Treatment of flue gases for carbon capture and storage: analysis

of a process based on carbonic anhydrase

Russo M.E., Olivieri G., Napoli F., Marzocchella A., Salatino P.

Chemical Engineering Department - Università degli Studi di Napoli Federico II - ITALY

1. Introduction

Sequestration of CO2 by absorption in water and conversion into stable bi/carbonates has been

receiving attention as an effective Carbon Capture and Storage (CCS) technology among

post-combustion treatments. The most attractive perspective is the safe and stable

sequestration of the captured carbon into solid carbonates that could be exploited as building

materials. The research has been directed towards the optimization of the main steps of the

process: i) the absorption of CO2 into the aqueous phase; ii) the precipitation of carbonate

minerals from the enriched aqueous stream supplied with a proper metal ion source. The first

step is strongly limited as regards both the absorption capacity of the liquid phase and the rate

of CO2 conversion in carbonic acid. Regarding the latter issue, several authors proposed a bio-

mimetic approach: the Carbonic Anhydrase (CA) enzyme catalysis for the hydration of the

dissolved CO2. The enzyme is ubiquitous in nature and it is able to rapidly convert CO2 into

bicarbonate ion as well as to catalyse the inverse reaction (turnover close to 106 s

-1).

The key features of the CA-assisted CO2 capture processes are: i) the water where the CO2

dissolves and converts; ii) CO2 conversion catalysed by the enzyme; iii) carbon distribution

among CO2, HCO3-, CO3

-- as a function of the pH; iv) the metal ion source to sequester

carbonate.

The enzyme can be made available as dissolved or confined. The latter solution is pursued for

process intensification because it allows to increase the enzyme load of the reactor - by

immobilization on solid carriers or on membranes – then to increase the specific potentiality of

the absorption unit.

The selection criterion of the water stream must take into account the following issues: a) the

buffering capacity of the liquid phase; b) the metal source to form carbonates; c) the required

mass flow rate.

The pH of the absorbing liquid phase has to be high enough to increase the CO2 absorption

capacity of the aqueous stream [1]. Bond et al. [2] assessed the activity of CA in a synthetic

seawater. They pointed out that the effects of ionic species, high salinity, sulphates and

nitrates – transported from the flue gas – on enzymes activity was negligible. Mirjafari et al.

[3] reported a study adopting synthetic brines in lab-scale batch devices. They showed that the

carbonate precipitation rate in the presence of the enzymes increased provided constant pH that

guarantees high carbonate ion fraction in the solution. Favre et al. [4] highlighted the synergistic

effects of the CA and the buffer system: the enhancement of hydration rate due to the adoption

of the enzyme must be properly offset by the action of the buffer, if this is not the case

exceeding catalysts activity leads to the reduction of solid carbonate formation rate.

A rough assessment of the mass flow rate of the metal ion bearing water was reported by

Bond et al. [2]. Given a typical flue gas stream from coal fired power plant (300 MW(e)), the

stoichiometric calcium request is satisfied by 18 106 tonseawater/day (one order of magnitude

larger than the cooling water of the same plant).

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Liu et al. [5] reported investigation on feasibility of CA-based carbon capture and mineral

sequestration adopting produced waters or natural brines from deep reservoirs. Dissolved CO2

was continuously converted in a fixed bed reactor loaded with immobilised CA, solid

carbonate formed in a separate unit where the bi/carbonate bearing stream was mixed with the

brine.

The present contribute reports a part of a research program aimed at studying a CA-assisted

CO2 absorption process based on two step operation. The first step is an enzymatic absorption

unit, in particular a three-phase system: the solid phase is the carrier with immobilised CA;

the liquid phase, the aqueous stream bearing Ca++

; the gas phase, the exhausted flue gas. The

second step is a carbonate recovery unit where the liquid phase is processed. This study

focuses on a preliminary analysis of the process. The main phenomena occurring during the

CA-assisted CO2 absorption and carbonate precipitation in the presence of seawater as metal ion

source have been considered. As a first attempt, the absorption/precipitation process occurs in a

mixed device. The performances of seawater as metal ion source and the extent of the beneficial

effect of the CA enzyme on the process have been assessed.

2. Model assumption and equations

Figure 1 shows a sketch of the CA-assisted CO2 absorption and carbonate precipitation process

occurring in a single apparatus.

NCO2

Seawater

Seawater

Bi/carbonate

CaCO3 (s)

E0

Flue gas

gas phase liquid phase

Gas

NCO2

Seawater

Seawater

Bi/carbonate

CaCO3 (s)

E0

Flue gas

gas phase liquid phase

Gas

Fig. 1 Sketch of the absorption/precipitation unit. E0: carbon anhydrase activity. NCO2: CO2

gas-liquid flux.

Main assumptions of the model are hereby reported.

A) The unit is well-mixed with respect to liquid and is operated under continuous conditions.

The time-space referred to the liquid phase volume is

B) The unit is well mixed and differential with respect to the gas phase. Accordingly, the CO2

concentration in the flue gas stream was constant and equal to the flue gas content

(PCO2=0.15 atm).

C) Seawater at the equilibrium with atmospheric air has been adopted as aqueous phase. The

composition is [6]:

-5 - -3

2 3

-- -4 + -9

3

- -6 ++

[CO ] = 10 mol/kg [HCO ] =1.77 10 mol/kg

[CO ] = 2.6 10 mol/kg [H ] = 6.3 10 mol/kg

[OH ] =9.6 10 mol/kg [Ca ]=0.01028 mol/kg

(I)

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Italian Section of the Combustion Institute

The contribution of magnesium to the precipitation of carbonates has been neglected.

D) The CO2 flux between the gas and the bulk liquid phase (NCO2, mol/s kg) is estimated as

2

2

CO l l 2 2

2 s.w. CO

N =K a ([CO ]*-[CO ])

[CO ]*=H P (II)

Klal is the product between the mass transfer rate and the specific interfacial area between

liquid and gas phases, Hsw (=0.0028 mol/kg atm) the Henry constant for CO2 dissolution in

seawater at 25°C [6].

E) The reactions occurring in the liquid phase are [6]:

- +

2 2 3

- -

2 3

-- + -

3 3

- - --

3 3 2

++ --

3 3( )

1) CO +H O HCO +H

2) CO +OH HCO

3) CO +H HCO

4) HCO +OH CO H O

5) Ca +CO CaCOs

(III)

F) The kinetic expressions of the reactions 1) - 4) in (III) are [6]:

-1 - + 4

d1 d1 2 d1 i1 i1 3 i1

- 3 - 4 1

d2 d2 2 d2 i2 i2 3 i2

d3 d3

r =k [CO ] k = 0.037 s r =k [HCO ][H ] k = 2.66 10 kg/ mol s

r =k [CO ][OH ] k 4.05 10 kg / mol s r =k [HCO ] k 1.76 10 s

r =k [

-- + 10 - 1

3 d3 i3 i3 3 i3

- - 9 -- 5 1

d4 d4 3 d4 i4 i4 3 i4

CO ][H ] k 5 10 kg / mol s r =k [HCO ] k 59.4 s

r =k [HCO ][OH ] k 6 10 kg / mol s r =k [CO ] k 3.06 10 s

(IV)

G) The CO2 hydration reaction catalysed by the CA

- +

2 2 3CO +H O HCO +H

E

(V)

has been described by means of the Michaelis and Menten kinetic model

cat 0 2

E

m 2

k E [CO ]r =

K +[CO ] (VI)

where Km has been set at 0.0174 mol/kg, in agreement with results reported by Mirjafari et

al. [4]. The CA activity E0 has been set at 0.001mol/kg. The value reported in the literature

regarding kcat ranges over a quite large interval. Accordingly, a sensitivity analysis has

been carried out changing kcat between 102 and 10

6 s

-1.

The reversible nature of the reaction (V) has been modelled by multiplying rE by *

1 1(1-K K ) , where *

1 3 2K [HCO ][H ] [CO ] and K1=kd1/ki1.

H) The growth kinetic of the calcite crystals has been expressed as the product between the

specific surface area Ac of the crystal and the net specific growth rate. The latter being the

difference between the growth and the dissolution rates of the crystals. Accordingly, the

growth and dissolution rates are:

n 7 2

growth growth

6 2

diss diss diss

R =k Ω-1 k =4.6 10 mol / s m n 2.22            

R =k (1 ) k 4.6 10 mol / s m

growth

(VII)

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PTSE 2010

where the saturation ratio is defined as Ca++

][CO3--]/Kps [7, 8]. The value of the

solubility product Kps for calcite in seawater was set at 4.27·107(mol/kg)

2 [6].

The growth surface area (A) has been estimated as the product between Ac seeding

concentration (ms). Assuming spherical particles characterised by diameter 10-3

m and

ms=0.1 kg/kgseawater, the A results 0.22m2/kgseawater. The precipitated calcite has been

assumed negligible with respect to ms. Accordingly, the A is constant in this study.

I) The buffer system adopted is able to accept protons produced by the dissociation of the

carbonic acid. Accordingly, the pH is constant and equal to 8.2, typical of seawater. The

ion product of water is Kw=6.06 10-14

(mol/kg)2 [6].

The model is based on the mass balance equations on the dissolved species extended to the

liquid phase operated under the assumption A-H):

2

*2 1

2 IN 2 i1 d1 i2 d2 E CO1

- *- -3 13 IN 3 i1 d1 i2 d2 E d3 i3 d4 i4

1

--

-- --3

3 IN 3 d3 i3 d4 i4 diss

d[CO ] 1 K= ([CO ] -[CO ])+r -r +r -r -r 1- +N

Kdt τ

d[HCO ] 1 K= ([HCO ] -[HCO ])-r +r -r +r +r 1- +r -r -r +r

Kdt τ

d[CO ] 1= ([CO ] -[CO ])-r +r +r -r +A(R (

dt τ

growth

++

++ ++

IN diss growth

Ω)-R (Ω))

d[Ca ] 1= ([Ca ] -[Ca ])+A(R (Ω)-R (Ω))

dt τ

(VIII)

where the subscript “IN” refers to the liquid feeding stream. The species concentration at t=0

and in liquid feeding stream are set at the values reported in (I). Model computation has been

performed with Mathematica® (Wolfram Research) software package.

The performance of the unit under steady state conditions has been expressed in terms of the

total captured carbon () and the fraction () of carbon sequestered as solid calcite, defined as:

2 3 3

=( [ ] [ ] [ ])[ ]

[ ] [ ]θ=

[ ] [ ] [ ] [ ]

IN IN

IN

IC Ca CaIC

Ca Ca

IC CO HCO CO

(IX)

3. Results

A preliminary analysis has been carried out about an unit operated under batch conditions and

assumptions B-H). In particular, the pH=constant constraint was removed. Results showed

that the pH decreased approaching 5.7 and that calcite precipitation was hindered by decrease

of below 1 as a consequence of the decrease of carbonate ions concentration.

Model results reported hereby refer to the system described in section 2 by setting: i) Klal at

0.005 and 0.05s-1

[9]; ii) E0 at zero and at 0.001 molCA/kg; iii) between 102 and 10

4 s.

Figure 2 shows the transient behaviour of dissolved CO2, bicarbonate and carbonate ions

concentrations in the liquid phase up to steady state conditions establishment. Simulations in

Fig. 2 have been carried out by setting Klal =0.05s-1

and E0=0 (A) and E0=0.001molCA/kg (B),

kcat=102s

-1. As expected, the larger the space-time the higher the concentration of total

dissolved inorganic carbon. For a given , the enzyme converts a large fraction of the

absorbed CO2 into bicarbonate. Increasing space-time (s, not shown the outlet aqueous

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Italian Section of the Combustion Institute

stream is at the equilibrium with the gas phase and, as expected, its composition does not

depend on enzyme concentration.

A

5000 10000 15000 20000 25000 30000

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

5000 10000 15000 20000 25000 30000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

5000 10000 15000 20000 25000 30000

0.05

0.1

0.15

0.2

time, stime, s time, s

=100 s

10000 s5000 s1000 s

[CO2], mol/kg [HCO3-], mol/kg [CO3

--], mol/kg

5000 10000 15000 20000 25000 30000

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

5000 10000 15000 20000 25000 30000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

5000 10000 15000 20000 25000 30000

0.05

0.1

0.15

0.2

time, stime, s time, s

=100 s

10000 s5000 s1000 s

[CO2], mol/kg [HCO3-], mol/kg [CO3

--], mol/kg

5000 10000 15000 20000 25000 30000

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

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0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

5000 10000 15000 20000 25000 30000

0.05

0.1

0.15

0.2

time, stime, s time, s

=100 s

10000 s5000 s1000 s

[CO2], mol/kg [HCO3-], mol/kg [CO3

--], mol/kg

B

5000 10000 15000 20000 25000 30000

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

5000 10000 15000 20000 25000 30000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

5000 10000 15000 20000 25000 30000

0.05

0.1

0.15

0.2

[CO2], mol/kg [HCO3-], mol/kg

time, s

=100 s

10000 s

5000 s

1000 s

time, s time, s

[CO3--], mol/kg

5000 10000 15000 20000 25000 30000

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

5000 10000 15000 20000 25000 30000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

5000 10000 15000 20000 25000 30000

0.05

0.1

0.15

0.2

[CO2], mol/kg [HCO3-], mol/kg

time, s

=100 s

10000 s

5000 s

1000 s

time, s time, s

[CO3--], mol/kg

5000 10000 15000 20000 25000 30000

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

5000 10000 15000 20000 25000 30000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

5000 10000 15000 20000 25000 30000

0.05

0.1

0.15

0.2

[CO2], mol/kg [HCO3-], mol/kg

time, s

=100 s

10000 s

5000 s

1000 s

time, s time, s

[CO3--], mol/kg

Fig. 2 Transient behaviour of dissolved carbon species for different values of the space-

time Klal=0.05 s-1

. A) E0=0. B) E0=0.001 mol/kg, kcat=102 s

-1.

The effects of the enzyme are highlighted in Fig. 3. The rate of carbon capture () and the

fraction of carbon () sequestered as solid calcite under steady state conditions are reported as

a function of the space-time in Fig. 3 A and B, respectively.

4. Discussion

Providing a constant pH for a given space-time, the rapid CO2 conversion catalysed by the

enzyme allows to obtain larger bi/carbonate concentrations (see Fig. 2). Such improvement is

clearly depicted in Fig. 3A: providing outlet composition far from the equilibrium the total

amount of absorbed CO2 increases in the presence of the enzyme.

The efficiency in carbon sequestration is reported in Fig. 3B in terms of vs The plots have

a maximum at the maximum exploitation of calcium available in seawater: the maximum at

=2·103 and 1·10

3s for Klal = 0.005 and 0.05s

-1, respectively. Comparing plots a and b it

results that the sequestered carbon fraction decreases as the mass transfer rate increases.

This is due to the increase of dissolved carbon although the amount of solid calcite

precipitated does not increase.

The analysis of Fig. 3B shows that the enzyme did not influence the amount of calcite formed.

In fact, the vs. plots related to different values of kcat – b2) are very close each

other. A beneficial effect of the enzyme on at fixed is limited at the left region with

respect to the maximum. Under these conditions the growth rate of calcite is positively

influenced by the faster production of carbonate ions. Simulations carried out changing kcat

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PTSE 2010

over four order of magnitude (curves b1 and b2) show that is not appreciably affected by

kcat since the characteristic time of the enzyme reaction, calculated as Km/(kcat E0), is lower

than 0.18s.

Results suggest that the enzyme activity reduces the seawater flow rate required for the CO2

absorption. Under the adopted assumptions, the model suggests that Ca++

concentration in

seawater is not sufficient to sequester the absorbed CO2.

0.0

0.5

1.0

1.5

2.0

2.5

0 2 4 6 8 10

x 10-3, s

x 1

04 , mol

/(kg

s) bb2

A

0.0

0.5

1.0

1.5

2.0

2.5

0 2 4 6 8 10

x 10-3, s

x 1

04 , mol

/(kg

s) bb2

A

0

2

4

6

8

10

12

0 1 2 3 4 5 6 7 8 9 10x 10-3, s

,

%

ab

b2b1

B

0

2

4

6

8

10

12

0 1 2 3 4 5 6 7 8 9 10x 10-3, s

,

%

ab

b2b1

ab

b2b1

B

Fig. 3 A: rate of carbon capture as a function of the space-time. B: sequestered fraction of

captured carbon as a function of the space-time. Line: a) Klal =0.005 s-1

, E0=0; b)

Klal = 0.05 s-1

, E0 = 0; b1) Klal = 0.05 s-1

, E0 = 0.001 mol/kg, kcat = 102 s

-1;

b2) Klal = 0.05 s-1

, E0=0.001 mol/kg, kcat=106 s

-1

Acknowledgements

Financial support from “ENEL Ingegneria e Innovazione S.p.A.” is acknowledged.

References

1. Dilmore R., Griffith C., Liu Z., Soong Y., Hedges S.W., Koepsel R., Ataai M.: Int J Gre-

enhouse Gas Control, 3:401 (2009).

2. Bond G., Stringer J., Brandvold D., Simsek F.A., Medina M.-G., Egeland G.: Energy &

Fuels, 15:309 (2001).

3 Mirjafari P., Asghari K., Mahinpey N.: Ind. Eng. Chem. Res. 46: 921 (2007)

4 Favre N., Christ M.L., Pierre A. C.: J Mol Catal B: Enzymatic. 60: 163 (2009)

5. Liu N., Bond G. M., Abel T.A., McPherson B.J., Stringer J.: Fuel Proc Technol. 86:1625

(2005).

6. Zeebe R. E., Wolf-Gladrow D.: CO2 in seawater: equilibrium, kinetics, isotopes. Elsevier

Oceanography Series. Amsterdam, The Netherlands (2001).

7 Zhong S., Mucci A.: Geochim Cosmochim Acta 78: 283 (1993)

8 Gledhill D.K., Morse J. W.: Geochim Cosmochim Acta. 70: 5802 (2006)

9 Chisti M.Y.: Airlift bioreactors. Elsevier Applied Biotechnology Series. Amsterdam, The

Netherlands (1989)

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