CO2 Capture from Flue gas using Amino acid salt solutions

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CO2 Capture from Flue gas using Amino acid salt solutions
Lerche, Benedicte Mai
Publication date:
2012
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Lerche, B. M. (2012). CO Capture from Flue gas using Amino acid salt solutions. Technical University of
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Denmark.

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CO Capture from Flue gas using
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Amino acid salt solutions

Ph.D. Thesis
By

Benedicte Mai Lerche
2012

CERE ‐ Center for Energy Resources Engineering
Department of Chemical and Biochemical Engineering
Technical University of Denmark
DK‐2800 Kgs. Lyngby

“As the true method of knowledge is experiment, the true
faculty of knowing must be the faculty which experiences.
This faculty I treat of”.
–William Blake, All Religions Are One, “The Argument” (1788).

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Summary

By implementing carbon dioxide (CO ) capture, it is possible to clean the flue gas
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from coal-fired power plants, so that it is almost completely free of this
greenhouse gas. The most advanced techniques towards practical application are
based on chemical absorption, where CO in the flue gas is chemically bond by a
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solvent, usually an aqueous solution of amines, resulting in a cleaned gas being
released to the atmosphere. Heating of the solution, desorbs the CO and
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regenerates the solvent, which is then ready for a new round of absorption. The
captured CO can then be compressed and sent to storage.
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Typical solvents for the process are based on aqueous solutions of alkanolamines,
such as mono-ethanolamine (MEA), but their use implies economic disadvantages
and environmental complications. Amino acid salt solutions have emerged as an
alternative to the alkanolamines, partly because they are naturally occurring
substances, and partly because they have desirable properties, such as lower vapor
pressures and higher stability against oxidative degradation. One important feature
of these new solvents is the formation of solids upon CO absorption, which
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happens especially at higher CO loadings and/or amino acid salt concentrations.
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The formation of solids poses challenges, but it also holds the promise for
improving the efficiency of the capture process.

This project focuses on phase equilibrium experiments of five systems CO +
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amino acid salt + H O, at conditions relevant for the CO capture process. Also,
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attention is given to the chemical compositions of the precipitations, which forms
as a result of CO absorption into the five amino acid salt solutions.
2

Phase equilibrium data are needed to develop safe and economically viable
capture processes. Two different experimental apparatuses were used. One was
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developed specifically for this project and is based on an analytical semi-flow
method. The other has recently been developed in another project, and is based on
a synthetic method. The semi-flow method was used for measurements at 10 kPa
CO partial pressure, total pressure equal to the atmospheric pressure and
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temperatures of 298 K, 313 K and 323 K; these conditions cover the absorber part
of the capture process. The apparatus based on the synthetic method was used to
obtain data at different pressures and temperatures up to 353 K (80 ºC), which is
the maximum operation temperature of some of the components in the apparatus.
Ideally, measurements in the full temperature range for desorption, which usually
needs approximately 393 K (120 ºC), would be desirable.

Using the 2 apparatuses, CO solubility in aqueous solutions of MEA and the
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potassium salts of taurine, glycine, L-alanine, L-proline and L-lysine were
examined. It was found that all of the tested amino acid salt solutions have the
potential to replace MEA. Nevertheless, solutions of the potassium salt of L-lysine
showed the best properties.

The chapters of this thesis are organized as follows:

Chapter 1 is a brief introduction to the central issues of this work, setting the
subject of the project in perspective to the issues of global warming and CO
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capture and storage (CCS).

Chapter 2 explains the principle behind the chemical absorption of CO from flue
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gas, emphasizing on the issues involving the solvent.

Chapter 3 is dedicated to the concept of using amino acids salt solutions as
solvents in CO capture, it includes a description of general amino acids chemistry
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and an outline of the chemical reactions involved in CO absorption into amino
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acids salt solutions.

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Chapter 4 deals with the description and validation of the new apparatus for
measuring of CO solubility based on the semi-flow method. A validation study of
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CO solubility in aqueous solutions of MEA is presented.
2

Chapter 5 focuses on the determination of the chemical compositions of the
precipitations, which arise in the five amino acid salt solutions upon CO
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absorption. The solutions were saturated with CO at 298 K, using the apparatus
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based on the semi-flow method. The precipitates were isolated and analyzed with
XRD.

Chapter 6 concerns a CO solubility study using the apparatus based on the semi-
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flow method; CO solubility were examined in aqueous solutions of MEA and the
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five amino acid salts, with concentrations between 2 and 8 molal and temperatures
around 298 K, 333 K and 323 K.

Chapter 7 describes a CO solubility study using the apparatus based on the
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synthetic method; CO solubility in aqueous solutions of the potassium salts of L-
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lysine and L-proline, with concentrations of 3.5 molal and 7 molal respectively,
were studied at CO partial pressures up to about 760 kPa, and temperatures
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around 313 K, 333 K and 353 K. Similar data were obtained with solutions of 7
molal MEA, at the same temperatures and at CO partial pressures up to around
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2.3 MPa

Chapter 8, concludes on the accomplishments of the project, and finally addresses
considerations concerning future work.
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Resumé (Dansk)

Ved at implementere kuldioxid (CO ) fangst eller såkaldt ”CO capture” er det
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muligt, at rense røggassen fra kulfyrede kraftværker, således at den næsten er helt
fri for denne drivhusgas. De teknikker, som er længst udviklede frem mod
praktisk anvendelse, bygger på kemisk absorption, hvor CO i røggassen bindes
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kemisk af en opløsning kaldet et solvent, sædvanligvis en vandig opløsning af
aminer, hvilket resulterer i, at en renset gas frigives til atmosfæren. Opvarmning
af opløsningen desorberer CO gassen igen og regenerer solventet, som så er klar
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til en ny runde absorption. Den indfangede CO kan derefter komprimeres og
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deponeres. Typiske solventer til denne proces er baseret på vandige opløsninger af
alkanolaminer, såsom mono-ethanolamin (MEA), men deres anvendelse
indebærer økonomiske ulemper og miljømæssige komplikationer. Aminosyre salt
opløsninger har vundet interesse, som alternativ til alkanolaminerne, dels fordi de
er naturligt forekomne, og dels fordi de har ønskværdige egenskaber; såsom
lavere damptryk og højre stabilitet over for oxidativ nedbrydning. Et vigtigt træk
ved disse nye solventer er dannelsen af udfældninger som følge af CO
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absorptionen, hvilket sker især ved høj CO optagelse og/eller høje aminosyre salt
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koncentrationer. Dannelsen af udfældning er en udfordring, men kan muligvis
også forbedre effektiviteten af ”CO capture” processen.
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Arbejdet i dette projekt vedrører faseligevægts eksperimenter for fem systemer
CO + aminosyre salt + H O, ved forhold som er relevante for ”CO capture”
2 2 2
processen. Desuden fokuseres der på den kemiske sammensætning af de
udfældninger, der dannes, ved CO absorption i de fem aminosyre saltopløsninger.
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Faseligevægts data er nødvendige, for at udvikle sikre og økonomisk rentable
”CO capture” processer. To forskellige eksperimentelle apparaturer blev anvendt.
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