Performance of a combined CaO-based sorbent and catalyst on H2 production, via sorption enhanced methane steam reforming

Chemical Engineering Journal 264 (2015) 697–705

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Performance of a combined CaO-based sorbent and catalyst on H2 production, via sorption enhanced methane steam reforming Ana L. García-Lario ⇑, Gemma S. Grasa, Ramón Murillo Instituto de Carboquímica, (CSIC), Miguel Luesma Castán 4, 50018 Zaragoza, Spain

h i g h l i g h t s  A hybrid CO2 sorbent and reforming catalyst was synthesized through physical mixing.  It presents a CO2 carrying capacity over 20 wt.% with respect the CaO in the solid.  H2 percentages close to equilibrium are achieved even for a steam to carbon ratio as low as 1.5.  It was tested cyclically under realistic conditions with H2 yields over 90 vol.%

a r t i c l e

i n f o

Article history: Received 15 September 2014 Received in revised form 18 November 2014 Accepted 23 November 2014 Available online 29 November 2014 Keywords: H2 production CO2 capture Sorption Enhanced Reforming Multifunctional catalyst

a b s t r a c t The performance of an hybrid material CO2-sorbent and reforming catalyst under Sorption Enhanced Reforming of methane has been assessed. The material was synthesised through physical mixing of CaO, NiO and calcium cement aluminate (varying the proportion CaO/NiO to produce three different solids). The materials, that presented a stable CO2 carrying capacity of 20 wt.% of the total CaO in the solid, were able to reach gas product composition very close to thermodynamical equilibrium (at 650 °C, steam to carbon ratio, S/C, of 3 and 1200 h 1 CH4 spatial velocity), with H2 composition over 94 vol.% (dry basis). In addition, it has also been demonstrated that the hybrid materials with NiO wt.% contents of 14 and 18.5 can fulfill the thermodinamical equilibrium at S/C ratios as low as 1.5. Finally the 18.5% NiO material has been tested in cyclic operation, under realistic conditions by regenerating the sorbent in oxidizing conditions. The cycled material showed slight increase of NiO crystal size that resulted in slight loose of activity after cycling. However, H2 percentages over 90 vol.% (dry basis) were always obtained. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Hydrogen is a raw material used in several industrial processes as ammonia and methanol synthesis, hydro cracking and hydro processing in refineries, hydrogenation of ethylene, metallurgical processes or glass production. Its demand is expected to increase once the recent proposed applications as low-emission fuel (for gas turbines and combustion engines i.e.) become a proven alternative as energy conversion systems for electricity production [1]. Around 50% of the H2 produced worldwide present a fossil fuel origin as it is produced via Steam Methane Reforming (SMR) [1,2]. This is an energy intensive process performed in several steps at severe conditions regarding to operating pressure and temperature (reforming of methane with steam takes place at 800–900 °C and

⇑ Corresponding author. Tel.: +34 976 733977. E-mail addresses: [email protected] (A.L. (G.S. Grasa), [email protected] (R. Murillo). http://dx.doi.org/10.1016/j.cej.2014.11.116 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.

García-Lario),

[email protected]

15–30 bar and shift reaction is conducted in secondary reactors at a lower temperature around 200–400 °C). Moreover, it is reported that around 8 ton CO2 per ton H2 produced via Steam Methane Reforming are emitted [3]. In this context, large scale hydrogen production processes including CO2 capture at a reduced cost and more efficiently than existing processes, constitute a great challenge in the climate change mitigation route [4–7]. As an alternative to the SMR, the Sorption Enhanced Reforming (SER) process proposes the addition of a Ca-based CO2 acceptor to the commercial SMR catalyst so that the reforming, shift and CO2 removal processes take place simultaneously in the reactor [8]. The combination of chemical and separation reactions simplifies the process, improves efficiency and enhances conversion and hydrogen yield. According to SER equilibrium (whose reactions are shown in Fig. 1), product gas with a H2 content around 96 v.% dry basis (d.b.) is possible in a wide range of temperatures from 650 to 750 °C when using CaO-based sorbents in a single reaction step [8–12]. Several configurations have been proposed for the process. One of the preferred configurations for scaling up, proposes

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the use of two interconnected fluidized beds operating at atmospheric pressure [8,13]. In this way, the reformer/carbonator reactor would operate at around 650 °C, and the gas stream exiting this reactor would be mainly comprised by H2, steam and low content of unconverted CH4, CO and CO2. To operate the process cyclically, it is required a regeneration step, and a common method to supply the energy required to calcine the sorbent in the calciner reactor is the oxy-combustion of a gas fuel operating at temperatures close to 900 °C [14–16]. The scheme of this process is shown in Fig. 1. An important operation parameter in the process configuration depicted above, besides reforming temperature and pressure, is the steam to carbon ratio (S/C) in the reformer. The S/C ratio has a great influence over product gas composition and, according to equilibrium, high S/C ratios in reforming processes improve methane conversion, promote the CO2 production and decrease CO selectivity due to the reverse water gas shift reaction that is equilibrium limited. In SER, the trend for the gas composition is similar to that shown in SMR but thanks to the reaction of CO2 with CaO, the changes of CO2 concentration are less abrupt at different steam to methane ratios [8,9]. However, from the perspective of the global performance of the H2 production plant in terms of equivalent H2 efficiency and CO2 emissions, a compromise between H2 purity, CH4 conversion in the reformer and energy efficiency of the global process should be reached. In this way, low S/C ratios would be preferable in a self sustained SER plant for H2 production where the steam needs in the reformer are generated thanks to the integration of the high temperature energy exceeding sources of the process within a steam cycle; and the energy required in the calciner to regenerate the sorbent is provided by the oxy-combustion of the off gas of a PSA unit that purifies the H2 rich gas obtained in the reformer unit [16,17]. A critical issue for the development of the SER process is the availability of materials able to sustain the reactions involved in the process, these materials are: the catalyst and the CO2 acceptor sorbent. Regarding to the catalyst selection, it is required that it presents high catalytic activity at a range of temperatures lower than typical temperatures for SMR and it must prove its stability along multiple oxidation/reduction periods as the regeneration in the calciner will take place in an oxidant environment. Finally its reduction kinetics in presence of steam and at low temperature must be reasonable [18]. Traditionally, Ni acting as active phase supported on diverse inert materials has been used in methane reforming because it presents a high activity and it is more economical than other precious metals [3,19]. Several materials such as a-Al2O3, that is commonly used in SMR due to its stability at high temperatures and its low price, have been proposed as inert

support [3,20]. However, the possible formation of NiAl2O4 that is not easily reducible at low temperatures and it is produced by NiO and Al2O3 reaction during the calcination stage, impulse the search for alternative catalysts and supports [21–24]. For example, a NiO/NiAl2O4 catalyst that has been proved to be able to reach gas compositions close to equilibrium in CH4 reforming experiments after being submitted to oxidation/reduction cycles [21]. Other supports that contain alkaline metals as Mg [25–27] have been proposed to operate even at lower temperatures than 500 °C, and a NiO/CaAl4O7 catalyst, whose stability in oxidation/reduction cycles has been also assessed, has been proposed as a reforming catalyst for SER [18]. Concerning to the sorbent selection, its preferred characteristics are a sufficient CO2 carrying capacity stable with the number of carbonation/calcination cycles; durability comparable to the catalysts, to minimize the requirements of purge of the system to renew the spent sorbent; and adequate kinetics for CO2 capture and regeneration. From a thermodynamical point of view, CaO-based sorbents are in principle one of the most suitable CO2 acceptors as they present a good ability to react with CO2 at low partial pressures at moderate temperature and also present fast kinetics [8,14]. Therefore, there is an important amount of works in the literature using natural CaO-based sorbents (limestone or dolomite) on SER, [9,12,25,28,29] as reviewed by Harrison [8]. However their well-known decay of CO2 capture capacity with the number of reaction cycles [30–32] led to an intense work on the development of Ca-based synthetic sorbents [8,33]. In particular, sorbents synthesized doped with Al present a high stability after several carbonation/calcination cycles due to the formation of stable mixed metal oxide compounds as for example mayenite, Ca12Al14O33, that provides a stable framework between the CaO particles and reduces the sintering of active CaO [34–40]. As reviewed by Li et al. [40] the calcium precursor has an important effect on its reactivity, and the utilization of CaO precursors sintering resistant, or nano-scale CaO showed promising results. Different routes have been reported in the literature to incorporate the inert support on CaO particles, [33,40] being the most common methods the sol-gel combustion synthesis, [34–36] sol mixing (one soluble powder with one insoluble powder), [37] and mixing, calcining and pelletization [38]. Following this last method Manovic and Anthony, with the objective to produce an stable CaO-based sorbent, and also attractive from the economical and production point of view, prepared a CaO/Ca12Al14O33 sorbent by physical mixture of natural CaO and calcium aluminate cement as binder obtaining an efficient porous solid that presented satisfactory performance in long multicycle testing [41].

Fig. 1. Simplified Sorption Enhanced Reforming process (SER) scheme.

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Recently, with the objective to reduce the amount of inert material in the SER Ca-looping system that would decrease the volume of the reactors, and also would affect positively to the energy balance of the process, the works that aim the use of a single particle that contains both CaO sorbent and reforming catalyst have increased [42–45]. The concept of the hybrid material could be as shown in Fig. 2, and the intimate contact of catalyst and sorbent in the solid could also improve the kinetics of the SER process. Satrio et al. designed spherical core-shell pellets that consisted of a natural sorbent (lime or dolomite) enclosed in an alumina shell that was loaded with Ni via impregnation methods [42,46]. These materials that contained around 10 wt.% CaO and 6 wt.% Ni were tested satisfactorily in a fixed bed reactor with H2 in the gas stream reaching the equilibrium composition [47]. However the cyclic testing of the materials showed the typical loose in carrying capacity of the natural CO2 sorbent core. Moreover the ratio inert/sorbent/catalyst would not allow for a full-scale design of the process. Martavaltzi and Lemonidou eliminated the alumina support and designed a multifunctional material formed by NiO–CaO–Ca12Al14O33 where the proportion CaO/NiO is a key parameter on the H2 content in the product gas [43]. They showed that 16 wt.% Ni–CaO–Ca12Al14O33 was able to produce a gas stream containing 90 vol.% H2 (d.b.) at 700 °C, S/C ratio 3.4 and around 500 CH4 h 1 spatial velocity. However, although the material acted as an efficient CO2 sorbent under SER conditions, its functionality as catalyst was limited, resulting in a slightly lower CH4 conversion than equilibrium predictions. Chanburanasiri et al. also produced all in one solids, but in this case through incipient wetness technique using natural CaO and hydrotalcite (MK30-K) as both, support and CO2 sorbent [44]. They tested different NiO loads in fixed bed reactor at a steam to methane ratio of 3. The conclusions of their study confirmed that CaO is a good support for Nickel catalyst what eliminates the use of traditional support thereby minimizing the size of reactor. Nevertheless although the catalytic activity of NiO/CaO solid was less than NiO/Al2O3 catalyst, the hydrogen concentration at exhaust gases was relatively high (80 vol.% db) at 600 °C and a CH4 spatial velocity of around 600 h 1. Jong-Nam Kim et al. prepared a NiO–CaO–Ca12Al14O33 combining a precipitation and hydration technique [45]. Again, the formation of Ca12Al14O33 provided stability at the solid as it was reported by this group. They also reported good performance of their material under SER conditions, comparable to the physical mixture of separate solids (catalyst and sorbent). Solid with 7 wt.% Ni showed good behavior at a CH4 spatial velocity of around 1200 h 1, and reached hydrogen concentrations of 95 vol.% (d.b.) in the exhaust gas and good repeatability was achieved after 4 cycles for this hybrid material. Physical mixing has been also proposed as a valid method to prepare hybrid materials, using calcium aluminate cement as a binder to produce CuO–CaO pellets following a similar procedure as for the synthetic sorbents [38,48,49]. Following the physical mixing route, the objective of the present work is to synthesize a new hybrid material containing both the CaO-based CO2 sorbent and the reforming catalyst, using

Fig. 2. Conceptual representation of an hybrid sorbent-catalyst material.

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calcium aluminate cement as a binder and source of mayenite. The stability of the solids in terms of CO2 carrying capacity has been determined in multicycle carbonation/calcination tests. The functionality of these solids under SER conditions as a function of their composition in terms of NiO load and CaO content has been satisfactorily proved. To achieve this objective, the materials have been tested in a micro-reactor under SER conditions, and the effect of S/C ratio on gas composition has been evaluated. The best material has been submitted to consecutive reforming/carbonation and oxidation/calcination cycles to determine the suitability of the materials synthesized in continuous cyclic operation.

2. Experimental CaO derived from the calcination of natural limestone (2 h, at 900 °C, >95% CaO in the calcined material), and powdered NiO (99.99% Sigma Aldrich) were the starting materials for the hybrid solid, that was prepared by physical mixing. Three materials were prepared varying the proportion of CaO/NiO. In all of them, a 10 wt.% of calcium aluminate cement (Electroland) with respect to the CaO content was added. The calcium aluminate cement acted as a binder and a source of Ca12Al14O33 after calcination of the solids, in a similar way as the description given by Manovic et al [41]. The three powdered solids were mixed in a baker with water. The resulting mixture was air-dried for 24 h. Finally, the samples were crushed and sieved (200–400 lm). The solids have been named according to their relative weight percentage of NiO, this is: 18.5NiO (18.5 wt.% NiO, and 74 wt.% CaO); 14NiO (14 wt.% NiO, with a 78.5 wt.% CaO) and 9NiO (9 wt.% NiO, with a 82 wt.% CaO). The materials have been texturally, physically and chemically characterized in their calcined form (at 800 °C in air). The so called ‘fresh’ solids were directly calcined after preparation and then characterized. The ‘used’ solids were calcined after being submitted to SER process. The BET surface areas were determined in a Micromeritics ASAP-2020 by nitrogen adsorption at 196 °C. The density of the solids has been determined by means of a pycnometer AccuPyc II 1340 while Mercury Porosimeter has been applied to determine pore volume with a Quantachrome Pore Master. XRD analysis were conducted in an X-ray diffractometer Bruker D8Advance, using the Cu Ka radiation, to define the crystalline structure and the crystallite size of the solids prepared. Hitachi S-3400 N scanning electron microscopy (SEM) was used to observe the morphologies of the prepared solids (surface morphology and cross-section), the metal distribution was evaluated using energy dispersive X-ray (EDX) analysis. Temperature programmed reduction (TPR) of the fresh materials were carried out in order to determine the reduction temperature of the reducible phases in the solid. The TPR analysis was done in a Micromeritics PulseChemisorb 2700 apparatus heating the sample at 20 °C/min from room temperature to 950 °C in 10% H2 in Ar. The CO2 carrying capacity of the hybrid materials was assessed in an atmospheric TGA apparatus able to operate cyclically. The TGA apparatus consisted of a quartz tube reactor with a platinum basket suspended from it, inside a two-zone furnace. The furnace can be moved up and down by means of a pneumatic piston, and the position of the furnace with respect to the platinum basket allowed the alternation between calcination and carbonation conditions. The temperature and sample weight were continuously recorded on a computer and the reacting gas mixture was regulated by mass flow controllers and fed in through the bottom of the quartz tube. Steam was generated by external electric heating of the water flow controlled by a liquid mass flow controller and then introduced into the reaction atmosphere for some specific tests. Around 5 mg of hybrid material were placed in the platinum

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basket on every test that consisted of 100 reaction cycles. The experimental routine consisted on the calcination of the material at 900 °C in a 70 vol.% CO2 in air for 7 min., and the carbonation of the solid was carried out at 650 °C in a 15 vol.% CO2 in air for 5 min. Also, some specific samples have been submitted to calcination/carbonation tests that included a 15 vol.% of steam during the carbonation stage. The catalytic activity of the hybrid material, as well as its performance under SER conditions, were determined in a fixed bed reactor system described in detail by Garcia-Lario et al. [21] The reactor is a quartz tube, 4 mm of inner diameter, electrically heated by a cylindrical oven. Two thermocouples measure and control the bed temperature. Around 500 mg of the solid sample was introduced in the reactor on each test that resulted in around 40 mm of bed height. CH4, N2 and water were feeding into the system by mass flow controllers being the water previously evaporated at 200 °C. Once the exhaust steam is condensed, the gas products were analyzed by a gas chromatograph (Varian CP-3800). All the experiments were conducted under atmospheric pressure at 650 °C. Previously to the reforming test, the solids were reduced at 650 °C in a flow of 30 vol.% H2/N2 for 1 h. The experiments carried out, whose CH4 spatial velocity and S/C ratios are compiled in Table 1, served to assess the performance of the hybrid material with respect to H2 production via SER, and also to evaluate the catalytic activity of the solids once the sorbent part of the material has been saturated (SMR). Finally, one of the hybrid materials that fulfilled equilibrium compositions, was selected to carry out a multi cyclic test (7 cycles in total) where the solid was alternatively submitted to SER conditions (at 650 °C, S/C 3 and 550 h 1 CH4) to regeneration conditions carried out at 875 °C in air. An intermediate stage of pre-reduction at 650 °C and 10 v.% H2 was carried out between every calcination and SER stage. The experimental results were compared to the equilibrium predictions obtained by means of the process simulator Aspen HysysÒ at similar experimental conditions.

3. Results and discussion The BET surface area of the fresh materials synthesized along with XRD results (crystalline phase and NiO particle size) are summarized in Table 2. As it is expected, the BET surface area of the solids is slightly lower than the BET surface area of the calcined limestone (15 m2/g), however the differences are not significant. The data are consistent with BET areas of hybrid solids reported in the literature prepared through mixing, as for example, the 9 m2/g BET surface reported for solids that contained 25 wt.% NiO, by Martavaltzi et al. [43]. And, they are even higher than those prepared through impregnation, as for example Chanburanasari et al. who obtained a BET area of 4 m2/g for a 12.5 wt.% Ni load [44]. According to the experimental results, the BET area does not follow a significant diminishing trend when the Ni content is increased, in contrast with data reported in the literature [43]. The crystalline phases of fresh materials obtained by XRD analysis denote that no mixed phases are found. This indicates that CaO and NiO do not interact, and although nickel might react to form NiAl2O4, the presence of CaO avoids this reaction [50,51]. Therefore, only CaO and NiO were detected along with small amount Table 1 Experimental conditions for the SER and SMR experiments. wt.% NiO

CH4 (h

9 14 18.5 18.5

1200 1200 1200 550

1

)

SER

SMR

Comment

S/C = 3 S/C = 3, 1.5 S/C = 3, 1.5 S/C = 3

S/C = 3 S/C = 3, 1.5 S/C = 3, 1.5 S/C = 3

7 cycles

of Ca12Al14O33 due to the addition of cement aluminates, in concordance with results published in the literature [38] and a negligible amount of Ca(OH)2 that was formed after the calcination due to the atmospheric moisture. Regarding to the NiO crystallite size, that depends on several parameters such as nickel amount, catalyst support, and preparation method or reduction process [52,53] and that will affect the catalyst activity and resistance to coke deposition [52], its average value is in concordance to other hybrid solids reported in the literature (for example, 37.7 nm for 25 wt.% NiO hybrid materials prepared by Martavaltzi et al. [43]) and no relation was found between the nickel load and NiO particle size. However, as it was expected, a small increment in NiO crystal size was found after the solid was cycled between reduction environment to oxidant conditions during the sorbent regeneration stage (see Tables 2 and 3 for comparison). The results from TPR for the three fresh solids with different NiO content showed a single peak at about 400 °C with a hump nearly 500 °C that corresponds to free NiO [53,54]. It was not identified a hump at higher temperatures that would have indicated that part of the NiO could be buried in the CaO structure [50]. These results are in agreement with XRD patterns which showed that the only reducible phase in the hybrid material was NiO. Table 3 shows the characterization of the so called 18.5NiO solid after one reaction cycle (including SER stage and calcination stages) and after five reaction cycles. According to XRD analysis, besides CaO and NiO, the presence of Ca12Al14O33 was also confirmed. As it can be seen BET surface area does not suffer an important decrease with the number of reaction cycles and similar results have been found with respect solid porosity that is around 50% after 5 reaction cycles. The density of the solid decreases slightly as long as the number of reaction cycles increases and in spite of the NiO crystal size that slightly increases with the number of the cycles as is expected, there are no signals of important sintering phenomena. This has been corroborated through SEM analysis of the superficial and cross section of the solids, some pictures corresponding to the 18.5NiO material have been included as examples in Fig. 3. The surface image of the fresh material shows a good nickel distribution (Fig. 3a), but the cross section proofs three different morphological phases (Fig. 3b): phase 1 rich in Ca, phase 2 rich in Ni and phase 3 mixture of Ca and Ni. Al presence was determined in all particles. The formation of different phases in hybrid solids has been also detected by other authors [43] and, the different phase distribution is an expected result due to the low Ca–Ni interaction [53,55]. Slight sintering was found on superficial images (Fig. 3c) of the cycled material. Its cross section pictures revealed a porous material with the nickel inside the particle being accessible to the reacting gas through the channels along the particle (Fig. 3d). To determine the effect that the NiO content has on the CO2 carrying capacity of the hybrid solids, the materials have been submitted to calcination/carbonation cycles that include a demanding calcination stage carried out at high temperature and in a concentrated CO2 stream (900 °C and 70 v.% CO2), and a short carbonation time (at 650 °C, 15 v.% CO2 for 5 min). The results shown in Fig. 4a are represented in terms of mass fraction conversion referred to the total CaO present in the solid material, although part of it will be in the mayenite formed after the first calcination. The results obtained have been compared with the CO2 carrying capacity that presents the parent limestone source of CaO for the hybrid materials. As it can be seen in Fig. 4a, the materials still present the typical decay in sorption capacity with the number of calcination/ carbonation cycles that has been widely described in the literature as typical from natural CaO-based sorbent [8,30,31]. However, after the initial rapid decay, the CO2 carrying capacity of the solids tends to stabilize and remains at 12 wt.% with respect to the total mass of CaO in the solid after 100 cycles (9 wt.% with respect to

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A.L. García-Lario et al. / Chemical Engineering Journal 264 (2015) 697–705 Table 2 Fresh materials characterization.

a

Solid

NiO (wt.%)

SBET (m2 g

Fresh

9 14 18.5

12 ± 1.7 13 ± 1.8 14 ± 1.9

1

)

NiO dp (nm)

Crystalline phase

36 38 26

NiO, CaO, Ca12Al14O33a, Ca(OH)2a NiO, CaO, Ca12Al14O33a, Ca(OH)2a NiO, CaO, Ca12Al14O33a, Ca(OH)2a

Lesser extent.

Table 3 Characterization of 18.5NiO after reaction cycles. Solid

e (%)

SBET (m2 g

1st cycle 5th cycle

51.6 50.3

17.1 13.2

Surface

1

)

Cross-section

(a)

(b)

(c)

(d)

Fresh

5 Cycle

Fig. 3. SEM surface and cross-section images of the 18.5NiO material, fresh and used after 5th cycle.

the total mass of the hybrid material). As it can be seen the carrying capacity of the synthetic solids is highly stable after around 20 reaction cycles, and it almost doubles the capacity of the natural lime. It suggests that the Ca12Al14O33 formed contributes to maintain the porous structure of the material, in line with the results from Hg porosimetry and the BET surface area that do not diminishes significantly after 5 reaction cycles. The NiO content in the

q (g/cm3)

NiO (dp) nm

Crystalline phase

3.2 2.6

38.5 39.5

NiO, CaO, Ca12Al14O33 NiO, CaO, Ca12Al14O33

hybrid material did not have important influence on the stability, or the CO2 carrying capacity of the materials. This is in line with the results presented by Martavaltzi and Lemonidou [43] that found an optimum for a material with a 20 wt.% NiO, but with minor differences among the materials that they developed (NiO contents varied from 10 to 25 wt.%). On view of these results, the 18.5NiO material was selected for an additional calcination/carbonation test that included steam in the carbonation reaction atmosphere (15 vol.% steam, 15 vol.% CO2 in air, 650 °C and 5 min reaction time). Fig. 4b shows how in these conditions, that could be representative of a SER process operated in continuous mode in a dual fluidized bed system. The residual capacity of the material is stabilized around 20 wt.% with respect to the total CaO in the solid (15 wt.% with respect to the total mass of the hybrid material). Although it is not possible to compare directly the CO2 carrying capacity of the materials tested with results published in the literature, as carbonation conditions in terms of carbonation time and/or [56–58] CO2 partial pressure [59] and also the calcination conditions have a very important impact on sorbent CO2 residual carrying capacity, it is important to highlight the stability of the materials tested at a high number of cycles that is in agreement with the results reported by solid sorbents prepared by physical mixing [37,38,43,60], and also by sorbents prepared by precipitation and hydration [45]. To evaluate the performance of the synthesized solids as function of their NiO content with respect to the SER process, and also their catalytic activity in SMR (once the active CaO in the solid has reacted to form CaCO3 and the reaction takes place only thanks to the catalytic part of the solid), several reforming tests have been carried out, whose experimental conditions are compiled in Table 1.

Fig. 4. (a) Evolution of CO2 carrying capacity, expressed in wt.% CaO conversion, of the hybrid materials and natural CaO source. Calcination 900 °C, 70 vol.% CO2 in air, carbonation at 650 °C, 15 vol.% CO2 in air. (b) Evolution of CO2 carrying capacity of the 18.5NiO material. Calcination under identical conditions as in figure (a), carbonation carried out at 650 °C, 15 vol.% CO2, 15 vol.% H2O in air.

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In first place, the effect of the material NiO content, on product gas composition at 650 °C, CH4 space velocity of 1200 h 1 and for a given steam to carbon ratio of 3 has been assessed. Fig. 5 shows an example of the experimental results obtained in the micro fixed-bed reactor, in terms of evolution of the product gas composition with time for the 18.5NiO material. In this figure, the continuous lines represent the experimental data, the long dash lines symbolize SER equilibrium according to HysysÒ calculus referred to experimental conditions, and dotted lines indicate the SMR equilibrium also according to HysysÒ calculations. As it can be seen in Fig. 5, the experimental curves present the three typical stages widely described in the literature for SER experiments (see for example the review by Harrison) [8,9,12]. These are: the prebreakthrough period (where reforming, shift and carbonation reactions take place at the same time), breakthrough (the sorbent starts to be saturated and CO2 removal starts to decrease until the sorbent is totally saturated) and post-breakthrough (the sorbent is saturated so CO2 sorption is no longer effective and only Steam Methane Reforming takes place). As it can be seen in Fig. 5, in this experiment H2 concentration reaches the 94 vol.% (d.b.), in close agreement with the equilibrium predictions. This composition is typical of a pre-break-through period where the CaO is reacting with CO2 and shifting the equilibriums toward H2 production. Low concentration of CO, CO2 and CH4 are measured during this period. CH4 conversion is approximately 98% and the productivity performance is around 0.15 kgH2/kghybrid solid
hour. These values are consistent with data of hybrid solids reported in literature. As instance Kim et al. [45] get a gas composition close to equilibrium concentration values reaching a productivity performance of around 0.13 kgH2/kghybrid solid hour while Martavaltzi et all [43] achieve a productivity performance of around 0.041 kgH2/kghybrid solid
hour and also a lower methane conversion was obtained (around 80%). After this initial stage, H2 concentration follows a gradual drop until it is stabilized in the so called post-breakthrough period at around 80 vol.% (d.b.), while CO2 concentration increases up to 11 vol.% At this point, the carbonation is not effective as the sorbent part of the hybrid solid is saturated (similar trend to CO2 is followed by CO as expected) and gas composition corresponds to that predicted by the SMR equilibrium. The gas composition remains stable, and the catalytic part of the hybrid material is able to maintain its reforming activity. Similar experimental curves (with the three reaction periods) are obtained for the 14NiO and the 9NiO hybrid materials at a ratio S/C = 3 and a space velocity of 1200 h 1 CH4. Fig. 6 resumes the experimental results in terms of H2 vol.% concentration in d.b., for the pre-breakthrough period (SER conditions) and the post-breakthrough (SMR

Fig. 5. Evolution of gas composition with time for the experiment carried out with the 18.5NiO material, at 650 °C, S/C = 3 and 1200 h 1 CH4 spatial velocity.

conditions). The experimental values are compared with the equilibrium predictions. It can be seen that the 14NiO material fulfills the equilibrium predictions producing a 93.5 vol.% H2 d.b. gas product, and presenting a CH4 conversion around 98%. The material with the lower metal load (9NiO material), presented a CH4 conversion around 75%, that resulted in a 90 vol.% H2 (d.b.) in the gas product stream, that is slightly lower than equilibrium predictions. The activity of the 9NiO material once the sorbent part has been saturated is also limited, and the H2 concentration in the gas stream in the so called post-breakthrough period is sensibly lower than that predicted by the equilibrium (65 v.% H2 d.b. with respect to the 76 v.% d.b. predicted by the equilibrium) and does not remain stable in the experiment. It is clear that there is a minimum amount of NiO required in the solid to maintain its activity either in SER or SMR. This result is in line with studies compiled in the literature, where lower CH4 conversion at lower Ni loads was also found for other authors in hybrid solids prepared by different synthesis routes. For example, Martavaltzi and Lemonidou [43] found that increasing the nickel load in the solid leads to increased the conversion at both pre-breakthrough and post-breakthrough, reaching a maximum peak when the NiO content in the solid was 20 wt.%, at 650 °C and S/C = 3.4. Under these conditions, they obtained a 90 vol.% H2 gas stream, which is slightly below equilibrium predictions. Chanburanasiri et al. [44] prepared hybrid solids by incipient wetness technique and studied at 600 °C and S/C = 3 the effect of changes in nickel loads over the solid. These authors also found that it is necessary a minimum load on nickel in the hybrid solid to be active enough as reforming catalyst. Radfarnia and Illuta [61], varied the NiO content in their materials between 12 and 25 NiO wt.% in their solids, being the material with the highest load the material that had better performance. Kim et al. [45] synthesized solids with a Ni content between 3 and 10 wt.%, reaching the best performance close to equilibrium predictions (at 650 °C, S/C 3.1, 1000 h 1 CH4) with a 7 wt.% Ni material. Once the 9NiO has been discarded due to its slightly low performance at the most favorable reaction conditions tested in this work, the behavior of the 14NiO and 18.5NiO materials at a lower S/C ratio = 1.5 (at 650 °C and 1200 h 1 CH4) under SER conditions has been evaluated. The results in terms of H production, that are compiled in Fig. 6, show that the gas composition in both experiments fulfills the equilibrium predictions for the pre-breakthrough period achieving both solids a gas product with around 84 vol.%. H2 (d.b.) while methane conversion values for this experiments (66% and 69.6% respectively) that are close to the predicted by the equilibrium (72%). Once the sorbent part of the solid is in form of CaCO3 and the capture of CO2 is not longer efficient, the 14NiO and 18.5NiO materials behave different. Although the 18.5NiO is able to reach the reforming equilibrium for the SMR with a H2 production of 71 vol.% on d.b., the gas composition produced by the 14NiO material is well below the equilibrium predictions with only a 61 vol.% H2 in the product gas stream. It is well known that CO2 removal makes possible to use less active catalyst [11] as the carbonation reaction shifts the equilibriums toward H2 formation, improving the performance of the catalyst promoting its activity at SER step at less favorable conditions for the catalyst (lower steam to methane ratio). The possibility to work at a very low steam to methane ratio (S/C = 1.5) at usual nickel content (14–18.5 wt.% NiO) is a novel, and a very interesting result for hybrid materials, as the results reported in the literature comprise experimental conditions with S/C ratios between 3 and 5 [43,45,61]. A material able to operate satisfactorily at this low S/C ratio would allow minimizing the energy penalty of the process that is mainly associated to the steam consumption either in the reformer, or in the calciner. Also, a sorbent-catalyst able to work at such as low S/C values would allow for the design of an energy self-sustained SER plant where

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Fig. 6. H2 gas product content in d.b. under Sorption Enhanced Reforming conditions (SER), and Steam Methane Reforming (SMR), for the experiments carried out with the 9, 14, 18.5NiO materials and at S/C ratios of 1.5 and 3. Reaction temperature 650 °C, and 1200 h 1 CH4. Discontinuous lines represent equilibrium composition.

the off-gas from the H2 purification unit is used to provide the energy necessary in the calciner reactor [16,17]. With respect to the solids performance once the sorbent part of the hybrid material is no longer active for CO2 capture, as it can be seen also in Fig. 6, the amount of Ni is critical for the activity of the catalyst. As it has been confirmed experimentally, high S/C favors the reforming reaction for both the SER and SMR conditions. In this way, for an S/C = 3, equilibrium compositions are achieved with the 18.5NiO regardless the activity of the sorbent part of the solid. When reducing the ratio S/C down to 1.5, it can be seen that the 18.5NiO hybrid material is able to produce a gas stream that fulfills the thermodynamical equilibrium at the SMR conditions maintaining the steady state for more than one hour. It has been experimentally observed that catalyst deactivation depends strongly on the NiO load of the solid for different S/C values. While the 18.5NiO solid is able to act not only as SER catalyst but also as a SMR catalyst at an S/C as low as 1.5, the 14.5NiO material does not achieve equilibrium compositions for S/C = 3 and lower S/C rates, being the catalyst part of the solid not able to convert CH4 as would be marked by the thermodynamics. However, as described above, the presence of active CaO during the SER period is able to push up the activity of the hybrid material and the H2 composition at S/C = 1.5 corresponds to that predicted by the thermodynamics for 14.5NiO solid. Finally once it has been proved the suitable performance of the hybrid materials synthesised with respect to H2 production in a SER process, a cyclic experiment has been carried out with the 18.5NiO solid. The experiment comprised a SER stage at 550 h 1 CH4, at S/C = 3 and 650 °C. Once the sorbent was saturated the stability of the material under SMR was also evaluated. The calcination/regeneration of the material was carried out in air at 875 °C, under these conditions the CaCO3 will be decomposed to CaO and CO2 and the catalytic part of the solid will be oxidized to NiO. To fulfill cyclic operation, a NiO pre-reduction step in 10 vol.% H2 at 650 °C was carried out previously to the following SER stage. The experimental information extracted from this SER/calcination-oxidation cycles will be critical to assess the performance of the hybrid material. While the behavior of the CO2 sorbent has been widely analyzed as function of reaction (SER and calcination) conditions, [8] there are very scarce data on the behavior of the reforming catalyst after being exposed cyclically to reducing/oxidizing environments [18,21]. In fact, the results presented in the literature that include the cyclic testing of hybrid catalyst-sorbent materials are regenerated either under reducing conditions as Albrecht et al. that kept the same reaction atmosphere for the SER and the regeneration stages and increased the reaction temperature up to 850 °C to carry out the calcination

of the CaCO3 formed; or under inert atmosphere and at a relatively low temperature [45,61]. Up to seven cycles were tested with the 18.5NiO material, and the results in terms of H2 and CO2 content in gas product are shown in Fig. 7. As it can be seen, as a consequence of the cyclic operation alternating between reforming/carbonation and regeneration/calcination, the length of the pre-breakthrough period slightly diminishes with increasing the number of reaction cycles. This indicates that the sorbent part of the material still has not reached its residual CO2 carrying capacity. At this point it is necessary to clarify however, that due to gas analysis limitations, the compositions plotted in Fig. 7 are obtained after around 45 min of reforming reaction, therefore, the decrease in CO2 capture capacity is relatively low (a good indicative of the stability of the sorbent). With respect to the composition of the gas phase, the differences among the reaction cycles are not representative and in every case H2 is over 90 v.% (d.b.). During this period, the productivity performance is around 0.08 kgH2/kghybrid solid
hour. Once the CO2 capture is not efficient, and the gas composition tends to the typical composition of SMR the differences along cycles increase. For this step in every case, the gas does not fulfill equilibrium composition, and there is a noticeable deactivation of the catalyst as long as the number of cycles increases. The slight increase in NiO crystal size that has been determined for the 18.5NiO trough XRD revealed a slight sintering of the solid that must be responsible of the poorer performance of the catalytic part of the material as long as reaction cycles increases. However, and in a

Fig. 7. Evolution of H2, and CO2 content in the gas product stream with time and number of reaction cycles. Reaction conditions: 650 °C, S/C = 3 and 550 h 1 CH4. The discontinuous lines represent equilibrium composition.

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similar trend as found for the experiments carried at lower S/C, the presence of CaO in the material allows reaching H2 production close to equilibrium values in SER period. 4. Conclusions

[15]

[16]

In this work hybrid materials containing the CO2 sorbent and reforming catalyst have been synthesised through physical mixing of CaO, powdered NiO and calcium aluminate cement that acted as a binder and a source of Ca12Al14O33. The materials showed a CO2 carrying capacity of around 20 wt.% of calcined sorbent after being submitted to 80 carbonation/calcination cycles (calcining conditions 900 °C and 70 v.% CO2). The effect of NiO content, and S/C ratio on H2 yield was analyzed. As it was shown in the experimental results, the 14NiO and 18.5NiO materials were able to reach equilibrium compositions for a range of S/C between 1.5 and 3 at a CH4 spatial velocity of 1200 h 1. Finally, the 18.5NiO material has been successfully tested cyclically alternating from SER conditions to regeneration (oxidation conditions), and it was able to reach H2 close to equilibrium compositions (over 90 v.% H2 d.b.) showing a low deactivation with increasing the reaction cycles.

[17]

Acknowledgments

[25]

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This work acknowledges the support by the R+D Spanish National Program from the Spanish Ministry of Economy and Competitiveness under project ENE2012-37936-C02-01, the European Regional Development Fund (FEDER program) and the Regional Aragon Government (DGA) under the research groups support program. The research leading to these results has also received funding from the European Union Seventh Framework Programme FP7 under Grant agreement n° 608512 ASCENT Project. A.L. GarcíaLario acknowledges the FPI fellowship (project ENE 2009-11353, BES-2010-032636 financed by Spanish Ministry of Science and Innovation – MICINN).

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