CO 2 sequestration via direct mineral carbonation of Mg-silicates. Natalie Johnson GCEP Symposium 4 October 2011

Transcription

1 CO 2 sequestration via direct mineral carbonation of Mg-silicates Natalie Johnson GCEP Symposium 4 October 2011

2 CO 2 release/year (Gt) 2 CCS: Part of climate change mitigation Projection based on current emissions Trajectory required to maintain 450 ppm End-use efficiency (appliances, cars, etc.) 52% Power plant efficiency 5% Renewables (solar, wind, hydro) 20% Biofuels 3% Nuclear 10% CCS 10% International Energy Agency, 2009

3 3 Mineral carbonation is stable over the long term Physical trapping (A) Structural Residual (pore-level) Chemical trapping Solubility (B) Mineral (C) Mg, Ca, and Fe-silicates Shale caprock Sandstone Deep saline aquifer Image by Pablo Garcia del Real Chemical trapping, specifically mineral carbonation, is the most stable way to sequester carbon over the long-term, but will only proceed in a significant way in specific types of geologic settings. Even then, the reaction has limitations.

4 ΔG 4 Mineral Carbonation: A natural geochemical process Carbon -400 kj/mol 2 m Carbon Dioxide Mg-Carbonate -60 to 180 kj/mol 1 mm 1 m Photos courtesy of Pablo Garcia del Real and USGS archives

5 5 CO 2 storage capacity is enormous Mineral name olivine 2015 pyroxene 1404 plagioclase 436 serpentine 1233 Pruess et al Potential CO 2 fixed (kg/m 3 mineral) World serpentine deposits World basalt deposits: 5% of land surface By carbonating all the olivine in the world, we d capture about 500,000 GT CO 2, >15,000 years at current emission levels.

6 6 Mineral carbonation challenges Reaction is kinetically limited Limiting step? Catalysts/activators/inhibitors? Silica is less soluble than carbonates Reaches saturation first May inhibit reaction in several ways Volume increase is significant +94% silicates to carbonates and silica +28% silicates to carbonates In nature, dissolution and precipitation occur in different places can we engineer this?

7 7 Carbonation chemistry Mineral Carbonation Mg 2 SiO 4 + 2CO 2 2MgCO 3 + SiO kj/mol Mineral Dissolution Mg 2 SiO 4 + 4H + 2Mg 2+ + SiO (aq) 2 + 2H 2 O CO 2 Dissolution and Dissociation CO 2 + H 2 O HCO 3- + H + Secondary Phase Precipitation Mg 2+ + HCO - 3 MgCO 3 + H + SiO 2 (aq) SiO 2 (am)

8 8 Experiments to further understand kinetics Allows for fluid sampling at reaction conditions Study short- and long-term dissolution and carbonation kinetics Lemke 2008 Photos courtesy of Robert Rosenbauer

9 9 Experimental details Batch reactions Reach saturation No product losses Conditions mimic a natural system T = 60 o C P = 100 bar CO 2, ml headspace 20:1 water:rock ratio (by mass) 0.5M NaCl ph = 3-6, controlled by CO 2 and rock System is primarily composed of a CO 2 -saturated aqueous phase containing solid olivine, with some excess CO 2.

10 [Mg] (ppm) Magnesite Saturation Index 10 Reactions form Mg-carbonate (magnesite) μm time (days) 1 μm 1 µm

11 [Si] (ppm) Amorphous Silica Saturation Index 11 Solutions reached silica saturation quickly μm SiO μm time (days) 0 10 μm

12 olivine dissolution rate (mol/s/cm 2 ) 12 Dissolution rate decreases with increasing Ω SiO2 1E-09 1E-10 Mg 1E-11 1E-12 1E-13 Si Gibbs Free Energy of SiO 2 Dissolution (kj/mol) Rate depends on the driving force (thermodynamics) Dissolution rate should not depend on saturation of a secondary phase (silica)

13 [Mg] (ppm) 13 Unless the olivine surface is like silica Bearat et al SiO 2 -saturated H 2 O SiO 2 xh 2 O No net dissolution of Si-rich layer (no driving force) Slow Mg diffusion across Si-rich layer (Mg, Fe) 2 SiO The combination of these effects will cause the olivine dissolution rate, as measured by either Mg or Si in solution, to go to zero time (days)

14 Olivine dissolution rate (mol/s/cm^2) 14 Evidence for the Si-rich layer is plentiful Si-rich, Mg-depleted reacted surface Unreacted olivine Si Mg 1E-10 1E-11 1E-12 Si Mg Initial incongruent dissolution Reacted olivine Binding Energy 0 1E time (hrs) H + Mg +2 Metal-proton exchange Si +4 O -2

15 15 Summary Magnesite can form at mild T, P with no harsh additives Abundant evidence exists for the formation of a Si-rich layer on the olivine surface (under these conditions) Silica saturation state has significant effect on olivine dissolution rate Rate decreases with increasing saturation Effect diminishes after silica begins precipitating 1. Geochemical models should be modified to include the dependence on SiO 2 saturation 2. Laboratory experiments should be focused on SiO 2 management

16 16 Ongoing and future work Si-rich layer: leached or precipitated? Invesitgate concentration profile of Mg XPS depth profile Ion microprobe Activators and catalysts Organic acids (i.e. salicylic) Titanium Surface processes

17 17 Thank you Acknowledgements Prof. Gordon E. Brown, Jr. Prof. Kate Maher Prof. Dennis Bird Dr. Bob Rosenbauer, Dr. Yousif Kharaka Dr. Burt Thomas To hear more: stop by posters 52, 53, and 54