Capturing CO2

Capturing C02

The capture of CO2 from a stationary source, such as, a power plant involves separating and purifying CO2 from the bulk of the flue gas stream rather than allowing it to be released to the atmosphere. The purified CO2 stream is then available for geological storage.

The main sources suitable for CO2 capture are: industrial processes, electricity generation, and in the future, hydrogen production from fossil fuel sources.Industrial processes that lend themselves to CO2 capture include natural-gas processing, ammonia production, and cement manufacture, however the total quantity of CO2 produced by these processes is relatively small. A far larger source of CO2, accounting for one- third of total CO2emissions in Australia, is fossil-fuelled electricity generation. Research is underway on the capture of CO2 from this source.

Technologies for capturing CO2 from electricity generation fall into three categories: post-combustion, pre-combustion, generally referred to as Integrated Gasification Combined Cycle or IGCC, and oxyfuels (in which a power plant's fuel is burnt in oxygen rather than air).

A range of capture equipment and technologies can be used in these processes. Research is underway into the four main technologies namely:

- Absorption, where CO2 is selectively absorbed into liquid solvents;
- Membranes, where CO2 is separated by semipermeable plastic or ceramic membranes;
- Adsorption, where CO2 is separated using specially designed solid particles; and
- Low Temperature Processes, where separation is achieved by chilling and/or freezing the gas stream.

While the capture of CO2 for geosequestration is a relatively new concept, CO2 capture for commercial markets has been practised in Australia and overseas for many years. CO2 is captured from natural gas wells in south-east South Australia, near Mt Gambier and in southern Victoria, near Port Campbell. The CO2 is then used for various commercial processes including carbonation of beverages and dry-ice production.

Following capture, CO2 is usually transported from the source, such as a power station, to the geological storage site in a compressed form via a pipeline. It can also be transported by truck, rail or ships depending on the location of both the source and the geological storage site and injected via pipeline deep underground.

Geological storage of CO2 secures the gas deep underground in a geological reservoir. In addition to the careful selection of a suitable geological reservoir, a comprehensive monitoring system is required initially to ensure that the gas is safely contained. Geological reservoirs into which CO2 can be injected include depleted oil and natural gas fields, and deep saline formations.

Since the stored CO2 will be less dense than the water in and around the reservoir rocks, it needs to be geologically trapped to ensure that it does not reach the surface. The exact trapping mechanism depends on the geology.In depleted oil and gas reservoirs geological traps contain the CO2. In some cases these are anticlines, in other cases fault traps.

In the case of deep saline formations with no distinct geological traps, an impermeable caprock above the underground reservoir is needed and the CO2 is contained by the groundwater flow. This is known as hydrodynamic trapping.

Solubility and mineral trapping are two other very important mechanisms. Solubility trapping involves the dissolution of CO2 into the saline water in the reservoir. In mineral trapping the CO2 reacts with minerals in the rocks to form stable carbonate minerals. The first commercial- scale project dedicated to CO2 storage in a geological reservoir has been in operation at the Sleipner West Field since 1996. Sleipner West is a natural gas field with a high concentration of CO2. It is operated by Statoil and located in the North Sea about 250 km off the coast of Norway.

CO2 is usually transported from a source, such as a power station, to the geological storage site in a compressed form via a pipeline. It can also be transported by truck, rail, or in the case of a geological storage site, deep beneath the seabed, by ocean tanker.It is injected from a tanker, truck or pipeline deep underground into the geological reservoir.