Kinetic modeling of NiO-based oxygen carriers for the sorption enhanced chemical looping steam CH4 reforming

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Kinetic modeling of NiO-based oxygen carriers for the sorption enhanced chemical looping steam CH4 reforming D. Ipsakisa, E. Heracleousb, L. Silvesterc, D. B. Bukurc,d, Α. Α. Lemonidoua,* a

Department of Chemical Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece School of Science & Technology, International Hellenic University (IHU), 57001 Thessaloniki, Greece c Texas A&M University at Qatar, Chemical Engineering Program, Education City 23874, Doha, Qatar d Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, USA b

Abstract The reduction and oxidation (redox) kinetics of four NiO-based oxygen carriers (OCs) under chemical looping conditions are presented and discussed in this study. The 40% NiO OCs were supported on Al2O3, TiO2, SiO2 and ZrO2 and were isothermally tested in a thermogravimetric unit connected with a mass spectrometer (TGA-MS) in 20 redox cycles (reduction under 15% CH4/He at 650oC/oxidation under air at 800oC). Several solid-state kinetic models (chemical reaction controlled, geometrical/volumetric contraction, diffusion limited, nucleation growth and random pore models) were consecutively and optimally screened for each OC and for different redox cycles. As was found, metal-support interactions occurring between NiO and Al2O3, TiO2 supports lead to a reduction controlled by chemical reaction and was quantitatively captured through the “Unreacted Shrinking Core Model”. On the contrary, reduction of NiO supported on SiO2, TiO2 is controlled by “nucleation and nuclei growth” and Avrami-Erofeev kinetics fitted the respective experimental data. Regarding oxidation, the present study revealed that it is controlled by nucleation and “Avrami-Erofeev” (NiO/Al2O3, NiO/TiO2) and “Prout-Tompkins” (NiO/ZrO2) kinetic models provided an optimal fitting. In all studied cases, the progressing of cycles revealed structural material modification that were summed up to significant variations in OCs’ redox rates and related kinetics. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 11th Panhellenic Scientific Conference on Chemical Engineering. Keywords: NiO-based oxygen carriers; redox solid-state kinetics; chemical looping reforming;

*Corresponding author. Tel.: +30-2310-996273; fax: +30-2310-996184. E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 11th Panhellenic Scientific Conference on Chemical Engineering.

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Nomenclature f(X) k RD/OD : T t Wox Wred Wt X

Empirical function that correlates with the solid-state kinetics Arrhenius kinetic parameter, min-1 Reduction/Oxidation degree, % Reduction/oxidation temperature, K Time, min Oxygen carrier weight at the oxidized (or initial) state in each cycle (NiO), mg Oxygen carrier weight at theoretical fully reduced state (Ni0), mg Oxygen carrier weight at time t, mg Solid conversion (or equivalently the reduction/oxidation degree)

1. Introduction Cornerstone on novel and promising technologies featuring hydrocarbons processing towards power production is H2 that is prevalently produced through steam methane reforming (SMR). Although mature and applied in industry, SMR is both energy and operationally demanding since it requires i) a continuous heat supply in order to sustain the endothermic reforming reactions at 850oC (further contributing to CO2 emissions due to CH4 combustion) and ii) extensive downstream processing due to the need to eliminate the undesired CO/CO2 [1,2]. Chemical Looping Reforming (CLR) is an intensified and compact process that follows similar operating principles with Chemical Looping Combustion (CLC) and effectively alleviates the SMR disadvantages previously mentioned [3]. Further intensification of the process can also be achieved with the use of a CaO-based sorbent for the in-situ CO2-capture that can provide an even high H2 purity with low energy demands (Sorption Enhanced Chemical Looping Reforming, SE-CLR) as shown in earlier studies of our research group [4,5]. Figure 1 describes the basic operating scheme of the proposed concept. Methane and steam are co-fed to the reformer that operates at significantly lower temperatures than the conventional process. A MexOy based OC is reduced by CH4, producing CO and H2 (main reaction pathway) and to a smaller extent CO2 and H2O due to the lower NiO/CH4 ratio as compared to CLC process [3]. The reduced OC (in the form of MexOy-1) catalyzes the reforming of CH4, forming additional H2, CO and CO2 based on a series of reactions (table A1 at the appendix with MexOy shown as NiO). The CaO-based sorbent captures CO2 as it is produced and drives equilibrium towards high H2 production, while providing significant heat to sustain the endothermic reforming reactions. In the second reactor, the regeneration of the reduced metal and the formed CaCO3 takes place along with residual C removal. There, MexOy-1 oxidation provides the necessary heat to sustain the endothermic CaO regeneration [4]. An important research area that complements to the above novel process is the evaluation of kinetic models for both reduction and oxidation stages regarding CLR concept (without the use of the sorbent material). On this behalf, current state-of-the-art focuses on solid-state models that can effectively capture gas-solid interactions at both reactors. Syed-Hassan and Li studied the reduction of NiO by H2 and CH4 (supported in SiO2 and unsupported) and concluded that the selection of support can significantly affect reduction proceeding [6]. Son and Kim [7], evaluated a series of solid-state models for the redox kinetics of Ni-, Fe- and Ni-Fe- based OCs, and showed that the “Unreacted Shrinking Core Model” can be applied under acceptable fitting. On a more systematic way, Bollas et al.[8,9] delved greatly on the reduction of NiO under different supports (α-Al2O3, γ-Al2O3, MgAl2O4, YSZ etc.). As they reported, the systematic evaluation of several kinetic models and the final selection of the most optimal should also encompass the OC physicochemical features that can be qualitatively reviewed only through the evaluation of several redox cycles. On this behalf, the present study evaluates the redox kinetics of four OCs (40% NiO supported on Al2O3, TiO2, SiO2 and ZrO2) in different cycles. The ultimate selection of kinetic models for both stages is based (among others), also on the inherent physicochemical features of the OCs (e.g. metal-support interactions). Several kinetic models are screened, providing a wide view of the continuous redox operation under chemical looping conditions.

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Depl. Air+CO 2

H2 Mex Oy-1 (s) T=650CC REFORMER REACTOR

T=850CC

CaCO3 (s)

REGEN. V REACTOR

MexOy (s) CaO (s)

CH4

H2 O

Air

Fig. 1. Sorption enhanced chemical looping steam methane reforming concept

2. Kinetic modeling methodology Figure 2a shows the kinetic modeling methodology that was followed in this study. The first step in this methodology refers to the evaluation and selection of experimental data (OC weight from TGA and H2, CO, CO2 and CH4 from MS signals), derived during the performed isothermal redox experiments. These results are fed to a computational platform as input data and the screening of several solid-state kinetic models is applied. Optimal kinetic models (along with their respective parameters) is provided to a parallel step where the overall evaluation is based on statistical data (e.g. minimization of error) and on inherent OC physicochemical characteristics (qualitative analysis). 2.1. Experimental methodology The four NiO-based OCs were prepared by wet impregnation of commercial γ-Al2O3, SiO2, TiO2 and ZrO2 supports (Saint Gobain – NORPRO) with aqueous solution of nickel nitrate. The mixture was heated slowly in a rotary vapor apparatus (under vacuum) until evaporation of the solvent and the solids were dried overnight at 100°C before calcined at 650°C for 4h in air flow. More information on material preparation can be derived from a recent study of this group [10]. All materials were tested for their redox characteristics in a thermobalance unit (SDT Q600, TA Instruments) and Fig. 2b presents the variations of the OC weight through time. Initially, 10mg of the respective OC (dp<300μm) are placed on a tin-plate and temperature is gradually increased at 650oC (15oC/min under He).

(a)

(b)

105

He

15% CH4/He

Air o

o

o

800 c

800 c

650 c

Weight (%)

100

95

90

85

0

10

20

30

40

50

60

70

80

90 100

Time, min Fig. 2. a) Kinetic methodology applied in OTM redox evaluation, b) typical weight profile during reduction and oxidation stages

Next, 15% CH4/He are introduced and reduction takes place for 20min. During this stage, the OC weight is decreased due to [O] removal until a minimum weight point (end of reduction). At that point, solid carbon deposition due to CH4 decomposition (catalyzed by metallic Ni) takes place and the OC weight is increased. At the

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end of reduction, temperature is increased at 800oC (10oC/min under He flushing) and air is introduced to the system for 20min. During oxidation, carbon is completely removed and simultaneously Ni is re-oxidized to its initial stage. Figure 3 shows the overall results for the reduction and oxidation degree of all OCs. As can be seen, the reduction of NiO/SiO2, NiO/TiO2 and NiO/ZrO2 is nearly complete (95-100%) and stable in all cycles, whereas the reduction degree of NiO/Al2O3 is increasing with the number of cycles, where it remains stable after cycle 12th. This gradual increase was attributed to the formation of NiAl2O4, which has a low reducibility. The latter was identified through XRD measurements of the fresh and used samples as recently discussed in [10, 11]. Regarding the OCs’ oxidation degree, the simultaneous and complete metal oxidation and carbon removal was reported for all OCs, and materials regained their initial weight without any modification. More information on these experiments can be found in related studies [10,11], where physicochemical characterization was deeply discussed.

(a)

(b)

110

Degree of reduction (%)

100 90 80 70 60

Ni-Al Ni-Si Ni-Ti Ni-Zr

50 40

0

2

4

6

8

10

12

14

16

18

20

22

Cycle #

Fig. 3. a) Reduction and b) oxidation degree of the four OTMs under 20 consecutive redox cycles

2.2. Mathematical kinetic modeling Based on the abovementioned analysis, the proposed mathematical modeling framework aims to the utilization of the recorded experimental data towards OC kinetic evaluation. The reduction and oxidation degree (RD/OD) of each OC is calculated according to their weight data and provided through equations (1) and (2). On the same trend, the reduction/oxidation rate that is related with the OC conversion profile is shown in equation (3) and depends on the Arrhenius kinetic parameter (k) and on an empirical function (f(X)) that solely provides insights on the related redox kinetic mechanism [11]: RD (t ), % 

Wox  Wt  100 Wox  Wred

OD (t ), % 

Wt  Wred  100 Wox  Wred

dX  k (T )  f ( X ) dt

(1)

(2)

(3)

whereas Wox denotes the weight of the OC at the fully oxidized state (or equivalently the initial state at each cycle) in mg, Wred the weight of the OC at the theoretical fully reduced state (Ni0) in mg, Wt the weight of the OC at the time step t in mg, X the solid’s conversion (i.e. the OC’s reduction/oxidation degree), t the time in min, k the Arrhenius parameter in min-1, T denotes the reduction/oxidation temperature in K and f(X) the empirical function that correlates the solid-state kinetics (usually includes between 0-2 unknown parameters).

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The optimal fitting of the reduction/oxidation degree was ensured by the minimization of the squared error between experimental and simulated data that led to the estimation of the Arrhenius parameter and the respective f(X) function. The latter was screened through a library of available kinetic models from literature (random pore models, nucleation and nuclei growth models, diffusion models, chemical reaction models, shrinking core models) that were presented and discussed in detail in [11]. It is noted that prior kinetic modeling, the absence of external and internal mass transfer was ensured by the Mears and Weisz-Prater criteria respectively. 3. Results & analysis Figure 4 presents the RD profile of the four materials for 4 selected cycles Experimental data are presented with the open points while the modeling results by the continuous lines. Similarly, Fig.5 shows the respective OD profiles for all materials except NiO/SiO2 at 3 different cycles. It is noted that during oxidation deposited carbon was simultaneously removed thus OD is in a sense “masked” during weight transition data. The latter is the reason for not including NiO/SiO2 oxidation profile on the present analysis. 3.1. Reduction stage NiO/Αl2O3: As can be seen in Fig.4a, cycles 2 and 5 are characterized by low RDs due to the presence of the hardly reducible NiAl2O4. Gradually, RD increases and stabilizes after cycle 12 at values around 85%. Simultaneously, a decrease in the reduction time with cycles is recorded, from 1.65-1.80min at ~1min, which is quantitatively captured through the increase of Arrhenius kinetic parameter (table 1). The kinetic modeling evaluation concluded to the “Unreacted Shrinking Core Model” for all related cycles. An insignificant deviation that appears at the end of reduction was attributed to a possible diffusion-limited reduction only at the first cycles, which disappears at latter ones [11]. NiO/ZrO2: In all cycles, a steady RD at ~95% is recorded with an average reduction time of 1.2-1.3min. The analysis of the reduction profile further revealed that two different kinetic areas are observed. One fast reduction area up to RDs ~ 30-40% at times~0.40 min (stage a) and a slow area that is prolonged till the end of reduction. The first stage of reduction was found to be controlled by the “nucleation and nuclei growth rate” and described via the 3rd order of Avrami-Erofeev kinetics. As nucleis of Ni0 increase in size and volume, it seems that CH4 is diffusionlimited towards reaching the available and free NiO-sites and hence a change of the reduction mechanism is possible. Though, further material characterization could be required in order to verify this assumption. NiO/TiO2: On this material, NiO reduction was found to take place simultaneously with C deposition (due to CH4 decomposition) after an RD~60%. Hence, while the reduction was complete as shown in Fig.3a, the actual reduction profile was masked by C deposition and extracted up to that point only. Overall, no significant changes were observed through cycles and reduction proceeds under a similar rate at all cycles. Reduction kinetics were evaluated through the “Unreacted Shrinking Core Model” as in the case of NiO/Al2O3. In a previous study [10], it was shown that NiTiO3 was formed due to metal-support interaction, and indeed the two materials seem to conform on the kinetic mechanism evaluation of this study. NiO/SiO2: The reduction rate of this material was gradually increased and completed at <1.2min after cycle 10th. As in the case of NiO/ZrO2, no metal-support interaction was observed, with the reduction kinetics to follow a 4th order Avrami-Erofeev modeling approach. The different order (4th instead of 3rd in the case of NiO/ZrO2) is qualitatively depicted on the slower rate during the onset of reduction (nucleation period). After a certain point, nuclei growth rate is increased and forces reduction to be completed at rather short time (k Arrhenius parameter was increased with cycles as shown in table 1).

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3.2. Oxidation stage NiO/Αl2O3: The oxidation of this material was completed at times between 1.5–2.0min for all cycles (quite fast as compared to the other OCs). Contrary with the case of reduction, the oxidation rate was decreased with cycles and this variation was depicted on the respective change to the kinetic Arrhenius parameter (table 1). A significant point that is observed at all materials is the fact that oxidation proceeds in two stages; one fast one till ODs ~90% and a slow one (controlled by diffusion) till the end of oxidation. A 4th order Avrami-Erofeev kinetic model was found to optimally describe the oxidation profile and the deviation at higher ODs is only related to diffusion limitations. NiO/ZrO2: As in the case of reduction, the oxidation degree profiles are similar in all cycles, confirming the stability of this material. Again, a fast and a slow stage are observed and the overall profile was fitted via the «ProutTompkins» kinetic model. As shown in table 1, the increase of the kinetic parameter with cycles denotes the increase of the oxidation rate in contrast with the case of NiO/Al2O3. NiO/TiO2: In contrast with the other materials, the initial (t=0) ODs are in the range of <5% denoting the nonparallel Ni oxidation and C removal. In all cycles, a similar oxidation rate was reported (captured through the similar k parameter value with cycles) and the 2nd order Avrami-Erofeev was optimally fitted. The lower order is depicted on the higher oxidation rate of this material over NiO/Al2O3 (4th order Avrami-Erofeev model). 1.0

1.0

0.8

Reduction Degree

Reduction Degree

Cycle # 2

0.6 0.4

NiO-SiO2 NiO-ZrO2 NiO-Al2O3

0.2

NiO-TiO2

0.0 0.0

0.4

0.8

1.2

1.6

0.8 0.6 0.4

NiO-SiO2 NiO-ZrO2

0.2

NiO-Al2O3 NiO-TiO2

0.0 0.0

2.0

Cycle # 5

0.4

Time, min

1.2

1.6

2.0

1.0

1.0

Cycle # 10

Cycle # 20

0.8

Reduction Degree

Reduction Degree

0.8

Time, min

0.6 0.4

NiO-SiO2 NiO-ZrO2 NiO-Al2O3

0.2 0.0 0.0

NiO-TiO2

0.4

0.8

1.2

Time, min

1.6

2.0

NiO-SiO2 NiO-ZrO2

0.8

NiO-Al2O3 NiO-TiO2

0.6 0.4 0.2 0.0 0.0

0.4

0.8

1.2

1.6

Time, min

Fig. 4. Reduction degree profiles for NiO supported in Al2O3, ZrO2, TiO2 and SiO2 at 4 different cycles

2.0

Ipsakis et al. / Materials Today: Proceedings 5 (2018) 27353–27361

NiO-ZrO2

0.6

NiO-Al2O3 NiO-TiO2

0.4 0.2

Cycle # 5 0.0

1.0

1.0

0.8

0.8

0.6

1

2

3

4

5

NiO-ZrO2 NiO-Al2O3 NiO-TiO2

0.4 0.2

Cycle # 10 0.0

0

Oxidation Degree

0.8

Oxidation Degree

Oxidation Degree

1.0

0

1

2

3

Time, min

Time, min

4

5

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NiO-ZrO2

0.6

NiO-Al2O3 NiO-TiO2

0.4 0.2

Cycle # 20 0.0

0

1

2

3

4

5

Time, min

Fig. 5. Oxidation degree profile for NiO supported in Al2O3, ZrO2 and TiO2 at 3 different cycles

Table 1. Optimal kinetic models in NiO reduction/Ni oxidation stages OC

Kinetic Model

Arrhenius Parameter, ki*

NiO/Al2O3 (Reduction) (Oxidation)

NiO/ZrO2 (Reduction)

Grain Model: f ( X )  (1  X ) 2 / 3

k2=0.528, k5=0.771, k10=1.506, k20=1.83

Avrami Erofeev:

k5=0.619, k10=0.484, k20=0.424

Avrami Erofeev:

k2a=1.549, k2b=0.872

f ( X )  4  (1  X )  ( log(1  X ))3 / 4

f a ( X )  3  (1  X )  ( log(1  X ))2 / 3 1-D Diffusion Model: f b ( X )  1 / 2 X

k5a=1.727, k5b=0.832 k10a=1.897, k10b=0.862 k20a=2.201, k20b=0.892

(Oxidation)

Prout-Tompkins:

f ( X )  X  (1  X )

k5=1.707, k10=2.086, k20=2.361

NiO/TiO2 (Reduction) (Oxidation)

NiO/SiO2 (Reduction)

Grain Model: f ( X )  (1  X ) 2 / 3

k2=1.335, k5=1.152, k10=1.089, k20=1.095

Avrami Erofeev:

k5=1.658, k10=1.685, k20=1.619

Avrami Erofeev:

k2=0.841, k5=0.842, k10=0.972, k20=0.986

f ( X )  2  (1  X )  ( log(1  X ))1 / 2

f ( X )  4  (1  X )  ( log(1  X ))3/ 4

4. Discussion Based on the overall redox kinetic evaluation of the four OCs, a series of important conclusions were possible to be made. During reduction in cycles 2 and 5 (Figs 4a,b), the reduction rate of NiO for RDs <30% was higher in NiO/TiO2, followed by NiO/ZrO2, ΝiO/Al2O3 and ΝiO/SiO2. In the next cycles however, the reduction rate is significantly increased in ΝiO/Al2O3, whereas no significant change is reported for the other three OCs. Based on literature studies, it can be supported that NiO reduction is faster as the number of cycles is increasing, possibly due to surface reconstruction and changes in the pore size prior to material stabilization [12]. These textural modifications seem to weaken the support effect, as all reduction profiles tend to resemble with increasing cycles as shown in Fig.4d and further discussed in [11]. As was further discussed, in the case where supports strongly interact with NiO (such as Al2O3 and TiO2), reduction proceeds via the “Unreacted Shrinking Core Model”. There, reduction is initiated on the available and unreacted NiO sites rather instantly, with practically insignificant nucleation rate and

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reduction is controlled by chemical reaction (the slow step). On the other hand, insignificant metal-support interactions (NiO/ZrO2 and NiO/SiO2), leads to reduction proceeding via nucleation and nuclei growth, which forms the slow governing step. This mechanism involves the initial formation of nuclei clusters of Ni0, which occurs with a low rate during induction (captured through the Avrami-Erofeev order for the two OCs), and the fast sequential growth of the formed clusters. Regarding Ni oxidation, it was shown that NiO/TiO2 exhibits the highest rate, whereas NiO/ZrO2 the slowest one, possibly due to the more intense diffusion limitations (they are gradually eliminated through cycles). The most pronounced change is the acceleration of oxidation of NiO on ZrO2 that tends to meet that of NiO/Al2O3. In agreement with what was observed in the reduction stage, it seems that during oxidation, the modifications that the materials undergo with continuous cycling reduce the support and diffusion effects and OD profiles seem to resemble each other in all OCs [11]. 5. Conclusions In the present study, the redox behavior of NiO-based oxygen carriers that are meant to be used in Soprtion Enhanced Chemical Looping Reforming was evaluated through a rigorous kinetic modeling strategy. As was presented, the nature of supports can indeed affect kinetic performance with a) the “Unreacted Shrinking Core Model” to quantitatively explain the reduction profiles of NiO/Al2O3 and NiO/TiO2 (only these materials experienced metal-support interaction at initial redox cycles), b) the Avrami-Erofeev model to fit the reduction profiles of NiO/SiO2 and ZrO2. Further on, the oxidation profiles revealed a fast stage controlled by chemical reaction and a slow stage controlled by diffusion. In all cases (both different cycles and OCs) the Avrami-Erofeev (NiO/Al2O3 and NiO/TiO2) and Prout-Tompkins models (NiO/ZrO2) optimally fitted the respective profiles. As a direct relation to the inherent physicochemical features, the progressing of cycles revealed structural modifications that were depicted on the variation of redox rates (captured through the kinetic constants). Such a rigorous analysis aims to form a reactor-designed basis that is required in a scaled-up sorption enhanced chemical looping reforming (CLR) process. Acknowledgements This work was made possible by NPRP grant 5-420-2-166 from QNRF (member of Qatar Foundation). The statements made herein are solely the responsibility of the authors Appendix A. The complete reaction scheme of sorption enhanced chemical looping steam methane reforming is provided through reactions R1-R14 in table A1 at both consecutive stages (reforming/regeneration). Table A 1. Reaction scheme involved in sorption enhanced chemical looping steam methane reforming Reduction/Reforming Stage 4NiO + CH4 → 4Ni+CO2+2H2O

(R1)

NiO + CH4 → Ni+CO+2H2

(R2)

NiO + CO → Ni+CO2

(R3)

NiO + H2 → Ni+H2O

(R4)

CH4 → C+2H2

(R5)

CH4 + H2O → CO+3H2

(R6)

CH4 +CO2 → 2CO+2H2

(R7)

2CO ↔ CO2+C

(R8)

C + H2O → CO+H2

(R9)

CO + H2O ↔ CO2+H2

(R10)

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(R11)

Oxidation/Regeneration Stage Ni +1/2O2 → NiO

(R12)

C +O2 → CO2

(R13)

CaCO3 ↔ CaO+CO2

(R14)

References [1]

M.H. Shariati, F. Farhadi, Exergy Analysis of Waste Heat Recovery Section in Steam-Natural Gas Reforming Process, Energy & Fuels. 29 (2015) 3322–3327. doi:10.1021/ef5027983.

[2]

U. Ahmed, U. Zahid, Y.S. Jeong, C.J. Lee, C. Han, IGCC process intensification for simultaneous power generation and CO2 capture,

[3]

J. Adanez, A. Abad, F. Garcia-Labiano, P. Gayan, L.F. De Diego, Progress in chemical-looping combustion and reforming

[4]

A. Antzara, E. Heracleous, D.B. Bukur, A.A. Lemonidou, Thermodynamic analysis of hydrogen production via chemical looping steam

[5]

A. Antzara, E. Heracleous, A.A. Lemonidou, Improving the stability of synthetic CaO-based CO2 sorbents by structural

[6]

S.S.A. Syed-Hassan, C.Z. Li, NiO reduction with hydrogen and light hydrocarbons: Contrast between SiO2-supported and unsupported

[7]

S.R. Son, S.D. Kim, Chemical-looping combustion with NiO and Fe2O3 in a thermobalance and circulating fluidized bed reactor with

[8]

O. Nordness, L. Han, Z. Zhou, G.M. Bollas, High-Pressure Chemical-Looping of Methane and Synthesis Gas with Ni and Cu Oxygen

[9]

Z. Zhou, L. Han, G.M. Bollas, Kinetics of NiO reduction by H2 and Ni oxidation at conditions relevant to chemical-looping

[10]

L. Silvester, D. Ipsakis, A. Antzara, E. Heracleous, A.A. Lemonidou, D.B. Bukur, Development of NiO-Based Oxygen Carrier

Chem. Eng. Process. Process Intensif. 101 (2016) 72–86. doi:10.1016/j.cep.2015.12.012. technologies, Prog. Energy Combust. Sci. 38 (2012) 215–282. doi:10.1016/j.pecs.2011.09.001. methane reforming coupled with in situ CO2 capture, Int. J. Greenh. Gas Control. 32 (2015) 115–128. doi:10.1016/j.ijggc.2014.11.010. promoters, Appl. Energy. 156 (2015) 331–343. doi:10.1016/j.apenergy.2015.07.026. NiO nanoparticles, Appl. Catal. A Gen. 398 (2011) 187–194. doi:10.1016/j.apcata.2011.03.033. double loops, in: Ind. Eng. Chem. Res., 2006: pp. 2689–2696. doi:10.1021/ie050919x. Carriers, Energy and Fuels. 30 (2016) 504–514. doi:10.1021/acs.energyfuels.5b01986. combustion and reforming, Int. J. Hydrogen Energy. 39 (2014) 8535–8556. doi:10.1016/j.ijhydene.2014.03.161. Materials: Effect of Support on Redox Behavior and Carbon Deposition in Methane, Energy and Fuels. 30 (2016). doi:10.1021/acs.energyfuels.6b01520. [11]

D. Ipsakis, E. Heracleous, L. Silvester, D.B. Bukur, A.A. Lemonidou, Reduction and oxidation kinetic modeling of NiO-based oxygen

[12]

M. Ishida, H. Jin, T. Okamoto, A fundamental study of a new kind of medium material for chemical-looping combustion, Energy and

transfer materials, Chem. Eng. J. 308 (2017) 840–852. doi:10.1016/j.cej.2016.09.114. Fuels. 10 (1996) 958–963. http://www.scopus.com/inward/record.url?eid=2-s2.03743091636&partnerID=40&md5=3e4a91636ca131cd347cd5093f5a331b.