SYNTHETIC, CALCIUM-BASED SYNTHETIC, CALCIUM-BASED SORBENTS FOR THE CAPTURE SORBENTS FOR THE CAPTURE

OF COOF CO22

SUMMARY OF RESULTS AND SUMMARY OF RESULTS AND MODELLINGMODELLING

J. S. Dennis and R. PaccianiJ. S. Dennis and R. PaccianiUniversity of CambridgeUniversity of Cambridge

Department of Chemical EngineeringDepartment of Chemical Engineering

jsd3@cam.ac.ukjsd3@cam.ac.ukrp339@cam.ac.ukrp339@cam.ac.uk

• The challenge is to find a sorbent which The challenge is to find a sorbent which

can be reused many times.can be reused many times.

• Natural limestone (mainly CaCONatural limestone (mainly CaCO33) degrades. ) degrades.

How can it be improved, based on a fundamental How can it be improved, based on a fundamental understanding of the reactions involved? Synthetic sorbents?understanding of the reactions involved? Synthetic sorbents?

ZECA – Generation of HZECA – Generation of H22 from coal and pure CO from coal and pure CO2 2 for for

sequestrationsequestration

Gasifier Reformer Calciner

4 H2

Fuel Cell

Air

CaCO3

CaO

C(s)

Work

Shift Reactor

CO + 3H2

+ H2O

CH4

2H2

2H2O

CO2

2H2O

COCO22 + CaO + CaO CaCO CaCO33**

* * The percentage completion of this reaction is theThe percentage completion of this reaction is the

carrying capacitycarrying capacity of the solid sorbent. of the solid sorbent.

Either mol CO2/mol CaO or g CO2/g calcined sorbent

KEY REACTIONSKEY REACTIONS

CalcinationCalcinationCaCOCaCO33 CaO + CO CaO + CO22

CarbonationCarbonation

C + 2HC + 2H2 2 CH CH44HydrogasificationHydrogasification

CHCH44 + H + H22OO CO + 3H CO + 3H22Reforming Reforming

CO + HCO + H22OO COCO22 + H + H22Watergas Shift Watergas Shift Separation of Separation of

HH22 and CO and CO22

Regeneration Regeneration of sorbent: of sorbent:

COCO22 to storage to storage

Typical experimental conditions:– Atmospheric pressure.Atmospheric pressure.– T = 600 – 900T = 600 – 900ooC (constant for an experiment). C (constant for an experiment). – Partial pressure of COPartial pressure of CO22 in N in N22 = 0 (calcination) and 0.14-0.8 bar (carbonation). = 0 (calcination) and 0.14-0.8 bar (carbonation).– Sorbent particles dSorbent particles dp p = 500-710 = 500-710 m.m.

fluidised bed

EXPERIMENTAL APPARATUSEXPERIMENTAL APPARATUS

Measure the total uptake of COMeasure the total uptake of CO22 by by the sorbent on each cycle:the sorbent on each cycle:

Carbonation

Calcination Calcination

sorbentof mass initial

flowrate mass Gasarea)(Grey X ncarbonatio

Carbonation

T = 750oC

CYCLING EXPERIMENTCYCLING EXPERIMENT

CARRYING CAPACITY OF NATURAL SORBENTS CARRYING CAPACITY OF NATURAL SORBENTS Limestone vs. Eggshell

Extended cycles of Extended cycles of calcination and calcination and carbonationcarbonation

Uptake of COUptake of CO22

confirmed by XRD confirmed by XRD analysis of analysis of carbonated material.carbonated material.

Surprising Surprising similarities for similarities for disparate materials!disparate materials!

INITIAL SIMPLE MODEL: REACTION IN PARTICLEINITIAL SIMPLE MODEL: REACTION IN PARTICLE

Macropore Macropore volumevolume

Micro/mesopore Micro/mesopore (BJH) volume (BJH) volume

GrainGrain (~ 400 nm)(~ 400 nm)LimestoneLimestone

Particle (~ 3 mm)Particle (~ 3 mm)

Micro/mesopore Micro/mesopore (5 – 100 nm)(5 – 100 nm)

Small pores fill Small pores fill up with CaCOup with CaCO3 3

when reaction when reaction largely stopslargely stops

BJH PORE SIZE DISTRIBUTION (Limestone)BJH PORE SIZE DISTRIBUTION (Limestone)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 0.5 1 1.5 2 2.5

(1 0)

(2 1)

(30 29)

(16 15)

Log Log ddpp / nm / nm

dV /

d (L

og

dV /

d (L

og dd

pp) c

m)

cm33 /g/

g

Closer examination of pores below 100 nm

Same trends in other materials:

chalk

eggshell

dolomite.

BJH valid for volume BJH valid for volume in in ddporepore = 2 – 200 nm. = 2 – 200 nm.

Increasing cyclesIncreasing cycles

y = 1.0746x + 0.1335R2 = 0.8204

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

For range of limestones: Purbeck, Cadomin, Penrith, Glen Morrison, Havelock.Best fit regression line fitted.

FRACTIONAL CARRYING CAPACITYFRACTIONAL CARRYING CAPACITY F

ract

ion

al C

arry

ing

Cap

acit

y (E

xptl

.)

Theoretical Conversion of BJH Volume

Typical residual constant conversion of ~13% is typical of limestone.

Conclusion holds for Conclusion holds for other materials – chalk, other materials – chalk, eggshell, etc. These have eggshell, etc. These have muchmuch different/larger different/larger macropore volumes than macropore volumes than limestone but limestone but comparablecomparable micropore micropore volumes. volumes.

BUTBUT they have similar they have similar capacity and decay of capacity and decay of capacity with cycle.capacity with cycle.

Fennell, Pacciani, Dennis, Davidson & Hayhurst, Energy Fuels, 21, 2072 (2007)

• The uptake of limestone decreases with increasing number of cycles.

• This is caused by loss in pore volume contained in the small pores (< 200 nm dia.) – sintering occurs.

• This is valid for other natural sorbents (e.g. dolomite and chalk) which have similar pore size distributions.

• Can this be improved upon?Can this be improved upon?

RESULTS - NATURAL SORBENTSRESULTS - NATURAL SORBENTS

• Aim: - a porous particle, resistant to loss of micropores by sintering effects, with high, constant reactivity over large number of cycles.

• Explored CaO supported on inert materials to provide – mechanical strength

– to investigate if an inert support will prevent micropore migration/agglomeration during calcination

• Methods investigated*:– Impregnation Impregnation

– Mechanical mixingMechanical mixing

– CoprecipitationCoprecipitation

– Hydrolysis:Hydrolysis:• CaO dispersed on MgO or CaCaO dispersed on MgO or Ca1212AlAl1414OO3333 (mayenite) (mayenite)

PREPARATION OF SYNTHETIC SORBENTSPREPARATION OF SYNTHETIC SORBENTS

*Full details: Pacciani, Müller, Davidson, Dennis & Hayhurst, Can. J. Chem. Eng., 86, 356 (2008)

HA-85-850

Pacciani, Müller, Davidson, Dennis & Hayhurst, Can. J. Chem. Eng., 86, 356 (2008)

BBJH analysis measures the volume in pores with dpore < 200 nm.

RESULTS - NOVEL SORBENTSRESULTS - NOVEL SORBENTSC

O2 U

pta

ke, g

CO

2/g

so

rben

t

T = 750oC, carbonation:14 mol% CO2 in N2, calcination: 100 mol% N2

HA-85-850

Number of cycles

RESULTS - NOVEL SORBENTSRESULTS - NOVEL SORBENTSC

O2 U

pta

ke, g

CO

2/g

so

rben

t

New sorbent - capacity loss much less. Capacity increases with [CO2]

Micropore volume increases with [CO2]

Natural sorbent - uptake degrades with no. of cycles of sorption &

regeneration. Insensitive to [CO2]

Micropore volume continuously decreases

Pacciani, Müller, Davidson, Dennis & Hayhurst, A.I.Ch.E.J., paper accepted, July 2008

Some Observations from Some Observations from ModellingModelling

Initial AssumptionsInitial Assumptions

• Particle contains cylindrical pores: surface area and pore volume is distributed by pore radius, as determined by N2 adsorption (BET/BJH) and Hg porosimetry

• Effectiveness factor for particle = 1 (no intra-particle diffusional gradients of CO2 in gas phase, across diameter of a particle)

• Particle is isothermal

• Rate of reaction at CaO/CaCO3 interface is first order in (Ci – C*)

• Pores react independently - no overlapping effects from impinging fronts

• Pseudo-steady state

MODELLINGMODELLING

MODELLINGMODELLING

• Rate of increase in radius of CaCO3/CaO interface for jth pore size:

• Reaction rate in all pores, initial size roj:

dt

dr

r

r

V

SQ

r

r

D

kr

CCkV

dt

dr

cj

soj

cj

CaO

jj

ji

cj

s

cj

CaOcj

,ln1

)(

C* - equilibrium concentration

Ds - diffusivity of CO2 in CaCO3 product

ri,j

Ci

C

rcj

product CaCO3

Cross-section of Reacting Pore - Initial Radius roj

unreacted CaO

pore space

MODEL VALIDATIONMODEL VALIDATION

• Tested on results from the carbonation of eggshell

• Parameters: Overall particle radius, = 0.605 mm Temperature = 750oC

Bulk CO2 concentration = 1.985 mol/m3

Equilibrium CO2 concentration = 1.067 mol/m3

Ratio of molar volumes product/reactant = 2.183

Diffusivity in product layer, Ds = 4.0 10-13 m2/s First order rate constant, k = 2.6 10-4 cm/s Nominal, total BET area = 17.3 m2/g (calcined)

Reducing Ds to 4 10-16 m2/s

Rate vs. time for Eggshell, 1st Calcination

0.00E+00

1.00E-05

2.00E-05

3.00E-05

4.00E-05

5.00E-05

0 100 200 300 400 500 600

time, s

rate

, m

ol C

O2/s

/g

RESULTSRESULTS

Experimental

Cylindrical pore model

Rat

e, m

ol

CO

2/s

/g s

orb

ent

Rate vs. time for Eggshell, 1st Calcination

0.00E+00

1.00E-05

2.00E-05

3.00E-05

4.00E-05

5.00E-05

0 100 200 300 400 500 600

time, s

rate

, mo

l CO

2/s

/g c

alc

ine

d

ma

teri

al

RESULTSRESULTS

Experimental

Slit pore model

Uptake vs. time for Eggshell, 1st Calcination

0

0.004

0.008

0.012

0.016

0 100 200 300 400 500 600

time, s

up

take

, m

ol

CO

2/g

cal

cin

ed

mat

eria

l

RESULTSRESULTS

Experimental

Slit pore model

Cylindrical pore model

A Model for the Prediction of A Model for the Prediction of Maximum UptakeMaximum Uptake

MODEL : Stage I – UPTAKE WITHIN GRAINMODEL : Stage I – UPTAKE WITHIN GRAIN

Macropore Macropore volumevolume

Micropore (BJH) Micropore (BJH) volume volume

GrainGrain (~ 400 nm)(~ 400 nm)LimestoneLimestone

Particle (~ 3 mm)Particle (~ 3 mm)

Micropore (5 – Micropore (5 – 100 nm)100 nm)

Internal Internal micropores fill micropores fill up with CaCOup with CaCO3 3

- fast- fast

RESULTS FOR HA-85-850 - Conversion of Grains OnlyRESULTS FOR HA-85-850 - Conversion of Grains Only

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.1 0.2 0.3 0.4 0.5 0.6

Predicted Uptake in Grains, g CO2/g sorbent

Exp

erim

enta

l U

pta

ke,

g C

O2/

g

sorb

ent

PCO2: 0.14 – 0.80 bar

Cycles: 1 – 23

T: 750 & 850oC

MODEL: Stage II – PREDICTION OF MAXIMUM MODEL: Stage II – PREDICTION OF MAXIMUM PRODUCT LAYER THICKNESS AROUND GRAINPRODUCT LAYER THICKNESS AROUND GRAIN

Unreacted CaOUnreacted CaO

Reaction Reaction interfaceinterface

Nanocrystallites Nanocrystallites of CaCOof CaCO33 build build

up as productup as product

Internal micropore Internal micropore already filled with CaCOalready filled with CaCO33

dd

Elastic modulus, E*

101.173/12

4

d

EE

GPaGPa

d ~ 15 nm ~ 0.5 - 1 J/m2

Slowly growing Slowly growing product layer around product layer around grain develops tensile grain develops tensile hoop stresshoop stress

MODEL: Stage II – PREDICTION OF MAXIMUM MODEL: Stage II – PREDICTION OF MAXIMUM PRODUCT LAYER THICKNESS AROUND GRAINPRODUCT LAYER THICKNESS AROUND GRAIN

• Cannot be a Cannot be a solidsolid product layer – how would pores fill? product layer – how would pores fill?

• Product layer around grain consists of nanocrystallites: very high Product layer around grain consists of nanocrystallites: very high elastic modulus owing to small size (~ 15 nm) and interface energy elastic modulus owing to small size (~ 15 nm) and interface energy (~ 0.5 J/m(~ 0.5 J/m22). Treat as thin shell under tension.). Treat as thin shell under tension.

• Thermodynamic model Thermodynamic model (adaptation of Duo, Grace, Clift & Seville)(adaptation of Duo, Grace, Clift & Seville): : GGoveralloverall = = GGreactionreaction

+ + GGnucleationnucleation

+ Mechanical strain energy stored in the product layer+ Mechanical strain energy stored in the product layer

• Gives an expression for Gives an expression for maximum maximum product layer thickness, product layer thickness, hh, as , as function of [COfunction of [CO22], ], TT, , ZZ = vol product/vol reactant = vol product/vol reactant etc.etc.

MAXIMUM PRODUCT THICKNESS, MAXIMUM PRODUCT THICKNESS, hh, AROUND , AROUND GRAINGRAIN

),(

),(

)ln(

12

3

2

3

2

1

02

xXfy

xXfZ

Zf

pRTGG

d

yG

R

h

i

i

coTreaction

ooreaction

grain

structural parameterstructural parameter

vol product/vol reactant allowing for conversion in Stage Ivol product/vol reactant allowing for conversion in Stage I

mol CaCOmol CaCO33/m/m33 in new nanocrystals formed in Stage II in new nanocrystals formed in Stage II

xx mass fraction CaO in original sorbentmass fraction CaO in original sorbent

oo yield stress of layeryield stress of layer

RESULTS FOR HA-85-850 - Grains plus Limiting Surface RESULTS FOR HA-85-850 - Grains plus Limiting Surface Layer Around GrainsLayer Around Grains

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.1 0.2 0.3 0.4 0.5 0.6

Predicted Uptake, Grains + Surface Layer g CO2/g sorbent

Exp

erim

enta

l U

pta

ke,

g C

O2/

g s

orb

ent

PCO2: 0.14 – 0.80 bar

Cycles: 1 – 23

T: 750 & 850oC

SULPHATION BEHAVIOUR, HA-85-850SULPHATION BEHAVIOUR, HA-85-850

T = 850oCbatch w = 0.03 gdp = 500 - 710 m5.2% O2

2200 ppm SO2

N2 at balance

0.000

0.100

0.200

0.300

0.400

0.500

0 100 200 300 400 500 600 700

Time [s]

SO

2 u

pta

ke

[gS

O2/

g cal

cin

ed s

orb

ent]

HA-85-850

Purbecklimestone

Good sulphation behaviour: maximum approximates to ~ 49% molar conversion of CaO to CaSO4 filling total porosity (~ 55% overall)

Gsulphation >> Gcarbonation

so sulphation reaction evidently not limited by mechanical work

What Happens in Practice?What Happens in Practice?

STEM HAADF TOMOGRAPHY - NanoengineeringSTEM HAADF TOMOGRAPHY - Nanoengineering

A grain from our synthetic sorbent showing pores in 5 - 50 nm range

Collaboration with Prof. Paul Midgeley, Materials Science

e.g. Midgley, P.A., Science, 309, 2195 (2005)

• Natural sorbents:– Deactivate (viz. lose their pore volume) after small number of cycles.

– Insensitive to changes in [CO2] during carbonation.

• Novel sorbent:– Stable, higher uptake than natural sorbents over a large number of cycles.– Able to develop new pore volume with number of cycles.

– Uptake increases with [CO2] during carbonation:

• way of regenerating pore volume.

• useful in systems with relatively high [CO2]

– High capacity for sulphur dioxide. • Modelling:

– A possible explanation of our experimental observations– To be verified by STEM HAADF

CONCLUSIONSCONCLUSIONS

ACKNOWLEDGEMENTSACKNOWLEDGEMENTS

• Miss R. Pacciani

• Dr. Stuart Scott

• Dr. P. Fennell (Imperial College London)

• Professor J. F. Davidson

• Professor Allan Hayhurst

• Prof. P. A. Midgeley

• Dr. C. Müller

• Prof. R. Kandiyoti, Prof. D. Dugwell, Dr. N. Paterson (Imperial College London)

• Dr. E. J. Anthony (CANMET)

• Cambridge European Trust

• EPSRC

bBJH analysis measures the volume in pores with dpore < 200 nm.

T = 750oC, carbonation:14 mol% CO2 in N2, calcination: 100 mol% N2

RESULTS - NATURAL SORBENTSRESULTS - NATURAL SORBENTS

Same trends in BJH Same trends in BJH volume of other volume of other natural materials:natural materials:

chalkchalk

eggshelleggshell

dolomite.dolomite.