Presentation by Rachel White (Port & Coastal Solutions Ltd, UK & Australia), at the Delft3D User Days, during Delft Software Days - Edition 2022. Monday, 14 November 2022.

1. Rachel White, Port and Coastal Solutions
Phil Shepperd, Coastal Science
Modelling the effects of Magnesium
Hydroxide Addition

2. Project Overview
• In support of a negative-CO2-
emissions hydrogen R&D
project being conducted by PH,
PCS and CoSci were
commissioned to develop and
apply a suite of numerical
modelling tools to assess the
effect of Magnesium Hydroxide
(MH) addition.
• Pilot study considering addition
of MH to an existing discharge
at Hayle in St Ives Bay, UK.
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3. Study Objectives
To main study objectives were to assess:
• the impact of MH addition on pH (<8.5) and suspended solids (<100
mg/l);
• the effect of the MH addition on Carbon Dioxide Removal (CDR) from the
atmosphere; and
• the likely area of detectable change around the outfall.
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4. Modelling Tools
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Delft3D-WAQ
Non Hydrostatic
Delft3D-FLOW
Delft3D-PART
Delft3D-FLOW
CORMIX
Delft3D-WAQ
pH, AT, DIC and
pCO2 sea
SSC
Nearfield plume
characteristics
Test addition
and survey

5. Hydrodynamic model setup
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6. Modelled Flows
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Peak flood and ebb occurs approximately 3 hours either side of HW

7. 7
Determination of pCO2SEA from seawater DIC, pH, and temperature
DIC = [CO2aq ] + [H2CO3] + [HCO3‾] + [CO3
2‾]
pCO2SEA = DIC / (K0 (1 + K1/[H+] + K1K2/[H+]2)) where H+ = 10-pH
Simplified determination of [H+] (Follows, 2006)
Carbonate alkalinity defined as AC = [HCO3
-] + 2[CO3
2-] can be approximated by;
AC ≈ AT – [B(OH)4
-] – [OH-] where [OH-] = kw/H+
Air-Sea Gas Exchange - simplified expression given for CO2 by Wanninkhof (2014)
Flux, F = 7.7 × 10–4 <U10
2> ΔpCO2 where ΔpCO2 = pCO2SEA - pCO2AIR
Water Quality Modelling – MH addition

8. Simple terms (from a non-chemist)
• We have 5 variables, (DIC, pCO2, AT, CO2sea and pH), if we know 2 of them
we can calculate the other 3.
• MH is alkaline
• MH addition will therefore increase pH
• A more alkaline sea can absorb more CO2 from the atmosphere = Carbon
Dioxide Removal (CDR)
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9. Dissolution
MH is not that soluble…
Particulate MH is
represented by two particle
class sizes, with an
assumption that the
number of particles is split
equally between the two
sizes.
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The mass and surface area of the particles are conserved on
hydrodynamic mixing and the particle numbers and sizes are updated each
model timestep to account for the addition from the discharge and the
dissolution.

10. Model Simulations
• Background assessment – No discharge to give baseline flux of
CO2 for summer and winter conditions
• Existing Hayle discharge – what the MH addition runs are
assessed against for CDR
• MH addition assessment
• Addition of 158 kg/day fully dissolved MH
• Addition of 158 kg/day fully dissolved plus 112 kg/day (low dosing) in
particulate form
• Addition of 158 kg/day fully dissolved plus 842 kg/day (high dosing) in
particulate form
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11. Effect on pH
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95th percentile change < 0.001,
positive values show an increase.
Partially dissolved – highest dosing, winter conditions

12. Effect on CO2 Flux
95th percentile change in CO2 flux,
More than 10% locally close to outfall, 1% across the model domain
(increase of 201 tonnes/6 months relative to background of 20,665)
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Partially dissolved – highest dosing, winter conditions

13. Effect on SSC
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95th percentile change in SSC
Change is << 1mg/l
Partially dissolved – highest dosing, winter conditions

14. Nearfield Plume Modelling
• High resolution grid with 1 m x 1 m resolution sits in existing domain using
DD (1:45 refinement) over approx. 300 x 300 m.
• Run in 3D with 16 vertical z-layers.
• For stability it was necessary to run both grids in 3D
• Max depth in inner grid was 22 mODN, while outer grid was 76 mODN
• Irregular vertical layers were selected to give a vertical resolution of
around 3 m in the inner grid (7-8 layers in the inner grid)
• Inner grid run in non-hydrostatic mode
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15. Model grids
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16. Nearfield plume dispersion
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The discharge (DWF 0.228 m3/s) was split over the 4 outfall heads, applied with a zero salinity
Low Flow Typical Flow

17. Nearfield WAQ model
Modelled the discharge over two semi-diurnal tides (12.5 hours) for spring
and neap tides.
Only fully dissolved MH was considered
Looked at the changes in pH, AT and pCO2.
Detectable limits of these parameters are as follows:
• pH: 0.01;
• AT: 5 µmol; and
• pCO2 = 10 µatm.
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18. Nearfield pH effect
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Close to limit of detectability local to risers, near the bed. Slack flow conditions give best chance!

19. Results from nearfield plume model
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The area of detectable change is limited to a small area local to the
outfall ports (within 20 to 30 m of the outfall ports) within 3-6 m of the
bed.
Thermal stratification is not included in the model; seasonal thermocline
could potentially increase the detectability of any signal near the bed;
Momentum was not applied at the outfall since details of the outfall
design are not known; if the outfall is orientated directly upwards some
momentum would occur and this could increase the depth over which
any signal is detectable.

20. Dosing at the treatment site
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21. Measuring offshore
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22. Conclusions from study
• no consenting issues relating to the addition of MH on the pH and
suspended solids were identified at the dosing rates considered.
• with respect to potential for increased CO2 uptake resulting from the
addition of MH, increases across the domain are of the order of 200
tonnes in six months (and with some additional increases of the
order of 20-40 tonnes expected outside the model domain).
• in the nearfield the plume is very difficult to detect with signatures in
pH, AT and pCO2 close to instrument accuracy.
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23. Acknowledgements
We would like to thank the following:
Arjen Markus (Deltares) for his assistance in developing the DELWAQ
processes library (Delft3D-WAQ) and for his persistence in finding a solution
to the dissolution problem!
Erik de Goede and Frank Platzek for their technical support in developing
the non-hydrostatic model
Steve Rackley, Will Burts, Mike Kelland and the wider PH team
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