Homogeneous combustion of fuel-lean syngas mixtures over platinum at elevated pressures and preheats

Combustion and Flame 160 (2013) 155–169

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Combustion and Flame j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m b u s t fl a m e

Homogeneous combustion of fuel-lean syngas mixtures over platinum at elevated pressures and preheats Xin Zheng, John Mantzaras ⇑, Rolf Bombach Paul Scherrer Institute, Combustion Research, CH-5232 Villigen PSI, Switzerland

a r t i c l e

i n f o

Article history: Received 4 July 2012 Received in revised form 3 September 2012 Accepted 4 September 2012 Available online 1 October 2012 Keywords: Syngas hetero-/homogeneous combustion over platinum Homogeneous ignition High-pressure catalytic combustion In situ Raman and LIF measurements

a b s t r a c t The gaseous oxidation of H2/CO/CO2/O2/N2 mixtures was investigated experimentally and numerically in a platinum-coated channel at fuel-lean stoichiometries (equivalence ratios u 6 0.30), H2:CO molar ratios 0.47–4.54, pressures 2–14 bar, and reactant preheats up to 736 K. Two-dimensional laser induced fluorescence of the OH radical monitored the homogeneous (gaseous) combustion, while 1-D Raman spectroscopy assessed the heterogeneous (catalytic) conversion of H2 and CO. Numerical simulations, which were carried out with a 2-D elliptic code and detailed hetero-/homogeneous reaction schemes, reproduced the measured onset of homogeneous ignition, the ensuing flame shapes, and the mass-transport-limited catalytic conversion of H2 and CO. Additional simulations in practical tubular channels with 1 mm diameter have shown that gaseous oxidation was suppressed at atmospheric pressure due to the intrinsic slow gas-phase ignition kinetics in conjunction with the competition from the catalytic pathway for H2 and CO consumption. At pressures p > 4 bar, homogeneous combustion was largely controlled by flame propagation characteristics due to the near-wall confinement of the established flames. The decrease in laminar mass burning rates at p > 4 bar led to a push of the gaseous combustion zone close to the channel wall, to leakage of H2 and CO through the flame and, finally, to subsequent catalytic conversion of the leaked fuel components. Radical heterogeneous reactions promoted mildly the onset of homogeneous ignition at p P 2 bar due to the net desorptive flux of OH over the gaseous induction zone. The catalytically produced H2O had a strong kinetic impact on homogeneous combustion by inhibiting the gaseous oxidation of both H2 and CO at high H2:CO ratios and by promoting CO gaseous oxidation at low H2:CO ratios. The catalytically produced CO2 always inhibited kinetically the gaseous combustion of H2 and CO, although its effect was much weaker compared to that of H2O. Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction Increasing concerns regarding sustainable and secure energy supply have incited a global interest for syngas fuels. Syngas mainly consists of varying proportions of carbon monoxide and hydrogen, and can be produced from fossil fuels or from diverse renewable biological feedstocks via biothermal or thermochemical processing. High-pressure and high-preheat syngas combustion, in particular, is currently under intense investigation for application in gas turbines of power generation systems employing fuel decarbonization (pre-combustion CO2 capture) as a means to reduce greenhouse gas emissions [1,2]. Although lean premixed combustion is the main approach for gas-fired power plants, catalytic combustion methodologies are increasingly explored in the last years due to their enhanced combustion stability at very fuel-lean equivalence ratios and the resulting ultra-low NOx emissions [3–5]. In the fuel-lean catalytically ⇑ Corresponding author. Fax: +41 56 3102199. E-mail address: [email protected] (J. Mantzaras).

stabilized thermal combustion (CST) concept, fractional fuel conversion is achieved in a heterogeneous (catalytic) reactor, while the remaining fuel is combusted in a subsequent homogeneous (gas-phase) burnout zone [5]. CST is particularly suited for low calorific value syngas-based fuels, due to the enhanced combustion stability at the ensuing moderate reaction temperatures [6,7]. Therefore, application of hetero-/homogeneous combustion to syngas fuels is an attractive option for renewable and clean power generation. Moreover, combined hetero-/homogeneous combustion has been shown [8,9] to suppress most of the intrinsic flame instabilities appearing in non-catalytic (pure homogeneous combustion) channel-flow reactors [10,11]. Future utilization of syngas catalytic combustion relies on the development of active and stable catalysts as well as on the understanding of the heterogeneous and homogeneous syngas kinetics under industrially-relevant operating conditions. In contrast to the extensive investigations of syngas homogeneous chemistry over broad ranges of mixture compositions and pressures, which have been reviewed in [12], there is a clear lack of corresponding studies in combined hetero-/homogeneous syngas combustion.

0010-2180/$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.combustflame.2012.09.001

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Nomenclature b L Lek _o m p s_ k T Vk,x, Vk,y W Wk

channel half-height, Fig. 1 channel length, Fig. 1 Lewis number of gaseous species k (thermal over mass diffusivity) laminar mass burning rate, Eq. (4) pressure heterogeneous molar production rate of gaseous species k, Eq. (1) temperature diffusion velocity components of gaseous species k, Eq. (1) channel width, Fig. 1b molecular weight of gaseous species k, Eqs. (1) and (3)

The non-negligible contribution of homogeneous chemistry (even in practical catalytic reactor geometries with large surface-to-volume ratios [13]), either at the moderate pressures (up to 5 bar) relevant to micro-reactors [14] or especially at the elevated pressures (15 bar) of gas turbines [15], requires additional validation of the gas-phase syngas kinetic models [12] that were developed in the absence of heterogeneous reactions. To facilitate the validation of kinetics, we have introduced the methodology of in situ spatially resolved 1-D Raman measurements of major gas phase species concentrations across the boundary layer of a catalytically-coated channel, along with 2-D laser induced fluorescence (LIF) of the OH radical [16–18]. These measurements, when compared to detailed numerical simulations, have allowed for direct assessment of the catalytic and gas-phase reactivities. Using the aforementioned approach, Appel et al. [16] evaluated several heterogeneous and homogeneous kinetic schemes for atmospheric-pressure combustion of fuel-lean H2/air mixtures over Pt and for non-preheated reactants (gas inlet temperatures of 310 K). It was shown that heterogeneous reactions inhibited homogeneous ignition mainly via competitive fuel depletion, while the corresponding inhibition via radical adsorption/ desorption catalytic reactions was modest. Mantzaras et al. [17] extended the validation of hydrogen gaseous kinetic schemes to pressures up to 10 bar, for fuel-lean stoichiometries and non-preheated reactants. Therein, the observed suppression of gas-phase combustion at pressures p P 4 bar was due to the combined effects of intrinsic homogeneous kinetics, competition by the catalytic pathway for hydrogen consumption, and inhibition from catalytically-produced major products (notably H2O). Ghermay et al. [18] further investigated combustion of fuel-lean H2/air mixtures over Pt at high preheats (up to 773 K) and gas-turbine relevant pressures up to 15 bar. For pressures above 12 bar – and even for the highest examined preheats – the heterogeneous reaction pathway was strongly favored against the homogeneous one, thus suppressing flame formation. Homogeneous ignition of fuel-lean and fuel-rich H2/air mixtures at atmospheric pressure has been numerically investigated over stagnation flow Pt-coated surfaces in Bui et al. [19], establishing the dependence of the ignition temperature on equivalence ratio. Finally, addition of hydrogen to less reactive hydrocarbon fuels, such as methane, has been shown to greatly extend the performance of catalytic microreactors [20]. In terms of syngas combustion, numerical investigation with detailed hetero-/homogeneous reaction schemes was initially reported in Mantzaras [21] at pressures up to 15 bar. Ghermay et al. [22] subsequently investigated experimentally and numerically the homogeneous kinetics of fuel-lean CO/H2 mixtures over Pt at pressures of 1–5 bar (relevant to microreactors), inlet temperatures up to 874 K, and a constant H2:CO molar ratio of 1:2 (pertinent to syngas production via coal gasification). It was therein

Yk x, y, z

mass fraction of gaseous species k, Eq. (1) streamwise, transverse and lateral coordinates, Fig. 1

Greek symbols C surface site density, Eq. (3) ck sticking coefficient of gaseous species k, Eq. (3) hi coverage of surface species i q gas density u fuel-to-air equivalence ratio Subscripts IN inlet ig ignition demonstrated that the presence of CO could facilitate flame propagation towards the center of the catalytic channel, thus leading (for a given channel geometry) to enhanced mass consumption rates for syngas fuels when compared to pure hydrogen ones. Detailed syngas homogeneous ignition experiments at pressures and preheats relevant to gas turbines and, moreover, at varying H2:CO ratios (referring to different degrees of hydrocarbon fuel decarbonization) have not yet been reported in the literature. The present work undertakes a first investigation of high-pressure (up to 14 bar) and high preheat (up to 736 K) syngas homogeneous ignition over platinum at fuel-lean stoichiometries (equivalence ratios, u, up to 0.3) and a wide range of H2:CO molar ratios (0.47–4.54). Planar LIF of the OH radical monitored the onset of homogeneous ignition in a Pt-coated catalytic channel, while 1-D Raman measurements of major species concentrations across the channel boundary layer yielded the catalytic H2 and CO consumption. Numerical simulations were performed with a 2-D full elliptic code that included detailed hetero-/homogeneous chemical reaction schemes and transport. Main objectives were to validate gas-phase chemical reaction mechanisms at operating conditions relevant to catalytic combustion power generation systems, to investigate the coupling of heterogeneous and homogeneous syngas kinetics, and to clarify the effect of fuel composition (H2:CO ratio) on gas-phase ignition and flame propagation characteristics in the confined catalytic channel geometry. The onset of homogeneous ignition inside the catalytic reactor, in particular, was of great interest for syngas (or generally for hydrogen-rich fuels) as the presence of gaseous combustion has been shown to moderate the superadiabatic surface temperatures attained due to the catalytic conversion of the diffusionally imbalanced hydrogen fuel component [16,21]. The findings of the present study could further facilitate the design of syngas catalytic burners and address reactor thermal management issues. This article is organized as follows. The experimental and numerical methodologies are introduced in Sections 2 and 3, respectively. In Section 4.1, measurements are compared against numerical predictions in order to assess the aptness of the applied kinetic models and to reveal the effects of pressure, preheat, and fuel composition on gas-phase combustion. The coupling of heterogeneous and homogeneous chemistry is subsequently addressed in Section 4.2, followed by discussion of gas-phase ignition and flame propagation characteristics in Section 4.3.

2. Experimental 2.1. High pressure test rig The test-rig has been employed in previous high-pressure combustion experiments of methane and hydrogen fuels [13,18,23]

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such that a brief summary is provided hereafter, with emphasis on specific modifications introduced for the present syngas studies. Hetero-/homogeneous combustion experiments were carried out in an optically accessible channel-flow reactor with a length, width and height of 300 mm (L), 104 mm (W) and 7 mm (2b), respectively (see Fig. 1). The reactor comprised two 9-mm-thick Si[SiC] horizontal ceramic plates and two 3-mm-thick vertical quartz windows (Fig. 1b) and was positioned inside a cylindrical stainless steel tank that yielded the desired pressurization. The inner surfaces of both Si[SiC] ceramic plates were firstly coated with a 1.5 lm thick non-porous Al2O3 layer and subsequently with a 2.2 lm thick Pt layer by means of plasma vapor deposition (PVD). The absence of surface porosity was verified with total area and active surface area measurements via Brunauer–Emmett–Teller (BET) Kr-physisorption and CO-chemisorption measurements, respectively, while post-combustion X-ray photoelectron spectroscopy (XPS) analysis verified the presence of only Pt at the catalyst surface and the absence of bulk Al or Si [13]. A water-cooled metal plate was attached to the reactor entry (Fig. 1a) to suppress the superadiabatic surface temperatures attained during catalytic combustion of syngas fuels with high hydrogen content [16,21]; such superadiabatic temperatures were a result of the diffusional imbalance of hydrogen (at lean stoichiometries, the Lewis number – ratio of thermal over mass diffusivity – of hydrogen is LeH2  0.3). On the other hand, the central and rear sections of the Si[SiC] plates (100 < x < 300 mm) were heated by two resistive coils positioned above the plates in order to counteract the external heat losses. Surface temperatures along the x–y symmetry plane were measured by S-type thermocouples (12 for each plate), embedded 0.9 mm beneath the catalyst surface through holes eroded from the outer non-catalytic Si[SiC] surfaces (Fig. 1a). Syngas fuels with high hydrogen content necessitated the design of a special reactant supply section upstream of the channel

O2,N2

Power Thermocouple feedthroughs feedthroughs Quartz Pt-coated windows x surfaces A

CO,H2,CO2

TC B

TC A

Exhaust

TC C

y

Preheater

2b =7

TC D

Ls=200 Heater Static Water coils mixers cooling Flushing N2

LIF laser sheet

Insulation L=300 Heater A coils

Flow straightener

Water cooling

Pressure throttle

(a) y

Cross section A-A

z

35 3 Quartz windows Inconel steel frame

W =104

110

9

High pressure tank

50 Si[SiC] ceramic plates

(b) Fig. 1. (a) Schematic of the high-pressure test rig and (b) cross section of the catalytic channel reactor. All distances are in mm.

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reactor, in order to reduce the risk of autoignition and at the same time achieve good mixing. O2 and N2 from pressurized bottles were electrically preheated and then mixed with room temperature CO and H2 in two sequential static mixers (Fig. 1a). To attain the high preheat temperatures necessary for the present study, the steel pipe section driving the preheated O2/N2 mixture to the static mixers and was also externally heated by an electric coil (Fig. 1a). Varying amounts of CO2 diluent were further added in the H2/CO fuel stream, so as to avoid autoignition of the reactive mixture inside the flow straightening section. The resulting O2/N2/H2/CO/CO2 reactant mixture was subsequently driven into a flow straightening section, which comprised a rectangular steel duct (200 mm (Ls)  104 mm (W)  7 mm (2b)) positioned upstream of the channel reactor and equipped with cross-flow grids to produce a uniform velocity (Fig. 1a). Mitigation of autoignition was attested by a series of four sheathed K-type thermocouples (TCA to TCD in Fig. 1a) that monitored the mixture temperature from the point of H2/CO/CO2 injection into the preheated O2/N2 stream, down to the reactor entry. Moreover, flow uniformity was assessed by hot-wire velocimetry measurements at the exit of the stand-alone flow straightening unit. Thermocouple TCD, positioned 1 mm upstream of the reactor entry, provided the inlet temperature required for the numerical simulations. Two 35-mm thick quartz windows on the sides of the high-pressure cylindrical tank (Fig. 1b), aligned with respect to the reactor windows, provided optical access for the ensuing Raman and planar OH-LIF measurements. Experiments were performed at pressures 2–14 bar, equivalence ratios 0.17–0.30 (based on the combined amounts of H2 and CO) and H2:CO molar ratios 0.47–4.54. The investigated conditions are summarized in Table 1. The Reynolds number for each condition (ReIN), based on the uniform inlet properties and the channel hydraulic diameter (=13.1 mm), was maintained below 3000. All flows were laminar, as turbulent catalytic combustion studies have shown that the flow laminarization induced by the heat transfer from the hot catalytic walls guaranteed laminar conditions at ReIN considerably higher than 5000 [24,25]. 2.2. Laser diagnostics The optical setups for the spectroscopic measurements (planar OH-LIF and 1-D Raman) are depicted in Fig. 2. OH-LIF detected gas-phase combustion, whereas spontaneous Raman assessed the catalytic processes preceding the onset of homogeneous ignition. For LIF, the 532 nm second harmonic beam of a pulsed Nd:YAG laser (Quantel TDL90 NBP2UVT3) pumped a tunable dye laser (Quantel TDL90). The dye output radiation was frequency-doubled to 285 nm with a pulse energy of 0.5 mJ, sufficiently low to avoid saturation of the A(v = 1) X(v = 0) OH transition. A cylindrical lens telescope and a 1-mm slit mask transformed the excitation beam into a 0.3 mm thick light sheet, which propagated counterflow along the x–y symmetry plane of the reactor (see Figs. 1a and 2). Fluorescence from both (1–1) and (0–0) OH transitions at 308 and 314 nm, respectively, was collected at 90° through the reactor and tank side windows with an ICCD camera (LaVision Imager Compact HiRes IRO, 1392  1024 pixels). Channel areas of 100  7 mm2 were recorded on a 628  44 pixel area of the CCD detector chip. The camera was traversed axially to map the entire 300 mm reactor extent. Given the steady operating conditions, 400 LIF images were averaged at each measuring location to increase the signal-to-noise ratio. For the Raman measurements, a high repetition rate (up to 2 kHz) frequency-doubled Nd:YLF pulsed laser (k = 526.5 nm, Quantronix Darwin Duo) was employed, with a pulse duration and energy of 130 ns and 43 mJ, respectively. The 526.5 nm beam was focused by an f = 150 mm cylindrical lens into a vertical line (0.3 mm thick), which spanned the entire 7 mm channel height

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Table 1 Experimental parameters.a Case

p (bar)

TIN (K)

u

UIN (m/s)

CO (vol.%)

H2 (vol.%)

CO2 (vol.%)

H2:CO

xig,exp (mm)

xig,sim (mm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

2 2 2 4 4 4 8 8 8 8 12 12 12 12 14 14 14 14

441 443 444 558 557 555 725 716 703 660 719 736 732 651 399 400 401 652

0.29 0.30 0.30 0.20 0.19 0.21 0.18 0.18 0.18 0.17 0.19 0.19 0.19 0.20 0.25 0.25 0.25 0.18

3.23 3.07 2.87 3.03 2.97 2.97 2.14 2.10 2.02 1.97 1.70 1.58 1.54 1.43 0.72 0.72 0.72 1.54

1.54 2.49 4.53 1.18 1.94 3.14 1.38 2.16 3.34 4.21 1.39 1.93 3.32 4.46 5.50 3.84 2.18 2.42

7.03 6.20 4.61 4.73 3.97 3.16 4.91 4.37 3.36 2.14 5.38 4.62 3.37 2.28 2.60 4.28 5.96 4.03

13.86 12.88 9.88 15.11 14.36 14.27 12.96 12.03 9.86 12.67 14.95 10.18 9.89 11.43 15.37 15.40 15.39 11.71

4.54 2.50 1.02 4.00 2.04 1.00 3.57 2.04 1.01 0.51 3.84 2.38 1.01 0.51 0.47 1.11 2.70 1.67

44 48 48 105 110 114 85 85 89 128 85 85 92 129 – – – 146

46 50 50 98 97 100 90 89 90 114 91 92 95 114 – – – 132

a Pressure, inlet temperature, equivalence ratio u, CO, H2 and CO2 vol.% content at reactor inlet, H2:CO volumetric ratio, experimental (xig,exp) and simulated (xig,sim) homogeneous ignition distances. Oxygen can be deduced from u, while the balance is N2.

The Raman spectrometer slit was set to 200 lm, yielding a 500 lm separation of the CO and N2 signals at the detector plane. This resulted in good light gathering power while keeping crosstalk below 5%, thus allowing simple subtraction of the CO/N2 overlap when determining the weaker CO signal. The image intensifier employed a GaAsP-Gen3 photocathode, which yielded quantum efficiencies above 35% for N2 and all other molecules with lower Raman shift, while maintaining quantum efficiency above 20% for the H2 signal. Combining the factors of Raman cross section, which was much larger for H2 than for N2 (by a factor of 3.86), with the frequency factor (m3 for quantum detectors) and the quantum efficiency of the photocathode (0.6), a hydrogen sensitivity clearly above that of nitrogen was calculated. Nevertheless, the signal of hydrogen remained modest with the noise being dominated by detection shot noise. The standard deviation of the hydrogen signal was 4% or less of its maximum reported value. Measurement accuracy was estimated to be ±3% for species compositions P3 vol.% and ±8% for compositions as low as 0.5 vol.%; values less than 0.5 vol.% entailed larger inaccuracies. Raman data closer than 0.7 mm to both catalyst surfaces were discarded due to low signal-to-noise ratios. 3. Numerical simulation

Fig. 2. Optical layouts for the planar OH-LIF and the 1-D Raman measurements. All distances and focal lengths are in mm.

and was moderately offset in the lateral direction (z = 15 mm) to increase the collection angle and minimize thermal beam steering, as in [16–18]. Two f = 300 mm lenses collected the scattered light at 50° with respect to the sending optical path, and focused it to the entrance slit of a 25 cm spectrograph (Chromex-250i) equipped with an intensified CCD camera (Princeton Instruments PI-MAX1024GIII). As the operating conditions were steady, Raman signals of up to 200,000 pulses were integrated on the detector chip. The laser repetition rate, the number of integrated pulses, and the temporal width of the camera intensifier gate have been chosen to optimize the signal-to-noise ratio. Data were acquired for x > 10 mm, with emphasis on the reactor length preceding the onset of homogeneous ignition, by traversing an optical table supporting the sending and collecting optics, including the Nd:YLF laser (Fig. 2).

The flow was simulated with a 2-D steady elliptic CFD code (details have been provided in [15–18,26]). The 300  7 mm2 (in x and y, respectively) reactor domain was discretized with 680  100 grid points. The governing equations in 2-D Cartesian coordinates for a steady laminar channel-flow with heterogeneous and homogeneous reactions have been provided elsewhere [16,18] and are not repeated here. Temperature, velocity and species compositions were uniform (see Table 1) at the reactor entry (x = 0). The interfacial boundary conditions for gas-phase species and temperature at the lower and upper catalytic walls (y = 0 and y = 2b) were:

ðqY k V k;y Þy¼0 ¼ W k ðs_ k Þy¼0 ;

ðqY k V k;y Þy¼2b ¼ W k ðs_ k Þy¼2b

ð1Þ

and

Tðx; y ¼ 0Þ ¼ T W;L ðxÞ;

Tðx; y ¼ 2bÞ ¼ T W;U ðxÞ;

ð2Þ

respectively. TW,U (x) and TW,L (x) were the temperature profiles of the upper and lower wall, respectively, fitted through the 12 thermocouple measurements of each plate; q was the gas density; Wk, s_ k and Vk,x, Vk,y were the molecular weight, catalytic molar pro-

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duction rate and diffusion velocity components of the kth gaseous species, respectively. Mixture-average diffusion including thermal diffusion for the light species H2 and H was used to model Vk,x, Vk,y [27]. No-slip was applied for both velocity components at the gas-wall interfaces. At the outflow (x = L) the transverse velocity was set to zero and zero-Neumann conditions were used for all other scalars. For surface chemistry the CO/H2 scheme on Pt from Deutschmann et al. [28] has been used, augmented with HCOO(s) reactions from [29]. The resulting syngas heterogeneous reaction mechanism (shown in Table A1 of Appendix A) has been validated in our recent [30] pure heterogeneous kinetic syngas studies. Finally, the formulation in Dogwiler et al. [31] with a modified Motz-Wise correction was employed for the adsorption rate constant kad,k of the kth gaseous species:





ck 1 kad;k ¼ 1  ck hPt =2 Cm

sffiffiffiffiffiffiffiffiffiffiffiffiffiffi RT ; 2p W k

ð3Þ

where ck and Wk are the sticking coefficient and molecular weight of the kth gaseous species, respectively, hPt the Pt surface coverage, C = 2.7  109 mol/cm2 the total surface site density, m the sum of surface reactants’ stoichiometric coefficients, R the universal gas constant, and T the surface temperature. The gas-phase mechanism for H2/CO consisted of 36 reversible reactions involving 13 species and is shown in Table A2 of Appendix A. Reactions for CO (R24–R36) were modeled with the elementary CO/HCO reaction subset in Li et al. [32] while H2 reactions (R1– R23) were taken from the latest H2/O2 kinetic model of Burke et al. [33]. Species thermodynamic data were also provided in [32,33]. Surface and gaseous reaction rates were evaluated with CHEMKIN packages [34,35], while for species transport the CHEMKIN database [27] was used. 4. Results and discussion 4.1. Comparisons between measurements and predictions Mixture preheats (TIN) up to 736 K were obtained (Table 1); with increasing preheat, the fuel-to-air equivalence ratios were generally reduced so as to avoid autoignition inside the static mixers and/or flame flashback towards the flow straightening section (Fig. 1a). To this purpose, equivalence ratios in the range 0.17 6 u 6 0.30 were investigated. Measured and predicted 2-D OH distributions over the entire 300  7 mm2 reactor domain are presented in Fig. 3 for all cases in Table 1. The measured OH images in Fig. 3 were constructed by connecting 100-mm-long overlapping LIF images recorded at different positions, as stated in Section 2.2. The established flames were open, i.e. comprised two separate branches extending nearly parallel to the catalytic walls, as also observed in earlier pure hydrogen homogeneous combustion studies over Pt [16,18]. This was in direct contrast to earlier fuel-lean methane [15] and propane [36] studies, whereby closed flames were formed in the same reactor. As discussed in [16], this was predominantly an outcome of the diffusional imbalance of hydrogen (LeH2 < 1): the transport of heat from the hot walls to the flowing gas was less effective than the transport of hydrogen from the channel core towards the walls, thus confining the flames in the near-wall regions. The OH-LIF data in Fig. 3 clearly indicated that the presence of CO, at the specified compositions in Table 1, did not alter this specific feature of hydrogen flames. The ignition positions (xig) marked with green arrows in Fig. 3 have been defined in both measurements and predictions as the far-upstream locations whereby the OH concentration reached 5% of its maximum value over the entire reactor domain 300  7 mm2. OH concentrations dropped substantially with rising

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pressure; from 290 ppmv at 2 bar to 1.6 ppmv at 12 bar and finally to sub-ppmv values at 14 bar (Fig. 3). Modest temperature differences between the upper and lower catalytic walls led to a slight asymmetry between the two flame branches (see Fig. 3 and the provided wall temperature profiles in Figs. 4 and 5); in all cases the upper walls were hotter by as much as 50 K, such that homogeneous ignition was always defined at the upper wall. The measured homogeneous ignition distances were well reproduced by the model for all cases with p 6 12 bar (whereby flames were established in the reactor), with the differences between measured and predicted ignition distances being less than 12% (see Table 1). It is pointed out that homogeneous ignition at p = 14 bar could not be determined with the LIF data, as these cases pertained to subppmv OH concentrations, which were not amenable to the planar LIF technique. Nonetheless, quantitative agreement at p = 14 bar was not of great concern since at this pressure gas-phase combustion was minimal, as will be elaborated below. Good agreement between measurements and predictions was thus established not only for the homogeneous ignition distances but also for the ensuing flame shapes and lengths. Such comparisons clearly demonstrated the aptness of the employed hetero-/homogeneous reaction mechanisms for syngas mixtures at high pressures and preheats relevant to gas turbine applications. Predicted axial profiles of catalytic (C) and gaseous (G) conversion rates of H2 and CO, along with the y-averaged (over the 7 mm channel height) mass fractions of these two species are shown in Fig. 4 for the p = 2 and 4 bar cases, in Fig. 5 for the 8 and 12 bar cases and finally in Fig. 6 for the 14 bar cases. The C rates in Figs. 4–6 accounted for the catalytic conversion on both channel surfaces, while the G rates were calculated by integrating the volumetric gaseous H2 and CO conversions over the 7 mm channel height. Furthermore, to better compare the extent of catalytic (C) and gas-phase (G) conversions between cases at different operating conditions (equivalence ratios, mass flow rates), the conversion rates in Figs. 4–6 have been normalized by the corresponding inlet H2 and CO mass fluxes of each case i.e. qINUINYH2,IN and qINUINYCO,IN, respectively. As evidenced in Figs. 4 and 5, over the extent of the gaseous induction zone (x < xig) the contribution of the homogeneous reaction pathway was minor compared to that of the heterogeneous pathway. The normalized C conversion rates of H2 were, irrespective of the particular H2:CO ratio, always higher than those of CO over the length x < xig (see Figs. 4–6). This was attributed to the higher molecular diffusivity of hydrogen and not to its higher catalytic reactivity, since the catalytic conversions of both fuel components were, in all examined cases, mass transport limited (this will be elaborated in the forthcoming Fig. 7). At distances x P xig, the rise of homogeneous conversion (G) progressively suppressed the heterogeneous conversion (C) by depleting both H2 and CO fuel components before they could diffuse from the channel core to the catalytic walls. The catalytic suppression efficiency was thus directly linked to the gas-phase oxidation rates, which were in turn intricately dependent on pressure, fuel composition (H2:CO ratio) and preheat, issues that will be addressed in Section 4.2. In contrast to the nearly complete suppression of C conversions well-downstream of the homogeneous ignition location at 2 bar (Fig. 4a), there was an increased H2 and CO leakage through the flame zones towards the catalyst surfaces at p P 4 bar, as manifested by the non-vanishing C rates at x > xig in Figs. 4b and 5a and b. This led to combined heterogeneous and homogeneous conversion of both H2 and CO. Moreover, the C conversions after dropping monotonically with increasing axial distance, started rising at the rear channel section (x > 200 mm) as a result of the diminishing G conversions (see Figs. 4b and 5a and b). The results in Figs. 4 and 5 indicated that in order to enhance homogeneous combustion at elevated pressures and hence to suppress catalytic conversion at x > xig, increased mixture preheats

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Fig. 3. (a) LIF-measured and (b) predicted 2-D maps of the OH radical for all cases in Table 1. The vertical arrows denote the onset of homogeneous ignition and the color bars provide the computed OH in ppmv.

Fig. 4. Computed axial profiles of catalytic (C) and gas-phase (G) conversion rates for H2 (solid lines) and CO (dashed lines) normalized by the corresponding inlet species mass fluxes; computed average (over the 7 mm channel height) axial profiles of the H2 and CO mass fractions (Y) normalized by the corresponding inlet mass fractions; thermocouple measurements (upper wall: squares; lower wall: circles) and fitted axial temperature profiles through the measurements (lines). The vertical arrows define the computed location of homogeneous ignition (xig). Cases 1–6 in Table 1.

were necessary. As evidenced in Figs. 3 and 4a, low preheats (<450 K) were already sufficient to establish strong flames at 2 bar, whereas at 8 and 12 bar lower gas-phase conversion rates were obtained even with much higher preheats (>600 K), as shown

by the G curves in Fig. 5a and b. At 14 bar (Fig. 6), gas-phase reactions were fully suppressed at low preheats and could only be marginally increased at 654 K (Case 18). Moreover, when transitioning (for a given pressure) from H2-rich fuels to CO-rich fuels, a stronger

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Fig. 5. Computed axial profiles of catalytic (C) and gas-phase (G) conversion rates for H2 (solid lines) and CO (dashed lines) normalized by the corresponding inlet species mass fluxes; computed average (over the 7 mm channel height) axial profiles of the H2 and CO mass fractions (Y) normalized by the corresponding inlet mass fractions; thermocouple measurements (upper wall: squares; lower wall: circles) and fitted axial temperature profiles through the measurements (lines). The vertical arrows define the computed location of homogeneous ignition (xig). Cases 7–14 in Table 1.

Fig. 7. Predicted (lines) and Raman-measured (symbols) transverse profiles of H2 and CO mole fractions at three selected axial positions: (a–c) Case 4 and (d–f) Case 14 in Table 1. H2 (solid lines, triangles); CO (dashed-lines, circles).

Fig. 6. Computed axial profiles of catalytic (C) and gas-phase (G) conversion rates for H2 (solid lines) and CO (dashed lines) normalized by the corresponding inlet species mass fluxes; computed average (over the 7 mm channel height) axial profiles of the H2 and CO mass fractions (Y) normalized by the corresponding inlet mass fractions; thermocouple measurements (upper wall: squares; lower wall: circles) and fitted axial temperature profiles through the measurements (lines). The vertical arrows define the computed location of homogeneous ignition (xig). Cases 15–18 (p = 14 bar) in Table 1.

quenching of homogeneous reactions was observed (e.g., compare in Fig. 5a Cases 7 and 10 and in Fig. 5b Cases 11 and 14). As a result, the increased fuel leakage through the weak gaseous combustion

zones for Cases 10 and 14 (H2:CO ratio 0.51) led to an appreciable rise in catalytic conversion (by 10% for H2 and 20% for CO) when compared to the H2-richer Cases 7–9 and 11–13, respectively. However, this composition effect was negligible for H2:CO P 1.0 as seen by comparing Cases 7–9 or Cases 11–13. To assess the impact of pressure, preheat, and fuel composition on homogeneous chemistry, it was essential to ensure that the underlying catalytic processes over the gaseous induction zones x < xig were also well-captured by the heterogeneous kinetic model. This was a cardinal requirement, since an incorrect prediction of the catalytic H2 and CO conversions over x 6 xig could greatly affect the location of homogeneous ignition and hence falsify the gaseous kinetics [37]. To this direction, the Raman measurements ensured

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that the catalytic processes preceding homogeneous ignition were also well predicted. Raman-measured and computed transverse profiles of H2 and CO mole fractions are compared in Fig. 7 for two cases at 4 and 12 bar and for three axial positions preceding the onset of homogeneous ignition. The predictions in Fig. 7 yielded a catalytic conversion close to the transport-limit for both H2 and CO, as evidenced by the practically vanishing concentrations of these species near both walls (y = 0 and 7 mm). This was also well reproduced by the measurements, despite the lack of Raman data closer than 0.7 mm to the catalytic walls. It is finally noted that the surface temperatures needed to achieve homogeneous ignition (see Figs. 4 and 5) were much higher than the transition temperature of around 750 K, below which the heterogeneous chemistry coupling between H2 and CO was strong and could lead to CO poisoning of the catalyst [21,30]. Thus, the high surface temperatures in all examined cases greatly facilitated the present homogeneous ignition studies, as the performance of catalytic kinetic models at surface temperatures T < 750 K was quite demanding. Additional manifestation for the absence of CO surface poisoning is illustrated in the surface coverage plots in Fig. 8, for three cases at 8 bar at different H2:CO ratios. Main coverage is O(s) and Pt(s). With rising CO content, CO(s) increased; however, both CO(s) and H(s) were several orders of magnitude lower than O(s) and Pt(s). This indicated that the catalytic reactions of both H2 and CO were fully lit-off over the entire reactor length. 4.2. Homogeneous combustion in confined channels The conditions in Table 1 and hence the results in Figs. 3–6 pertained to different operating parameters (mass throughputs, equivalence ratios and surface temperatures). These differences were essential to stabilize flames inside the channel reactor at conditions of broadly varying mixture preheats and pressures, without the danger of autoignition and/or flashback. Nonetheless, to clarify the coupling of heterogeneous and homogeneous kinetics and the impact of homogeneous chemistry and transport on gas-phase oxida-

Fig. 8. Surface coverage at 8 bar (Cases 7, 9 and 10 in Table 1). Solid lines: upper wall; dashed lines: lower wall.

tion over Pt, a comparative study was necessary between cases where only a limited number of parameters changed while the remaining were maintained constant. Consequently, detailed simulations have been performed using the validated hetero-/homogeneous chemistries of Section 4.1, in channel geometries relevant to practical honeycomb reactors. A cylindrical channel was used in the following simulations, with diameter d = 1.0 mm and length of 200 mm (sufficient to achieve at least 99.5% H2 and CO conversion for all examined conditions). Pressures in the range 1–15 bar were investigated, with inlet gas temperatures of 673 and 773 K, a wall temperature of 1350 K, and a constant mass flux of 42.4 kg/m2 s. The latter corresponded to a gas velocity of 5.5-5.9 m/s (depending on the H2:CO ratio) at p = 15 bar and TIN = 673 K. The selected inlet temperature and mass flux were relevant to gas turbine operating conditions, while the surface temperature of 1350 K was typically the highest tolerable by reactor materials and catalyst for long-term operation. Predicted streamwise profiles of catalytic (C) and gaseous (G) conversion rates for both fuel components are illustrated in Fig. 9 at pressures of 1, 4, 8 and 15 bar, equivalence ratio u = 0.3, preheat TIN = 673 K and H2:CO molar ratios varying from 0.25 to 4.0. Simulations with the higher investigated inlet temperature TIN = 773 K gave the same qualitative trends and hence are not presented. Ignition distances (xig) were determined via the OH radical concentration according to the procedure outlined in Section 4.1. As shown in Fig. 9, the ignition distances (xig) decreased with rising pressure. At p = 1 bar, the G conversions were very weak, however, a homogeneous ignition could still be defined. At pressures p P 4 bar, the G conversions became stronger (although their magnitudes did not increase monotonically with rising pressure) and ignition occurred close to the reactor entry such that the resulting xig were nearly the same. With rising CO content, homogeneous ignition was delayed, as seen by comparing the xig in Fig. 9a–c. Moreover, the OH-defined

Fig. 9. Predicted catalytic (C) and gas-phase (G) H2 and CO conversion rates in a cylindrical channel with diameter 1 mm, mass flux of 42.4 kg/m2 s, fuel H2:CO ratios of 4:1, 1:1 and 1:4, inlet temperature TIN = 673 K and equivalence ratio u = 0.3. Results are shown for pressures of 1, 4, 8 and 15 bar.

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xig always corresponded to ignition of the hydrogen component (Fig. 9); CO ignition followed that of H2, as the gas-phase oxidation of CO crucially depended on radicals – notably OH produced by H2 chemistry. However, the delay between the two sequential H2 and CO ignitions changed with pressure as well as fuel composition. For an H2:CO ratio of 1:4, the H2 and CO ignitions at p P 4 bar were nearly simultaneous (Fig. 9c). It was further evident that, for both fuel components, catalytic and gas-phase conversions occurred in parallel. Even well-downstream the location of homogeneous ignition, catalytic conversion persisted: H2 and CO leaked through the gaseous combustion zone to be subsequently converted on the catalyst surface. This effect was more pronounced compared to the optically accessible channel results in Section 4.1 (Figs. 4–6), due to the larger surface-tovolume ratio of the present 1 mm diameter channel. Following homogeneous ignition, the axial extent of the established flames, as indicated by the G conversion rate profiles in Fig. 9, exhibited strong dependence on pressure and H2:CO ratio. The peak gas-phase oxidation rate was a non-monotonic function of pressure, being highest at 4 bar and dropping significantly at higher pressures; moreover, this happened despite the fact that, for a given H2:CO ratio, the homogeneous ignition locations (xig) were nearly the same for all pressures p P 4 bar (see Fig. 9). The magnitude of homogeneous oxidation rates at p > 4 bar, was also dependent on fuel composition. For the H2-rich fuels (H2:CO ratio of 4:1), gas-phase oxidation rates were only slightly lower at 8 bar when compared to 15 bar (Fig. 9a). When reducing the H2:CO ratio to 1:1 (Fig. 9b), the CO gas-phase oxidation rate for 8 bar surpassed the corresponding one for 15 bar at x  50 mm; farther downstream, the peak CO oxidation rate for 8 bar became about twice the one for 15 bar. Additional decrease of the H2 content (H2:CO ratio of 1:4) shifted the gas-phase conversion rates at 8 bar towards those at 4 bar, and intensified the magnitude difference between the oxidation rates at 8 and 15 bar (Fig. 9c). In addition to the conversion profiles in Fig. 9, 2-D distributions of H2 and CO mole fractions and isocontours of H2 and CO gasphase reaction rates for pressures 4 and 15 bar are illustrated in Figs. 10 and 11, respectively. Seven contours are provided with the outermost one corresponding to 10% of the maximum reaction rate of each case. As evidenced by the reaction rate isocontours, the H2 gaseous oxidation zones were confined close to the catalytic plates, while the corresponding CO zones extended from the catalytic walls to the centerline, occupying the full radial reactor domain. The mismatch in the extent of H2 and CO inner reaction rate isocontours (corresponding to the higher reaction rates) in Figs. 10b and c and 11c led to the occurrence of two peaks in the CO gas-phase conversion plots for H2:CO = 1:1 and 1:4 in Fig. 9b and c. The first peaks of CO gas-phase oxidation curves coincided with those of the H2 curves, indicating that H2 reactions promoted CO oxidation in these regions. As also stated in Section 4.1, the confinement of the H2 oxidation zone near the walls was largely an outcome of the heat and mass diffusion imbalance of H2 (LeH2 < 1) [16,18]. However, at 4 bar the H2 reaction rates extended to a larger degree away from the wall compared to the 15 bar cases (see Figs. 10 and 11). As will be explained in the forthcoming Section 4.3.2, this was a result of the corresponding higher burning rates at 4 bar compared to those at 15 bar: the 4 bar flames could therefore stabilize farther away from the wall towards the channel center, since the radial mass fluxes supplying reactants to the flames were higher there (directly linked to the higher magnitude of the H2 reaction rates, as provided in the captions of Figs. 10 and 11). Finally, the reduced reaction rates at high pressures (p > 4 bar) required a considerably longer reaction zone to convert the same fuel mass throughputs. The mole fraction maps in Figs. 10 and 11 indicated that hydrogen was depleted within the first quarter of the reactor due to its

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Fig. 10. Computed 2-D color maps of the H2 and CO mole fractions, isocontours of H2, CO gas-phase reaction rates for a u = 0.30 H2/CO/air mixtures in a d = 1 mm cylinder reactor at p = 4 bar, a mass flux of 42.4 kg/m2 s, constant wall temperature Twall = 1350 K and inlet temperature TIN = 673 K; r = 0 denotes the channel centerline and r = 0.5 mm the channel wall. (a) H2:CO = 4:1, (b) H2:CO = 1:1 and (c) H2:CO = 1:4. The maximum gas-phase H2 consumption rates are 3.3  103, 1.9  103 and 4.2  104 kmol/m3 s in (a) to (c), respectively. The outermost contours refer to lower reaction rates; the increment between successive contours is 4.0  104, 2.3  104 and 4.5  105 kmol/m3 s in (a) to (c), respectively. The maximum gas-phase CO consumption rates are 3.4  104, 7.8  104 and 1.2  103 kmol/m3 s in (a) to (c), respectively; the increment between successive contours is 4.3  105, 9.7  105 and 1.5  104 kmol/m3 s in (a) to (c), respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

high catalytic conversion (driven by its high molecular diffusivity since its consumption was mass transport limited) and also due to its high gas-phase reactivity. The strong coupling of CO and H2 gaseous kinetics yielded significant CO oxidation over the same reactor extent. A combined catalytic and gas-phase reaction flux diagram at x = 22 mm is shown in Fig. 12a for the case in Fig. 10b; therein, normalized reaction rates are provided for the major steps. Reaction fluxes at x = 78 mm, far downstream of the complete H2 consumption, are shown for the same case in Fig. 12b. At this position, the product H2O was crucially involved in the reaction sequence to sustain the radical pool and in particular the OH radical, which controlled the CO gas-phase oxidation. Catalytic oxidation was found to be much more efficient, as seen by comparing the relative magnitudes of the H2 and CO adsorption rates to the corresponding gaseous consumption rates in Fig. 12 (see also Fig. 9). The impact of pressure and H2:CO composition on the rich homogeneous oxidation characteristics seen in Fig. 9 will be elaborated in Section 4.3. The coupling of heterogeneous and homogeneous kinetics via competitive reactant depletion, radical adsorption/desorption reactions, and major species formation is finally discussed. Previous work [18] has identified the key interactions of the two reac-

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Fig. 11. Computed 2-D color maps of the H2 and CO mole fractions, isocontours of H2, CO gas-phase reaction rates for a u = 0.30 H2/CO/air mixtures in a d = 1 mm cylinder reactor at p = 15 bar, a mass flux of 42.4 kg/m2 s, constant wall temperature Twall = 1350 K and inlet temperature TIN = 673 K; r = 0 denotes the channel centerline and r = 0.5 mm the channel wall. (a) H2:CO = 4:1, (b) H2:CO = 1:1 and (c) H2:CO = 1:4. The maximum gas-phase H2 consumption rates are 1.8  103, 1.2  103 and 4  104 kmol/m3 s in (a) to (c), respectively. The outermost contours refer to lower reaction rates; the increment between successive contours is 1.8  104, 1.5  104 and 4.9  105 kmol/m3 s in (a) to (c), respectively. The maximum gas-phase CO consumption rates are 1.2  104, 2.8  104 and 3.8  104 kmol/m3 s in (a) to (c), respectively; the increment between successive contours is 1.2  105, 3.5  105 and 3.4  105 kmol/m3 s in (a) to (c), respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

tion pathways for hydrogen fuel. These interactions included competitive fuel consumption and adsorption/desorption of minor and major species. Regarding the effects of competitive reactant depletion, the delayed homogeneous ignition at atmospheric pressure led to higher heterogeneous consumption that in turn strongly inhibited homogeneous oxidation, as evidenced in Fig. 9. Therein, the catalytic (C) consumption rates at 1 bar were always the highest among all cases and the corresponding homogeneous (G) consumptions were largely vanished. At p P 4 bar, gas-phase ignition occurred near the reactor entry. Such a substantial decrease of the ignition distance resulted in flames established far upstream, thus favoring homogeneous fuel consumption and attenuating the inhibiting effect of catalytic fuel consumption. The radical hetero-/homogeneous coupling was modest and could be clarified with the traverse OH profiles in Fig. 13 at 1 and 4 bar for an H2:CO molar ratio of 1:1. At atmospheric pressure, the very weak gaseous oxidation zone was aided through the OH flux from the surface, as shown by the positive OH transverse gradient on the wall (net-desorptive OH fluxes). With the formation of strong gaseous combustion zones at 4 bar, the OH flux was still net-desorptive over the gaseous induction zone (x < xig), but turned to net-adsorptive following homogeneous ignition. Comparison of the predictions in Fig. 9 to additional simulations without the inclusion of OH adsorption/desorption steps demonstrated that OH produced catalytically over the gaseous induction zone promoted (albeit to a small degree) homogeneous ignition under all examined conditions. On the other hand, the post-ignition flame

Fig. 12. Hetero-/homogeneous reaction fluxes for a u = 0.30 H2/CO/air (H2:CO molar ratio of 1:1) mixtures in a d = 1 mm cylinder reactor at p = 4 bar, mass flux of 42.4 kg/m2 s, constant wall temperature Twall = 1350 K and inlet temperature TIN = 673 K (conditions as in Fig. 10b). (a) x = 22 mm and (b) x = 78 mm. The reaction fluxes are normalized with respect to the rates of H2 þ OH () H2 O þ H and CO þ OH () CO2 þ H in (a) and (b), respectively.

Fig. 13. Computed traverse profiles of OH mole fraction at various axial positions for a u = 0.30 H2/CO/air (H2:CO = 1:1) mixture in a d = 1 mm cylindrical channel with a mass flux of 42.4 kg/m2 s, constant wall temperature Twall = 1350 K, inlet temperature TIN = 673 K and two pressures (p = 1 and 4 bar); r = 0 denotes the channel centerline and r = 0.5 mm the channel wall. For clarity, the OH profiles at 1 bar have been multiplied by 5.

propagation was inhibited at the axial locations where the OH flux was net adsorptive. In terms of homogeneous ignition, the OH coupling effects were the strongest at 1 bar (whereby gaseous com-

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bustion was the weakest) and drastically decreased with rising pressure. Characteristically, at 1 bar the removal of OH adsorption/desorption reactions increased xig by 50% while it marginally increased xig for pressures p P 2 bar. Moreover, the aforementioned effects were only weakly dependent on H2:CO ratio. Finally, simulations have shown that the effect of other radicals (O and H) was much less crucial when compared to that of OH. The impact of the major species H2O and CO2, produced from the catalytic pathway, on the gaseous oxidation was finally assessed. Simulations were carried out at two different H2:CO ratios (4:1 and 1:4) by replacing the catalytically-produced H2O or CO2 with the fictitious species H2 O and CO2 that had the same thermodynamic and transport properties as H2O and CO2, respectively, but did not participate in any gas-phase reaction. The gaseous pathway was still allowed to produce normal H2O and CO2. Comparisons at p = 4 bar with predictions from the original catalytic reaction scheme are illustrated in Fig. 14a and b regarding the effect of H2 O and in Fig. 14c and d regarding CO2 . The homogeneous processes in Fig. 14a and b were strongly affected by H2 O and, moreover, they exhibited a complex dependence on H2:CO ratio. On the other hand, the homogeneous processes in Fig. 14c and d were significantly less affected by CO2 . The gaseous oxidation of both H2 and CO was substantially suppressed by catalytically-produced H2O for H2:CO = 4:1 (Fig. 14a). However, for H2:CO = 1:4 the gaseous H2 oxidation was marginally affected while the corresponding CO oxidation was significantly promoted (Fig. 14b). The kinetic effect of H2O was a result of the competition between the radical termination reaction R11 (due to its high third body efficiency, xH2O = 14, see footnote in Table A2) and the radical branching step R5 (the net of this reaction proceeded in the reverse direction). Reaction flux analysis at elevated pressures p P 4 bar indicated that, for sufficiently large H2:CO ratios, the catalytic production of copious amounts of H2O rendered R11 dominant in the aforementioned reaction competition and dramatically reduced the radical pool. Thus, the oxidation of both fuel components (Fig. 14a) was inhibited. On the other hand, for CO-rich mixtures (H2:CO = 1:4) the enhancement on the radical termination step R11 was modest due to significant reduction of catalytically-produced H2O and the branching step R5 increased in significance: this resulted in a net OH radical production which was higher for the case of normal H2O compared to H2 O . The en-

Fig. 14. Comparison of predicted gas-phase H2 and CO conversion rates between the original catalytic reaction scheme (solid lines) and the modified catalytic reaction scheme (dotted lines) with fictitious species H2 O in (a) and (b), and CO2 in (c) and (d). Computations in a cylindrical channel with diameter 1 mm, mass flux of 42.4 kg/m2 s, H2:CO ratios of 4:1 and 1:4, inlet temperature TIN = 673 K, pressure 4 bar and equivalence ratio u = 0.3.

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hanced OH production, in turn, substantially promoted the CO oxidation (Fig. 14b). However, H2 consumption was marginally affected (Fig. 14b); this was because the increase in the forward rate of the main hydrogen consumption reaction R4 was balanced by a corresponding increase in the reverse rate due to the presence of higher amounts of H2O. The chemical impact of CO2 was only inhibitive for the homogeneous oxidation of both H2 and CO (Fig. 14c and d) due to its enhanced third body efficiency in R11 (xCO2 = 3.8); this effect was obviously stronger for higher CO contents in the fuel mixture (Fig. 14d). Finally, it could be shown that the pressure dependence of the H2O and CO2 effects discussed above was weak. 4.3. Homogeneous combustion characteristics The foregoing discussion in Section 4.2 and specifically in Figs. 8–10 has shown an intricate dependence of the homogeneous ignition characteristics primarily on pressure but also on H2:CO ratio. To clarify these effects, simulations have been performed in this section using as numerical platform ideal reactors. 4.3.1. Ignition delays At first, pure gas-phase ignition characteristics (without the inclusion of catalytic reactions) of syngas fuels were investigated. Ignition delays, defined as the inflection points in the temperature–time histories, have been calculated in a spatially homogeneous constant pressure reactor (batch reactor) using the Senkin package of CHEMKIN [38]. Inverse ignition delays (quantities proportional to the gaseous reactivities) are plotted in Fig. 15 for pressures 1–15 bar, u of 0.3 and 0.2, initial temperatures To = 950–1350 K, and H2:CO molar ratios of 4, 2, 0.5 and 0.25. The selected initial temperature range mimicked the gas preheating and heat transfer from the hot catalytic walls occurring in practical channel reactors. Similar to previous findings on pure H2 fuel [18], the syngas inverse ignition delays (reactivities) followed a nonmonotonic relationship with pressure, possessing a critical pressure (pcr) where the maximum reactivities occurred. The critical pressure was a function of the initial gas temperature (the higher the initial temperature, the larger the pcr value) and had a weaker dependence on fuel composition. Changing the initial gas temperature to 1350 K and 950 K shifted the critical pressure pcr beyond 15 bar and below 1 bar, respectively (i.e. outside of the pressure range of interest for this study). The profiles in Fig. 15 pertaining to the highest initial temperatures (e.g., To = 1250 K or 1350 K) were appropriate to explain the impact of pressure on the ignition distances (the xig exhibited similar characteristics as the ignition delays) in the channel of Fig. 9, which had a constant wall temperature of 1350 K. At p = 1 bar, the gaseous conversion (G) was practically suppressed in Fig. 9, since the low gas-phase reactivities (elongated ignition delays) at this pressure (see Fig. 15) allowed for appreciable H2 and CO catalytic conversion that in turn deprived fuel from the homogeneous pathway thus inhibiting gaseous combustion. However, as the pressure increased, the shorter ignition delays (see Fig. 15) allowed for appreciable homogeneous fuel conversion as the gaseous pathway could now compete more favorably against the catalytic pathway for fuel consumption. Nonetheless, the particular nonmonotonic behavior of the gaseous conversions with rising pressure (significant drop in G above p = 4 bar, see Fig. 9) could not be explained with the ignition characteristics in Fig. 15; this was because the ignition delays either decreased (at 1350 K) or at least changed modestly (at 1250 K) with rising pressure at p P 4 bar. To explain the G drop at p P 4 bar in Fig. 9, arguments based on flame propagation characteristics will be brought about in the following Section 4.3.2.

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Fig. 15. Inverse ignition delays of CO/H2/air mixtures in a spatially homogeneous isobaric batch reactor with the total equivalence ratio u = 0.3 (solid lines) and u = 0.2 (dashed lines), initial temperatures ranging between 950 and 1350 K and pressures of 1–15 bar.

The profiles in Fig. 15 at moderate temperatures (61150 K) were suitable for the interpretation of the experimental findings in Figs. 4–6, as the plate temperatures and preheats in the optically accessible reactor were considerably lower than those used in the tubular reactor simulation. In such moderate temperature cases, the change of the critical pressures towards the lower end of the 1–15 bar range (Fig. 15) resulted in reduced reactivity (increased ignition delays) at high pressures; this could in turn explain the lack of vigorous flames at p = 14 bar seen in Figs. 3 and 6. The observed ignition characteristics in Fig. 15 at low to modest temperatures were an outcome of the competition between the radical branching and termination steps (R1, R11, R18 and R20 in Table A2) as clarified in [18]. The CO-involved radical reactions (R24 and R26) were secondary in affecting ignition characteristics when compared to H2 reactions. Moreover, the CO reactions relied on H2 chemistry to provide necessary radicals O and HO2. Therefore, a rising CO content resulted in decreased reactivity of the CO/H2 mixtures, thus explaining the predicted effects on homogeneous ignition for mixtures with different H2:CO compositions in Fig. 9.

4.3.2. Mass burning rates Laminar burning rates have been computed using the 1-D premix flame code of CHEMKIN [39]. Results are illustrated in Fig. 16 for equivalence ratios 0.2–0.3, H2:CO molar ratios of 4:1–1:4, pressures of 1–15 bar, and a fresh gas temperature To = 673 K. The selected equivalence ratio range and initial gas temperature were suitable for simulating the tubular reactor of Section 4.2 wherein flames were stabilized close to the entry (see Fig. 9) and nearly parallel to the walls via supply of fresh unburned gas from the channel core at 673 K preheat. As shown in Fig. 16, the mass burning rates firstly increased with rising pressure, peaked at a transition pressure ptr, then decreased with increasing pressure, and finally became modestly dependent on pressure. Peak burning rates of H2-rich fuels are always higher than those of CO-rich fuels: the peak mass burning rate for H2:CO = 4:1 is the highest and decreases with dropping H2:CO ratio for each equivalence ratio. Moreover, the transition pressure ptr for H2:CO = 4:1 is the lowest and a reduction in H2:CO ratio drives ptr to higher values. Increasing the pressure above the transition point leads to burning rates of CO-rich fuels crossing over those of H2-rich fuels due to the combination of higher ptr and weaker pressure dependence of CO rich fuels than those of H2 rich fuels. For instance, the mass burning rate for H2:CO = 1:4 (u = 0.25) at 15 bar is the highest among all examined H2:CO molar ratios at this pressure, as evidenced in

Fig. 16. Computed laminar mass burning rates plotted as a function of pressure for 1-D freely propagating H2/CO/air flames with H2:CO molar ratios ranging from 4:1 to 1:4. The total equivalence ratios are u = 0.30, 0.25 and 0.20 and the fresh mixture temperature is To = 673 K.

Fig. 16b. A rise of the equivalence ratio to 0.3 increased the absolute mass burning rates and shifted the transition pressure and the crossover points to higher values or even above the present upper pressure limit of 15 bar as shown in Fig. 16a. Conversely, lowering the equivalence to 0.2 (Fig. 16c) significantly decreased the mass burning rates and shifted the transition pressures and cross-over points to lower values. The profiles of the lower equivalence ratios (u = 0.25 or 0.20) were suitable to explain the predicted flame propagation in the cylindrical reactor (u = 0.30) and the optically accessible planar reactor (u 6 0.30), since the upstream heterogeneous consumption reduced the availability of fuel for gaseous reactions at the location of homogeneous ignition. Moreover, the unequal diffusivities of the two fuel components could reduce the local H2:CO ratio when compared to this ratio in the initial feed. The profiles of H2:CO = 4:1 or 2:1 at u = 0.25 (Fig. 16b) captured the trends in

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Fig. 9a, where the homogeneous conversions (G) at 8 and 15 bar were much lower than those of 4 bar. This was due to the drop of the mass burning rates at these higher pressures (Fig. 16b) that in turn pushed the gaseous combustion zones closer to the walls, such that larger amounts of H2 and CO leaked through the flame zones to be subsequently consumed on the catalytic surface. Reducing the H2:CO ratio resulted in decreased burning rates at low pressures. The shift of the transition pressure to higher values and the weaker pressure dependence at low H2:CO ratios enhanced the burning rates at high pressures compared to those of mixtures with higher H2:CO ratios. The resulting behavior in Fig. 16 was then consistent with the findings in Fig. 9b and c. It is also noted that an asymptotic study of gaseous combustion over stagnation flow catalytic surfaces has reported a similar behavior [40]. This study has shown that a progressive increase of the strain rate pushed the flame towards the wall, leading to catalytic conversion of the leaked fuel and thus to weaker gaseous combustion. In the channel-low geometry of Fig. 9, the aerodynamic strain (characterized by the imposed radial fuel fluxes) was the same at all pressures since the mass throughput was maintained constant for all conditions; however, the laminar burning rates dropped appreciably at the highest examined pressures, leading to the same result i.e. a push of the flames towards the catalytic wall and increased fuel leakage through the gaseous combustion zones. The profiles for u = 0.20 in Fig. 16c could be used to interpret the observations in the optically accessible reactor (Figs. 3–6), whereby measurements were performed mostly with u around 0.2 for all studied pressures (except for 2 bar). The observed increased drop of gas-phase oxidation with rising pressure could be partially attributed to the reduced equivalence ratio, but predominantly to the lower mass burning rates, even though the preheat increased from approximately 400 to 600 K as the pressure increased. The additional CO2 dilutions with rising pressure (imposed to avoid autoignition in the mixing section) further intensified the aforementioned restraint of gaseous combustion. The

complete suppression of gaseous combustion at 14 bar for Cases 15–17 (shown in Figs. 3 and 6) had also an analog in the aforementioned stagnation flow studies over catalytic surfaces [40]; therein a sufficient increase of the strain rate could push the flame against the stagnation surface and eventually extinguish it. In terms of channel-geometries, flame extinguishment has also been reported under intense turbulence flow conditions [25,41] due to the increased radial transport of reactants. The impact of the H2:CO ratio on the non-monotonic pressure dependence was evident in Fig. 16 as the transition pressure ptr shifted to higher values with dropping H2:CO ratio. This influence was attributed to the reduced global reaction order with rising _ o were CO content in syngas [12]. In [12] the mass burning rates m modeled as: n

_ o ¼ qo SL / p2 m

pffiffiffiffiffiffiffiffiffi



qo a exp 

 Ea =R ; 2T rz

ð4Þ

where a is the mixture thermal diffusivity, qo the unburned gas density, n the overall reaction order, Ea the effective activation energy, R the gas constant and Trz the reaction zone temperature. Variation of H2:CO ratio did not change the monotonic drop of the reaction order n with respect to pressure but changed its rate of drop. It was shown in [12] that the reaction order for H2:CO = 1:4 decreased much slower with pressure than the one for H2:CO = 4:1, such that the reaction order of the former mixture overtook the reaction order of the latter at about 2 bar. At 15 bar this led to a reaction order n  1.1 for H2:CO = 1:4, which was nearly twice the value for H2:CO = 4:1 (n  0.6). Such a composition dependence of the mass burning rate exemplified the importance of the CO reaction CO þ HO2 () CO2 þ OH (R26), which produced OH to compensate the termination reaction (R11) at high pressures and high CO contents. The foregoing discussion of mass burning rates could clarify the combustion behavior in the cylindrical channel (Figs. 9–11) and the planar reactor (Figs. 3–6) with the variation of pressure at

Table A1 Heterogeneous chemical reaction mechanism.a Reactions S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S22 S23 S24 S25 S26 S27

H2 þ 2PtðsÞ ) 2HðsÞ 2HðsÞ ) H2 þ 2PtðsÞ H þ PtðsÞ ) HðsÞ O2 þ 2PtðsÞ ) 2OðsÞ O2 þ 2PtðsÞ ) 2OðsÞ 2OðsÞ ) O2 þ 2PtðsÞ O þ PtðsÞ ) OðsÞ H2 O þ PtðsÞ ) H2 OðsÞ H2 OðsÞ ) H2 O þ PtðsÞ OH þ PtðsÞ ) OHðsÞ OHðsÞ ) OH þ PtðsÞ HðsÞ þ OðsÞ ) OHðsÞ þ PtðsÞ OHðsÞ þ PtðsÞ ) HðsÞ þ OðsÞ HðsÞ þ OHðsÞ () H2 OðsÞ þ PtðsÞ OHðsÞ þ OHðsÞ () H2 OðsÞ þ OðsÞ CO þ PtðsÞ ) COðsÞ COðsÞ ) CO þ PtðsÞ CO2 ðsÞ ) CO2 þ PtðsÞ COðsÞ þ OðsÞ ) CO2 ðsÞ þ PtðsÞ CðsÞ þ OðsÞ ) COðsÞ þ PtðsÞ COðsÞ þ PtðsÞ ) CðsÞ þ OðsÞ OHðsÞ þ COðsÞ ) HCOOðsÞ þ PtðsÞ HCOOðsÞ þ PtðsÞ ) OHðsÞ þ COðsÞ HCOOðsÞ þ OðsÞ ) OHðsÞ þ CO2 ðsÞ OHðsÞ þ CO2 ðsÞ ) HCOOðsÞ þ OðsÞ HCOOðsÞ þ PtðsÞ ) CO2 ðsÞ þ HðsÞ CO2 ðsÞ þ HðsÞ ) HCOOðsÞ þ PtðsÞ

A

b

Ea

4.46E+10 3.70E+21 1.00E+00 1.80E+21 2.30E02 3.70E+21 1.00E+00 7.50E01 1.00E+13 1.00E+00 1.00E+13 3.70E+20 1.00E+21 3.70E+21 3.70E+21 8.40E01 2.13E+13 1.00E+13 3.70E+20 3.70E+21 1.00E+18 3.70E+21 1.33E+21 3.70E+21 2.79E+21 3.70E+21 2.79E+21

0.5 0 0 0.5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 67,400–6000hH 0 0 0 213,200–60,000hO 0 0 40,300 0 192,800 70,500 130,690 17,400 48,200 0 136,190–33,000hCO 20,500 108,000–33,000hCO 62,800 184,000 94,200 870 0 151,050 0 90,050

a From [28,29]. The surface site density is C = 2.7  10-9 mol/cm2. In the surface and desorption reactions, the reaction rate coefficient is k = ATbexp(Ea/RT), A [mol cm K s] and E [J/mol]. In all adsorption reactions, except S1 and S4, A denotes a sticking coefficient. Reactions S4 and S5 are duplicate. S1 has an order of one with respect to platinum. S16 has an order of two with respect to the platinum. The suffix (s) denotes a surface species.

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p P 4 bar and of H2:CO ratio. The transition pressure ptr was a critical parameter in determining the combustion efficiency and thus great attention should be paid on its effects when designing and operating hetero-/homogeneous combustion systems. 5. Conclusions The hetero-/homogeneous combustion of fuel-lean H2/CO mixtures over Pt was investigated experimentally at H2:CO molar ratios 0.47–4.54 and turbine-relevant conditions, specifically pressures up to 14 bar and preheat temperatures up to 736 K. Complementary 2-D simulations were performed to evaluate the performance of hetero-/homogeneous kinetic models in terms of their capacity to reproduce the homogeneous combustion characteristics. Assessment of the pressure and composition effects on the gaseous oxidation processes was further achieved with detailed simulations in a practical cylindrical channel with a diameter of 1 mm. The key results of this study are summarized below. (1) Predictions with the employed detailed hetero-/homogeneous kinetic schemes satisfactorily reproduced the LIF-OH measured gas-phase ignition distances and the flame shapes for all examined conditions. Mass transport limited catalytic conversion of CO and H2 was attested by both the Raman measurements and model predictions. The results indicated that enhanced preheats were required to sustain gaseous combustion at elevated pressures. At 14 bar, gaseous combustion was fully suppressed at 400 K preheat and only mildly restored at 658 K preheat. Measured and predicted flame shapes for all investigated H2:CO ratios were open (i.e. formed two separate branches) and this was largely an outcome of the diffusional imbalance of the H2 fuel component (LeH2 < 1). The addition of CO up to an H2:CO ratio of 0.5 did not alter this particular feature of hydrogen gas-phase combustion. (2) For a given pressure, a stronger quenching of homogeneous reactions was observed with increasing CO content, leading to increased CO and H2 leakage through the gaseous combustion zone to be subsequently converted catalytically on the channel walls. This increased fuel leakage through the gaseous combustion zone led to a noticeable increase in catalytic conversion (by 10% for H2 and 20% for CO) for the H2:CO ratio of 0.5 when compared to the H2:CO ratio of 4. Moreover, this composition effect was negligible for ratios H2:CO P 1.0. (3) Gas-phase ignition of H2 was achieved slightly upstream that of CO, despite the fact that over the gaseous induction zone the catalytic depletion of H2 was higher than the corresponding one of CO due to the larger molecular diffusivity of the former species. Simulations in the optically accessible channel geometry as well in a tubular channel with 1 mm diameter indicated that the initiation of gas-phase CO oxidation reactions necessitated H2 gas-phase oxidation reactions for the supply of crucial OH radicals. However, at the rear of the CO gaseous oxidation zones whereby H2 was fully converted, the upstream produced H2O (via the catalytic and gaseous pathways) facilitated the radical pool buildup for the CO gaseous oxidation. (4) The OH radical adsorption/desorption reactions promoted mildly homogeneous ignition at p P 2 bar due to the netdesorptive OH flux over the gaseous induction zone. However, the effect of these reactions on the downstream gaseous combustion zone was determined by the sign of the OH flux (net desorptive or adsorptive). The hetero-/homogeneous radical coupling was insensitive to the specific H2:CO

ratio. The kinetic impact of the catalytically-produced H2O on the gaseous oxidation of H2 and CO strongly varied with H2:CO ratio. CO gas-phase oxidation was inhibited by catalytically-produced H2O at high H2:CO ratios and was promoted at low H2:CO ratios. On the other hand, the heterogeneous production of CO2 always inhibited gasphase oxidation of H2 and CO but its effect was weaker when compared to that of H2O. (5) Computations in a tubular channel with 1 mm diameter were carried out under a constant mass flux and preheat relevant to large scale gas turbine applications. The gaseous oxidation was suppressed at 1 bar, preheat of 673 K, and constant wall temperature of 1350 K. This was due to the increased catalytic fuel consumption over the extended gaseous pre-ignition period, which in turn competitively deprived fuel from the homogeneous pathway. At elevated pressures, homogeneous ignitions occurred farther upstream (compared to 1 bar) and flame propagation characteristics gained importance in determining the strength

Table A2 Homogeneous chemical reaction mechanism.a Reactions

A

b

Ea

R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11

1.04E+14 3.82E+12 8.79E+14 2.16E+08 3.34E+04 4.58E+19 6.16E+15 4.71E+18 6.06E+27 1.01E+26 4.65E+12 6.37E+20 2.75E+06 7.08E+13 2.85E+10 2.89E+13 4.20E+14 1.30E+11 2.00E+12 2.49E+24 2.41E+13 4.82E+13 9.55E+06 1.74E+12 7.59E+13 1.80E+10 1.55E+24 2.53E+12 3.01E+13 2.23E+05 4.75E+11 7.58E+12 7.23E+13 3.02E+13 3.00E+13 3.02E+13 3.00E+13 3.00E+12 3.13E+13

0.00 0.00 0.00 1.51 2.42 1.40 0.50 1.00 3.32 2.44 0.44 1.72 2.09 0.00 1.00 0.00 0.00 0.00 0.90 –2.30 0.00 0.00 2.00 0.00 0.00 0.00 2.79 0.00 0.00 1.90 0.70 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

15,286 7948 19,170 3430 1930 104,380 0 0 120,790 120,180 0 525 1451 295 723.9 497 11,982 1629.3 48,749 48,749 3970 7950 3970 318 7270 2380 4190 47,700 23,000 1160 14,900 410 0 0 0 0 0 0 0

R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 R22 R23 R24 R25 R26 R27 R28 R29 R30 R31 R32 R33 R34 R35 R36

H + O2 = O + OH O + H2 = H + OH O + H2 = H + OH H2 + OH = H2O+H OH + OH = O + H2O H2 + M = H + H + M O + O + M = O2 + M O + H + M = OH + M H2O + M = H + OH + M H2O + H2O = H + OH + H2O H+O2(+M) = HO2(+M) H + O2(+M) = HO2(+M) HO2+H = H2 + O2 HO2 + H = OH + OH HO2 + O = O2 + OH HO2 + OH = H2O+O2 HO2 + HO2 = H2O2 + O2 HO2 + HO2 = H2O2 + O2 H2O2(+M) = OH + OH(+M) H2O2(+M) = OH + OH(+M) H2O2 + H = H2O + OH H2O2 + H = HO2 + H2 H2O2 + O = OH + HO2 H2O2 + OH = HO2 + H2O H2O2 + OH = HO2 + H2O CO + O(+M) = CO2(+M) CO + O(+M) = CO2(+M) CO + O2 = CO2 + O CO + HO2 = CO2 + OH CO + OH = CO2 + H HCO + M = H + CO + M HCO + O2 = CO + HO2 HCO + H = CO + H2 HCO + O = CO + OH HCO + O = CO2 + H HCO + OH = CO + H2O HCO + HO2 = CO2 + OH + H HCO + HCO = H2 + CO + CO HCO + HCO = CH2O + CO

a R1–23 from [33] and R24–R36 from [32]. Reaction rate k = ATbexp(Ea/RT), A mol cm K s, E cal/mol. Third body efficiencies in reactions R6–R8 are xH2O = 12.0, xH2 = 2.5, xCO = 1.9 and xCO2 = 3.8; in R9: xH2O = 0.0, xH2 = 3.0, xO2 = 1.5, xCO = 1.9, xCO2 = 3.8 and xN2 = 2.0; in R11: xH2O = 14.0, xH2 = 2.0, xO2 = 0.78, xCO = 1.9, and xCO2 = 3.8; in R18: xH2O = 7.5, xH2O2 = 7.7, xH2 = 3.7, xO2 = 1.2, xCO = 2.8, xCO2 = 1.6 and xN2 = 1.5; in R24: xH2O = 12.0, xH2 = 2.5, xCO = 1.9 and xCO2 = 3.8; in R28: xH2O = 6.0, xH2 = 2.5, xCO = 1.9 and xCO2 = 3.8. Reaction pairs (R2, R3), (R16, R17) and (R22, R23) are duplicate. R11 and R18 are Troe reactions centered at 0.5 and 0.42, respectively, and R24 a pressure-dependent reaction (second entries are the low pressure limits).

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of the established flames. The reduced mass burning rates at p > 4 bar confined the combustion zones near the walls and subsequently led to a leakage of fuel through the near-wall flames, thus appreciably reducing the homogeneous fuel conversion. (6) Gaseous ignition and propagation characteristics were quantitatively analyzed with the aid of homogeneous batch reactor and 1-D freely propagating flame codes, respectively. The predictions could explain the observations in the optically accessible reactor and the results in the cylindrical channel geometry regarding the variation of gaseous fuel conversion under a wide range of pressures, temperatures, and H2:CO ratios. In contrast to the well-known monotonic increase of ignition delays with rising CO fuel fraction, the mass burning rates could either increase or decrease with increased CO fuel fraction, depending on the pressure. This was attributed to the weakening pressure dependence of the effective global reaction order used to describe the mass burning rates when reducing the H2:CO ratio.

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