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Thermodynamical aspects of ocean sequestration

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					Chemical & Bioengineering




 Thermodynamical aspects of ocean sequestration




                                            Wolfgang Arlt
                      Chair of Separation Science & Technology


                                 Wolfgang.Arlt@cbi.uni-erlangen.de


                                                                     Prof. Arlt

    Friedrich-Alexander-University Erlangen-Nuremberg                  1
capture of CO2 in electric power stations
        fired with fossile energy




                     .                      Prof. Arlt

                                              2
                   efficiency drop        captured CO2
• Nat. gas CC: 56% => 47%            (370 => 60 g CO2/kWh)
• Coal fired:      46% => 33%        (720 => 150 g CO2/kWh)
• Gasif. CC:       46%=> 38%         (710 => 130 g CO2/kWh)


                                                          CC: combined cycle



    It is the task of thermodynamicists to reduce this drop of efficency.




                                                                     Lysen, Utrecht



                CO2 capture in power plants reduces efficiency                Prof. Arlt

                                                                                3
      Natural dissolution in the sea water

The sink for CO2 are mainly the oceans, absorbing CO2 in a
buffered system. The surface is affected most.




                               .                             Prof. Arlt

                                                               4
Alkalinity there are two definitions for alkalinity:
3- dimensional dynamical-statistical atmosphere model
- The alkalinity of seawater is the sum of the concentrations of anions that
     •spatial resolution pH of seawater.
  accept protons at theof 10° (2°) in latitude and 51° (4°) in longitude
     •multibasin zonally averaged ocean model
- Another definition for the alkalinity is that it is the difference between the
     •sea ice model which operates with latitudional resolution of 2,5°
  concentrations of total cation and total anion that do NOT exchange H+ in the
     •terrestrial vegetation model
  pH range of seawater.
                                              (Lawrence Livermore National Lab model)
Ocean Biogeochemistry: biogeochemical tracers considered in the model
    •phosphate PO42- and oxygen O2
    •alkalinity ALK
    •dissolved inorganic carbon DIC
    •two types of dissolved organic carbon DOC


advective, diffusive and convective transports for every tracer
at the sea surface DIC and O2 are exchanged with the atmosphere
                                                                                 Brovkin


                what is simulated by climate models like CLIMBER-2?             Prof. Arlt

                                                                                  5
                                                            Units are Pg for carbon
                                                            storages and Pg/yr for
                                                            carbon fluxes

                                                            The oceanic and
                                                            terrestrial carbon
                                                            storages are 38320 and
                                                            1930 Pg C

                                                            oceanic and terrestrial
                                                            net primary productivities
                                                            are similar 44 and 55
                                                            PgC/yr respectively




                                                               P = 1015; NPP = net
                                                               primary production;
                                                               POC =particulate
Brovkin                                                        organic carbon

          Scheme of the global carbon cycle in preindustrial steady state     Prof. Arlt

                                                                                6
driving force: partial pressure of CO2 in the air and in the surface water = Henry Law

gas exchange coefficient estimated to 0,06 mol m-2 yr-1 10-6 atm-1 [Siegenthaler 1986]

PCO2 in the surface water is calculated from surface temperature, salinity, DIC and
ALK in accordance with Millero [1995]

dissolution of CaCO3


Biological processes in the euphotic zone (the upper 100 meters) are explicitly
resolved using the model for plankton dynamics. Availability of nitrate and in some
places of iron (IronEx) biological productivity




It was proposed by John Martin, researcher at Moss Landing Marine Laboratories (MLML), that
adding iron to the oceanic waters would increase plankton growth to a significant degree. In
1993 and 1995, researchers at MLML conducted research experiments called IronEx I and
IronEx II in which they tested this hypothesis.                                          Brovkin


                                           .
                        thermodynamics of dissolution in the sea water                   Prof. Arlt

                                                                                           7
Downwelling and upwelling currents are produced by density differences in sea
 water, e.g.
•at the north pole pure water ice is produced what causes an increase of density in
 the remaining sea water.
•in Gibraltar, denser sea water from the „cooking pot“ Mediterranean flows into the
 Atlantic ocean.


Flows can be very high.




Diffusion is important but does not really contribute to mixing.




                             mixing effects with deep water                    Prof. Arlt

                                                                                 8
The vertical tracer mixing coefficient varies from
0.2 cm2/s          [Wickett 2003]
 1 cm2/s           [Radhakrishnan 2003]
0,1-0,3 cm2/s      [Wong, 1996]


... at the surface to 10.0 cm2/s at depth. Since model momentum mixing (viscous)
coefficients are larger than physically justifiable, smallest value is used, what is
relatively free of numerically induced noise. [Wickett 2003]




                                  uncertainties in diffusion                           Prof. Arlt

                                                                                         9
Man-made, two phase dissolution in the sea
                water


•impact on microorganism
•available data and modelling




                                .            Prof. Arlt

                                               10
1. inject liquid CO2 into liquid seawater
    (plume, dissolution of drops)
2. inject gaseous CO2 into liquid seawater
    (plume, dissolution of bubbles)
3. dispose adsorbed CO2 into liquid seawater
4. single phase, saturated at the land




                            how CO2 is injected in the ocean   Prof. Arlt

                                                                 11
         deep sea                                         flat water

                                                           CO 2 gas

liquid CO2          liquid CO2                   200 ~ 400 m
                                                 1 phase or 2
                                                 phases

                    1000 ~ 3000 m



    >3400 m

CO 2 -hydrate       liquid CO2




                Where to leave the CO2: ocean (the natural option)     Prof. Arlt

                                                                         12
                    The uplift velocity is a function of the
                    viscosity and density of the sea water
                    and the size of the bubbles. The density
                    of seawater is increased by dissolved
                    CO2.
                    The size of the bubbles is extended by
                    decreasing pressure and reduced by
                    dissolved gas. So a gas transfer
                    coefficient must be known.




airlift principle by Kajishima [1997]                Prof. Arlt

                                                       13
There are 2 principal topics:
•thermodynamics of hydrate formation
•dynamics of hydrate formation


for the dynamics the following data is needed
1. diffusion coefficient of CO2 in seawater (assumed to be 2x10-9 m2s-1)
2. diffusion of water thru the hydrate film (1,1 x 10-12 m2/s) [Rad..]




                      hydrate formation (Radhakrishnan,Kajishima)          Prof. Arlt

                                                                             14
                                                    The CO2 may form a
                                                    hydration shell from a
                                                    symmetrical dodecahedral
                                                    arrangement of 18 water
                                                    molecules where each CO2
                                                    oxygen atom is hydrogen
                                                    bonded to three water
                                                    molecules.




                                                Clathrates exist near 0°-+10°C
                                                and higher pressures 40-1000 bar



Clathrates are known between water and different gases, among them methane.

                                    Clathrate                             Prof. Arlt

                                                                            15
impact of dissolved CO2 on marine
             organisms




                                    www.ieagreen.org.uk


                .                              Prof. Arlt

                                                 16
consider the marine organism as a membrane


CO2 affects
•the transport of respiratory gases in the organism
•the formation of the calcareous skeleton




                                            .         Prof. Arlt

                                                        17
Thermodynamic data in literature




                .                  Prof. Arlt

                                     18
[Roy, 1995 /Subow, 1931] define sea water as

                     compound      conc in g/ kg seawater

                     NaCl          26,518
                     MgCl2         2,447

                     MgSO4         3,305

                     CaCl2         1,141

                     KCl           0,725

                     NaHCO3        0,202

                     NaBr          0,083


Approximation of data is made by an empirically extended virial equation by
Zhenhao Duan.



                                         .
                                 what is seawater?                            Prof. Arlt

                                                                                19
The CO2-solubility in pure water is known from 0,05 to 30 MPa and from 0 to
100°C. Some data are on seawater.
Clatharates are formed below 10°C and 4,4 MPa. [Diamond 2003]


Liquid densities of CO2/pure water 4 to 20°C by [Teng 1997],
for water/CO2/NaCl at more than 130°C by Gehring [1986]


We did not check for viscosities.




                                    thermodynamic data                        Prof. Arlt

                                                                                20
                 solid
         4°C


      10°C

                            0°C


                             2°C




                                          (Brewer, Science 1999)
Approximate density of sea water

      Densities of pure liquid and solid Carbon Dioxide     Prof. Arlt

                                                              21
                                         known   unknown
density of water + CO2                   X
density of sea water + CO2                       X
solubility of CO2 in water at 5°C         X
solubility of CO2 in sea water at 5°C     X      X
pH of CO2-water/seawater                  X
impact of pH-change on marine life               X
deep sea current                          X
surface current                           X
diffusion                                 X      X
biological conversion of CO2              X      X
alternative power processes               X      X
effective absorption procedures 10->1 vol%       X
total energy balance                             X



                           What is known?                  Prof. Arlt

                                                             22
                         „Everything“ is in the ocean.



CO2-hydrogencarbonate-carbonate equilibrium
                   .                                     Prof. Arlt

                                                           23
     equilibrium of carbonate salts
24




                                      http://ocean.stanford.edu/bomc/chem/lecture_10.pdf.
          Prof. Arlt
                                                                       J.P.Riley, G.Skirrow: Chemical Oceanography, Vol. 2, page 145
These data vary with water depth, too. Fluctuation in pH about 0,15.


                 pH and alkalinity of surface water in the Pacific                                                                Prof. Arlt

                                                                                                                                       25
                                                   http://ocean.stanford.edu/bomc/chem/lecture_10.pdf.
alkalinity as a function of locus and depth   Prof. Arlt

                                                26
„Sea water“ is a different liquid a different locations and different depths. A flexible
model is needed.


Most thermodynamical data for carbon dioxide and water is known, but few is
available for sea water.
Small changes in density are important for downwelling currents. This density must
be known precisely as a function of CO2 concentration.


The thermodynamical model is more or less the semi-empirical EoS by Duan, what
only reflects phase equilibrium data.
A more general model is needed.


Most transport data is interpolated and approximated. This is especially true for
hydrate formation and plume.



                                          Résumé
                                             .                                      Prof. Arlt

                                                                                      27
  W enn D u ein build a ship, illst, drum the men together to divide the
„If you want toSchiff bauen wdon‘t trom m le nicht die M änner zusa m m en, um
 work and give orders. Instead, teach die Sehnsucht
die A rbeit einzuteilen, sondern lehre Sie them to yearn for the last and
 endless w eiten
nach dem sea. M eer.“                                    Antoine E xupéry
                                                    A ntoine de Saintde Saint Exupéry
courtesy: BASF


                                      The end.                                    Prof. Arlt

                                                                                    28
Arlt, not Saint Exupéry   Prof. Arlt

                            29

				
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