Carbon Capture in Molten Salts

A new process for CCS based on Ca-looping chemistry

Summary

Espen Olsena, Viktorija Tomkutea, Asbjørn Solheimb aDep. Mathematical Sciences and Technology, UMB, N-1432 Ås, NorwaybDep. Energy Conversion and Materials, SINTEF Materials and Chemistry, N-0314 Oslo, Norway

Carbon Capture in Molten Salts (CCMS) has been demonstrated on the laboratory scale. Absorption of CO2 from a simulated combustion gas is shown to exhibit extremely efficient characteristics, absorbing up to 99.97% of CO2 from a simulated flue gas in a reactor column of 10 cm length. Desorption proceeds to 100% so all CaO is regenerated in reactive state overcoming the main challenge in conventional Ca-looping. If the process characteristics are scalable to larger scale reactors exhibiting similar efficiency, selective capture and release of CO2 from a wide range of gas compostitions is possible. This opens for a large number of applications.

Introduction and Theory

Experimental

Conclusions and further work

Results

The Ca-looping principle1 relies on displacement of the equilibrium described by Eq.(1) (M denotes an alkaline-earth metal) by thermal cycling at elevated temperatures (650 - 1000°C). This minimizes fundamental losses due to low temperature waste heat.

The process is performed in FBR-reactors and is being developed on the demonstration scale2. The main obstacles for successful commercial implementation is deactivation of CaO powders by decomposition and sintering introduced by thermal cycling.

A dedicated laboratory set up involving a high-sensitivity FTIR gas analyzer and TGA functionality is used. A simulated flue gas (0-100% CO2 in N2) is fed to a 10 cm column of molten salt containing CaO. The gas composition before and after absorption is analyzed with high accuracy

References: 1 Chem. Eng. Res. Des. 89, (2011), 836-8552 www.caoling.eu3 Norwegian patent No. 20092083

(s) MCO (g) CO (s) MO 32

Figure 1: The Gibbs free energy of reaction (1) vs. temperature and alkali-earth cation.

v

Ni

SS

CaF2/NaF/CaO/CaCO3

N2+CO2(+ SO2) N2

150

mm

50 mm

Figure 3: Details of the reaction chamber. Outer sleeve of steel, inner crucible and feed tube of Ni.

Gas composition(FTIR)

Temperature

Gas in (MFC)

Tubular ceramic furnace (1250°C)

Weight (TGA)

Reactor chamber

Gas out (MFM)

Figure 2: The experimental setup, schematically depicted. The absorption-desorption processes are monitored by gravimetry (TGA) and mass balance by gas analysis (FTIR).

The CCMS idea: By (partly) dissolving the active substances in a supersaturated molten salt, highly reactive absorbing CaO is constantly regenerated as described by Eq.(2). (M denotes an alkaline-earth metal)

The CCMS project aims at:

• To develop a new and patented process for carbon capture. 3

• Establish the scientific foundation for industrialization. Time frame: 5-10 years.

s)(diss,MCO (g)CO s)(diss, MO 32

Figure 5: Repeated absorption-desorption cycling (4x, 800°C/950°C) from a simulated flue gas (N2+27% CO2) in a chloride based absorbing liquid (CaCl2+5%CaO). The content of CO2 in the gas emitted is shown in the bottom panel while the mass of the reaction vessel (―) as well as temperature (―) is shown in the top panel.

Figure 6: The conversion efficiency of the cycling between CaO and CaCO3 during absorption (•) and desorption (•) in each of the cycles from Fig.5. The decarbonation of CaCO3 by forming CaO and CO2 is reaching 100% efficiency in all the cycles while the conversion of CaO to CaCO3 during absorption shows a rising trend with each cycle, contrary to the loss in reactivity experienced in solid state Ca-looping.

Figure 7: Absorption with subsequent desorption of CO2 from a simulated flue gas (N2+27% CO2) in a fluoride based liquid (NaF/CaF2/10% CaO) at 820°C. Desorption at 1150°C. The content of CO2 in the gas emitted from the reactor (―) and temperature (―).

The CCMS process works as predicted from fundamental thermodynamic modeling. 5 cycles has been completed with 100% conversion efficiency from CaCO3 to CaO. The present results are promising indicating the potential for CCS from a wide variety of gas compositions from different sources. Focus will be now be directed towards construction of a lab pilot reactor for continous operation

(1)

(2)

-300

-200

-100

0

100

200

300

0 500 1000 1500 2000

ΔG°/

[kJ/

mol

]

Temperature/°C

Mg

Ca

Sr

Ba

Figure 4: Schematic set up of a pilot scale reactor for continous operation.

CaCO3-enriched molten salt

CaO- enriched molten salt

CO2N2. H2O etc

Flue gasN2, CO2, etc

CaCO3 → CaO+ CO2CaO → CaCO3 + CO2

~700°C

~900°C

heat

heat