High-Pressure Chemical-Looping of Methane and Synthesis Gas with

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High-Pressure Chemical-Looping of Methane and Synthesis Gas with Ni and Cu Oxygen Carriers Oscar Nordness, Lu Han, Zhiquan Zhou, and George M. Bollas* Department of Chemical & Biomolecular Engineering, University of Connecticut, 191 Auditorium Road, Unit 3222, Storrs, Connecticut 06269-3222, United States S Supporting Information *

ABSTRACT: This paper presents an experimental study of high-pressure chemical-looping combustion (CLC) of methane and synthesis gas using supported Cu and Ni oxygen carriers. The experiments were performed in an isothermal, fixed-bed reactor at the pressure range of 1−10 bar. The analysis showed that at elevated pressures, the reactivity of the CLC oxygen carriers deviates from that at atmospheric pressure. Formation of solid carbon was found favorable at high pressures for both oxygen carriers, though more extensively with Cu materials. An empirical kinetic model was used to capture the effect of pressure on the reduction and oxidation reactions. The objective of this work is to derive a kinetic model that can accurately capture the idiosyncrasies of high-pressure CLC, which can guide process design studies of CLC integration into power plants.

1. INTRODUCTION The power generation sector is responsible for one-third of anthropogenic CO2 emissions and, correspondingly, it is a major contributor to the greenhouse gas effect.1 Carbon capture and sequestration is considered an immediate solution to control CO2 emissions from fossil fuel combustion. However, existing carbon capture methods entail high energy penalties for the power plant, reducing its thermal efficiency by at least 8−12%.2 Chemical-looping combustion (CLC) has been identified as one of the most promising CO2 capture technologies with low cost and high capture efficiency.3 CLC is conceptually achieved through the pairing of two reactorsa reducer and an oxidizerutilizing a metal oxide (oxygen carrier) as the oxygen transport agent. The desired characteristics of a suitable oxygen carrier are high reactivity toward reduction and oxidation reactions, high CO2 selectivity, stability over multiple cycles, low cost, and environmental benignity.4 Among the over 700 carriers already tested,5 Ni and Cu oxygen carriers have been the most extensively studied materials. Ni-based oxygen carriers have shown high reactivity toward gaseous fuels and high melting point. Cu-based oxygen carriers present high reactivity and oxygen transfer capacity. Moreover, CuO is capable of spontaneously uncoupling its oxygen at high temperatures, which allows for the combustion of solid fuels. In this work, these two oxygen carriers are studied in terms of their performance at high pressure. CLC technology holds substantial potential for incorporation in existing power plants utilizing coal6 and natural gas.7 Coal is gasified at high pressure with steam and oxygen producing a syngas fuel,9 which is burned downstream the process to produce power. In combined cycle gasification systems without CLC, CO2 must be removed separately resulting in a high energy penalty.8 This energy penalty can be significantly reduced through the combination of integrated gasification combined cycle (IGCC) with CLC, as proposed by Rezvani et al.,9 wherein syngas leaves the gasifier and is sent to the CLC reactor. The CLC exhaust streams are then sent to a heat © 2015 American Chemical Society

recovery steam generator (HRSG) unit, whereby the CO2-rich flow is condensed and compressed. Since the high pressure in the gasifier contributes to an increase in thermal efficiency, high-pressure (20−35 bar) CLC reactors downstream of the gasifier are necessary to maintain cycle efficiency in the CLC power plant and reduce the amount of additional power required to compress CO2 to pipeline or sequestration pressures.10,11 The natural gas-fired combined cycle (NGCC) uses combustion turbine engines generators, heat-recovery steam generators (or boilers), and steam turbine generators to convert natural gas into electricity. The typical inlet pressures to the gas turbines in the NGCC are in the range of 10−20 bar,12−14 and higher thermal efficiencies can be achieved with higher turbine temperatures and pressures.15 Similarly, high system pressure is also preferred due to the requirement for sequestration and compression of CO2. The most widely recognized reactor configuration for CLC consists of two interconnected fluidized beds, namely a fuel reactor and an air reactor, with the oxygen carrier circulating between the two reactors.5 Interconnected fluidized gas/solid systems are difficult to operate at high pressure and temperature, because of the difficulty in gas/solid separations, loop sealing between the two reactors, solids attrition, and issues with the stability of a certain hydrodynamic regime at high pressure. To avoid these problems, fixed-bed, alternating-flow CLC reactor concepts were introduced by Noorman et al.16 Fixed-bed reactor designs were studied theoretically for power generation and hydrogen production in the literature, indicating thermodynamic feasibility for gaseous systems.6,16,20−25 The main advantages of CLC fixed beds are the elimination of gas/solid separation steps and their capability to operate at high pressures without significantly affecting the flow regime. Among the disadvantages of fixed beds is their relatively high pressure drop Received: September 2, 2015 Revised: November 17, 2015 Published: November 17, 2015 504

DOI: 10.1021/acs.energyfuels.5b01986 Energy Fuels 2016, 30, 504−514

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of laboratory experiments with syngas and natural gas at high pressure. The incorporation of CLC kinetics derived from experiments conducted at such pressures could substantially improve the accuracy of these models. The objective of this work is to explore the effect of pressure on the reactivity of Ni and Cu oxygen carrier reduction with syngas or methane and oxidation with air. The effects of high pressure on the gas conversion, selectivity, and extent of carbon formation are investigated in a bench-scale fixed-bed reactor. The experimental results are used to derive reaction kinetics for CLC with Cu and Ni oxygen carriers under high pressure, on the basis of solid-state models previously derived at atmospheric conditions.

along the reactor, which necessitates the use of large oxygen carrier particles, typically in the range of 2−10 mm in diameter.6,16,26 Correspondingly, the reaction rates inside these large particles are limited by intraparticle diffusion and external transfer effects,27,28 which need to be accounted for in the mathematical models of large-scale fixed or moving bed processes. In the literature, various configurations of CLC used in integrated combined cycles were proposed, and process simulation tools were employed to analyze key process metrics (e.g., oxygen carrier type and air reactor temperature) on the overall efficiency.18,19,29−31 In these studies, detailed reaction kinetics was neglected in favor of thermodynamic calculations, due to lack of available data at high-pressure operation.31 In a study by van Sint Annaland and co-workers,32 a coal-fired integrated combined cycle power plant was designed based on a dynamic reactor analysis of the CLC reactor. Porrazzo et al.33 incorporated hydrodynamic and kinetic mechanisms to model a CLC system in Aspen Plus model, finding that the assumptions of fast kinetics and complete gas/solid contact can underpredict the solid inventory. Kolbitsch et al.34 developed a model of a 120 kW CLC reactor system and determined that conversion of solids inside the air and fuel reactors is dependent on the kinetics of the oxygen carrier oxidation and reduction reactions, which controls the overall solids inventory in the reactors. Therefore, it is desirable to incorporate reaction kinetics in the modeling analyses for more accurate process design and sizing of large-scale CLC units. Experimental studies of high-pressure CLC report conflicting results on the effect of operating pressure on the heterogeneous kinetics. Deshpande et al.35 studied high-pressure CLC using a Fe2O3/TiO2 oxygen carrier in a PTGA. They reported a 2-fold increase in the rate of reduction of Fe2O3/TiO2 using H2 and a 5-fold increase in the rate of reduction using CH4 when the reactor pressure increases by 10-fold. Jin and Ishida36−39 studied the effects of elevated pressure on the CLC of H2, natural gas, and syngas using NiO/NiAl2O4 and CoO/NiO in a PTGA and a fixed-bed reactor. Kinetics taken from reduction with H2 in the PTGA favored elevated pressures of 3 and 9 bar.37 Contradictory to the results from Deshpande et al.,35 the rate of reduction with CH4/H2O decreased at higher pressures because of the pressure independence on the CH4 steam reforming reaction.36,38 The reduction of NiO with simulated syngas in a fixed-bed reactor generated more CH4 at elevated pressures, which led to a higher yield of solid carbon.39 Siriwardane et al.40 tested the reactivity of NiO/bentonite with simulated syngas and found consistent CO2 production across successive cycles with a flue gas composition of CO2/CO = 8:1. Elevated pressure resulted in more complete syngas combustion throughout the oxygen carrier reduction period. In contrast, using Cu-, Fe-, and Ni-based oxygen carriers in a PTGA with CO and H2 at pressures up to 30 bar, GarcíaLabiano et al.11,41 observed a negative correlation between pressure and reduction rate for all the oxygen carriers studied, which was contributed to an internal restructuring of the Al2O3 support. Another conclusion drawn from this work was that the kinetic parameters estimated from experiments at one pressure were not sufficient to predict the data at different pressures. The analysis by Hamers et al.42 supported this finding and found that the pressure decreases the rate of the reduction and oxidation reactions, which are kinetically controlled. In summary, the ability of the models of IGCC and NGCC with CLC to represent real world processes is limited by a lack

2. EXPERIMENTAL SECTION 2.1. Oxygen Carrier Preparation. Oxygen carriers were prepared via incipient wetness impregnation as described in Zhou et al.43−45 Cu(NO3)2 was added to a SiO2 gel (Sigma-Aldrich) that had been previously dried overnight at 110 °C to remove moisture. The SiO2 gel particles had an average size of 100 μm and BET surface area of 498 m2/g. The resulting mixture was stirred at room temperature, then dried for 12 h at 120 °C followed by calcination for 3 h at 600 °C. Two successive impregnation steps were employed in order to increase the CuO loading. After double impregnation, the sample was sintered at 950 °C for 6 h to enhance its mechanical strength. Once sintered the oxygen carrier was then sieved to a particle size range of 50−150 μm. NiO was prepared through the addition of Ni(NO3)2 (Sigma-Aldrich) to a premade Al2O3−SiO2 support (W.R. Grace & Co.) with mean particle size of 70 μm and BET surface area of 160 m2/g. The mixture was stirred at room temperature followed by drying at 120 °C for 12 h. This was followed by calcination at 800 °C for 5 h. The resulting oxygen carrier was then sieved to a particle size of 50−150 μm. The stability of both oxygen carriers was tested for over 30 redox cycles in a TGA and a fixed-bed reactor, showing good stability. The physical properties of the fresh oxygen carriers are shown in Table 1.

Table 1. Characterization of the Fresh NiO and CuO Oxygen Carriers properties 2

BET surface area (m /g) pore size (Å) active MeO content (%) XRD identified phases particle size (μm)

NiO/γ-Al2O3−SiO2

CuO/SiO2

94 116 21 NiO, NiAl2O4, Al2O3, SiO2 50−150

6.493 219.5 37 CuO, SiO2 50−150

As discussed in previous work,34 the stability of the oxygen carriers used in this work is very good. In particular, BET and XPS/XRD measurements showed that the CuO oxygen carrier was stable up to the 30 cycles tested. Surface area and pore size of the NiO oxygen carrier remained unchanged for up to 10 cycles tested.28 2.2. Experimental Procedure. Experiments were performed with 2.2 g of oxygen carriers in the bench-scale, fixed-bed reactor shown in Figure 1. Successive cycles of CLC were executed by alternating the gases fed to the reactor, in which gas flow rates were either 100 or 500 sccm depending on the experimental approach. The CLC reduction cycle was initiated by feeding a fuel gas (either 10% CH4/Ar or a mixture of 5% CO and 5% H2 in Ar) to the reactor. After each reduction, the reactor was purged with pure Ar and fed with an oxidizing gas (20% O2/Ar) to commence the oxidation cycle. Subsequently, the reactor was purged to begin the next cycle. Several successive cycles were completed proving experimental repeatability. Experiments were performed with two oxygen carriers (NiO and CuO), two reducing fuels (methane and syngas), and three total pressures (1, 5, and 10 bar). As shown in Figure 1, the reactor pressure (1, 5, or 10 bar) was controlled with a back pressure regulator placed at the reactor exit. An electric furnace maintained the reaction zone at a relatively isothermal bed temperature of 800 °C, measured by a 505


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3. KINETIC ANALYSIS We used the dynamic 1D homogeneous reactor model, presented in Han et al.,27,28 which assumes that mass transfer limitations within the particle are negligible, compared to the gas/solid reaction rates. This assumption is appropriate, given that the particle sizes used in the fixed-bed experiments are in the range of 50−150 μm. The reduction kinetics for the Cuand Ni-based oxygen carriers was developed and validated at atmospheric pressure over a range of relevant temperatures, particle sizes, and fuel compositions.43−49 The corresponding reaction networks are shown in Tables 3 and 4 and discussed in Table 3. Chemical-Looping Reduction and Oxidation Reactions with Cu Oxygen Carrier44 (R1) (R2) (R3) (R4) (R5)

Figure 1. Fixed-bed chemical-looping reactor used in this study. thermocouple placed inside the oxygen carrier bed. The maximum temperature variation during reduction or oxidation was 20 °C. Each of the experiments was repeated three times with excellent repeatability, shown with error bars of the standard error of the experiments of Table 2.

(R6) (R7)

Table 2. Summary of the Experimental Setup and Conditions of This Work reactor i.d. solid loading total flow rate temperature reactor pressure oxidation feed reduction feed inert purge

(R8)

9.9 mm 2.2 g 100, 500 sccm 800 °C 1, 5, 10 bar 20% O2 in Ar 10% CH4 in Ar, 5% H2, and 5% CO in Ar Ar

(R17) (R18) (R19)

Reduction Reactions of CuO 2CuO + H2 → Cu2O + H2O (R9) 4CuO → 2Cu2O + O2 Cu2O + H2 → 2Cu + H2O (R10) CO2 + CH4 → 2CO + 2H2 2CuO + CO → Cu2O + CO2 (R11) CO + H2O ↔ CO2 + H2 Cu2O + CO → 2Cu + CO2 (R12) CH4 ↔ 2H2 + C CH4 + 6CuO → 3Cu2O + (R13) C + CO2 ↔ 2CO CO + 2H2O CH4 + 4CuO → 2Cu2O + (R14) CH4 + 2O2 → CO2 + 2H2O 2H2 + CO2 CH4 + 2CuO → Cu2O + 2H2 (R15) 2H2 + O2 → 2H2O + CO CH4 + 4Cu2O → 8Cu + (R16) 2CO + O2 → 2CO2 2H2O + CO2 Oxidation Reactions of Cu O2 + 4Cu → 2Cu2O (R20) O2 + 2C → 2CO O2 + 2Cu2O → 4CuO (R21) O2 + 2CO → 2CO2 O2 + C → CO2

Table 4. Chemical-Looping Reduction and Oxidation Reactions with Ni Oxygen Carrier28,43,45,46 Reduction Reactions of NiO (R7′) CH4 + H2O ↔ 3H2 + CO H2 + NiO → Ni + H2O (R8′) CO + H2O ↔ H2 + CO2 CO + NiO → Ni + CO2 H2 + NiAl2O4 → H2O + (R9′) CH4 + CO2 ↔ 2CO + 2H2 Ni + Al2O3 (R4′) CO + NiAl2O4 → CO2 + (R10′) CH4 ↔ 2H2 + C Ni + Al2O3 (R5′) CH4 + NiO → Ni + 2H2 + (R11′) C + H2O ↔ CO + H2 CO (R6′) CH4 + 4NiAl2O4 → CO2 + (R12′) C + CO2 ↔ 2CO 2H2O + 4Ni + 4Al2O3 Oxidation Reactions of Ni (R13′) O2 + 2Ni → 2NiO (R15′) O2 + 2C → 2CO (R16′) O2 + 2CO → 2CO2 (R14′) O2 + C → CO2

(R1′) (R2′) (R3′)

High-pressure reactor conditions led to a reduced gas velocity inside the oxygen carrier bed, when experiments were performed with the same total gas flow rate as those performed at atmospheric pressure. The decrease in gas velocity led to an increase in gas residence time, thus diluting the information on the effect of pressure on reactivity and selectivity. A thorough experimental approach was required to separate the effects of pressure and residence time on CLC reactivity and selectivity. This was achieved through a combination of two types of experiments. First, the effect of the reduced gas velocity inside the reactor bed was overcome by increasing the cycle time to reach the solid conversions of the experiments performed at atmospheric pressure. Experiments were designed, in which the cycle time for reduction/inert purging/oxidation were determined for the highest reactor pressure (smallest gas velocities) to be 60/8/18 min for the experiments with syngas and 3/6/8 min for the experiments with CH4, respectively. These experiments were completed with flow rate fixed at 100 sccm to enable the study of the effects of pressure and gas velocity separately. Second, the pressure and total gas flow rate (keeping the fuel molar fraction the same) were increased 5 times to maintain the residence time at the different operating pressures. The total amount of gas entering the reactor was held constant by reducing the oxidation, reduction, and purging periods 5-fold. The carbon balance was closed for every reduction−oxidation cycle by integrating the CO, CO2, and CH4 readings exiting the reactor over time and ensuring the total carbon leaving the system was equivalent to the carbon entering with the fuel source. The solid carbon yield was calculated by dividing the amount of carbon exiting the reactor during the oxidation stage by the total carbon entering the system. These experiments allowed the effects of pressure to be studied at constant gas velocity.

detail in several works by Zhou et al.43−45,47 Tables 3 and 4 present all the relevant reaction pathways for NiO and CuO in reduction by CH4, H2, and CO. This kinetic model was previously shown to accurately predict CLC reduction at different operating temperatures and reacting gas compositions. For the oxidation step, Zhou et al.45 showed that the shrinking core model is appropriate for the oxidation of Ni, which we extend here to the modeling of the oxidation of Cu and Cu2O. Also, the combustion of the solid carbon in this work is modeled using the kinetics of Keskitalo et al.50 and Subramaniam and Varama.51 As shown in Table 1, the NiO oxygen carrier contained NiAl2O4 spinel, the reactions of which are included in Table 3 and modeled using the kinetics reported in Han et al.28 506


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Figure 2. Reduction of CuO with syngas and oxidation of Cu at constant flow rate at 1 bar (a) and 5 bar (b) and at constant residence time at 5 bar (c). Symbols are the experimental data averaged from three independent runs. Lines depict the corresponding prediction of the kinetic model described in Section 4.5, with the kinetic parameters listed in Appendix, Table A.1.

Section 2 was indeed appropriate. The mathematical model and parameter estimation were developed and implemented using the commercial software package gPROMS.52

The kinetic models for all the reactions of Tables 3 and 4 are given in the Appendix. The kinetics expressions were based on previous work, augmented with eq 1 to semiempirically describe the impact of high pressure on the reactivity of the NiO and CuO oxygen carriers. According to this correlation, which was originally proposed by García-Labiano et al.11 for copper-, iron-, and nickel-based oxygen carriers, the kinetic rate at high pressure, kiP, is inversely related to the total pressure, P, inside the reactor: ki Pd

kPi =

4. RESULTS AND DISCUSSION 4.1. Reduction of CuO with Syngas. Figure 2 presents the experimental results from the reduction of the CuO oxygen carrier with syngas at different pressures under constant flow rate (a,b) and at high pressure under constant residence time (c). In the experiments performed at atmospheric pressure (Figure 2a), the syngas fuel was completely oxidized by the CuO and after ∼15 min of reduction, breakthrough of the syngas was observed. The concentration of H2 increased with a relatively slower rate compared to CO, because of the higher reactivity between H2 and CuO.44,53,54 Reduction of CuO by syngas at atmospheric pressure produced a minor amount of solid carbon (1.9 mol %), which was oxidized to CO/CO2 in the following oxidation cycle (Figure 2a). When the reactor pressure was increased to 5 bar at constant flow rate (Figure 2b), the CO2 selectivity was high for longer reduction times because of the decrease in gas velocity and corresponding increase in gas residence time. The emissions of CO and CO2 during oxidation were higher in Figure 2b than in Figure 2a, because more solid carbon (17 mol %) was deposited during the reduction step at high pressure compared to atmospheric pressure (1.9 mol %). The increase in carbon formation with reactor pressure can be explained by the increase in partial pressures, which increases the rate of the Boudouard reaction. Figure 2c shows the reduction of CuO by syngas at high pressure with the same residence time as the experiment performed at atmospheric pressure. The gas profiles of Figure 2c followed similar trends with those shown in Figure 2b, characterized by the initial high concentration of CO2 followed by the breakthrough of H2 and CO, with simultaneous CO2 production due to carbon formation reactions. The time interval to completely reduce and oxidize the oxygen carrier is shorter in Figure 2c than in Figure 2a and Figure 2b because the flow rate was increased to 5 times its original value. More solid carbon formation was observed in the experiment shown in Figure 2c as compared to that of Figure 2a, revealing that at constant residence time, higher reactor pressure is correlated to more solid carbon formation. In summary, high-pressure operation of CLC with CuO promoted the deposition of solid carbon during reduction with syngas fuel. A general decrease in the reduction reactions at higher reactor pressures was observed, which is consistent with

(1)

i

where k is the kinetic rate at atmospheric pressure and d is a fitted parameter for each oxygen carrier and reaction. Only the kinetics for the oxygen carrier reduction and oxidation reactions are modified using eq 1, namely R1−R9, R17, and R18 for the Cu oxygen carrier (Appendix, Table A.1) and R1′−R6′ and R13′ for the Ni oxygen carrier (Table A.2). A systematic procedure was employed to design highpressure experiments in order to estimate d of eq 1. The first step involved the execution of experiments at atmospheric pressure, to estimate the reduction kinetics of the NiAl2O4 spinel (R3′, R4′, and R6′ of Table 4) and the oxidation kinetics (R17−R21 of Table 3 and R13′−R16′ of Table 4). The kinetics for all the remaining reactions at atmospheric pressure, listed in Tables 3 and 4, were fixed to the values reported in Han et al.46 and Zhou et al.43,44 The next step was to perform reduction experiments with syngas at high pressure and constant flow rate, to estimate the value of d for the reduction reactions of NiO (and NiAl2O4) and CuO, by H2 and CO (R1−R4, R1′−R4′). In this step, ki was kept at the values previously estimated from atmospheric pressure experiments. The same procedure was followed for the estimation of the values of d for the oxidation reactions, R17, R18, and R13′, using the values of ki estimated from atmospheric oxidation experiments. The third step was to execute reduction experiments with CH4 at high pressure and constant flow rate, and estimate the values of d for reactions R5−R8, R5′, and R6′, with ki estimated from atmospheric experiments. In the final step, we validated the accuracy of the kinetic model, under different flow conditions. High-pressure experiments with syngas and CH4, conducted at an increased flow rate keeping the gas residence time constant to that at atmospheric pressure, were used for validation. These experiments were predicted using the developed kinetics, without further parameter estimation. The objective of this strategy was to confirm that the experiment design strategy discussed in 507


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Figure 3. Reduction of CuO with CH4 and oxidation of Cu at a fixed flow rate at 1 bar (a), 5 bar (b), and 10 bar (c) and at fixed residence time at 5 bar (d). Symbols are the experimental data averaged from three independent runs. Lines depict the corresponding prediction of the kinetic model described in Section 4.5, with the kinetic parameters listed in Appendix, Table A.1.

several studies.11,42 The same conclusion was drawn for the oxidation reactions of Cu and Cu2O. 4.2. Reduction of CuO with CH4. The experimental results for the reduction of CuO by CH4 at different pressures under constant flow rate and constant residence time are shown in Figure 3. At atmospheric pressure (Figure 3a), significant fuel slip was observed, which can be explained by the initial slow reduction rate of CuO by CH4.44 The extent of carbon deposition during the reduction of CuO by CH4 at atmospheric pressure was insignificant, indicating a negligible rate for CH4 decomposition over Cu, Cu2O, or CuO. In the experiments performed at 5 and 10 bar at constant flow rate (Figure 3b,c), complete CH4 conversion was observed and the fuel slip was eliminated, showing that the rate of CH4 conversion increased with reactor pressure. The selectivity toward solid carbon was higher at 5 bar (Figure 3b) and 10 bar (Figure 3c) than at atmospheric pressure (Figure 3a). The high-pressure experiment performed at constant residence time (Figure 3d) had a higher CO2 selectivity, more solid carbon formation, and a significantly smaller fuel slip compared to the experiment at atmospheric pressure (Figure 3a). In summary, the results of Figure 3 show that the conversion of CH4 to CO2 improves with increasing pressure. The high-pressure operation also promoted the solid carbon formation reactions. 4.3. Reduction of NiO with syngas. As shown in Figure 4a, the reduction of NiO by syngas at atmospheric pressure proceeded in two stages. The first stage in Figure 4a corresponds to the reduction of the NiO in the oxygen carrier. The fraction of the active Ni in the oxygen carrier was estimated to be ∼50 wt % of the total impregnated Ni, from thermogravimetric analysis done at atmospheric pressure, as

detailed in previous work.28,45 The second stage of the experiment shown in Figure 4a proceeded with slower kinetics and was attributed to the NiAl2O4 spinel phase being reduced.28 During this second stage, the concentration of H2 and CO gradually increased to their inlet values, indicating that the oxygen carrier was completely reduced at the end of each reduction cycle. At atmospheric pressure, the reduction of NiO by syngas did not yield solid carbon. The reduction of NiO by syngas at high pressure and constant flow rate (Figure 4b) exhibited the same two-stage behavior, where NiO reduction was followed by a slower reduction of NiAl2O4. The breakthrough of CO and H2 occurred later in the experiment of Figure 4b than in that of Figure 4a, due to the increase in gas residence time. Solid carbon formation was observed in the reduction of NiO by syngas at 5 bar (Figure 4b), similarly to what was previously observed with CuO. The increased pressure also eliminated the initial slip of unconverted CO during reduction. The experiments presented in Figure 4c were performed at 5 bar, with the gas residence time constant to its value calculated for the atmospheric pressure experiments (Figure 4a). The extent of carbon formation increased from the experiment of Figure 4a to that of Figure 4c, which is consistent with the constant flow rate experiments. Overall, we showed that the high-pressure reduction of NiO by syngas promotes the deposition of solid carbon on the oxygen carrier. 4.4. Reduction of NiO with CH4. The NiO oxygen carrier was highly reactive in the oxidation of CH4 at atmospheric pressure (Figure 5a). The conversion of CH4 was near 100% for the entire duration of the reduction step, but the selectivity toward CO2 drops to ∼0% at the end of reduction. This was 508


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Figure 4. Reduction of NiO with syngas and oxidation of Ni at a fixed flow rate at 1 bar (a) and 5 bar (b) and at fixed residence time at 5 bar (c). Symbols are the experimental data averaged from three independent runs. Lines depict the corresponding prediction of the kinetic model described in Section 4.5 with the kinetic parameters listed in Appendix, Table A.2.

Figure 5. Reduction of NiO with CH4 and oxidation of Ni at a fixed flow rate at 1 bar (a), 5 bar (b), and 10 bar (c) and at fixed residence time at 5 bar (d). Symbols are the experimental data averaged from three independent runs. Lines depict the corresponding prediction of the kinetic model described in Section 4.5 with the kinetic parameters listed in Appendix, Table A.2.

This was due to the improved axial mixing inside the fixed bed, as result of the higher reactor pressure and corresponding longer residence time. As discussed in Han et al.,46 fixed-bed processes with smaller Péclet numbers achieve higher fuel conversion and CO2 selectivity. In Figure 5a−c, the Péclet numbers were decreased by increasing the reactor pressure, and so a higher CO2 selectivity was observed. CO and H2 are purged out of the reactor slowly, as for instance shown in Figure 5c, due to reduced gas velocities of the reactor operation at 10 bar. When the experiments were performed at 5 bar with the higher flow rate, to match the residence time of the atmospheric pressure experiments (Figure 5d), the fuel slip and solid carbon selectivity increased. The concentration of CO2 during reduction at high pressure (Figure 5d) was lower than that at atmospheric pressure (Figure 5a), indicating that the

due to the higher activity for the Ni-catalyzed methane cracking reaction. In the beginning of the reduction step, there was sufficient NiO to completely oxidize the CH4 and associated gas species into CO2 and H2O. When most of the NiO in the bed was reduced, the methane cracking reaction was dominant and produced high concentrations of H2, shown in Figure 5a. From Figure 5a, it is evident that carbon deposition poses a major challenge for atmospheric CLC processes operating with Ni-based oxygen carriers and CH4 fuel. Figure 5b,c presents the experimental results at 5 and 10 bar under constant flow rate. The solid carbon selectivity remained relatively the same between experiments at 1 and 5 bar (Figure 5b) and decreased by one-third between 5 and 10 bar (Figure 5c). The CO2 capture efficiency of the oxygen carrier increased with reactor pressure under constant flow rate. 509


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Table A.1. Kinetic Parameters for the High-Pressure Reduction and Oxidation Reactions of the CuO Oxygen Carrier, Listed in Table 3 (R1)

′ r1 = a0k1/P1.74n(1 − XCuO)(− ln(1 − XCuO))(1 − 1/ n)C H2CCuO

(R2)

′ r2 = a0k 2/P 2.91nXCuO(1 − XCu2O)(− ln(1 − XCu2O))1 − 1/ n C H2CCuO

(R3)

′ r3 = a0k 3/P1.36n(1 − XCuO)(− ln(1 − XCuO))1 − 1/ n CCOCCuO

(R4)

′ r4 = a0k4 /P1.06nXCuO(1 − XCu2O)(− ln(1 − XCu2O))1 − 1/ n CCOCCuO

(R5)

′ r5 = a0k5/P 0.46n(1 − XCuO)(− ln(1 − XCuO))(1 − 1/ n)CCH4CCuO

(R6)

′ r6 = a0k6/P 0.39n(1 − XCuO)(− ln(1 − XCuO))1 − 1/ n CCH4CCuO

(R7)

′ r7 = a0k 7/P 0.49n(1 − XCuO)(− ln(1 − XCuO))1 − 1/ n CCH4CCuO

(R8)

′ r8 = a0k 8/P 0.34nXCuO(1 − XCu2O)(− ln(1 − XCu2O))1 − 1/ n CCH4CCuO

(R9)

′ r9 = a0k 9n(1 − XCuO)(− ln(1 − XCuO))1 − 1/ n (PO2,eq − PO2)CCuO

(R10)

⎛ 2 2 PCO2PCH4 ⎞ r10 = k10⎜PCO PH2 − ⎟CCu K10 ⎠ ⎝

(R11)

⎛ PCOPH2O ⎞ r11 = k11⎜PCO2PH2 − ⎟CCu K11 ⎠ ⎝

(R12)

⎛ PH2 ⎞ r12 = k12K CH4,12⎜⎜PCH4 − 2 ⎟⎟ K12 ⎠ ⎝

(R13)

r13 =

k14(600PO2)0.5 PCH4 1 + (600PO2)0.5 + 500PCO2 + 950PH2O

(R15)

r15 =

⎞2 ⎛ PCO2 ⎟ ⎜⎜1 + K CO,13PCO + PCOK CO,13K CO2,13 ⎟⎠ ⎝

⎛ PCO k13 P ⎞ 2 ⎜ − CO ⎟ K CO,13K CO2,13 ⎝ PCO K13 ⎠

(R14)

r14 =

⎛ ⎞2 P 3/2 ⎜1 + H2 + K CH ,12PCH ⎟ ⎜ 4 4⎟ K H2,12 ⎝ ⎠

k15K H0.52 PH0.52 K O2,15PO2 0.5k15K H0.52 PH0.52 + K O2,15PO2(1 + K H0.52 PH0.52 )

(R16)

r16 = k16PCOPO0.52 /(1 + K COPCO +

(R17)

′ r17 = a0k17/P1.00(1 − XCu)2/3 CO2CCu

(R18)

′ 2O r18 = a0k18/P1.00(1 − XCu2O)2/3 CO2CCu

(R19)

r19 = a0k19(1 − XC)1/2 CO2CC′

(R20)

r20 = a0k 20(1 − XC)1/2 CO2CC′

(R21)

r21 = k 21CO2CCO2/(1 + K CO,21CCO)

K O2,16PO2 )2

solid carbon (R17−R21 of Table 3) were estimated from the oxidation data of Figure 2a. Using eq 1 and ki estimated from the experiments performed at atmospheric pressure, the values of d for reactions R1−R4, R17, and R18 were fitted to match the data of Figure 2b. The estimated pressure factors, shown in Appendix, Table A.1, are greater than 1 for the reduction reactions and equal to 1 for the oxidation reactions. Thus, the oxygen carrier reaction rates were negatively affected by the increase in reactor pressure, which is consistent with the work of Hamers et al.42 Next, the values of d for the reduction reactions of CuO by CH4 (R5−R8) were estimated from the high-pressure data of Figure 3b,c, using all the parameters previously derived. As shown in Table A.1, the values of d for these reactions were generally less than 1 but greater than 0, indicating that the rate of CH4 consumption was less affected by the increasing reactor pressure, than the consumption rates for H2 and CO. This also explains why the slip of unconverted CH4 was eliminated in all the high-pressure experiments, but

conversion of CH4 to CO2 was reduced when pressure was increased. The experiments of Figure 5 show that reduction of NiO by CH4 at high pressure favored the formation of solid carbon. This is consistent with the results of the reduction experiments with CuO (Section 4.2). The data collected from these experiments at variable pressure and flow rates are used in the next section for validation of the kinetic model. Possible solutions to reduce carbon deposition during high-pressure CLC are to limit the oxygen carrier conversion during the reduction cycle,55 dilute the fuel with H2O or CO2,6 and optimize the oxygen carrier, by using bimetallic materials.56 4.5. Kinetics of High-Pressure CuO and NiO Reduction. The development of kinetic models at atmospheric pressure for the reduction of CuO and NiO is discussed in detail in Han et al.46 and Zhou et al.44 In this work, the reported kinetics of Zhou et al.44 is used to describe the reduction of CuO and Cu2O and associated catalytic reactions of Table 3. The values of ki for the oxidation of Cu, Cu2O, and 510


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Energy & Fuels Table A.2. Kinetic Parameters for the High-Pressure Reduction and Oxidation Reactions of the NiO Oxygen Carrier, Listed in Table 4 (R1′)

′ r1 = a0k1/P1.12n(1 − XNiO)(− ln(1 − XNiO))1 − 1/ n C H2C NiO

(R2′)

′ r2 = a0k 2/P 0.85nX(1 − XNiO)(− ln(1 − XNiO))1 − 1/ n CCOC NiO

(R3′)

′ 2O4 r3 = a0k 3/P1.03(1 − X NiAl2O4)2/3 C H2C NiAl

(R4′)

′ 2O4 r4 = a0k4 /P 0.68(1 − X NiAl2O4)2/3 CCOC NiAl

(R5′)

′ r5 = a0k5/P 0.59n(1 − XNiO)(− ln(1 − XNiO))1 − 1/ n CCH4C NiO

(R6′)

′ 2O4 r6 = a0k5/P 0.90(1 − X NiAl2O4)2/3 CCH4C NiAl

P3 P ⎞ k7 ⎛ ⎜PCH PH O − H2 CO ⎟ ⎜ K 7 ⎟⎠ PH2.52 ⎝ 4 2

(R7′)

r7 =

2 ⎛ K P ⎞ ⎜⎜1 + K CO,7PCO + K H ,7PH + K CH ,7PCH + H2O,7 H2O ⎟⎟ 2 2 4 4 PH2 ⎝ ⎠

(R8′)

PH PCO ⎞ k8 ⎛ ⎜PCOPH2O − 2 2 ⎟ PH2 ⎝ K8 ⎠

r8 =

2 ⎛ K P ⎞ ⎜⎜1 + K CO,8PCO + K H ,8PH + K CH ,8PCH + H2O,8 H2O ⎟⎟ 2 2 4 4 PH2 ⎠ ⎝

(R9′)

2 ⎛ PCO PH22 ⎞ ⎟/(1 + K CH ,9PCH ) r9 = k 9K CH4,9⎜⎜PCH4PCO2 − 4 4 K 9 ⎟⎠ ⎝

(R10′)

⎛ PH2 ⎞ r10 = k10K CH4,10⎜⎜PCH4 − 2 ⎟⎟ K10 ⎠ ⎝

(R11′)

⎛ PH O P ⎞ ⎜⎜ 2 − CO ⎟⎟ r11 = K H2O,11 ⎝ PH2 K11 ⎠ k11

(R12′)

r12 =

⎛ ⎞2 P 3/2 ⎜1 + H2 + K CH ,10PCH ⎟ ⎜ 4 4⎟ K H2,10 ⎝ ⎠ ⎛ ⎞2 PH2O PH3/2 2 ⎟ ⎜1 + K CH ,11PCH + + ⎜ 4 4 PH2K H2O,9 K H2,9 ⎟⎠ ⎝

⎛ PCO2 P ⎞ k12 − CO ⎟ ⎜ K CO,12KCO,10 ⎝ PCO K12 ⎠

(R13′)

′ r13 = a0k13/P1.02(1 − XNi)2/3 CO2CNi

(R14′)

r14 = a0k14(1 − XC)1/2 CO2CC′

(R15′)

r15 = a0k15(1 − XC)1/2 CO2CC′

(R16′)

r16 = k16CO2CCO/(1 + K CO,16CCO)

the slip of unconverted CO remained (Figure 3b,c). In the final step, we utilized the estimated kinetic and pressure parameters to predict the experiments performed at constant residence time. These predictions are displayed in Figures 2c and 3d. By the close agreement between the model predictions with the data, we have demonstrated that developed kinetic and reactor models accurately capture the high-pressure CLC experiments with the CuO oxygen carrier. A similar approach was followed for the CLC data with the NiO oxygen carrier. The values of ki for the reduction of NiAl2O4 spinel (R3′, R4′, and R6′ of Table 4) and oxidation of Ni and solid carbon (R13′−R16′ of Table 4) were estimated from experiments conducted at atmospheric pressure, using the NiO reduction and catalytic reactions kinetic parameters reported in Han et al.46 Subsequently, the values of d for reactions R1′−R4′ and R13′ were estimated from the highpressure syngas-fed experiments (Figure 4b), using ki as previously estimated from atmospheric pressure. Shown in Appendix, Table A.2, the estimated values of d for the reduction

⎞2 ⎛ PCO2 ⎟ ⎜⎜1 + K CO,12PCO + PCOK CO,10K CO,12 ⎟⎠ ⎝

by syngas and oxidation by O2 are close to 1, indicating that increasing reactor pressure lowers the reaction rates. We used the high-pressure data from the experiments of NiO with CH4, Figure 5b,c, to estimate the values of d for reactions R5′, R6′, and R13′. The NiO kinetic model was then used for the prediction of the experiments performed at constant residence time (Figures 4c and 5d). Again, a good agreement between the model and the experiment data was achieved. The empirical model of eq 1 and the procedure of experiment design Section 3 are adequate to evaluate the effect of pressure on the reactivity of different oxygen carriers. The kinetic models derived from this work can be used with confidence for extrapolation to other particle sizes, temperatures, and fuel compositions.

5. CONCLUSIONS The reactivity of CuO and NiO oxygen carriers at high-pressure operation was measured in a fixed-bed reactor under various total pressures and flow rates. Two types of experiments were conducted to analyze the influence of reactor pressure on the 511


Article

Energy & Fuels Notes

gas/solid reactions. The experiments showed that high-pressure operation increased the formation of solid carbon over both oxygen carriers, increased the fuel conversion for the CuO oxygen carrier, and decreased the CO2 selectivity for the NiO oxygen carrier. The experimental results were well-predicted by a numerical model, using a semiempirical correlation for the effect of pressure in the kinetic expression. The results of the modeling work confirmed that the oxygen carrier reduction and oxidation kinetics decreased with increasing total pressure. The kinetics determined from the high-pressure experiments can be used for the design, scale-up, and optimization of pressurized CLC systems.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grant No. 1054718. Support by W.R. Grace & Co. by providing the Al2O3/SiO2 matrices is gratefully acknowledged.

■

NOTATION a0 surface area of the oxygen carrier (m2/kg OC) Ci concentration of gas species i (mol/m3) C′CuO solid concentration of initial CuO (kg CuO/kg OC) C′Cu solid concentration of Cu (kg Cu/kg OC) C′NiO solid concentration of initial NiO (kg NiO/kg OC) C′NiAl2O4 solid concentration of initial NiAl2O4 (kg NiAl2O4/ kg OC) i gas species (CH4, H2, H2O, CO, CO2, O2, Ar, N2) j chemical reaction kj reaction rate constant for reaction j Ki,j adsorption constant for gas species i in reaction j Kj equilibrium constant for reaction j OC oxygen carrier P total pressure (bar) Pi partial pressure of species i (bar) n Avrami exponent for the Avrami−Erofe’ev model rj rate of reaction j (mol/kg OC·s) XCuO conversion of CuO XCu2O conversion of Cu2O XNiO conversion of NiO

■

APPENDIX: KINETICS OF HIGH-PRESSURE CLC EXPERIMENTS WITH CUO AND NIO As detailed in Section 4.5, the effect of pressure on the reaction rates was calculated using eq 1 and fitting the kinetic parameters to the high-pressure CLC experiments. In Tables A.3 and A.4, the estimated values of d for the reduction and oxidation reactions and their 95% confidence intervals are summarized. Table A.3. Confidence Intervals (95%) of the Estimated Values of d for the Reactions Listed in Table 3 reaction

estimate

lower bound

upper bound

(R1) (R2) (R3) (R4) (R5) (R6) (R7) (R8) (R17) (R18)

1.74 2.91 1.36 1.06 0.46 0.39 0.49 0.34 1.00 1.00

1.43 2.71 0.99 0.94 0.40 0.34 0.46 0.28 0.91 0.91

2.05 3.11 1.73 1.12 0.53 0.44 0.52 0.41 1.09 1.09

■

Table A.4. Confidence Intervals (95%) of the Estimated Values of d for the Reactions Listed in Table 4

■

reaction

estimate

lower bound

upper bound

(R1′) (R2′) (R3′) (R4′) (R5′) (R6′) (R13′)

1.12 0.85 1.03 0.68 0.59 0.90 1.02

0.86 0.60 0.95 0.63 0.49 0.71 0.84

1.38 1.10 1.10 0.72 0.68 1.09 1.20

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b01986. Stability of CuO/SiO2 oxygen carriers after 30 cycles in TGA (Figure S1), stability of NiO/Al2O3 oxygen carriers after 30 cycles in TGA (Figure S2), cyclic results of NiO reduction with CH4 in fixed-bed reactor (Figure S3), and cyclic results of CuO reduction with CH4 in fixed-bed reactor (Figure S4) (PDF)

■

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 512


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High-Pressure Chemical-Looping of Methane and Synthesis Gas with

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