Post combustion

Post combustion CO₂ capture is a process where the CO₂ is separated, or removed, from a flue gas containing CO₂ mixed with other gasses.

Post combustion capture technology can be added to existing coal or gas power plants or factories that emit large quantities of CO₂.

The purpose of the process is to get a gas of pure CO₂ that can be safely stored.

How it works

Post-combustion CO₂ capture is a process where the CO₂ is removed from a gas mixture after the combustion of a fossil fuel.

When a fossil fuel like coal, oil or natural gas is combusted in a traditional power plant, or a vehicle engine, the flue gas will contain some CO₂, typically in the range from a few percent to ten percent. The rest will be mainly nitrogen and water vapour.

There are several options for separating out the CO₂ from this gas mixture by post-combustion CO₂ capture. The most common process is absorption.

An absorption-based capture process is based on a chemical reaction between CO₂ and a suitable chemical, also called an absorbent. The flue gas is brought into contact with a chemical absorbent which has an ability to react with the CO₂. This process takes place inside a scrubber column, where the flue gas from the power plant is mixed with an absorbent dissolved in water. Typical absorbents that are used today are amines and carbonates.

After the absorption process, the absorbent and the CO₂ are separated in a regeneration column. The result is then a stream of pure CO₂ and a second stream of absorbent that can be recycled to the scrubber column.

Post-combustion CO2 capture

Post-combustion CO₂ capture can also be performed by adsorption instead of absorption. The difference is that in absorption CO₂ reacts with a liquid chemical, while in adsorption the CO₂ will be attached to the surface of a chemical, also called adsorbent, which in most cases is a solid material.

Available literature often refers to post-combustion CO₂ capture by sorbents, which means capture by either absorption or adsorption.

Post-combustion CO₂ capture can also be performed in a membrane module. A membrane has the ability to let some molecules pass through while other molecules are stopped. This can be used to separate CO₂ from the other molecules in the flue gas.

Today, absorption is a more mature technology than adsorption or membranes when it comes to post combustion CO₂ capture. The current proposed large-scale CO₂ capture plants are to be based on absorption. With further research and development adsorption and membranes will continue to advance, becoming cost effective alternatives in the future.

Absorption

 

Amine Scrubbing for CO2 Capture (Rochelle 2009)

The scrubber column is designed to ensure that the exhaust gas and the absorbent are brought into close contact with each other. When the CO₂ comes into close contact with the absorbent, a reaction occurs creating a new stable compound. Other components in the flue gas such as nitrogen will not react with the absorbent and are vented to the atmosphere as a cleaned gas-stream with low CO₂ content

After the absorption process, the absorbent and the CO₂ are separated in a regeneration column. When heated, CO₂ is liberated from the absorbent. The CO₂ exits the regeneration column in the form of a gas stream of high CO₂ purity, which can then be transported to a CO₂ storage site. The absorbent has now been regenerated and contains little CO₂, allowing it to be pumped back to the absorber and be reused to capture continuing the cycle. Approximately 80 to 90 percent of the CO₂ from a power plant can typically be removed by post-combustion CO₂ capture. 

Below two mature CO₂ absorption techniques utilizing different absorbents are described:

Detailed Amine Solvent Solution Capture Process

Aqueous amine capture of CO₂ was first patented in 1930 by R. R. Bottoms; and have since become a standard industrial process for the production of CO₂. The process involves an absorber and desorber column. Flue gas containing relatively low amounts of CO­_­2 (>15 wt%) at ambient temperatures, is fed into the bottom of an absorber column. Here lean amine solution is showered down from above and reacts with the CO₂ to form a weak acid and water-soluble salt. N₂ and other gasses exit through the top of the absorber.The now rich loaded CO₂ exits the bottom of the absorber and is pumped to the stripper. Here the amine solution is heated to approximately 110^oc, this reverses the reaction that occurred in the absorber and breaks the chemical bonds between the CO₂ and the solvent. CO­₂, water vapour and a small amount of solvent exit through the top of the stripper to be dried and compressed for transport. After the stripping process lean solvent is returned to the absorber and the process is repeated.

Detailed Chilled Ammonia Capture Process

Ammonia has been used in the power generations industry in the past as a desulphurisation technology. Ammonia reacts with SiO₂ to produce ammonia sulphite. This process is known as the Walther-process and can be used to produce fertilisers. Ammonia may also be used as an efficient and cost effective technology for CO₂ capture. Ammonia reacts with CO­₂ to produce ammonia bicarbonate. The flue gas must first be cooled to ~ 1^oc to reduce ammonia release in the absorber, and reducing the flue gas volume. The chilled flue gas is forced upward against lean ammonia down comers. The now rich ammonia slurry is compressed and pumped to the stripper. The main advantage of the chilled ammonia system is that the CO₂ can be stripped at high pressures and moderate temperature; reducing compression costs for transport. Ammonia is also cheap and abundant, with a high CO₂ capacity and no degradation during the absorption / desorbtion process.

Advantages

Mature technology
The big advantage of post-combustion CO₂ capture is that the technology is much more mature than the alternatives of pre-combustion capture and oxyfuel combustion with CO₂ capture.

CO₂ capture has been demonstrated in the laboratory and in pilot plants for many years, and it has been proved that the technology works. But there are no large CO₂ capture plants today at the scale of a commercial coal power plant. There are, however, plans for building large-scale CCS demonstration plants worldwide. Because of the relative well advanced maturity of amine absorption, it is believe that the first large-scale CO₂ capture plants will mainly be based on post-combustion CO₂ capture by amine absorption.

Existing CO₂ emission sources
Post-combustion CO₂ capture technologies can easily be added to existing CO₂ emissions sources. None, or minor, modifications are required for implementing post-combustion CO₂ capture to a coal or gas power plant or a factory with large CO₂ emissions.

Pre-combustion and oxyfuel CO₂ capture technologies require a large degree of interaction with the process that generates the CO₂ emissions. Post-combustion CO₂ capture will therefore continue to be the preferred CO₂ capture path for existing CO₂ sources even if pre-combustion or oxyfuel CO₂ capture should develop to be the preferred choice for new power plants and factories.

Challenges

Energy penalty
There is one main challenge with existing post-combustion CO₂ capture technologies; they use a lot of energy.

When the first CO₂ capture plant based on amine absorption is put into operation around 2015, it is estimated that 10 percent [1] of the energy produced by the power plant will be consumed in the CCS process. Most of this energy will be consumed in the regeneration unit where energy is needed to heat the mixture of amine and CO₂.

The challenge is to develop new solvents that will require less energy. Worldwide research programs are looking for ways to improve existing solvents and develop new ones.

The high energy consumption for post-combustion CO₂ capture can pave the way for pre-combustion or oxyfuel CO₂ capture in the future as well as more novel technologies that are unmature today.

CO₂ capture cost
The energy loss represents a considerable cost, and the CO₂ capture cost today is the largest CCS cost component [1].

A prerequisite for establishing CCS as a main tool for combatting global warming is to reduce the costs, and that means reducing the costs of capture in particular.

Reducing the energy penalty will reduce the capture cost, but research activities are ongoing to reduce capital costs of materials and equipment by inventing improved or new materials.

Large-scale demonstration
CO₂ capture is a widely available technology in the laboratory and in small pilot plants. However, it is expensive and it has not been demonstrated on a large-scale yet.

This is about to change as research and industry worldwide are putting an increasing effort into making CO₂ capture technology ready for large-scale deployment. But we are still waiting for the first investment decision for building a large-scale CO₂ capture plant.

Capture ready
Adding a post-combustion CO₂ capture plant to an existing CO₂ source requires that there is space available for the capture plant. That is not always possible because some factories and coal power plants are built in industrial areas with limited space available.

It will take some years before CO₂ capture becomes a standard part of factories and fossil fuel power plants. Until that happens it is very important to ensure that new factories and power plants are built with sufficient space available to allow for construction of a CO₂ capture plant in the future.

Technology providers

There are currently no large-scale CO₂ capture plants. Smaller plants exist, but it is still a large technical challenge to build a capture plant of the size required for a coal power plant.

The largest providers of processing equipment for CO₂ capture are Fluor Daniel (USA), ABB Lummus (USA) and Mitsubishi Heavy Industries (Japan). They have all developed post-combustion CO₂ capture technologies based on absorption by amines.

There are several other actors that are interested in projecting and constructing CO₂ capture plants. The Norwegian company Aker Clean Carbon launched a new project called "Just Catch" in 2005. This project is based on optimising known post-combustion technology by using amine absorption.

The Norwegian company Sargas is also developing CO₂ capture technology. Their concept is based on combustion of fossil fuel in a pressurised boiler and CO₂ capture by carbonates.

Alstom has started a comprehensive RD&D program to develop a very promising post-combustion technology called "chilled ammonia process". Their technology is is based on post-combustion CO₂ capture with ammonium carbonate as an absorbent. The advantage of the "chilled ammonia" technology is that it requires much less energy than the absorption by amines. The challenge is that the process is more complex than the amine process, and so far there are still unresolved issues.

HTC Purenergy is another company that has established a comprehensive program to develop post-combustion CO₂ capture technology.

It is believed that CCS will become an important tool to combat global warming in the future. In addition to the companies mentioned here, several companies worldwide regard CCS as a new business opportunity and have started to develop new post-combustion CO₂ capture technology.

References

Carbon Capture & Storage: Assessing the Economics McKinsey & Company. 2008

Rochelle (2009) Amine Scrubbing for CO₂ Capture. Science 25 September 2009: 1652-1654