English
中文1. Background
Increasing atmospheric concentrations of carbon dioxide (CO2) is regarded as one of the main causes of anthropogenic climate change. This byproduct from combustion of fossil fuels traps reflection of heat from the Earth back to the Sun, in effect acting as a thermal greenhouse roof surrounding our planet in the Milky Way. Scientists have coupled this to a number of potentially devastating consequences like globally rising sea levels.
The political process to mitigate these threats has come to the spotlight through a series of UN summits. The aim is to reach consensus and binding agreements on a cap limiting the amount of CO2 emitted worldwide. Each nation will have their individual target.
A family of methods developed for removing CO2 from a mix of exhaust gases goes under the name CO2 storage. Also known as sequestration or simply capture, it works primarily at coal fired power facilities and is currently in the planning stages for a number of plants in different parts of the world. This paper will take a look at such methodologies from a number of aspects.
The basic infrastructure for a CO2 storage is shown in figure 1. CO2 is removed from an industrial processing plant based on fossil fuels and transported either for reuse or permanent storage. There might be a substantial amount of hydrogen that can be used as a byproduct.
The technique can in theory collect up to 90 % of CO2 emissions from an otherwise heavily polluting coal fired power plant. Countries with a reliance on coal for their electricity and heating can benefit greatly from the technology. However, it does so at a rather extensive cost, requiring up to as much as 40 % of the energy produced by the facility.
Another seemingly unresolved issue concerns how to deal with the stored carbon dioxide. While some of it can be used in alternative sites or injected into oil wells for enhanced oil recovery, a large amount of CO2 will apparently need to be buried into geological or ocean storage with unknown long term consequences. This creates a problem similar to the issue of radioactive waste from nuclear power.
This report presents a literature survey of the technique of CO2 storage. Information about the different methods, arguments for and against and some contemporary projects will be presented in subsequent chapters.
2. Capture
The state of the art divides CO2 capture techniques into three categories: post combustion, oxyfuel and pre combustion. Figure 2 illustrates the principles of each in the energy cycle of a power plant.
2.1 Post Combustion
As the name is indicative of, post combustion removal takes place in a step coming after the fuel has been burned. The reactant mix of gases is bubbled through a tube filled with a liquid possessing solvent
characteristics, most likely ammonia. A chemical reaction stores the carbon in the fluent. When this step has become saturated, intake of CO2 is halted while a gaseous steam is run through the column to extract the captured emissions. This is illustrated in Figure 3.
Post combustion is easier to install to existing facilities, but more expensive to run. Some estimates state that a post-combustion step may reduce the efficiency of a coal facility for electricity to as low as 35 (Davidson, 2007). Huge energy requirements and high costs of acquiring high quality solvents are the main obstacles of the technology.
There are some tests underway. Australia with a high dependence on coal based energy has equipped a plant in Victoria (Novinite, 2008) and the Tarong station in Queensland (CSIRO, 2008) as some of their first testing beds. The EU has tested usage of the method in their joint CASTOR project in Denmark (EU, 2004), while the U.S. demonstrated post combustion removal with ammonia pillars in Kenosha County (WI) and Shadyside (OH) (Sciam, 2009).
2.2 Oxyfuel
Whereas combustion in general is done in untreated air, the oxyfuel method works by creating an ideal 100 % oxygen environment where the burning of fuel takes place. The difference is that the products of the reaction will be a mix consisting almost entirely CO2 and water vapour, making it much easier to separate carbon dioxide for removal. This requires a preprocessing step where liquid oxygen is extracted from the air. The process has been illustrated in Figure 4.
Advantages of the oxyfuel method are that in principle 100% of CO2 emissions can be captured at a very low running cost at the plant and that nitrogen emissions are removed as well. The current obstacle is the complex and costly process of acquiring purified oxygen from air. The need for high temperatures during combustion also poses special challenges to the plant.
A number of pilot oxyfuel plants have been erected during latter years. The first site was at Schwarze in Germany (BBC, 2008) and another one has been set up at Lacq in Southern France (Guardian, 2009). This seems to be the method of choice of the three at this time.
2.3 Pre Combustion
Pre combustion techniques divert the carbon from the fuel compound before it is burned in a chemical reaction producing carbon monoxide and hydrogen. The CO gas is then saturated with water yielding CO2, while the hydrogen goes to the reactor as the fuel or for other uses. The operating scheme is depicted in Figure 5.
This is a known technique requiring relatively little investment in research with the potential to reduce 90 – 95 % of CO2 emissions (Scottish Centre for Carbon Storage, 2009). The major disadvantage is that a plant will have to be retrofitted with an expensive additional chemical reactor. There will be problems with nitrogen emissions as well.
Literature can’t identify many pre combustion projects in the world. It has been mentioned in conjunction with a site in Belgium (Nuon, 2007), but not at the major CO2 storage projects. Focus appears to be on the other methods.
3. Transport and Storage
The captured carbon dioxide must be transported to locations where they can be safely deposited. This creates a need for an infrastructure capable of carrying large amounts safely across nations or even continents. It has been estimated that pipelines will handle the biggest part of such movements.
In America, there is a well developed network of especially dedicated CO2 pipes already in place. Figure 6 shows the current CO2 pipeline map for major lines in continental United States.
The existing connections to oil fields that have been built since the 1970’s can be extended with further CO2 fitting if necessary.
Figure 6. American CO2 major pipelines (CRS, 2007).
The European network is not as well developed. However, a doctoral thesis shows that a capacity for sending required amounts of CO2 across the continent can be created at a moderate cost by adjusting existing lines for oil and gas (Damen, 2007). Some public investment will be necessary to create market conditions for the system.
Concern has been raging over what to do with the sequestrated carbon dioxide. Some propose that it be stored in deep sea beds, where it is transported by pipes or container tankers. Norway is currently doing a study to determine whether a site in the North Sea as shown in Figure 7 can be used as the main collection point for European CO2 storage (CS, 2009).
Other options include injecting CO2 into oil fields for the so called enhanced oil recovery method, geological storage primarily in sandstone saline fields and putting it into coal mines (GEUS, 2004).
4. Discussion
CO2 storage techniques have become a highly pursued topic for researchers, policy makers and industry. If it works, they promise removal of a very high percentage of the CO2 emission from fossil fuel fired power plants, factories, etc. A number of sites are in operation to test and give valuable information for future installations.
The public has not been equally enthusiastic about the idea. It should be noted that several environmental organisations have voiced their opposition to this technique (BMU, 2008). The uncertainty of storing such large amounts in the ground or in the sea has also lead to protests by citizens living nearby proposed collection sites.
My personal opinion is that this at best is a way to buy time for the necessary transition to renewable energy generation. While it seems better to use this than not to use it, it does not address the other detrimental consequences of coal based technologies including waste, public health and infringements into valuable natural habits for mining. I can not see it as more than a small part of the solution to the problems facing the world’s environment today.
Disregarding pre combustion as it does not appear to be considered by the community, both methods have their advantages and disadvantages. Post combustion would be easier to implement and might be most suitable on small to medium sized factories as it does not require an expensive additional facility. It has been used mostly on such already existing installations in the world. Large scale new installations with more resources available such as the plants in Schanze and Lacq seem to opt for the oxyfuel option. Their hope is that the price of acquiring oxygen from air will fall as technology improves.
Oxyfuel capture offers the important advantage of removing nitrogen compunds (NOx) from the exhaust. As it at present holds promise of the cleanest method presenting virtually no greenhouse gases to the atmosphere, I must conclude that it would be the best of the studied techniques. The economic case for it will grow as research is done on oxygen separation.
5. Conclusion
CO2 storage is an exciting idea to remove carbon dioxide otherwise headed to the atmosphere from major plants. This paper studied the principles and how it is being used around the world. A special focus was put on the different technical methods.
It was found that oxyfuel provides the best reduction in emissions, in particular as it also diverts nitrogen from the outlet. It is at present more expensive than post combustion and therefore used mostly in very big plants. There is hope that this price can fall as better ways to obtain the necessary purified oxygen are developed. I expect oxyfuel to be the technology of choice for the future.
