current status of r&d in post combustion co2 capture · 2012. 5. 22. · •only potassium and...

Current status of R&D in post combustion CO2 captureKaj Thomsen, Ph.D.Center for Energy Resources Engineering, CEREDTU Chemical EngineeringTechnical University of Denmark

DTU Chemical Engineering, Technical University of Denmark

Outline•Choice of solvent

–Alkanolamines–Alkali carbonates–Ammonia–Amino acid salts–Ionic liquids

•Thermodynamic modelling•Process simulation

26.01.2011Current status of R&D in post combustion CO2 capture – Chalmers Energy Conference2


Alkanolamines•Organic compounds with an alcohol functionalgroup and an amine functional group–MEA MonoEthanolAmine (primary amine)–MDEA MethylDiEthanolAmine (tertiary)–DGA DiGlycolAmine (primary amine)–DIPA DiIsoPropanolAmine (secondary)–DEEA DiEthylEthanolAmine (tertiary)–EEA EthylEthanolAmine (secondary)

•Aqueous solutions of alkanolamines or mixtures of alkanolamines

•Promoters such as AMP and Piperazine



Chemistry•Carbamate formation

•The R1R2NCOO- ion is the carbamate ion•The formation of carbamate is fast and exothermal–Fast kinetics is desired–Exothermal reaction is not desired

•Only primary and secondary amines form carbamates


1 2 3 2 1 2R R NH HCO H O R R NCOO− −+ ↔ +


Carbamate formation•The reaction rate of carbamate formingalkanolamines, such as MEA, with CO2 is a factor 1000 higher than the reaction rate of a tertiaryamine such as MDEA.

•Tertiary alkanolamines are bases. They reactwith water to form protonated alkanolamine and hydroxide.

•Carbamate formation goes through a maximumat a certain loading.

•Capture processes are operated in the loadingrange where carbamate formation is significant.



Known technology•Alkanolamines have been used for CO2 removalfrom gas mixtures since the 1930’s.

•Alkanolamines replaced alkali carbonates due to their ability to absorb more CO2 per kg solvent and their higher absorption rate.

•Known industrial processes operate at othertemperatures and pressures than those relevant for CO2 capture.

•Some processes are operated in closed circuitswhere the emission of degradation products is not a problem



Degradation of alkanolamines•Susceptible to thermal and oxidative degradation•Alkanolamines react and degrade in contact withoxygen, CO2, and SO2.

•Degradation products:–Oxazolidinones and imidazolidinones–Piperazine and carboxylic acids–Amides–Amines and nitrosamines - carcinogens–Aldehydes–Heat stable salts - and more



Alkali carbonates•Oldest known method of removing CO2 and other acid gases from gas mixtures.

•Only potassium and sodium carbonates are used due to low solubility of other alkali carbonates.

•In the 1950’s the ”hot potassium carbonate” process or Benfield process was developed by Benson and Field. –Operates at 60/130°C.–Absorption pressure: 3 bar CO2 pressure.

•Simpel chemistry: carbonate is converted to bicarbonate



Carbonate advantages•No carbamate formation

–Low desorption energy required–Absorption rate can be improved by promoters such as DEA or Piperazine

•No degradation problems•Well known technology

–Requires modification of process conditions•Large capacity when KHCO3 is precipitated in the process



Aqueous ammonia•Based on a patent from E. Gal, 2006•The technology has similarities with the Solvay process

•Different versions of the process are being developed:–Chilled Ammonia Process (original)–Ammonia Process (CSIRO)–Ammonia Process (KIER, Korea)–Others

•A 26 wt% ammonia solution has very large capacity for CO2 absorption.



Chemistry•Carbamate ions are formed

–The energy associated with forming and breaking the carbamate is apparently muchlower than for alkanolamines

•The formation of solids is part of the process in some variations of the process. Three differentsolids can form:–(NH4)2CO3·H2O Ammonium carbonate–(NH4)2CO3·2NH4HCO3 Sesquicarbonate–NH4HCO3 Ammonium bicarbonate

•No known degradation of ammonia



Chilled Ammonia Process•The process requires chilling of several streamswhich costs energy

•The low temperature results in:–Condensation of moisture from flue gas–Reduction of volume of flue gas–Relatively low reaction rate–Formation of solids

•The desorber is operated at high pressure–Less evaporation of water and ammonia–Less compression of carbon dioxide required



Partial pressures duringabsorption


0

0,01

0,02

0,03

0,04

0,05

0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

Part

ial P

ress

ure

, b

ar

Loading (mol CO2/ mol NH3)

CO2

NH3

10°C, 1 bar total pressure, 27

(NH4)2CO3·H2O

NH4HCO3

The graph was calculated with the Extended UNIQUAC model of Thomsen and Rasmussen Chem. Eng. Sci. 54 (1999) 1787-1802


Amount of solids formed duringabsorption


0

2

4

6

8

10

12

14

16

18

20

0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

mo

l sa

lt

Loading (mol CO2/ mol NH3)

NH4HCO3

(NH4)2CO3·H2O

10°C, 1 bar total pressure, 27 wt % NH3The graph was calculated with the Extended UNIQUAC model of Thomsen and Rasmussen Chem. Eng. Sci. 54 (1999) 1787-1802


Gibbs phase rule•F = C – P + 2F = Degrees of freedomC = Number of independent components= 3P = Number of phases

•P = 1 gas phase, 1 liquid phase, 2 solid phases•F = 3 – 4 + 2 = 1•The only degree of freedom is the composition of the solid phase

•The compositions of liquid and gas phases have to remain constant as long as there are 4 phasesand T and P are fixed!



Amino acid salts•Amino acid salt properties

–Not volatile like ammonia and alkanolamines–Same functional groups like alkanolamines–Some are highly soluble in water–Temperature dependency of acid constants–Might decompose like alkanolamines

•Few examined systems–Potassium taurate–Potassium sarcosinate–Potassium glycinate–Potassium methionate



Ionic liquids•Still on a very basic level•Task specific ionic liquids are being developed

–Contain eg amine groups–Might be hydrofobic–Not volatile

•No convincing results have yet been publishedfor ionic liquids



Thermodynamic modelling•Purpose: To simulate and optimize the captureprocess.

•The thermodynamic model should reproduce the experimental data and be valid for interpolation and extrapolation.

•Electrolyte solutions are modelled using activitycoefficient models for the liquid phase and equation of state for the gas phase–Electrolyte NRTL (ASPEN)–Extended UNIQUAC (DTU, Lyngby)



CCS process simulation•ASPEN plus modelling

–Equilibrium stage approach • Using ASPEN’s own thermodynamics• Using a user thermodynamic model (DTU)

–Simple rate based approach•Promax modelling•Detailed In-house packages

–NTNU/SINTEF, Trondheim, Norway (CO2SIM)–University of Texas at Austin–DTU, Lyngby, Denmark (CAPCO2)



Conclusion•Alkanolamine and alkali carbonate processes arebeing adapted to be used for carbon capture

•Processes using ammonia apparently have someadvantages and are tested extensively

•Amino acid salt solutions have great potential but need more research

•Task specific ionic liquids for selective absorption of carbon dioxide have not been presented yet


current status of r&d in post combustion co2 capture · 2012. 5. 22. · •only potassium and...

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