Carbon dioxide (CO2) emissions are released into the atmosphere when fossil fuels, such as oil, gas and coal, are burned. Many human activities rely upon the combustion of these fuels; for example, burning coal is essential to run power stations, and the combustion of other fuels derived from the distillation of petroleum (e.g. kerosene, diesel, gasoline) is required for road and air transport. The recognition that CO2 emissions are responsible for environmentally damaging effects on the climate has lead to many suggestions as to how the adverse impacts can be limited (climate change mitigation action), whilst ensuring the benefits from existing methods of industrial and energy production are maintained.
These suggestions can be broadly divided into two groups. Firstly, those that argue for a radical change in the way we generate power, a shift to technologies that do not require the burning of fossil fuels, but are instead based upon renewable sources of energy such as the sun, wind and waves. The second group regards a wholesale shift in energy production to be unrealistic for a variety of reasons and instead argues that improvements in technology can allow for the continued use of fossil fuels. These technologies decrease the amount of CO2 released into the atmosphere for every unit of power generated.
How does CCS work?
CCS is a mitigation technology which allows for the continued use of fossil fuels, but limits the quantity of CO2 emissions released into the atmopshere. It is described as a 'bridging technology' that can help limit CO2 emissions until other cleaner technologies become commercially viable.
CCS involves three main phases:
capturing the CO2 produced from industrial or power generation processes, at a specific installation or power plant, and compressing it in a liquid or dense state;
transporting the CO2 to an underground geological formation that has specific geological characteristics ('suitable storage site');
injecting the CO2 into the suitable storage site, which generally is a depleted oil and gas field or a saline aquifer. This process is often also referred to as "sequestration".
Capture of CO2
The capture phase can operate in three ways:
Post-combustion capture: the CO2 is captured from the flue gas once the fuel has been burnt. This method requires no change in the process that produces the CO2 and therefore could be used to capture CO2 from plants that have been built without CCS in mind - an operation known as 'retrofitting'.
Pre-combustion capture: the fuel is reacted with oxygen and/or steam to produce a mixture of hydrogen and CO2. The CO2 is then removed and the hydrogen is used to generate power. This method reduces the amount of CO2 released into the atmosphere during the power generation process.
Oxyfuel-combustion: the fuel is combusted with oxygen resulting in a flue gas that consists of CO2 and water vapour. These can then be separated. Oxyfuel has the advantage that it can be readily used to capture CO2 from plants that have been set up without carbon capture and it also allows 100% of the CO2 to be captured.
Currently, capture technologies are still in the demonstration phase. However, elements of the various capture options have been used in other industries. For example, the separation of CO2 from natural gas - as required by pre-combustion and oxyfuel-methods - has been commercially used in fertilizer manufacturing and hydrogen production. It has also been used commercially for a process called Enhanced Oil Recovery (EOR). This is a practice which relies upon the injection of CO2 into oil fields to increase pressure in order to facilitate oil extraction and make the reservoir more productive. This procedure is important as it is a commercially viable process that can be drawn upon to provide techniques for storing CO2 (see below).
Transport
Once captured and compressed, the CO2 is to be transported to a suitable site for long-term storage. Suitable storage sites are generally onshore or offshore depleted oil and gas fields or saline aquifers. Some of these sites can be located at a significant distance from the capture point, therefore requiring a transport system to be in place in order to convey the captured CO2 to the storage site.
Although ships have been suggested as a means of transporting CO2 offshore, or where particularly long distances are involved, the most cost effective method of transporting CO2 is via pressurised pipelines.
For the purpose of CO2 transport via pipelines, the CO2 must conform to certain purity standards, as impurities in the CO2 could change its properties and may interfere with the transport infrastructure. Pipelines must also meet some construction, design and maintenance requirements as well as health and safety targets. Specific permits are also likely to be necessary in order to lay CO2 pipelines, according with the relevant national, regional and international legislation.
Storage
The most likely sites for CO2 storage are depleted oil and gas fields or saline aquifers. The technologies used - for example, well drilling and reservoir modelling - are similar to extractive industries such as gas and oil production. These sites are generally located more than 800 metres underground and consist of porous rock. Storage sites are chosen so the CO2 is constrained by a dense rock layer above the storage layer ("cap rock") and, in some instances, saline water present in the storage site will dissolve the CO2 . Initially the pressure and temperature will maintain the CO2 in a liquid state, but in the long term, CO2 is expected to become geologically trapped.
In addition, the well that allowed the injection of the CO2 will also have to be capped to prevent leakages.
No matter how careful the scientific assessment of the geological characteristics of the area, there will always be a risk that some CO2 will escape from the storage site. In order to minimise these risks a long-term programme of monitoring will have to be implemented.
Financial Costs of CCS and Energy Penalty
CCS is still a very expensive process. Costs include the initial investment for the CCS plant and related technology, as well as for the transport infrastructure. In addition, power plants using CCS consume more energy than normal power stations, as a high proportion of energy is used during the capture phase (often referred to as the 'energy penalty'). All these factors could make the construction of CCS power-stations less attractive and potentially prohibitively expensive, especially for developing countries. There are several methods being developed to provide financial incentives for developing countries to build CCS power stations, but as yet, how they will work is unclear (see below). It is hoped that technological developments will eventually reduce the costs associated with CCS technology.
In order to incentivise commercial investment in CCS, governments are attempting to develop emission trading schemes (ETS). This places a cost on the company for carbon that is emitted, thus creating an incentive for the company to invest in technologies that reduce this cost - such as CCS. The detail of how these schemes (often called "cap and trade") work varies, but the European Union scheme allows governments to place a limit on what a plant is allowed to emit. Where an operator wishes to emit more than this amount they have to purchase allowances on the open market, if they emit less than the allowed amount then they are able to sell the unused allowance.