Experimental Study of Chemical-Looping Reforming in a Fixed-Bed

Article pubs.acs.org/EF

Experimental Study of Chemical-Looping Reforming in a Fixed-Bed Reactor: Performance Investigation of Different Oxygen Carriers on Al2O3 and TiO2 Support E. Karimi,† H. R. Forutan,† M. Saidi,† M. R. Rahimpour,*,†,‡ and A. Shariati† †

Chemical Engineering Department, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz 71345, Iran Department of Chemical Engineering and Material Sciences, University of California, Davis, California 95616, United States

‡

ABSTRACT: This study examines the hydrogen production by the steam reforming of methane integrated to chemical-looping reforming (CLR) as a novel technology in a fixed-bed reactor at 700−1200 °C. The particles are present in two consecutive oxidation and reduction steps. In the reduction step, the oxygen carrier is reduced with the fuel, which, in turn, is partially oxidized to H2 and CO (synthesis gas), and in the oxidation step, the reduced oxygen carrier is reoxidized with oxygen (O2 + argon). The oxygen carriers Fe, Mn, Co, and Cu using inert materials Al2O3 and TiO2 as a support are prepared by the precipitation method. The samples are analyzed using energy-dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), and X-ray diffraction (XRD) to check the carrier specifications before and after the process. The main goal of this study is investigation of the reactivity of different metal oxides on Al2O3 and TiO2 support. The conversion of fuel into products depends upon the type of oxygen carriers and experimental conditions. All of the oxygen carriers show favorable hydrogen production over the 3 cycle experiments at optimum temperature. Among used metals, Fe has the highest hydrogen yield. The optimum temperature for maximum conversion of methane over Fe-, Mn-, Co-, and Cu-based carriers is nearly 1025, 1030, 900, and 800 °C, respectively. According to experimental results, at higher temperatures, Fe- and Mn-based carriers have better performance, but at lower temperatures, the Cu-based carrier is more efficient compared to other carriers. The comparison of supports represents that the reactivity of Al2O3 is better than TiO2, so that the conversion of the fuel over Fe/Al2O3 is 95−100% in comparison to Fe/TiO2, which is 78−80%.

1. INTRODUCTION Nowadays, hydrogen is introduced as an important energy carrier in different applications, such as internal combustion engines (ICEs), fired boilers, gas turbines, and microcombustors. Efforts to improve hydrogen generation have emerged as an attractive field. The only waste product of hydrogen combustion is water; thus, it is called clean fuel and has no negative impact on the environment.1 Therefore, it is an appropriate energy carrier that can be used in gas turbine or combustion engines and other applications.2 Synthesis gas is a fuel gas mixture containing H2 and CO. Synthesis gas is mainly produced from all kinds of fossil fuels, natural gas, and conventional steam reforming (SR). The SR process is widely used for synthesis gas production.3 In the steam reformer, synthesis gas is produced by reforming of methane with water.4 Besides SR, it is suggested that chemical-looping reforming (CLR) can be applied to produce hydrogen directly.5 CLR includes partial oxidation and reforming of light hydrocarbon fuels. Two separated reactors are designed in the CLR technique, including air and fuel reactors. A metal oxygen carrier transports the required oxygen between the reactors.6 The main objective of this research is improvement of steam methane reforming (SMR) by application of the CLR concept as a novel technology and focusing on the performance of the oxygen carriers. This study considers the performance of different oxygen carriers, such as supported on Al2O3 and TiO2. For more assessment, the effect of the temperature and oxygen consumption on the reforming efficiency has been determined. © 2014 American Chemical Society

Followed by carbon deposition characteristics, sintering and agglomeration are analyzed experimentally.

2. TECHNICAL BACKGROUND 2.1. SMR. SMR as one of the major routes of hydrogen production is a type of chemical synthesis.1 Methane converts to synthesis gas by exposing it to a catalyst (usually nickel) at high temperatures and pressures. The operating temperature is about 950−1100 °C, and outlet pressure may be high, up to 100 bar. This process can improve production of hydrogen or synthesis gas compared to hydrogen produced from renewable energy sources or solid fossil fuels.4 Figure 1 shows a schematic diagram of the conventional SR reactor. 2.2. CLR. Hydrogen production can be modified by application of the CLR technique as will be discussed in the following sections. CLR was proposed in 2001 by Mattisson and Lyngfelt.7 CLR involves the partial oxidation process of methane, where a solid oxygen carrier is used as a source of undiluted oxygen.2 The CLR system is made of two interconnected reactors, designated as air and fuel reactors. In the fuel reactor, the fuel gas (CnH2m) is partially oxidized to CO and H2 by a metal oxide (MeO) that will be reduced to a metal (Me), where “Me” represents the (typically metal-based) oxygen carrier. Subsequently, the reduced oxide is transferred Received: July 23, 2013 Revised: March 12, 2014 Published: March 26, 2014 2811

dx.doi.org/10.1021/ef5003765 | Energy Fuels 2014, 28, 2811−2820

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Figure 2. General scheme of the CLR process.

Figure 1. Schematic diagram of the conventional SR reactor.

CLR process. Development of oxygen carrier materials with excellent reactivity and sufficient rates of oxidation and reduction is an essential issue in the CLR process. Also, the oxygen carrier should have adequate stability in consecutive cycle reactions under high temperatures, remarkable mechanical strength to attrition, agglomeration, and wear. It should also be environmentally, technically, and economically feasible and capable of converting the fuel to CO and H2 when the air/fuel ratio reduces.2,10,11 Moreover, because of the high temperature of the CLR reactions, the melting point of the oxygen carrier should be high enough to stand during the process. Among different oxygen carriers, Cu has the lower melting point. Because the melting point of iron oxides and cobalt oxides are high enough, they are known as the high withstand oxygen carriers at the CLR operating temperature.12 Jerndal et al.13 showed that metal oxides based on Ni, Cu, Fe, Mn, Co, and W and sulfates of Ba and Sr are applicable oxygen carriers when methane is used as fuel. Mattisson et al.14 concluded that CuOand NiO-based carriers have high reactivity with CH4, while Mn3O4 and CoO are the least reactive oxygen carriers. Mattisson and Lyngfelt investigated the performance of different possible oxygen carriers, such as NiO/Ni, Mn3O4/MnO, Fe2O3/Fe3O4, Cu2O/Cu, and CoO/Co, in chemical-looping combustion (CLC) in interconnected fluidized beds. Their studies showed that the obtained conversion using NiO/Ni and Fe2O3/Fe3O4 at high temperatures is complete but the other three systems (Mn3O4/MnO, Cu2O/Cu, and CoO/Co) are unstable at the high decomposition temperature.7 Generally, suitable metal oxides are combined with an inert material as a support. The support increases the porosity and surface area, improves the catalyst structure, and also helps to increase reactivity as well as strongly influence the deactivation of the catalyst. Therefore, choosing an appropriate support is an important factor in the preparation of oxygen carriers.15−17 Al2O3 and TiO2 are used as the most commercial CLR catalyst supports because of their high mechanical resistance, proper chemical and physical stability, and considerable melting point at high temperatures in the CLR process.1,16,18,19 In other related work, Chen et al.1 studied experimentally the effect of supports in the chemical-looping hydrogen generation using oxygen carrier Fe2O3 supported on Al2O3 and TiO2 in a batch fluidized bed. The reactivity of the Fe-based supported on

to the air reactor where it is oxidized with air, and the regenerated material is recycled to the process. The main reactions that occur with different contributions in the fuel and air reactors are as follows:2,4 fuel reactor CH4 + MeO ↔ CO + 2H 2 + Me

(1)

air reactor

1 O2 ↔ MeO (2) 2 The amount of released or required energy in each reactor depends upon the nature of the oxygen carrier and fuel.8 Because the oxidation reaction in the air reactor is highly exothermic, the required heat for the reduction reactions and also the required heat for the steam and carbon dioxide reforming, which occur simultaneously in the fuel reactor, can be provided from the exothermic oxidation reaction. Carbon dioxide reforming is preferred in comparison to SR because not only is it more effective on coke removing but it also leads to CO2 capture and decreases greenhouse gas emission.6 Me +

SR CH4 + H 2O → CO + 3H 2

ΔH298 = 206 kJ/mol (3)

CO2 reforming CH4 + CO2 → 2CO + 2H 2

ΔH298 = 247.3 kJ/mol (4)

The amount of adsorbed oxygen from the oxygen carrier determines the synthesis gas composition in the fuel reactor. Outlet gas from fuel reactor contains a gas mixture of CH4, CO2, CO, and H2.2,9 Figure 2 shows a general scheme of the CLR process. Solid carbon deposits during the reduction step of the oxygen carrier. In the air reactor, because of the high concentration of oxygen, the bonds of carbon deposited on the oxygen carriers break and, thus, some CO2 would be formed. Because of the easier accessibility to suitable oxygen carriers, relatively low cost, and environmentally sound, the CLR process can be used widely in large-scale industrial processes. The oxygen carrier plays a vital role in the performance of the 2812


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Table 2. Experimental Reaction Conditions particle temperature (°C) purge gas reactant gas (reduction) method

Figure 3. Summary of steps for catalyst preparation.

oxygen carrier

Fe/Al2O3 Fe/TiO2 Co/Al2O3 Co/TiO2 40−60 53.2 2.7 123

40−60 42.9 3.0 105

40−60 39.3 4.2 87.8

40−60 31.3 5.1 79.8

2.1

5

10.3

13

initial: 25 °C ramp: 5 °C/min to the target temperature isothermal

each experiment of CLR. In fact, the time of each test and the methane conversion mainly depend upon the oxygen transport capacity of the oxygen carriers. The most significant characteristic of a successful oxygen carrier is its reactivity in both reduction and oxidation cycles. In this work, the precipitation method was used to prepare suitable oxygen carriers. This method is based on the precipitation by changing the pH level of a nitrate solution. With using this method, a good distribution of the high active metal content on the support internal surface occurs. A weighted amount of metal nitrate and Al(NO3)3· 9H2O or TiO2 were dissolved in a beaker with distilled water. Because solubility of TiO2 in water is limited, using acidic conditions is a common method to overcome this problem. Therefore, TiO2 was dissolved in water by diluted H2SO4. When the salts were completely dissolved, the desired amount of NH4OH was added slowly until the solution pH reached 8−9. The solution was heated to 70−80 °C and sufficiently stirred. When the solution was homogeneous with the suitable viscosity, it was loaded in a water bath for 10 h. Consequently, the precipitate was filtered and washed with distilled water. After that, the paste mixture was dried for 6 h at 100 °C in the oven, because the surface water should be evaporated. In the first, the synthesized materials were calcined in an electrical furnace under air atmosphere conditions by increasing the temperature from ambient conditions up to 700 °C, kept at this temperature for 2 h, followed by a second calcination step under air atmosphere conditions at 1250 °C (for Fe, Mn, and Co) and 1000 °C (for Cu) with a ramp of 5 °C/min, and remained for 6 h at the final temperature. This procedure was applied for increasing the catalyst strength because of the high vapor pressure of water. The synthesis of the oxygen carriers is schematically shown in Figure 3.

Table 1. Characterization of the Fresh Oxygen Carrier Particles theoretical weight ratio (wt %) porosity (%) real density (g/mL) BET specific surface area (m2/g) crushing strength (N/mm)

Fe, Mn, Co, and Cu on the Al2O3 support isothermal hold (700, 750, 800, 850, 900, 950, 1000, 1030, 1050, 1100, 1150, and 1200 °C) Ar CH4 and CO2

Al2O3 was better than TiO2, because Al2O3 provides a more porous regular structure and, as a result, higher surface area. Bradford et al.20 compared the performance of MgO, TiO2, and SiO2 as a support for methane reforming with carbon dioxide. Their results showed a strong metal−support interaction in the TiO2 case.

3. EXPERIMENTAL SECTION 3.1. Oxygen Carrier Preparation. Oxygen carrier preparation was begun in Japan in 1994, and after 2000, the studies developed by groups at Chalmers University of Technology in Sweden and at ́ Instituto de Carboquimica−Consejo Superior de Investigaciones ́ Cientificas (ICB−CSIC) Zaragoza in Spain.21 To determine the required precursor amounts, a molar balance should be considered in

Figure 4. Reactor system for CLR of natural gas with hydrogen production. 2813


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Figure 5. SEM pictures of the fresh and reacted oxygen carrier particles after multi-cycle experiments: (a) fresh Fe/Al2O3, (b) reacted Fe/Al2O3, (c) fresh Fe/TiO2, (d) reacted Fe/TiO2, (e) fresh Co/Al2O3, (f) reacted Co/Al2O3, (g) fresh Co/TiO2, and (h) reacted Co/TiO2. 3.2. Characterization of Oxygen Carriers. The oxygen carriers were characterized by X-ray diffraction (XRD) to investigative the new phase formation in the oxygen carrier after cycles and comparing them to the oxygen carriers present before cycles. XRD patterns were discovered by a powder diffractometer (Bruker D8 Advance, Germany) using Cu Kα radiation, operated at 40 kV and 40 mA. The samples were scanned in a step-scan mode, in the angular 2θ range of 10−90°, with a step size rating of 0.05°/s. The morphology of the oxygen carriers was considered in the fresh and reacted samples using scanning electron microscopy [SEM, Vega 2 Tescan (Czech Republic) microscope instrument]. The same instrument was also used to check energy-dispersive X-ray analysis (EDX) of each oxygen carrier. In addition to the specific surface area [Brunauer−Emmett− Teller (BET)], porosity of the fresh oxygen carriers was studied, using an ASAP 2010 instrument (Micrometrics). The crushing strength and real density were measured using a digital force gauge machine (Shimpo FGN-5). Table 1 represents some measured structural

properties of these particles, such as percent porosity, real density, specific surface area (BET), and crushing strength. As reported in Table 1, the specific surface areas of Fe/Al2O3, Fe/TiO2, Co/Al2O3, and Co/TiO2 are 123, 105, 87.8, and 79.8 m2/g, respectively. It is clear that the Fe-based oxygen carrier exhibits the maximum surface area, while the Cu-based oxygen carrier has the lowest surface area. The crushing strength of the prepared fresh particles is highly dependent upon the type of active metal oxide and the inert material used as a binder (support); i.e., the porosity of the particles is increased by the addition of inert fillers. The high crushing strength of particles can be explained by the lower porosity of these particles. The Fe-based oxygen carrier showed a low crushing strength value, especially when it was prepared using Al2O3 and sintered at temperatures above 1025 °C. Generally, it is observed that Al2O3-supported oxygen carriers have the higher specific surface area and lower crushing strength compared to TiO2-supported oxygen carriers. 2814


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Figure 7. Oxygen consumption during 3 oxidation reaction cycles at 900 °C: (a) Fe/Al2O3 and (b) Fe/TiO2.

Figure 6. H2/CO ratio at 900 °C: (a) Fe/Al2O3 and (b) Fe/TiO2.

Table 3. Composition of the Solid Particles (Fresh and Reacted) oxygen carrier

estimated (wt %)

Fe/Al2O3

40:60

Fe/TiO2

40:60

Co/Al2O3

40:60

Co/TiO2

40:60

component Fe Al O impurities Fe Ti O impurities Co Al O impurities Co Ti O impurities

fresh (wt %) 40.66 30.64 28.70 0 40.01 30.50 29.49 0 39.41 30.93 29.66 0 40.05 34.50 25.23 0

Table 4. Main Crystalline Phase before and after 3 Cycle Experiments

reacted (wt %) 40.46 30.02 28.77 0.75 39.72 30.05 29.34 0.89 39.01 28.62 29.82 2.55 28.18 43.74 25.25 2.83

oxygen carrier

main crystalline phase

Fe/Al2O3 (fresh) Fe/Al2O3 (reacted) Fe/TiO2 (fresh) Fe/TiO2 (reacted) Co/Al2O3 (fresh) Co/Al2O3 (reacted) Co/TiO2 (fresh) Co/Al2O3 (reacted)

Fe3O4, Fe2O3, FeO, Fe, FeAl, and Al2O3 Fe3O4, FeO, Fe, and FeAl2O4 Fe3O4, Fe2O3, FeO, FeTiO3, Ti3O5, TiO2, and TiO Fe3O4, Fe2O3, FeO, Fe, FeTiO3, Ti3O5, TiO2, and TiO Co3O4, Co, and Al2O3 CoO, Co, and CoAl2O4 Co3O4 and TiO2 Co, Co2Ti4O, CoTi2O5, CoTiO3, Ti3O5, TiO2, and TiO

an electrical furnace for heating the system to the desired operating temperature. Figure 4 shows a schematic diagram of the fabricated setup. In the first step, which is called the oxidation step, oxygen is transferred from the reformer air to the oxygen carrier. The oxidation stream considered 20% O2 and 80% Ar. The gas flow into the reactor was controlled using an electronic low-pressure mass flow controller (LP-MFC). Then, in the transition step, the residual oxygen from the previous step in the reactor must be pushed out using argon. Argon was used as an inert carrier gas for dilution of the methane and oxygen, and a recycle stream was considered to provide a sufficient reaction time. Finally, in the reduction step, oxygen is transferred from the oxygen carrier to the fuel. In most experiments, the reducing gas composition was 40% CH4, 10% CO2, 3% water (steam), and 47% Ar. The reactions in the oxidation and reduction steps occurred at

3.3. Reactor System. The reactor design of the CLR reformer must be done carefully, because direct contact between the oxygen carrier and the gas phase is important. In the present work, multi-cycle reduction−oxidation reactions (redox) were conducted in a cylindrical fixed-bed micro-reactor. The reactor was made of stainless steel, which has an inner diameter of 12 mm. The reformer reactor was located in 2815


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Figure 8. Volumetric percentage profiles of the product in the second reduction cycle at 900 °C: (a) Fe/Al2O3, (b) Fe/TiO2, (c) Co/Al2O3, and (d) Co/TiO2. atmospheric pressure in the reformer tube and catalyst located inside the reactor. In this work, different oxygen carriers on various supports are synthesized. The oxygen carrier is loaded in a crucible. This instrument allows for rapid cycling of a small amount of carrier in a repeatable way with acceptable manual work. The initial weight of the used metal sample was 2.1 g. The oxygen carriers were then sieved to achieve a particle size of 20−40 mesh. Before starting oxidation reactions, samples of Fe or Co on Al2O3 or TiO2 were preheated (5 °C/min) under an inert (argon) atmosphere to the desired temperature (e.g., 700−1200 °C), because increasing the temperature of the reforming reaction zone enhances the reformer efficiency. Because the temperature difference between the furnace and reformer tube reactor is limited to a rather low value, this could be found by socalled heat-exchange reforming, but it is questionable whether this case of the process is commercially attractive.18,20,22 According to the experimentally estimated lifetime for each particle and also the reaction time of burning of methane with using oxygen carrier, the operation time has been considered at 6 h to prevent oxygen carrier particle agglomeration. The outlet gas stream of the reduction step involved H2, CO, and a small percentage of CH4 and CO2. An outlet gaseous sample containing a mixture of species was injected to gas chromatography (GC). A summary of the reaction conditions is presented in Table 2.

of the particles were made by the precipitation method. The composition and their suitability of the solid particles before (fresh catalyst) and after (reacted catalyst) the CLR process are represented in Table 3. The impurities that are represented in this table contain mostly carbon and a negligible amount of other impurities. The EDX analysis confirms that the synthesized oxygen carriers are highly stable under CLR operating conditions. 4.2. Effect of the Support. One of the most important parameters in the catalytic systems is the stability of the support under reaction conditions. It means that, at temperatures around the desired reaction temperature, there should be no phase changes in the support structure, because any phase changes in the support accelerates catalyst deactivation and, as a result, decreases the profitability in metal/support systems. The common routes of support-related deactivation, which lead to the decrease in activity and selectivity of catalysts, are decreasing the surface area of the support and sintering the active metal particles. However, investigation of XRD results (Table 4) represents that the main deactivation route of the support involves absorption of the metal catalyst by the support to form metal−support product phases, such as metal TiO2. This may be due to unsuitable dispersion of active sites on TiO2 compared to Al2O3, causing a blockage of the pores in this support and confirming the results obtained from the XRD analysis.

4. RESULTS AND DISCUSSION 4.1. Oxygen Carrier Particles. EDX is an analytical method used for analyzing the chemical components, including Fe and Co on Al2O3 and TiO2, where the weight ratios of the solid reactants to binder of Al2O3 and TiO2 were set at 4:6. All 2816


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figure 7, the percentage of consumed oxygen through the second cycle is much higher than those through the first and third cycles. The volumetric percentage profiles of products in the outlet gas stream (inert-gas-free basis) through the second cycle of the reduction step (methane reforming) are shown in panels a−d of Figure 8. As shown in this figure, the methane conversion to hydrogen using the Al2O3 support for both Fe and Co catalysts is higher than that using TiO2-supported catalysts. The better performance of Al2O3 compared to TiO2 can be considered from several perspectives. First, the required time to reach a certain conversion over Al2O3 is lower than that over TiO2. Second, hydrogen production for both carriers (Fe and Co based) on the Al2O3 support is much greater than that on the TiO2 support. Third, the required time to reach the maximum hydrogen production is equal for both supports when the same carriers are used. It is also evident that the required time to reach the maximum value of methane conversion decreases when Co is used as an oxygen carrier. The maximum conversion for Co/Al2O3 is 12% higher than that for Fe/Al2O3. On the basis of reported results in Figure 8, Co generally has a better performance than Fe when a specific support is used at 900 °C. Panels a and b of Figure 9 illustrate the methane conversion using different oxygen carriers at a temperature range of 700− 1200 °C. Increasing the temperature improves methane

SEM can present the change in the surface morphology and the roughness of the catalysts; i.e., these pictures show the effect of reduction and oxidation reactions and their chemical and physical transformation details. Figure 5 shows that the SEM pictures of oxygen carriers (Fe/Al2O3, Fe/TiO2, Co/Al2O3, and Co/TiO2) were taken before and after multi-cycle experiments. Interestingly, the pore size of the oxygen carriers on the Al2O3 support was obviously larger than the TiO2 support. As reported in Table 1, because the TiO2 support presents a small surface area, it has a low thermal stability. As observed in this figure, the active sites of carriers are occupied by coke and carbon deposits and mostly deactivated after CLR cycles. Because in every cycle of the CLR process, the oxygen carriers endure main chemical and structural changes at high temperatures, it is observed that the mainly active sites lose after the third cycle. In this study, four metal oxides on Al2O3 and TiO2 support are used as oxygen carriers in 3 cycles of redox. In each cycle, the minimum required reduction time for each carrier to reach complete methane conversion was suggested as 60 min, while this time for the oxidation step was 35 min. It should be noted that, to determine the optimum operating temperature for each carrier, all experiments are performed at the temperature range of 700−1200 °C. Figure 6 illustrates the hydrogen/carbon monoxide ratio (H2/CO) in the effluent gas stream of the reduction step in 3 cycles of Fe/TiO2 and Fe/Al2O3 versus time at 900 °C. As shown in this figure, the H2/CO ratio in the second cycle is higher than two others. It is due to the fact that the oxygen carrier reactivity in the first cycle is different because of some instability, but through the next cycles, more hydrogen is generated. Indeed, it can be concluded that the particles need 1 cycle to reach a more favorable structure. Therefore, in the following, the second cycle is used to study the effect of other related parameters. According to panels a and b of Figure 6, it is observed that the required time to attain the maximum H2/CO ratio for the TiO2-supported oxygen carrier is lower than that for the Al2O3-supported oxygen carrier. The maximum point for Fe/Al2O3 and Fe/TiO2 occurs at 38 and 15 min, respectively. Also, as indicated in this figure, the hydrogen conversion is reduced by increasing the time because of catalyst deactivation through the third cycle. As expected in the XRD analysis, this decreasing rate for the TiO2-supported oxygen carrier is more severe than that for the Al2O3-supported oxygen carrier. As the time passes and also through different cycles, in the same reaction conditions, the rate of hydrogen production has been reduced, even after reoxidizing the carrier. This continued decreasing trend shows the deactivation of carriers. Panels a and b of Figure 7 indicate the rate of absorbed oxygen by reduced carriers through 3 cycles of the oxidation process over Fe/Al2O3 and Fe/TiO2 at 900 °C. The inlet gas to the oxidation step contains 20% oxygen and 80% argon. Because most of the active sites are reduced in the first reduction reaction, active site regeneration is more difficult in the next oxidation reactions. Generally, Al2O3 as a support provides a porous structure and higher surface area for the CLR process; therefore, the oxygen consumption by Fe/Al2O3 is higher than that by Fe/TiO2. It is noticeable that the maximum oxygen consumption by Fe/TiO2 is 1.2 times more than that by Fe/Al2O3. Also, because of the higher porosity, the rate of oxygen absorption for Fe/Al2O3 at the beginning of oxidation reactions is lower than that for Fe/TiO2. As represented in this

Figure 9. Comparison of the CLR performance under different operating temperatures: (a) Fe/Al2O3 and Fe/TiO2 and (b) Co/Al2O3 and Co/TiO2. 2817


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Figure 10. CO/CO2 ratio at 900 °C through the second reduction cycle: (a) Fe/Al2O3, (b) Fe/TiO2, (c) Co/Al2O3, and (d) Co/TiO2.

the reduction time to reach a maximum amount. Figure 11 shows that the highest H2/CO ratio belongs to the Fe-based carrier compared to other metals, such as Mn, Cu, and Co. The maximum ratio of H2/CO for Fe-, Mn-, Cu-, and Co-based carriers is 3.8, 2.98, 3.1, and 2.5, respectively. Although Fe has the highest hydrogen production, the required time to reach this maximum point is longer than other carriers. According to this figure, at the beginning of the second cycle, all carriers have the same performance. Also, the required time to reach the maximum H2/CO ratio over Mn, Co, and Cu is approximately the same. Because of catalyst deactivation, the hydrogen production decreases with increasing the time. The main reason for catalyst deactivation in this stage is sintering phenomena because of the high operating temperature. The deactivation rates for Co/Al2O3 and Cu/Al2O3 oxygen carriers are more severe than those for Fe/Al2O3 and Mn/Al2O3 oxygen carriers. The experimental results of the CLR process through the second cycle using Fe/Al2O3, Mn/Al2O3, Co/Al2O3, and Cu/ Al2O3 as the oxygen carrier have been put into a maximum methane conversion−temperature diagram in Figure 12. As represented in this figure, generally, Fe- and Mn-based carriers have the most conversion among other carriers. High reactivity in both reduction and oxidation cycles and high resistance to agglomeration are the important properties of Fe and Mn.

conversion to reach a maximum value. However, as illustrated in this figure, increasing the temperature more than a certain value reduces the catalyst active sites and, as a result, decreases the methane conversion. Therefore, the maximum conversion can be considered as an optimum point for each carrier. The maximum methane conversion in the presence of Fe- and Co-based oxygen carriers happens at 1025 and 900 °C, respectively. On the basis of reported conversions in Figure 9, the optimum operating temperature for different oxygen carriers is independent of the support type and can only be affected by the oxygen carrier properties. The optimum temperature for the Fe-based oxygen carrier is higher than that for the Co-based oxygen carrier. To obtain a free CO2 stream in the outlet of the reduction step, it is necessary to achieve a complete methane conversion. Because the complete conversion of gaseous fuels cannot be achieved in a single-stage reactor, the CO/CO2 ratio should be maximized. The amount of the CO/CO2 ratio for Co/Al2O3, Co/TiO2, Fe/Al2O3, and Fe/TiO2 through the second reduction cycle is shown in Figure 10. This figure represents that generally the carriers supported on Al2O3 have a higher CO/CO2 ratio compared to carriers supported on TiO2. 4.3. Effect of the Oxygen Carrier. Figure 11 represents H2/CO in the effluent gas stream (inert-gas-free basis) of the second reduction cycle for different oxygen carriers at 800 °C. It is observed that the H2/CO ratio is enhanced by increasing 2818


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the highest hydrogen yield. Also, the conversion of the oxygen carriers supported on Al2O3 is better than TiO2 because Al2O3 provides a more porous structure and a higher surface area for the CLR process. The results indicate that the maximum methane conversion using the Fe-, Mn-, Co-, and Cu-based carriers happens at nearly 1025, 1030, 900, and 800 °C, respectively. At higher temperatures, Fe- and Mn-based carriers have better performance, but the Cu-based carrier is more efficient compared to other carriers at lower temperatures. Investigation of SEM and XRD images shows that the active sites of carriers are occupied by coke deposits, most of which are deactivated after CLR cycles. Finally, the experimental results confirm that the CLR concept as a novel technology can improve hydrogen production in the SR process and should be further investigated for wide industrial applications.

■

Figure 11. H2/CO ratio through the second reduction cycle using Fe-, Mn-, Co-, and Cu-based oxygen carriers at 800 °C.

AUTHOR INFORMATION

Corresponding Author

*Telephone: +98-711-2303071. Fax: +98-711-6287294. E-mail: [email protected] and/or [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 12. Maximum methane conversion at different temperatures using the Fe-, Mn-, Co-, and Cu-based carriers on Al2O3 as a support.

The maximum methane conversion using the Fe-, Mn-, Co-, and Cu-based carriers happens at nearly 1025, 1030, 900, and 800 °C, respectively. At higher temperatures, Fe- and Mn-based carriers have a better efficiency. For example, the maximum conversion for Fe at its optimum temperature is approximately 98%. At low temperatures, Cu-based carriers are highly reactive in both reduction and oxidation reactions and have the best conversion among the four presented metals on Al2O3. According to this figure, the conversion rate of Cu is enhanced quickly by increasing the temperature and, after 800 °C, the reactivity of the Cu is extremely decreased, reaching to only 10%. This manner is due to the fact that the Cu-based carrier agglomerates and exhibits low reactivity at high temperatures. Although the Co/Al2O3 performance is similar to Cu results, it is an attractive oxygen carrier for the CLR process at 900 °C.

5. CONCLUSION In this work, CLR technology is investigated in a fixed-bed reactor at 700−1200 °C. The oxygen carriers are made of Fe, Mn, Co, and Cu supported on Al2O3 and TiO2 by application of the precipitation method. All experiments are performed in 3 cycles to compare the reactivity and stability of the oxygen carrier through the CLR process. Among used metals, Fe has 2819


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Article

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Experimental Study of Chemical-Looping Reforming in a Fixed-Bed

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