Adsorption and Methanation of Flue Gas CO2 with Dual Functional

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Adsorption and methanation of flue gas CO2 with dual functional catalytic materials: a parametric study Qinghe Zheng, Robert J. Farrauto, and Anh Chau Nguyen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01275 • Publication Date (Web): 06 Jun 2016 Downloaded from http://pubs.acs.org on June 6, 2016

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Industrial & Engineering Chemistry Research

Adsorption and methanation of flue gas CO2 with dual functional catalytic materials: a parametric study Qinghe Zheng,*† Robert Farrauto, † and Anh Chau Nguyen† †

Department of Earth and Environmental Engineering, Columbia University, New York 10027, United States.

KEYWORDS: CO2 adsorption and methanation, dual functional solid materials, nano dispersed calcium oxide and ruthenium

ABSTRACT: Conventional carbon capture and sequestration (CCS) in aqueous alkanolamine solutions is an energy-intensive process for power plant flue gas CO2 treatment. We report a laboratory parametric study on the utilization of CO2 from simulated natural gas post-combustion effluent by cyclic adsorption and methanation using a single dual functional material (DFM). The DFM is composed of nano-dispersed CaO and Ru metal supported on an 𝛾Al2O3 carrier (5%Ru,10%CaO/Al2O3), respectively functioning as the CO2 adsorbent and methanation catalyst. The stored H2 used for methanation is assumed to be produced by water electrolysis using excess alternative wind or solar energy. The effects of DFM preparation methods, Al2O3 carrier materials (with different shapes and properties), and adsorption and methanation conditions (feed

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compositions, flow rates, and reaction temperatures) were examined. The DFM samples, prepared using chloride precursor salts, showed stable performance under cyclic operating conditions. Reaction conditions were explored for the optimized CO2 utilization efficiency.

1. INTRODUCTION Increasing greenhouse gas emissions (GHG) continues causing concerns about global warming, ocean acidification, and other environmental problems.1,2 Anthropogenic CO2 emission from fossil fuel combustion and industrial processes contributed about 78% to the total GHG emission increase between 1970 and 2010.3 Scientists have been persistently seeking effective approaches to the capture of CO2 from post-combustion effluents, such as flue gas. The current industrial scale separation technology is based on the CO2 scrubbing (absorption) using alkanolamine, in a thermal swing reactor.4 The liquid amine-based technology has several shortcomings including large equipment size, severe instrument corrosion, high regeneration energy, and solvent degradation.5, 6 Major advances have been achieved toward the development of various adsorbent materials for CO2 capture through physisorption at low temperature (25 ~ 150 oC) and high pressure. Adsorbents studied include zeolites,7,8 carbon-based materials,9,10 and metal organic framework (MOFs) structures.11,12 However, these materials often adsorb steam preferentially over CO2, while possessing insufficient CO2 adsorption capacity at atmospheric pressure and slightly elevated temperatures. Therefore, they have questionable value for a demanding adsorption condition such as in a post-combustion flue gas environment where high concentrations of steam and air are present.4 Alternatively, CO2 capture using dry alkali metal-based materials are suggested for post-combustion gas treatment.13-18 Typical processes include chemical looping, a cyclic process of reversible carbonation and decarbonation of calcium oxide with CO2, as


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described by Eq. (1).19,20 This cyclic process is operated at temperatures between 500 and 900 oC, with issues such as slow carbonation/decarbonation, and sorbent deterioration with an increasing number of cycles.21-23 CaO s +CO2 g ⇌ CaCO3 s ΔH 0 =-‐‑178 kJ/mol

(1)

The applicability of conventional sorption/desorption- based carbon capture and sequestration (CCS) technology is challenged by significant technical, energy-, and cost-related issues. A conceptual process design with both sorption and chemical conversion of flue gas CO2 on the same material could potentially be a more feasible solution. CO2 capture based on heterogeneous adsorption on solids at elevated temperature can be an attractive approach with lower cost and energy consumption. We have demonstrated a dual function solid material containing an adsorbent and a catalyst that captures CO2, from a simulated flue gas, and upon hydrogenation produces methane which is recycled to the inlet of the power plant thereby reducing the overall consumption of natural gas. The CO2 capture and conversion to CH4 occurs in the same reactor, at flue gas temperatures. Excess stored renewable H2 is used to methanate the captured CO2.24,25 The stored H2, assumed generated by water electrolysis, is one way of utilizing excess solar and wind energy out of phase with its direct use for electricity. The DFM is composed of nano-dispersed calcium oxide (CaO) and ruthenium (Ru) metal dispersed on a high surface area carrier (Al2O3). A two step-process involving adsorption of CO2 in a steam/air-containing simulated flue gas followed by methanation, both at 320 oC and 1 bar, is further examined in this paper. Methanation occurs, via a process whereby the adsorbed CO2 spills over to the Ru sites where renewable stored H2 is added and catalytically converts it to synthetic natural gas or CH4. The approach has the following advantages: (1) The reactor containing DFM can be positioned in the flue gas utilizing its sensible heat; (2) Methanation


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occurs off-line at the same temperature generating CH4 at 100% selectivity; (3) The process utilizes renewable H2 generated via electrolysis from electricity generated from solar and/or wind energy out of phase with its direct use; (4) Significant CO2 capture and CH4 production, with stable adsorption/methanation performance has been demonstrated24 and process parameters identified in this paper. Both CO2 adsorption and methanation are exothermic processes, according to the equations below. Eq. (2) is given according to a previously proposed model for CO2 chemisorption on nano-dispersed CaO in the temperature range 300 oC~ 350 oC. Its nano-dispersed state (~ 3 nm) promotes chemisorption rather than carbonate formation.26 When supported on γ-Al2O3, the dispersed CaO adsorbs CO2 exclusively as a reversible bound structure, which differs from the monodentate carbonates27-30 formed on bulk CaO at the same temperature condition (300 oC). These observations explain the rapid adsorption and regenerability of dispersed CaO/Al2O3 as an effective CO2 adsorbent in DFM. CaO/Al2 O3 +CO2 ⇌ CO2 ⋯CaO/Al2 O3

(2)

The hydrogenation (methanation) of CO2, also known as the Sabatier reaction (Eq. 3), is an exothermic process that can be used to selectively produce methane catalytically, preferably from H2 produced from renewable resources.31 CO2 +4H2 ⇌ CH4 +2H2 O ΔH 0 =-‐‑165 kJ/mol

(3)

Supported Ni and PGM metals (e.g. Ru, Rh, Pd) on various supports (TiO2, SiO2, Al2O3, CeO2, ZrO2) have been studied for this reaction.32 Ni-based catalysts are often preferred but deactivate at low temperatures due to the interaction of the metal particles with CO and the formation of mobile nickel subcarbonyls.33 Furthermore, in a flue gas atmosphere the Ni is oxidized and cannot be adequately reduced at temperatures where methanation is thermodynamically


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favorable (< 350 oC). Supported Ru is known to be an active and stable methanation catalyst34-38, and can be reduced rapidly. It has been reported that the CO2 reduction on supported Ru/Al2O3 catalysts is cluster size-dependent, with higher Ru loadings (> 1 wt%) and moderate dispersions favoring the selectivity toward CH4 formation.39 The methanation of CO2 on Ru is consistent with an Eley-Rideal mechanism.25 Ru metal also exhibits CO2 adsorption in an activated process.25, 40 It has been suggested that CO2 hydrogenation on Ru is initiated by a dissociative adsorption of CO2 (Eq. 4) to CO and O, followed by dissociation of the latter species to C and O (Eq. 5) and successive conversion of C to CH4 through the final reaction (Eq. 6).41 Ru has CO2 adsorption capacity but only in its metallic state. The spill-over step of the pre-adsorbed CO2 on CaO to the adjacent metallic Ru sites (generated upon the addition of H2) is critical for the enhanced methanation yield in the DFM application. CO2 ads. → CO ads. + O(ads.)

(4)

CO ads. → C ads. + O(ads.)

(5)

C ads. +4H(ads.) → CH4

(6)

The present paper is focused on a laboratory-parametric and scale-up study of CO2 capture and methanation from a simulated natural gas power plant effluent. With proliferation of the exploration of natural gas resources and the development of hydraulic fracturing technology, there has been a substantial increase in the number of operational natural gas-fired power plants in U.S. over the past few years.42 The clean nature of the natural gas reduces the concentration of SO2 and eliminates the particulate matter characteristic of coal-based effluent systems. The exhaust gas from the turbine will cool from about 650 oC, to atmospheric conditions.43 Thus the DFM reactor can be located at any desired position and temperature in the exhaust. The effects


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of the main flue gas parameters including temperature, flow rate, O2 and steam concentrations on the DFM performance have been studied. The study includes the following aspects: (1) preparation of the 5%Ru,10%CaO DFM on several γ-Al2O3 carriers for effective industrial mass production and minimized reactor pressure drop; (2) parametric and lab aging studies including reaction temperature, flow rate, and feed gas composition for conceptual process optimization; (3) the cyclic testing protocol utilizing a simulated gas mixture of 7.5% CO2, 15% steam, 4.5% O2, and N2 in balance (all in vol-%) at flue gas temperature (320 oC) and pressure (1 bar). Methanation was conducted with 5% H2 in N2 to insure safe operation in the lab. The study provides direction for a process design.

2. EXPERIMENTAL SECTION 2.1. Material preparation The DFM samples, with a target composition of 5%Ru,10%CaO/Al2O3 were prepared by incipient wetness impregnation followed by co-precipitation with an alkaline solution. The schematic preparation process flow is shown in Figure 1. Ruthenium (III) Chloride hydrate (RuCl3⋅xH2O, 99.9% PGM basis, Ru 38% min, Alfa Aesar) and calcium chloride (CaCl2, anhydrous, powder, 99.99% trace metals basis, Sigma-Aldrich) were selected as the precursor salts, and dissolved in water with an appropriate amount of HCl added for pH control close to 0±0.2. Powder-, spherical (beads)-, and cylindrical pellet-forms of γ-Al2O3 (Table 1) were tested as DFM carriers. Shaped carriers, with particulate sizes around 5 mm, allow a preferred reactor design with minimized pressure drop. The physical properties of the carrier must also insure high effectiveness factors. The co-impregnation of ruthenium and calcium oxide precursor salts were first performed on the degassed γ-Al2O3 carrier, followed by drying in air at 120 oC for 3 hrs. An


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aqueous NaOH (≥ 97%, Sigma-Aldrich) solution was used to precipitate and fix the Ca and Ru hydroxides on the Al2O3 at 80 oC for 3 hrs. The precipitates were water washed to remove the chloride precursors and filtered. Silver nitrate was added to the filtrate to insure chloride removal as noted by the absence of turbidity. The DFM was dried at 120 oC for 3 hrs in air, followed by calcination in air at 400 oC for 3 hrs. RuOx pre-reduction was carried out with a flow of 5 vol-% H2/N2 at 400 oC for 150 min to form the catalytically active metallic state. Calcination temperatures were always lower than 400 oC to prevent formation of volatile Ru oxides. The deposition uniformity of CaO on Al2O3 were assessed by using a calcium indicator (Taylor), with universally dyed pink pellets indicating uniformed deposition. While uniform Ru deposition was confirmed by its own brown-dark brown color after impregnation. In some cases, DFM powders were pressed into tablets or pellets with a die (Specac Atlas 13 mm Evacuable Pellet Die) under a hydraulic pressure of 5 tons.

Figure 1. Schematic process flow for the catalyst preparation of 5%Ru,10%CaO/Al2O3 DFM on different Al2O3 support materials.


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Table 1 Material properties of the alumina supports used for catalyst preparation. Support alumina

BET surface area

Pore volume

Packing density

Diameter

Water uptake

m2/g

mL/g

g/mL

mm

mL H2O/g

BASF precursor powder*

143

N/A

0.93

N/A

0.74

SASOL TH200 pellets

110

0.73

0.59

5.08

0.69

SASOL TH100 pellets

140

0.66

0.58

5.07

0.76

BASF SAS200 spheres

N/A

N/A

N/A

3.18

0.52

*BASF powder was subsequently tableted.

2.2. CO2 adsorption-methanation cycle tests CO2 adsorption-methanation tests were performed in a fixed bed flow reactor (Fig. 2). A standard quartz tube reactor (O.D of 25 mm, (I.D. of 20 mm, length of 570 mm) was connected to the reaction system through Swagelok Ultra-Torr Vacuum fittings housed in a microthermal furnace (Mellen MTSC12.5R-.75x18-1Z) with temperature feed-back control. The Omega thermocouples (K type) were placed close to the inlet of the DFM bed. Ten grams of DFM were secured in the middle of the quartz reactor while the downstream reactor space was filled with degassed glass beads (Fisher Scientific, DI of 6 mm) to reduce the reactor dead volume and improve analytical response frequencies. Inlet gas flow rates and compositions were controlled by multiple gas rotameters (Key Instruments) with precise linear calibrations using specific gas carriers. Water was injected using a syringe pump (Cole-Palmer) into the heated system with a temperature maintained at 125 oC to generate steam. This was mixed with the other pre-heated feed gas components. During the test, the unreacted water (and water produced from methanation) was condensed in an ice cold trap, while the dry product gas mixture flowed into an Enerac Model 700 analyzer equipped with electrochemical and non-dispersive-infrared sensors for frequent (1 Hz) on-line remote analysis of the product gas composition. Among the dry gas products, the volume percentages of CH4, CO, O2, and CO2 were detected, while the


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material balances were ensured by confirming the flow rate detected by a mass flow meter placed upstream of the gas analyzer. All the tests were performed at ambient pressure with negligible pressure drop observed from the inlet to outlet.

Figure 2. Reactor setup for the CO2 adsorption-methanation cycle tests and process parameter study. The schematic protocol of CO2 adsorption-methanation cycle tests is shown in Figure 3. Two steps were involved in each cycle, i.e. CO2 adsorption and followed by methanation of the adsorbed CO2 (with added H2). For the CO2 adsorption step, a gas mixture (Vol %) composed of 7.5% CO2, 15% steam, 4.5% O2, and N2 balance, at a total gas hourly space velocity (GHSV) of 11, 236 h-1, simulating the flue gas content of a natural gas fired power plant, was fed to the reactor. CO2 adsorption was performed at 320oC and 1 atm. for varying times as indicated. For the methanation step, the reaction gas feed was switched to 5% H2 and N2 balance, at a varying total GHSV. In between the adsorption and the methanation step the reactor was purged with pure N2 for 3 min. to prevent mixing of O2 and H2. For ultimate commercial use it is expected


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that pure H2 will be used and generated by water splitting driven by an alternative energy source, such as excess solar energy. The CO2 methanation was carried out at 320 oC (same temperature as that in the adsorption step) and 1 atm. for around 60 min to complete conversion of adsorbed CO2 to CH4. The amount of CH4 produced was determined from the integrated CH4 volume percentage present in the dry product. The CO2 captured was calculated by subtracting the system background (product CO2 flow rate achieved by flowing the adsorption feed to an empty reactor) with the adsorption test result.

Figure 3. Schematic reaction process flow for CO2 adsorption-methanation cycle tests. 2.3. Process parametric study for the adsorption and methanation The effects of process parameters on CO2 adsorption and methanation were studied with the DFM supported on Al2O3 pellets (SASOL TH200). The process parameters investigated included adsorption time on stream (TOS), reaction temperature, adsorption feed flow rate, methanation feed flow rate, and the presence and absence of steam and/or O2 in the adsorption feed stream. The standard reaction condition was the same as described in Section 2.2. With each process


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parameter varied, the other reaction conditions were maintained the same. The parametric variables are shown in the accompanying figures.

3. RESULTS AND DISCUSSION 3.1. Cyclic tests of CO2 adsorption and conversion with 5%Ru,10%CaO DFM on various Al2O3 carriers Table 2. Average performance (in 10 cycles, with stable activity) for CO2 adsorption and methanation for 5%Ru,10%CaO/Al2O3 on different Al2O3 carriers. The performance of # I~IV materials were evaluated by integration of the adsorption and methanation curves.

#

Catalyst

CO2 capture capacity a

Methanation capacity

I

5%Ru,10%CaO/Al2O3 tableted powder (BASF)

11.18

2.08

48.3%

II

5%Ru,10%CaO/Al2O3 pellets (TH200)

6.08

2.26

91.2%

III

5%Ru,10%CaO/Al2O3 pellets (TH100)

7.50

3.27

86.6%

IV

5%Ru,10%CaO/Al2O3 beads (SAS200)

8.00

1.90

57.7%

b

CO2 conv. efficiency c

Annotations: a

CO2 capture capacity at adsorption step (g CO2/ kg DFM) ≡ grams of CO2 captured / kg of DFM material;

b

Methanation capacity at methanation step (g CH4/ kg DFM) ≡ grams of CH4 produced/ kg of DFM material;

c

CO2 conversion efficiency (mol CH4/ mol CO2, %) ≡ Moles of CH4 produced/ Moles of CO2 captured.

The CO2 adsorption-methanation cycle tests were performed with DFM samples prepared on different Al2O3 supports (tableted powder, pellets with different pore volumes, and beads), with the best results achieved on SASOL TH100 and TH200 pellet supports. For each material, indicated in Table 2, the amount of CO2 adsorbed and O2 consumed during the adsorption step and CH4 production were measured as a function of reaction time on stream (TOS) for a total of 10 cycles. The CO2 adsorption, methanation rates and capacities varied with the various carrier


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materials, reflecting pore diffusion of CO2 to the dispersed CaO sites, H2 reduction of the RuOx and desorption and pore diffusion of CH4 into the bulk gas. The rate limiting step at this time appears to be reduction of the RuOx to the metallic state. Generally, stable performances were observed for all four catalysts during and after 10 cycles. The slightly higher CO2 adsorption and the lower CH4 production in early cycles suggests the possibility of unreactive carbonate formation on the CaO causing some “temporary deactivation”. This can be reversed by longer methanation times or higher H2 pressure. The average performance over 10 cycles is given in Table 2. Standard deviation of the mean for each cycle test was calculated for the accuracy of the result. Take the adsorption-methanation cycle test (10 cycles) with 5% Ru 10% CaO/Al2O3 (TH200) for example, the standard deviation of the mean respectively for CO2 adsorption, O2 consumption, and CH4 production were 0.86 mL, 2.9 mL, and 0.66 mL. Among the Ru-CaO based materials studied, DFM supported on BASF tableted powder sample showed the best CO2 adsorption, likely due to its small powder particle size free from pore diffusion, while DFM supported on SASOL alumina pellets showed the best methanation performance. The high specific surface area and larger average pore size of SASOL alumina pellets may contribute to the improved DFM performance by enhancing the metal dispersion and the gas diffusion within the catalyst. In our study, the shaped samples are loosely packed in the reactor, and the CO2 adsorption exothermic heat is negligible due to the limited adsorption capacity. As a consequence, the heat transfer effects in our reactor are negligible. Similarly, the extent of the exotherm in the methanation step is small due to the limited amount of CO2 methanated. In the industrial application, heat transfer effects need to be taken into consideration in the reactor design because both adsorption and the subsequent methanation steps are highly exothermic.


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Figure 4. CO2 adsorption and methanation performance of 5%Ru10%CaO/Al2O3 (SASOL TH100) for cyclic testing. The amounts of (a) CO2 captured and (b) O2 consumed at the adsorption step, and (c) CH4 produced during the methanation step were plotted against process time on stream (TOS). CO2 adsorption and methanation temperatures for all studies was maintained at 320 oC.

The performance of 5%Ru10%CaO/Al2O3 (TH100) sample during the CO2-adsorptionmethanation cycle tests (10 cycles) is shown in Figure 4. During the adsorption step, the nanodispersed CaO sites were able to adsorb and bind CO2 molecules, resulting in the formation of a labile (reversible) structure of chemisorbed CO2 on CaO compared to bulk carbonate. In the presence of both O2 and steam, ruthenium metal undergoes partial oxidation, rendering it inactive for CO2 adsorption. This is reversed by in situ reduction with H2 added during the methanation step. Both CO2 adsorption and ruthenium oxidation are rapid processes, occurring within the first 15 minutes. In contrast, the conversion of adsorbed CO2 to methane is a slower process (40-60 minutes) due to the reduction of the Ru oxide to catalytically active metal. After the recovery to its active metallic state, ruthenium was able to catalyze the conversion of CO2 that had migrated (spilled-over) from the CaO to CH4 and 2H2O. 3.2. Effects of reaction parameters on the CO2 capture and methanation capabilities


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3.2.1. Variation of adsorption time on stream (TOS)

Figure 5. Effect of adsorption time on stream (TOS) (60 min, 20 min, or 10 min) on CO2 capture and methanation capabilities. The amounts of (a) CO2 captured and (b) O2 consumed at the adsorption step, and (c) CH4 produced during the methanation step were plotted against process time on stream (TOS). DFM material loading: 5%Ru,10%CaO/Al2O3 pellets (SASOL TH200) of around 10 g. Adsorption condition: 7.5% CO2, 15% steam, 4.5% O2, N2 bal. (vol-%); GHSV=11236 h-1; 320 oC; 1 bar. Methanation condition: 5 vol-% H2/ N2; GHSV= 2611 h-1; 320 o

C; 1 bar, 60 min. From Figure 5 it is clear that the adsorption of CO2 and the oxidation of Ru are rapid and

essentially complete within the first 15-20 minutes of the reaction. Further increases in TOS did not result in significant increases in reaction. Increases beyond 20 minutes during adsorption should be avoided since this leads to more extensive oxidation of the Ru that delays onset of methanation as shown in 5c between 10 (blue) and 20 (red) minutes of air exposure. However, more methane is produced with longer hydrogenation times (compare blue, red and green curves). These differences will likely be eliminated in a commercial plant that operates with pure H2 rather than the 5% used in these experiments. 3.2.2. Variation of adsorption and methanation temperatures


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Figure 6. Effect of reaction temperature (350 oC, 320 oC, 300 oC, or 280 oC) on the CO2 capture and methanation capabilities. The amounts of (a) CO2 captured and (b) O2 consumed at the adsorption step, and (c) CH4 produced during the methanation step were plotted against process time on stream (TOS). DFM material loading: 5%Ru,10%CaO/Al2O3 pellets (SASOL TH200) of around 10 g. Adsorption condition: 7.5% CO2, 15% steam, 4.5% O2, N2 bal. (vol-%); GHSV=11236 h-1; 1 bar; 20 min. Methanation condition: 5 vol-% H2/ N2; GHSV= 2611 h-1; 1 bar, 60 min. Legends refer to volumes of respective gases. Both adsorption and methanation are exothermic processes and thus are thermodynamically favored at low temperatures, while reaction kinetics follow the opposite trend. Figure 6 shows the effect of adsorption temperature on the DFM performance. With increasing temperature for both adsorption and methanation between 280 oC to 350 oC, CO2 adsorption slightly decreased, while ruthenium oxidation accelerates. Little methanation occurs at 280 oC while the optimum rate occurs around 320 oC. Extensive ruthenium oxidation occurs at the highest temperature (350 o

C), and therefore higher temperatures should be avoided. 3.2.3. Variation of adsorption feed flow rate


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Figure 7. Effect of adsorption feed flow rate (48.21, 40.00, 32.36, or 26.00 L/hr) on the CO2 capture and methanation capabilities. The amounts of (a) CO2 captured and (b) O2 consumed at the adsorption step, and (c) CH4 produced during the methanation step were plotted against process time on stream (TOS). DFM loading: 5%Ru,10%CaO/Al2O3 pellets (SASOL TH200) of around 10 g. Adsorption condition: 7.5% CO2, 15% steam, 4.5% O2, N2 bal. (vol-%); 320 oC; 1 bar; 20 min. Methanation condition: 5 vol-% H2/ N2; GHSV= 2611 h-1; 320 oC, 1 bar, 60 min. Legends refer to volumes of respective gases. The effects of adsorption feed flow rates on the performance are plotted in Figure 7. Both CO2 adsorption (a) and O2 consumption (b) increase with increased feed flow rate indicating adsorption and oxidation rates are relatively fast. At lower flow rates (lower linear velocities) some bulk mass transfer dominates the process. It is clear from Figure 6c that methanation has a lower rate than CO2 adsorption given the larger TOS required for conversion. This is undoubtedly due to the slow reduction of the Ru oxides to the metallic and active methanation state. The extent of methane produced parallels the amount of CO2 adsorbed for the adsorption flow rate 48.21 L/hr, but further improvements in the reactor design are possible. It’s necessary to compare the methane yield by taking the overall CO2 amount into account, even though the CO2 converted/CO2 total is very low. This is due to the fact that CO2 adsorption


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is essentially 100% during the first 5~10 min of the adsorption step. Beyond this point the CO2 adsorption continues to decrease. In order to maximize the amount of CO2 trapped this adsorption process was continued until the bed was saturated. The CO2 converted/CO2 total for the two processes with 48.21 L/hr and 26 L/hr total flow rates are respectively 2.62% and 2.40%. In a larger bed of material and a higher H2 partial pressure, which would be the case in an optimized process these percentages would be much higher. 3.2.4. Variation of methanation feed flow rate

Figure 8. Effect of methanation (H2/N2) feed flow rate (22.4, 11.2, 5.6, or 2.8 L/hr) on the methanation capability. The amounts of (a) CO2 captured and (b) O2 consumed at the adsorption step, and (c) CH4 produced during the methanation step were plotted against process time on stream (TOS). DFM catalyst loading: 5%Ru,10%CaO/Al2O3 pellets (SASOL TH200) of around 10 g. Adsorption condition: 7.5% CO2, 15% steam, 4.5% O2, N2 bal. (vol-%); GHSV=11236 h-1; 320 oC; 1 bar; 20 min. Methanation condition: 5 vol-% H2/ N2; 320 oC, 1 bar, 60 min. Legends refer to volumes of respective gases. Figure 8c demonstrates the delicate balance between low and high H2 flow rates required for methanation. Moderate flow rates provide sufficient residence time with low mass transfer resistance resulting in higher methane production. The highest flow rate produces the highest


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methanation rate with sharper and narrower methane peaks at low TOS. This suggests that higher average H2 concentrations will increase the rate of the reaction by enhancing the reduction rate for RuOx. Also our published kinetic rate model for methanation (Eley-Rideal mechanism) shows a strong dependence on H2 partial pressure.25 However, methane production is low due to the following reasons: (1) a higher methanation flow rate means higher space velocity, and smaller residential time for the reaction rate that is kinetically limited, resulting in lower conversion, (2) H2 flow during the methanation step had two functions: one was reducing and regenerating the Ru species oxidized during the previous adsorption step, the other was the hydrogenation agent. Therefore, the reactor should be designed with a high H2 linear velocity but with sufficient residence time to allow for more complete methanation. 3.2.5. Influences of steam and/or O2 in the CO2 feed The most important consideration in the scale-up study was the presence of O2 in the CO2 adsorption feed. Fig. 9 shows the adsorption/conversion of CO2 as the adsorption feed composition is varied. The presence of steam (without O2) slightly decreases the CO2 adsorption (blue 30.38 ml vs purple in 34.75 in Figure 9a). The combination of O2 and steam (red vs. purple) has the most dramatic effect of decreasing both the CO2 adsorption and subsequently, decreasing methane produced. With O2 present, the Ru sites are oxidized making them inactive towards CO2 adsorption. Therefore, with lower CO2 adsorbed less methane is produced (9c). The DFM is most effective for feeds without steam or O2. This is shown by the immediate and enhanced CH4 production with the two non-O2 feed conditions (purple and blue lines in Figure 9c). In the real flue gas applications, the exposure of DFM to O2 cannot be avoided. However, a more O2-tolerant catalyst such as Rh is an expensive alternative to Ru. Furthermore, there are


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applications for CO2 capture where the feed is O2-free such as in brewery exhausts, rich burn engines, and water gas shift reaction processes.

Figure 9. Effect of the presence of steam and/or O2 in adsorption feed on the CO2 adsorption and methanation capability. The amounts of (a) CO2 captured and (b) O2 consumed at the adsorption step, and (c) CH4 produced during the methanation step were plotted against process time on stream (TOS). DFM material loading: 5%Ru10%CaO/Al2O3 pellets (SASOL TH200) of around 10 g. Adsorption condition: GHSV=11236 h-1; 320 oC; 1 bar; 20 min. Methanation condition: 5 vol-% H2/ N2; GHSV= 2611 h-1; 320 oC, 1 bar, 60 min. In summary, the optimum conceptual process parameters for the CO2 adsorption and methanation were achieved as listed below. (1) Adsorption condition: 7.5% CO2, 15% steam, 4.5% O2, N2 bal. (vol-%); GHSV=11236 h-1; 320 oC; 1 bar; 20 min-TOS; (2) Methanation condition: 5 vol-% H2/ N2; GHSV= 2611 h-1; 320 oC, 1 bar, 60 min-TOS. It must be mentioned that in a real application H2 will be 100% and thus the limitations using 5% H2 in N2 will likely disappear.


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Figure 10. Proposed schematic mechanism of the surface reactions on Ru,CaO/Al2O3 DFM for CO2 adsorption and methanation. The Ru,CaO/Al2O3 DFM surface reaction mechanisms for CO2 capture and methanation are postulated in Figure 10. The co-impregnation and co-precipitation preparation methods allow uniform depositions of the CaO and Ru nanoparticles on the Al2O3 support. During the CO2 adsorption step, the Ru catalyst is exposed to a CO2-containing feed in the presence of steam and O2. CO2 adsorption onto the CaO sites is rapid and equally rapid is ruthenium oxidation preventing it from adsorbing CO2. The presence of steam slightly suppresses the CO2 adsorption by competing with CO2 for the occupation of the CaO sites. Subsequently, the reaction feed is switched to H2 for the methanation process, involving the following elementary steps: (1) regeneration of catalytically active ruthenium metal by reduction of its oxide; (2) spill-over of adsorbed CO2 from the CaO to the adjacent reduced Ru metal sites; (3) dissociative chemisorption of H2 on Ru; (4) dissociation of CO2 and H2 to complexes, and (5) catalytic conversion to CH4.


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4. CONCLUSION A laboratory parametric and conceptual scale-up study of CO2 capture and conversion to methane with dual functional material (DFM) is reported. The study highlights (1) the demonstration of an effective DFM preparation method for meeting the industrial-scaled packed bed reactor and processing requirements, (2) the exploration of an effective Al2O3 particulate carrier material for enhanced DFM performance, and (3) the performance of a process (reaction) laboratory parametric study for optimized CO2 utilization. DFM samples (powders, pellets and spheres) with a composition of 5%Ru,10%CaO/Al2O3 were prepared and evaluated. The DFM cyclic performance tests for CO2 adsorption and methanation were conducted in a simulated power plant flue gas (including CO2, steam, and O2). The effects of CO2 adsorption time on stream (TOS), reaction temperature, adsorption and methanation feed flow rates, and the presence of O2 and/or steam in the adsorption feed were investigated. Compared to traditionally used nitrate salts, chloride salts (especially chloride salt of ruthenium) showed a more uniform penetration into the Al2O3 particulates. DFM materials showed stable performance during both the CO2 adsorption and methanation after cyclic conditions. The Ru-CaO system, made on the two SASOL materials TH100 and TH200 Al2O3 exhibited the best performance. CO2 adsorption on nano-dispersed CaO sites and O2 consumption by the Ru metallic sites are rapid processes, essentially complete within the first 15 minutes of TOS. Increasing the adsorption TOS negatively affected the subsequent catalytic methanation by “over-oxidizing” the ruthenium active sites. The reaction temperature influenced the adsorption/reaction thermodynamics and kinetics, with the optimum temperature for both adsorption and


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methanation being 320 oC. The adsorption feed flow rate influenced the performance by altering the gas diffusion pathways within the compact DFM particulate samples. High H2 methanation feed flow rates, resulted in an increased rate of reduction of the Ru oxides to the active metallic state but the decrease in methane produced was due to residence time limitations. Maximum methane production was achieved with a moderate methanation flow rate. The presence of steam in the adsorption feed slightly lowered the CO2 adsorption capacity of CaO by a likely competitive adsorption mechanism. The presence of O2 in the feed significantly decreases both adsorption of CO2 and methanation. The methanation activity was reduced by a delay in the reduction of the over oxidized Ru. Surface reaction mechanisms on the DFM materials are proposed. This technology addresses reduced CO2 emissions, more efficient use of synthetic natural gas and utilizes H2 produced from renewable sources. Further study on the exploration of advanced adsorption/methanation materials and optimized DFM composition is currently being developed. ACKNOWLEDGEMENT Financial support by Anglo American is greatly appreciated. The authors would like to thank BASF and SASOL for supplying the Al2O3 support materials. AUTHOR INFORMATION Corresponding Author * Email: [email protected] (Q. Zheng) ABBREVIATIONS DFM = dual functional material BET = Brunauer–Emmett–Teller


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Step I: CO2 adsorption


Step II: CO2 methanation

Scale-up process parameters Renewable H2 CO2, O2, N2, Steam, etc. H

Adsorbent (CaO)

Methanation catalyst (Ru)

Natural gas power plant Support (𝜸Al2O3) Dual functional material (DFM) ACS Paragon Plus Environment

H

Synthetic natural gas

Adsorption and Methanation of Flue Gas CO2 with Dual Functional

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