Hetero-/homogeneous combustion of ethane/air mixtures over platinum at pressures up to 14 bar

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1 Available online at Proceedings of the Combustion Institute 34 (2013) Proceedings of the Combustion Institute Hetero-/homogeneous combustion of ethane/air mixtures over platinum at pressures up to 14 bar Xin Zheng, John Mantzaras, Rolf Bombach Paul Scherrer Institute, Combustion Research, CH-5232 Villigen PSI, Switzerland Available online 14 June 2012 Abstract The hetero-/homogeneous combustion of fuel-lean ethane/air mixtures over platinum was investigated experimentally and numerically at pressures of 1 14 bar, equivalence ratios of , and surface temperatures ranging from 700 to 1300 K. Experiments were carried out in an optically accessible channel-flow reactor and included in situ 1-D Raman measurements of major gas phase species concentrations across the channel boundary layer for determining the catalytic reactivity, and planar laser induced fluorescence (LIF) of the OH radical for assessing homogeneous ignition. Numerical simulations were performed with a 2-D CFD code with detailed hetero-/homogeneous C 2 kinetic mechanisms and transport. An appropriately amended heterogeneous reaction scheme has been proposed, which captured the increase of ethane catalytic reactivity with rising pressure. This scheme, when coupled to a gas-phase reaction mechanism, reproduced the combustion processes over the reactor extent whereby both heterogeneous and homogeneous reactions were significant and moreover, provided good agreement to the measured homogeneous ignition locations. The validated hetero-/homogeneous kinetic schemes were suitable for modeling the catalytic combustion of ethane at elevated pressures and temperatures relevant to either microreactors or large-scale gas turbine reactors in power generation systems. It was further shown that the pressure dependence of the ethane catalytic reactivity was substantially stronger compared to that of methane, at temperatures up to 1000 K. Implications for high-pressure catalytic combustion of natural gas were finally drawn. Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Ethane heterogeneous and homogeneous combustion; Catalytic reactivity on platinum; Homogeneous ignition; In situ Raman and OH-LIF 1. Introduction Combined hetero-/homogeneous combustion has demonstrated the potential of ultra-low NO x emissions in power generation systems ranging from large-scale gas turbine burners to microreactors for portable power generation. Commercial utilization of such combustion technologies requires knowledge of the heterogeneous and moderate-temperature homogeneous kinetics of conventional fuels (e.g. natural gas) under realistic operating conditions, so as to facilitate reactor Corresponding author. Fax: address: ioannis.mantzaras@psi.ch (J. Mantzaras) /$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

2 2280 X. Zheng et al. / Proceedings of the Combustion Institute 34 (2013) design and delineate operational envelops. In the past decade, development and validation of heterogeneous methane kinetics have been accomplished over a wide range of operating conditions [1 11]. For ethane, however, which is another important natural gas component (comprising up to 16% per volume, depending on gas provenance), studies were mainly limited to partial oxidation catalytic kinetics due to the industrial use of ethane as the primary feedstock for ethylene production in short-contact-time catalytic reactors. The first detailed ethane heterogeneous kinetic scheme, consisting of 82 elementary reactions among 19 surface species over Pt, was reported by Zerkle et al. [12] with reaction rates based on literature data and appropriate fitting to measurements; their model predictions yielded good agreement to earlier atmospheric-pressure measurements [13]. Subsequently, Donsi et al. [14] constructed a scheme for ethane partial oxidation over Pt and Pt/Sn catalysts, by combining detailed hydrogen and carbon monoxide kinetics with lumped steps of ethane heterogeneous reactions. Their model reproduced experiments in a catalytic monolith at pressures up to 10 bar. More recently, Vincent et al. [15] extended the work of Zerkle et al. [12] to a more detailed scheme comprising 283 elementary reactions among 35 surface species, with kinetic parameters determined by Density Function Theory (DFT) and the UBI-QEP method. Furthermore, kinetics of ethane catalytic partial oxidation has also been studied over other catalysts, e.g. metal oxides which are low-cost substitutes for noble metals [16]. In contrast to partial oxidation, fewer studies are reported for ethane total oxidation relevant to large- or micro-scale power generation. Early work [17] proposed a global step having first order kinetics with respect to ethane and zeroth order with respect to oxygen concentration, for various catalysts (Ni, Pd and Pt) on the basis of subatmospheric-pressure, fuel-lean experiments in a recirculating batch reactor. Deshmukh et al. [18] suggested a three-step mechanism with kinetic parameters estimated by using the bond-order conservation theory and their kinetic predictions agreed well with experimental literature data at atmospheric pressure. The aforementioned kinetics was only tested for partial oxidation or total oxidation at low pressures. Variation in operating pressure may break down their validity under microreactor (up to 6 bar) [19] or gas turbine (16 bar) [20] operating conditions. Therefore, extension of the available heterogeneous kinetic schemes to the high pressure and temperature total oxidation conditions necessitates further investigation. The present work continues previous kinetic studies of H 2 [21], syngas [22], methane [10] and propane [23] by experimentally and numerically investigating heterogeneous and homogeneous combustion of ethane over platinum in a channel-flow reactor. The main objective is to establish suitable heterogeneous and homogeneous kinetic schemes for ethane, which are capable of reproducing key catalytic and gas-phase combustion processes at elevated pressures. Particular interests are to improve fundamental understanding regarding the impact of pressure on the heterogeneous kinetics and on the hetero-/homogeneous chemistry coupling, and thus advance the development of natural-gas fired catalytic combustion systems. 2. Experimental methodology 2.1. Test rig and channel flow reactor Experiments were performed in an optically accessible channel flow reactor (Fig. 1 and details in [10]), consisting of two horizontal, platinumcoated Si[SiC] ceramic plates (300 mm long, 9 mm thick, and placed 7 mm apart) and two vertical quartz windows (300 mm long and 3 mm thick). The inner Si[SiC] surfaces were coated via plasma vapor deposition with a 1.5 lm thick non-porous Al 2 O 3 layer, followed by a 2.2 lm thick Pt layer. Measurements of the total and active catalyst surface areas with BET (Kr-physisorption) and CO-chemisorption, respectively, have verified the absence of surface porosity, while post-combustion X-ray photoelectron spectroscopy (XPS) attested the presence of only Pt and the lack of bulk Si or Al at the catalyst surface [10]. A water-cooled metal block was attached to the reactor entry (Fig. 1) to control the surface temperatures and thus to allow for kineticallycontrolled catalytic ethane conversion, away from the mass-transport-limit. Surface temperatures Fig. 1. High-pressure test rig and optical layout of Raman and OH-LIF (all distances in mm).

3 X. Zheng et al. / Proceedings of the Combustion Institute 34 (2013) Table 1 Experimental conditions. a Case p (bar) u C 2 H 6 (%vol) T IN (K) U IN (m/s) Re IN a Pressure, equivalence ratio, % vol. ethane, inlet temperature, velocity and Reynolds number. along the x y symmetry plane were monitored by S-type thermocouples (12 for each plate) embedded 0.9 mm beneath the catalyst surface through holes eroded from the outer uncoated Si[SiC] surfaces. Compressed air was electrically preheated, mixed with ethane in two sequential static mixers and the resultant flow was driven into a rectangular steel duct (200 mm long, 104 mm wide and 7 mm high) equipped with cross-flow grids to produce a uniform flow at the reactor entry. Flows of air and ethane were controlled by two mass flow meters while the incoming gas temperature was monitored by a sheathed K-type thermocouple, positioned at the channel inlet (x = 0 mm). The reactor assembly was mounted inside a high-pressure stainless steel tank. Two quartz windows (350 mm long, 50 mm high and 35 mm thick) on the tank provided optical access through both reactor sides (Fig. 1). Two additional quartz windows, one at the reactor exhaust and the other at the rear flange of the tank, offered an additional optical access in the streamwise direction, which was used for the LIF excitation beam. The experimental conditions are presented in Table 1, with pressure and equivalence ratios ranging from 1 to 14 bar and 0.11 to 0.49, respectively. The provided Reynolds numbers (Re IN ) based on the uniform inlet properties and the channel hydraulic diameter (=13.1 mm) manifested laminar flow conditions. It is emphasized that laminar conditions were guaranteed even at Re IN 5000 due to the strong flow laminarization induced by the heat transfer from the hot catalytic plates [24] Laser diagnostics The Raman and LIF set-up (Fig. 1) is similar to that employed in recent H 2 /air studies [25]. Planar OH-LIF was used to detect gas-phase combustion. Excitation (k = 285 nm) was achieved by a frequency-doubled Nd:YAG pulsed laser (Quantel YG981E20-CL), which pumped a tunable dye laser (Quantel TDL90 NBP2UVT3). A cylindrical lens telescope and a 1 mm slit mask transformed the 285 nm beam into a light sheet propagating counterflow, along the x y symmetry plane of the reactor. Fluorescence from both (1 1) and (0 0) OH transitions at 308 and 314 nm, respectively, were collected at 90 with an ICCD camera (LaVision Imager Compact HiRes IRO). Each recorded LIF image corresponded to a 100 7mm 2 channel area and the camera was traversed axially to map the entire 300 mm reactor length. Given the steady operating conditions, 400 images were averaged at each measuring location. For the Raman measurements, a pulsed Nd:YLF laser (Quantronix Darwin Duo, k = nm) was operated at khz with pulse duration and energy of 130 ns and mj, respectively. The k = nm beam was focused through the side windows into a vertical line (0.3 mm thick) by an f = 150 mm cylindrical lens. The focal line spanned the entire 7 mm channel height and was offset laterally (z = 15 mm) to increase the collection angle and minimize thermal beam steering, as in [21 23]. Two f = 300 mm lenses collected the scattered light at 50 angle with respect to the sending optical path and focused it on a 25 cm imaging spectrograph (Chromex-250i) equipped with an ICCD camera (Princeton Instruments PI-MAX1024GIII). Signal of 200, ,000 pulses was integrated on the detector chip. Effective Raman cross sections, which included transmission efficiencies, were evaluated by recording the signals of pure ethane, air, and completely burnt gases of known composition. Raman data were acquired at different positions by traversing axially a table supporting the sending and collecting optics, and also the Nd:YLF laser (Fig. 1). Measurement accuracies were ±5% for species molar fractions down to 1%, while lower concentrations entailed larger uncertainties. Finally, data points closer than 0.7 mm to either wall were discarded due to low Raman signal-to-noise ratios. 3. Chemistry and flow simulations Chemistry was modeled with detailed heterogeneous and homogeneous reaction mechanisms. An

4 2282 X. Zheng et al. / Proceedings of the Combustion Institute 34 (2013) Table 2 Heterogeneous reaction mechanism for ethane oxidation on platinum. Reactions S 0 /A n E a Reference Adsorption and desorption reactions R1 C 2 H 6 + 2Pt(s)! C 2 H 6 (s) 3.0E [14] ab R2 C 2 H 6 (s)! C 2 H 6 + 2Pt(s) 1.0E ,900 [12] R3 CH 4 + 2Pt(s)! CH 3 (s) + H(s) 1.0E [4] ab R4 O 2 + 2Pt(s)! O(s) + O(s) 2.3E [4] a R5 O 2 + 2Pt(s)! O(s) + O(s) 1.8E [4] R6 2O(s)! O 2 + 2Pt(s) 3.2E , h 0 [4] R7 CO + Pt(s)! CO(s) 1.6E [4] a R8 CO(s)! CO + Pt(s) 1.0E ,500 [4] R9 OH + Pt(s)! OH(s) [4] a R10 OH(s)! OH + Pt(s) 1.0E ,800 [4] R11 H 2 O + Pt(s)! H 2 O(s) 7.5E [4] a R12 H 2 O(s)! H 2 O + Pt(s) 1.0E ,300 [4] R13 CO 2 (s)! Pt(s) + CO 2 1.0E ,500 [4] Surface reactions R14 C 2 H 6 (s)! C 2 H 5 (s) + H(s) 1.0E ,700 [26] R15 C 2 H 6 (s) + O(s)! C 2 H 5 (s) + OH(s) + Pt(s) 1.0E ,100 [12] R16 C 2 H 5 (s) + 6Pt(s)! 2C(s) + 5H(s) 5.0E ,000 [14] b R17 CH 3 (s) + Pt(s)! CH 2 (s) + H(s) 3.7E ,000 [4] R18 CH 2 (s) + Pt(s)! CH(s) + H(s) 3.7E ,000 [4] R19 CH(s) + Pt(s)! C(s) + H(s) 3.7E ,000 [4] R20 H(s) + O(s) M OH(s) + Pt(s) 3.7E ,500 [4] R21 H(s) + OH(s) M H 2 O(s) + Pt(s) 3.7E ,400 [4] R22 OH(s) + OH(s) M H 2 O(s) + O(s) 3.7E ,200 [4] R23 C(s) + O(s)! CO(s) + Pt(s) 3.7E ,800 [4] R24 CO(s) + O(s)! CO 2 (s) + PT(s) 3.7E ,000 [4] a Reaction rates are in terms of sticking coefficients (S 0 ). b Reaction order with respect to Pt(s) is 1.1, 2.3 and 1.0 for R1, R3 and R16, respectively. Units: S 0 [ ], A [cm, s, K], E a [J/mol], coverage h [ ]. ethane surface reaction mechanism has been constructed based on reactions of ethane and its derivatives, collected by compiling literature data [12,14,26] and further including CH 4 /CO/H 2 reactions on Pt [4]. The added ethane reactions had only been tested for atmospheric-pressure partial oxidation conditions and thus their applicability has not been previously assessed for deep oxidation at elevated pressures. In order to reproduce the measurements, the reaction order of ethane adsorption with respect to Pt was increased to 1.1 from its original value of 1.0; this change was supported by detailed sensitivity analysis as will be elaborated in Section 4.1. While the reaction order modification had a negligible impact at p 6 2 bar, it had a progressively larger effect with rising pressure above 2 bar. It is further emphasized that the order of hydrocarbon adsorption with respect to Pt is crucial in capturing the correct pressure dependence of the catalytic reactivity. Earlier methane studies [10] showed that the good performance of the therein tested catalytic scheme [4] at elevated pressures cardinally depended on the 2.3 order of methane adsorption with respect to Pt. The resulting ethane heterogeneous scheme comprised 24 reactions among 14 surface species (see Table 2), with surface site density C = mol/cm 2. Methane reactions in Table 2 were included for completeness, as this species was part of the ethane gas-phase mechanism; however, the ensuing ethane hetero-/homogeneous combustion simulations were practically unaffected by these reactions. The gas phase mechanism, involving 192 reactions among 35 species, was extracted from a detailed C 3 combustion mechanism [27] by removing C 3 reactions as they were largely irrelevant under the present very lean conditions. Species thermodynamic and transport data were also taken from [27]. Surface and gas-phase reaction rates were evaluated with CHEMKIN [28,29]. Finally, a mixture-average diffusion model was used for species transport. A steady elliptic 2-D CFD code (details given in [21,22]) simulated the 300 7mm 2 (x y) channel domain. Uniform gas temperature, velocity, and species composition were applied at the inlet (x = 0) and prescribed temperatures at the gaswall interfaces (y = 0 and 7 mm), constructed by polynomial curve fits to the measured 12 surface temperatures per plate. Given the particularly large gas-phase and catalytic mechanisms, it was imperative to first use the solution of a fast parabolic code [5] as initial guess to the elliptic code. In the presence of strong gaseous combustion, the parabolic code always yielded homogeneous

5 X. Zheng et al. / Proceedings of the Combustion Institute 34 (2013) ignition distances longer than those of the elliptic code, by up to 2 cm (comparisons between elliptic and parabolic homogeneous ignition predictions have been elaborated in [5]). Despite the good initial guess, maximum 15 CPU days were still necessary for convergence of the elliptic code in each examined case, in a Xeon 3.0 MHz processor. 4. Results and discussion Experimental and numerical results are compared against each other. For Cases 6-10 in Table 1, flames were anchored inside the channel reactor as manifested by the OH-LIF measurements, thus allowing validation of homogeneous kinetics. On the other hand, the absence of flames for the lower-wall temperature Cases 1-5 allowed evaluation of heterogeneous kinetics Pressure effect on catalytic reactivity Computed catalytic (C) and gaseous (G) hydrogen consumption rates (the latter integrated over the 7 mm channel-height) as well as measured surface temperatures are presented in Fig. 2 for four selected cases without appreciable gas-phase chemistry contribution. For the same cases, predicted and measured transverse molefraction profiles for C 2 H 6 and the major product H 2 O are shown in Fig. 3 at three selected streamwise positions x 6 80 mm, whereby the gas-phase contribution on ethane conversion was negligible. The magnitudes of the catalytic conversions (C)in Fig. 2 bear the combined effects of pressure, equivalence ratio, gas inlet temperature and velocity, and surface temperatures. The pressure effect, of particular interest herein, will be numerically delineated in the next section by fixing all other parameters. The vertical arrows marked x ag in Fig. 2 denote the onset of appreciable gas-phase ethane conversion (therein G amounts to 5% of the C conversion). By limiting the heterogeneous reactivity investigation to x < x ag, potential falsification of the heterogeneous kinetics by gaseous chemistry is eliminated. It is clarified that the onset of appreciable gaseous conversion does not imply the onset of vigorous exothermic gaseous reactions: it was shown [10,23] that the incomplete oxidation of hydrocarbons to CO can still yield significant gaseous fuel consumption without appreciable exothermicity and hence without the establishment of a flame. As seen in Fig. 3, predictions of the herein-constructed heterogeneous scheme are in good agreement with the Raman measurements of C 2 H 6 and H 2 O mole fractions at pressures up to 12 bar; however, simulations in Fig. 3 without the corrected C 2 H 6 adsorption (with the 1.1 platinum order), overpredict substantially the catalytic reac- Fig. 2. Measured wall temperatures (upper-wall: squares and fitted solid lines, lower-wall: circles and fitted dashed lines), and computed catalytic (C) and gasphase (G) ethane conversions for four flameless cases in Table 1. Fig. 3. Measured (symbols) and predicted (dashed-lines and solid-lines for the original and the herein proposed reaction scheme, respectively) transverse profiles of C 2 H 6 (triangles) and H 2 O (circles) mole fractions, at three axial locations for Cases 2 5. For Case 2, the solid and dashed-line predictions practically overlap.

6 2284 X. Zheng et al. / Proceedings of the Combustion Institute 34 (2013) tivity at p > 2 bar. Such in situ, highly-resolved measurements of gas-phase species concentrations across the catalyst boundary layer have been provided for the first time, allowing for direct evaluation of the ethane heterogeneous kinetics. It is noted that mass-transport-limited conversion was not approached for pressures up to 12 bar and wall temperatures up to 1100 K, as manifested by the appreciable C 2 H 6 concentrations near both walls (Fig. 3). The attained kinetically-controlled catalytic conversion was also facilitated by the larger than unity Lewis number of ethane (for fuel-lean C 2 H 6 /air stoichiometries, Le C2H6 1.8), which in turn led to strongly underadiabatic surface temperatures at the upstream parts of the reactor. This effect, present during fuel-lean catalytic combustion of higher hydrocarbons, was also discussed in earlier propane studies [23]. The sensitivity analysis (SA) in Fig. 4(a) identified ethane adsorption (R1) as the most sensitive catalytic reaction controlling the consumption of ethane at all pressures; this SA was constructed using the surface perfectly stirred reactor (SPSR) code of CHEMKIN [30] at u = 0.3, constant wall temperature of 900 K, and a residence time of 0.1 s. The limiting reaction in ethane catalytic oxidation is thus its adsorption: _s ad ¼ k ad ½C 2 H 6 Š m w½ Ch PtŠ n ; ð1þ Fig. 4. (a) Sensitivity analysis, SPSR computations for a = 0.3 ethane/air mixture (reaction numbering as in Table 2), (b) SPSR-computed temperatures for CH 4 / C 2 H 6 mixtures versus inlet temperature, for three % vol. C 2 H 6 contents in the total fuel at 15 bar, and (c) SPSRcomputed fuel conversions for CH 4 /air, C 2 H 6 /air and C 3 H 8 /air mixtures (u = 0.3) versus pressure. where k ad is the adsorption rate constant, [C 2 H 6 ] w the ethane concentration at the gas-wall interface, U the total surface site density ( mol/ cm 2 ), h Pt the Pt coverage, while m = 1.0 and n = 1.1 are the reaction orders regarding ethane and Pt, respectively. As previous catalytic reactivity studies of lean propane/air [23] and methane/ air mixtures [10] have indicated, the rise in oxygen partial pressure at elevated pressures increases the O(s) surface coverage at the expense of Pt(s). This in turn reduces the catalytic reaction rate due to the drop in available free platinum surface concentration ([Uh Pt ] in Eq. (1)). On the other hand, a rise in pressure increases proportionally the concentration of the limiting reactant at the wall ([C 2 H 6 ] w in Eq. (1)), thus promoting ethane adsorption. These two competing factors determine the overall dependence of the catalytic reactivity on pressure, as also shown for methane [10] and propane [23]. To facilitate the forthcoming discussion, the negative pressure dependence of h Pt is represented as [h Pt ] p b, where b is a positive number that is not necessary constant, but function of local conditions. According to Eq. (1), the adsorption reaction becomes a product of the positive pressure dependent gas-phase concentration, ½C 2 H 6 Š m w pm, and the negative pressure dependent free-site coverage, [h Pt ] n p nb. Thus, the catalytic reactivity assumes an overall pressure dependence p m nb with m > nb, such that a total positive pressure dependence is guaranteed. This pressure effect is well-reproduced by the proposed kinetic scheme, as the comparisons in Fig. 3 indicate. Crucial for this performance is the adopted 1.1 reaction order with respect to Pt(s) for C 2 H 6 adsorption (R1 in Table 2). Having established appropriate ethane highpressure kinetics, comparisons of heterogeneous reactivities for C 2 H 6 and CH 4 and C 3 H 8 (main components of natural gas) were performed using SPSR [30] simulations; the equivalence ratio for all cases was u = 0.3, the surface-to-volume ratio was 2.86 cm 1 (equal to that of the channel in Fig. 1), the reactor temperature was constant at 900 or 1000 K, pressure varied from 1 to 15 bar, and residence time increased proportionally with rising pressure (s = 10 ms at 1 bar) to maintain a constant reactor mass throughput. The CH 4 and C 3 H 8 high-pressure heterogeneous schemes were taken from [4,23], respectively. Reactivity is shown in terms of fuel conversion versus pressure in Fig. 4(c). It is evident that reactivity drops in the order of C 3 H 8, C 2 H 6 and CH 4 and, most importantly, relative differences between the fuel reactivities increase with rising pressure. For the examined temperatures, methane conversion reaches a nearly constant value above a certain pressure, while C 2 H 6 and C 3 H 8 conversions strongly increase with rising pressure up to 15 bar. The continuous rise of C 2 H 6 and C 3 H 8

7 X. Zheng et al. / Proceedings of the Combustion Institute 34 (2013) reactivities with pressure indicates that these fuel components could facilitate the natural gas lightoff at elevated pressures relevant to many power generation systems. This is further illustrated by additional SPSR simulations at a fixed pressure of 15 bar and inlet temperatures varying from 700 to 900 K. Computed reactor (or outlet) temperature is plotted in Fig. 4(b) as a function of inlet temperature, demonstrating a reduction of the light-off temperature by about 20 and 50 K when replacing volumetrically CH 4 with 5% or 16% C 2 H 6 (contents typical to various natural gas compositions). It is clarified, however, that using the scheme in Table 2 for natural gas would also entail further validation experiments with CH 4 /C 2 H 6 mixtures. Nonetheless, the results indicate that the presence of ethane is highly beneficial for the light-off of natural-gas-fueled high pressure systems Homogeneous ignition Predicted and LIF-measured 2-D OH distributions are presented in Fig. 5 for Cases 6 11, wherein flames are anchored inside the reactor; vertical arrows indicate locations of homogenous ignition (x ig ), defined as the far-upstream positions whereby OH rises to 5% of its maximum value inside the reactor. Differences between measured and predicted x ig are less than 8%, apart from Case 8 (16%). Moreover, simulations mildly underpredict the ignition distance at p < 6 bar and overpredict it at higher pressures. Comparisons of flame sweep angles, which are linked to laminar flame speeds, are also in good agreement. The predicted flames are thinner than the measured ones, particularly at higher pressures, but this is largely attributed to the fact that OH-LIF images have not been calibrated with adsorption measurements to have quantitative traits. As the heterogeneous kinetics was already validated in the foregoing section, the comparison in Fig. 5 allows for direct evaluation of homogeneous kinetics. The overall good agreement demonstrated that the selected kinetic scheme is suitable for describing hetero-/homogeneous high pressure ethane combustion. In Fig. 6, heterogeneous and homogeneous conversions rates of ethane are plotted for Cases 7, 8 and 9. Catalytic conversion is only considerable in the first few centimeters, and is subsequently overtaken by gas-phase reactions well-upstream of the homogeneous ignition location. As explained previously, the appreciable gas-phase ethane conversion upstream of x ig in Fig. 6 is due to the incomplete reaction of C 2 H 6 to CO. It is also evident that the surface temperatures in Fig. 6 are 100 K higher than those in the flameless cases in Fig. 2, a necessary condition for flame establishment. Additional hetero-/homogeneous validations are provided in Fig. 7 by comparing predicted transverse profiles of ethane and water against Raman measurements upstream of homogeneous ignition, in regions whereby homogeneous reactions are still non-negligible. Catalytic fuel conversion is kinetically controlled in Fig. 7(a) (d) (manifested by the non-zero concentration of C 2 H 6 at both walls), while in Fig. 7(e) and (f) the nearly zero transverse wall gradients of C 2 H 6 (dx C2H6 =dy 0aty = 0 and 7 mm) indicate negligible catalytic fuel conversion and predominant gas-phase conversion (as also seen in Fig. 6 at the corresponding streamwise positions). The good agreement between measurements and predictions in Fig. 7, particularly in regions where both reaction pathways contribute to fuel consumption, demonstrates that the combined heter- Fig. 5. (a) LIF-measured and (b) predicted OH distributions for Cases 6 10 in Table 1. The color bars provide predicted OH in ppmv. Fig. 6. Measured wall temperatures (upper wall: triangles and fitted solid lines, lower wall: circles and fitted dashed lines) and computed catalytic (C) and gas-phase (G) ethane conversions for three cases with vigorous homogeneous combustion. For clarity, the C rates are multiplied by three.

8 2286 X. Zheng et al. / Proceedings of the Combustion Institute 34 (2013) was suitable for ethane/air total oxidation over Pt at elevated pressures relevant to both microreactors and gas turbines. Finally, the pressure dependence of the ethane catalytic reactivity was considerably stronger compared to that of CH 4, exemplifying the advantages of high-pressure catalytic combustion of natural gas. Acknowledgments Fig. 7. Measured (symbols) and predicted (lines) transverse profiles of C 2 H 6 (triangles) and H 2 O (circles) mole fractions at selected streamwise positions for Cases 8 and 9. ogeneous and homogeneous kinetic schemes used herein reproduce the underlying combustion processes over the pressure and temperature ranges of interest. The impact of catalytically-produced major species and radical adsorption/desorption reactions on homogeneous ignition was finally investigated. Sensitivity analyses were conducted in an SPSR at pressures 1-14 bar and constant reactor temperature of 1000 K. The most important surface reactions (negative sensitivity) affecting homogeneous ignition were the adsorption of C 2 H 6 and O 2 as they determined the catalytic fuel consumption which in turn reduced the availability of fuel for the gaseous pathway. In terms of radicals, their adsorption/desorption reactions played only a minor effect, in agreement to earlier methane studies [10]. 5. Conclusions The catalytic combustion of ethane/air mixtures over Pt was investigated in a channel-flow reactor under fuel-lean equivalence ratios of , pressures of 1 14 bar, and surface temperatures up to 1300 K. A heterogeneous reaction scheme, with ethane adsorption having an order of 1.1 with respect to Pt(s), was constructed. This scheme reproduced the Raman-measured transverse major species profiles across the reactor boundary layer at conditions whereby only heterogeneous reactions were important, and for all examined pressures and temperatures. Coupling of the catalytic scheme with an elementary homogeneous reaction scheme resulted in good predictions over the reactor zones where combined hetero-/homogeneous reactions were important, and also yielded homogeneous ignition distances within 10% to those measured by planar OH- LIF. The validated hetero-/homogeneous kinetics Support was provided by Swiss-Electric Research and the Swiss Competence Center of Energy and Mobility (CCEM). We thank Mr. Rene Kaufmann for assistance in the experiments. References [1] O. Deutschmann, R. Schmidt, F. Behrendt, J. Warnatz, Proc. Combust. Inst. 26 (1996) [2] U. Dogwiler, J. Mantzaras, P. Benz, B. Kaeppeli, R. Bombach, A. Arnold, Proc. Combust. Inst. 27 (1998) [3] U. Dogwiler, P. Benz, J. Mantzaras, Combust. Flame 116 (1999) [4] O. Deutschmann, L. Maier, U. Riedel, A.H. Stroeman, R.W. Dibble, Catal. Today 59 (2000) [5] J. Mantzaras, C. Appel, P. Benz, Proc. Combust. Inst. 28 (2000) [6] G. Vesser, J. Frauhammer, Chem. Eng. Sci. 55 (2000) [7] A.B. Mhadeshwar, P. Aghalayam, V. Papavassiliou, D.G. Vlachos, Proc. Combust. Inst. 29 (2002) [8] M. Reinke, J. Mantzaras, R. Schaeren, R. Bombach, W. Kreutner, A. Inauen, Proc. Combust. Inst. 29 (2002) [9] P. Aghalayam, Y.K. Park, N. Fernandes, V. Papavassiliou, A.B. Mhadeshwar, D.G. Vlachos, J. Catal. 213 (2003) [10] M. Reinke, J. Mantzaras, R. Schaeren, R. Bombach, A. Inauen, S. Schenker, Combust. Flame 136 (2004) [11] J. Koop, O. Deutschmann, Appl. Catal., B 91 (2009) [12] D.K. Zerkle, M.D. Allendorf, M. Wolf, O. Deutschmann, J. Catal. 196 (2000) [13] A.S. Bodke, S.S. Bharadwaj, L.D. Schmidt, J. Catal. 179 (1998) [14] F. Donsi, K.A. Williams, L.D. Schmidt, Ind. Eng. Chem. Res. 44 (2005) [15] R.S. Vincent, R.P. Lindstedt, N.A. Malik, I.A.B. Reid, B.E. Messenger, J. Catal. 260 (2008) [16] R. Grabowski, Catal. Rev. 48 (2006) [17] M. Aryafar, F. Zaera, Catal. Lett. 48 (1997) [18] S.R. Deshmukh, D.G. Vlachos, Combust. Flame 149 (2007) [19] S. Karagiannidis, J. Mantzaras, G. Jackson, K. Boulouchos, Proc. Combust. Inst. 31 (2007) [20] R. Carroni, T. Griffin, J. Mantzaras, M. Reinke, Catal. Today 83 (2003)

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