Ce-Î³Al2O3 Oxygen Carrier for Chemical Looping

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Fluidizable NiO/Ce-γAl2O3 Oxygen Carrier for Chemical Looping Combustion Shamseldin A. Mohamed,‡ Mohammad R. Quddus,§ Shaikh A. Razzak,† Mohammad M. Hossain,*,† and Hugo I. de Lasa§ Department of Chemical Engineering and KACST-TIC CCS, and ‡Center of Research Excellence in Renewable Energy, King Fahd University of Petroleum & Minerals, Dhahran 34464, Saudi Arabia § Department of Chemical and Biochemical Engineering, The University of Western Ontario, London, Ontario, Canada N6A 5B9 Downloaded by UNIV OF NEBRASKA-LINCOLN on September 14, 2015 | http://pubs.acs.org Publication Date (Web): August 26, 2015 | doi: 10.1021/acs.energyfuels.5b01212

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ABSTRACT: This communication presents the effects of Ce modification on NiO/Ce-γAl2O3 oxygen carrier for chemicallooping combustion (CLC). The NiO/Ce-γAl2O3 samples are prepared by an incipient wetness technique using successive metal loading. The reduction characteristics of the prepared oxygen carriers are evaluated in repeated temperature-programmed reduction/oxidation cycles. The results show that the presence of Ce minimizes metal/support interaction and improves the reducibility and oxygen carrying capacity of the oxygen carriers. As a result, almost 94 wt % nickel oxides on Ce-γAl2O3 is reduced as opposed to only 77 wt % of nickel oxides on bare γAl2O3. The presence of Ce also provides stable reduction performance over repeated oxidation/reduction cycles, which is essential for a CLC process. The sustained reducibility of the samples is mainly due to the absence of agglomeration and the formation of easily reducible nickel oxide species instead of nickel aluminate. The EDX mapping of the oxygen carrier shows a uniform nickel dispersion on the support. The NiO/Ce-γAl2O3 oxygen carriers are further evaluated in a fluidized CREC Riser Simulator using methane as fuel and air for reoxidation. The Ce containing oxygen carrier shows excellent reactivity and stability over the repeated fluidized CLC cycles. The XRD analysis of fresh and used oxygen carrier samples shows unchanged crystal structures of the nickel particles, indicating stable properties of the NiO/Ce-γAl2O3 in the repeated CLC cycles.

1. INTRODUCTION The combustion of fossil fuel is the main source of carbon dioxide; it represents about 57% of the world anthropogenic greenhouse gas emissions.1 The ratification of the Kyoto Protocol in 1997 set the goal to reduce the greenhouse gas emissions by 5.2% during 2008 to 2012 compared to the levels of 1990. Although the Kyoto protocol failed to achieve much of its goals, measures need to be taken to reduce the CO2 emissions drastically.2 The possible options for the reduction of CO2 emissions are (1) efficient utilization of energy, (2) switching to high hydrogen to carbon ratio fuels such as natural gas, (3) increasing the contribution of renewable clean energy sources (wind power, solar energy, etc.), and (4) carbon dioxide capture and storage. Unfortunately, renewable energy alternatives are not ready to replace fossil fuels because of their high cost and the risks associated with the large scale applications. Therefore, the world has to continue to rely on fossil fuels in the near future. According to the international energy agency, fossil fuels will provide 80% of the world energy demand in the first part of the 21st century.3 Therefore, there is an urgent need for technologies to capture/store CO2 from fossil fuel combustion processes in order to minimize the greenhouse gas emission effects. In this regard, chemical looping combustion (CLC) is considered as a very promising, efficient, and economical technology for CO2 capture without any energy penalty.4−6 In CLC, the fuel combustion is carried out in two interconnected fluidized bed reactors called a fuel reactor and an air reactor, respectively. A metal oxide oxygen carrier is being circulated © XXXX American Chemical Society

between the two reactors to transport oxygen from the air reactor to the fuel reactor. The gaseous outlet of the fuel reactor consists of carbon dioxide, water vapor, and traces of carbon monoxide. Since there are no nitrogen or nitrogen compounds present in the flue gas, the carbon dioxide can be easily captured by condensing the water vapor.5 Thus, the application of CLC can successfully eliminate the cost of the additional energy, as required by other carbon dioxide separation processes. In addition, the NOx formation in the CLC process is also minimal, given that fuel combustion takes place without any flame.7 The availability of a suitable oxygen carrier is considered as one of the major challenges for large scale application of CLC. The selection of the oxygen carrier also affects the performance of the combustion process. The net amount of heat produced mainly relies on the type of metal oxide used in the oxygen carrier.7 The oxygen carrying capacity of the oxygen carrier also determines the solid circulation rate between the two reactors. Thus, the oxygen carrier must have high oxygen carrying capacity, and high reactivity in both oxidation and reduction reactions. It should also be fluidizable, resistant to agglomeration, resilient to attrition, stable, and low cost.4,8,9 Transition metal oxides such as nickel, copper, manganese, and iron have been investigated as active components for oxygen carriers due to their good reduction/oxidation properties and potential ability of high fuel conversion.10−16 Although Received: May 30, 2015 Revised: August 17, 2015

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DOI: 10.1021/acs.energyfuels.5b01212 Energy Fuels XXXX, XXX, XXX−XXX

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Downloaded by UNIV OF NEBRASKA-LINCOLN on September 14, 2015 | http://pubs.acs.org Publication Date (Web): August 26, 2015 | doi: 10.1021/acs.energyfuels.5b01212

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Al2O3, facilitating high dispersion of Ni crystallites, oxygen carrying capacity, and reactivity.36 Keeping in mind the above positive effects of rare-earth elements, the present study is focused on developing Ce (also a rare-earth element) modified NiO/Ce-γAl2O3 oxygen carriers for CLC. Before nickel loading, γ-Al2O3 was modified with a small amount of Ce in order to achieve thermal stability of the support material and to minimize the metal (Ni)/support (γAl2O3) interaction. It is hypothesized that Ce not only thermally stabilizes the support γ-Al2O3 but also creates a barrier phase between γ-Al2O3 and nickel, helping the formation of desirable NiO and minimizing the formation of undesirable nickel aluminate on γ-Al2O3. The synthesized NiO/ Ce-γAl2O3 oxygen carrier(s) materials have been characterized using various physicochemical techniques (such as XRD, TPR, pulse chemisorption, and SEM-EDX) to elucidate the modificational effects of Ce. The reactivity and stability of the NiO/Ce-γAl2O3 oxygen carrier particles was established in a CREC fluidized Riser Simulator using CH4 and air for reduction and reoxidation cycles, respectively.

the particles of these metal oxides can be used directly as oxygen carrier, the bulk metal oxides often lack the sintering resistance and mechanical strength to withstand the repeated oxidation/reduction cycles.17,18 Consequently, they are dispersed on support materials such as Al2O3, NiAl2O4, MgAl2O4, SiO2, TiO2, and ZrO2. The present research group first, and later Adanez and his collaborators, published two comprehensive review articles summarizing the research activities of various oxygen carriers developments.4,5 In the open literature, nickel-Al2O3 based oxygen carriers have been extensively studied due to their excellent reactivity and high melting temperature.19−22 Recently, Cabello et al.23 investigated NiO(12%)/CaAl2O4 oxygen carriers prepared by an impregnation method. The performance of these oxygen carriers was evaluated in a 500 Wth CLC unit using methane as a fuel. A high fuel combustion efficiency was achieved at comparatively low oxygen carriers to fuel ratios. Tseng et al.24 prepared cNiFe composites using a sol gel method. This oxygen carrier showed better reduction and reoxidation activities than those of the physically mixed m-NiFe materials. However, in repeated CLC cycles, both the m-NiFe and c-NiFe materials suffered from reactivity drops due to agglomeration. On the contrary, cNiFe (40%) on SiC displayed improved thermal stability and reactivity over the CLC cycles. Huijun et al. 25 used mechanically mixed NiO/Al2O3 oxygen carriers for biomass gasification using chemical looping, while Silvester et al.26 tested two nickel based oxygen carriers supported on alumina and zirconia, respectively for the application in chemical looping reforming. According to literature studies, the main issue related to a nickel/Al2O3 oxygen carrier is the lack of stability due to the formation of nickel aluminate (NiAl2O4) by NiO/support interactions.27,28 The formation of NiAl2O4 severely affects the performance of the oxygen carrier because of the low reactivity and poor fuel conversion ability of NiAl2O4. To avoid the formation of NiAl2O4, other support materials (YSZ and MgAl2O4) have been investigated due to their low interactions with NiO. These materials must be calcined at high temperatures beyond 1400 °C to achieve high mechanical strength. Despite improving mechanical strength, high temperatures calcination may decrease the reactivity of the oxygen carrier due to increasing the support/metal interactions.29,30 NiAl2O4 itself can be used as a support for NiO to achieve high reactivity with gaseous fuels, stable performance under high temperature, and agglomeration resistance.31,32 However, the use of NiAl2O4 as support increases the cost of the oxygen carrier significantly. Chemical treatment of the support can also be applied to minimize NiAl2O4 formation. In such cases, the support is coated with other compounds prior to loading of the active metal. Baek et al. reported that the modification of γAl2O3 support with MgO can improve the oxygen transfer capacity and the performance of the Ni-based oxygen carrier.30 Jerndal et al. tested the effects of MgO and CaO addition to αalumina support and concluded that MgO coating on the support prior to NiO loading improves the fuel conversion ability.33 In other studies, Co and La were found to be useful at improving the oxygen carrier reducibility as a result of decreased support/metal interactions.28,34,35 It has been reported that the catalyst modifiers based on rare-earth elements such as La help stabilize γ-Al2O3 by preventing phase transformation at high working temperature, as required in CLC.27,28,36−38 La also preserves the porous structure of γ-

2. EXPERIMENTAL SECTION 2.1. Preparation of oxygen carrier. The oxygen carrier samples were prepared by incipient wetness technique using γ-Al2O3 as a support. Nickel nitrate hexahydrate and cerium nitrate hexahydrate were used as precursors for nickel and cerium oxides, respectively. Before nickel loading, the support γ-Al2O3 was first modified with Ce in order to improve its thermal stability. The amount of Ce was varied between 0.5 to 5 wt % and found optimum thermal stability can be achieved at approximately 1 (one) wt % Ce loading. The 20 wt % highest limit of nickel loading on Ce-γAl2O3 was selected aiming at possible highest amount of oxygen carrying capacity without any metal sintering or agglomeration.8,19,36 The main steps involved in metal (Ce/Ni) loading are impregnation of the support, drying, reduction and calcination. The same procedure is used for Ce or Ni loading on alumina. For impregnation, Ce(NO3)2.6H2O or Ni(NO3)3.6H2O powder was dissolved in ethanol. The solution was slowly added (by using a syringe) to the alumina support which was placed in a sealed conical flask under vacuum and at continuous stirring. The resultant paste was dried overnight at ambient conditions. After drying, the sample was moved to a fluidized bed reactor located inside a furnace. The sample was reduced by flowing hydrogen/argon mixture (10 mol percent hydrogen) under fluidized condition. During the reduction, the temperature was increased from room temperature to 750 °C over 4 h period and maintained at this temperature for another 8 h, then brought back to ambient conditions. The hydrogen-thermal treatment decomposes the nitrates Ce(NO3)2 to oxides (NiO and/or NiAl2O4), and finally to the metallic form.39 The three steps (support impregnation, drying and reduction) were repeated until 20% nickel loading was achieved. In each cycle 5 wt % nickel was loaded to the Ce modified γ-Al2O3 support. After reaching the 20 wt %, the prepared samples were calcined in the presence of air. The temperature was increased to 750 °C applying the same temperature ramp used in the reduction step. The SEM-EDX analysis of a prepared NiO (20%)/Ce-γAl2O3 sample showed that the actual metal loading was within 5% error band. Previously, the present research group also obtained similar results of a NiO/α-Al2O3 sample prepared by incipient wetness method and analyzed by using SEMEDX and ICP techniques.8 The measured particle size of the prepared oxygen carriers was ranging from 10 to110 μm, with a Sauter mean diameter of 95 μm. 2.2. Characterization of the oxygen carriers. 2.2.1. Temperature-programmed reduction and reoxidation. The temperatureprogrammed reduction (TPR) and temperature-programmed reoxidation (TPO) experiments were conducted using a Micromeritics AutoChem II 2920 Analyzer, USA. For the TPR/TPO analysis, B


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Energy & Fuels approximately 100−200 mg sample was loaded in a “U” shaped quartz reactor located inside a furnace. In order to perform TPR, the oxygen carrier sample was first degassed by circulating helium through the reactor. After degassing, the gas flow was switched to a gas mixture of 10 mol % hydrogen in argon at a rate of 50 mL/min. The oxygen carrier sample was heated from ambient to 900 °C with a heating rate of 10 °C/min. TPO experiments were conducted following each TPR experiments to reoxidize the reduced sample. Before starting TPO, the system was cooled down to room temperature under helium flow. For the TPO, a stream of 5 mol % O2 and 95 mol % helium gas was circulated through the reduced oxygen carrier bed at a rate of 50 mL/min. The bed temperature was increased from ambient to 900 °C at a heating rate of 10 °C/min. A thermal conductivity detector (TCD) was used to measure the concentration of the outgoing gas. The TCD data was further processed to obtain the hydrogen consumption during the reduction reaction, which was used to find the percentage of nickel oxide reduction. 2.2.2. Pulse chemisorption. Hydrogen pulse chemisorption tests were carried out to estimate the percent metal dispersion, average active area and average crystal size. The pulse tests were conducted in between the TPR and TPO cycles, i.e, the oxygen carrier at reduced condition. Before pulsing hydrogen, the system was flushed by flowing argon through the bed. For adsorption, a series of hydrogen pulses (1 mL) were injected to the oxygen carrier bed at ambient temperature. The amount of the hydrogen adsorption was determined by analyzing gas passing across the TCD. The amount of chemically adsorbed hydrogen on the oxygen carrier active sites was used to find the percent metal dispersion, average active area and the average particle size. 2.2.3. XRD analysis. The crystalline phases of the oxygen carriers were identified by using a Rigaku Miniflex diffractometer with Ni filtered Cu K radiation (λ = 1.5406 Å). The X-ray intensity was recorded at 30 °C using a 0.05° step for 2θ ranges between 10° and 90°. The crystalline phases were identified according to the JCPDS files (JCPDS: Joint Committee on Powder Diffraction Standards). 2.2.4. SEM-EDX analysis. JEOL JSM-840 model scanning electron microscope (SEM) analyzer was employed to determine morphology of the fresh and used oxygen carriers. For analysis, the oxygen carrier sample was dispersed on a copper stub then coated with gold film for 2 min. The distribution of the nickel particles over the alumina support was obtained using energy-dispersive X-ray (EDX) coupled with the SEM. 2.3. CLC in fluidized CREC Riser Simulator. The reactivity, stability and regenerability of the oxygen carriers were established using a CREC Riser Simulator operated under the conditions as expected in a large scale fluidized CLC unit.5,8 The CREC Riser Simulator is a bench scale mini-fluidized bed reactor consisting of (i) a 50 mL reactor basket, (ii) heaters (upper and lower), (iii) a vacuum box and (iv) an impeller.40 The reactor is placed between the upper and lower heaters to maintain uniform temperature of the oxygen carrier bed. The reactor is connected to the vacuum box (1000 mL) by a four port automatic control valve to ensure precise termination of the reaction following prespecified reaction time. The vacuum box also serves as a product container and directly connected to a GC for product analysis. The impeller, placed on top of the reactor, can rotate up to 7000 rpm speed. The rotation of the impeller forces the gas to flow outward of the impeller center and downward into the reactor annulus, leading to the fluidization of the solid oxygen carriers and ensure intimate contact between gaseous fuel and solid phase oxygen (oxygen carrier). For the CLC experiments, 0.4 g of oxygen carrier was loaded in the reactor basket. After leak test, the system was purged with argon to flash out the air completely. Meanwhile, the temperature program was started to heat the reactor to the desired temperature (up to 700 °C). During the heating period, argon flow was maintained to avoid any interference of air into the system. The argon flow was stopped as the reactor attained to the desired temperature. The reactor was isolated from the vacuum box when it reached to approximately atmospheric pressure. The vacuum box was further evacuated down to 3.7 psi using

a vacuum pump. At this stage the impeller was turned on and fuel (CH4) was injected into the reactor using a preloaded gastight syringe. The pressure profile of the reactor was recorded during the combustion of methane using a pressure transducer. At the end of the prespecified reaction time, the isolation valve was automatically opened up and transferred all the reactor contents into the vacuum box. The immediate decrease of the reactor pressure confirmed the complete transfer of the reactor contents and ensured the termination of the reaction after the prespecified time. Finally, the product was analyzed using gas chromatography. The reduced oxygen carrier was regenerated (oxidized) using an air flow at a specific temperature during preset reaction time.

3. RESULTS AND DISCUSSION 3.1. Reducibility and oxygen carrying capacity. The TPR/TPO experiments were conducted to demonstrate the reduction and reoxidation characteristics of the prepared oxygen carriers. They also allow estimating the oxygen carrying capacity of the oxygen carriers. Figure 1A displays the reduction profiles of support γAl2O3, unmodified NiO/γAl2O3, and Ce modified NiO/Ce(1)-γAl2O3

Figure 1. TPR profiles of (A) (i) γAl2O3, (ii) NiO/γAl2O3, and (iii) NiO/Ce-γAl2O3, and (B) (i) NiO/Ce(1)-γAl2O3 and (ii) NiO/Ce(5)γAl2O3.

samples. It is important to mention here that both the unpromoted and Ce promoted oxygen carrier samples were synthesized using the same amount of nickel (20 wt %) loading. One can see from Figure 1A that the hydrogen consumption by the support γAl2O3 is almost zero, indicating that the support itself is not capable of carrying oxygen to the reduction process. The nickel containing samples show significant amount of hydrogen consumption. The NiO/γAl2O3 sample shows a relatively wide asymmetric reduction profile between 400 and 700 °C with the peak temperature around 660 °C. There is a long tail on the right side of the TPR profile (of NiO/γAl2O3), C


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increasing the percentage of the Ce from 1 to 5 wt % has negative effects on the reducibility of nickel oxides. With higher Ce loading, the height of the first reduction peak was significantly decreased while the height of the second peak was considerably increased. In addition to that, an extra small peak at around 600 °C also appeared. Similar behavior was reported by Solsona et al.,47,48 where they found that the reducibility of nickel oxide species decreased when increasing the amount of ceria and the reduction peaks shift to higher temperature. The small peak at 600 °C represents the availability of the surface oxygen of the cerium oxide. This is believed to be the consequence of the interaction between the cerium and nickel oxides, which reduces comparatively at higher temperature. Bortolozzi et al. also observed similar negative effects of excessive CeO2 loading on the catalyst reducibility.49 They found that the presence of small amounts of cerium enhances the ethanol conversion until the cerium to nickel ratio reaches 0.17 and that the presence of cerium decreases the conversion at higher ratios.49 The previous results suggest there is an optimal amount of cerium that can maximize the performance of the oxygen carrier. This effect was proposed by several researchers.50−52 The decreasing trend of the reducibility of NiO species suggests the presence of SMSI-like interactions (Strong metal/support interaction) between the cerium and nickel oxides, which leads to the decrease of the NiO activity. SMSI-like interactions between the cerium oxide and nickel were suggested by Chung et al.53 and Zhuang et al.43 The addition of 5 wt % ceria increases the total metal loaded to the support to around 25 wt %, including NiO, which might have caused agglomeration and sintering of the oxygen carrier when the metal loading goes beyond 20 wt %.19,36 Elbaba et al.54 also suggested that sintering might be responsible for such a drop in the NiO activity with an excessive amount of ceria promoter. Another potential reason for the decrease of NiO activity is the blockage or coverage of active NiO sites with excess amounts of ceria.46,55 The amount of oxygen available during the reduction process was calculated by integrating the area under the reduction curve. The total oxygen consumed as well as the available oxygen content below 600 °C is provided in Table 1. In the

indicating a significant amount of nickel species reduction beyond the peak temperature. This also suggests the presence of more than one nickel oxide species on the γAl2O3 support. Previously, Richardson and his co-workers reported similar reduction profile of nickel oxide supported on alumina.39 According to Richardson et al., the nickel oxide phases reduce in the temperature range 325−700 °C.39 Nickel aluminate reduces at relatively higher temperature (above 650 °C). Rynkowski et al. also suggested the same reduction behavior of a Ni-alumina catalyst for steam reforming of methane.41 They showed that NiO supported on alumina and a spinel of NiAl2O4 differ in reducibility. By TPR and XRD analysis, it was demonstrated that nickel has a tendency to interact with the alumina support to form nickel aluminate, which reduces around 780 °C. Jin et al. showed that the formation of NiAl2O4 on a NiO/γAl2O3 oxygen carrier is mainly responsible for the low reduction, given that NiAl2O4 can be stable up to 900 °C.42 In the context of the present study, after Ce modification, the reduction peak was shifted to lower temperature (which is favorable for CLC), where the relatively weak attached nickel oxide particles reduced easily. The sharper and taller reduction peak of the Ce modified sample indicates the availability of homogeneous nickel oxides on the Ce-γAl2O3 surface. The shift in the peak temperature and the improved reducibility is considered to be the promotional effects of Ce that help to minimize the interactions between the nickel and the support leading to the formation of easily reducible nickel oxides.43 The species reduced above 720 °C are mainly the strongly attached nickel oxide particles on the support.5,27,36 The promotional effects of Ce on the oxygen carrier can be attributed to the following factors: (i) First, the addition of Ce to γ-Al2O3 support improves the thermal stability of the support and minimizes the phase transition of the support from γ-Al2O3 to α-Al2O3, which has much less surface area and thus lower activity. The XRD results of the Ce promoted oxygen carrier clearly indicate the presence of γ-Al2O3, with no phase transformation even after the thermal treatment of the support (shown in section 3.3). Previous studies also reported the positive role of the rare-earth metals, such as La and Ce in preventing the phase transformation35 and surface area loss of support γ-Al2O3 that was used in methanation44 and reforming catalysts.45 (ii) Second, the introduction of Ce improves the dispersion of the nickel oxide species over the Ce-γAl2O3 support, and accordingly, the number of active sites increases. The pulse chemisorption analysis (in the following section 3.2) also confirmed the improved nickel dispersion after Ce modification. (iii) Third, the reducibility of the nickel oxide species is enhanced by inhibiting the formation of NiAl2O4, which can only be reduced at very high temperature. XRD results show no formation of nickel aluminate for the ceria promoted oxygen carriers. Previously, Liu et al.46 also reported enhanced reducibility of Ni/La-SBA-15 and Ni−Ce/SBA-15 catalysts in comparison with the Ni/SBA-15 catalysts, applied in ammonia decomposition. The authors suggested that the smaller crystal size of NiO on the Ce/La modified SBA-15 was mainly attributed to the improved reducibility. In order to investigate the effects of the amount of Ce loading, a second sample was prepared with 5 wt % Ce loading while keeping the nickel loading the same at 20 wt %. Figure 1B compares the reduction behavior of two Ce modified nickel based samples. The first sample NiO/Ce(1)-γAl2O3 was modified with 1 wt % Ce, while the second sample NiO/ Ce(5)-γAl2O3 was modified with 5 wt % Ce. Apparently,

Table 1. Oxygen Carrying Capacity of the Prepared Oxygen Carriers Amount of oxygen released (mmol/g) Sample

600 °C

Total

NiO/γAl2O3 NiO/Ce(1)-γAl2O3 NiO/Ce(5)-γAl2O3

0.61 1.88 0.86

1.83 1.35 1.72

2.44 3.23 2.58

context of CLC, the availability of reducible oxygen at low temperature is highly desired in order to facilitate the complete combustion of the fuels. The sample modified with 1 wt % Ce gives the highest amount of oxygen release for both the total content and the content below 600 °C temperature. As mentioned in the Experimental Section, the TPR/TPO cycles experimentally simulate the repeated combustion and regeneration reactions during the CLC process. The cyclic TPR/TPO experiments were conducted to measure the percentage reduction and the oxygen carrier stability over multiple redox cycles. The amount of hydrogen consumed was used to calculate the number of reducible species as follows: D


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Energy & Fuels WNi =

MWNiVH2 vρg

understand the surface chemistry and the effect of controlling variables on the oxygen carrying properties. 3.2. Metal dispersion and crystal size. Table 2 summarizes hydrogen pulse chemisorption analysis results as

(1)

where WNi represents the weight of reducible species, MWNi the molecular weight of reducible species, VH2 the volume of hydrogen consumed at STP, v the stoichiometric number based on the following reaction stoichiometry: NiO + H2→ Ni + H2O, and ρg the gas molar volume at STP. Furthermore, the percentage reduction can be calculated as W %Reduction = Ni × 100% WO (2)

Table 2. Dispersion and Crystal Size


where WNi represents the weight of the reducible species and WO is the actual metal amount on the oxygen carrier. Figure 2 plots the percentage reduction of nickel species on NiO/γAl2O3 and NiO/Ce(1)-γAl2O3 samples during the

Sample

TPR/TPO cycle (No.)

Dispersion (%)

Crystal size (nm)

NiO/Ce(1)-γAl2O3 NiO/Ce(1)-γAl2O3 NiO/Ce(5)-γAl2O3 NiO/Ce(5)-γAl2O3

1 10 1 10

1.86 1.67 1.63 1.64

50.4 50.7 51.5 51.9

obtained for oxygen carrier samples after the first reduction cycle and after 10 TPR/TPO cycles. As mentioned earlier, in typical pulse chemisorption experiments, hydrogen pulses were injected to the oxygen carrier sample to be chemisorbed and the excess hydrogen volume was recorded to calculate the dispersion and crystal size. The chemically adsorbed hydrogen on the active sites is used for the calculation of the dispersion percentage according to the following equation: %D =

CX Wf

(3)

where C is a constant, X is the total chemisorbed hydrogen, W is the percentage of weight metal, and f is the reduced metal fraction. The crystal average size (dv) can be calculated using the following formula: dv = Figure 2. Reduction percentage of nickel oxide of NiO/Ce(1)-γAl2O3 and NiO/γAl2O3 samples over repeated TPR/TPO cycles.

ϕVm 1 × Sm %D

(4)

where φ is the particle shape constant, Sm is the average surface area of the exposed metal surface per metal atom surface, and Vm is the volume of metal atoms. Apparently, increasing the percentage of Ce has minimal impact on the metal dispersion, as can be seen from Table 2. This observation is consistent, as previously confirmed by Liu et al.46 However, one can see that both the metal dispersion and nickel crystal sizes remained stable over the repeated TPR/TPO cycles, which further confirms the oxygen carrier stability under the repeated oxidation/reduction cycles and the absence of agglomeration. 3.3. XRD analysis. XRD analysis was performed using fresh oxygen carrier samples and reduced samples after 10 reduction/ oxidation cycles. The tests were carried out to detect any phase transformation (γAl2O3 to αAl2O3) during the preparation and/ or redox cycles, which is crucial for the oxygen carrier stability and deactivation resistance, because αAl2O3 has low surface area as compared to γAl2O3. Figure 3 displays the diffraction patterns of fresh and reduced (after 10 TPR/TPO cycles) NiO/Ce(1)-γAl2O3 and NiO/ Ce(5)-γAl2O3 samples. The XRD patterns have not shown any peaks corresponding to CeO2 or CeAlO3, which indicates a uniform dispersion of the cerium phases on the support. This could also be as a result of the relatively low concentration of Ce. The presence of γAl2O3 is confirmed in all the samples (fresh and reduced), which indicates that there is no transformation of the γAl2O3 support to αAl2O3 and explains the stability of the oxygen carrier over the repeated cycles.45 For the fresh oxygen carrier samples, the diffraction peaks other than those for the support (at 37.6°, 43.5°, and 75.8°) are assigned to nickel oxide. The diffraction peak at 43.5° for fresh NiO/Ce(5)-γAl2O3 sample disappeared. This could be a result

repeated TPR/TPO cycles carried out up to 900 °C. As can be seen in this figure, approximately 94 wt % nickel conversion was achieved using the Ce modified sample as opposed to only 77 wt % of nickel oxide species on γAl2O3. The Ce modified sample also demonstrates consistent percentage reduction of nickel oxide over the cyclic TPR/TPO experiments, indicating that the sample is stable in the repeated redox cycles. This observation suggests that no phase transformation (for γAl2O3) has taken place during the cyclic redox process. Therefore, a significant improvement of the oxygen carrying capacity was observed using the NiO/Ce-γAl2O3 oxygen carrier. The addition of Ce helps the formation of easily reducible nickel oxide species and minimizes nickel support interaction, which promotes the formation of less-reactive nickel aluminates. Previously, the present research group demonstrated improved reduction/reoxidation behavior of a La modified nickel material.36 It was shown that the presence of La created a barrier phase which minimized the interaction between nickel and support alumina, resulting in superior reducibility and oxygen carrying capacity in repeated CLC cycles. Based on the above analysis and discussion, it is obvious that the variation in the TPR profiles and the oxygen carrying capacity of the nickel based materials are mainly due to the presence of varied concentrations of nickel oxide species. Even though both the Ce promoted and unpromoted samples were prepared with the same amount of nickel loading, the presence of Ce turned out to be the determining factor for the formation of easily reducible NiO. Thus, it would be highly desirable to E


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circulating air at 550 °C for 15 min. The reaction time was varied from 10 to 60 s to demonstrate the effects of contact time on the conversion. The repeated methane combustion and oxygen carrier regeneration cycles were performed using the same oxygen carrier sample to test the stability of the oxygen carrier over multiple CLC cycles. The combustion product samples were analyzed using gas chromatography. The gaseous product mainly contains CO2 and H2O and a trace amount of CO. The very low concentration of CO is due to the complete combustion of methane with NiO.56 Another reason for the absence of CO in the product might come from the fact that hydrogen and CO are very reactive with the nickel oxide, as confirmed by the TPR and several previous studies.28,57,58 As a result any traces of CO formed due to incomplete combustion were further converted to CO2 by a water-gas shift reaction. Based on the pressure profile and product analysis, one can consider the following reactions in the fluidized bed CLC process with methane:59−61 Fuel combustion

Figure 3. XRD patterns of the prepared oxygen carriers.

CH4 + 4NiO → 4Ni + CO2 + 2H 2O

of the overlapping of the two peaks at 43.5° and 45.6° for NiO and γAl2O3, respectively.45,56 No major diffraction peaks for nickel aluminate were observed. For confirmation, the XRD for the reduced samples at 750 °C was also plotted in Figure 3. It clearly shows that the modification of the support with cerium oxide minimizes the formation of nickel aluminate. For the reduced samples, the diffraction lines confirm the presence of the support and reduced nickel (Ni) with no detection of nickel aluminate, which cannot be reduced at 750 °C, and this further confirms the promotional effects of cerium oxide. Thus, the XRD analysis confirmed the successful loading of NiO on CeγAl2O3 with no transformation of the support and little or no formation of the difficult reducible nickel aluminate species. 3.4. SEM-EDX analysis. Scanning electron microscopy (SEM) was performed coupled with energy dispersive X-ray analysis (EDX). The elemental distribution images can be used to visualize the quality of the dispersion. Figure 4 shows the

H 2 + NiO → Ni + H 2O

CO + NiO → Ni + CO2

CH4 + NiO → Ni + CO + 2H 2

Reforming CH4 + H 2O → CO + 3H 2 CH4 + CO2 → 2CO + 2H 2

Decomposition CH4 → C + 2H 2

Water-gas shift reaction CO + H 2O → CO2 + H 2

In the context of the present study, the direct contribution of cerium oxide(s) in fuel combustion can be considered as minimal; therefore, it did not appear in the above reactions steps. The small quantity of Ce (1 wt %) mainly interacts with alumina and itself does not carry a significant amount of the oxygen for fuel combustion. The TPR analysis (Figure 1) and XRD patterns (Figure 3) of NiO/Ce(1)-γAl2O3 support the noncontribution of cerium oxide(s), as both these characterizations have not shown any peak corresponding to CeO2 or CeAlO3. Following the reduction (in fuel combustion step), the reduced oxygen carrier was regenerated by flowing air for 15 min to reoxidize the metal after the combustion process. Consequently, the following reactions can be assumed during the regeneration process of the oxygen carrier.56,59

Figure 4. Nickel distribution of (a) NiO/Ce(1)-γAl2O3 and (b) NiO/ Ce(5)-γAl2O3.

nickel element distribution over the oxygen carrier samples. It is evident that the nickel particles are well dispersed on the CeγAl2O3 support. This indicates superior dispersion of the nickel oxide over the Ce-γAl2O3 support. The addition of cerium oxide improves the surface area and the dispersion of the nickel, which enhances the oxygen carrier reducibility, as confirmed by Liu et al.46 3.5. Fluidized CLC. Following the promising characteristics, the NiO/Ce(1)-γAl2O3 oxygen carrier was further evaluated in a fluidized CREC Riser Simulator using methane as fuel. After each combustion cycle the oxygen carrier was regenerated by

Ni + 1/2O2 → NiO

C + O2 → CO2

The conversion of methane and nickel oxide are the two important parameters assessed from the CLC experiments. Methane conversion is calculated from the product analysis data considering CO2 is the only carbon containing species in the methane combustion with nickel oxide: F


Article

Energy & Fuels XCH 4 =

CCO2, out CCH 4, out + CCO2, out

× 100% (5)

where CCH4,out and CCO2,out are the final concentrations of methane and carbon dioxide, respectively, after the desired reaction time. Nickel oxide conversion is calculated on the basis of the number of moles of reacted oxygen, the weight of the carrier, and the nickel composition of the carrier according to the following equation:


XNiO =

4 × NCH 4o × XCH4

(

w × XW , NiO MWNiO

)

× 100% (6)

where NCH4o is the initial number of moles of methane injected into the reactor, XCH4 is the conversion of methane, w is the mass of oxygen carrier loaded into the reactor, XW,NiO is the fraction of nickel oxide present in the oxygen carrier sample, and MWNiO is the molecular weight of nickel oxide. Figure 5 shows the effects of reaction temperature on the conversion of the oxygen carrier NiO/Ce(1)-γAl2O3. The

Figure 6. Effect of reaction time on NiO conversion for NiO/Ce(1)γAl2O3.

Figure 7. CH4 conversions over multiple CLC cycles using Ce promoted NiO/Ce(1)-γAl2O3 and unpromoted NiO/γAl2O3 oxygen carriers (repeated cycles were reproducible within ±1.9% standard deviation). Figure 5. Effect of reaction temperature on NiO conversion for NiO/ Ce(1)-γAl2O3.

reaction temperature was varied between 550 and 700 °C at a constant reaction time of 40 s. One can see in Figure 5 that the conversion of the oxygen carrier increased with reaction temperature, reflecting the availability of reducible nickel oxide in the oxygen carrier. The conversion of the oxygen carrier was also increased with increasing reaction time as shown in Figure 6. Figure 7 and Figure 8 plot the methane and nickel oxide conversions, respectively, during the repeated CLC cycles in the fluidized CREC Riser Simulator. It is clear from Figure 7 that the methane conversion increased substantially from about 50% to 88% after modification of the support with cerium oxide. This is due to the improvements in the nickel dispersion and the minimization of nickel aluminate formation, which leads to the formation of easily reducible nickel oxide species as shown in TRP/TPO analysis. The oxygen carrier (NiO) conversion also increased from 44% to 76% after the Ce modification (Figure 8). In the case of the unmodified nickel based oxygen carrier (NiO/γAl2O3), the reduction process was limited by the formation of nickel aluminate, that is usually formed when nickel oxide is loaded on alumina.

Figure 8. NiO conversions over multiple CLC cycles using Ce promoted NiO/Ce(1)-γAl2O3 and unpromoted NiO/γAl2O3 oxygen carriers (repeated cycles were reproducible within ±1.8% standard deviation).

4. CONCLUSION In this investigation fluidizable NiO/Ce-γAl2O3 oxygen carriers were prepared, characterized, and evaluated in a chemicalG


Article


Energy & Fuels looping combustion (CLC) cycle of methane using a fluidized CREC Riser Simulator. TPR/TPO analysis revealed that the Ce modification improved the thermal stability of support γAl2O3 and helped minimize the formation of nickel aluminate. However, excess Ce resulted in synergetic interactions between cerium oxide and the metal oxide, leading to the increase of the reduction temperature of the nickel oxide. The SEM-EDX and hydrogen pulse chemisorptions analysis show a uniform distribution of nickel species on the Ce modified γAl2O3. NiO on 1 wt % Ce modified γAl2O3 showed the highest reducibility (94%). It also reduced below 600 °C, which is highly favorable for CLC application. This sample demonstrates sustained reactivity over the repeated redox cycles. The pulse chemisorption shows a high dispersion of nickel crystal on the Ce modified support which remained unchanged with no sign of agglomeration. The XRD analysis further confirms the stability of the NiO/Ce(1 wt %)-γAl2O3 oxygen carrier with no phase transformation of the support. The NiO/Ce(1 wt %)-γAl2O3 sample displayed excellent reactivity and stability in repeated CLC cycles in a fluidized bed CREC Riser Simulator using methane as fuel and air to regenerate the reduced oxygen carrier. The combustion of methane with NiO/ Ce(1 wt %)-γAl2O3 mainly gives CO2 as combustion product, indicating the complete combustion capability of the synthesized oxygen carriers.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +966-13-860-1478. Fax: +966-13-860-4234. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was funded by the National plan for Science, Technology and Innovation (MAARIFAH)King Abdulaziz City for Science and Technologythrough the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM)the Kingdom of Saudi Arabia, Award number 11-ENV1655-04.

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Ce-Î³Al2O3 Oxygen Carrier for Chemical Looping

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