Development of Ni- and CaO-based mono- and bi-functional catalyst and sorbent materials for Sorption Enhanced Steam Methane Reforming: Performance over 200 cycles and attrition tests

Fuel Processing Technology 195 (2019) 106160

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Research article

Development of Ni- and CaO-based mono- and bi-functional catalyst and sorbent materials for Sorption Enhanced Steam Methane Reforming: Performance over 200 cycles and attrition tests

T

A. Di Giulianoa,⁎, K. Galluccia, S.S. Kazib, F. Giancaterinoa, A. Di Carloa, C. Coursonc, J. Meyerb, L. Di Feliceb a b c

University of L'Aquila, Department of Industrial and Computer Engineering and Economics, 18 via G. Gronchi, 67100 L'Aquila, Italy IFE Department of Environmental Industrial Processes, Institute for Energy Technology, Instituttveien 18, NO-2007 Kjeller, Norway University of Strasbourg, Institut de Chimie et Procédés pour l'Énergie, l'Environnement et la Santé, ICPEES, 25 rue Becquerel, 67087 Strasbourg cedex 2, France

ARTICLE INFO

ABSTRACT

Keywords: Sorption Enhanced Steam Methane Reforming Combined sorbent-catalyst material Multicycle process Industrially relevant conditions Resistance to attrition

Sorption Enhanced Steam Methane Reforming (SESMR) involves carbon-capture and natural gas exploitation to produce H2. A 2-particle system (Ni-catalyst and CaO-based sorbent particles), and a CSCM (obtained by granulating powders of that 2-particle system) were studied in multicycle SESMR/regeneration at atmospheric pressure, with reforming at mild temperature (650 °C) and oxidative regeneration under pure CO2 (925 °C), conditions relevant for hypothetical industrial-scale SESMR. 200 cycles (>500 h non-stop per test) were performed for both systems in a packed bed reactor, highlighting a satisfactory catalytic stability and a decrease in sorption capacity explained by after-test characterizations (PXRD, BET-BJH methods, SEM-EDS, TEM). In view of industrial applications in fluidized-beds, resistance to attrition of investigated materials was determined according to ASTM D5757-11, at SESMR/regeneration process temperatures: results compared well with a reference Fluid Catalytic Cracking (FCC) catalyst, purposely designed for applications in industrial fluidized-beds.

1. Introduction IPCC (United Nations Intergovernmental Panel on Climate Change) acknowledged the coupling of CCS (Carbon Capture and Storage) with fossil-fuel energy systems as an affordable strategy to face the problem of global warming [1–3]. In the USA, the shift in power sector from coal to natural gas [4,5] allowed their CO2 emissions to decrease down to 1995 levels [6], demonstrating that economy can grow even under greenhouse-gases mitigation policies [4,7]. H2 has been increasingly considered as a more virtuous energy vector than fossil fuels, thanks to developments in fuel cells and automotive applications [8,9], in addition to several well-known usages (petroleum refining, syntheses of commodity chemicals and gas-to-liquid syntheses [10–14]). Positive elements from these three statements (CCS coupling with fossil-fuels, natural gas exploitation, usage of H2) are exalted by Sorption Enhanced Steam Methane Reforming (SESMR), that is to say a process intensification of Steam Methane Reforming (SMR), due to in-

situ CO2 capture by a high-temperature solid sorbent, to produce highpurity H2 [13,15–17]. SMR (Reaction 1) always occurs together with Water Gas Shift (WGS, Reaction 2) [18–20], producing H2 and CO2 [21,22]; in-situ CO2 sorption shifts thermodynamic equilibria of WGS (Reaction 2) and SMR (Reaction 1) towards products, enhancing H2 production [22–29]. As the solid sorbent eventually gets saturated, a continuously enhanced H2 production requires a cyclic alternation between process conditions for SESMR and sorbent regeneration, from the solid point of view [30]; this relevantly influences the choice of SMR catalyst and CO2 sorbent, as well as the reactor configuration [31,32]. Metallic nickel (Ni) and calcium oxide (CaO) are suitable active phases for reforming catalysis and CO2 capture by carbonation (Reaction 3), respectively [32]. With these phases, SESMR can occur as a nearly autothermal process at 650 °C and atmospheric pressure [33], milder conditions than industrial SMR (800–1000 °C and 1.5–3.0 MPa) [13,16], while CaO is usually regenerated by calcination (reverse of Reaction 3) at higher temperature, depending on the CO2 partial pressure in the regenerative atmosphere [34,35]. From here onwards, SESMR is intended

⁎ Corresponding author at: University of L'Aquila, Department of Industrial and Computer Engineering and Economics, 18 via G. Gronchi, Nucleo Industriale di Pile, 67100 L'Aquila, Italy. E-mail address: [email protected] (A. Di Giuliano).

https://doi.org/10.1016/j.fuproc.2019.106160 Received 19 April 2019; Received in revised form 4 July 2019; Accepted 16 July 2019 Available online 24 July 2019 0378-3820/ © 2019 Elsevier B.V. All rights reserved.

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as the simultaneous occurring of Reactions 1, 2, 3.

CH 4(g) + H2 O(v)

H0298 K = 206.2 kJ mol

CO(g) + 3H2(g)

1

(Reaction 1)

CO(g) + H2 O(v)

CO2(g) + H2(g)

H0298 K =

41.2 kJ mol

1

(Reaction 2)

CO2(g) + CaO(s)

CaCO3(s)

H0298 K

=

178.2 kJ mol

1

(Reaction 3) A recent review [32] gathered several experimental results on SESMR by Ni- and CaO-based materials, in both forms of “2-particles systems” (solo-sorbent plus solo-catalyst particles inside the reactor) and Ni-CaO Combined Sorbent-Catalyst Materials (CSCM, with Ni and CaO on a unique bifunctional kind of particles): several researchers faced the issue of catalytic and sorbent stability in a SESMR/regeneration multicycle process, although most of them used mild regeneration conditions at laboratory scale under inert atmosphere [36], allowing lower regeneration temperatures (e.g. 800–850 °C) according to the equilibrium of Reaction 3 [34]. The same review [32] highlighted that, among several solutions, a hypothetical configuration on commercial-scale for SESMR is the dual circulating fluidized bed, with one reactor working as a reformer and the second as an oxyfuel calciner, therefore in the presence of high partial pressure of CO2, requiring higher temperatures (e.g. 900–950 °C), and steam in the regeneration step [37,38]. As a consequence, materials for SESMR at commercial-scale should be chemically and physically stable at these severe conditions, as well as mechanically resistant to attrition [39]. The issue of CaO-regeneration for SESMR with industrially relevant conditions was lately faced by Di Giuliano et al. [35], by means of multicycle SESMR/regeneration tests in an automated packed bed test rig, able to perform hundreds of cycles: a purposely synthesized CSCM (Ni-CaO-mayenite optimized for multicycle SESMR with regenerations in N2 at 850 °C [15,31]) underwent a catalytic deactivation because of severe regenerations (pure CO2 at 925 °C, atmospheric pressure), while sorption functionality of CaO appeared stable in multicycle carbonation/calcination tests by Thermo-Gravimetric Analysis (TGA). Di Felice et al. [40] prepared a CSCM by granulation of mixed powders made of a hydrothermally synthesized CaO-mayenite sorbent and a commercial Ni-based catalyst, then compared it with a 2-particles system made of identical – but physically separated – powders of the same two materials. Their CSCM and 2-particles system were tested for multicycle SESMR/regeneration experiments in a TGA reactor, using oxidative (CO2- and H2O-rich) and high-temperature (850–925 °C) regeneration conditions. Promising results, in terms of both catalytic and CO2 sorption stability throughout cycles, were obtained when regeneration was carried out in pure CO2 and an intermediate reduction step in 50/50 vol% H2/N2 at 850 °C was performed before each SESMR stage. This paper aims to extend the investigation on materials presented in Di Felice et al. [40] by means of: (i) testing the chemical stability of the same CSCM in multicycle SESMR/oxidative-regeneration tests (over 200 cycles), carried out by the automated packed bed rig of Di Giuliano et al. [30,35]; (ii) testing a 2-particles system made of commercial Nibased catalyst and CaO-mayenite sorbent, exposed to the same cycling conditions; (iii) characterizing all studied materials before and after these multicycle tests, so to better understand their behaviours; (iv) determining the mechanical stability of these materials to investigate their suitability in fluidized bed reactors, via dedicated attrition tests based on ASTM D5757-11. Compared to Di Felice et al. [40], this work deals with experimental upscaling of tests from 0.025 g (in TGA) to 12 g (in a packed bed reactor), with an increase of the cycles number per test – from 100 to over 200 – and attrition tests at high temperatures of interest for SESMR or sorbent-regeneration. Overall, this new experimental campaign allowed deepening the investigation of chemical and mechanical durability of SESMR materials at industrially relevant

Fig. 1. Schematization of the experimental apparatus for attrition tests. Table 1 Packed beds features in multicycle SESM/regeneration tests. Test

Material

2-p-test

CaO-based sorbent + Commercial Ni-catalyst CSCM

CSCM-test

Mass [g] 9 + 3 12

Particle diameter [μm] 200–300 120–180 200–300

conditions. Both 2-particles system and CSCM tested in this work expressed remarkable catalytic stability and good resistance to attrition, marking an important step forward from the previous work [40] and becoming an interesting subject for further studies at higher scales. This study descends from the recently concluded European Research project ASCENT (Advanced Novel Cycles for Efficient Novel Technologies, grant agreement n° 608512) [41] and aims to give momentum to some relevant results obtained in its framework. 2. Materials and methods 2.1. Materials synthesis methods The production of CaO-based sorbent and CSCM particles have been detailed in a previous work [40]. In short, the granulated sorbent (nominal 30 wt% CaO supported on mayenite) was prepared by hydrothermal synthesis, using Ca(OH)2 (Emsure® ACS, Reag.Ph Eur, purity 96%) and AlO(OH) (Sasol, dispersal P2) as precursors. They were mixed with deionized water in an autoclave at 150 °C for 5 h. After drying, the obtained mass was granulated using an aqueous solution of 10 wt% polyvinyl alcohol as binder in a high shear granulator, then sieved to obtain the appropriate particle size of 200–300 μm before calcination in air at 1000 °C for 1 h (heating rate 5 °C min−1). For 2-particles system tests, this sorbent was physically mixed with a pre-grinded and sieved (120–180 μm) commercial reforming catalyst consisting of Ni supported on MgAl2O4 – Al2O3. 2

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Table 2 Experimental conditions for multicycle SESMR/regeneration tests (temperature variations are intended with T1 as the starting temperature and T2 as the ending temperature of the heating ramp at the given Speed followed by Dwells at T2 of the given duration; S/Cin is the inlet steam to carbon molar ratio).

Pre-reduction Cooling 1 Steaming Reforming Cleaning Regeneration Cooling 2 Reduction a b

T1 [°C]

T2 [°C]

Heating rate [°C min−1]

20 850 850 650 650 650 650 925 925 850

850 850 650 650 650 650 925 925 850 850

5 5

10 5

Dwell at T2 [min]

FCH4, in [Nml min−1]

FCO2, in [Nml min−1]

60 2 20 or 40b 2

75 150 150

15 30

FN2, in [Nml min−1]

FH2, in [Nml min−1]

100 100 190 150

100 100 10

150 150 100

S/Cin [#]

4a 4a

100

3 in the first 7 cycles of CSCM-test. 2-p-test: 40 min in cycles 1, 40, 80, 140, 179, 208, 20 min for all other cycles. CSCM-test: 40 min in cycles 1, 20, 21, 63, 111, 148, 200; 20 min for all other cycles.

As far as the granulated CSCM is concerned, sorbent powder and pre-grinded commercial Ni-based catalyst were mixed to obtain a nominal CaO and Ni loading in the blend of 18 wt% and 5.2 wt% respectively (sorbent/catalyst weight ratio of 1.5 in the final CSCM [40]); this mix was then granulated and sieved (200–300 μm) before calcination at 800 °C (5 °C min−1, 3 h dwell). Note that this catalyst was not specifically designed for SESMR and/or to coexist with CaO in the same particle. The particle size of both catalyst and sorbent is chosen to meet the requirements of the fluidized bed reactor technology which represents the final application for SESMR. It is worth mentioning that the CaO-mayenite sorbent and the CSCM studied in this paper both belong to batches produced at tens-of-kg scale, in the framework of ASCENT project [41]. Further upscaling of the adopted granulation technique is conceivable due to its well spread use at industrial scale.

2.2.4. TEM Transmission Electron Microscopy (TEM) micrographs were acquired by a JEOL 2100 LaB6 (lanthanum hexaboride filament) operating at 200 kV, with a punctual resolution equal to 0.2 nm in parallel mode and 2–3 nm in STEM (Scanning Transmission Electron Microscopy) mode, equipped with a SDD detector (30 mm2) for elemental analysis by in-situ EDS. About 10 mg of powdery samples were firstly ultrasonicated for 5 min in 50 ml of ethanol, so to obtain a suspension, 3 or 4 drops of which impregnated the sample-holder (a polymeric membrane sustained by a copper grid). After ethanol natural evaporation, sonicated particles resulted as dispersed on the membrane. 2.2.5. TGA Thermo-Gravimetric Analysis (TGA) was performed to identify the total sorption capacity (as gCO2/100 g of material) of the prepared sorbent and CSCM prior to packed bed test. This information has been used to estimate the final CaO conversion from packed bed reactor tests (as indicated in Section 2.3.2 and discussed in Sections 3.2.1, 3.2.2 and Tables 4, 5). A custom-made TGA device [44] loaded with 27 mg of material was exposed to a 15/50/35 vol% CO2/H2O/N2 gas mixture flow of 500 Nml min−1 at 650 °C for 25 min (N2 gas purity: 99.999 vol %; CO2 gas purity: 99.999 vol%). Due to a known increase in the CO2 sorption capacity of the sorbent during the first 2–3 cycles [40], the total sorption capacity for this material was taken at cycle 5. To allow for this measurement, sorbent has been regenerated in 20/50/30 CO2/ H2O/N2 gas mixture flow of 500 Nml min−1 at 850 °C. Measured sorption capacities have been assumed as the reference values to estimate the total amount of CaO available for CO2 capture in both materials, on the basis of Reaction 3 stoichiometry.

2.2. Characterization 2.2.1. PXRD Powder X-ray Diffraction (PXRD) data were collected on a Bruker D8 Advance, using a CuKα radiation source (λ = 1.5406 Å) at 40 kV and 40 mA equipped with a Göbbel mirror. The diffractograms were collected in the 2-theta range of 10°–90° with steps of 0.02°. The EVA software was used for phase identification. Rietveld refinements were performed with the TOPAS software for quantitative phase analysis and nickel crystal size calculation, as described in a previous work [40]. 2.2.2. Porosimetry A MICROMERITICS ASAP 2420 surface area and porosity analyser recorded N2 adsorption and desorption isotherms at −196 °C, performing calculations after BET and BJH methods by ASAP 2420 software v2.09. Other details on the experimental procedure can be found in [15,31,42,43]. Considered measures are BET surface area (SBET), BJH cumulative pore volumes (VBJH), BJH pore volume distributions with respect to pore sizes, and BJH averaged cylindrical pore diameter (Dav,BJH). N2 desorption data are used for BJH mesoporosity assessment (VBJH and Dav,BJH).

2.2.6. Attrition tests Particle mechanical resistance of materials for SESMR, for their applicability in fluidized beds, was evaluated by procedure and device (Fig. 1) based on ASTM D5757-11 (Standard Test Method for Determination of Attrition and Abrasion of Powdered Catalysts by Air Jets), so to constitute a jet system where interaction between particles, promoted by exchange of gas momentum, are dominant. That device (Fig. 1) includes a stainless-steel attrition vertical tube having at its base three equidistant nozzles, a stainless-steel settling chamber flanged on the attrition tube top, and with conical ends connected with a fines collector. The experimental rig is externally heated by means of an electric furnace, to carry out tests at temperatures of SESMR or calcination steps (650–850 °C). A Fluid Catalytic Cracking (FCC) zeolite catalyst was tested at 650 °C and used as reference to evaluate performances of materials studied in this work, tested in the following conditions:

2.2.3. SEM/EDS Scanning Electron Microscopy (SEM) micrographs were recorded by a PHILIPS XL30CP device, equipped with an OXFORD ENERGY 250 INCAx-act LN2-free detector for elemental analyses by in-situ Energy Xray Dispersive Spectrometry (EDS). Samples in the form of powder were observed on their external surface, as well as on their internal crosssectional area. Further details on this method are given elsewhere [15,31,35,42]. 3

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(a)

(b)

Al

Ca

(c)

Fig. 2. SEM-EDS of fresh CaO-based sorbent on external surface (a) and on cross-sectional area (b,c).

• reduced commercial Ni-based catalyst, at 650 °C and 850 °C; • calcined CaO-based sorbent, at 850 °C; • carbonated and reduced CSCM, at 650 °C; • calcined and reduced CSCM, at 850 °C;

so to cause attrition; the cross-section expansion of the settling chamber allows only fine particles to be entrained and collected in the terminal flask of fines collector. Recovered fines mass in this flask was measured after 1 h (mfines(1 h)) and after 5 h (mfines(5 h)), as well as residual mass (mr) in the attrition tube at the end of the 5 h test. The Air Jet Index (AJI) was then calculated as the percent attrition loss after a given time (Eqs. (1), (2)). To attest the test reliability, the overall sample mass recovery (η) was also calculated (Eq. (3)):

About 50 g of a powdery sample (ms) were placed on the bottom of the attrition tube; N2 flowed upwards through the nozzles (10 l min−1 at the operating temperature) and therefore through the solid particles, 4

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(a)

(b)

Al

Ca

Mg

Ni

Fig. 3. SEM-EDS of fresh CSCM on external surface (a) and on cross-sectional area (b).

AJI (1 h) = AJI (5 h) = =

mfines (1 h) ms mfines (5 h) ms

mr + mfines (5 h) ms

100 100

100

ADVANCE OPTIMA CALDOS 17 module, measuring H2 volumetric concentration by a thermal conductibility detector). Each inlet gas comes from a bottle of the pure species, each flowrate regulated by BROKHORST mass flowmeter; steam is produced and mixed with those gases in an evaporation chamber, fed with distilled water at a proper flowrate regulated by a KDS LEGATO 110 syringe pump. The automated control system of this rig allows to: (i) record online dry procuct flowrate Ftot,out, concentrations Yi, out, together with packed bed temperature, with high sampling frequency (0.2 Hz); (ii) manage cyclic switch of desired process conditions, so to carry out long duration SESMR/regeneration multicycle tests, with a high number of cycles. Recorded measurements allow calculating outlet molar flowrates of H2, CH4, CO, CO2 (Eq. (4)).

(1) (2) (3)

2.3. Multicycle SESMR/regeneration tests 2.3.1. Experimental procedure The automated packed bed reactor test rig, used for multicycle SESMR/regeneration tests is fully described elsewhere [30,31,42]. In short, it consists of a vertical tubular reactor feedable with H2 (99.999 vol% purity), CH4 (99.995 vol% purity), N2 (99.9995 vol% purity), CO2 (99.99 vol% purity) and steam, filled with investigated particles, which form the active packed bed, heated by a cylindrical furnace (controlled by a thermocouple inside the bed); downstream, the gaseous product stream was cooled and dried, then overall molar flowrate (Ftot,out) and dry volumetric percentages of H2, CH4, CO, CO2 (Yi, out) are measured online (ABB online analysis system, equipped with an ADVANCE OPTIMA URAS 14 module, measuring CO, CO2, CH4 volumetric concentrations by non-dispersive infrared, and an

Fi, out =

Yi, out Ftot , out ; i = CH4 , H2 , CO , CO2 100

(4)

Two tests were carried out on packed beds made of 2-particles system (2-p-test) or of CSCM (CSCM-test). Specific features of tested packed beds are summarized in Table 1. To expose the materials to relevant process conditions, the following measures were adopted: (i) reforming inlet stream containing only CH4 and steam, without diluting inert gases; (ii) severe regenerations in an oxidative environment (pure CO2 at 925 °C). As previously proposed [40], a reduction step in 50/50 vol% H2/N2 at 850 °C was applied to 5

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Fig. 4. TEM micrographs on fresh CSCM.

of Ni in the 2-p-test bed vs. 0.624 g of Ni for the CSCM-test, i.e. a more stringent condition for reforming catalysis) and in a longer pre-breakthrough time, which was useful to better detect possible deactivation trends during the sorption-enhanced reforming phase. A direct comparison between the CSCM and its equivalent 2-particles system was already carried out in [40]. Reforming performances were evaluated in terms of CH4 conversion (χCH4, Eq. (5)) and outlet concentrations of H2, CH4, CO, CO2 on dry basis (Yi,out).

Table 3 Results from CHEMCAD® simulations on SESMR and SMR multiple equilibria systems based on equilibrium constants as functions of temperature reported in [45] (650 °C, 1 atm, 75 Nml min−1 CH4 and steam/carbon molar ratio = 4 in the feeding stream). Yi,out [vol%

SMR SESMR

χCH4 [%]

dry]

CH4

H2

CO

CO2

1.1 0.3

77.4 96.9

8.6 1.3

12.9 1.5

96 99

CH4

=

FCH4, in FCH4, out ·100 FCH4, in

(5)

In order to gain insight into the catalyst performance after sorbent saturation (post-breakthrough period), reforming step duration was increased from 20 min to 40 min for selected cycles (see Table 2). Reported χCH4 and Yi,out of pre-breakthrough were calculated by averaging measurements acquired during the 3rd minute of each reforming step; in addition, post-breakthrough performance was evaluated in the same way, reporting the average measurement of the 36th minute in each longer reforming step. The choice of these representative minutes for pre- and post-breakthrough allowed avoiding effects from process transients (at the beginning and the end of each reforming step) and comparing data from different cycles univocally. For both 2-p-test and CSCM-test, CO2 and H2 breakthrough curves from the first and the last

reduce NiO – formed during oxidative sorbent regeneration – to metallic Ni, so as to reactivate the catalyst before each SESMR step. The full sequence of process conditions is detailed in Table 2: steps from “Cooling 1” to “Reduction” constituted the cycled session, repeated 208 times in the 2-p-test and 200 times in CSCM-test, for total non-stop durations of 530 h and 511 h, respectively. It is worth mentioning here that the sorbent/catalyst mass ratio in the 2-p-test was doubled with respect to that in the CSCM (2-particles system sorbent/catalyst mass ratio = 3, instead of 1.5 for the CSCM). The increased sorbent/catalyst mass ratio used in this work for the 2-ptest resulted in a lower relative amount of Ni in the packed bed (0.390 g 6

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Fig. 5. 2-p-test: averaged pre- and post-breakthrough values of H2 and CO2 fractions in the product stream and of CH4 conversion, as functions of cycle number (a); first and last breakthrough curves of H2 and CO2 (b).

cycle were also compared.

Table 4 Carbon balance results on long SESMR steps of 2-p-test, with estimations of captured CO2 by the CaO-based sorbent (Γ) in the packed bed (m0 = 9 g), also expressed as CaO conversion on the basis of actual CaO fraction in the sorbent (m0CaO = 2.74 g, according to TGA result). Cycle

ΔC [mol]

1 40 80 140 179 208

4.70 4.65 4.41 3.86 3.65 3.57

-2

10 10-2 10-2 10-2 10-2 10-2

ΔC

Γ

[% of NC,in]

[gCO2/100 gsorbent]

35.5 34.7 33.0 28.9 27.3 26.7

23.0 22.7 21.6 18.9 17.8 17.5

2.3.2. Carbon balances An overall molar carbon balance was performed on the packed bed reactor, for each long reforming step (40 min). Assumptions in force for these balances follow: (i) only CH4 flowrate brings carbon into the reactor (FCH4,in, constant with respect to time); (ii) carbon leaves the reactor by flowrates of CO, CO2 and CH4, (FCO,out, FCO2,out, FCH4,out), determined as functions of time (Eq. (4)); (iii) carbon deposition is negligible due to the relatively high inlet steam/carbon ratio. Carbon moles entering the reactor (NC,in) in each whole long reforming step were calculated by numerical integration with respect to time of FCH4,in. Analogously, total carbon moles leaving the reactor in the same reforming step (NC,out) were obtained from integrations of FCO,out, FCO2,out and FCH4,out with respect to time. The carbon accumulation term ΔC estimates the moles of atomic carbon held in the packed

χCaO [%] 96 95 90 79 75 73

7

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Fig. 6. CSCM-test: averaged pre- and post-breakthrough values of H2 and CO2 fractions in the product stream and of CH4 conversion, as functions of cycle number (a); first and last breakthrough curves of H2 and CO2 (b).

bed and therefore the moles of CO2 captured by Reaction 3 (ΔCO2), i.e. moles of converted CaO in the solid (ΔCaO) (Eq. (6)). This allows the further estimation, at the end of each long reforming step, of: (i) Γ, grams of captured CO2 per 100 g of sorbent material (Eq. (7), with m0 = 9 g of sorbent for the 2-p-test and 12 g of CSCM for the CSCMtest); (ii) CaO conversion in the active packed bed (χCaO, Eq. (8)), with respect to CaO initially available for CO2 capture in the fresh bed (m0CaO, calculated on the basis of sorption capacity values measured by TGA). CaO conversion can be used to fairly compare the two multicycle tests carried out in the automated packed bed rig.

C=

CO2 =

CaO = NC, in

NC, out

=

CaO

CO2 MMCO2 100 m0 =

CaOMMCaO 100 0 mCaO

(7) (8)

3. Results and discussion 3.1. Characterization of fresh materials Both 2-particles system and CSCM before test have been characterized by PXRD, BET-BJH methods and SEM analysis. SEM

(6) 8

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3.2. Multicycle SESMR/regeneration tests

Table 5 Carbon balance results on long SESMR steps on CSCM-test, with estimations of captured CO2 (Γ) by the whole CSCM packed bed (m0 = 12 g), also expressed as CaO conversion on the basis of actual CaO fraction in the CSCM (m0CaO = 2.29 g, according to TGA result). Cycle

1 21 63 111 148 200

χCaO

ΔC

ΔC

Γ

[mol]

[% of NC,in]

[gCO2/100 gCSCM]

3.17 2.24 1.53 1.25 9.76 7.66

10-2 10-2 10-2 10-2 10-3 10-3

23.9 16.7 11.5 9.3 7.3 5.7

In order to evaluate experimental outcomes from multicycle SESMR/regeneration tests, calculations of thermodynamic thresholds were carried out by CHEMCAD® for SESMR and SMR (results in Table 3). An equilibrium reactor (EREA UnitOp) simulated reforming experimental conditions described in Section 2.3.1, according to equilibrium constant functions with respect to temperature for Reactions 1, 2, 3 (reported by Aloisi et al. [45]).

11.6 8.2 5.6 4.6 3.6 2.8

[%] 77 54 55 37 31 24

3.2.1. 2-p-test Fig. 5a shows the performance of the 2-particles system in multicycle SESMR/regeneration (Table 2). In the whole 2-p-test, YH2,out in the pre-breakthrough was higher than 93 vol% dry, a value slightly lower than thermodynamic limits of SESMR (Table 3); similarly, YH2,out in the post-breakthrough was close to thermodynamic threshold of SMR (Table 3). As far as CH4 conversion is concerned, pre-breakthrough values underwent a slight decrease from 97% to 94% in the first 50 cycles, then stabilizing at 92–93%; post-breakthrough CH4 conversion started from 94% and then stabilized at around 82–84% until the end of the test (Fig. 5a). Therefore, a slight catalyst deactivation is observed in both pre- and post-breakthrough periods during SESMR/ regeneration cycles, preventing to attain thermodynamic equilibrium for the 2-p system. Although characteristic pre- and post- breakthrough sessions clearly occurred for all cycles, sharper increase and decrease could be observed in the outlet concentrations of CO2 and H2, respectively (i.e. shorter pre-breakthrough), as evidenced by comparing CO2 and H2 breakthrough curves from cycle 208 with those from cycle 1 (Fig. 5b). This is in agreement with carbon balance on long reforming steps of the 2-ptest (ΔC in Table 4); in fact, the total carbon moles retained in the active packed bed decreased with cycle number progression. Conversion of CaO reported in Table 4 is referred to CaO quantity equivalent to the total sorbent CO2 capture capacity of 23.9 gCO2/100 gsorbent, measured by TGA test on the CaO-based sorbent, prior to the packed bed 2-p-test (see Section 2.2.5).

Table 6 Results of PXRD including Rietveld refinement analysis, on post-test materials. Catalyst after 2-p-test Catalyst phases Ni [wt%] Ni crystallite [nm] NixMg(1−x)O2 [wt%] MgAl2O4 [wt%] Al2O3 [wt%] Sorbent phases CaO [wt%] Ca12Al14O33 [wt%]

10.1 70 2.8 80.7 4.8 – 1.6

Sorbent CSCM after 2-p-test after test

25.0 75.0

4.3 40 14.6 25.1 – 3.3 52.7

micrograph on external surface of CaO-based sorbent (Fig. 2a) located some cubic structures in the order of 1 μm within a prevalent crumbled solid: elemental Al an Ca in different ratios appeared in SEM-EDS analyses, depending on the probed zone, in agreement with the presence of both Ca12Al14O33 and CaO detected by PXRD. Remarkably, SEM micrograph on cross-sectional area of CaO-based sorbent (Fig. 2b) evidenced a stratification of several porous shells upon an originating nucleus, ascribable to the granulation process. SEM-EDS on cross-sectional area of CaO-based sorbent (Fig. 2c) showed two morphologies (laminar and granular), also confirming the presence of elemental Ca and Al with different ratios in different zones, in further agreement with PXRD analyses. MgAl2O4, Al2O3 and metallic Ni phases were detected in the catalyst material by PXRD. As for the sorbent, CaO and mayenite (Ca12Al14O3) were observed. The CSCM showed a coexistence of an active CO2 absorption phase (CaO), an active reforming catalyst (Ni) and a composite support made of MgAl2O4, Al2O3 and Ca12Al14O3. These findings confirmed previously reported analyses on fresh materials [40] and are not further detailed in this work. SEM-EDS micrographs of CSCM in Fig. 3 evidenced two different morphologies aggregated in the same particle: the more compact one was identified as the Ni-based catalyst (Fig. 3a, Spectrum 2) and the more granular as CaO-based sorbent (Fig. 3a, Spectrum 3). SEM-EDS on cross-sectional area of a CSCM particle (Fig. 3b) confirmed these interpretations, locating elemental Ca in more granular zones (related to CaO-based sorbent) and elemental Ni and Mg only in the more compact ones (related to Ni-based catalyst). TEM micrographs of fresh CSCM (Fig. 4) showed a compact portion with NiO detected in central, darker zones (Fig. 4a, b); NiO crystals were better shown in Fig. 4d (a magnification of Fig. 4b), where support shells around NiO crystals were observed; Fig. 4c showed the overlapping of several particles, made of different metal oxides, causing moiré pattern.

3.2.2. CSCM-test The CSCM exhibited a satisfactory performance, with both H2 concentration and CH4 conversion being steadily high throughout the whole 200 SESMR/regeneration cycles (Fig. 6a). The first 7 cycles were carried out with inlet steam/carbon molar ratio equal to 3 instead of 4; these cycles were considered in all respects as a relevant part of the test, since they allowed highlighting effects from inlet steam/carbon molar ratio and the ability of the CSCM to work at different process conditions. A positive influence on H2 concentration and CH4 conversion came from the increase of inlet steam/carbon molar ratio: in the first 7 cycles (steam/carbon = 3) pre-breakthrough YH2,out was 96 vol%dry and pre-breakthrough CH4 conversion was in the range 90–92%; during the rest of the test (steam/carbon = 4) pre-breakthrough YH2,out was 97 vol%dry, and pre-breakthrough CH4 conversion was within the range 93–97%. Pre- and post-breakthrough experimental outcomes are close to respective equilibrium thresholds (Table 3), thus highlighting a very good reactive performance of the CSCM. However, breakthrough curves of H2 and CO2 from the first and the last cycle (Fig. 6b) showed that the duration of sorption enhanced reforming period decreased with cycles progression (the momentary H2 increase after breakthrough is ascribable to temperature fluctuations recorded in the reactor during the CSCM-test); as for the 2-p system, the lowering of pre-breakthrough duration is in agreement with carbon balance (ΔC in Table 5). Conversion of CaO reported in Table 5 is referred to the total CaO quantity equivalent to CO2 capture capacity of 15.0 gCO2/100 gCSCM, measured by a TGA test on the CSCM, prior to the packed bed CSCM-test (see Section 2.2.5). 9

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(a)

(b)

Al

Ca

Mg

Ni

Al

(c)

Mg

Ni

Fig. 7. SEM-EDS of the 2-particles system after 208 SESMR/regeneration cycles on external surface (a) and on cross-sectional areas (b,c).

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Fig. 8. TEM micrographs on the 2-particles system after 208 SESMR/regeneration cycles.

3.2.3. Comparative analysis Values of H2 concentration and CH4 conversion in Figs. 5a and 6a prove the satisfying stability of reforming catalysis in both multicycle tests, showing that the commercial Ni-based catalyst suffered only limited deactivation. In previous studies [31,35] a different kind of NiCaO-based combined material (synthesized at laboratory scale by wet mixing and wet impregnation methods) was tested with in same packed bed set-up used in this work: it expressed an excellent stability throughout 204 cycles of SESMR followed by regenerations in N2 at 850 °C [31]; on the other hand, when tested with the very same procedure used in this work, it suffered sintering of Ni catalytic phase, with consequent deactivation because of oxidative regeneration conditions during cycles [35]. This highlights how regeneration conditions may affect performances of a given Ni active phase, suggesting that the choice of a commercial catalyst to bring Ni in 2-particle system and CSCM here investigated was relevant. In greater detail, without prejudice to the satisfying catalytic stability in both tests, small differences emerged, ascribable to the different Ni contents in the two packed beds (having the same inlet flows of reactants): (i) pre-breakthrough CH4 conversion was more stable during the 200 cycles for the CSCM (Fig. 6a), while a slightly more relevant decrease occurred in the 2-ptest (Fig. 5a); (ii) pre-breakthrough H2 concentration resulted higher in the CSCM-test (Fig. 6b), with respect to the same quantity measured in the 2-p-test (Fig. 5b). As far as CO2 capture performances are concerned, the lower sorbent/catalyst mass ratio in the packed bed contributed from the first cycle to the shorter duration of pre-breakthrough periods in the CSCMtest (Fig. 6b), when compared to 2-p-test (Fig. 5b). Therefore, CaO conversion estimated by C balance (Eq. (8)) was assumed as the common parameter to fairly compare performances from these two multicycle tests (results in Tables 4 and 5). In the first reforming cycle, estimation of CaO conversion was 96% for the CaO-sorbent in the 2-particles system (Table 4) and 77% for the CSCM (Table 5). Lower sorption capacity values in packed beds as compared to TGA were previously observed by other authors [46]. This could be ascribed to the different gas/solid contact occurring in a TGA equipment and in a continuous packed bed chemical reactor: the former is conceivable as a bulk of solid particles in which gaseous species percolate by diffusion, the latter as system in which gases are mainly subjected to convective transport through the bed and towards reactor exit. The second situation appears as less favourable to the capture of CO2, as found in studies by Aloisi et al. [36] and Di Giuliano et al. [35]

on an Axial Dispersion Plug Flow Reactor (ADPFR) dynamic model and a Particle Grain Model (PGM), both validated with experimental data. For a given CSCM, the PGM fitted well TGA experimental data for CO2 capture, with total conversion of CaO; on the other hand, the ADPFR model - used to simulate SESMR in a packed bed made of the same CSCM - resulted in an uncomplete CaO conversion of the whole bed after the breakthrough, noticeably with the same carbonation kinetic parameters used in the PGM. The variable performance of the same CSCM was therefore ascribable to the process configurations and conditions mathematically described in the ADPFR dynamic model and the PGM, respectively. When the cycle number increased, the two systems showed different behaviours, and in particular a more marked decrease in CaO conversion for the CSCM as compared to the 2-particle system (Table 5 vs. Table 4). This different trend is in agreement with the more marked decrease of pre-breakthrough duration for the CSCM (Fig. 6b) in comparison to that of 2-particle system (Fig. 5b). These observations can be explained by after-test PXRD characterizations with Rietveld refining, detailed in the following, which highlighted a quantitative depletion of CaO active phase in spent materials from both tests, more important in the CSCM case. 3.3. After-test characterization Materials after multicycle 2-p-test and CSCM-test were characterized by PXRD including Rietveld refinement, SEM-EDS and TEM; in addition, measures by BET and BJH methods were carried out to evaluate and compare pores properties of the CSCM before- and aftertest. The after-test materials are expected to be in calcined and reduced state since tests ended after the reduction step of the sequence shown in Table 2. As far as PXRD with Rietveld refinement is concerned, main results are reported in Table 6 and commented in the following subsections. However, it is worth to preliminary note that some Ca(OH)2 was found as result of parasitic CaO reaction with H2O occurring before the PXRD analysis for both the 2-p sorbent and the CSCM; to allow for a direct comparison of all materials before and after test, Ca(OH)2 has been reconverted to equivalent CaO (Eq. (9)) and all mass fractions normalized to the resulting mass change.

wt %CaO = wt %Ca (OH )2

11

MMCaO MMCa (OH )2

(9)

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(a)

(b)

Al

Ca

Mg

Ni

Fig. 9. SEM-EDS of CSCM after 200 SESMR/regeneration cycles on external surface (a) and on cross-sectional area (b).

3.3.1. 2-Particles system With regard to PXRD analyses of the 2-particles system after-test, sorbent and catalyst were pre-separated using a magnet so that specific information was obtained for each material (Table 6). CaO in the spent sorbent resulted to be 25 wt%, lower than the expected initial 30 wt% but in good agreement with the reported decrease in sorption capacity in Table 4. As for the catalyst, a limited formation of NiO-MgO solid solution is detected (2.8 wt%, 23% of which being NiO) and MgAl2O4 is present at 80.7 wt%, together with 4.8 wt% Al2O3. A residual 1.6 wt% mayenite phase was found as effect of incomplete catalyst-sorbent segregation before analysis. Although nickel is effectively reduced after the reduction step (about 95% of it is found as metallic species), a Ni crystallite size of 70 nm is estimated, which is rather high compared to the values previously reported before and after cycling for the same catalytic system (12 and 50 nm) [40]. SEM-EDS on both external surfaces (Fig. 7a) and cross-sectional areas (Fig. 7b and c) located the two components of the 2-particles system; moreover, SEM-EDS at higher magnification highlighted the formation of coarser Ni grains within the Ni-based catalyst particles (Fig. 7c), absent in the corresponding fresh sample (Fig. 3c). TEM micrographs, coupled with STEM-EDS analyses, confirmed the formation

of this agglomeration of metallic Ni (darker spheroids and hexagons in Fig. 8, roughly within the range 15–100 nm). This evidence, in conjunction with the higher crystallite size detected by PXRD (Table 6), can explain the slight decrease in CH4 conversion with cycle number progression (Fig. 5a). No carbonaceous formations were detected (e.g. carbon whiskers, carbon fouling), confirming the hypothesis of assimilation of ∆C to captured moles of CO2 (Eq. (6)) in carbon balances summarized in Tables 4 and 5. 3.3.2. CSCM A more evident CaO depletion for the case of CSCM was evidenced by PXRD (Table 6), which corresponds to a decrease of residual CO2 capture capacity, in agreement with results reported in Table 5 and Fig. 6. On the other hand, 52.7 wt% mayenite (Ca12Al14O33) and 25.1 wt% magnesium aluminate (MgAl2O4) – which are higher and lower than the expected initial values of 42 and 34 wt%, respectively – were found. This evidence confirms the previously inferred mechanism of catalyst and sorbent phases rearrangement in the granulated CSCM [40]: MgAl2O4 and CaO react producing additional Ca12Al14O33 and MgO, while the latter in turn forms a MgO-NiO solid solution (containing 25.4 mol% of NiO in the sample after test). Note that Al2O3 12

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Fig. 10. TEM micrographs on CSCM after 200 SESMR/regeneration cycles.

MgO solid solution following nickel oxidation. While the formation of NiO-MgO solid solution decreases the quantity of active Ni0 available for reforming, it provides a relatively small amount of Ni segregated as small particles on the surface of the support, which might result in limited sintering [47]. After-test SEM on both external surfaces (Fig. 9a) and cross-sectional areas (Fig. 9b) did not evidence morphological modifications due to SESMR/regeneration cycles, in comparison to the as-synthesized state (Fig. 3), except for the qualitative observation of voids in the form of cracks appearing more frequently after the test. TEM micrographs on after test CSCM, interpreted by STEM-EDS analyses, confirmed the presence of two kinds of sub-structures (Fig. 10a): one based on Ni particles (Fig. 10d), mainly coupled with a matrix containing Mg and O (Fig. 10b, more heterogeneous structure on the right), appearing as darker spheroids (Fig. 10c); the other organized in bigger Ca/Al/O plates (Fig. 10a, right side and Fig. 10b, left side). As for 2-particles system after test, no carbonaceous formations were detected (e.g. carbon whiskers, carbon fouling). A comparison was carried out between surface area and pores properties of CSCM in its fresh state and after multicycle SESMR/regeneration tests (Table 7): having values in the fresh state as a

Table 7 Results from BET and BJH methods for the CSCM. Material

SBET [m2 g−1]

VBJH [cm3 g−1]

Dav,BJH [nm]

CSCM (fresh) CSCM (after test)

10.5 4.1

0.080 0.022

28.6 17.3

phase is not detected after test, possibly indicating a direct reaction between this phase and CaO as additional reaction path to produce mayenite, although formation of amorphous (undetectable) Al2O3 cannot be excluded. From PXRD analysis (Table 6), the residual metallic nickel available for the reaction accounts for 4.3 wt%, while 14.6 wt% NiO-MgO solid solution (25.4 mol% NiO) is estimated. A Ni crystallite size of 40 nm is reported, which compares favourably with that of the spent catalyst from 2-p test (70 nm) and corroborates the evidence that the catalytic activity of the CSCM does not decrease during 200 cycles of operation. CSCM aging may be supposed longer than that of the 2-particles system, more catalyst being available for the same reactant inlet flowrates, although active nickel is more evidently depleted by formation of NiO13

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The sorbent and catalyst show attrition results similar to those of the reference material, the former being slightly above and the latter below the AJI (1) and AJI (5 h) of the FCC zeolite catalyst. This performance is satisfactory, as FCC zeolite catalysts are used in industrial fluidized bed reactors at temperatures and pressures close to those foreseen in the SESMR looping cycle. The reduced CSCM, in both carbonated and calcined forms – the latter being slightly less resistant – shows AJI values similar to the calcined sorbent. 4. Conclusions This paper detailed the results obtained by testing sorbent and catalyst materials – separated in a 2-particles system or merged in a multifunctional combined catalyst and sorbent (CSCM) – for 200 SESMR/regeneration cycles in a packed bed reactor. While marking a step forward on the development of these materials, from 100 cycles in TGA to 200 cycles (>500 h operation) in a packed bed complemented by attrition tests, it highlights the importance of physical, chemical and mechanical stability of the different materials involved. Multi-cycle chemical stability in relevant operating conditions, satisfactory SESMR performance in packed bed reactor environment, and relatively low attrition jet index resulting in acceptable mechanical stability, have been verified. However, due to cycling at high temperature of both the sorbent (carbonation-calcination) and the catalyst (oxidation-reduction) functions, the material is continuously subjected to physical modification and chemical reactions, which in this work have brought to a decrease in available CaO and eventually to a lower CO2 sorption capacity of the material during cycles; this was found to be particularly evident for the CSCM, where the different catalyst and sorbent phases are in more intimate contact. Physical modifications and chemical alterations also involved the catalytic function, with a partial transformation of active Ni into other solid phases. On the whole, this did not undermine catalytic activity of tested materials at the severe process conditions used in this work; however, it points out that chemical stability of combined materials is more challenging than for the case of mixed (segregated) particle systems, because of the coexistence of active nickel, calcium oxide and support phases in the same particle. Of note, the inert support plays an important role stabilizing both active functions; therefore, simple and highly stable support formulations are recommended for further studies aimed at improving the multicycle performance of combined materials. Further steps will be dedicated to extend these satisfactory results by:

Fig. 11. BJH pore volume distribution with respect to pore diameter of CSCM in fresh state (lighter squares) and after test (darker squares).

reference, the after-test CSCM underwent a marked decrease in surface area (SBET), pore volume (VBJH) and average pore size (Dav,BJH). This was caused by a strong drop of mesoporosity and macroporosity, as inferred by BJH pore volume distribution with respect to pores diameter (Fig. 11). This did not appreciably influence the catalytic activity throughout cycles, as proved by CH4 conversion values in Fig. 6a. 3.4. Attrition tests In addition to activity and physico-chemical stability, this work has aimed to investigate the mechanical stability of the sorbent, catalyst and CSCM, in view of their further use in fluidized beds. Table 8 summarizes results obtained for attrition tests: recovery values η allow considering all tests as consistent. Assuming an FCC zeolite catalyst as a reference, the comparison with investigated materials (commercial Nibased catalyst, CaO-based sorbent and CSCM particles) indicates acceptable attrition resistances for their use in fluidized bed (Table 8). As a general trend, the temperature change from 650 °C to 850 °C does not produce significant variations of attrition tests results (Table 8). The sorbent has been attrition-tested in calcined state, which is well known to be softer and might erode quickly than the carbonated form. For the case of calcined sorbent and calcined and reduced CSCM, the attrition test has been repeated twice (reported as Test n.1 or n.2 in Table 8), using particles synthesized in different production batches. This allows to prove the reproducibility of the agglomeration step and at the same time the repeatability of the attrition test.

1) investigating oxidative calcinating conditions other than pure CO2, since in real operation the calciner can be characterized by high H2O concentration as well as presence of O2 – i.e. in the case of a fuel combusted inside the calciner for direct heat supply during sorbent regeneration; 2) the design of new catalytic functions that can directly cycle between calcination and reforming process conditions, without the need of a pre-reduction step before SESMR

Table 8 Attrition tests results. Material

State

FCC zeolite catalyst (reference material) Commercial Ni-based catalyst Commercial Ni-based catalyst CaO-based sorbent, Test n.1 CaO-based sorbent, Test n.2 CSCM CSCM, Test n.1 CSCM, Test n.2

Reduced Reduced Calcined Calcined Carbonated and reduced Calcined and reduced Calcined and reduced

Size range [μm]

T [°C]

AJI (1 h) [%]

AJI (5 h) [%]

η [%]

75–212 100–200 100–200 100–200 100–200 100–200 100–200 100–200

650 650 850 850 850 650 850 850

3.2 2.7 2.5 4.8 5.9 6.4 6.8 4.4

8.1 5.5 6.1 8.9 9.8 8.5 9.2 10.3

96 95 99 94 97 94 96 98

14

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Nomenclature

CSCM studied in this work at tens-of-kg scale.

Abbreviations

References

ADPFR AJI ASCENT ASTM BET BJH CCS CSCM EDS FCC IPCC PGM PXRD SEM SESMR SMR STEM TEM TGA USA WGS

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Axial Dispersion Plug Flow Reactor Air Jet Index Advanced Novel Cycles for Efficient Novel Technologies American Society for Testing and Materials Brunauer-Emmett-Teller Barrett-Joyner-Halenda Carbon Capture and Storage Combined Sorbent-Catalyst Material Energy Dispersive X-ray Spectroscopy Fluid Catalytic Cracking United Nations Intergovernmental Panel on Climate Change Particle Grain Model Powder X-ray Diffraction Scanning Electron Microscopy Sorption-Enhanced Steam Methane Reforming Steam Methane Reforming Scanning Transmission Electron Microscopy Transmission Electron Microscopy Thermo-Gravimetric Analysis United States of America Water Gas Shift

Symbols Dav,BJH F m MM NC S/C SBET T VBJH Y

average pores diameter by BJH method [nm] molar flowrate [Nml min−1] mass [g] molar mass [g mol−1] carbon cumulated moles (carbon balances) [mol] steam to carbon molar ratio [mol mol−1] specific surface from BET method [m2 g−1] temperature [°C] specific volume by BJH method [cm3 g−1] concentration of gaseous components on dry basis [vol%dry]

Greek letters Γ estimation of captured CO2 [gCO2 per 100 g of sorbent material] ΔC carbon accumulation [mol] ΔCaO estimated converted moles of CaO [mol] ΔCO2 estimated captured moles of CO2 [mol] η sample mass recovery (attrition test) [%] χ conversion [%] Subscripts 0 amb fines i in out r s tot

and superscripts initial ambient recovered fines (attrition tests) generic chemical species inlet outlet residual mass (attrition tests) powdery sample (attrition tests) total

Acknowledgements The research leading to these results has received funding from the European Union's Seventh Framework Programme ASCENT grant agreement n° [608512]. Authors thank the ASCENT partner MARION TECHNOLOGIES S.A. (Parc Technologique Delta Sud, 55 rue Louis Pasteur, 09340 Verniolle, France) for producing CaO-based sorbent and 15

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