Post-combustion
Capturing the carbon dioxide using the post-combustion method requires the CO2 generated by the plant to be bubbled through solvent (for example, ammonium carbonate). This separates the CO2 and the flue gases. The flue gases (the mix of gases coming out the chimney after the coal is burned) are then released, and the carbon dioxide is held in the solvent. The CO2 then needs to be separated from the solvent. To do this, the pressure and temperature of the solvent is altered by an injection of steam. The CO2 becomes a gas and is ready to be transported. The solvent can be used again in the capture process.
Post-combustion can be retrofitted to old power stations, which gives it an obvious advantage.
ii) Oxy-fuel combustion
If fossil fuels are burnt in pure oxygen, then the waste gases produced are primarily water vapour and carbon dioxide. The water vapour can then be condensed, leaving just the CO2. The carbon dioxide is then ready to be transported.
The main challenge in this lies in creating an atmosphere to burn fossil fuels in that is composed of pure oxygen. This process is often performed cryogenically (at a temperature below -150 °C) for the best results.
iii) Pre-combustion
This method involves removing the CO2 before combustion to create CO2 free emissions. To do this, the coal must be gasified, in other words, heated to extreme temperatures so that it turns into a gas. The resulting gas is then transformed into a synthetic gas: a gas composed primarily of carbon monoxide and hydrogen. The carbon monoxide is then combined with water. This reaction produces CO2. An amine - a type of organic compound - is then used to separate the carbon dioxide. The main by-product, hydrogen, can afterwards be used as energy source.
At present, pre-combustion can only be used in purpose-built coal-gasification power stations and cannot be used in conjunction with conventional coal fired plants.
2. Transportation
Once captured, the carbon dioxide needs to be transported to the storage sites under the North Sea. There will be a network of pipelines built to do this. The CO2 is then put under high pressure, which compresses the gas into a dense form. This is the most efficient way to transport the carbon dioxide through the pipeline network.
3. Storage
CO2 storage sites are typically located several kilometers below the Earth’s surface, and are carefully selected and monitored to ensure they remain suitable for permanent CO2 storage. These sites can be depleted gas and oil fields, deep saline aquifer formations, or declining oil fields. If a declining oil field is used, the CO2 becomes part of a process known as Enhanced Oil Recovery (EOR). By injecting CO2 into the oil field, the amount of oil recovered is increased.
For storage, CO2 is injected into microscopic spaces in porous rock located at the storage site. Over time, the CO2 binds chemically to the rock. This process is very similar to that which has kept oil and gas held under the sea for thousands of years. The CO2 is then safely and permanently stored, meaning it cannot contribute to global warming and climate change.