A Global Village
Issue 3 » Energy & Environment

Dissolving CO2

Amine–Based Absorption Technology

Cheng Seong Khor, Imperial College London

Large-scale commercial deployment of CCS presents a major challenge. The closest to market technology with several pilot plants worldwide is CO2 capture with amine-based solvents. However, there exist major cost and technical challenges to be overcome, and environmental risks associated with amine emissions to be understood, before a sector-wide rollout of this technology is viable.

The UK government’s recent announcement of a major capture and storage (CCS) demonstration programme is a promising indication that CCS is a high priority in the suite of CO2 mitigation initiatives under consideration to meet the UK’s reduction targets for CO2 emissions. As a strategy for climate change mitigation, CCS is most cost-effective for large stationary sources of CO2 such as fossil-fuel-fired power plants that account for roughly 80% of global CO2 emissions from such facilities. Furthermore, this percentage is poised to increase in the future to service the emerging electrification of the transportation sector. However, there has been hitherto no CO2 capture implementation at a large power plant of a scale of hundreds of megawatts although designs of such systems have been rigorously studied and proposed.

The key idea of amine-based absorption is to capture CO2 from the exhaust flue gas streams of coal-fired power plants via the absorption of CO2 into a liquid solvent to form a concentrated CO2 stream that can be transported to a storage or sequestration site. Arguably the most attractive method amongst a diverse range of alternatives, amine-based CO2 capture has a long history and hence technical maturity. There are, however, major cost-related technical challenges to be overcome, and environmental impacts to be understood, before use of this absorption technology is widespread.

Amine Solvents
In a modern coal-fired power plant using pulverized coal, the heat released as a result of combustion generates steam that drives a turbine generator, thus producing electricity. CO2 is formed as part of the hot exhaust flue gas generated from the combustion process, which chiefly contains nitrogen with small amounts of water vapour and compounds formed from impurities in coal that include SO2, NOx, and particulate matter. The CO2 is captured ‘post-combustion’ from the flue gas via chemical reaction with a liquid solvent – a process called absorption.

To date, most commercial carbon dioxide separation plants use monoethanolamine (MEA) solvent which was historically developed about 80 years ago to remove acidic impurities from natural gas. MEA is an organic base that reacts chemically with carbon dioxide, which is slightly acidic. Acidic gases, such as NOx and SO2, must be removed first as they will react with MEA, thus diminishing the amount of active MEA for carbon capture.

CO2 removal via absorption takes place in a CO2 scrubbing column. The flue gas is cleaned (or ‘scrubbed’) by forcing it to come into contact with MEA. This may be done in three ways: bubbling flue gas through the MEA solution, dispensing droplets of MEA solution in the midst of flue gas, or blowing the flue gas over a thin film of MEA supported on a surface. Up to 90% of the CO2 present is absorbed via this process. As the solvent is expensive, it is reclaimed in a solvent regeneration column by applying heat to the CO2-rich solvent solution for release (or ‘stripping’) of almost pure CO2 molecules.

Applying high pressure then liquefies the removed CO2, making it easier and much less costly to transport than in gaseous form. The pressurised CO2 is stored by various methods including injecting it deep underground for geological sequestration. The amine solution that is now low in CO2 concentration is then recycled to the CO2 scrubbing column and the cycle repeats.

A Solution for CO2 Mitigation?
Absorption-based technology is attractive mainly because it is compatible with existing infrastructure for energy and power systems. As it removes carbon dioxide after combustion, this technology can be handled almost like a standalone unit that is easily incorporated into an existing power plant without major structural and technological modifications.

Much remains
unknown about
how much of the
volatile amine
solvent is emitted
to the atmosphere

The technical operation of amine-based absorption is particularly simple compared to alternative existing technologies. Absorption takes place at ordinary temperature and pressure, thus presents a minimal energy demand. The chemistry of MEA renders it effective for dilute CO2 streams, making it particularly suited for flue gas from coal combustion which typically contains about 10% - 12% CO2 by volume.

For new plants, amine-based absorption technology can be integrated into the design of the plant and hot flue gas can be used to supply energy that runs the carbon-capture cycle. In particular, heat from the flue gas can be used to strip off CO2 from solvent, thus reducing energy demand of the whole process.

But…
The capital and operating costs for absorption-based CO2 capture technologies are substantial. Penalties are incurred from both the heat energy input required to reclaim the solvent and the energy required for the CO2 compression and transport process. These costs may amount to a capture penalty of 8-9 percentage points below the plant lower heating value (which is a property of a fuel, defined as the amount of heat released by combusting a specified quantity).

Furthermore, a sizeable amount of the solvent (0.35–2.0 kg) is consumed per ton of carbon dioxide captured, resulting in significant costs associated with solvent make-up to maintain the operation. These are in addition to costs associated with the transportation of exhaust gases and solvents.

And the Environmental Cost?
Much remains unknown about how much of the volatile amine solvent is emitted to the atmosphere and what happens to these emission compounds in the atmosphere. We are equally in the dark about whether the emission of amines and its degradation products are deposited close to their source. It is known that most of the molecules involved are soluble in water. Therefore it is possible that amine emissions will mix with rainwater and immediately enter the ecosystem.

A recent research article by a team of researchers at Imperial College London1 attempts to shed some light on our current knowledge of the environmental impacts of the absorption-based technologies. Preliminary findings unfortunately indicate that many of the products formed as a result of the degradation of the amine solvent will be harmful to both human health and the environment, to the extent of being potentially carcinogenic. Active research on this is currently being undertaken to understand the health effects and also to look for a new solvent for carbon capture.

In summary, before alternative sustainable carbon-neutral energy sources are found, there is a pressing mandate for the global community to curtail carbon dioxide emissions from power plants, which collectively contribute overwhelmingly to carbon emissions. Absorption-based carbon capture technology with amine solvents may be an effective technology well suited for this purpose. However, lingering uncertainties surrounding the environmental and heath impacts of this technology should be addressed quickly to ensure that we are fully confident of the capability of this technology as an intermediate solution to combat global climate change.

Cheng Seong Khor is a PhD student in the Department of Chemical Engineering at Imperial College London.

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