Current status of R&D in post combustion CO 2 capture

Transcription

1 Current status of R&D in post combustion CO 2 capture Kaj Thomsen, Ph.D. Center for Energy Resources Engineering, CERE DTU Chemical Engineering Technical University of Denmark

2 Outline Choice of solvent Alkanolamines Alkali carbonates Ammonia Amino acid salts Ionic liquids Thermodynamic modelling Process simulation 2

3 Alkanolamines Organic compounds with an alcohol functional group 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 3

4 Chemistry Carbamate formation R R NH + HCO H O + R R NCOO The R 1 R 2 NCOO - 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 4

5 Carbamate formation The reaction rate of carbamate forming alkanolamines, such as MEA, with CO 2 is a factor 1000 higher than the reaction rate of a tertiary amine such as MDEA. Tertiary alkanolamines are bases. They react with water to form protonated alkanolamine and hydroxide. Carbamate formation goes through a maximum at a certain loading. Capture processes are operated in the loading range where carbamate formation is significant. 5

6 Known technology Alkanolamines have been used for CO 2 removal from gas mixtures since the 1930 s. Alkanolamines replaced alkali carbonates due to their ability to absorb more CO 2 per kg solvent and their higher absorption rate. Known industrial processes operate at other temperatures and pressures than those relevant for CO 2 capture. Some processes are operated in closed circuits where the emission of degradation products is not a problem 6

7 Degradation of alkanolamines Susceptible to thermal and oxidative degradation Alkanolamines react and degrade in contact with oxygen, CO 2, and SO 2. Degradation products: Oxazolidinones and imidazolidinones Piperazine and carboxylic acids Amides Amines and nitrosamines - carcinogens Aldehydes Heat stable salts - and more 7

8 Alkali carbonates Oldest known method of removing CO 2 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 CO 2 pressure. Simpel chemistry: carbonate is converted to bicarbonate 8

9 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 KHCO 3 is precipitated in the process 9

10 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 CO 2 absorption. 10

11 Chemistry Carbamate ions are formed The energy associated with forming and breaking the carbamate is apparently much lower than for alkanolamines The formation of solids is part of the process in some variations of the process. Three different solids can form: (NH 4 ) 2 CO 3 H 2 O Ammonium carbonate (NH 4 ) 2 CO 3 2NH 4 HCO 3 Sesquicarbonate NH 4 HCO 3 Ammonium bicarbonate No known degradation of ammonia 11

12 Chilled Ammonia Process The process requires chilling of several streams which 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 12

13 Partial pressures during absorption Partial Pressure, bar 0,05 0,04 0,03 0,02 CO2 NH3 10 C, 1 bar total pressure, 27 (NH 4 ) 2 CO 3 H 2 O NH 4 HCO 3 The graph was calculated with the Extended UNIQUAC model of Thomsen and Rasmussen Chem. Eng. Sci. 54 (1999) ,01 0 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 Loading (mol CO 2 / mol NH 3 ) 13

14 Amount of solids formed during absorption mol salt C, 1 bar total pressure, 27 wt % NH 3 NH 4 HCO 3 The graph was calculated with the Extended UNIQUAC model of Thomsen and Rasmussen Chem. Eng. Sci. 54 (1999) (NH 4 ) 2 CO 3 H 2 O ,3 0,4 0,5 0,6 0,7 0,8 0,9 1 Loading (mol CO 2 / mol NH 3 ) 14

15 Gibbs phase rule F = C P + 2 F = Degrees of freedom C = Number of independent components= 3 P = Number of phases P = 1 gas phase, 1 liquid phase, 2 solid phases F = = 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 phases and T and P are fixed! 15

16 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 16

17 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 published for ionic liquids 17

18 Thermodynamic modelling Purpose: To simulate and optimize the capture process. The thermodynamic model should reproduce the experimental data and be valid for interpolation and extrapolation. Electrolyte solutions are modelled using activity coefficient models for the liquid phase and equation of state for the gas phase Electrolyte NRTL (ASPEN) Extended UNIQUAC (DTU, Lyngby) 18

19 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) 19 Current status of R&D in post combustion CO2 capture Chalmers Energy Conference

20 Conclusion Alkanolamine and alkali carbonate processes are being adapted to be used for carbon capture Processes using ammonia apparently have some advantages 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 20

21