High-Pressure Redox Behavior of Iron-Oxide ... - ACS Publications

Jan 30, 2015 - For gas to liquid (GTL)-type applications, partial oxidation of methane (CH4) is a viable route for conversion of natural gas to valuab...
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High-Pressure Redox Behavior of Iron-Oxide-Based Oxygen Carriers for Syngas Generation from Methane Niranjani Deshpande, Ankita Majumder, Lang Qin, and L.-S. Fan* William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, 151 West Woodruff Avenue, Columbus, Ohio 43210, United States ABSTRACT: For gas to liquid (GTL)-type applications, partial oxidation of methane (CH4) is a viable route for conversion of natural gas to valuable chemicals. The oxygen for this partial oxidation process can be supplied using solid oxygen carriers, which can be single or mixed metal oxides. A particular partial oxidation scheme for CH4 conversion consists of two reactors, using Febased oxygen carrier particles, which circulate within the two units and undergo cyclic reduction−oxidation (redox) reactions. The solid carriers, therefore, serve as a vehicle for oxygen between the units, enabling clean conversion of the fossil fuel with high-purity product streams generated. Unlike the conventional combustion and/or gasification, the gaseous products of the two reactors are inherently separated. This allows for minimization of downstream processing and gas separation, making it a highly efficient energy conversion process. For applications involving high-pressure downstream processing (such as producing syngas as an intermediate feedstock for liquid fuels and chemical synthesis), it is advantageous to operate this gas−solid partial oxidation system at elevated pressures. Thus, it is desirable to study the effect of the pressure on the reaction kinetics of the various reactions involved. In this study, the high-pressure experiments were conducted for reduction and oxidation of oxygen carrier particles in a specialized thermogravimetric analyzer. The relative reaction rates are computed for all experiments conducted in the range of 1−10 atm. Specifically, the rate of reduction under H2 is observed to double when the pressure was increased from 1 to 10 atm compared to a 5-fold increase in the reduction rate under CH4. By comparison, the oxidation reaction rate under air is observed to increase by ∼50%. The reduced and oxidized samples are analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Brunauer−Emmett−Teller (BET) techniques to determine the role of pressure in producing a more reactive particle, which explains the superior reaction rates observed at elevated pressures.

1. INTRODUCTION Transition metal oxides are one of the most technologically versatile materials that have found their applications in various fields. In electronics, they are used in making conductor and semiconductor materials.1 In electrochemistry, they have applications in solid oxide fuel cells, lithium-ion batteries, etc.2,3 However, perhaps they are most widely used in the chemical industry as catalysts, catalyst precursors, and oxygen donors in various processes.4 Selective oxidation, dehydrogenation,5 and chemical looping6 constitute some of the most important processes that are based on the reduction−oxidation (redox) properties of the transition metal oxides. In these processes, lattice oxygen from the metal oxides participates in the reaction, while the vacancies left behind are replenished by molecular oxygen. The redox behavior of the metal oxides influences their crystal phases and their morphologies and, consequently, their optical, electrical, and chemical properties. In the chemical industry, partial and selective oxidation processes often need to be operated at increased pressure for the downstream product applications and the process economics. For example, in the recent past, a portion of the chemical industry has shifted its focus toward natural gas or methane (CH4) for the synthesis of valuable chemicals, such as gasoline, methyl tert butyl ether (MTBE), alcohols, and oxygenates, through such oxidation processes.7 A number of important processes, such as methanol, ammonia, and Fischer− Tropsch syntheses, which are used to synthesize these valuable chemicals, use syngas as their feedstock.8 Syngas for these processes is preferably derived from CH4 because of the lower © XXXX American Chemical Society

capital costs and the higher efficiency of CH4 to syngas conversion systems compared to coal-derived syngas. CH4 is preserved in natural gas fields at high pressures. Also, processes such as Fischer−Tropsch and methanol syntheses, which use syngas as their feedstock, operate at elevated pressures between 2 and 4 MPa.9,10 Thus, for the conventional syngas generation at ambient pressure, the syngas needs to be compressed prior to being introduced in the system. Hence, it is economically beneficial to carry out syngas generation processes at pressures compatible to downstream applications to minimize the energy losses associated with compressing the syngas feedstock for these processes. Gas-to-liquid (GTL) processes, such as syntheses of gasoline, diesel, and methanol, also require a hydrogen-rich syngas feed with a 2:1 ratio of hydrogen/carbon monoxide (H2/CO).11 Existing syngas generation processes, such as steam methane reforming, autothermal reforming, and catalytic partial oxidation of methane, are unable to achieve the required syngas quality in a single unit and need additional processing steps.8 Thus, the single step partial oxidation of CH4 over metal oxides offers an attractive alternative to the existing CH4-to-syngas conversion methods, and the concept is widely used in the process of chemical looping reforming. The economic advantages and the limited knowledge of highpressure partial oxidation of CH4 necessitate a comprehensive Received: November 20, 2014 Revised: January 29, 2015

A

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pressure on the reaction rates. The present study is intended to accomplish this objective. Developing the solid oxide-based redox system for highpressure partial oxidation of CH4 is a multi-optimization problem, which requires careful manipulation of each operating parameter to maximize performance and to reduce the overall cost. These parameters include (but are not limited to) the gas/ solid loading ratio, choice of reactor operation, such as moving bed versus fluidized bed, co-current and countercurrent gas solid flow, and precise control of the gas and solid residence times in each reactor to obtain the desired oxidation state of the solids. Each of these parameters is equally crucial and deserves separate attention and in-depth analysis. Nevertheless, in this study, the reduction and oxidation kinetics of ITCMO oxygen carriers developed at OSU have been studied at pressures ranging from 1 to 10 atm using H2/CH4 for reduction and air for oxidation. Change in reaction kinetics may influence the reducer sizes, processing capacity, and consequently, process economics. The purpose of this study is to demonstrate the effect of the pressure on the reaction rates of the ITCMO particles for CH4-to-syngas conversion. Although the reducing environment of interest is CH4, H2 has been used as the reducing gas for a major part of the study because the H2 reduction reaction is well-understood and relatively easier to operate. It provides a clear understanding of the kinetics without the interference of coking, which is observed with CH4 as the reducing gas. Furthermore, the effect of the pressure that is discussed in the case of H2 can be extrapolated to other reducing environments. The results presented are that of kinetic experiments carried out in a thermogravimetric apparatus. It has been demonstrated in this work that higher pressures are kinetically favorable for H2 and CH4 reduction and to a lesser extent air reoxidation of the metal oxides. This work also briefly discusses the advent of coking and its response to elevated pressures.

study of the impact of elevated pressures on the reactions involved. Chemical looping has been regarded as one of the most promising technologies in the U.S. Department of Energy’s CO2 capture roadmap.12 The technology is based on hightemperature cyclic redox reactions of metal-oxide-based oxygen carriers between two or more reactors (reducer, combustor, and oxidizer) for the conversion of carbonaceous fuel to generate electricity and/or H2.13 It is designed to produce a sequestration-ready stream of CO2 from the reducer. Thus far, the chemical looping process has been used to generate H2 and/or electricity using syngas, coal, and biomass as feedstock.14−16 Nevertheless, it is a highly versatile process and can be used for syngas generation using natural gas/CH4 as its feedstock. CH4 is converted to syngas in the reducer via oxygen transferred from the metal-oxide-based oxygen carriers. Such a process was conceptualized at The Ohio State University (OSU) for the utilization of shale gas, termed as the shale gas to syngas (STS) process.17 The schematic of this process is seen in Figure 1. Syngas with a 2:1 H2/CO ratio can be obtained by

2. SYNGAS GENERATION AT ELEVATED PRESSURES: THERMODYNAMIC ANALYSIS The thermodynamic analysis of the reducer was conducted using HSC Chemistry (OutoKumptu Research Oy, version 6.0). As stated earlier, the process of partially oxidizing CH4 for production of syngas using the oxygen carrier particles causes coking or C soot formation. Operating the system at elevated pressures exacerbates this condition. To understand the thermodynamic equilibrium limits of operating this system at elevated pressures and its effect on the soot formation, the reducer species were simulated via Gibbs free energy minimization at elevated pressures (up to 10 atm). The Fe− Ti bimetallic system is used for simulating the ITCMO oxygen carrier particles. Conversion of CH4 to syngas is carried out by partial oxidation using hematite (Fe2O3) as the reactive phase from the ITCMO oxygen carriers. The titanium oxide (TiO2) phase is assumed to be non-reactive. In its simplest form, the theoretical desirable reaction is

Figure 1. Schematic of the Fe-oxide-based system for syngas generation from partial oxidation of CH4.

controlling the oxygen carrier circulation rate in the system and thereby the extent of reduction of the oxygen carriers in the reducer. This process has been demonstrated experimentally at various scales at atmospheric pressure.17 Therefore, syngas generation via chemical looping can be developed into an efficient, economic, and environmentally friendly process that overcomes the issues associated with the existing processes. However, experimental kinetic investigations for the effect of the pressure on this system are relatively sparse. Most existing studies on CH4-to-syngas conversion using metal oxides are focused on nickel (Ni)-based complex oxides, because of its catalytic capabilities for the reforming reaction.9,18,19 Some of the other metal oxides studied for CH4-to-syngas application include copper (Cu), iron (Fe), and manganese (Mn).20,21 This study has been conducted from the perspective of such redox systems using Fe-based oxides, which have the potential to be more economical if operated at elevated pressures. The bimetallic system has been investigated at OSU for the STS process in the form of iron−titanium complex metal oxide (ITCMO) particles designed for this process. The operating conditions of the STS process have been determined through detailed thermodynamic and process analyses.17 Therefore, it is essential that the ITCMO particles be tested for the effect of

1/3Fe2O3 + CH4 → CO + 2H 2 + 2/3Fe

(1)

Thus, CH4 is partially oxidized to form syngas, a mixture of CO and H2, and Fe2O3 is completely reduced to elemental Fe. However, thermodynamically, the reduction of Fe2O3 goes through the progressively different reduced phases of iron oxide, namely, magnetite (Fe3O4), wüstite (FeO), and finally the completely reduced form of elemental Fe. All of these B

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processing, such as Fischer−Tropsch synthesis. The carbon deposition can be managed by careful manipulation of the gas/ solid ratios in the moving bed reducer reactor system.17 Nevertheless, the success of this partial oxidation system rests equally on the kinetic factors affecting the reaction. The highpressure operation of Fe-based partial oxidation will require the basic understanding of the manner in which pressure affects the kinetics of each of the reactions involved. Therefore, in the following sections, the effect of the pressure on the reduction and oxidation of Fe−Ti oxygen carriers is investigated.

phases are likely to be present. Similarly, along with the formation of H2 and CO, CH4 oxidation also results in complete combustion (to CO2 and H2O) as well as formation of elemental C. Accordingly, the reactive system was simulated with the following species in the gaseous state: CH4, H2, CO, CO2, and H2O, and following species in the solid state: C, Fe, FeO, Fe3O4, Fe2O3, Fe2TiO5, and FeTiO3. Isothermal and isobaric systems were simulated at 950 °C and 1, 5, and 10 atm. The solid loading was assumed to be in the forms of fully oxidized Fe and Ti metallic species. The gaseous input of CH4 was incrementally added, and the outlet species were analyzed for solid and gas equilibrium compositions at minimum Gibbs free energy, at fixed T and P values. As expected, TiO 2 remains largely unreacted, while compounds of Fe undergo sequential reduction with increasing CH4 loading, from fully oxidized to fully reduced form, in both pure Fe−O as well as Fe−Ti−O complex phases. The equilibrium amounts of FeTiO3 and Fe2TiO5 are found to be negligible, and therefore, for simplicity, only pure Fe−O phases are considered for the remainder of this discussion. For all pressures, overall CH4 conversion was found to be >99% for the range of gas/solid ratios tested, which was varied between 0.05 and 1.5 mol of CH4/mol of Fe2O3. This range was chosen because of the fact that all four oxidation states of Fe are found to exist in this range. The conversion was found to increase as the gas/solid ratio was increased. As expected, solid C formation is observed simultaneously with the formation of elemental Fe. This C amount is higher at higher pressures. This is in agreement with our experimental findings, discussed further in section 4.2. For example, at the CH4/Fe2O3 ratio of 1.5, a comparison of C formed at 10 and 1 atm reveals that the equilibrium C amount at 10 atm is approximately 8 times that of 1 atm. The same comparison between 5 and 1 atm shows that equilibrium C formation at the same ratio is 4.5 times that of 1 atm. The C deposition is shown as a function of the gas/ solid ratio in Figure 2 at 5 atm and 950 °C. The figure clearly shows the simultaneous onset of elemental Fe formation and C deposition.

3. EXPERIMENTAL SECTION A magnetic suspension balance (MSB, Rubotherm GmbH, US-200400162) was used for the high-pressure thermogravimetric analysis (TGA) experiments. The schematic of the MSB setup is shown in Figure 3. Different pure gas bottles (H2, CH4, N2, and air) were connected through a battery of mass flow controllers and valves to the TGA assembly. A pressure transducer upstream of the sample cell measures and records the pressure of the sample during the experiment. Downstream of the sample cell, a back-pressure regulator (BPR) is installed to regulate the pressure in the sample cell. The gas was preheated prior to entering the TGA assembly by means of heating tapes. The unique working principle of the MSB allows the sample weight and the balance to be connected via magnetic coupling, and therefore, the balance is isolated from (and not affected by) the reaction environment. This principle allows for the use of highpressure and highly corrosive environments in the sample cell. The section of the TGA housing the magnetic coupling is maintained at 140 °C by means of a heat jacket connected to an oil bath. The reaction temperature in the sample cell is maintained independently by means of an electric furnace. The gas mass flow rates were fixed such that the gas space velocity experienced by the sample was constant at all pressure experiments (∼1 min−1 for the volume of the sample cell). The sample was heated in the inert flow of N2. When the reaction pressure and temperature were achieved, the gases were switched to introduce the reaction mixture in the sample cell. The sample weight, temperature, and pressure were recorded as a function of time. Knowing the composition of the oxygen carrier particle, the theoretical maximum weight loss (complete reduction) and gain (complete oxidation) were calculated. These numbers were used to calculate the extent of each reaction. Specifically, for each sample, the red ) is theoretical maximum weight loss during reduction (Δwmax calculated on the basis of the active weight of the sample. During the experiment, the weight change as a result of reduction (Δwr) at any instant is then used to compute the extent of reduction by the following formula:

extent of reduction =

Δwr red Δwmax

× 100%

Similarly, in the case of oxidation extent of oxidation =

Δwo ox × 100% Δwmax

where Δwo is the weight change due to oxidation and Δwox max is the theoretical maximum weight gain during oxidation. Through the remainder of this paper, a general term X is used to indicate the reaction conversion or extent of reaction for reduction and oxidation alike, as applicable to the discussion. The X values were plotted for each experiment versus time (minutes), to obtain the thermogravimetric conversion curves. These curves were then used to compute the instantaneous rates of reaction, denoted by dX/dt, per minute. Samples of ITCMO particles (0.1 g) were collected after selected experiments and mechanically crushed to a powdered form and sieved to the appropriate particle size. X-ray diffraction (XRD) was performed on the powdered specimens with a Rigaku SmartLab X-

Figure 2. Simulated equilibrium iron oxide phases and fractional carbon deposition as a function of inlet gas/solid ratios at elevated pressure, with T = 950 °C and P = 5 atm.

In addition, this thermodynamic analysis reveals that an increase in the system pressure from 1 to 10 atm results in an increase in the formation of CO2 and H2O, along with a slight decrease in the formation of desirable H2 and CO as well as overall CH4 conversion. However, an increase in the system pressure is found to have a favorable impact on the equilibrium H2/CO ratio, which is desired at ∼2 for downstream C

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Figure 3. Schematic of the experimental setup of the MSB. ray diffractometer. Additionally, specimens were also examined by scanning electron microscopy (SEM) using e-beam imaging of a FEI Helios NanoLab600 DualBeam system. To obtain high-quality crosssection imaging, focused ion beam (FIB) was generated using a gallium ion source at an accelerating voltage of 30 kV for crosssectional milling for in situ SEM observation. The two-dimensional (2D) material mapping was obtained using Oxford energy-dispersive X-ray spectrometry (EDS) at an accelerating voltage of 20 kV. Additionally, a NOVA 4200e Quantachrome Brunauer−Emmett− Teller (BET) analyzer was used to measure the pore volume and surface area of samples using N2 sorption.

the system, at three different partial values of PPH2 of 1, 1.5, and 3 atm. At a constant value of PPH2, the increase in the overall system pressure resulted in a decrease in the mole fraction of H2 (YH2). The rate of reaction is found to decrease with an increase in the total pressure of the system at each value of constant PPH2. Similar results have been reported in the past on ́ metal oxide reduction reactions. For example, Garcia-Labiano et al. have reported a decrease in the reduction reaction rate with an increase in the system pressure at PPH2 and PPCO values of 1 atm for Fe-, Cu-, and Ni-based metal oxides.22 The negative effect of the pressure on various reactions has been previously observed by other researchers and explained by factors such as an increase in product gas volume upon reaction or increased diffusion resistance through the product layer at higher pressures.23−25 Thus, the same set of data is plotted in two different ways. In Figure 4, the rates of reaction are plotted against the total system pressure, and in Figure 5, the same rate values are plotted against the respective YH2 values. For example, in comparison of the experiments conducted at the

4. RESULTS AND DISCUSSION To study the effect of the pressure on the rates of reactions involved in the partial oxidation system, isothermal experiments of reduction and oxidation were conducted in the reducing environment of H2 and CH4 and the oxidizing environment of air. The rates of reactions were then compared by calculating the rates from the solid conversion curves obtained from TGA. 4.1. Reduction in H2. The reduction of the oxygen carrier samples was conducted isothermally at 900 °C using H2 as the reducing agent, at various operating conditions of gas concentration and partial pressure. The parameters studied for H2 reduction include partial pressure (PPH2), total system pressure, and mole fraction of H2 (YH2). The weight loss of the sample corresponds to the total amount of oxygen lost by the sample during reduction. Therefore, the reduction conversion is calculated assuming 100% reduction at complete weight loss of the sample or the most reduced state of Fe. In this manner, reduction conversion was calculated and plotted. The rates of reaction are graphically calculated at X = 0.5 and 0.75 reduction conversion values at the various conditions tested, and exhibit similar trends. 4.1.1. Constant Partial Pressure of H2 (PPH2). The isothermal isobaric experiments were conducted to observe the rates of reaction at different total gas pressures, with a constant partial pressure of the reducing agent. The TGA conversion curves were compared at different total pressures of

Figure 4. Effect of the total system pressure on rates of reduction at X = 0.75 and constant partial pressure of H2, with T = 900 °C. D

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values are plotted in this manner as a function of YH2, the plots are linear, indicating a direct proportionality between the rate and mole fraction of reacting gas. However, the plots converge at lower YH2 values (which correspond to the high-pressure experiments, toward the positive x-axis direction in Figure 4). Regardless of the manner of analysis, it is plainly seen that, at higher values of constant partial pressure of the reducing gas, a higher rate of the reduction of solid oxide ITCMO particles is achieved. 4.1.2. Constant Mole Fraction of H2 (YH2). Similarly, the reduction experiments were conducted at various pressures between 1 and 10 atm by keeping YH2 constant at 50% to study the rate of reduction of the ITCMO particles at the varied pressures. In this case, the value of PPH2 also inevitably increased with the increase in the pressure. It is observed that, as the pressure is increased, the slope of the conversion curves increases, indicating higher reaction rates. This is in contrast to ́ the previously reported findings by Garcia-Labiano et al., who report a slight decrease in reaction rates with an increase in the pressure at a constant mole fraction of CO at a 10% value.22 It must be noted in the outset that the rate of reaction is determined by a combination of various factors, such as reactive gas partial pressure, change in diffusivity of the reactant and product gas through the porous particle at elevated pressures, relative superficial velocity of gas with respect to solids, etc. As stated before, in the present study, a constant gas linear velocity (space velocity) was used for all experiments. The rates were determined graphically at a fixed conversion value for all of the curves and are plotted in Figure 6 at X = 0.5 and 0.75. From

Figure 5. Effect of the mole fraction of reducing gas (YH2) on rates of reduction at X = 0.75 and constant partial pressures, with T = 900 °C.

system pressure of 3 atm, the three different experiments at this pressure value would correspond to the three different values of PPH2 of 1, 1.5, and 3 atm. The rates of reduction for these three experiments fall on a vertical straight line of Figure 4, at a x axis value of 3 atm. At this system pressure of 3 atm, the mole fractions YH2 are, however, widely different. PPH2 = 1 atm at the system pressure of 3 atm results in a YH2 value of 33%. PPH2 = 1.5 atm at the system pressure of 3 atm is at a YH2 of 50%. Finally, at PPH2 = 3 atm, YH2 is obviously 100%. This is true for all of the values of the pressure along the x axis of Figure 4; i.e., the curve for PPH2 = 3 atm is always at the highest mole fraction of the reducing gas (YH2), which seems to be a crucial factor contributing to the superior rates of the reaction observed. The entire set of experimental conditions is given in Table 1. Table 1. YH2 as a Function of the Total System Pressure and PPH2 for Section 4.1.1 (%) PPH2 (atm) total pressure (atm) 1 3 5 7 8 10

1 100 33.33 20 12.50 10

1.5

50 30 18.75 15

3

100 60 42.86 37.50 30

Figure 6. Effect of the sysyem pressure on rates of reduction at X = 0.5 and 0.75 and constant mole fraction of reducing gas YH2 = 50%, with T = 900 °C.

Figure 6, it can be concluded that there is more than a 100% increase in the reaction rate as the pressure is increased from 1 to 10 atm, when operating at the same mole fraction of the reducing gas, namely, H2. 4.1.3. Constant Pressure of the System. Finally, the reduction reactions were studied for the reaction kinetics under a constant pressure. For this set of tests, the pressure of the system was maintained at 5 atm and YH2 was increased from 30 to 100%, thus increasing the partial pressure of the reducing gas (PPH2). If the reaction is conducted under a fixed pressure, the rate of reaction is found to increase as expected, with an increase in PPH2; this is shown in Figure 7. 4.2. Reduction in CH4. The same experiments as section 4.1 were repeated with CH4 as the reducing gas instead of H2. As indicated in section 2, it was observed that the reduction of

The curves of rate versus system pressure seem to converge at higher pressures. This can be explained by the fact that the PPH2 values chosen here are relatively low, and therefore, at high system pressures, the YH2 values are closer. In contrast, at a lower system pressure, the rate curves are further apart, which correspond to the disparity in YH2 values at those conditions (see Table 1), which increases at a lower pressure. In Figure 5, the same data are plotted against the mole fraction of H2 (YH2). Following the same logic, the rates can be compared at similar values of YH2. In the vicinity of YH2 = 30− 33%, even though the mole fraction of H2 is so similar, the system pressure values for the three curves are 3, 5, and 10 atm. Therefore, it is observed that the higher PPH2 values expectedly play a part in increasing the rate of reduction. When the rate E

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Figure 7. Effect of the partial pressure of reducing gas PPH2 on (a) conversion curves obtained and (b) rates of reduction at a constant system pressure P = 5 atm, with T = 900 °C.

that the reaction halted at lower conversions, owing to the higher amount of C deposition. These conversion curves were used to compute the reaction rates at different pressures. The evaluation of three separate rate values is warranted for the three distinct stages of reduction. Accordingly, the rate values were calculated from the conversion data obtained. These rate values are plotted in Figure 9. It can be appreciated that reaction rates for stages I

oxygen carrier particles with CH4 as the reducing agent results in the formation of elemental C. This is evidenced by the soot formation and weight increase of the sample beyond a certain point. The C deposition is observed when the oxygen carrier reaches a certain degree of reduction conversion and is always after the elemental Fe phase has been formed. Thus, to study the reaction kinetics of the reduction of oxygen carrier materials in the presence of CH4, the reaction is arrested at or before the initiation of C deposition. Accordingly, experiments were conducted by adjusting the procedure and allowing for the maximum possible reduction of particles, until the onset of C deposition. The reduction of Fe2O3 to Fe proceeds through sequential steps of various oxidation states. Here, complete loss of oxygen is considered as 100% conversion. Stage I corresponds to Fe2O3 to Fe3O4 conversion, which translates to 11% reduction conversion (or X = 0.11). Stage II corresponds to Fe3O4 to FeO conversion, which translates to 33% reduction conversion (X = 0.33). At conversions higher than 33%, stage III is initiated, which results in the formation of elemental Fe.20 Unlike reduction in H2, in the case of CH4 reduction, these three stages have three distinct reaction rates, as seen in Figure 8. Further, the different stages react differently to an increase in the pressure in terms of the rate of reaction. The rate of each reaction stage is studied at various pressures between 1 and 10 atm. At higher pressures, the rate disparity between the three stages is less pronounced, giving a faster overall conversion obtained without three distinct rate stages. It was also observed

Figure 9. Effect of the system pressure on the reaction rate for the three-step reduction with CH4 as the reducing gas, with YCH4 = 50%, T = 950 °C, and P = 1−10 atm.

and III go through a maxima in the range of the pressure tested here. However, the rate of stage II increases exponentially with pressure. Because stage II is the slowest reaction stage, it is the overall rate-determining step, and therefore, any change in the rate of stage II overwhelmingly affects the overall rate of the reduction reaction. For example, it can be seen from Figure 8 that, at 10 atm, 33% reduction is achieved in almost 1/7 of the time taken at 1 atm and, similarly, 60% reduction is achieved in 1 /3 of the time. An increase in the pressure, specifically at a constant mole fraction of the reacting gas, results in an increase in the concentration of the active species in the gas phase. This increased concentration of gaseous species inevitably results in an increased reaction rate, which is also reported earlier in section 4.1.2 (for H2). Specifically, the role of pressure in the reaction rate is observed to be particularly pronounced in the case of reduction of ITCMO particles with CH4. The effect of the pressure on reduction kinetics for a chemical looping combustion (CLC) scenario has previously been investigated ́ by Garcia-Labiano et al.22 They studied the reduction kinetics on Fe-, Cu-, and Ni-based oxygen carriers at pressures up to 30

Figure 8. Reduction conversion curves obtained using CH4 from TGA between 1 and 10 atm at a constant mole fraction of reducing gas YCH4 = 50%, with T = 950 °C. F

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reaction to correct the weight to reflect reaction alone. Figure 10 shows the example of this correction applied to compute the conversion.

atm. At a low constant mole fraction of 10% H2 and CO, no significant increase in the reaction rate was observed with an increase in the pressure. However, it was noted that the actual reaction rate was influenced to a certain degree by several factors, of which an important factor was “gas dispersion” that occurs particularly during the initial stage of the reacting gas introduction to the sample cell. The use of a constant molar flow rate across all pressures resulted in a progressively increased gas dispersion effect in the reaction cell at increased pressures. To minimize the progressive gas dispersion effect because of an increase in pressures in the present study, the ITCMO particles were reduced at a constant space velocity across all pressures. The use of the constant space velocity leads to an increase in the reaction rate of the reduction reaction of the ITCMO particle with increased pressure, as seen in Figure 9. For comparison purposes, experiments have also been carried out for the reduction reaction of ITCMO particles with CH4 under the condition of a constant molar flow rate between 1 and 10 atm in the present study. In this case, the space velocity at a higher pressure is appreciably lower, giving rise to a larger “gas dispersion” effect. In such experiments, the difference in reaction rates is found to diminish significantly, confirming that the negligible effect of increased pressure on the reduction rate observed in ref 22 is attributed to the “gas dispersion” effect with pressures. 4.2.1. Pressure Correction. The sample weight measurement in the MSB is extremely sensitive to pressure changes in the cell. The system pressure is regulated by the BPR situated downstream of the sample cell. Usually, this BPR enables the system to be maintained at a steady pressure value for all of the experiments. Thus, all of the changes in the sample weight measurement are “true” weight changes, attributed to the reaction alone. However, the reduction reaction in the presence of CH4 is a volume expansion reaction, where 1 mol of reactant gas is converted to 3 mol of product according to reaction 1 (other side reactions, such as complete combustion, also result in volume expansion). The rate of this volume expansion is directly proportional to the rate of the reaction. In the fixed volume of the sample cell of MSB, rapid gas volume expansion thus results in a temporary increase in the system pressure. The faster reaction results in more volume expansion and, thus, has a more pronounced effect on the temporary system pressure change. The three stages in the CH4 reduction reaction occur at different reaction rates, as shown in section 4.1. The difference in these rates is also reflected in the trend in the system pressure. During stage I, the pressure increases because of volume expansion (faster reaction = more pronounced effect on the pressure). At the onset of stage II, there is a sudden appreciable decrease in the rate of the reduction reaction, and therefore, the pressure buildup starts dissipating by returning to the set point value (and further shows a slight increase after stage III initiation). These pressure changes in the system inevitably affect the measurement of sample weight, causing it to change. Therefore, to discern the “true” weight change of the sample because of reaction alone, it becomes necessary to correct the measured sample weight for fluctuation because of the pressure effect. Thus, a correlation was established between the system pressure and sample weight in the absence of the reaction, by changing the pressure set point externally in inert gas and recording the changes in sample weight. This correlation was applied to the measured sample weight during the reduction

Figure 10. Pressure data and reduction conversion obtained on the basis of the original data and the data obtained by applying pressure correction. Reducing gas = CH4, with YCH4 = 50%, T = 950 °C, and P = 8 atm.

4.3. Air Oxidation. The effect of the pressure was also studied for the oxidation reaction for the combustor block discussed in section 1. The oxidation reactions were carried out using air as the oxidizing agent. The sample ITCMO particles were subjected to reduction in H2, followed by air oxidation at constant pressure values (1, 5, and 10 atm). The oxidation conversion curves are compared in Figure 11. The increasing

Figure 11. Oxidation conversion curves obtained from TGA between 1 and 10 atm at YO2 = 0.1 and T = 900 °C.

trend of the reaction rate with the pressure is apparent with air oxidation too, although it is not as pronounced as reduction reactions, mainly because of the fact that the oxidation reaction with O2 is extremely fast to begin with, with the total oxidation completed in less than 8 min even at ambient pressure at the conditions tested here. This slight increase in the oxidation rate with the pressure is in agreement with a similar rate increase observed by Jin and Ishida in the case of Ni-based oxygen carrier particles.26 In the case of two different Ni-based particles, the effect of the pressure was reported to be more pronounced on reduction rates than oxidation, when H2 was used as a reducing gas and O2 (air) was used as an oxidizing gas, between 1 and 9 atm. G

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Energy & Fuels 4.4. XRD, SEM, EDS, and BET Analyses. The samples were subjected to XRD analysis after complete reduction (H2) and oxidation, at the lowest and highest pressures tested, viz., 1 and 10 atm. Complete reduction of the sample is unattainable in CH4; therefore, H2 was used as the reducing medium. The diffraction spectra are shown in Figure 12. The samples after

through an in-depth study of the morphological changes on the reaction surface at the microscopic level at elevated pressures. The reacted particles were studied using SEM and elemental mapping according to the technique outlined in section 3. More details of this technique are available elsewhere.27 Panels a and b of Figure 13 show the reduced samples produced at 1 and 10 atm, respectively. Figure 13a shows a typical non-uniform microparticle produced at ambient pressure: a denser part of Ti-rich oxides with a particle size of 40 μm and a porous part containing comparable amounts of Fe and Ti oxides with an average grain size of 1−2 μm. It is apparent that subjecting the oxygen carrier particles to a high pressure results in more porous particles upon reduction; an average grain size of 500 nm can be clearly seen in the cross-section (Figure 13b). This strongly suggests that a higher pressure results in a higher surface porosity compared to ambient pressure, which may explain the difference in the reaction rate. Figure 14 shows the difference between samples treated under 1 and 10 atm. After reduction, the particles processed at 1 atm have a grain size of 1 μm and higher pressures lead to a smaller grain size of ∼500 nm. Consequently, a reaction pressure at 10 atm can largely promote the increase of the overall surface area. These samples were further tested for surface area and pore volume measurements using a BET analyzer. The Barrett−Joyner− Halenda (BJH) pore size distribution method was used to find the values of the total surface area and pore volume for the four samples tested here. From the analysis, it was discovered that the increase in the pressure from 1 to 10 atm resulted in an increase in both surface area and pore volume values for reduced as well as oxidized samples. For the reduced samples, the increase in the pressure resulted in a surface area change from 7.036 to 7.227 m2/g, whereas the pore volume values increased from 0.014 to 0.022 cm3/g. The same trend was observed for oxidized samples, where the change was more pronounced, going from 4.726 to 15.507 m2/g of surface area values as well as from 0.025 to 0.117 cm3/g of the total pore volume. All of these observations serve to explain the increased reduction rates for oxygen carrier particles with an increased pressure. Using this preliminary analysis, we conclude that reduction carried out at elevated pressures results in particles with a superior surface and intraparticle morphology as well as a uniform small grain structure, which contribute to the high reaction rates observed. A detailed analysis of the relationship between the particle morphology and the reaction rates will result in a greater understanding of the mechanism of highpressure redox reactions involved in a Fe-based partial oxidation system. Such a study is currently underway and will be published separately. It should be noted that, upon reoxidation, no significant morphological difference was found in samples treated at 1 and 10 atm, which is consistent with the small difference in the oxidation reaction rates observed.

Figure 12. XRD analysis of (a) reduced and (b) oxidized samples at 1 and 10 atm and 900 °C. A reducing environment is under H2 with YH2 = 0.5, and an oxidizing environment is air.

reduction exhibit complete reduction in the iron oxide phase at both pressures tested. In addition, the peaks corresponding to TiO2 are diminished at an elevated pressure compared to that at an ambient pressure, and the peak corresponding to Fe remains unchanged. The ambient pressure sample exhibits distinct TiO2 peaks along with two prominent peaks characteristic of Fe. In the case of oxidized samples, completely oxidized phases are identified as Fe2O3 and TiO2. In addition, trace amounts of complex species are formed at ambient pressures, namely, Fe2TiO5 and FeTiO3. These complex species are also found to be significantly less at the sample oxidized at a pressure of 10 atm. These differences in the samples point toward possible mechanistic differences in the reactions conducted at ambient pressure versus high pressure, which might be contributing toward a faster reaction rate at high pressures. However, the exact nature of these differences may only be understood

5. CONCLUDING REMARKS For chemical synthesis applications, the Fe-based partial oxidation process can be applied to produce pressurized syngas from CH4. One such process developed at OSU has been termed as the STS process. The use of the STS process for downstream GTL applications requires the system to be operated at elevated pressures. Thus, it is imperative to investigate the effect of the pressure on the operation of such a system, specifically, on the metal oxide particles that supply the H

DOI: 10.1021/ef5025998 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 13. SEM and EDS elemental mapping of cross-sections of reduced particles. Samples reduced under H2, with YH2 = 0.5 and T = 900 °C: (a) 1 atm and (b) 10 atm.

oxidation of reduced ITCMO particles, albeit to a smaller degree. The reacted particles were analyzed using SEM, XRD, and BET techniques to understand the morphological changes on surface and intraparticle levels and the role of these changes in the observed differences in reaction rates. This analysis indicates that conducting the reduction reactions at elevated pressures results in a product particle that shows more uniformity with respect to grain sizes, porosity, and reactive iron oxide distribution, with increased surface and intraparticle porosities. The formation in this uniform particle significantly contributes to the superior reaction kinetics. One of the major issues of syngas production using CH4 is the formation of C soot. However, the soot deposition can be managed or eliminated entirely using the control of various other process parameters, such as gas to solid loading, reactor residence times, gas injection location, and mode of gas−solid contact. Therefore, the advantages of faster reaction kinetics at higher pressures can be realized by circumventing the soot deposition through precise process operation. The advantages of operating the Fe-based partial oxidation system at elevated pressures include increased processing capacity or reduced reactor sizes and capital cost. A high-pressure reactor operation is thus desired for the metal oxide partial oxidation process system. However, the specific operating pressures to be used in this process that are optimal can be determined through rigorous experimental testing on various scales and thorough process and economic analyses.

Figure 14. Surface grains in samples reduced under H2, with YH2 = 50% and T = 900 °C: (a) 1 atm and (b) 10 atm.

oxygen. This study was conducted to determine the effect of the pressure on the rates of reduction and oxidation reactions for OSU’s ITCMO particles developed for applications such as the STS process. Extensive testing was carried out on the reducer block at the conditions amenable to such a process. It is found that an increase in the mole fraction or partial pressure of the reducing gas has a favorable effect on the rate of the reduction reaction of the ITCMO particles over the wide range of conditions tested. Overall, operating the system at a higher pressure results in superior reduction reaction rates. Although the primary goal of the present work is to ascertain the effect of the pressure on the reaction rates of ITCMO particles for CH4 partial oxidation using the STS process, an extensive parametric study for reduction was also carried out using H2 as the reducing gas. It is found that the experiments carried out with CH4 exhibit the same trends with the variation of the pressure, and therefore, the pressure effects can be extrapolated. When the experimental methodology to eliminate any gas dispersion effects was chosen, the kinetic advantage of a higher pressure operation was ascertained in this study. With CH4 as the reducing gas, the ITCMO particles exhibit three distinct reaction stages for reduction. Stage II, which is the slowest and therefore rate-determining stage, is the most sensitive to a change in the pressure, undergoing a 10-fold increase in the rate with the increase in the pressure from 1 to 10 atm. Thus, any increase in the pressure results in a favorable change in the rate of the reduction reaction by CH4. The improvement in the reaction rate is also observed in the case of

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 614-688-3262. Fax: 614-292-3769. E-mail: fan.1@ osu.edu. Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/ef5025998 Energy Fuels XXXX, XXX, XXX−XXX