Effect of CO2 on the Methane Combustion over a Perovskyte Catalyst

Sep 13, 2013 - *E-mail: [email protected]. ... (1, 2) This stream can be sequestered or used in other applications, such as ...... AIChE Journal (201...
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Effect of CO2 on the Methane Combustion over a Perovskyte Catalyst at High Pressure A. Di Benedetto,† P. S. Barbato,*,‡ and G. Landi‡ †

Department of Chemical, Materials and Industrial Production Engineering (DICMAPI), University of Naples Federico II, Piazzale Tecchio 80, 80125 Naples, Italy ‡ Institute of Researches on Combustion, National Research Council (CNR), Piazzale Tecchio 80, 80125 Naples, Italy ABSTRACT: The effect of the CO2 content (up to 30%) on the methane catalytic combustion over a perovskyte-based monolith has been investigated at high pressure (up to 8 bar). Catalytic tests have been performed by varying the preheating temperature and the operating pressure. The CO2 adsorption over the catalytic surface has been evaluated through temperatureprogrammed desorption tests coupled to a kinetic model. Results show that the presence of CO2 retards ignition, shifting the ignition pressure at higher values. However, once ignition has been attained, the catalytic surface temperature increases upon an increasing CO2 content. These results have been explained by a double role of CO2: on one hand, it affects the ignition through adsorption over the catalytic surface, and on the other hand, it decreases the gas thermal diffusivity, thus reducing the velocity of heat removal from the surface.

1. INTRODUCTION Among the CO2 capture and sequestration technologies for reducing CO2 emissions, the oxy-combustion appears promising because it allows for production of a product stream that contains only CO2 and water vapor, thus significantly reducing the cost of removing CO2 from the combustion exhaust.1,2 This stream can be sequestered or used in other applications, such as enhanced oil recovery or other chemical processes. Accordingly, the “advanced zero emissions power plant” (AZEP) project addresses the development of a novel “zero emissions”, gas-turbine-based, power generation process to reduce local and global CO2 emissions in the most costeffective way.3 The oxy-combustion process is carried out in stoichiometric fuel/O2 with a large exhaust gas recycle (EGR) in the feed, which eventually dilutes in CO2 and H2O for temperature moderation.4 Moreover, the use of pure oxygen rather than air circumvents the formation of thermal and prompt NOx. Stoichiometric feed allows for reduction of the oxygen demand and minimization of excess oxygen in the exhaust stream, which is beneficial because excess oxygen represents a potential corrosion concern for sequestration applications that transport CO2 via a pipeline.5 The high dilution of the feed stream with CO2 and H2O inhibits homogeneous combustion, leading to an increase in the lean blow-out limit of more than 150 K, as compared to the results obtained with air.3 The combustion reactivity is significantly reduced when CO2 is present in the feed.6 Griffin et al.3 also showed that the high dilution with H2O and CO2 leads to an incomplete burnout of the combustion intermediate CO, increasing the CO emissions when stoichiometric O2/fuel ratios are used. Recently, it has also been showed that the use of CH4/O2/ CO2 mixtures even at high CO2 concentrations is unsafe.6,7 Indeed, it has been found that when igniting a CH4/O2/CO2 mixture at a stoichiometric ratio, even in the presence of 40%, © 2013 American Chemical Society

anomalous behaviors arise, leading to over-adiabatic peak pressure up to hundreds of bars [the combustion-induced rapid phase transition (cRPT) phenomenon]. To overcome the technological (flame stabilization and combustion efficiency) and safety issues (preventing cRPT phenomena), catalytic combustion can be proposed. Indeed, catalytic combustion allows for complete and stable combustion at much lower temperatures and also with very high dilution outside the flammability and cRPT limits. In previous papers,8,9 we studied the catalytic combustion of methane under pressure over perovskite. We showed that, after the catalytic ignition, the homogeneous reaction is activated, allowing for complete methane conversion. The impact of CO2 on catalytic combustion of methane was addressed by a few studies, such as those conducted at atmospheric pressure and low temperatures (up to 600 °C) on Pd.10,11 The operation of the catalytic combustion at conditions relevant to oxy-combustion applications is still an open issue, especially on non-noble metal catalysts. Indeed, it is not clear how CO2 and H2O affect the catalytic ignition and also how they affect the activation of the homogeneous reaction. Some works have been performed over Pt, both experimental and theoretical. Renike et al.12,13 studied the effect of CO2 and H2O on the catalytic and gas-phase chemistry and also the homogeneous ignition of CH4/air and H2O- and CO2-diluted CH4/O2 mixtures over Pt, at high pressure (up to 16 bar). They found that CO2 had only a minor chemical impact on the catalytic pathway and homogeneous ignition, because of the weak interaction of carbon dioxide with Pt. Nevertheless, the impact of CO2 on the thermal balance of the reactor was not addressed in the literature concerning catalytic combustion and Received: June 4, 2013 Published: September 13, 2013 6017

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Figure 1. Schematic drawing of the catalytic reactor with indications of flow path and thermocouple placement.

also in the aforementioned works12,13 conducted at pressure above atmospheric pressure. Other works have been devoted to the evaluation of the role of CO2 dilution on the ignition of the homogeneous reaction in the catalytic partial oxidation of CH4/O2 mixtures diluted with H2O and CO2.14,15 In this work, we aim at quantifying the role of CO2 addition to the CH4/O2/N2 mixture in the ignition of the catalytic combustion of methane over a perovskite catalyst at high pressure, eventually understanding the nature of the role of CO2 (kinetic, transport, and/or thermal). To this end, we performed activity tests in a lab-scale reactor and analyzed the results in conjunction with temperatureprogrammed desorption of CO2 (CO2-TPD) tests. We also developed a model of the CO2-TPD for estimating the role of the sites in the CO2 adsorption.

temperatures believed to be more indicative of the surface than gas temperatures because the thermocouples limit the gas flow in the channel. Moreover, T1, positioned in the inlet duct, measure the temperature of the incoming cold gases, and T6, positioned in contact to the second thermal shield, measure the temperature of the hot gases leaving the reactor. Finally, another thermocouple (T2) is inserted at the middle of the first thermal shield. All of the thermocouples were sealed thanks to two multiple-hole ceramic glands (Conax Buffalo, MHC series). Combustion tests were conducted in a lab-scale homemade setup designed to work at pressures up to 12 bar. The test rig is described in more details in ref 8. With respect to this previous configuration, a mass flow controller was added to control the CO2 feed. The analysis system (ABB AO2000) is equipped with four modules for the online and continuous analysis of the main gas species (CH4, CO2, and CO by infrared detectors; O2 by a paramagnetic detector; detection limits, about 10 ppm for CH4, CO, and CO2 and 50 ppm for O2) and with a cross-sensitivity correction. Experiments were conducted at different preheating conditions and increasing the pressure from approximately 1 to 12 bar. The mass flow rate was kept constant (i.e., the volumetric flow rate decreased by increasing pressure). In particular, N2 has been substituted by an equal CO2 volumetric flow rate. The conditions adopted are summarized in Table 1. Some tests were repeated and furnished results differing

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Lean methane combustion under selfsustained conditions was performed over a 20 wt % LaMnO3/La-γAl2O3 catalyst supported on a 900 cpsi honeycomb cordierite monolith (NGK) in the shape of a cylinder (length, 50 mm; diameter, 11 mm). The preparation procedure has already been reported in a previous work.16 Briefly, a dip-coating procedure was adopted to first coat the cordierite substrate (NGK) with a La2O3-stabilized γ-Al2O3 layer [Brunauer−Emmett−Teller (BET) specific surface area (SSA) = 201 m2/g]. Then, the active perovskite phase was deposited on the stabilized alumina washcoat by means of successive impregnation cycles with La(NO3)3·6H2O (Aldrich, >99.99%) and (CH3CO2)2Mn· 4H2O (Aldrich, >99%) salts in aqueous solution. The samples were dried in a stove at 120 °C and calcined at 800 °C for 3 h under flowing air. The adopted procedure allowed us to deposit about 1.35 g of catalyst (BET SSA = 103 m2/g) onto the substrate. Characterization of this type of structured catalyst has previously been reported.17,18 2.2. Catalytic Tests under Pressure. The adopted reactor configuration is shown in Figure 1. Briefly, two mullite foams, acting as thermal shields, were placed at the two ends of the active monolith. A ceramic wool tape was used to wrap the monolith as well as the foams before inserting the three objects in the cylindrical stainless-steel reactor (L, 70 mm; de, 25.4 mm; di, 12 mm), thus avoiding the gases bypass. The reactor was heated by means of an heating jacket (Tyco Thermal Controls) equipped with a proportional−integral−derivative (PID) controller. Six thermocouples (T1−T6) were inserted in the reactor in both the catalytic section and the inlet/outlet sections to measure the overall axial temperature profile (Figure 1). In particular, three thermocouples were positioned at 5 mm from the inlet of the monolith (T3), at the middle (T4), and at 5 mm to the end of the monolith (T5) in the central channel. These thermocouples measure

Table 1. Operating Conditions Adopted for Experimental Tests temperature of the jacket (°C) CH4 (%) O2 (%) CO2 (%) N2 (%) thermal power (Wth) Qtot (splh) Rein, at STP GHSV, at STP P (bar)

525−600 3.0 10.0 0−30 57−87 16 60 12 1.71 × 104 1−11

within the experimental error. Conditions adopted, even if not fulfilling those required for real applications (no nitrogen dilution and stoichometric CH4/O2 ratio), allow us to discriminate the effect of CO2. 2.3. TPD: Experimental and Modeling Methods. CO2-TPD experiments were carried out using a Micromeritics Autochem II 2020 analyzer equipped with a TCD detector. The sample, about 100 mg, was first pre-treated 1 h at 450 °C in flowing air and then contacted for 45 min at room temperature with a mixture of 15% CO2/He. After 30 6018

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The temperature increase in the TPD experiment is linear with time (t)

min of He purging, allowing for the physisorbed CO2 evacuation, the sample was heated 10 °C min−1 up to 850 °C. We developed a model of CO2-TPD by assuming that CO2 adsorbs over three catalytic sites, giving rise to the following desorption steps: k1

(CO2 − σ1) → CO2 + σ1 k2

(CO2 − σ2) → CO2 + σ2 k3

(CO2 − σ3) → CO2 + σ3

T = T ° + βt

(19)

where β = 10 °C/min. Equations 11, 12, 13, and 17 with the initial conditions (eqs 14−16, and 18) were solved by means of a Runge−Kutta method. The square root of the mean square error (SRMSE) normalized by the maximum value of the TPD curve (TPDmax) was computed to quantify the differences between the experimental and model TPD curves:

(1) (2) (3)

The rates of steps 1−3 are

r1 = k1θ1n1

(4)

r2 = k 2θ2 n2

(5)

r3 = k 3θ3n3

(6)

SRMSE =

(7)

⎛ Edes2 ⎞ k 2 = k 2° exp⎜ − ⎟ ⎝ RT ⎠

(8)

⎛ Edes3 ⎞ k 3 = k 3° exp⎜ − ⎟ ⎝ RT ⎠

(9)

(10)

The unsteady balance equations on the fraction (θ1, θ2, and θ3) read

⎛ Edes1 ⎞ n dθ1 = − k1° exp⎜ − ⎟θ1 1 dt ⎝ RT ⎠

(11)

⎛ Edes2 ⎞ n dθ2 = − k 2° exp⎜ − ⎟θ2 2 dt ⎝ RT ⎠

(12)

⎛ Edes3 ⎞ n dθ3 = − k 3° exp⎜ − ⎟θ3 3 dt ⎝ RT ⎠

(13)

The initial conditions are

t=0

θ1 = θ1°

(14)

t=0

θ2 = θ2°

(15)

t=0

θ3 = θ3°

(16)

The CO2 molar balance equation is the following, assuming that the reactor is perfectly mixed and pseudo-steady-state conditions:

⎛ Edes1 ⎞ n CCO2 1 dCCO2 − = k1° exp⎜− ⎟θ1 1 + k 2° C ° dt C °τ ⎝ RT ⎠ ⎛ Edes3 ⎞ n ⎛ Edes2 ⎞ n exp⎜− ⎟θ3 3 ⎟θ2 2 + k 3° exp⎜ − ⎝ RT ⎠ ⎝ RT ⎠ (17) where C° is the total gas concentration in mol/g and τ is the residence time (s). The initial condition of the CO2 molar balance is the following: t=0

CCO2 = 0

N

∑ (TPDexp(i) − TPDmodel (i))2 (20)

i=1 19

then the two

3. RESULTS 3.1. Catalytic Tests. We tested the effect of the preheating temperature on the ignition/extinction behavior when the pressure is increased/decreased, in the presence and absence of CO2 (5 and 30 vol %). In Figure 2, the methane conversion is plotted as a function of the pressure at different values of the gas preheating temperature and CO2 content in the feed. At the temperature equal to 525 °C, ignition is not found in the whole pressure range investigated, with fuel conversion being lower than 10%, even at 10 bar. At 550 °C, an hysteresis behavior is found with ignition occurring at P = 7 bar, when CO2 is not present in the feed. On the contrary, by feeding only 5 vol % CO2, ignition does not occur up to 9 bar. Methane conversion is significantly lower than that obtained in the absence of CO2. At a higher temperature (575 °C), ignition is also found in the presence of 5 vol % CO2. It is worth noting that a slight difference of the ignition pressure is found (6 and 7 bar in the absence and presence of 5 vol % CO2, respectively). Ignition of the feed containing 30 vol % CO2 is observed only at a preheating temperature equal to 600 °C and pressure equal to 6 bar. At this temperature, mixtures without CO2 and with 5 vol % CO2 show exactly the same behavior. Starting from the ignited branch and reducing the operating pressure, the methane conversion also remains complete at a pressure lower than the ignition value and at a pressure down to the atmospheric pressure. No quenching is found when CO2 is not fed. Conversely, in the presence of 5 vol % CO2, quenching is found at P = 1 atm and T = 575 °C. When the preheating temperature increases, extinction is not found any more at 5 vol % CO2. As stated above, upon increasing the CO2 content (30 vol %), ignition is found only at a preheating temperature equal to 600 °C and P = 6 bar. In this case, quenching is found at P = 1 atm. In Figure 3, the ignition pressure is reported as a function of the preheating temperature at different CO2 contents. The ignition pressure decreases with the preheating temperature. On this plot, the pressures at which quenching is found are also plotted, thus identifying the range of pressures at which the CH4 combustion is sustained. 3.2. Investigation on the Role of CO2. From the above results, it appears that the presence of CO2 in the gas stream has the effect of retarding ignition and also promoting quenching. The role of CO2 may be addressed to the thermal,

where we assume that the activation energy of all of the steps may be variable with the site concentration. ° + dEdesθ Edes = Edes

1 N

If SRMSE ≤ 0.045, as reported by Kanervo et al., curves may be considered in good agreement.

where θ1, θ2, and θ3 are the fractions of site (CO2 − σ1), (CO2 − σ2), and (CO2 − σ3), respectively, with respect to the total number of sites [i.e., θi = (CO2 − σi)/total number of sites]. The kinetic constants are evaluated as follows:

⎛ Edes1 ⎞ k1 = k1° exp⎜ − ⎟ ⎝ RT ⎠

1 TPDmax

(18) 6019

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Figure 3. Ignition (full symbol) and quenching (empty symbol) pressures as a function of the preheating temperature at different contents of CO2: (red squares) 0% CO2, (black circles) 5% CO2, and (green triangles) 30% CO2. Operating conditions: Q, 60 slph; CH4, 3 vol %; and O2, 10 vol %.

Table 2. Properties of N2 and CO2 at 800 °C and 1 bar parameter

CO2

N2

λ (cal m−1 K−1 s−1) cp (cal mol−1 K−1) ρ (g/m3) α (m2/s)

1.44 × 10−2 13.2 0.50 5.0 × 10−5

1.54 × 10−2 7.9 0.32 2.2 × 10−4

Figure 2. CH4 conversion as function of the pressure, at different values of the preheating temperature: (red squares) 0 % CO2, (black circles) 5% CO2, and (green triangles) 30% CO2. Operating conditions: Q, 60 slph; CH4, 3 vol %; and O2, 10 vol %.

Figure 4. Thermal diffusivity of N2, CO2, and their mixtures calculated at 800 °C and 1 bar.

transport, and/or chemical properties of CO2. In the following, we discuss the role of CO2. 3.2.1. Thermal and Transport Roles. In the literature, it is widely reported that the presence of CO2 affects the homogeneous combustion.6 This is mainly related to the different properties of CO2 with respect to N2. In Table 2, the values of the thermal conductivity (λ), the specific heat (cp), the density (ρ), and the thermal diffusivity (α) for CO2 and N2 are given. It is shown that the heat capacity of CO2 is higher than that of N2, suggesting that the temperatures reached may be much lower. In Figure 4, the thermal diffusivity of CO2, N2, and their mixtures (α) is plotted. Upon increasing the CO2 content, the

thermal diffusivity decreases. The difference in the thermal diffusivity suggests that the heat produced on the catalyst surface is removed faster in the presence of N2 than in the presence of CO2. In Figure 5, the temperature measured in three locations along the catalytic reactor (T3, T4, and T5; see Figure 1) is plotted as a function of the operating pressure at different CO2 contents at 600 °C of the preheating temperature. Two different behaviors are found, depending upon the pressure. At low pressure, when ignition is not reached, the temperature is slightly affected by the CO2 content; in particular, temperatures slightly decrease by increasing the CO2 concentration. Conversely, at a higher pressure, when 6020

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Figure 6. Axial temperature profiles under ignited conditions for different CO2 feeding concentrations. Operating conditions: Q, 60 slph; CH4, 3 vol %; O2, 10 vol %; preheating temperature, 600 °C; and operating pressure, 6 bar.

catalyst temperature. This result may be mainly addressed to the reduced thermal diffusivity of CO2 richer mixtures, while the possible shift of the temperature maxima downstream is expected to have only a minor impact on the differences in the values of T4−T6. With the temperature of the catalyst surface not being so different in the presence of CO2 under not ignited conditions, the ignition retard cannot be explained when taking into account only the thermal role of CO2. We then investigated the effect of CO2 adsorption over the catalyst. 3.2.2. Chemical Role. The role of CO2 adsorption in affecting the catalytic combustion of methane has been studied over Pt catalysts. The kinetic scheme of Deutschmann et al.20 does not include a CO2 adsorption step. Reinke et al.12 compared the numerical results implementing this kinetic mechanism to the experimental results. They found a quite good agreement, confirming the negligible role of CO2 adsorption. Reinke et al.12 also added kinetic steps to take into account the CO2 adsorption over Pt, according to the steps proposed by Zerkle et al.21 However, they found that in the temperature range of 600−1600 K with CO2 dilutions up to 30 vol %, the addition of these steps has a negligible influence on the catalytic reactivity. This is due to a weak interaction between CO2 and Pt. On the contrary, it is known that perovskites can adsorb CO2 in significant amounts.20 To investigate the role of CO2 adsorption over perovskite catalysts, we performed a TPD. CO2-TPD have been generally used to titrate basic sites of perovskites.22 In this work, we study the interaction of carbon dioxide with the catalyst surface to relate CO2 adsorption to the effect of CO2 on the catalytic performance. In Figure 7, CO2 desorbed as a function of the temperature is shown. Three peaks are found (α, β, and γ) with measured maxima at about 180, 550, and 700 °C, respectively. The lowtemperature peak is commonly addressed to monodentate carbonate decomposition, while the higher temperature peaks can be more likely related to the decomposition of bidentate carbonates.20 From this result, it is possible to conclude that the CO2 adsorption may have a kinetic role at temperatures lower than 900 °C and, then, mainly during ignition.

Figure 5. Temperature (T3, T4, and T5) as measured in three locations (see Figure 1) as a function of the operating pressure at different values of the CO2 content: (red squares) 0% CO2, (black circles) 5% CO2, and (green triangles) 30% CO2. Operating conditions: Q, 60 slph; CH4, 3 vol %; O2, 10 vol %; and preheating temperature, 600 °C.

ignition occurs, the higher the CO2 content, the higher the temperature. Furthermore, this difference reduces upon increasing the pressure. It is worth noting that the adiabatic temperature decreases by increasing the CO2 content. By considering that the measured temperatures are believed to be more indicative of the surface than gas-phase temperatures from these results, we may conclude that, at ignited conditions, the major role of CO2 is to reduce the heat removal rate from the catalytic surface because of the decreased thermal diffusivity. At higher pressure, this effect is reduced because most of the reaction occurs in the gas phase,9 which is conversely affected by the increased specific heat of CO2. Besides the effect of thermal diffusivity, the effect of the specific heat also contributes to the change of the thermal profiles. In Figure 6, the temperature profiles obtained at the same preheating temperature (600 °C) and operating pressure (6 bar) are shown at different values of the CO2 content. Actually, in the first zone (preheating zone, non-catalytic foam; see Figure 1), upon increasing the CO2 content, the measured temperature is lower, according to the increased heat capacity of the CO2 richer mixtures, which cause, moreover, a possible slight shift of the reaction front, which, in all of the cases, is likely positioned at the inlet of the catalyst (i.e., before the T3 measured point). In the reaction zone (catalytic part of the monolith), the higher the CO2 content, the higher the 6021

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From these data, it is found that CO2 adsorption mainly affects the catalytic reaction at low−medium temperatures because peaks β and γ are found at 400 and 600 °C, respectively (Table 3), which are comparable to those necessary to ignite the fuel mixture. However, catalytic results (Figure 2) show that the addition of low amounts (5 vol %) of carbon dioxide (not providing a significant thermal effect) mainly affects ignition up to 550 °C, thus suggesting that the species involved in methane ignition are those related to peak β. It is also worth noting that, in the high-temperature reaction zone (∼900−1000 °C), CO2 is not adsorbed on the catalyst surface.

4. CONCLUSION Experiments performed over a LaMnO3-based structured catalyst at high pressure show that the reactor performances are strongly sensitive to the operating pressure, the preheating temperature, and the CO2 content. At a fixed value of the preheating temperature, the ignition pressure increases when increasing the CO2 content and, at the same time, CO2 addition slightly reduces the operability windows by increasing the quenching pressure (or equivalently the quenching temperature). Under ignited reaction conditions, the surface temperature increases when increasing the CO2 content. We addressed this behavior to the reduced heat removal rate from the surface because of the decrease of the thermal diffusivity. To evaluate the role of CO2 in retarding ignition (increase of the ignition pressure), we performed tests of CO2-TPD. We found that a significant CO2 content is adsorbed in the temperature range of 400−900 °C, suggesting that it may affect the catalytic reaction, mainly in the ignition phase. In particular, our results suggest that sites desorbing CO2 at medium temperature (about 500 °C) are mainly involved in the activation of methane, causing ignition. As a conclusion, we may assert the following: (1) the CO2 content in the feed mixture retards ignition because of CO2 adsorption over the catalyst surface at temperatures lower than 600 °C; (2) the presence of CO2 leads to a reduction of the operability window; (3) at ignited conditions, the effect of CO2 is due to thermal effects because the CO2 adsorption is negligible; and (4) at ignited conditions, the lower value of the thermal diffusivity in the presence of CO2 causes an increase of the catalytic surface temperature.

Figure 7. Experimental (red curve) and predicted (black curve) CO2 evolution curves during CO2-TPD analysis: (blue line) predicted contribution of the α peak, (green line) predicted contribution of the β peak, and (pink line) predicted contribution of the γ peak.

Table 3. Values of the Kinetic Parameters reaction

1

2

3

θ° E°des (J mol−1 K−1) dEdes (J mol−1 K−1) k° (min−1) Tpeak (K) n

0.06 27000 115000 500 360 1

0.155 55000 0 60 700 1

0.0185 52500 0 1550 900 1

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

In Figure 7, the TPD pattern as computed by the kinetic model is also shown. According to the model, the three peaks (α, β, and γ) have been addressed to three different sites, with the kinetic parameter values given in Table 3. Corresponding desorption curves are reported in Figure 7 as well. We found that the agreement between the experimental and model TPD curves is quite satisfactory because the SRMSE value is found to be equal to 0.03, satisfying the criterion of Karnevo et al.19 As shown in Table 3, only step 1 has an activation energy variable with the coverage degree. The step of CO2 desorption from site α exhibits an activation energy that depends upon the site concentration, thus suggesting the presence of heterogeneous sites over the catalyst.

■

ACKNOWLEDGMENTS This work was financially supported by the Ministry of Economic Development, National Research Council (MiSECNR) “CO2 Capture−Clean Coal” Project (Italy).

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