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.

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.