Power plant exhaust recycled in the lab.

Oct 25, 2011

Stevens, JG, P Gomez, RA Bourne, TC Drage, MW George and M Poliakoff. 2011. Could the energy cost of using supercritical fluids be mitigated by using CO2 from carbon capture and storage (CCS)? Green Chemistry http://dx.doi.org/10.1039/c1gc15503b.

Synopsis by Wim Thielemans

The potential to reuse captured carbon dioxide from power plant exhaust/emissions as a mixer for other chemical reactions is shown in a unique study.

In the future, coal and natural gas fired power plants may be able to provide an abundant supply of liquid carbon dioxide (CO2) – a valuable agent used in chemical reactions in a growing number of industrial applications.

A preliminary study of this unique way to recycle and reuse carbon dioxide finds the main impurities in the power plant CO2 won't prohibit its commercial use. The results are explained in the journal Green Chemistry.

Carbon dioxide is commonly known as the greenhouse gas formed by burning fossil fuels. Energy producers intend to capture and store the carbon dioxide they produce. To do this, they'll convert the CO2 gas into its supercritical state – that is, changing the gas into a liquid by using extreme heat and pressure.

Converting carbon dioxide gas into a liquid is one way power plants can meet the anticipated tougher emissions standards in an effort to counteract climate change. As a liquid, carbon dioxide is also a safe solvent in other chemical production processes. It has already found applications in decaffeinating coffee and in dry cleaning.

A main advantage of supercritical carbon dioxide is that it helps produce a purer end product without toxic residue. In a typical chemical reaction with it, lowering the pressure at the end of the reaction turns the supercritical carbon dioxide fluid back into a gas. The chemical product produced during the reaction is immediately recovered. If all starting material is converted to the final product, no further purification of the end product is needed. 

The supercritical state is reached by heating and compressing carbon dioxide to above its critical point of 88oF and 1071 pounds per square inch (psi). Supercritical fluids behave as liquids for dissolving chemicals but allow the chemicals to move around at high speeds as they would in gases.

Unfortunately, compressing carbon dioxide to above its critical point is energy-intensive. This makes it usually too costly for industrial-scale reactions.

However, coal and natural gas fired power plants provide a cheap source of compressed carbon dioxide. To reduce their carbon dioxide emissions, energy producers intend to capture and store the carbon dioxide they produce. Because large power plants easily produce more than half a ton of carbon dioxide per second, compressing it reduces the required space for storage. It is this compressed carbon dioxide that could be used as a solvent in industrial processes.

Researchers in the United Kingdom investigated the effect of impurities found in carbon dioxide from power plants on a chemical reaction. They tested the supercritical carbon dioxide in a reaction at a modern industrial production facility. The facility does not use supercritical carbon dioxide because the high compression costs proved uneconomical.

In this work, none of the major impurities – nitrogen, water and carbon monoxide – posed insurmountable problems at the concentration likely to be found in power plant exhaust. Water and carbon monoxide did reduce the activity of the catalyst metal used to speed up the reaction. Increasing the temperature restored the catalyst's activity.

This work is very relevant to industry and is a very promising first step toward reusing captured carbon dioxide. But, as the authors pointed out, it is only preliminary, and only one reaction was studied. Also, many impurities found in power plant carbon dioxide in very small quantities were not investigated.

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