How CCS Works

There are three key techniques for capturing CO2 from fossil fuel power plants - post-combustion, pre-combustion and oxyfuel combustion capture.

While ScottishPower is interested in the development of each of these techniques, we are focussing our efforts and resources on post-combustion capture on coal. This is the type of capture and fuel best suited to our existing generation fleet and the areas specified by the UK Government's CCS Demonstration Competition.

In demonstrating the viability of post-combustion capture on an existing coal power plant, we aim to provide a retrofit solution capable of drastically reducing emissions from the estimated 50,000 fossil fuel plants currently in operation worldwide.

Post-combustion

Post-combustion technology captures CO2 from the power station flue gas after the fossil fuel has been burnt. Before the flue gas is vented to the atmosphere it passes through an absorber tower, where special chemicals that stick to CO2 molecules (usually amines) are showered down on the gas, scrubbing the CO2 out of the flue gas.

The CO2 rich amines are then heated to release the ‘pure' CO2 to be compressed into a liquid, ready for transportation away from the site. Once the amines cool they are then recycled back around the capture system to be used over again.

This is probably the best understood capture technology, as the basic techniques have been used for many years to capture CO2 for the fizzy drinks industry, and industry has over 60 years experience working with amine solvents. The challenge is in scaling up the techniques and processes involved to handle the vast quantities of CO2 produced by an industrial power station.

Pre-combustion

This method is normally applied to Integrated Gasification Combined Cycle (IGCC) power plants coal-fired power plants that are actually run on hydrogen gas. It works by capturing CO2 before burning the coal. Coal is gently heated to release a synthetic gas (syngas) made up of carbon monoxide and hydrogen. The carbon monoxide (CO) is then reacted with water to produce more hydrogen and CO2 , which can be separated out and compressed into liquid ready for transportation. The hydrogen produced is used in the power plant to generate electricity.

Pre-combustion technology is not as well-developed as post-combustion but it could offer an extremely efficient method of running a coal-fired station in the future. The big disadvantage of pre-combustion is that it cannot be retro-fitted to the older coal power plants that currently provide the majority of the world's fossil fuel power.

Oxyfuel combustion

Oxyfuel combustion involves burning fossil fuels in pure oxygen as opposed to air. This results in a waste gas that is made almost entirely of CO2 and water. The water is condensed out while the CO2 is compressed for transportation.

Oxyfuel combustion can result in a very high rate of CO2 capture, however takes a great deal of energy to produce the pure oxygen which makes this a relatively inefficient process at the moment, however much work is being done to improve this technique.