Carbon Capture, Transport, and Storage- An Overview of Techniques

08/30/2010

The principle of carbon capture and storage (CCS) applies to the largest industrial emitters of carbon dioxide, such as thermal power plants, iron and steel plants, petrochemical plants, and oil refineries. The process involves capturing CO₂ and trapping it underground on a long-term basis. Carbon capture requires first isolating it from other components that are discharged in the smoke. A number of technologies are used to do this. Next, the gas has to be transported to a permanent storage site. Once again, there are a number of options. Here is an overview.

Capturing or Isolating CO₂

Unfortunately, smoke from large-scale industrial plants such as refineries or power plants does not contain only carbon dioxide that could be trapped in a jar. CO₂ capture is complex and particularly costly, because it has to be separated from the other constituents of the smoke, such as water vapor or nitrogen. The IPCC estimates that this part of the process accounts for 2/3 of the overall cost of capture and storage.

While it is feasible to avoid this phase by capturing and storing all the smoke, it could pose other problems. Flue gases only contain an average of 3-15% of CO₂. Isolating the CO₂ means that there is less bulk to transport and store, which reduces costs and optimizes storage space.
A number of techniques are currently available:

   • Postcombustion
This consists of extracting CO₂ from flue gases, often using a liquid chemical solvent. The solvent binds with CO₂ and the resulting complex is extracted from the rest of the smoke. Next, the two are separated in turn using a thermal process to regenerate the solvent. This technique is now well developed. One advantage is that it can be used in existing plants as it does not require any modifications to smoke-emitting plants. However, it has high energy requirements (large volumes of smoke have to be processed) and is costly.

   • Precombustion
The idea here is to remove carbon from the fuel before combustion. This is done by converting the fuel into a synthetic gas mostly comprising carbon monoxide and hydrogen. Next, water vapor is added; this reacts with the carbon monoxide, converting it to CO₂. The CO₂ and hydrogen are then separated using a solvent. The hydrogen is used to produce energy without any CO₂ emissions. This technique is already used on an industrial scale. It is less energy-intensive than postcombustion. However, it requires specific equipment that is still being developed. This equipment must be planned for when the relevant industrial plant is still at the design stage.

   • Oxy-fuel combustion
The conventional combustion scenario involves using air, but this process generates large volumes of smoke in which the CO₂ is highly diluted. This makes it more expensive to extract. By contrast, oxy-fuel combustion, which is still in the development phase, generates smoke that contains mostly CO₂ and water, which can easily be separated through water condensation. Oxy-fuel combustion involves replacing the air used during combustion with pure oxygen. This technique is both more cost-effective and energy efficient than other technologies. Another advantage is that it can be used in existing facilities.

The main disadvantage of this method lies in the fact that extracting pure oxygen from the air upstream is highly energy-intensive and involves relatively high CO₂ capture costs.

However, this stage has considerable potential for improvement with the new chemical loop process. This technology involves using a metal oxide to provide the oxygen required for combustion. In concrete terms, the chemical loop comprises two connecting reactors. In the first reactor, a metal is oxidized when it comes into contact with air. This metal oxide is injected into the second reactor - the combustion chamber, which also contains fuel. The fuel then uses the oxygen carried by the metal and converts it into a mixture of CO₂ and water, which are easy to separate. The regenerated metal is then reinjected into the first reactor and a new cycle can begin. This solution uses less energy and reduces capture costs. However, it is still in the experimental phase and has only been tested on a reduced scale under laboratory conditions.

All these technologies require more in-depth study to reduce capture costs so that it can be used on a larger scale.

CO₂ Transport

This is not in itself a very complicated procedure because well-established natural gas transport techniques can be applied to captured CO₂. There are two main solutions:

   • Transport by pipeline involves compressing the CO₂ until it is in an almost liquid state. This type of transport is already in use in the United States, where over 40 million tons of CO₂ are carried over a 4000-km long pipelne network every year.

   • CO₂ in liquid form can also be transported by ship or truck.

Transport costs vary from 1-10, depending on the distance covered and the means of transport used.

Three Storage Locations

According to the IEA Greenhouse Gas group, geological storage capacity worldwide is 1,200-10,000 Gt of CO₂. Given that annual CO₂ emissions caused by human activity amount to 30 Gt, there is plenty of capacity.

A reservoir is needed to trap carbon gas on a long-term basis.

   • It must be impermeable- the gas must remain trapped over a very long period, potentially thousands of years.

   • It must be deep- at a depth of 1000 meters, CO₂ is almost liquid. It is denser and therefore more of it can be stored in the same space.

Bearing these criteria in mind, there are three types of storage location that are currently being investigated:

   • End-of-life oil or natural gas reservoirs

These oil and gas reservoirs have proven capable of retaining hydrocarbons for thousands of years with no leakage. The oil and gas industry has extensive experience with these sites. For example, oil companies already inject CO₂ into oil fields to reduce viscosity, improve mobility, and increase recovery rates. The already existing infrastructure for extracting gas and oil can be used to store CO₂, which would reduce costs. Unfortunately, requirements far outstrip storage volumes worldwide.

   • Coal seams too far underground to be worked

Injecting CO₂ into unworkable coal seams does not actually involve storage, but rather CO₂ absorption into the coal. Provided that the coal seam is well covered by impermeable layers, this process can be used to both store CO₂ and recover methane. However, this type of storage needs further research and testing.

   • Saline aquifers

The term ‘aquifer' refers to a geological formation comprising porous, permeable rock saturated with water. The most superficial aquifers contain soft water used to supply drinking water. Deeper aquifers are saturated with salt water that is unsuitable for drinking. The storage potential of saline aquifers seems promising, but more in-depth work is required to ensure all conditions for long-term storage are met. Saline aquifers can extend over thousands of kilometers, thus offering significant storage capacity. However, to be usable, they need to be covered with impermeable layers and located at depths of over 800 meters.

Ocean storage of CO₂ is another option¹.

True and False. Deep ocean storage of CO₂ through injection is effectively possible. The gas is dissolved in water and therefore does not spread into the atmosphere. However, this is not a permanent solution. Over time, the natural concentration of CO₂ in water and the air would be restored. In just a few hundred years, stored CO₂ would return to the atmosphere. Furthermore, increasing the concentration of CO₂ in water would acidify the immediate environment. This option would therefore cause serious damage to marine flora and fauna.

[1] Source ZERO