Oxygen Removal from Oxy-combustion Flue Gas for CO2 Purification

Jan 18, 2018 - Typical allowable O2 concentration in recovered CO2 for certain applications including Enhanced Oil Recovery (EOR) is as low as 100 ppm...
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Oxygen Removal from Oxy-combustion Flue Gas for CO2 Purification via Catalytic Methane Oxidation Qinghe Zheng, Shaojun Zhou, Marty Lail, and Kelly Amato Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04577 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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Oxygen Removal from Oxy-combustion Flue Gas for CO2 Purification via Catalytic Methane Oxidation Qinghe Zheng, Shaojun Zhou, Marty Lail*, Kelly Amato Energy Technology Division, RTI International, Durham, NC 27709 KEYWORDS: Oxy-combustion; Flue Gas; Oxygen Removal; Catalytic Methane Oxidation; Natural Gas.

ABSTRACT: Typical allowable O2 concentration in recovered CO2 for certain applications including Enhanced Oil Recovery (EOR) is as low as 100 ppmv. The removal of high content O2 (3-5%) in oxy-combustion flue gas requires additional compression work in conventional downstream CO2 purification process. RTI hereby proposes to develop a novel technology for flue gas O2 removal based on catalytic oxidation of natural gas. Preliminary catalytic tests were performed over various supported Pd and Pt catalysts under simulated oxy-combustion flue gas conditions, with the addition of stoichiometric amount of CH4 as model compound for natural gas. Among the studied catalysts, Pd supported on zeolite H-ZSM-5 showed the highest oxygen conversion at relevant condition. Compared to Pt catalysts, Pd catalysts generally showed higher oxidation reaction kinetics. The catalytic oxidation activity can be further increased by optimizing reaction gas hourly space velocity (GHSV) and total reaction pressure.

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1. INTRODUCTION Oxy-combustion is one of the most promising combustion technologies for CO2 capture from coal-fired power plants due to its advantages from both technical and economic aspects. Conventional coal-fired boilers use air for combustion, in which N2 from air dilutes the CO2 concentration in the flue gas. In comparison, during oxy-combustion, coal is burned in a N2-lean and CO2-rich environment, which is achieved by feeding the combustor with an O2-rich stream (95% purity O2 acquired from a cryogenic Air Separation Unit, ASU) mixed with recycled flue gases.1 Oxy-combustion flue gas consists of predominantly CO2 and condensable water, whereas conventional air combustion flue gases are N2-rich with only about 15 vol-% of CO2.2 The high CO2 concentration and the significantly lowered N2 concentration in the oxy-combustion raw flue gases are unique features that reduce the energy consumption and capital cost of CO2 capture when compared to other combustion technologies.3,4 Oxy-combustion flue gas also has lower contents of NO and SO2.5 The flow of unrecycled CO2 still contains water vapor, impurities particularly some acid gases, including SO3, SO2, HCl and NOx produced as byproducts of combustion, and non-condensable gases such as O2, N2 and Ar originated from air infiltration.6,7 Flue gas cleaning (FGC, i.e. removal of condensable gases and water) and CO2 purification (i.e. removal of non-condensable gases) are needed to achieve the required CO2 purity before further CO2 compression and pipeline transportation.8 Gas impurities have great influences on the design, operation and optimization through their impacts on the thermodynamic properties of the flue gas stream.5,9 The current removals of O2 and other non-condensable impurities from oxy-combustion flue gas are through a downstream Compression and Purification Unit (CPU). After the flue gas is de-dusted and cooled, it is

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compressed to 3 MPa and fed to a cryogenic separation unit, where the stream is cooled down to about -54 oC which is close to the carbon dioxide triple point (-56 oC), while the impurities containing mainly N2, Ar and O2 (3-5%), are removed to increase the CO2 concentration from 70~ 75 mol-% (dry basis) to above 95 mol-% pure.3,10 The high content oxygen (3-5%) in oxy-combustion flue gas requires additional compression work. O2 in the presence of H2O can increase cathodic reactions, causing thinning of the CO2 pipeline. Because of this, the typical allowable O2 concentration in recovered CO2 for Enhanced Oil Recovery (EOR) application is 0.01 vol-% (100 ppmv), while operating pipelines tend to follow stricter standard in the 10-40 ppmv range. O2 can also cause the injection points of EOR to overheat due to exothermic reactions with the hydrocarbons in the oil well. In addition, high O2 content can facilitate the growth of aerobic bacteria in the reservoir and at the injection points. In sequestration applications, O2 can react with SO2 to form H2SO4, and with NO to form NO2, which in water leads to the formation of HNO2. Dissolved O2 can also react with the cap rock if it contains iron, manganese, and other metals. If dissolved ferrous ions are present in water, ferric hydroxide can be formed, which causes potential plugging of the pore space.11 To our knowledge, few technologies for O2 removal from oxy-combustion were based on chemical/catalytic approach except for a recent Air Liquide patent application.12 The patent describes a method of depleting O2 from oxy-combustion flue gas using catalytic oxidation of hydrocarbons, over a palladium and/or platinum catalyst supported by alumina. It can be certainly concluded that the proposed technology application is underdeveloped and is in great need of extensive scientific evaluation for any potential commercial opportunity. Although limited research effort has been made on this specific topic, catalytic oxidation/combustion of methane and other hydrocarbon species have been extensively studied

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for nearly half a decade.13 One application was catalytic methane combustion in gas turbine combustors for effective energy production with reduced emissions.14 Another application is catalytic abatement of non-methane hydrocarbons (NMHCs) and methane emissions from gasoline and natural gas vehicles.15 Supported palladium (Pd) and platinum (Pt) are well recognized as the most active oxidation catalysts.16 High surface area γ-Al2O3 was the commonly used support material for oxidation catalyst preparation. It was also reported that the addition of cerium improved the oxidation performance of Pt/Al2O3 and Pd/Al2O3 catalysts by suppressing the formation of partial oxidation product CO.17 Moreover, Pd/ZSM-5 was previously reported to have high activity for methane combustion, with FT-IR study showing that methane molecules can be adsorbed and activated on acidic bridging hydroxyl groups on ZSM-5.18 Pd catalysts supported on ZSM-5 zeolite with both Brønsted and Lewis sites may have better activity than Pd catalyst supported on Al2O3, where no Brønsted site is present.19 The present paper provides preliminary results on removing O2 from oxy-combustion flue gas by using catalytic oxidation of methane as a model compound for natural gas. Catalytic oxidation tests were performed under simulated oxy-combustion flue gas conditions. Pd and Pt with low to high loadings (0.5-3 wt-%) on different support materials including γ-Al2O3, CeO2-ZrO2 (denoted as CZO), and H-ZSM-5 zeolite were prepared by impregnation method. The catalyst samples were screened for optimized O2 removal performance. A more specific kinetic study was performed under relavant conditions for the studied oxy-combustion application. The asproposed technology features (1) single stage catalytic process for deep O2 depletion from 3 vol% to 100 ppmv, (2) moderate temperature oxidation development for optimized O2 abatement efficiency and energy saving, (3) pressurized catalytic process for lowered flow rate and enhanced reaction kinetics.

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2. EXPERIMENTAL METHODS Pd and Pt with different loadings (0.5, 1, and 3 wt-%) supported on γ-Al2O3 (SASOL CATALOX SBa-90), CeO2-ZrO2 (denoted as CZO), and H-ZSM-5 zeolite (ZEOCAT PZ-2/50H) were prepared by standard incipient wetness impregnation method. The physical and chemical properties of the powder support materials provided by the manufacturers are summarized in the supplemental information section. Palladium (II) nitrate hydrate (Pd(NO3)2∙ xH2O, 99.8% metal basis, Pd 39% min, Alfa Aesar) and Tetraammineplatinum (II) nitrate (Pt(NH3)4(NO3)2, 99.995 trace metal basis, Sigma-Aldrich) were respectively used as the Pd and Pt precursors. Incipient wetness impregnation was performed on dry support samples (oven dried at 150 oC for 2 hrs) with corresponding amounts of metal precursor salt aqueous solution till the supports were saturated. The impregnated samples were then dried in oven at 150 oC for 2 hrs, followed by calcination at 550 oC for 3 hrs in air. The elemental compositions of the as-prepared catalyst samples were measured by x-ray fluorescence analyses (XRF) using a Thermo Scientific PERFORMX WDXRF instrument. The instrument was equipped with two detectors and seven analyzer crystals to achieve a broad elemental range. The sample data was processed using UniQuant software. The sample measurements were repeated three times and the replicate results were within the 95 vol-%) with O2 concentration of < 100 ppmv. The advanced process is expected to enhance the energy efficiency related to oxycombustion flue gas processing, and to provide a purer CO2-stream for economic transport and storage, in order to meet the most stringent gas specification required by EOR and other applications.

3.2. Synthesis and characterization of Pd and Pt oxidation catalysts As discussed, Pd and Pt catalysts are the most active for catalytic oxidation of hydrocarbons. For the catalytic removal/depletion of flue gas O2 from oxy-combustion flue gas, a series of Pd and Pt catalysts on different supports including γAl2O3, CeO2-ZrO2 (CZO), and H-ZSM-5 zeolite, with metal loadings < 3 wt-% were made. The physical properties of the as-synthesized catalysts are summarized in Table 1. From the XRF result, it can be seen that the actual metal loadings of the as-prepared catalysts matched the targeted values. From CO-chemisorption result, with increasing metal loading, metal dispersion decreased while metal particle size increased. Higher metal dispersions were observed with Pt than Pd catalysts with the same metal loading (1 wt-%). Compared to Pd catalysts, Pt dispersions on all supports were generally higher.

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For Pd catalysts, metal particle size followed a general trend of Pd/CZO < Pd/Al2O3 < Pd/HZSM-5, at the same metal loadings. For Pt catalysts, similar trend was found as Pt/Al2O3 < Pt/CZO < Pt/H-ZSM-5. Previous study showed that metal size is strongly dependent on support selection and preparation method. For example, small crystallites can be produced by ion exchanging a small amount of palladium, less than 0.5 wt%, onto the high-surface-area Degussa alumina. Large crystallites can be produced by depositing either a large amount of palladium on alumina or other supports through impregnation.19 Table 1. Characterization of as-synthesized Pd and Pt catalysts on different supports. Metal loading a

Metal dispersion b

Metal particle size c

Ea d

wt-%

%

hemisphere, nm

kJ/mol

1% Pd/ γ-Al2O3

0.9

14.9

7.5

107.85

3% Pd/ γ-Al2O3

3.1

6.1

18.3

123.64

0.5% Pd/ CZO

0.5

38.9

2.9

97.29

1% Pd/ CZO

0.9

28.7

3.9

95.87

3% Pd/ CZO

3.0

14.1

8.0

98.78

1% Pd/ H-ZSM-5

1.1

6.6

16.9

99.61

3% Pd/ H-ZSM-5

3.1

3.0

37.8

105.85

1% Pt/γ-Al2O3

0.9

41.0

2.8

87.14

1% Pt/CZO

1.1

31.8

3.6

98.45

1% Pt/ H-ZSM-5

1.1

10.5

10.8

89.89

Catalyst sample

a

Precious group metal (Pd or Pt) loading measured by XRF;

b

Precious group metal (Pd or Pt) dispersion estimated by CO chemisorption measurement result, assuming stoichiometric factor of 1 for both Pd and Pt; c

Precious group metal (Pd or Pt) particle size estimated by CO chemisorption measurement result, assuming hemisphere shape. d

Methane oxidation activation energy Ea calculated from Arrhenius Plots in Figure 3(a).

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3.3. Catalytic methane oxidation under simulated oxy-combustion flue gas conditions Catalyst screening were performed with the as-synthesized catalysts under simulated oxycombustion flue gas condition with addition of stoichiometric amount of CH4 as model compound for natural gas. The total feed composition was 1.5% CH4, 3% O2, 10% N2, and balanced by CO2, all in vol-%. Figure 2 shows the conversion profiles of CH4 and O2 at reaction temperatures between 150 oC and 600 oC, at high GHSV of 120, 000 h-1. The studied catalysts all exhibited pronounced O2 conversion, especially Pd catalysts, which showed 100% O2 conversion below 550 oC. Among the studied catalysts, 3% Pd/H-ZSM-5 had the highest oxidation activity, with low T50 (temperature at which 50% conversion was reached) observed at 375 oC, and equilibrium conversion at 525 oC. As previously discussed, the pronounced oxidation activity of 3% Pd/H-ZSM-5 can be attributed to the presence of both Brønsted and Lewis acidic sites on the support, which facilitates the CH4 activation.19

Figure 2. CH4 and O2 conversions over Pt and Pd catalysts on various supports during catalytic oxidation of methane, at simulated dehydrated flue gas conditions. Reaction conditions: feed gas compositions (vol-%) of 1.5% CH4, 3% O2, 10% N2, and CO2 in balance; GHSV of 120,000 h-1; temperature at 150 to 600 oC, pressure at 1 atm.

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Pd catalysts with the same metal loadings generally exhibited higher oxidation activity than Pt catalysts. At the same metal loading of 1 wt-%, the T50s of Pd supported on CZO, Al2O3, and HZSM-5 were respectively 145 oC, 135 oC, and 160 oC lower than Pt on the same supports. This is probably due to the higher stability of Pd than Pt in oxidation environment. It was reported that in the presence of oxygen, Pd and Pt respectively oxidizes into PdO and PtO2. PdO forms between ca. 300-400 oC, being stable in air at atmospheric pressure up to about 800 oC. In contrast, PtO2 is highly unstable compared to PdO, and decomposes at much lower temperature, around 400 oC. .13 The higher oxidation activity of Pd catalysts compared to their Pt counterparts is probably due to the higher stability of PdO than PtO2 in oxidation environments.

Figure 3. CH4 oxidation Arrhenius Plots for studied Pd and Pt catalysts on different supports (a) normalized to total mass of the supported metal (Pd or Pt), and (b) normalized to exposed Pd or Pt atoms (using dispersion data from CO chemisorption). All measurements were performed under the same condition in Figure 2. To further illustrate the oxidation kinetics of studied catalysts, Figure 3 presents the Arrhenius Plots with reaction rate (consumption rate of O2 molecules normalized to total PGM mass) and reaction turnover frequency (TOF, consumption rate of O2 molecules normalized to exposed

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PGM atoms). The reaction activation energy were calculated and shown in Table 1. The measurements were performed in the kinetic control regime when CH4 and O2 conversion were below 10%. The Arrhenius Plots in Figure 3 (a) demonstrate the higher reaction kinetics of Pd catalysts than Pt catalysts. The activation energy Ea for 1% Pd on γAl2O3, CZO, and H-ZSM-5 were respectively 107.9, 95.9, and 99.6 kJ/mol, compared to 87.1, 98.5, 89.9 kJ/mol for 1% Pt on the same supports. When taking into account the metal dispersion (actual metal atoms exposed), TOFs as functions of inverse reaction temperature in Figure 3 (b) are plotted. For Pd catalysts on the same support, higher TOFs were observed with higher metal loadings, which have larger particle sizes. It was reported that the size dependence of specific catalytic activity is narrow and bell-shaped for catalytic metal oxidation.23 The strong size sensitivity originates from the strong influence of particle size on metal oxidation state, and reaction apparent activation energy.19 It is interesting to notice that all the CZO-supported Pd catalysts have the same activation energy regardless of different Pd loadings. It is likely that CeOx species facilitated the catalytic methane oxidation on Pd. The CZO (ceria-zirconia mixed oxide) is itself an active catalyst for methane combustion. Previous report showed that deposition of platinum or palladium on this support resulted in a strong increase in activity at low temperature (473-773 K). Methane oxidation takes place at the active ceria-zirconia/metal interface, according to a redox mechanism involving the reaction of dissociated methane with lattice oxygen from the support.24

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Figure 4. CH4 and O2 conversions at various space velocities during catalytic oxidation of methane over 3% Pd/H-ZSM-5, under simulated dehydrated flue gas conditions. Reaction conditions: feed gas compositions (vol-%) of 1.5% CH4, 3% O2, 10% N2, and CO2 in balance; GHSV of 6000, 12000, 60000, or 120000 h-1; temperature at 150-600 oC, pressure at 1 atm. Higher oxidation conversions can be achieved by tuning process parameters such as lowering space velocity. The effect of gas hourly space velocity (GHSV, feed total flow rate (mL/h) / catalyst volume (mL)) on the catalytic methane oxidation activity is shown in Figure 4. The catalytic oxidation activity of the best performing 3% Pd/H-ZSM-5 was examined under the same feed conditions but at GHSVs (6000, 12000, 60000, or 120000 h-1). It is obvious that at lower space velocity, some bulk mass transfer dominates the process. The T50 and equilibrium temperatures shifted from 375 oC to 300 oC, and 525 oC to 425 oC respectively, when GHSV was reduced from 120000 h-1 to 6000 h-1.

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Figure 5. Methane oxidation reaction activation energy as a function of absolute reaction pressure (1, 11, 21, and 31 bar). Reaction conditions: feed gas compositions (vol-%) of 1.5% CH4, 3% O2, 10% N2, and CO2 in balance; GHSV of 60000 h-1. The impact of reaction total pressure on the catalytic methane oxidation kinetics was further examined, as shown in Figure 5. The oxidation kinetics could be improved with increasing pressure, as it shows that the reaction activation energy increased from 74.4 to 119.8 kJ/mol as pressure increased from 1 to 31 bar. Reaction pressure can influenced the CH4 and O2 adsorptions on the active sites. Oxygen adsorption on palladium is extremely fast and irreversible below 400°C. Methane adsorption, on the other hand, is a slow, activated process.19 Increase of absolute reaction pressure may have increased the adsorption thermodynamic and kinetic especially for CH4 molecules on the catalyst surface. Hicks and colleagues reported the surface reaction mechanism on Pd/Al2O3 during methane oxidation.25 For our study, a slightly different reaction mechanism can be implicated because different support and feed condition were used, as shown in Figure 6. (a) Pd particles supported on H-ZSM-5 zeolite with surface acidic groups. (b) At room temperature, Pd adsorbs O2 upon exposure. (c) As reaction temperature increases, CH4 molecules decompose to surface carbon

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and gaseous hydrogen on Pd, and consume the adsorbed oxygen species [O]. This step can be facilitated by the CH4 adsorption on H-ZSM-5 support.18 On Pd/H-ZSM-5, methane first adsorbed on acidic hydroxyl group, resulting in high polarizability of methane (C-H activation). Then, the heterolytic dissociation of methane on strong acid or [AlO]-Pd2+ sites occurs.18 (d) Oxygen in the oxy-combustion stream is adsorbed on reduced Pd, and is readily to be consumed by activated CH4.

Figure 6. Proposed scheme of methane oxidation reaction on the surface of Pd/H-ZSM for oxygen depletion from oxy-combustion flue gas. It is summarized that the O2 impurity from oxy-combustion flue gas can be effectively depleted via catalytic oxidation of methane as a model compound of natural gas. The catalytic oxidation activity can be optimized by tuning catalyst compositions and process parameters including reaction temperature, gas hourly space velocity, and reaction pressure. However, the current study only demonstrates the feasibility of catalytic oxidation for oxygen removal. For

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complete technology development, research topics including sulfur poisoning, catalyst long term stability, and advanced catalyst testing at more realistic flue gas conditions will be performed in the future.

4. CONCLUSIONS The current study provided preliminary data for a new approach for removing O2 from oxycombustion flue gas for CO2 purification, by adopting the well-established catalytic methane oxidation reaction. Methane was used as a model compound for natural gas. Catalyst screening were performed with Pd and Pt catalysts on different supports (γAl2O3, CZO, and H-ZSM-5) under simulated oxy-combustion flue gas conditions. Among the studied catalysts, Pd supported on zeolite H-ZSM-5 showed the highest oxygen, with low T50 (temperature at which 50% conversion was reached) observed at 375 oC, and equilibrium conversion at 525 oC. Compared to Pt catalysts, Pd catalysts generally showed higher oxidation reaction kinetics. The catalytic oxidation activity can be further increased by optimizing reaction gas hourly space velocity (GHSV) and total reaction pressure. The study demonstrated it is highly feasible to complete remove O2 from oxy-combustion flue gas by catalytic oxidation approach.

Corresponding Author * [email protected] (Dr. Marty Lail) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally.

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ACKNOWLEDGMENT The authors would to thank Dr. Raghubir Gupta for consulting on the project and projectrelated proposals. Many thanks extend to Ms. Andrea McWilliams for the XRF measurements, Mr. Gary Howe for the instrument training and AuroChem-MS set-up, and Mr. Tim Bellamy for material lab assistance. ABBREVIATIONS EOR, enhanced oil recovery; GHSV, gas hourly space velocity; ASU, air separation unit; CPU, compression and purification unit; CZO, ceria oxide-zirconia oxide (CeO2-ZrO2); NMHC, non-methane hydrocarbons; XRF, x-ray fluorescence analysis; TCD, thermal conductivity detector; FGC, flue gas cleaning; BPR, back pressure regulator; GC, gas chromatography; PGM. precious group metal; TOF, turn-over frequency. REFERENCES (1)

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