Capturing carbon dioxide

Dear Control Engineering: Is it possible to scrub carbon dioxide from a flue gas stream? We seem to be able to capture other pollutants, why not this one?

It’s true that there are technologies capable of capturing or scrubbing virtually any airborne pollutant from a flue gas stream, except one. If you consider what might be going up the stack from some combustion process, there is a range of technologies available:

• Acid gasses can be neutralized by a reaction with an alkali reagent;
• Sulfur dioxide can be turned to gypsum via a reaction with limestone;
• Nitrous oxides can be broken down into harmless products through reaction with ammonia;
• Mercury can be captured with activated charcoal;
• Particulates can be captured in a baghouse or electrostatic precipitator; and
• Toxic gasses (dioxins, furans) can be captured with activated charcoal or prevented from forming by careful gas temperature regulation.

With enough technology, air coming out the stack can be cleaner than when it started. There’s just one problem, and that’s carbon dioxide. It’s hard to get rid of something that doesn’t react readily with many products, but there are ways to capture it.

The most common capture technology is amine scrubbing using monoethanolamine (MEA) or diethanolamine (DEA). The gas goes through a wet scrubber with a solution of the reagent and water with the following reaction:

C2H4OHNH2 + CO2 + H2O ↔ C2H4OHNH3+ + HCO3-

CO2 is captured in the solution, but it’s only temporary. When the solution is heated, the gas comes out and the solution can be used again. You still have the CO2, but at least it’s in a purer form. Of course you have to find someplace to put it, which can be pumping underground, into the ocean, etc. Large scale amine scrubbers are in operation at various places, but the technology is still considered experimental. Heating the solution to release the gas is energy intensive, the solution is corrosive, and eventually the amine oxidizes losing its effectiveness. Dow and Alstom, among others, have built demonstration projects and made significant advances, but getting rid of CO2 doesn’t have any easy answers.

–Peter Welander, process industries editor