CO2 capture by means of dolomite in hydrogen production from syn gas

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33 (2008) 3049 – 3055

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CO2 capture by means of dolomite in hydrogen production from syn gas Katia Gallucci, Stefano Stendardo, Pier Ugo Foscolo Chemical Engineering Department, University of L’Aquila, Monteluco di Roio 67040 L’Aquila, Italy

art i cle info

ab st rac t

Article history:

The production of pure hydrogen from renewable or fossil energy sources can be achieved

Received 15 October 2007

by purifying the syn gas obtained as a result of a steam gasification process. Depleting of

Received in revised form

the syn gas carbon-containing compounds by means of CO2 capture on calcined dolomite

13 February 2008

has been carried out. The dolomite behavior as CO2 sorbent and its regeneration has been

Accepted 11 March 2008

investigated by means of Linseis (TG-DTA) coupled with FT-IR spectrometer. Original and

Available online 16 May 2008

heat-treated dolomite has been characterized by XRF and SEM in order to detect the

Keywords: CO2 capture Dolomite Spherical grains Kinetic models

composition and the morphology. A simple modeling approach has been applied to the investigation of the CO2 sorption kinetic. The results of this work show good agreement between model predictions and experimental data obtained either by SEM micrographs of dolomite particles at different stages of the calcination/carbonation cycle, and by TG curves of CaO recarbonation as a function of time. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Among the renewable energy sources, biomass plays a strategic role, either as crops purposely cultivated for fuel production or power generation, as well as bio-residues and refuse-derived feedstocks that can profitably replace more conventional sources of energy, instead of being disposed at ever increasing costs. Steam gasification processes are able to convert the chemical energy of biomass into a hydrogen-rich syn gas containing up to about 50% by volume of hydrogen on dry basis [1]. Nowadays considerable interest is focused on a purehydrogen energy vector; renewable energy sources, such as biomass, are the only ones that could assure H2 production in a sustainable way [2–4]. In this respect, the steam gasification process (being very close to industrial exploitation) appears as an optimum candidate to furnish the raw syn gas, provided that a reliable and economically convenient process is

developed to extract hydrogen from the producer gas. This can be obtained either by the utilization of selective membranes permeable to hydrogen small molecules, or by capture of carbon-containing gas components by means of an appropriate sorbent [5]. Biomass gasification addressed to maximize the yield of a clean (substantially free of heavy organic compounds, collectively defined as tar) gaseous product needs to be performed at relatively high temperature (800–900 1C [6]): a thermally efficient process would imply the use of perm-selective membranes made of palladium or its alloys, and operation under pressure would have some technical and economic drawbacks. For such reasons, we are studying the practical feasibility of the syn gas treatment with calcined dolomite, to separate carbon dioxide, and regeneration of exhaust dolomite by calcination [7–13]. Magnesium oxide, also contained in calcined dolomite, does not contribute to carbon dioxide sequestration, because magnesium carbonate decomposes at much lower temperature

Corresponding author. Tel.: +39 0862 434237; fax: +39 0862 434203.

E-mail address: [email protected] (K. Gallucci). 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.03.039

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than calcium carbonate; however, inert MgO does contribute to stabilize the structure of the solid sorbent in a multi-cycle carbonation–calcination process [14]. The scope of this study is to investigate the behavior of dolomite as a CO2 sorbent in cyclic operation conditions and more specifically the optimum temperature levels and the recarbonation kinetics. The attention has been focused on solid particles observed by means of SEM and TG/DTA. A simple model is proposed to describe the CaO recarbonation phase.

2.

Experimental section

2.1.

Chemicals and sorbents

The gas used in TGA experiments are N2 with 99.999 vol% of purity, ambient air and CO2 (purity 99.99 vol%). The experimental study has been performed with sieved Pilkington dolomite as CO2 sorbent (kindly provided by Pilkington Ltd), with an average particle diameter of 165 mm (+150 mm–180 mm) and a measured particle density of 2665 kg/m3 (rD).

2.2.

Experimental procedure

The thermal behavior (weight losses, exo- and endothermic reaction temperatures and gas evolution) of dolomite as CO2 sorbent and its regeneration has been investigated by means of a Linseis TG-DTA coupled with FT-IR spectrometer. The DTA and TG curves of dolomite heat treated in different atmospheres up to 1000 1C (pure N2 flow rate of 2 l/min; stagnant ambient air; pure CO2 flow rate of 2 l/min), with a heating rate of 10 1C/min, have been obtained. The dolomite has been heat treated at 10 1C/min until 1000 1C and cooled in CO2 flow until 800 1C with a 30 min dwell time twice. Original and heat-treated dolomite has been also characterized by XRF and SEM with backscattered electrons, BSE, in order to detect the morphology and the composition profiles of polished samples of dolomite particles.

2.3.

Modeling approach

A mathematical model of a gas–solid reaction based on the assumption that the solid dolomite particles are made up of very small spherical grains is assumed to describe the recarbonation process of dolomite particles: the specific active surface, sCaO, is proportional to the number of calcium oxide/carbonate grains per unit particle volume, and to the surface of each grain corresponding to the volume occupied by the un-reacted calcium oxide, which in turn is assumed to keep a spherical shape. The experimental tests are characterized by a small size of the granular solid inventory (dpp200 mm), and a similar consideration applies to fluidized bed reactors, relevant to industrial applications based on cyclic operation: typically a calcinator and a solid sorption reactor are utilized, where the bed inventory is made to circulate [4]. As a result, in such cases intra-particle and boundary layer diffusion can be

33 (2008) 3049 – 3055

neglected as a first approximation, and reaction is assumed to occur uniformly throughout the particle. A linear dependency of the kinetic expression on CO2 gas concentration is also assumed. It may be concluded that the reaction rate could be well correlated by N0Ca

dX ¼ ksCaO ðCCO2  CCO2 eq Þ dt

(1)

where the amount of calcium g-atoms per unit particle volume, N0Ca, is obtained from the knowledge of the CaCO3 mass fraction, f, in fresh dolomite, and the particle density ðN0Ca ¼ f rD =MCaCO3 Þ: The equilibrium pressure of carbon dioxide (in atm) is known as a function of temperature [15]:   PCO2 eq 20 474 ; CCO2 eq ¼ (2) PCO2 eq ¼ 4:137  107 e  T RT where PCO2 is in atm and T in K. According to the above assumptions, when the initial (after calcination) CaO grain diameter, dCaO, is chosen, and its maximum reachable conversion, Xf, fixed, the following equations allow to express sCaO: sCaO ¼

N0CaO VCaO

 pd2CaO ðXf  XÞ2=3

ðp=6Þd3CaO

¼ N0CaO VCaO

6 ðX  XÞ2=3 dCaO f

(3)

The model equation (1) has been integrated analitically. Because of the increasing formation of calcium carbonate at the grain surface, diffusion through the product layer of calcium carbonate could become important. This phenomenon could explain calcium oxide conversion reaching almost a plateau at a value, X ¼ Xf, somewhat lower than one. The abrupt change in the slope of X(t) could be related to product layer diffusivity becoming the controlling step of the whole reaction–diffusion process. The effect of the product layer diffusion resistance on the reaction rate can be computed by means of the application of the shrinking core model at a single calcium containing grain [17]. The shrinking core model hypothesizes that reaction occurs in the outer coat of the grain and a thin reaction front is formed, moving towards the particle center and leaving backwards a product layer as the reation progresses. This picture takes into account a porous structure consisting in a grain matrix. By imposing a rate of diffusion through the product layer equal to the reaction rate at the core surface, a global kinetic relationship can be derived, expressed by the following equation: s0;CaOk0 ð1  XÞ2=3 ðCA  CAe Þ dX qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ p ffiffiffiffiffiffiffiffiffiffiffiffi N 0 3 dt 1 þ 0Cak d 1  X 1  3 1X 2DPL

CaO

(4)

1XþXZ

where Z is the calcium carbonate/oxide molar volume ratio, 41. The product layer mass transfer resistance can be:

 either considered since the beginning of the CO2 absorption process,

 or a transition should be imposed between ‘‘straight reaction’’ at the grain surface, as described by Eq. (1), and

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‘‘product layer diffusion followed by reaction (SRD model)’’ [18], as modeled by means of Eq. (4).

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The powder weight percent composition has been obtained by means of X-ray fluorescence spectrometer detecting all atomic mass ZX12 chemical elements (Table 1). The DTA curves highlight two endothermic peaks, corresponding to the heat absorbed by calcination of magnesium and calcium carbonate, with maxima at the temperatures Tp1 and Tp2, respectively. In Fig. 1, in correspondence to DTA peaks, the TG curve shows a weight loss of about 50%, close to the theoretical one for the complete decomposition of carbonates, calculated from the elemental analysis. In stagnant air the weight losses are slightly lower than in gas flow. No sintering processes take place after calcination, up to 1000 1C. The Tp values are

reported for each atmosphere in Table 2. It can be observed that Tp2, corresponding to CaCO3 calcination, increases in CO2 flow and decreases in N2 flow, as expected, with respect to stagnant air; Tp1, corresponding to MgCO3 calcination, decreases, whichever the flow, with respect to stagnant air. By comparing Table 1 and Fig. 1, we conclude that, at least with a fresh sample, magnesium and calcium carbonate are completely decomposed at relatively low temperature, and over time intervals of the order of a few minutes, which are compatible with a bubbling fluidized bed reactor design. In Fig. 2, the TG-DTA curves, simulating two cycles of pure CO2 (flow rate of 2 l/min) absorption and dolomite regeneration, have been reported. Three major peaks are obtained in the DTA signal, the endothermic peaks at 765 1C and at 940 1C, refer to the heat absorbed by magnesium carbonate and calcium carbonate, respectively. The exothermic peak at around 790 1C indicates that at this temperature the carbonation takes place with relatively high rate. This temperature is below the equilibrium value, Teq ¼ 894.25 1C according to the thermodynamic calculation by means of Eq. (2); so we are clearly inside the carbonation range. At the same time, it is observed that no sintering processes take place also in CO2 flow. This is also evidentiated for the dolomite sample by the pictures of SEM micrographs shown in Fig. 3, where small pores in calcined dolomite (b) and sintering bridges in calcined limestone (a) have been highlighted in the circle.

Table 1 – Elemental composition of Pilkington dolomite (loss of ignition 63%) by means of XRF

Table 2 – DTA peak temperatures: Tp1 MgCO3 and Tp2 CaCO3 decarbonation for different atmospheres of Fig. 1

In order to integrate Eq. (4) together with the initial and boundary conditions, to obtain CaO conversion as a function of time, we have used a standard code (MATHCADs) where a classical numerical algorithm for ODE solving according to Euler’s method is available.

3.

Results and discussion

3.1.

Experimental results

Z

Symbol

Concentration (%)

Abs. error (%)

12 20

Mg Ca Other

12.58 23.98 o0.35

0.045 0.030

N2 Air CO2

Tp1 (1C)

Tp2 (1C)

741 773 760

830 909 930

0 N2 Air CO2

N2 Air CO2

Tp2

DTA

TG

-10

DTA signal (µV)

Weight loss (mg)

Tp1

-20

-30

-40

-50 600

650

700

750 800 850 Temperature (°C)

900

950

1000

600

650

700

750 850 800 Temperature (°C)

900

950

Fig. 1 – TG/DTA curves (pure N2 flow rate of 2 l/min; stagnant ambient air; pure CO2 flow rate of 2 l/min).

1000

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1200

100

1100

75

1000

0

50

900

-5

0 -25 -50 -75

Temperature set up (°C)

125

25 DTA signal (µV)

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10

Temperature set up Weight loss DTA signal

5

-10

800

-15

700

-20

600

-25

500

-30

400

Weight loss (%)

3052

-35

-100

300

-40

-125

200

-45

-150

100

-50

-175

0

-55 0

20

40

60 80 time (min)

100

120

140

Fig. 2 – Two calcination–carbonation cycles with step temperature at 800 1C (pure CO2 flow rate of 2 l/min).

Fig. 3 – SEM analysis of calcined limestone (A) and dolomite (B).

Fig. 4 – SEM analysis of original particles of Pilkington dolomite.

A plausible explanation of the reduction of the final CO2 sorption level, observed in the second solid carbonation cycle is therefore linked to increasingly slower rate of reaction in the final phase of subsequent recarbonation steps due to an increase of product layer diffusivity phenomena as hypothesized in our model (Eq. (4)). In any case this phenomenon is of scarce relevance to industrial applications, because the

practical interest is to mainly observe the former stage of recarbonation, till the abrupt change of the slope is reached in the X(t) curve. In the original dolomite polished sample (Fig. 4) at increasing SEM magnification ratio, it is possible to observe a quite uniform composition, even if there are some impurities (metallic elements), and a structural uniformity; this does

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assumption, the initial surface area (after calcination), s0CaO, is found close to 2 m2/cm3. An average value for the intrinsic rate constant, k0 ¼ k/N0Ca, has been determined experimentally and reported in the literature; it is equal to 0.0595 cm4/mol s, with zero activation energy in the temperature range 823–998 K [16]. Experimental data of the first and second dolomite recarbonation steps have been extracted from Fig. 2 and are compared with model predictions. Fig. 6 refers to the simplest assumption that no diffusion limitations are present (Eq. (1)). Xf has been fixed preliminarily in agreement with experimental data. Results are shown in Fig. 6. At the beginning the calculated CaO conversion is slightly greater than the experimental data, notably as a result of neglecting any diffusive term. In a second time range, 90–180 s, the calculated conversion is lower than the experimental data and this could be explained by an estimated active surface lower than the actual one. When the conversion degree overcomes 0.7 for the first cycle, and 0.6 for the

0.9 0.8 0.7 CaO Conversion (X)

not occur at the boundaries of the particles, where some fractures are caused by mechanical stresses; micrographs highlight a crystalline structure of the virgin sorbent. In Fig. 5, polished samples treated in TG/DTA apparatus (two calcination–recarbonation cycles) in CO2 flow, are shown; it is possible to observe the presence of fine particles, diameter o20 mm, absent in the original dolomite, highlighting that dolomite, when subjected to repeated cycles of calcinations/recarbonation, becomes friable. The loss of mechanical resistance and the physical degradation of dolomite could also be a further explanation of why the degree of recarbonation decreases with the number of heat treatment cycles in DTA. Despite the significant fragmentation—due to the mechanical and thermal stresses—a grain structure in the whole mass of the particle is observed, and the experimental value of CaO grain size can be estimated of the order 1–2 mm. In the SEM–BSE pictures, an external carbonate shell (light zones) surrounding un-reacted calcium oxide (dark zones) can be individuated at grain scale. These results allowed to define a particle reaction model for dolomite recarbonation, as it is done in Section 2.3; it is worth noticing that, at the grain scale, once the overall grain surface is converted to CaCO3, the only possibility for CO2 to reach the un-reacted CaO core is by diffusing through the shell of the reaction product, a much slower mechanism than percolation through particle pores. These findings could justify why in the TG tests the degree of conversion of CaO never reaches 100% over limited time ranges. In the first cycle the maximum value of conversion is 0.54 mole of CO2/100 g of dolomite and in second one is 0.48 mole of CO2/100 g of dolomite (from Table 1 it is known that 0.6 mole of CaO/100 g of dolomite are present).

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0.6 0.5 0.4 0.3 T=800°C (1st step exp.) T=800°C (2nd step exp.)

0.2

3.2.

Results of numerical simulations

Relation (1) establishes the dependency of the specific active surface on the dolomite physical and chemical properties, the particle texture (the average grain size, dCaO), and the calcium oxide conversion. The grain size in our model is chosen to be included in the interval 1–2 mm, which agrees with measurements made on SEM micrographs as shown above; with this

st T=800°C (1 step model)

0.1

T=800°C (2nd step model)

0 0

100

200

300

400

500

time (s) Fig. 6 – TG/DTA experimental data and model prediction equation (1): first cycle (Xf ¼ 0.90) and second cycle (Xf ¼ 0.80) of dolomite recarbonation and spherical grain kinetic model (dCaO ¼ 1300 nm at 800 1C).

Fig. 5 – SEM analysis: on the left, particles of Pilkington dolomite after two cycles at 800 1C (100  ); in the middle, a single particle of Pilkington dolomite after two cycles at 800 1C (500  );on the right, a detail of previous particle (5000  ).

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1

1

0.9

0.9

0.8

0.8

0.7

0.7

0.6 0.5 0.4 Experimental data Model prediction

0.3

CaO Conversion (X)

CaO Conversion (X)

3054

0.6 0.5 0.4 Experimental data Model prediction

0.3

0.2

0.2

0.1

0.1

0

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0 0

100

200

300 time (s)

400

500

0

100

200

300 time (s)

400

500

Fig. 7 – TG/DTA experimental data and model prediction equation (4): first carbonation cycle (on the left, Xf ¼ 0.9) and second carbonation cycle (on the right, Xf ¼ 0.8) of dolomite recarbonation at 800 1C; DPL ¼ 3.5  109 cm2/s; dCaO ¼ 1300 nm.

second one, the experimental data are again lower than the model predictions. In order to improve the agreement between the experimental and calculated values, product layer diffusion should be considered: a threshold limit of CaO conversion, Xtrans, is fixed together to corresponding time value, which marks the transition between the rather rapid initial carbonation phase and the subsequent much slower approach to the final conversion value, Xf. For the former phase the reaction controlling mechanism (Eq. (1)) is kept valid, while for the latter phase the product layer diffusivity has been considered (Eq. (4)). Experimental data points and SRD model predictions (continuous lines) obtained according to the above assumptions are shown in Fig. 7, with reference to 800 1C; DPL ¼ 3.5  109 cm2/s; dCaO ¼ 1300 nm.

of a double mechanism: chemical reaction on the grain surface, followed by product layer diffusion and chemical reaction at the later stage of the sorption process. It is worth noticing that this model can account for the occasionally observed sigmoid-shaped CaO-conversion-versus-time curves, slow conversion at early stage then fast and at last slow, when diffusion through the solid layer of non–porous product around each grain is appreciable.

Acknowledgments The authors wish to thank the financial support of the Italian ministry for research under the project FISR-TEPSI. R E F E R E N C E S

4.

Conclusions

This experimental study shows the ability of calcined dolomite to be utilized as a CO2 sorbent by means of CaO recarbonation at a temperature of about 800 1C. According to DTA, it is possible to calcine dolomite practically at this same temperature with adequate gas flow. The degree of conversion of CaO in CaCO3 decreases with the number of calcination/recarbonation cycles as the dolomite degrades. The data obtained by TG/DTA tests show that with small particle sizes calcium oxide is converted to calcium carbonate over time intervals of the order of a few minutes, compatible with a bubbling fluidized bed reactor design and the SEM analysis of virgin and calcined dolomite samples have shown the presence of a particle grain structure, with average size of 1–2 mm. Good agreement has been found between SRD model predictions and experimental evidence obtained either by SEM micrographs of dolomite particles at different stages of the calcination/carbonation cycle, and by TG curves of CaO conversion as a function of time. This model is able to fit accurately the experimental kinetic data with the application

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