Thermodynamical aspects of ocean sequestration

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Thermodynamical aspects of ocean sequestration Powered By Docstoc
					Chemical & Bioengineering

 Thermodynamical aspects of ocean sequestration

                                            Wolfgang Arlt
                      Chair of Separation Science & Technology


                                                                     Prof. Arlt

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

                     .                      Prof. Arlt

                   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

      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

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

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

                                                            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

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

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

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

Man-made, two phase dissolution in the sea

•impact on microorganism
•available data and modelling

                                .            Prof. Arlt

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

         deep sea                                         flat water

                                                           CO 2 gas

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

                    1000 ~ 3000 m

    >3400 m

CO 2 -hydrate       liquid CO2

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

                    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
                    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

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

                                                    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

                                                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

impact of dissolved CO2 on marine


                .                              Prof. Arlt

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

Thermodynamic data in literature

                .                  Prof. Arlt

[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

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





                                          (Brewer, Science 1999)
Approximate density of sea water

      Densities of pure liquid and solid Carbon Dioxide     Prof. Arlt

                                         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

                         „Everything“ is in the ocean.

CO2-hydrogencarbonate-carbonate equilibrium
                   .                                     Prof. Arlt

     equilibrium of carbonate salts

          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

alkalinity as a function of locus and depth   Prof. Arlt

„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.

                                             .                                      Prof. Arlt

  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

Arlt, not Saint Exupéry   Prof. Arlt


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