SYNTHETIC, CALCIUM-BASED SORBENTS FOR THE CAPTURE OF CO 2 SUMMARY OF RESULTS AND MODELLING J. S. Dennis and R. Pacciani University of Cambridge Department of Chemical Engineering

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 CaCO 3 ) degrades. Natural limestone (mainly CaCO 3 ) 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 H 2 from coal and pure CO 2 for sequestration Gasifier Reformer Calciner 4 H 2 Fuel Cell Air CaCO 3 CaO C(s) Work Shift Reactor CO + 3H 2 + H 2 O CH 4 2H 2 2H 2 O CO 2 2H 2 O

CO 2 + CaO  CaCO 3 * * The percentage completion of this reaction is the * The percentage completion of this reaction is the carrying capacity of the solid sorbent. carrying capacity of the solid sorbent. Either mol CO 2 /mol CaO or g CO 2 /g calcined sorbent KEY REACTIONS Calcination CaCO 3  CaO + CO 2 Carbonation C + 2H 2  CH 4 Hydrogasification CH 4 + H 2 O  CO + 3H 2 Reforming CO + H 2 O CO 2 + H 2 Watergas Shift Separation of H 2 and CO 2 Regeneration of sorbent: CO 2 to storage

Typical experimental conditions: –Atmospheric pressure. –T = 600 – 900 o C (constant for an experiment). –Partial pressure of CO 2 in N 2 = 0 (calcination) and bar (carbonation). –Sorbent particles d p =  m. fluidised bed EXPERIMENTAL APPARATUS

Measure the total uptake of CO 2 by the sorbent on each cycle: Carbonation Calcination Carbonation T = 750 o C CYCLING EXPERIMENT

CARRYING CAPACITY OF NATURAL SORBENTS CARRYING CAPACITY OF NATURAL SORBENTS Limestone vs. Eggshell Extended cycles of calcination and carbonation Uptake of CO 2 confirmed by XRD analysis of carbonated material. Surprising similarities for disparate materials!

INITIAL SIMPLE MODEL: REACTION IN PARTICLE Macropore volume Micro/mesopore (BJH) volume Grain (~ 400 nm) Limestone Particle (~ 3 mm) Micro/mesopore (5 – 100 nm) Small pores fill up with CaCO 3 when reaction largely stops

BJH PORE SIZE DISTRIBUTION (Limestone) Log d p / nm dV / d (Log d p ) cm 3 /g Closer examination of pores below 100 nm Same trends in other materials: chalk eggshell dolomite. BJH valid for volume in d pore = 2 – 200 nm. Increasing cycles

For range of limestones: Purbeck, Cadomin, Penrith, Glen Morrison, Havelock. Best fit regression line fitted. FRACTIONAL CARRYING CAPACITY Fractional Carrying Capacity (Exptl.) Theoretical Conversion of BJH Volume Typical residual constant conversion of ~13% is typical of limestone. Conclusion holds for other materials – chalk, eggshell, etc. These have much different/larger macropore volumes than limestone but comparable micropore volumes. BUT they have similar capacity and decay of 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 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 –Mechanical mixing –Coprecipitation –Hydrolysis: CaO dispersed on MgO or Ca 12 Al 14 O 33 (mayenite)CaO dispersed on MgO or Ca 12 Al 14 O 33 (mayenite) PREPARATION OF SYNTHETIC SORBENTS *Full details: Pacciani, Müller, Davidson, Dennis & Hayhurst, Can. J. Chem. Eng., 86, 356 (2008)

HA Pacciani, Müller, Davidson, Dennis & Hayhurst, Can. J. Chem. Eng., 86, 356 (2008) BBJH analysis measures the volume in pores with d pore < 200 nm. RESULTS - NOVEL SORBENTS CO 2 Uptake, g CO 2 /g sorbent T = 750 o C, carbonation:14 mol% CO 2 in N 2, calcination: 100 mol% N 2

HA Number of cycles RESULTS - NOVEL SORBENTS CO 2 Uptake, g CO 2 /g sorbent New sorbent - capacity loss much less. Capacity increases with [CO 2 ] Micropore volume increases with [CO2] Natural sorbent - uptake degrades with no. of cycles of sorption & regeneration. Insensitive to [CO 2 ] Micropore volume continuously decreases Pacciani, Müller, Davidson, Dennis & Hayhurst, A.I.Ch.E.J., paper accepted, July 2008

Some Observations from Modelling Some Observations from Modelling

Initial Assumptions Particle contains cylindrical pores: surface area and pore volume is distributed by pore radius, as determined by N 2 adsorption (BET/BJH) and Hg porosimetry Effectiveness factor for particle = 1 (no intra-particle diffusional gradients of CO 2 in gas phase, across diameter of a particle) Particle is isothermal Rate of reaction at CaO/CaCO 3 interface is first order in (C i – C * ) Pores react independently - no overlapping effects from impinging fronts Pseudo-steady stateMODELLING

MODELLING Rate of increase in radius of CaCO 3 /CaO interface for j th pore size: Reaction rate in all pores, initial size r oj : C * - equilibrium concentration D s - diffusivity of CO 2 in CaCO 3 product r i,j CiCi C r cj product CaCO 3 Cross-section of Reacting Pore - Initial Radius r oj unreacted CaO pore space

MODEL VALIDATION Tested on results from the carbonation of eggshell Parameters: Overall particle radius, = mm Temperature = 750 o C Bulk CO 2 concentration = mol/m 3 Equilibrium CO 2 concentration = mol/m 3 Ratio of molar volumes product/reactant = Diffusivity in product layer, D s = 4.0  m 2 /s First order rate constant, k = 2.6  cm/s Nominal, total BET area = 17.3 m 2 /g (calcined)

Reducing D s to 4  m 2 /sRESULTS Experimental Cylindrical pore model Rate, mol CO 2 /s/g sorbent

RESULTS Experimental Slit pore model

RESULTS Experimental Slit pore model Cylindrical pore model

A Model for the Prediction of Maximum Uptake

MODEL : Stage I – UPTAKE WITHIN GRAIN Macropore volume Micropore (BJH) volume Grain (~ 400 nm) Limestone Particle (~ 3 mm) Micropore (5 – 100 nm) Internal micropores fill up with CaCO 3 - fast

RESULTS FOR HA Conversion of Grains Only P CO2 : 0.14 – 0.80 bar Cycles: 1 – 23 T: 750 & 850 o C

MODEL: Stage II – PREDICTION OF MAXIMUM PRODUCT LAYER THICKNESS AROUND GRAIN Unreacted CaO Reaction interface Nanocrystallites of CaCO 3 build up as product Internal micropore already filled with CaCO 3 d Elastic modulus, E * GPa d ~ 15 nm  ~ J/m 2 Slowly growing product layer around grain develops tensile hoop stress

MODEL: Stage II – PREDICTION OF MAXIMUM PRODUCT LAYER THICKNESS AROUND GRAIN Cannot be a solid product layer – how would pores fill? Cannot be a solid product layer – how would pores fill? Product layer around grain consists of nanocrystallites: very high elastic modulus owing to small size (~ 15 nm) and interface energy (~ 0.5 J/m 2 ). Treat as thin shell under tension. Product layer around grain consists of nanocrystallites: very high elastic modulus owing to small size (~ 15 nm) and interface energy (~ 0.5 J/m 2 ). Treat as thin shell under tension. Thermodynamic model (adaptation of Duo, Grace, Clift & Seville) : Thermodynamic model (adaptation of Duo, Grace, Clift & Seville) :   G overall =  G reaction +  G nucleation + Mechanical strain energy stored in the product layer + Mechanical strain energy stored in the product layer Gives an expression for maximum product layer thickness, h, as function of [CO 2 ], T, Z = vol product/vol reactant etc. Gives an expression for maximum product layer thickness, h, as function of [CO 2 ], T, Z = vol product/vol reactant etc.

MAXIMUM PRODUCT THICKNESS, h, AROUND GRAIN structural parameter structural parameter vol product/vol reactant allowing for conversion in Stage I mol CaCO 3 /m 3 in new nanocrystals formed in Stage II x mass fraction CaO in original sorbent  o yield stress of layer

RESULTS FOR HA Grains plus Limiting Surface Layer Around Grains P CO2 : 0.14 – 0.80 bar Cycles: 1 – 23 T: 750 & 850 o C

SULPHATION BEHAVIOUR, HA T = 850 o C batch w = 0.03 g d p =  m 5.2% O ppm SO 2 N 2 at balance Good sulphation behaviour: maximum approximates to ~ 49% molar conversion of CaO to CaSO 4  filling total porosity (~ 55% overall)  G sulphation >>  G carbonation so sulphation reaction evidently not limited by mechanical work

What Happens in Practice?

STEM HAADF TOMOGRAPHY - Nanoengineering A grain from our synthetic sorbent showing pores in 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 [CO 2 ] 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 [CO 2 ] during carbonation: way of regenerating pore volume. useful in systems with relatively high [CO 2 ] –High capacity for sulphur dioxide. Modelling: –A possible explanation of our experimental observations –To be verified by STEM HAADF CONCLUSIONS

ACKNOWLEDGEMENTS 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 d pore < 200 nm. T = 750 o C, carbonation:14 mol% CO 2 in N 2, calcination: 100 mol% N 2 RESULTS - NATURAL SORBENTS Same trends in BJH volume of other natural materials: chalkeggshelldolomite.