Scholarly article on topic 'Use of chemically and physically mixed iron and nickel oxides as oxygen carriers for gas combustion in a CLC process'

Use of chemically and physically mixed iron and nickel oxides as oxygen carriers for gas combustion in a CLC process Academic research paper on "Chemical engineering"

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{"Chemical looping combustion" / "CO2 capture" / "Oxygen carrier" / "Iron oxide" / "Nickel oxide"}

Abstract of research paper on Chemical engineering, author of scientific article — Miguel A. Pans, Pilar Gayán, Alberto Abad, Francisco García-Labiano, Luis F. de Diego, et al.

Abstract Different bimetallic Fe–Ni-based OCs have been prepared and evaluated in a TGA, a batch fluidised bed reactor, and a continuous CLC unit in order to analyse the effect of NiO content on the CLC performance when CH4 or a PSA-offgas was used as fuel. A set of experiments was conducted in continuous operation in a 500Wth CLC unit, firstly working with a chemically mixed OC, with the iron and nickel oxides impregnated over the same alumina particle, and secondly working with a physical mixture of two impregnated Fe- and Ni-based OCs. The results were also compared with those obtained with an unmixed Fe-based OC. The effect on the combustion efficiency of different operating conditions, such as fuel composition, oxygen carrier to fuel ratio and fuel reactor temperature has been determined in the continuous unit. It was found that the use of a chemically mixed OC had a negative effect on the combustion efficiency since the formation of Fe–Ni compounds reduced the catalytic effect of Ni addition. On the other hand, a physically mixed OC with 2% of NiO increased significantly the combustion efficiency at low temperatures.

Academic research paper on topic "Use of chemically and physically mixed iron and nickel oxides as oxygen carriers for gas combustion in a CLC process"

Use of chemically and physically mixed iron and nickel oxides as oxygen carriers for gas combustion in a CLC process^

Miguel A. Pans 1, Pilar Gayan *, Alberto Abad1, Francisco Garcia-Labiano 1, Luis F. de Diego1, Juan Adanez1

Department of Energy and Environment, Instituto de Carboquimica (C.S.I.C.), Miguel Luesma Castan 4, 50018 Zaragoza, Spain

ARTICLE INFO ABSTRACT

Different bimetallic Fe-Ni-based OCs have been prepared and evaluated in a TGA, a batch fluidised bed reactor, and a continuous CLC unit in order to analyse the effect of NiO content on the CLC performance when CH4 or a PSA-offgas was used as fuel. A set of experiments was conducted in continuous operation in a 500 Wth CLC unit, firstly working with a chemically mixed OC, with the iron and nickel oxides impregnated over the same alumina particle, and secondly working with a physical mixture of two impregnated Fe- and Ni-based OCs. The results were also compared with those obtained with an unmixed Fe-based OC. The effect on the combustion efficiency of different operating conditions, such as fuel composition, oxygen carrier to fuel ratio and fuel reactor temperature has been determined in the continuous unit. It was found that the use of a chemically mixed OC had a negative effect on the combustion efficiency since the formation of Fe-Ni compounds reduced the catalytic effect of Ni addition. On the other hand, a physically mixed OC with 2% of NiO increased significantly the combustion efficiency at low temperatures.

© 2013 The Authors. Published by Elsevier B.V. All rights reserved.

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Article history: Received 28 February 2013 Received in revised form 6 May 2013 Accepted 17 May 2013 Available online 13 June 2013

Keywords:

Chemical looping combustion CO2 capture Oxygen carrier Iron oxide Nickel oxide

1. Introduction

Climate change is among the largest environmental, social and economic challenge currently facing mankind. In so far as climate change is concerned, there is, today, overall consensus on the need to reduce greenhouse gas emissions globally by 50% by 2050. This represents a cut of at least 80% in the industrialized world. This will mean, from now until 2050, considerable re-organization of the way in which society works (work, transport, leisure, city planning, housing, electricity production). The sectors responsible for emission - power generation, industry, transport, buildings and construction - must all prepare the transition to a low-carbon economy [1]. CO2 capture and storage technologies (CCS) have been identified as one of the options necessary to overcome the anthropogenic emissions [2]. Among the different technologies for CO2 capture, the chemical-looping combustion (CLC) process was suggested as a worthy alternative to reduce the economic cost of CO2 capture [3].

CLC involves combustion of fuels with an oxygen carrier (OC), generally a metal oxide and a binder, which transfers oxygen from the air to the fuel by means of its exchange between two different reactors,

☆ This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Corresponding author. Tel.: +34 976 733977; fax: +34 976 733318.

E-mail address: pgayan@icb.csic.es (P. Gayan).

1 Tel.: +34 976 733 977; fax: +34 976 733 318.

avoiding in this way the direct contact between fuel and air. In conventional combustion the flue gas stream consists of carbon dioxide, steam and mostly nitrogen. Carbon capture in this combustion involves considerable energy for separating the CO2 from the N2. In CLC, CO2 separation is simply accomplished because the flue gas stream consists only of CO2 and steam. By steam condensation, a pure CO2 stream is produced. Therefore, CLC provides a sequestration ready CO2 stream without the need for using costly gas separation techniques. Moreover, the net chemical reaction and energy release are similar to that of conventional combustion of the fuel.

A CLC system is generally composed of two interconnected fluidized bed reactors (Fig. 1) designated as air (AR) and fuel reactors (FR), where the solid metal oxide particles are circulated between the reactors. In the fuel reactor, the fuel gas (CnH2m) is oxidized to CO2 and H2O by a metal oxide (MeO) that is reduced to a metal (Me) or a reduced form of MeO. The FR is typically a bubbling or circulating bed. The metal or reduced oxide is further transferred into the air reactor where it is oxidized with air, and the material regenerated is ready to start a new cycle. The flue gas leaving the air reactor contains N2 and unreacted O2. The exit gas from the fuel reactor contains only CO2 and H2O.

A large volume of knowledge on the CLC technology has been accumulated during recent years. CLC for gaseous fuels have successfully demonstrated in different CLC prototypes in the 10-140 kWth range using oxygen carriers based on nickel, cobalt, and copper oxides [4]. Many of the studies on CLC have focused on the development of appropriate oxygen carriers able to comply with the requirements imposed by the process. Tests with around 700 different materials based

0378-3820/$ - see front matter © 2013 The Authors. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/! 0.1016/j.fuproc.2013.05.013

(N2,O2

Air Reactor ^ (AR)

MeyOx.1

(CO2,H2O)

EuelReactor (FR)

Fig. 1. Schematic diagram of CLC.

on transition metal oxides, mixed oxides, and low-cost materials have been reported [4].

An oxygen carrier for CLC should have the following properties: high fuel conversion to CO2 and H2O; high reduction and oxidation rates; low tendency to agglomeration, fragmentation and attrition; low possibility of deactivation by carbon deposition or sulphur compounds; easy preparation to reduce costs; and it would be desired that the oxygen carrier will be environmental friendly.

Because of its low cost and environmental compatibility, Fe-based OC is considered an attractive option for CLC application. Other chemical characteristics are advantageous for the use of Fe-based oxygen carriers: low tendency to carbon deposition [5] and no risk of sulphide or sulphate formation at any sulphur containing gas concentration or operating temperature [4]. However, an important disadvantage is its low reactivity with methane [5]. But, it was observed that during the combustion of methane side reactions may occur, like the reforming reaction or the water-gas shift reaction, which generate CO and H2, gases which reacts faster with Fe-based oxygen carriers [5-7]. So, higher combustion efficiencies could be obtained increasing the reaction rate of the reforming reaction, and therefore, forcing the reaction to proceed via the intermediate products H2 and CO.

During the last years, it has been found that the combination of different oxygen-carrier materials may result in positive synergy effects, taking advantage of the favourable characteristics of each of them [4]. Different methods have been reported on literature to produce mixed oxides, such as the simple mixing [8-13], the impregnation of a second active phase onto existing particles [14] or the direct preparation of a multiple active phase material [8,9,15-24]. Considering this and in order to improve the reactivity of the Fe-based carriers with methane, some authors mixed them with a small amount ofa high reactive material. Ni-based oxygen-carriers have shown very high reactivity and good performance working at high temperatures (900-1100 °C). Several investigations [25,26] found that NiO particles reacted with CH4 through CO and H2 formation, because the metallic nickel formed on the particle surface enhanced the reforming of methane. The indication that nickel catalyzes the methane reforming and the fact that iron oxide reacts fast with CO and H2, suggest that a combination of both types of oxides may show synergy effects with an increased overall rate of reaction with respect to iron. This may have great implication in terms of the cost and safety of oxygen carriers since nickel is more expensive than other metal oxides, and the use of Ni-based oxygen-carriers may require safety measures because of its toxicity.

Bimetallic oxygen carriers of Fe-Ni have been prepared by different researchers. Materials containing Ni and Fe with a spinel structure were tested in a TGA by Lambert et al. [27]. They found that impregnating

NiO on a spinel material increased both oxygen-carrier capacity and reactivity of the resulting material. Lagerbom et al. [28] tested in a TGA a bimetallic Fe-Ni/Al2O3 oxygen-carrier and observed that addition of NiO to Fe2O3/Al2O3 particles improved the activity but decreased the mechanical strength. Son and Kim [22] carried out experiments in a continuous CFB using different Fe-Ni/bentonite particles. They found that the reactivity of the oxygen-carrier particles increased with increasing NiO content. The optimum ratio of NiO/Fe2O3 was found to be 3 (NiO/Fe2O3 = x75:25). In addition to mixed oxides with the spinel structure and bimetallic oxygen-carriers, other more complex metal oxides with perovskite structure have been proposed to be used as oxygen-carriers for the CLC process [29]. However, long-term chemical and mechanical properties of perovskite particles are largely unknown and further investigation with these new materials is needed to know its behaviour in continuous fluidized-bed reactors.

The addition of Ni-based particles in a bed of Fe-based particles has also been investigated. Several studies were performed with the addition of certain amount of nickel oxide using different experimental configurations as fixed bed [8], batch fluidized-bed reactor [10-12], 300 Wth circulating fluidized-bed reactor [13] and 500 Wth unit [9]. Johansson et al. [10] found that a bed of iron oxides with only 3 wt.% nickel oxide was sufficient to give a very high CH4 conversion. In addition, these researchers showed that the mixed-oxide system produced significantly more CO2 than the sum of the metal oxides that run separately, thus giving evidence of the synergy in using nickel oxide together with iron oxide. Very similar findings were also observed by Ryden et al. mixing NiO60-MgAl2O4 either in a bed of Fe2O360-MgZrO2 [12], in a bed of ilmenite [13] or waste products from the steel industry [11]. Ortiz et al. [9] reported an increase in the combustion efficiency in a continuous 500 Wth CLC prototype using PSA-offgas as fuel. Moghtaderi and Song [30] carried out a theoretical analysis of the kinetic parameters when physically mixed-metal oxides are used. No clear effect on the resulting kinetics of the mixture was achieved.

So, physical and chemical addition of Ni to a Fe-based OC has been previously investigated in the literature. However, a comparison between both Ni addition methods was never carried out before. So, this works the effects of Ni addition over a Fe-based OC that has been evaluated and compared using two different strategies: the chemical and the physical mixtures. Thus, different bimetallic OCs have been prepared by impregnation on an alumina support and evaluated in a TGA and batch fluidized bed reactor to analyse the effect of NiO content on the reactivity and gas product distribution. Moreover, a physically mixed OC, using a Fe-based and a Ni-based OC, was also evaluated in the combustion of fuel gases.

The effect of this Ni addition on the combustion efficiency was analysed in a 500 Wth CLC continuous unit using different combustion gases and operating conditions (temperature, oxygen carrier to fuel ratio and fuel composition). A first set of experiments were conducted working with a chemically mixed OC, with the iron and nickel oxides impregnated over the same alumina particle. A second set of experiments were performed working with a physical mixture of two impregnated Fe- and Ni-based OCs. The results were also compared with those obtained by the authors [31] with a highly reactive unmixed Fe-based OC developed and tested in a continuous CLC unit.

2. Experimental section

2.1. Oxygen carrier materials

The behaviour of several chemically mixed Fe-Ni oxygen carriers was analysed in this work together with a physically mixed Fe-Ni OC, using a Fe-based and a Ni-based OC. Fe-based OC was prepared by incipient wetness impregnation over commercial y-AI2O3 (Puralox NWa-155, Sasol Germany GmbH) particles of 0.1-0.32 mm with density of 1.3 g/cm3 and porosity of 55.4%. The details of the preparation of the Fe-based OC have been described elsewhere [31]. Ni-based OC was prepared over a-Al2O3 (obtained by calcination of y-AI2O3 at 1150 °C during 2 h) particles of 0.1-0.32 mm with density of 2.0 g/cm3 and porosities of 47.3% respectively. The details of the preparation of the Ni-based OC have been described elsewhere [32].

Bimetallic oxides were prepared by applying successive impregnation steps after the iron impregnation, followed by calcination at 550 °C in air atmosphere for 30 min, with a nickel nitrate solution in the exact concentration to produce carriers with different Fe:Ni ratios from 1.4 to 10. Finally the particles were sintered at 950 °C in air atmosphere for 1 h.

Table 1 shows the different oxygen carriers prepared. The samples were designated with the chemical symbols referred to the active metal oxides followed by their weight content and an indication of the mixture: WM refers to without mixed OC, CM is used for the chemical mixed bimetallic OC and PM is used for the Fe-Ni OC made by a physical mixture of the unmixed Fe-OC and the Ni-OC.

2.2. Oxygen carrier characterization

Several techniques have been used to characterize physically and chemically the fresh and after-used oxygen carrier particles. The metal active content for the CLC process was determined by complete reduction of the sample with hydrogen in TGA at 950 °C.

The oxygen transport capacity, ROC, was calculated assuming that Fe2O3 is reduced to FeO-Al2O3 and NiO to Ni in the CLC process.

The force needed to fracture a particle was determined using a Shimpo FGN-5X crushing strength apparatus. The mechanical strength was taken as the average value of at least 20 measurements. The real density of the particles was measured with a Micromeritics AccuPyc II 1340 helium pycnometer. The surface area of the oxygen carrier was determined by the Brunauer-Emmett-Teller (BET) method by adsorption/desorption of nitrogen at 77 K in a Micromeritics ASAP-2020 (Micromeritics Instruments Inc.), whereas the pore volume was measured by Hg intrusion in a Quantachrome PoreMaster 33. The identification of crystalline chemical species was carried out by powder X-ray diffractometer Bruker AXS graphite monochromator. The chemically mixed Fe-Ni oxygen carrier particles were also analysed in a scanning electron microscope (SEM) ISI DS-130 coupled to an ultra thin window PGT Prism detector for energy-dispersive X-ray (EDX) analysis.

2.3. Reactivity tests in TGA

The reactivity of the different oxygen carriers was determined in a TGA, CI electronics type, described elsewhere [33]. For the experiments, the oxygen carrier was loaded in a platinum basket and heated to the set operating temperature in air atmosphere. After weight stabilization, the experiment was started by exposing the oxygen carrier to alternating reducing and oxidizing conditions. To avoid mixing of combustible gas and air, nitrogen was introduced for 2 min after each reducing and oxidizing period.

The reactivity of the oxygen carrier was determined with different reducing gases: CH4, CO and H2 at different temperatures. The gas composition was 15 vol.% of the reducing gas. In the experiments with CH4, 20 vol.% H2O was introduced to avoid carbon formation by methane decomposition. Steam was incorporated to the gas stream by bubbling

Table 1

Main characteristics of the oxygen carriers.

Fe15-WM Fe15Ni2-CM Fe15Ni5-CM Fe15Ni12-CM Ni18-WM

Fe2O3a(wt.%) 15.2 15.5 14.9 14.2 -

NiO a (wt.%) - 2 5 12.3 18

Oxygen transport capacity b (%) 1.5 2 2.5 4.1 3.9

Crushing strength (N) 2.5 1.8 2.0 2.7 4.1

Real density (g/cm3) 3.9 3.9 3.8 4.2 4.3

Porosity (%) 50.5 51.8 48.4 45.6 42.5

Specific surface area, BET (m2/g) 39.1 44.2 56.3 19.9 7.0

Fresh a-Al2O3, Fe2O3 a-Al2O3, Fe2O3, NiFe2O4, a-Al2O3, Fe2O3, NiAl2O4, a-Al2O3, Fe2O3, NiAl2O4, a-Al2O3, NiO, NiAl2O4

NiAl2O4c NiFe2O4 NiFe2O4

TGA Oxidation a-Al2O3, Fe2O3 a-Al2O3, Fe2O3, NiFe2O4, a-Al2O3, Fe2O3, NiAl2O4, a-Al2O3, Fe2O3, NiAl2O4, -

NiAl2O4c NiFe2O4 NiFe2O4

Reduction a-Al2O3, FeO-Al2O3 a-Al2O3, FeO-Al2O3, a-Al2O3, FeO-Al2O3, a-Al2O3, FeO-Al2O3, -

NiAl2O4, FeNi3c NiAl2O4, FeNi3c NiAl2O4, FeNi3c

Batch Oxidation a-Al2O3, Fe2O3 - a-Al2O3, Fe2O3, NiAl2O4, a-Al2O3, Fe2O3, NiAl2O4, -

NiFe2O4c NiFe2O4c

Reduction a-Al2O3, FeO-Al2O3 - a-Al2O3, NiAl2O4, a-Al2O3, NiAl2O4, -

FeO-Al2O3, FeNi3c FeO-Al2O3, FeNi3c

CLC facility Particles from AR a-Al2O3, Fe2O3 a-Al2O3, Fe2O3, FeO-Al2O3 - - -

Particles from FR a-Al2O3, FeO-Al2O3 a-Al2O3, Fe2O3, FeO-Al2O3, - - -

NiFe2O4c

a Determined by TGA. b Including Fe and Ni contents. c Minor amounts.

through a water containing saturator at the selected temperature to reach the desired water concentration. Similarly, 20 vol.% CO2 was introduced together with CO to avoid carbon formation by the Boudouard reaction. In all cases, nitrogen was used to balance. For oxidation reaction, 100% air was used as reacting gas. The effect of temperature on the reactivity of the OCs was evaluated using the different fuel gases at 830 and 950 °C.The conversion of solids for the reduction reaction was calculated as:

The back-mixing in the system, which was illustrated by the transient changes in gas concentration during the first seconds of reaction, was considered in order to obtain the actual concentration of the gases in the bed. The correction was done using a method of deconvolution that takes into account the gas residence time distribution in the system [34].

2.5. ICB-CSIC-g1 facility

X — mox-m

r Rormox

being mox the mass of the fully oxidized solids, m the instantaneous mass of the sample and ROC the oxygen transport capacity of solids for the transformation between Fe2O3 and FeAl2O4 + NiO/Ni, given in Table 1.

2.4. Fluidized bed reactor

Several reduction-oxidation multicycles were carried out in a batch fluidized bed reactor to know the gas product distribution during reaction and the fluidization behaviour of the carrier. The experimental set-up has been described elsewhere [31].

The tests were carried out at 950 °C with an inlet superficial gas velocity into the reactor of 10 cm/s. The composition of the gas was 25 vol.% CH4 in N2 during reduction and 10-15 vol.% O2 in N2 during oxidation. The reduction periods were varied between 60 and 300 s. The oxidation periods necessary for complete oxidation varied between 600 and 1200 s. To avoid mixing of CH4 and O2, N2 was introduced for two minutes after each reducing and oxidizing periods. The fluidized bed was fed with 260-300 g of oxygen carrier with a particle size of 0.1 -0.3 mm.

The conversion of the oxygen carriers as a function of time during the reduction and oxidation periods was calculated from the gas outlet concentrations by the equations:

Reduction

Fig. 2 shows a schematic diagram of the continuous atmospheric CLC facility used in this work, which was designed and built at Instituto of Carboquimica (ICB-CSIC). The pilot plant was basically composed of two interconnected fluidized-bed reactors—the air and fuel reactors, a riser for solids transport from the air reactor to the fuel reactor, a solid valve to control the solids flow rate fed to the fuel reactor, a loop seal and a cyclone. This design allowed the variation and control of the solid circulation flow rate between both reactors.

In the FR (1) the oxygen carrier particles are reduced by the fuel. Reduced oxygen carrier particles overflowed into the AR (3) through a U-shaped fluidized bed loop seal (2), to avoid gas mixing between fuel and air. The oxidation of the carrier took place at the AR. Secondary air could be introduced at the top of the bubbling bed to help particle entrainment. N2 and unreacted O2 left the AR passing through a high-efficiency cyclone (5) and a filter (9) before the stack. The oxidized solid particles recovered by the cyclone were sent to a solids reservoir setting the oxygen carrier ready to start a new cycle. In addition, these particles avoid the leakage of gas between the FR and the riser. The regenerated oxygen carrier particles returned to the FR by gravity from the solids reservoir through a solids valve (7) which controlled the solids circulation flow-rate entering the FR. A diverting solids valve (6) located below the cyclone allowed the measurement of the solids flow rates at any time. Fine particles produced by fragmentation/attrition in the plant were recovered in the filters that were placed downstream of the FR and AR. The gas outlet streams of the FR and AR were drawn to respective on-line gas analysers to get continuous data of the gas composition. Detailed information about this experimental facility was described elsewhere [31].

Xred —

— f „ 7 (2PCO2,out + PCO,out + PH,O,out) i nn Ptot \ 2 2 '

Qout — QJ^I = Qn

1 PCH,,our PCO,,ouf PCO,ouf PH,,ouf PH,O,ouf ,

Oxidation

Xoxi — f 2 Q°Ut (rQnPO2,in-PO2,out— 1/2PCO,out —PCO2,ou^]dt (4)

* nnP Mr \Qnut /

Qout —

Qin 1-Po

CO2;out-PCO;out-PO2;out

where Xi is the conversion of the oxygen carrier, Q,n is the molar flow of the gas coming into the reactor, Qout is the molar flow of the gas leaving the reactor, Ptot is the total pressure, Pifin is the partial pressure of gas i incoming to the reactor, Piout is the partial pressure of gas i exiting the reactor, n0 are the moles of oxygen which can be removed from fully oxidized oxygen carrier, and t is the time. The last terms in Eq. (4) take into account the formation of CO and CO2 during the oxidation period due to the oxidation of C coming from the decomposition of CH4 in the reduction period.

Secondary Air

Gas analysis O2, CO, CO2

] 9 n^~stack

Gas analysis CH4, CO,, CO, H,

H 9 H* stack

1.-Fuel reactor

2.-Loop seal

3.-Air reactor

4.-Riser

5.-Cyclone

6.-Diverting solids valve

7.-Solids valve

8.-Water condenser

9.-Filter

10.-Furnace

N, CH. N, H, CO CO,

Fig. 2. Schematic diagram of the ICB-CSIC-gl facility.

The conversion for the oxidation reaction was calculated as Xo = 1 — Xr

The total solids inventory in the system was about 1.2 kg of solid material. The temperature in the air reactor was always kept constant at about 950 ± 15 °C. The inlet flow of fuel was 170 Nl h-1, which corresponds to an inlet gas velocity in the fuel reactor of 10 cm s-1. The inlet air flow in the AR was 720 Nl h-1 as primary air, (46 cm s-1 at 900 °C), and 150 Nl h-1 as secondary air. Nitrogen was used to fluidize the bottom loop seal (37.5 Nl h-1).

The effect of NiO addition on a Fe-based OC was analysed working with a chemically mixed Fe-Ni oxygen carrier (Fe15Ni2-CM), with both metals impregnated on the same alumina particle, and with a physically mixed Fe-Ni OC (Fe15Ni2-PM), adding a small amount of a Ni-based OC to the Fe-based OC bed to obtain a mixed OC with similar NiO content than the chemically mixed one (but maintaining constant the solids inventory in the system). The results obtained with the chemically and physically mixed OCs were compared with those obtained previously in the same facility using an unmixed Fe-based OC (Fe15-WM) [31].

Two different fuel gases were used with all the OCs, methane and a simulated PSA-offgas stream. The PSA-offgas was used in order to know the potential of the materials as oxygen carriers for a steam reforming process coupled with a CLC system (SR-CLC) [31,35]. The gas composition of the PSA-offgas was determined to be 12 vol.% of CH4, 18 vol.% of CO, 25 vol.% de H2 and 45 vol.% of CO2 taken from the final report of CACHET project (FP VI-019972) [36].

Table 2 shows a summary of the different operating conditions used in the experiments conducted with both fuels. In the test series of experiments carried out with both fuels, the fuel reactor temperature, the fuel flow, the solids circulation rate and the power of the plant varied.

The oxygen carrier-to-fuel ratio (4>) was defined by Eq. (6), as:

Fe2O3+NiO

Fe2O3+NiO

being the molar flow rate of the iron and nickel oxide and FFuei is the inlet molar flow rate of the fuel in the FR. Parameter b is the stoichiometric coefficient of the fuel gas mixture, calculated in Eq. (7) as:

b = 4xCH4 + xCO + xH2

Thus, the oxygen carrier-to-fuel ratio was defined as the ratio between the oxygen supplied and the oxygen needed to stoichiometrically react with the fuel flow. A value of the ^ ratio equal to unity means that the oxygen supplied by the solids is exactly the stoichiometric oxygen to fully convert the fuel gas to CO2 and H2O.

To analyse the effect of the ^ ratio, the experiments were carried out varying the flow of the fuel, but maintaining roughly constant the solids circulation flow-rate in each set of experiments. To maintain the total flow of gas entering to the fuel reactor, the corresponding flow of nitrogen was added in every case. Under all operating conditions, the ratio of the constituent gases of the PSA off-gas, i.e. CH4, H2, CO, CO2 was maintained constant. When the flow of the fuel was varied, the air to fuel ratio, the solids inventory per MWth (mFR) and the gas concentration were varied simultaneously.

To analyse the effect of FR temperature on combustion efficiency the experiments were carried out at two different FR temperatures, 830 and 880 °C.

To evaluate the behaviour of the oxygen carrier during the combustion tests, the combustion efficiency nc, defined in Eq. (8), was used as a key parameter. The combustion efficiency (nc) was defined as the ratio of the oxygen consumed by the gas leaving the FR to that consumed by the gas when the fuel is completely burnt to CO2 and

Table 2

Operating conditions and main data for the experiments carried out in the ICB-CSIC-gl facility using CH4 and PSA-offgas as fuels.

Test Fuel gasa(vol.%) Tfr fs (kg/h) + Power (Wth)

Fe15Ni2-CM

M-1 25.0 830 11.2 1.8 430

M-2 14.9 830 11.2 3.1 250

M-3 10.0 830 11.2 4.6 168

M-4 29.4 880 13.9 1.8 533

M-5 25.0 880 13.9 2.3 417

M-6 20.0 880 13.9 2.9 331

M-7 14.9 880 13.9 3.8 253

M-8 10.0 880 13.9 5.7 168

PSA-offgas

PSA-1 100.0 830 13.4 2.4 436

PSA-2 72.1 830 13.4 3.3 317

PSA-3 57.5 830 13.4 4.2 249

PSA-4 41.1 830 13.4 5.8 181

PSA-5 100.0 880 13.4 2.4 436

PSA-6 72.1 880 13.4 3.3 317

PSA-7 57.5 880 13.4 4.4 238

PSA-8 41.1 880 13.4 6.2 169

Fe15Ni2-PM

M-9 29.4 830 10.4 1.3 496

M-10 25.0 830 10.4 1.4 461

M-11 19.9 830 10.4 1.8 358

M-12 14.5 830 10.4 2.6 248

M-13 29.4 880 11.4 1.4 505

M-14 25.0 880 11.4 1.5 471

M-15 19.9 880 11.4 2.0 353

M-16 15.8 880 11.4 2.6 272

PSA-offgas

PSA-9 100.0 830 9.3 1.5 437

PSA-10 70.5 830 9.3 2.1 312

PSA-11 57.5 830 9.3 2.6 252

PSA-12 44.3 830 9.3 3.4 193

PSA-13 33.7 830 9.3 4.4 149

PSA-21 100.0 880 11.4 1.8 447

PSA-22 70.5 880 11.4 2.6 309

PSA-23 57.5 880 11.4 3.2 251

PSA-24 44.3 880 11.4 4.1 196

PSA-25 33.7 880 11.4 5.4 149

Fe15-WM

M-17 35 830 13.5 1.2 597

M-18 25 830 13.5 1.6 447

M-19 19.9 830 13.5 2.0 358

M-20 15.5 830 13.5 2.5 286

M-21 10 830 13.5 4.1 175

M-22 36.8 880 13.9 1.1 670

M-23 35 880 13.9 1.2 614

M-24 29.4 880 13.9 1.5 491

M-25 14.5 880 13.9 3.0 246

PSA-offgas

PSA-26 100 830 13.5 1.9 429

PSA-27 72.1 830 13.5 2.6 313

PSA-28 57.5 830 13.5 3.2 255

PSA-29 45.9 830 13.5 4.0 204

PSA-30 100 880 13.4 1.9 425

PSA-31 72.1 880 13.4 2.6 311

PSA-32 57.5 880 13.4 3.2 253

N2 to balance.

H2O. So, the ratio gives an idea about how the CLC operation is close or far from the full combustion of the fuel, i.e. nc = 100%.

( 2xCO2 + xCO + XH2O ) Fout ( 2xCO2 + xCO ). Fin n __v_2_2 / out_V_2_/ in 100

I Ic , s

(4xCH4 + XCO + xHjinFin

Fin being the molar flow of the inlet gas stream, Fout the molar flow of the outlet gas stream, and xi the molar fraction of the gas in the inlet or outlet gas stream.

XCH4 + xCO + xH2

3. Results and discussion

The behaviour of several chemically mixed Fe-Ni oxygen carriers was analysed in this work together with a physically mixed Fe-Ni OC, using a Fe-based and a Ni-based OC.

Table 1 shows the physical and chemical characteristics of the fresh OCs. As it can be seen, the NiO content affects the properties of the materials. The higher the NiO content, the higher the density and mechanical strength of the oxygen carriers prepared by impregnation and the lower the porosity and the superficial area.

XRD patterns of the fresh oxygen carriers showed the phase transformation of the alumina support during the calcination step and the formation of nickel aluminium spinel (NiAl2O4) and nickel ferrite (NiFe2O4) compounds for the nickel oxide. Moreover, in Fig. 3 SEM images of the fresh chemically mixed Fe-Ni OC particles are shown. The oxygen carrier particles exhibited an irregular shape, since impregnation of porous alumina particles has been carried out. The iron and nickel distributions inside the particles were also analysed by EDX in some particles embedded in resin epoxy, cut, and polished. A uniform distribution of iron and nickel through the particles was found.

3.1. Reactivity in TGA

The different samples of chemically mixed OCs were first characterized by TGA, in order to know their reactivity with different fuel gases (CH4, H2 and CO), since methane and PSA-offgas (a mixture of H2 + CO + CH4) were used as fuel in the continuous unit. The influence of the temperature was also studied.

For Fe-based oxygen carriers, different reduction reactions are possible depending on the reducing gas composition and temperature. For the data presented here, it was assumed that the weight variations observed in the TGA were mainly associated with the transformation Fe2O3/FeO-Al2O3. This assumption was confirmed by the results obtained by the XRD patterns, as it can be seen in Table 1, which shows that the final reduced form in the experiments carried out using CH4 was the form FeO-Al2O3.

Fig. 4 shows the reduction and oxidation reactivities at 950 °C using CH4 as reduction gas and air for oxidation for the chemically mixed OCs, together with the Fe15-WM and the Ni18-WM OCs for comparison purposes. As it can be seen, the effect of Ni addition on the bimetallic OC is negative, as higher amount of Ni in the mixed

Reduction Oxidation

X° 0.6

0.2 0.0

0 20 40 60 80 100 0 20 40 60 time /sec

Fig. 4. Effect of NiO addition on the reactivity of the different oxygen carriers at 950 °C. Reduction:15%CH4 + 20% H2O. Oxidation:air.

OC implies lower reactivity of the OC. These results can be explained by the formation of a Fe-Ni spinel compound in the bimetallic OCs with lower methane reactivity than the corresponding to Fe2O3 and NiO [37], as it could be seen in Table 1. XRD patterns show the formation of NiFe2O4 in the oxidation state and FeNi3 in the reduced stated in all the bimetallic OCS. This fact may lead to a solid conversion higher than 1, since it was assumed that the total iron content is reduced only to FeO-Al2O3 using CH4 as fuel gas.

The Fe15-WM and the Ni18-WM OCs had been previously confirmed as high oxygen carriers for fuel gases (CH4, H2 and CO), [31,32], as it can be seen in Fig. 4.

In spite of the formation of Fe-Ni compounds, the oxidation reactivities for all the bimetallic OC were high and very similar to the Fe15-WM OC, as it can be seen in Fig. 4.

The effect of the fuel gas was analysed for the bimetallic OCs using H2 and CO in the TGA. Fig. 5 shows the reduction and oxidation conversions versus time curves obtained using H2 as reacting gas, and air for the oxidation at 950 °C and for the Fe15-WM. As it can be seen, very high reactivities were obtained with all carriers. It must be pointed out that using H2 different reduction states can be reached depending on the ratios H2O/H2 used [38]. In the TGA conditions used

Fig. 3. SEM-EDX images of the chemically mixed Fe-Ni oxygen carrier particles; a) Fe15Ni2-CM; b) Fe15Ni5-CM; c) Fe15Ni12-CM.

X° °.6

Reduction

Oxidation

Reduction

Oxidation

// 132

7 1- Fe15-WM

| 2- Fe15Ni2-CM

| 3- Fe15Ni5-CM

I 4- Fe15Ni12-CM .....

0 20 40 60 80 100 0 20 time /sec

Fig. 5. Effect of NiO addition on the reactivity of the different oxygen carriers at 950 °C. Reduction:15% H2. Oxidation:air.

20 40 60 80 100 0

time /sec

Fig. 7. Effect of NiO addition on the reactivity of the different oxygen carriers at 830 °C. Reduction:15%CH4 + 20% H2O. Oxidation:air.

in this work, reduction up to Fe0 could be reached. Comparing with the values obtained with CH4 as reacting gas, it can be seen that higher reactivity was obtained working with H2 with all carriers.

Fig. 6 shows the reactivity curves for the mixed OCs and for the Fe15-WM, using CO as reacting gas at 950 °C. Similar results to CH4 were obtained for the effect of Ni content on the reactivity: higher NiO content implies lower reactivity of the mixed OC. Comparing Figs. 4,5 and 6, it can be seen that the highest reactivities were obtained with H2 as reacting gas, followed by CH4. The lowest reactivities were observed working with CO as reacting gas. These results agree with the ones obtained by Abad et al. [38] and Dueso et al. [39], who found the highest reactivities working with H2 and the lowest with CO using Ni-based oxygen carriers. In all cases, lower reactivities were achieved with those OCs with higher Ni contents.

The effect of temperature on the reactivity of the OCs was evaluated using the different fuel gases at 830 and 950 °C. Fig. 7 shows the reactivities obtained with CH4 as reacting gas at the low temperature for all the bimetallic OCs. As it can be seen comparing Figs. 4 and 7 an important effect of the reduction temperature was observed for the bimetallic OCs, especially as Ni content increases. This effect is due to the formation of Ni based compounds, NiAl2O4 and NiFe2O4, with a slower reactivity than NiO [40,41].

Thus, the bimetallic OCs used in this work need high operation temperatures, about 950 °C, to have reactivities as high as to the unmixed Fe15-WM, specially working with CO, and CH4 as fuel gases.

Reduction

Oxidation

0 20 40 60 80 100 0 20 time /sec

Fig. 6. Effect of NiO addition on the reactivity of the different oxygen carriers at 950 °C. Reduction:15%CO + 20% CO2. Oxidation:air.

3.2. Oxygen carrier behaviour in batch fluidized bed

Several reduction-oxidation multicycles were done with the chemically mixed oxygen carriers in the batch fluidized bed reactor, using CH4 as reducing gas, to determine the gas product distribution and to analyse the fluidization behaviour of the oxygen carriers with respect to agglomeration phenomena. The carriers tested were the Fe15Ni2-CM, Fe15Ni5-CM, and Fe15Ni12-CM. The results obtained were compared with the one achieved working with the Fe-based OC without mixed, Fe15-WM, at similar conditions.

Fig. 8 shows the outlet product gas distribution for a typical reduction period of 2 min working with the chemically mixed OCs and with the Fe15-WM at 950 °C using CH4 as fuel in the batch fluidized bed reactor. It was found, in all cases, a first period of full conversion of CH4, where CO2 and H2O were formed just immediately after the introduction of the reducing gas to the reactor. After this first period, CO2 and H2O concentrations begin to decrease as a result of the decrease of the oxygen transference rate. Because of this, the CH4, CO and H2 concentrations start to increase because to the oxygen carrier is unavailable to convert fully CH4 into CO2 and H2O, and the partial oxidation of methane takes place. In this period, it can be observed that as NiO content in the bimetallic OC increases, the CH4 concentration at the outlet of the reactor decreases, due to the capacity of the Ni active sites to catalyze the reforming reaction of methane. Thus, the selectivity of methane reaction to CO2 + H2O increased as the nickel content in the oxygen carrier increases. Small amounts of nickel oxide (< 5 wt.%) in the OC are sufficient to fully convert all the methane inlet flow. Similar results were found during the whole batch experiments indicating that the OC maintains its reactivity during cycling operation.

The multi-cycle tests carried out in the batch fluidized bed reactor were useful to determine the fluidization behaviour of the oxygen carrier with respect to the agglomeration phenomena. Although a high degree of conversion was reached in the cycling tests, neither of the bimetallic OCs show any agglomeration behaviour during operation. These results agreed with previous works carried out in continuous operation using the unmixed OC Fe15-WM, and other Fe-based materials [7,31,42] where the absence of agglomeration in the use of Fe-based materials for the CLC process was establish.

Once the reactivity and selectivity to CH4 combustion of the different bimetallic OCs have been determined in the TGA and in the batch fluidized bed a few conclusions can be drawn. By one hand, the higher Ni content in the chemically mixed FeNi OCs, the lower CH4, and CO reactivity. On the other hand, the selectivity to full conversion of methane increases as Ni content increases due to the catalytic effect of the reduced nickel. According to this, the chemically mixed FeNi OC, Fe15Ni2-CM, was selected to evaluate its behaviour

Time (s)

Fig. 8. Product gas distribution in dry basis during reduction for a typical cycle with bimetallic OCs. H2O concentration as measured in an FTIR analyser. T = 950 °C, Fuel = 25% CH4, solid conversion also shown.

in the continuous unit, since it has shown a high reactivity with hydrogen, methane, and CO, at 950 °C and full conversion of CH4 was reached in the fluidized bed facility during reduction periods.

3.3. Test in ICB-CSIC-g1 facility

The effect of Ni addition on the combustion efficiency was analysed in the 500 Wth CLC continuous unit using the unmixed Fe-OC, the chemically mixed Fe-Ni OC and the physically mixed Fe-Ni OC. A first set of experiments were conducted working with a chemically mixed OC, with the iron and nickel particles impregnated over the same alumina particle. A second set of experiments were performed working with a physical mixture of two impregnated Fe- and Ni-based OCs. Moreover,

these results were compared with those obtained in the same facility using an unmixed Fe-based OC, Fe15-WM, described in detail in a previous work [31 ].

As it was commented above, the Fe15Ni2-CM OC was selected for the continuous CLC facility experiments. A total of about 38 h at hot conditions, of which 32 corresponded to combustion conditions were conducted in the facility with the bimetallic oxygen carrier. Experiments were performed with CH4 and a simulated PSA-offgas as gas fuels for comparison purposes. The effect of the oxygen carrier-to-fuel ratio and the fuel reactor temperature on the combustion efficiency, nc, was analysed.

The gas product concentrations of the fuel and air reactors were measured by on line analysers. These gas concentrations were used to make carbon, hydrogen and oxygen mass balances over the whole reactor system. For better comparison, the results are presented in N2 free dry basis. CO and CO2 concentrations at the outlet of the AR were never detected in any test. Thus, no losses in CO2 capture were produced by carbon transfer to the AR, reaching 100% CO2 capture in the process.

Fig. 9a) shows the effect of ^ on the combustion efficiency using the Fe15Ni2-CM as oxygen carrier, working with CH4 as fuel at 830 °C and at 880 °C. As it can be observed full combustion of the fuel was reached at ^ values higher than 4 working at 830 °C. An important effect of the temperature can be seen also, especially at low ^ values.

Fig. 10a) shows the gas product distribution measured at the outlet of the FR in these tests. High CH4 and low CO and H2 concentrations were measured at low ^ values for both temperatures, 830 and 880 °C, when CH4 was used as fuel, indicating a negligible catalytic effect of Ni present in the Fe15Ni2-CM OC. An increase in the oxygen carrier to fuel ratio produced an increase in the conversion of methane as more oxygen is available for combustion of the fuel.

3.3.1. Effect of the fuel composition

A simulated PSA-offgas stream (a mixture of CH4, CO, and H2) was also used as fuel in the CLC continuous unit in order to prove the feasibility of the Fe15Ni2-CM oxygen carrier for a SR-CLC process. Fig. 9b) shows the effect of ^ on the combustion efficiency using the Fe15Ni2-CM as oxygen carrier, working with PSA-offgas as fuel at 830 °C and at 880 °C. Full combustion of the fuel working at 880 °C can be reached at ^ values higher than 4. An important effect of the fuel reactor temperature on the combustion efficiency was found, as CO is present in the PSA-offgas and this OC had a slow reactivity with this gas, especially at low temperatures.

Fig. 10b) shows the gas product distribution measured at the outlet of the FR in these tests. As it can be seen, at 830 °C, similar low concentrations of CH4, CO and H2 were obtained. At higher temperature, 880 °C, the CH4 concentration decrease, confirming the high effect of the temperature on the reactivity of the mixed OC.

The effect of the fuel composition on the combustion efficiency with the Fe15Ni2-CM can be analysed in Fig. 9. Lower combustion efficiencies were obtained using the PSA-offgas as fuel compared to CH4 as fuel, especially at the lower operation temperature. These results were due to the low reactivity of the Fe15Ni2-CM OC with the CO, one of the gases contained in the PSA tail gas, as it could be seen in experiments conducted in TGA.

3.3.2. Effect ofNi addition on a Fe-based OC

In order to analyse the effect of Ni addition on the behaviour of Fe-based OC, a second set of experiments were performed working with a physical mixture of two impregnated Fe- and Ni-based OCs, Fe15Ni2-PM. The results were compared with those obtained in the first set of experiments, the chemically mixed Fe15Ni2-CM OC, and also with previous obtained with unmixed Fe-based OC, Fe15-WM [31]. It was added the necessary amount of Ni18-WM to obtain a

Fig. 9. Effect of oxygen carrier to fuel ratio on the combustion efficiency with the Fe15Ni2-CM using: a) CH4 at 880 °C (-■-) and 830 °C (--□--); b) PSA-offgas at 880 °C (-•-) and 830 °C (--o--).

mixed OC with a similar percentage of Ni than the chemically mixed OC. A total of about 56 h at hot conditions, of which 50 corresponded to combustion conditions were carried out in the facility using the Fe15Ni2-PM oxygen carrier.

Fig. 11(a) and (b) shows a comparison of the combustion efficiency as a function of the ^ value using the three different OCs, i.e. Fe15Ni2-PM, Fe15Ni2-CM and Fe15WM, using CH4 as fuel at 830 and 880 °C, respectively. As it could be seen no substantial improvement of the combustion efficiency was reached working with the chemically mixed bimetallic oxygen carrier, Fe15Ni2-CM, in respect to the results obtained with the Fe-based oxygen carrier without

mixing, Fe15-WM. It can be seen that higher combustion efficiencies were obtained working at 830 °C with the Fe15Ni2-PM in respect to the ones obtained with the Fe15-WM. Small differences between the combustion efficiencies values were observed at 880 °C. Fig. 12 shows the effect of the oxygen carrier to fuel ratio on gas product concentration at the exit of the FR, using CH4 as fuel, at 830 °C, working with Fe15Ni2-PM. As it can be observed lower values of CH4 and higher H2 and CO concentrations were detected at the outlet of FR with the physically mixed material in respect to the ones obtained with the Fe15Ni2-CM, confirming the effect of NiO on the methane reforming when Ni is not bounded in the same particle.

Fig. 10. Effect of oxygen carrier to fuel ratio on the composition of outlet gas for the Fe15Ni2-CM, using: a) CH4andb) PSA-offgas as fuels, at 830 °C (empty dots) and 880 °C (filled dots).

Fig. 11. Effect of oxygen carrier to fuel ratio on the combustion efficiency using CH4 as fuel at 830 °C (a) and 880 °C (b) with: Fe15Ni2-PM (•••□•••), Fe15Ni2-CM (--□--) and Fe15-WM (-■-).

3.3.2.1. Effect of the fuel composition. Fig. 13 shows a comparison of the combustion efficiency as a function of ^ using the three different materials (Fe15-WM, Fe15Ni2-CM and Fe15Ni2-PM) as oxygen carrier, working with PSA-offgas as fuel, at 830 °C and at 880 °C, respectively. Comparing with the results obtained using CH4 as fuel it can be seen that higher combustion efficiency can be obtained working with CH4 as fuel, due to the catalytic effect of Ni and the low reactivity of Ni with CO. It can be observed the similar combustion efficiencies for the physically mixed carrier and the Fe-based OC without mixed. Full combustion of the fuel was reached with these OCs at ^ values of 2.5 at 880 °C. It can be also observed that the lowest values of the combustion efficiency were achieved working with the bimetallic oxygen carrier Fe15Ni2-CM at both temperatures.

3.4. Discussion

According to the results obtained with the Fe15Ni2-CM using CH4 or PSA-offgas as fuels, it could be said that worse results were obtained at both temperatures with the chemically mixed OC. Therefore, no improvement of the combustion efficiency was detected working with the chemically mixed OC in respect to the results obtained with a Fe-based OC without mixed. The formation of mixed compounds like the awaruite (FeNi3) or the trevorite (NiFe2O4) which could hinder the catalytic effect of Ni could be the cause of the poor results achieved with the Fe15Ni2-CM OC, as it could be seen in experiments conducted in TGA, where it was determined that the higher Ni content in the oxygen carrier the lower the reactivity of the carrier.

According to the results obtained with the physically mixed OC a positive effect was only measured when methane was used as fuel at low temperatures due to a Ni catalytic effect. Similar combustion efficiencies were found to respect with the results obtained with the Fe-based OC without mixed working with PSA-offgas.

The use of a specific OC has important implications for a CLC system. The reactivity of the solids determines the solids inventory in the system and the operating conditions needed. Table 3 shows the required values of ^ and solid inventories in the FR, mFR't to reach full combustion of the fuel, working with the different bimetallic OCs tested in the continuous 500 Wth unit at 830 and 880 °C, using CH4 and PSA-offgas as fuels.

As it can be observed, using CH4 as fuel, the highest solid inventory needed was obtained for the chemically mixed OC. A very low solid inventory (600 kg/MW) is required with the physically mixed OC at 880 °C.

Using PSA-offgas as fuel, higher solid inventories are needed with the chemically mixed OC, due to the formation of Ni based compounds, NiFe2O4, and NiAl2O4, with a slower reactivity.

The considerable improvement of the combustion efficiency achieved working with the physically mixed OC at low temperature, and the limited improvement obtained working with the chemically mixed OC at any temperature, can be explained by the fact that in the physical mixture of Fe-OC and Ni-OC, the Ni-OC particles not only act as a catalyst but also as an oxygen carrier, providing bulk oxygen for the reactions with methane. The highest amount of oxygen transported, the lowest solid inventory is required to reach full combustion of the fuel.

Fig. 12. Effect of oxygen carrier to fuel ratio on the composition of outlet gas for the Fe15Ni2-PM, using CH4 as fuel at 830 °C; CH4 (■); CO ("A"); H2 (--O--).

4. Conclusions

The effect of Ni addition over a Fe-based OC has been evaluated and compared using two different strategies: the chemical and the

Table 3

Solids inventory in the FR and ^ values necessaries to reach full combustion of the fuels, at 830 and 880 °C in the lCB-CSlC-g1 facility.

CH4 PSA-offgas

830 °C 880 °C 830 °C 880 °C

+ mh + m'pR + mh + mh

kg/MW kg/MW kg/MW kg/MW

Fe15Ni2-CM 4.5 1100 4.5 1100 >6.5 >1400 6.0 1300

Fe15Ni2-PM 2.3 600 2.3 600 4.0 975 2.5 600

Fig. 13. Effect of oxygen carrier to fuel ratio on the combustion efficiency using PSA-offgas as fuel at 830 °C (a) and 880 °C (b) with: Fe15Ni2-PM (—о—). Fe15Ni2-CM (•• О ••) and Fe15-WM (-•-).

physical mixtures. Thus, different bimetallic OCs have been prepared with the iron and nickel oxides impregnated over the same alumina support particle. Moreover, a physically mixed OC, using a Fe-based and a Ni-based OCs, was also evaluated in the combustion of fuel gases. The effect of this Ni addition on the combustion efficiency was analysed in a 500 Wth CLC continuous unit using methane and a PSA-offgas as fuel gases at different operating conditions.

It was found that the use of a chemically mixed Fe-Ni OC had a negative effect on the combustion efficiency compared to the results obtained with an unmixed Fe-based OC since the formation of Fe-Ni compounds reduced the catalytic effect of Ni addition.

However, the addition of 2 wt.% of NiO by a physical mixture of an impregnated OC can improve significantly the combustion efficiency of methane at low temperatures.

A considerable reduction of the solid inventories needed to reach full CH4 or PSA-offgas combustion was measured working with the Fe15Ni2-PM OC. In conclusion, in order to improve the behaviour of a Fe-based OC via NiO addition is preferable to mix physically a Ni-based OC and a Fe-based OC, with each metal supported on different particles, instead of the chemical addition by impregnation of NiO on Fe-based OC particles, supporting both metals on the same particle.

Acknowledgements

This paper is based on the work performed in the frame of the INNOCUOUS (Innovative Oxygen Carriers Uplifting Chemical - Looping Combustion) Project, funded by the European Commission under the seventh Framework Programme (Contract 241401). P. Gayan thanks to CSIC for the financial support of the project 201180E102. M.A. Pans thanks MICINN for the FPI fellowship.

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