An Accelerated Weathering of Limestone Reactor - Phil Renforth at the Alternative CCS Pathways Workshop, Oxford Martin School, 26 June 2014
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1) Accelerated weathering of limestone has the potential to store large volumes of carbon dioxide by increasing alkalinity in the oceans, but questions remain about environmental impacts and stability of stored carbon. 2) Laboratory experiments and geochemical modeling show that limestone dissolution rates and carbon dioxide uptake can be modeled and are dependent on factors like surface area, water volume, temperature, and gas-liquid interface. 3) At an industrial scale, accelerated weathering of limestone would require very large reactor volumes, on the order of 105-106 m3, to sequester a million tons of carbon dioxide per year.
![Kinetic Modelling
Gas mass transfer
CO2 hydration
Limestone dissolution
Considerable existing work deriving k1,k2,k3,ksp,DCO2 (T,S).
Rates r1 and r3 multiplied by interfacial area normalised to
solution volume.
Boundary layer z assumed ~ 60µm (Bolin 1960; Jähne 2012 )
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An Accelerated Weathering of Limestone Reactor - Phil Renforth at the Alternative CCS Pathways Workshop, Oxford Martin School, 26 June 2014
- 1. Dr. Phil Renforth RenforthP@Cardiff.ac.uk An Accelerated Weathering of Limestone reactor Alternative CCS technologies meeting (Oxford, UK) 26th June 2014
- 2. The potential of ocean alkalinity • Global potential of carbon storage as alkalinity not fully understood. • Large volumes of carbon may be stored. (~ 000’s GtCO2) Mineral Extraction and processing Accelerated weathering of limestone Terrestrial enhanced weathering Ocean enhanced weathering (and ocean liming) Electrochemical Conversion CO2 + - Paquay and Zeebe, 2013 Ilyina et al., 2013
- 3. Ocean alkalinity carbon storage questions • What is the environmental impact of a large scale AWL industry? • How stable is the carbon reservoir?
- 4. AWL as a reactor poor carbonate mineral solubility = Large mass ratio of seawater to CO2 (‘000s) • Pumping is likely to be one of the primary energy consuming activities • Minimise/or be limited by, vertical displacement of water • Lack of control • Probably a large reactor
- 5. AWL - Reactions CO2(g) CO2(aq) HCO3 - (aq) CO3 2- (aq) H2O(l) H+ (aq) H+ (aq) CaCO3(s) Ca2+ (aq) Gas Solution Solid Other species (% of total dissolved carbon species): MgHCO3 + (aq) (11%) NaHCO3 (aq) (8%) CaHCO3 + (aq) (2%) MgCO3 (aq) (0.6%) NaCO3 - (aq) (0.3%) High ionic strength of seawater YCa2+ 0.26 YHCO3- 0.68 YCO32- 0.21 CO3 2- (aq) (0.3%) CaCO3(s) + H2O(l) + CO2(g) Ca2+ (aq) + 2HCO3- (aq)
- 6. Approach PHREEQC modelling Gas Solution Solid Input: partial P, Volume, Temp, Input: Initial seawater chemistry, Volume, Temp, Input: mineral, moles to dissolve and/or saturation state Batch experiments
- 7. Experimental CO2 2.2L flat flange glass reaction vessel Artificial seawater Magnetic stirrer Limestone: The great Oolitie formation, Crushed and dry sieved to between 500 – 1000µm Sonicated in dionised water to remove microfines CO2 at 50,000 ppm
- 8. Kinetic Modelling Gas mass transfer CO2 hydration Limestone dissolution Considerable existing work deriving k1,k2,k3,ksp,DCO2 (T,S). Rates r1 and r3 multiplied by interfacial area normalised to solution volume. Boundary layer z assumed ~ 60µm (Bolin 1960; Jähne 2012 ) )(2)(2 1 aq k g COCO →← )][]([ )(2)(2 1 1 2 solgas CO COCO z kD r − ⋅ = +− +→←+ )()(3)(2)(2 2 aqaq k laq HHCOOHCO ]][[][ 3 ' 2222 +− −= HHCOkCOkr −+ +→← 2 )(3 2 )()(3 3 aqaq k s COCaCaCO −= −+ spk COCa kr ]][[ 1 2 3 2 33
- 9. Results comparison 0 10,000 20,000 30,000 40,000 50,000 60,000 0 2 4 6 ppmvCO2 Time (h) PHREEQC Model Results Experimental Data Vol solution/vol gas = 0.57 0 10,000 20,000 30,000 40,000 50,000 60,000 0 2 4 6 ppmvCO2 Time (h) Vol solution/vol gas = 1.9
- 10. AWL – Reactions Control CO2(g) CO2(aq) HCO3 - (aq) CO3 2- (aq) H2O(l) H+ (aq) H+ (aq) CaCO3(s) Ca2+ (aq) Gas Solution Solid Other species (% of total dissolved carbon species): MgHCO3 + (aq) (11%) NaHCO3 (aq) (8%) CaHCO3 + (aq) (2%) MgCO3 (aq) (0.6%) NaCO3 - (aq) (0.3%) High ionic strength of seawater YCa2+ 0.26 YHCO3- 0.68 YCO32- 0.21 CO3 2- (aq) (0.3%) CaCO3(s) + H2O(l) + CO2(g) Ca2+ (aq) + 2HCO3- (aq) Vgas Vliquid Vsolid Agas-liquid Aliquid-solid Volume CO2 in gas, Partial pressure
- 11. Integrated model PHREEQC Reactor model Initial cond. Outcomes pCO2, FCO2, T… Residency time Reactor volume Cursory reactor design Energy requirements Solid material handling Gas blowing and compression Seawater movement/ pumping Surface area creation (comminution), Short distance material transport, Stacking, Dewatering, Fuel Fun (water depth, interfacial area, recirculation) ~0.5 bar overpressure Fun (velocity head, frictional losses) Fun (residency time, reactor dimensions, Static mixing, material) Normalised as total thermal to net carbon sequestered GJt / tCO2
- 12. Sensitivity analysis 0.7 GJt / tCO2 1.9 GJt / tCO2
- 13. Conclusions and future work • Large reactor (105-106 m3 per million tonne CO2 p.a.) • Higher CO2 capture in colder waters, but larger reactor volume • Potential relative benefits of gas separation • Counter-flow for volume reduction?
- 14. Dr. Phil Renforth RenforthP@Cardiff.ac.uk Accelerated Weathering of Limestone reactor Alternative CCS technologies meeting (Oxford, UK) 26th June 2014 Thanks to: Prof. Richard Darton and Tim Kruger (Oxford)
- 15. Dr. Phil Renforth RenforthP@Cardiff.ac.uk -3E-06 -2E-06 -1E-06 -1E-20 1E-06 2E-06 3E-06 4E-06 5E-06 6E-06 0 200 400 600 800 1000 1200 1400 reactionspeed time (s) CO2 in gassing CO2 hydration Limestone dissolution
- 16. Example spread of existing research on biological response to changes in the carbonate system.