(24a) CO2 Capture Via Potassium Carbonate/MEA Solution

A great deal of concern has been expressed with regard to global climate change and its link to growing atmospheric concentrations of carbon dioxide. To decrease the impact of anthropogenic CO₂ on global climate, several strategies are under development that will potentially remove CO₂ from the atmosphere or decrease CO₂ emission. One such strategy involves the capture of CO₂ from large point sources (such as pulverized coal (PC) plants) and the long-term storage of CO₂ underground.  The post-combustion capture of CO₂ is a challenging application because of (1) the low pressure and dilute concentration (10 to 15 %) of the carbon dioxide dictate a high volume of gas to be treated and (2) trace impurities in the flue gas tend to reduce the effectiveness of the CO₂ absorbing processes.

          Much work has been studied in this area and has included development of amine-based liquid sorbents, solid amine-based sorbents, metal carbonate sorbents, ionic solution, chemical looping, membrane system and other novel technologies. The aqueous amine system is one of the commercial technologies for CO₂ capture for PC power plants.

The commonly used amine in scrubbing solutions contains monoethanolamine (2-aminoethanol) (NH₂CH₂CH₂OH) (MEA). The disadvantages of using MEA solutions include energy-intensive, amine degradation and corrosive.  The typical reaction is illustrated as follows:

2 MEA + H⁺    +  HCO₃ ^-    ¬>   MEA·H⁺    +    MEA·HCO₃^-

            Work also has been conducted in the area of using carbonate sorbents to  remove carbon dioxide from flue gas.  A carbon dioxide capture process using regenerable sorbents such as an alkali metal oxide or an alkali metal carbonate has been reported. Potassium carbonate (K₂CO₃) can be supported on a high surface area of activated alumina (Al₂O₃). The product formed from absorption of carbon dioxide is taken aside and heated to regenerate the sorbent. No significant chemical degradation is observed, but there is physical attrition. Loading occurs between from about 60⁰C to 100⁰C according to the following reaction equation.

K₂CO₃   +    H₂O   +   CO₂   ¬>    2 KHCO₃

Thermal desorption of potassium bicarbonate (KHCO₃) and the alumina support takes place at temperatures around 145⁰C to 150⁰C.

In this paper, a CO₂ capture process (US patent 7,842,126) is presented where CO₂ from combustion flue gas dissolves into an aqueous solution of amine and soluble potassium carbonate. Potassium carbonate rapidly reacts with amine-associated carbamate and water to form less-soluble potassium bicarbonate that precipitates from solution.  This reaction serves both to chemically regenerate the amine solution by removing the carbamate and to separate CO₂ in the form of solid bicarbonate precipitate.  Precipitated potassium bicarbonate is further physically separated from bulk solution and heated to temperatures greater than approximately 150 ºC to thermally regenerate back to solid potassium carbonate and release concentrated CO₂ gas.  Because of the absence of bulk aqueous solution, regeneration enthalpy requirements for separated bicarbonate solid is predicted to be low as compared to those reported for regeneration in traditional MEA-based absorption processes.  Following thermal regeneration of the separated solid, potassium carbonate is re-dissolved in potassium carbonate-lean aqueous amine solution, and the reactive solution is re-exposed to flue gas to close the process loop.  This process concept has the potential to achieve high CO₂ capture efficiency with relatively low regeneration energy requirements.

In addition to details description of capture methodology, preliminary laboratory results will be presented.