Combustion and Flame 160 (2013) 155–169

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Combustion and Flame

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

Xin Zheng, John Mantzaras ⇑, Rolf BombachPaul Scherrer Institute, Combustion Research, CH-5232 Villigen PSI, Switzerland

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 July 2012Received in revised form 3 September 2012Accepted 4 September 2012Available online 1 October 2012

Keywords:Syngas hetero-/homogeneous combustionover platinumHomogeneous ignitionHigh-pressure catalytic combustionIn situ Raman and LIF measurements

0010-2180/$ - see front matter � 2012 The Combusthttp://dx.doi.org/10.1016/j.combustflame.2012.09.001

⇑ Corresponding author. Fax: +41 56 3102199.E-mail address: ioannis.mantzaras@psi.ch (J. Mant

The gaseous oxidation of H2/CO/CO2/O2/N2 mixtures was investigated experimentally and numerically in aplatinum-coated channel at fuel-lean stoichiometries (equivalence ratios u 6 0.30), H2:CO molar ratios0.47–4.54, pressures 2–14 bar, and reactant preheats up to 736 K. Two-dimensional laser induced fluores-cence of the OH radical monitored the homogeneous (gaseous) combustion, while 1-D Raman spectros-copy assessed the heterogeneous (catalytic) conversion of H2 and CO. Numerical simulations, whichwere carried out with a 2-D elliptic code and detailed hetero-/homogeneous reaction schemes, reproducedthe measured onset of homogeneous ignition, the ensuing flame shapes, and the mass-transport-limitedcatalytic conversion of H2 and CO. Additional simulations in practical tubular channels with 1 mm diam-eter have shown that gaseous oxidation was suppressed at atmospheric pressure due to the intrinsic slowgas-phase ignition kinetics in conjunction with the competition from the catalytic pathway for H2 and COconsumption. At pressures p > 4 bar, homogeneous combustion was largely controlled by flame propaga-tion characteristics due to the near-wall confinement of the established flames. The decrease in laminarmass 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 leakedfuel components. Radical heterogeneous reactions promoted mildly the onset of homogeneous ignitionat p P 2 bar due to the net desorptive flux of OH over the gaseous induction zone. The catalytically pro-duced H2O had a strong kinetic impact on homogeneous combustion by inhibiting the gaseous oxidationof both H2 and CO at high H2:CO ratios and by promoting CO gaseous oxidation at low H2:CO ratios. Thecatalytically produced CO2 always inhibited kinetically the gaseous combustion of H2 and CO, although itseffect was much weaker compared to that of H2O.

� 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction

Increasing concerns regarding sustainable and secure energysupply have incited a global interest for syngas fuels. Syngasmainly consists of varying proportions of carbon monoxide andhydrogen, and can be produced from fossil fuels or from diverserenewable biological feedstocks via biothermal or thermochemicalprocessing. High-pressure and high-preheat syngas combustion, inparticular, is currently under intense investigation for applicationin gas turbines of power generation systems employing fuel decar-bonization (pre-combustion CO2 capture) as a means to reducegreenhouse gas emissions [1,2].

Although lean premixed combustion is the main approach forgas-fired power plants, catalytic combustion methodologies areincreasingly explored in the last years due to their enhanced com-bustion stability at very fuel-lean equivalence ratios and the result-ing ultra-low NOx emissions [3–5]. In the fuel-lean catalytically

ion Institute. Published by Elsevier

zaras).

stabilized thermal combustion (CST) concept, fractional fuel con-version is achieved in a heterogeneous (catalytic) reactor, whilethe remaining fuel is combusted in a subsequent homogeneous(gas-phase) burnout zone [5]. CST is particularly suited for low cal-orific value syngas-based fuels, due to the enhanced combustionstability at the ensuing moderate reaction temperatures [6,7].Therefore, application of hetero-/homogeneous combustion to syn-gas fuels is an attractive option for renewable and clean powergeneration. Moreover, combined hetero-/homogeneous combus-tion has been shown [8,9] to suppress most of the intrinsic flameinstabilities appearing in non-catalytic (pure homogeneous com-bustion) channel-flow reactors [10,11].

Future utilization of syngas catalytic combustion relies on thedevelopment of active and stable catalysts as well as on the under-standing of the heterogeneous and homogeneous syngas kineticsunder industrially-relevant operating conditions. In contrast tothe extensive investigations of syngas homogeneous chemistryover broad ranges of mixture compositions and pressures, whichhave been reviewed in [12], there is a clear lack of correspondingstudies in combined hetero-/homogeneous syngas combustion.

Inc. All rights reserved.

Nomenclature

b channel half-height, Fig. 1L channel length, Fig. 1Lek Lewis number of gaseous species k (thermal over mass

diffusivity)_mo laminar mass burning rate, Eq. (4)

p pressure_sk heterogeneous molar production rate of gaseous species

k, Eq. (1)T temperatureVk,x, Vk,y diffusion velocity components of gaseous species k, Eq.

(1)W channel width, Fig. 1bWk molecular weight of gaseous species k, Eqs. (1) and (3)

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

Greek symbolsC surface site density, Eq. (3)ck sticking coefficient of gaseous species k, Eq. (3)hi coverage of surface species iq gas densityu fuel-to-air equivalence ratio

SubscriptsIN inletig ignition

156 X. Zheng et al. / Combustion and Flame 160 (2013) 155–169

The non-negligible contribution of homogeneous chemistry (evenin practical catalytic reactor geometries with large surface-to-vol-ume ratios [13]), either at the moderate pressures (up to 5 bar) rel-evant to micro-reactors [14] or especially at the elevated pressures(�15 bar) of gas turbines [15], requires additional validation of thegas-phase syngas kinetic models [12] that were developed in theabsence of heterogeneous reactions.

To facilitate the validation of kinetics, we have introduced themethodology of in situ spatially resolved 1-D Raman measure-ments of major gas phase species concentrations across the bound-ary layer of a catalytically-coated channel, along with 2-D laserinduced fluorescence (LIF) of the OH radical [16–18]. These mea-surements, when compared to detailed numerical simulations,have allowed for direct assessment of the catalytic and gas-phasereactivities. Using the aforementioned approach, Appel et al. [16]evaluated several heterogeneous and homogeneous kineticschemes for atmospheric-pressure combustion of fuel-lean H2/airmixtures over Pt and for non-preheated reactants (gas inlet tem-peratures of �310 K). It was shown that heterogeneous reactionsinhibited homogeneous ignition mainly via competitive fuel deple-tion, while the corresponding inhibition via radical adsorption/desorption catalytic reactions was modest. Mantzaras et al. [17]extended the validation of hydrogen gaseous kinetic schemes topressures up to 10 bar, for fuel-lean stoichiometries and non-pre-heated reactants. Therein, the observed suppression of gas-phasecombustion at pressures p P 4 bar was due to the combined effectsof intrinsic homogeneous kinetics, competition by the catalyticpathway for hydrogen consumption, and inhibition from catalyti-cally-produced major products (notably H2O). Ghermay et al.[18] further investigated combustion of fuel-lean H2/air mixturesover Pt at high preheats (up to 773 K) and gas-turbine relevantpressures up to 15 bar. For pressures above 12 bar – and even forthe highest examined preheats – the heterogeneous reaction path-way was strongly favored against the homogeneous one, thus sup-pressing flame formation. Homogeneous ignition of fuel-lean andfuel-rich H2/air mixtures at atmospheric pressure has been numer-ically investigated over stagnation flow Pt-coated surfaces in Buiet al. [19], establishing the dependence of the ignition temperatureon equivalence ratio. Finally, addition of hydrogen to less reactivehydrocarbon fuels, such as methane, has been shown to greatly ex-tend the performance of catalytic microreactors [20].

In terms of syngas combustion, numerical investigation withdetailed hetero-/homogeneous reaction schemes was initially re-ported in Mantzaras [21] at pressures up to 15 bar. Ghermayet al. [22] subsequently investigated experimentally and numeri-cally the homogeneous kinetics of fuel-lean CO/H2 mixtures overPt at pressures of 1–5 bar (relevant to microreactors), inlet temper-atures up to 874 K, and a constant H2:CO molar ratio of 1:2 (perti-nent to syngas production via coal gasification). It was therein

demonstrated that the presence of CO could facilitate flame prop-agation towards the center of the catalytic channel, thus leading(for a given channel geometry) to enhanced mass consumptionrates for syngas fuels when compared to pure hydrogen ones. De-tailed syngas homogeneous ignition experiments at pressures andpreheats relevant to gas turbines and, moreover, at varying H2:COratios (referring to different degrees of hydrocarbon fuel decarbo-nization) have not yet been reported in the literature.

The present work undertakes a first investigation of high-pres-sure (up to 14 bar) and high preheat (up to 736 K) syngas homoge-neous ignition over platinum at fuel-lean stoichiometries(equivalence ratios, u, up to 0.3) and a wide range of H2:CO molar ra-tios (0.47–4.54). Planar LIF of the OH radical monitored the onset ofhomogeneous ignition in a Pt-coated catalytic channel, while 1-D Ra-man measurements of major species concentrations across the chan-nel boundary layer yielded the catalytic H2 and CO consumption.Numerical simulations were performed with a 2-D full elliptic codethat included detailed hetero-/homogeneous chemical reactionschemes and transport. Main objectives were to validate gas-phasechemical reaction mechanisms at operating conditions relevant tocatalytic combustion power generation systems, to investigate thecoupling of heterogeneous and homogeneous syngas kinetics, andto clarify the effect of fuel composition (H2:CO ratio) on gas-phaseignition and flame propagation characteristics in the confined cata-lytic channel geometry. The onset of homogeneous ignition insidethe catalytic reactor, in particular, was of great interest for syngas(or generally for hydrogen-rich fuels) as the presence of gaseouscombustion has been shown to moderate the superadiabatic surfacetemperatures attained due to the catalytic conversion of the diffu-sionally imbalanced hydrogen fuel component [16,21]. The findingsof the present study could further facilitate the design of syngas cat-alytic burners and address reactor thermal management issues.

This article is organized as follows. The experimental andnumerical methodologies are introduced in Sections 2 and 3,respectively. In Section 4.1, measurements are compared againstnumerical predictions in order to assess the aptness of the appliedkinetic models and to reveal the effects of pressure, preheat, andfuel composition on gas-phase combustion. The coupling of heter-ogeneous and homogeneous chemistry is subsequently addressedin Section 4.2, followed by discussion of gas-phase ignition andflame propagation characteristics in Section 4.3.

2. Experimental

2.1. High pressure test rig

The test-rig has been employed in previous high-pressure com-bustion experiments of methane and hydrogen fuels [13,18,23]

X. Zheng et al. / Combustion and Flame 160 (2013) 155–169 157

such that a brief summary is provided hereafter, with emphasis onspecific modifications introduced for the present syngas studies.Hetero-/homogeneous combustion experiments were carried outin an optically accessible channel-flow reactor with a length, widthand 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] hori-zontal ceramic plates and two 3-mm-thick vertical quartz win-dows (Fig. 1b) and was positioned inside a cylindrical stainlesssteel tank that yielded the desired pressurization. The inner sur-faces of both Si[SiC] ceramic plates were firstly coated with a1.5 lm thick non-porous Al2O3 layer and subsequently with a2.2 lm thick Pt layer by means of plasma vapor deposition(PVD). The absence of surface porosity was verified with total areaand active surface area measurements via Brunauer–Emmett–Tell-er (BET) Kr-physisorption and CO-chemisorption measurements,respectively, while post-combustion X-ray photoelectron spectros-copy (XPS) analysis verified the presence of only Pt at the catalystsurface 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 at-tained during catalytic combustion of syngas fuels with high hydro-gen content [16,21]; such superadiabatic temperatures were aresult of the diffusional imbalance of hydrogen (at lean stoichiome-tries, the Lewis number – ratio of thermal over mass diffusivity – ofhydrogen is LeH2 � 0.3). On the other hand, the central and rear sec-tions of the Si[SiC] plates (100 < x < 300 mm) were heated by tworesistive coils positioned above the plates in order to counteractthe external heat losses. Surface temperatures along the x–y sym-metry plane were measured by S-type thermocouples (12 for eachplate), embedded 0.9 mm beneath the catalyst surface throughholes eroded from the outer non-catalytic Si[SiC] surfaces (Fig. 1a).

Syngas fuels with high hydrogen content necessitated the de-sign of a special reactant supply section upstream of the channel

O2,N2CO,H2,CO2

Powerfeedthroughs

Heatercoils

Pressurethrottle

LIF laser sheet

Thermocouplefeedthroughs

Watercooling

Pt-coatedsurfaces

Exhaust

2b=7

Quartzwindows

L=300

High pres-sure tank

Insulation

Flow straightener

xy

Cross section A-Azy

Si[SiC]ceramicplates

W=104

Quartzwindows

35

Inconelsteel frame

3 50

A

AFlushing N2

TCA

TCD

110

9

Pre-heater

TCB

TCC

Staticmixers

Water cooling

Ls=200Heatercoils

(a)

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

reactor, in order to reduce the risk of autoignition and at the sametime achieve good mixing. O2 and N2 from pressurized bottles wereelectrically preheated and then mixed with room temperature COand H2 in two sequential static mixers (Fig. 1a). To attain the highpreheat temperatures necessary for the present study, the steelpipe section driving the preheated O2/N2 mixture to the static mix-ers and was also externally heated by an electric coil (Fig. 1a).Varying amounts of CO2 diluent were further added in the H2/COfuel stream, so as to avoid autoignition of the reactive mixture in-side the flow straightening section. The resulting O2/N2/H2/CO/CO2

reactant mixture was subsequently driven into a flow straighten-ing section, which comprised a rectangular steel duct (200 mm(Ls) � 104 mm (W) � 7 mm (2b)) positioned upstream of the chan-nel reactor and equipped with cross-flow grids to produce a uni-form velocity (Fig. 1a). Mitigation of autoignition was attested bya series of four sheathed K-type thermocouples (TCA to TCD inFig. 1a) that monitored the mixture temperature from the pointof H2/CO/CO2 injection into the preheated O2/N2 stream, down tothe reactor entry. Moreover, flow uniformity was assessed byhot-wire velocimetry measurements at the exit of the stand-aloneflow straightening unit. Thermocouple TCD, positioned 1 mm up-stream of the reactor entry, provided the inlet temperature re-quired for the numerical simulations.

Two 35-mm thick quartz windows on the sides of the high-pres-sure cylindrical tank (Fig. 1b), aligned with respect to the reactorwindows, provided optical access for the ensuing Raman and planarOH-LIF measurements. Experiments were performed at pressures2–14 bar, equivalence ratios 0.17–0.30 (based on the combinedamounts of H2 and CO) and H2:CO molar ratios 0.47–4.54. Theinvestigated conditions are summarized in Table 1. The Reynoldsnumber for each condition (ReIN), based on the uniform inlet prop-erties and the channel hydraulic diameter (=13.1 mm), was main-tained below 3000. All flows were laminar, as turbulent catalyticcombustion studies have shown that the flow laminarization in-duced by the heat transfer from the hot catalytic walls guaranteedlaminar conditions at ReIN considerably higher than 5000 [24,25].

2.2. Laser diagnostics

The optical setups for the spectroscopic measurements (planarOH-LIF and 1-D Raman) are depicted in Fig. 2. OH-LIF detectedgas-phase combustion, whereas spontaneous Raman assessed thecatalytic processes preceding the onset of homogeneous ignition.For LIF, the 532 nm second harmonic beam of a pulsed Nd:YAG la-ser (Quantel TDL90 NBP2UVT3) pumped a tunable dye laser(Quantel TDL90). The dye output radiation was frequency-doubledto 285 nm with a pulse energy of 0.5 mJ, sufficiently low to avoidsaturation of the A(v = 1) X(v = 0) OH transition. A cylindricallens telescope and a 1-mm slit mask transformed the excitationbeam into a 0.3 mm thick light sheet, which propagated counter-flow along the x–y symmetry plane of the reactor (see Figs. 1aand 2). Fluorescence from both (1–1) and (0–0) OH transitions at308 and 314 nm, respectively, was collected at 90� through thereactor and tank side windows with an ICCD camera (LaVision Im-ager Compact HiRes IRO, 1392 � 1024 pixels). Channel areas of100 � 7 mm2 were recorded on a 628 � 44 pixel area of the CCDdetector chip. The camera was traversed axially to map the entire300 mm reactor extent. Given the steady operating conditions, 400LIF images were averaged at each measuring location to increasethe signal-to-noise ratio.

For the Raman measurements, a high repetition rate (up to2 kHz) frequency-doubled Nd:YLF pulsed laser (k = 526.5 nm,Quantronix Darwin Duo) was employed, with a pulse durationand energy of 130 ns and 43 mJ, respectively. The 526.5 nm beamwas focused by an f = 150 mm cylindrical lens into a vertical line(�0.3 mm thick), which spanned the entire 7 mm channel height

Table 1Experimental 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 2 441 0.29 3.23 1.54 7.03 13.86 4.54 44 462 2 443 0.30 3.07 2.49 6.20 12.88 2.50 48 503 2 444 0.30 2.87 4.53 4.61 9.88 1.02 48 504 4 558 0.20 3.03 1.18 4.73 15.11 4.00 105 985 4 557 0.19 2.97 1.94 3.97 14.36 2.04 110 976 4 555 0.21 2.97 3.14 3.16 14.27 1.00 114 1007 8 725 0.18 2.14 1.38 4.91 12.96 3.57 85 908 8 716 0.18 2.10 2.16 4.37 12.03 2.04 85 899 8 703 0.18 2.02 3.34 3.36 9.86 1.01 89 90

10 8 660 0.17 1.97 4.21 2.14 12.67 0.51 128 11411 12 719 0.19 1.70 1.39 5.38 14.95 3.84 85 9112 12 736 0.19 1.58 1.93 4.62 10.18 2.38 85 9213 12 732 0.19 1.54 3.32 3.37 9.89 1.01 92 9514 12 651 0.20 1.43 4.46 2.28 11.43 0.51 129 11415 14 399 0.25 0.72 5.50 2.60 15.37 0.47 – –16 14 400 0.25 0.72 3.84 4.28 15.40 1.11 – –17 14 401 0.25 0.72 2.18 5.96 15.39 2.70 – –18 14 652 0.18 1.54 2.42 4.03 11.71 1.67 146 132

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.

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

158 X. Zheng et al. / Combustion and Flame 160 (2013) 155–169

and was moderately offset in the lateral direction (z = 15 mm) toincrease the collection angle and minimize thermal beam steering,as in [16–18]. Two f = 300 mm lenses collected the scattered lightat 50� with respect to the sending optical path, and focused it tothe entrance slit of a 25 cm spectrograph (Chromex-250i)equipped with an intensified CCD camera (Princeton InstrumentsPI-MAX1024GIII). As the operating conditions were steady, Ramansignals of up to 200,000 pulses were integrated on the detectorchip. The laser repetition rate, the number of integrated pulses,and the temporal width of the camera intensifier gate have beenchosen to optimize the signal-to-noise ratio. Data were acquiredfor x > 10 mm, with emphasis on the reactor length preceding theonset of homogeneous ignition, by traversing an optical table sup-porting the sending and collecting optics, including the Nd:YLF la-ser (Fig. 2).

The Raman spectrometer slit was set to 200 lm, yielding a500 lm separation of the CO and N2 signals at the detector plane.This resulted in good light gathering power while keeping cross-talk below 5%, thus allowing simple subtraction of the CO/N2 over-lap when determining the weaker CO signal. The image intensifieremployed a GaAsP-Gen3 photocathode, which yielded quantumefficiencies above 35% for N2 and all other molecules with lowerRaman shift, while maintaining quantum efficiency above 20% forthe H2 signal. Combining the factors of Raman cross section, whichwas much larger for H2 than for N2 (by a factor of 3.86), with thefrequency factor (m3 for quantum detectors) and the quantum effi-ciency of the photocathode (0.6), a hydrogen sensitivity clearlyabove that of nitrogen was calculated. Nevertheless, the signal ofhydrogen remained modest with the noise being dominated bydetection shot noise. The standard deviation of the hydrogen signalwas 4% or less of its maximum reported value. Measurement accu-racy was estimated to be ±3% for species compositions P3 vol.%and ±8% for compositions as low as 0.5 vol.%; values less than0.5 vol.% entailed larger inaccuracies. Raman data closer than�0.7 mm to both catalyst surfaces were discarded due to low sig-nal-to-noise ratios.

3. Numerical simulation

The flow was simulated with a 2-D steady elliptic CFD code (de-tails have been provided in [15–18,26]). The 300 � 7 mm2 (in x andy, respectively) reactor domain was discretized with 680 � 100grid points. The governing equations in 2-D Cartesian coordinatesfor a steady laminar channel-flow with heterogeneous and homo-geneous reactions have been provided elsewhere [16,18] and arenot repeated here. Temperature, velocity and species compositionswere uniform (see Table 1) at the reactor entry (x = 0). The interfa-cial boundary conditions for gas-phase species and temperature atthe lower and upper catalytic walls (y = 0 and y = 2b) were:

ðqYkVk;yÞy¼0 ¼Wkð_skÞy¼0; �ðqYkVk;yÞy¼2b ¼Wkð_skÞy¼2b ð1Þ

and

Tðx; y ¼ 0Þ ¼ TW;LðxÞ; Tðx; y ¼ 2bÞ ¼ TW;UðxÞ; ð2Þ

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

X. Zheng et al. / Combustion and Flame 160 (2013) 155–169 159

duction rate and diffusion velocity components of the kth gaseousspecies, respectively. Mixture-average diffusion including thermaldiffusion 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 thegas-wall interfaces. At the outflow (x = L) the transverse velocitywas set to zero and zero-Neumann conditions were used for allother scalars.

For surface chemistry the CO/H2 scheme on Pt from Deutsch-mann et al. [28] has been used, augmented with HCOO(s) reactionsfrom [29]. The resulting syngas heterogeneous reaction mecha-nism (shown in Table A1 of Appendix A) has been validated inour recent [30] pure heterogeneous kinetic syngas studies. Finally,the formulation in Dogwiler et al. [31] with a modified Motz-Wisecorrection was employed for the adsorption rate constant kad,k ofthe kth gaseous species:

kad;k ¼ck

1� ckhPt=2

� �1

Cm

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiRT

2pWk

s; ð3Þ

where ck and Wk are the sticking coefficient and molecular weightof the kth gaseous species, respectively, hPt the Pt surface coverage,C = 2.7 � 10�9 mol/cm2 the total surface site density, m the sum ofsurface reactants’ stoichiometric coefficients, R the universal gasconstant, and T the surface temperature.

The gas-phase mechanism for H2/CO consisted of 36 reversiblereactions involving 13 species and is shown in Table A2 of AppendixA. Reactions for CO (R24–R36) were modeled with the elementaryCO/HCO reaction subset in Li et al. [32] while H2 reactions (R1–R23) were taken from the latest H2/O2 kinetic model of Burkeet al. [33]. Species thermodynamic data were also provided in[32,33]. Surface and gaseous reaction rates were evaluated withCHEMKIN packages [34,35], while for species transport the CHEM-KIN 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 weregenerally reduced so as to avoid autoignition inside the static mix-ers and/or flame flashback towards the flow straightening section(Fig. 1a). To this purpose, equivalence ratios in the range0.17 6 u 6 0.30 were investigated. Measured and predicted 2-DOH distributions over the entire 300 � 7 mm2 reactor domain arepresented in Fig. 3 for all cases in Table 1. The measured OH imagesin Fig. 3 were constructed by connecting 100-mm-long overlap-ping LIF images recorded at different positions, as stated in Section2.2. The established flames were open, i.e. comprised two separatebranches extending nearly parallel to the catalytic walls, as alsoobserved in earlier pure hydrogen homogeneous combustion stud-ies over Pt [16,18]. This was in direct contrast to earlier fuel-leanmethane [15] and propane [36] studies, whereby closed flameswere formed in the same reactor. As discussed in [16], this waspredominantly an outcome of the diffusional imbalance of hydro-gen (LeH2 < 1): the transport of heat from the hot walls to the flow-ing gas was less effective than the transport of hydrogen from thechannel core towards the walls, thus confining the flames in thenear-wall regions. The OH-LIF data in Fig. 3 clearly indicated thatthe presence of CO, at the specified compositions in Table 1, didnot alter this specific feature of hydrogen flames.

The ignition positions (xig) marked with green arrows in Fig. 3have been defined in both measurements and predictions as thefar-upstream locations whereby the OH concentration reached5% of its maximum value over the entire reactor domain300 � 7 mm2. OH concentrations dropped substantially with rising

pressure; from 290 ppmv at 2 bar to 1.6 ppmv at 12 bar and finallyto sub-ppmv values at 14 bar (Fig. 3). Modest temperature differ-ences between the upper and lower catalytic walls led to a slightasymmetry between the two flame branches (see Fig. 3 and theprovided wall temperature profiles in Figs. 4 and 5); in all casesthe upper walls were hotter by as much as 50 K, such that homo-geneous ignition was always defined at the upper wall. The mea-sured homogeneous ignition distances were well reproduced bythe model for all cases with p 6 12 bar (whereby flames wereestablished in the reactor), with the differences between measuredand predicted ignition distances being less than 12% (see Table 1).It is pointed out that homogeneous ignition at p = 14 bar could notbe determined with the LIF data, as these cases pertained to sub-ppmv OH concentrations, which were not amenable to the planarLIF technique. Nonetheless, quantitative agreement at p = 14 barwas not of great concern since at this pressure gas-phase combus-tion was minimal, as will be elaborated below. Good agreement be-tween measurements and predictions was thus established notonly for the homogeneous ignition distances but also for the ensu-ing flame shapes and lengths. Such comparisons clearly demon-strated the aptness of the employed hetero-/homogeneousreaction mechanisms for syngas mixtures at high pressures andpreheats relevant to gas turbine applications.

Predicted axial profiles of catalytic (C) and gaseous (G) conver-sion rates of H2 and CO, along with the y-averaged (over the 7 mmchannel height) mass fractions of these two species are shown inFig. 4 for the p = 2 and 4 bar cases, in Fig. 5 for the 8 and 12 barcases 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 sur-faces, while the G rates were calculated by integrating the volu-metric gaseous H2 and CO conversions over the 7 mm channelheight. Furthermore, to better compare the extent of catalytic (C)and gas-phase (G) conversions between cases at different operatingconditions (equivalence ratios, mass flow rates), the conversionrates in Figs. 4–6 have been normalized by the corresponding inletH2 and CO mass fluxes of each case i.e. qINUINYH2,IN and qINUIN-

YCO,IN, respectively. As evidenced in Figs. 4 and 5, over the extentof the gaseous induction zone (x < xig) the contribution of thehomogeneous reaction pathway was minor compared to that ofthe heterogeneous pathway. The normalized C conversion ratesof H2 were, irrespective of the particular H2:CO ratio, always higherthan those of CO over the length x < xig (see Figs. 4–6). This wasattributed to the higher molecular diffusivity of hydrogen andnot to its higher catalytic reactivity, since the catalytic conversionsof both fuel components were, in all examined cases, mass trans-port 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) bydepleting both H2 and CO fuel components before they could dif-fuse from the channel core to the catalytic walls. The catalytic sup-pression efficiency was thus directly linked to the gas-phaseoxidation rates, which were in turn intricately dependent on pres-sure, fuel composition (H2:CO ratio) and preheat, issues that will beaddressed in Section 4.2. In contrast to the nearly complete sup-pression of C conversions well-downstream of the homogeneousignition location at 2 bar (Fig. 4a), there was an increased H2 andCO leakage through the flame zones towards the catalyst surfacesat p P 4 bar, as manifested by the non-vanishing C rates at x > xig inFigs. 4b and 5a and b. This led to combined heterogeneous andhomogeneous conversion of both H2 and CO. Moreover, the C con-versions after dropping monotonically with increasing axial dis-tance, started rising at the rear channel section (x > 200 mm) as aresult of the diminishing G conversions (see Figs. 4b and 5a andb). The results in Figs. 4 and 5 indicated that in order to enhancehomogeneous combustion at elevated pressures and hence to sup-press catalytic conversion at x > xig, increased mixture preheats

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 barsprovide 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 speciesmass 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 thecomputed location of homogeneous ignition (xig). Cases 1–6 in Table 1.

160 X. Zheng et al. / Combustion and Flame 160 (2013) 155–169

were necessary. As evidenced in Figs. 3 and 4a, low preheats(<450 K) were already sufficient to establish strong flames at2 bar, whereas at 8 and 12 bar lower gas-phase conversion rateswere 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 reac-tions were fully suppressed at low preheats and could only be mar-ginally increased at 654 K (Case 18). Moreover, when transitioning(for a given pressure) from H2-rich fuels to CO-rich fuels, a stronger

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 speciesmass 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 thecomputed location of homogeneous ignition (xig). Cases 7–14 in Table 1.

Fig. 6. Computed axial profiles of catalytic (C) and gas-phase (G) conversion ratesfor H2 (solid lines) and CO (dashed lines) normalized by the corresponding inletspecies mass fluxes; computed average (over the 7 mm channel height) axialprofiles of the H2 and CO mass fractions (Y) normalized by the corresponding inletmass fractions; thermocouple measurements (upper wall: squares; lower wall:circles) and fitted axial temperature profiles through the measurements (lines). Thevertical arrows define the computed location of homogeneous ignition (xig). Cases15–18 (p = 14 bar) 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).

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quenching of homogeneous reactions was observed (e.g., comparein 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 appreciablerise in catalytic conversion (by 10% for H2 and 20% for CO) whencompared to the H2-richer Cases 7–9 and 11–13, respectively.However, this composition effect was negligible for H2:CO P 1.0as seen by comparing Cases 7–9 or Cases 11–13.

To assess the impact of pressure, preheat, and fuel compositionon homogeneous chemistry, it was essential to ensure that theunderlying catalytic processes over the gaseous induction zonesx < xig were also well-captured by the heterogeneous kinetic mod-el. This was a cardinal requirement, since an incorrect prediction ofthe catalytic H2 and CO conversions over x 6 xig could greatly affectthe location of homogeneous ignition and hence falsify the gaseouskinetics [37]. To this direction, the Raman measurements ensured

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that the catalytic processes preceding homogeneous ignition werealso well predicted. Raman-measured and computed transverseprofiles of H2 and CO mole fractions are compared in Fig. 7 fortwo cases at 4 and 12 bar and for three axial positions precedingthe onset of homogeneous ignition. The predictions in Fig. 7yielded a catalytic conversion close to the transport-limit for bothH2 and CO, as evidenced by the practically vanishing concentra-tions of these species near both walls (y = 0 and 7 mm). This wasalso well reproduced by the measurements, despite the lack of Ra-man data closer than 0.7 mm to the catalytic walls. It is finallynoted that the surface temperatures needed to achieve homoge-neous ignition (see Figs. 4 and 5) were much higher than the tran-sition temperature of around 750 K, below which theheterogeneous chemistry coupling between H2 and CO was strongand could lead to CO poisoning of the catalyst [21,30]. Thus, thehigh surface temperatures in all examined cases greatly facilitatedthe present homogeneous ignition studies, as the performance ofcatalytic kinetic models at surface temperatures T < 750 K wasquite demanding.

Additional manifestation for the absence of CO surface poison-ing is illustrated in the surface coverage plots in Fig. 8, for threecases at 8 bar at different H2:CO ratios. Main coverage is O(s) andPt(s). With rising CO content, CO(s) increased; however, bothCO(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 per-tained to different operating parameters (mass throughputs, equiv-alence ratios and surface temperatures). These differences wereessential to stabilize flames inside the channel reactor at conditionsof broadly varying mixture preheats and pressures, without thedanger of autoignition and/or flashback. Nonetheless, to clarify thecoupling of heterogeneous and homogeneous kinetics and the im-pact 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: upperwall; dashed lines: lower wall.

tion over Pt, a comparative study was necessary between caseswhere only a limited number of parameters changed while theremaining were maintained constant. Consequently, detailed simu-lations have been performed using the validated hetero-/homoge-neous chemistries of Section 4.1, in channel geometries relevantto practical honeycomb reactors. A cylindrical channel was usedin the following simulations, with diameter d = 1.0 mm and lengthof 200 mm (sufficient to achieve at least 99.5% H2 and CO conversionfor all examined conditions). Pressures in the range 1–15 bar wereinvestigated, with inlet gas temperatures of 673 and 773 K, a walltemperature 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 (dependingon the H2:CO ratio) at p = 15 bar and TIN = 673 K. The selected inlettemperature and mass flux were relevant to gas turbine operatingconditions, while the surface temperature of 1350 K was typicallythe highest tolerable by reactor materials and catalyst for long-termoperation.

Predicted streamwise profiles of catalytic (C) and gaseous (G)conversion rates for both fuel components are illustrated in Fig. 9at pressures of 1, 4, 8 and 15 bar, equivalence ratio u = 0.3, preheatTIN = 673 K and H2:CO molar ratios varying from 0.25 to 4.0. Simu-lations with the higher investigated inlet temperature TIN = 773 Kgave the same qualitative trends and hence are not presented. Igni-tion distances (xig) were determined via the OH radical concentra-tion according to the procedure outlined in Section 4.1. As shownin Fig. 9, the ignition distances (xig) decreased with rising pressure.At p = 1 bar, the G conversions were very weak, however, a homo-geneous ignition could still be defined. At pressures p P 4 bar, theG conversions became stronger (although their magnitudes did notincrease monotonically with rising pressure) and ignition occurredclose to the reactor entry such that the resulting xig were nearly thesame.

With rising CO content, homogeneous ignition was delayed, asseen 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 acylindrical channel with diameter 1 mm, mass flux of 42.4 kg/m2 s, fuel H2:COratios of 4:1, 1:1 and 1:4, inlet temperature TIN = 673 K and equivalence ratiou = 0.3. Results are shown for pressures of 1, 4, 8 and 15 bar.

Fig. 10. Computed 2-D color maps of the H2 and CO mole fractions, isocontours ofH2, CO gas-phase reaction rates for a u = 0.30 H2/CO/air mixtures in a d = 1 mmcylinder reactor at p = 4 bar, a mass flux of 42.4 kg/m2 s, constant wall temperatureTwall = 1350 K and inlet temperature TIN = 673 K; r = 0 denotes the channel center-line 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 � 10�3,1.9 � 10�3 and 4.2 � 10�4 kmol/m3 s in (a) to (c), respectively. The outermostcontours refer to lower reaction rates; the increment between successive contoursis 4.0 � 10�4, 2.3 � 10�4 and 4.5 � 10�5 kmol/m3 s in (a) to (c), respectively. Themaximum gas-phase CO consumption rates are 3.4 � 10�4, 7.8 � 10�4 and1.2 � 10�3 kmol/m3 s in (a) to (c), respectively; the increment between successivecontours is 4.3 � 10�5, 9.7 � 10�5 and 1.5 � 10�4 kmol/m3 s in (a) to (c), respec-tively. (For interpretation of the references to color in this figure legend, the readeris referred to the web version of this article.)

X. Zheng et al. / Combustion and Flame 160 (2013) 155–169 163

xig always corresponded to ignition of the hydrogen component(Fig. 9); CO ignition followed that of H2, as the gas-phase oxidationof CO crucially depended on radicals – notably OH� produced byH2 chemistry. However, the delay between the two sequential H2

and CO ignitions changed with pressure as well as fuel composi-tion. For an H2:CO ratio of 1:4, the H2 and CO ignitions at p P 4 barwere nearly simultaneous (Fig. 9c).

It was further evident that, for both fuel components, catalyticand gas-phase conversions occurred in parallel. Even well-down-stream the location of homogeneous ignition, catalytic conversionpersisted: H2 and CO leaked through the gaseous combustion zoneto be subsequently converted on the catalyst surface. This effectwas more pronounced compared to the optically accessible chan-nel results in Section 4.1 (Figs. 4–6), due to the larger surface-to-volume ratio of the present 1 mm diameter channel.

Following homogeneous ignition, the axial extent of the estab-lished flames, as indicated by the G conversion rate profiles inFig. 9, exhibited strong dependence on pressure and H2:CO ratio.The peak gas-phase oxidation rate was a non-monotonic functionof pressure, being highest at 4 bar and dropping significantly athigher 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). Themagnitude of homogeneous oxidation rates at p > 4 bar, was alsodependent on fuel composition. For the H2-rich fuels (H2:CO ratioof 4:1), gas-phase oxidation rates were only slightly lower at8 bar when compared to 15 bar (Fig. 9a). When reducing theH2:CO ratio to 1:1 (Fig. 9b), the CO gas-phase oxidation rate for8 bar surpassed the corresponding one for 15 bar at x � 50 mm;farther downstream, the peak CO oxidation rate for 8 bar becameabout twice the one for 15 bar. Additional decrease of the H2 con-tent (H2:CO ratio of 1:4) shifted the gas-phase conversion rates at8 bar towards those at 4 bar, and intensified the magnitude differ-ence between the oxidation rates at 8 and 15 bar (Fig. 9c).

In addition to the conversion profiles in Fig. 9, 2-D distributionsof H2 and CO mole fractions and isocontours of H2 and CO gas-phase reaction rates for pressures 4 and 15 bar are illustrated inFigs. 10 and 11, respectively. Seven contours are provided withthe outermost one corresponding to 10% of the maximum reactionrate of each case. As evidenced by the reaction rate isocontours, theH2 gaseous oxidation zones were confined close to the catalyticplates, while the corresponding CO zones extended from the cata-lytic walls to the centerline, occupying the full radial reactor do-main. The mismatch in the extent of H2 and CO inner reactionrate isocontours (corresponding to the higher reaction rates) inFigs. 10b and c and 11c led to the occurrence of two peaks in theCO gas-phase conversion plots for H2:CO = 1:1 and 1:4 in Fig. 9band c. The first peaks of CO gas-phase oxidation curves coincidedwith those of the H2 curves, indicating that H2 reactions promotedCO oxidation in these regions. As also stated in Section 4.1, the con-finement of the H2 oxidation zone near the walls was largely anoutcome of the heat and mass diffusion imbalance of H2

(LeH2 < 1) [16,18]. However, at 4 bar the H2 reaction rates extendedto 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 Sec-tion 4.3.2, this was a result of the corresponding higher burningrates at 4 bar compared to those at 15 bar: the 4 bar flames couldtherefore stabilize farther away from the wall towards the channelcenter, since the radial mass fluxes supplying reactants to theflames were higher there (directly linked to the higher magnitudeof the H2 reaction rates, as provided in the captions of Figs. 10 and11). Finally, the reduced reaction rates at high pressures (p > 4 bar)required a considerably longer reaction zone to convert the samefuel mass throughputs.

The mole fraction maps in Figs. 10 and 11 indicated that hydro-gen was depleted within the first quarter of the reactor due to its

high catalytic conversion (driven by its high molecular diffusivitysince its consumption was mass transport limited) and also dueto its high gas-phase reactivity. The strong coupling of CO and H2

gaseous kinetics yielded significant CO oxidation over the samereactor extent. A combined catalytic and gas-phase reaction fluxdiagram at x = 22 mm is shown in Fig. 12a for the case inFig. 10b; therein, normalized reaction rates are provided for themajor steps. Reaction fluxes at x = 78 mm, far downstream of thecomplete H2 consumption, are shown for the same case inFig. 12b. At this position, the product H2O was crucially involvedin the reaction sequence to sustain the radical pool and in particu-lar the OH radical, which controlled the CO gas-phase oxidation.Catalytic oxidation was found to be much more efficient, as seenby comparing the relative magnitudes of the H2 and CO adsorptionrates to the corresponding gaseous consumption rates in Fig. 12(see also Fig. 9). The impact of pressure and H2:CO compositionon the rich homogeneous oxidation characteristics seen in Fig. 9will be elaborated in Section 4.3.

The coupling of heterogeneous and homogeneous kinetics viacompetitive reactant depletion, radical adsorption/desorptionreactions, and major species formation is finally discussed. Previ-ous work [18] has identified the key interactions of the two reac-

Fig. 13. Computed traverse profiles of OH mole fraction at various axial positionsfor a u = 0.30 H2/CO/air (H2:CO = 1:1) mixture in a d = 1 mm cylindrical channelwith a mass flux of 42.4 kg/m2 s, constant wall temperature Twall = 1350 K, inlettemperature TIN = 673 K and two pressures (p = 1 and 4 bar); r = 0 denotes thechannel centerline and r = 0.5 mm the channel wall. For clarity, the OH profiles at1 bar have been multiplied by 5.

Fig. 12. Hetero-/homogeneous reaction fluxes for a u = 0.30 H2/CO/air (H2:COmolar ratio of 1:1) mixtures in a d = 1 mm cylinder reactor at p = 4 bar, mass flux of42.4 kg/m2 s, constant wall temperature Twall = 1350 K and inlet temperatureTIN = 673 K (conditions as in Fig. 10b). (a) x = 22 mm and (b) x = 78 mm. Thereaction fluxes are normalized with respect to the rates of H2 þ OH() H2OþHand COþ OH() CO2 þ H in (a) and (b), respectively.

Fig. 11. Computed 2-D color maps of the H2 and CO mole fractions, isocontours ofH2, CO gas-phase reaction rates for a u = 0.30 H2/CO/air mixtures in a d = 1 mmcylinder reactor at p = 15 bar, a mass flux of 42.4 kg/m2 s, constant wall temper-ature Twall = 1350 K and inlet temperature TIN = 673 K; r = 0 denotes the channelcenterline 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 � 10�3,1.2 � 10�3 and 4 � 10�4 kmol/m3 s in (a) to (c), respectively. The outermostcontours refer to lower reaction rates; the increment between successive contoursis 1.8 � 10�4, 1.5 � 10�4 and 4.9 � 10�5 kmol/m3 s in (a) to (c), respectively. Themaximum gas-phase CO consumption rates are 1.2 � 10�4, 2.8 � 10�4 and3.8 � 10�4 kmol/m3 s in (a) to (c), respectively; the increment between successivecontours is 1.2 � 10�5, 3.5 � 10�5 and 3.4 � 10�5 kmol/m3 s in (a) to (c), respec-tively. (For interpretation of the references to color in this figure legend, the readeris referred to the web version of this article.)

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tion pathways for hydrogen fuel. These interactions included com-petitive fuel consumption and adsorption/desorption of minor andmajor species. Regarding the effects of competitive reactant deple-tion, the delayed homogeneous ignition at atmospheric pressureled to higher heterogeneous consumption that in turn stronglyinhibited homogeneous oxidation, as evidenced in Fig. 9. Therein,the catalytic (C) consumption rates at 1 bar were always the high-est among all cases and the corresponding homogeneous (G) con-sumptions were largely vanished. At p P 4 bar, gas-phaseignition occurred near the reactor entry. Such a substantial de-crease of the ignition distance resulted in flames established farupstream, thus favoring homogeneous fuel consumption andattenuating the inhibiting effect of catalytic fuel consumption.

The radical hetero-/homogeneous coupling was modest andcould be clarified with the traverse OH profiles in Fig. 13 at 1and 4 bar for an H2:CO molar ratio of 1:1. At atmospheric pressure,the very weak gaseous oxidation zone was aided through the OHflux from the surface, as shown by the positive OH transverse gra-dient on the wall (net-desorptive OH fluxes). With the formation ofstrong gaseous combustion zones at 4 bar, the OH flux was stillnet-desorptive over the gaseous induction zone (x < xig), but turnedto net-adsorptive following homogeneous ignition. Comparison ofthe predictions in Fig. 9 to additional simulations without theinclusion of OH adsorption/desorption steps demonstrated thatOH produced catalytically over the gaseous induction zone pro-moted (albeit to a small degree) homogeneous ignition under allexamined conditions. On the other hand, the post-ignition flame

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

X. Zheng et al. / Combustion and Flame 160 (2013) 155–169 165

bustion was the weakest) and drastically decreased with risingpressure. Characteristically, at 1 bar the removal of OH adsorp-tion/desorption reactions increased xig by �50% while it marginallyincreased xig for pressures p P 2 bar. Moreover, the aforemen-tioned 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 fromthe catalytic pathway, on the gaseous oxidation was finally as-sessed. 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 H2O� and CO�2 that had the same thermo-dynamic and transport properties as H2O and CO2, respectively, butdid not participate in any gas-phase reaction. The gaseous pathwaywas still allowed to produce normal H2O and CO2. Comparisons atp = 4 bar with predictions from the original catalytic reactionscheme are illustrated in Fig. 14a and b regarding the effect ofH2O� and in Fig. 14c and d regarding CO�2. The homogeneous pro-cesses in Fig. 14a and b were strongly affected by H2O� and, more-over, they exhibited a complex dependence on H2:CO ratio. On theother hand, the homogeneous processes in Fig. 14c and d were sig-nificantly less affected by CO�2.

The gaseous oxidation of both H2 and CO was substantially sup-pressed by catalytically-produced H2O for H2:CO = 4:1 (Fig. 14a).However, for H2:CO = 1:4 the gaseous H2 oxidation was marginallyaffected while the corresponding CO oxidation was significantlypromoted (Fig. 14b). The kinetic effect of H2O was a result of thecompetition between the radical termination reaction R11 (dueto its high third body efficiency, xH2O = 14, see footnote in TableA2) and the radical branching step R5 (the net of this reaction pro-ceeded in the reverse direction). Reaction flux analysis at elevatedpressures p P 4 bar indicated that, for sufficiently large H2:CO ra-tios, the catalytic production of copious amounts of H2O renderedR11 dominant in the aforementioned reaction competition anddramatically reduced the radical pool. Thus, the oxidation of bothfuel components (Fig. 14a) was inhibited. On the other hand, forCO-rich mixtures (H2:CO = 1:4) the enhancement on the radicaltermination step R11 was modest due to significant reduction ofcatalytically-produced H2O and the branching step R5 increasedin significance: this resulted in a net OH radical production whichwas higher for the case of normal H2O compared to H2O�. The en-

Fig. 14. Comparison of predicted gas-phase H2 and CO conversion rates betweenthe original catalytic reaction scheme (solid lines) and the modified catalyticreaction scheme (dotted lines) with fictitious species H2O� in (a) and (b), and CO�2 in(c) and (d). Computations in a cylindrical channel with diameter 1 mm, mass flux of42.4 kg/m2 s, H2:CO ratios of 4:1 and 1:4, inlet temperature TIN = 673 K, pressure 4bar and equivalence ratio u = 0.3.

hanced OH production, in turn, substantially promoted the CO oxi-dation (Fig. 14b). However, H2 consumption was marginallyaffected (Fig. 14b); this was because the increase in the forwardrate of the main hydrogen consumption reaction R4 was balancedby a corresponding increase in the reverse rate due to the presenceof higher amounts of H2O.

The chemical impact of CO2 was only inhibitive for the homoge-neous oxidation of both H2 and CO (Fig. 14c and d) due to its en-hanced third body efficiency in R11 (xCO2 = 3.8); this effect wasobviously stronger for higher CO contents in the fuel mixture(Fig. 14d). Finally, it could be shown that the pressure dependenceof 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 igni-tion characteristics primarily on pressure but also on H2:CO ratio.To clarify these effects, simulations have been performed in thissection using as numerical platform ideal reactors.

4.3.1. Ignition delaysAt 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 tempera-ture–time histories, have been calculated in a spatially homoge-neous constant pressure reactor (batch reactor) using the Senkinpackage of CHEMKIN [38]. Inverse ignition delays (quantitiesproportional to the gaseous reactivities) are plotted in Fig. 15 forpressures 1–15 bar, u of 0.3 and 0.2, initial temperaturesTo = 950–1350 K, and H2:CO molar ratios of 4, 2, 0.5 and 0.25. Theselected initial temperature range mimicked the gas preheatingand heat transfer from the hot catalytic walls occurring in practicalchannel reactors. Similar to previous findings on pure H2 fuel [18],the syngas inverse ignition delays (reactivities) followed a non-monotonic relationship with pressure, possessing a critical pres-sure (pcr) where the maximum reactivities occurred. The criticalpressure was a function of the initial gas temperature (the higherthe initial temperature, the larger the pcr value) and had a weakerdependence on fuel composition. Changing the initial gas tempera-ture to 1350 K and 950 K shifted the critical pressure pcr beyond15 bar and below 1 bar, respectively (i.e. outside of the pressurerange of interest for this study).

The profiles in Fig. 15 pertaining to the highest initial tempera-tures (e.g., To = 1250 K or 1350 K) were appropriate to explain theimpact of pressure on the ignition distances (the xig exhibited sim-ilar 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) atthis pressure (see Fig. 15) allowed for appreciable H2 and CO cata-lytic conversion that in turn deprived fuel from the homogeneouspathway thus inhibiting gaseous combustion. However, as thepressure increased, the shorter ignition delays (see Fig. 15) allowedfor appreciable homogeneous fuel conversion as the gaseouspathway could now compete more favorably against the catalyticpathway for fuel consumption. Nonetheless, the particular non-monotonic behavior of the gaseous conversions with rising pres-sure (significant drop in G above p = 4 bar, see Fig. 9) could notbe explained with the ignition characteristics in Fig. 15; this wasbecause the ignition delays either decreased (at 1350 K) or at leastchanged modestly (at 1250 K) with rising pressure at p P 4 bar. Toexplain the G drop at p P 4 bar in Fig. 9, arguments based on flamepropagation characteristics will be brought about in the followingSection 4.3.2.

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.

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The profiles in Fig. 15 at moderate temperatures (61150 K)were suitable for the interpretation of the experimental findingsin Figs. 4–6, as the plate temperatures and preheats in the opticallyaccessible reactor were considerably lower than those used in thetubular reactor simulation. In such moderate temperature cases,the change of the critical pressures towards the lower end of the1–15 bar range (Fig. 15) resulted in reduced reactivity (increasedignition delays) at high pressures; this could in turn explain thelack of vigorous flames at p = 14 bar seen in Figs. 3 and 6.

The observed ignition characteristics in Fig. 15 at low to modesttemperatures were an outcome of the competition between theradical branching and termination steps (R1, R11, R18 and R20 inTable A2) as clarified in [18]. The CO-involved radical reactions(R24 and R26) were secondary in affecting ignition characteristicswhen compared to H2 reactions. Moreover, the CO reactions reliedon H2 chemistry to provide necessary radicals O and HO2. There-fore, a rising CO content resulted in decreased reactivity of theCO/H2 mixtures, thus explaining the predicted effects on homoge-neous ignition for mixtures with different H2:CO compositions inFig. 9.

Fig. 16. Computed laminar mass burning rates plotted as a function of pressure for1-D freely propagating H2/CO/air flames with H2:CO molar ratios ranging from 4:1to 1:4. The total equivalence ratios are u = 0.30, 0.25 and 0.20 and the fresh mixturetemperature is To = 673 K.

4.3.2. Mass burning ratesLaminar burning rates have been computed using the 1-D pre-

mix flame code of CHEMKIN [39]. Results are illustrated in Fig. 16for equivalence ratios 0.2–0.3, H2:CO molar ratios of 4:1–1:4, pres-sures of 1–15 bar, and a fresh gas temperature To = 673 K. The se-lected equivalence ratio range and initial gas temperature weresuitable for simulating the tubular reactor of Section 4.2 whereinflames were stabilized close to the entry (see Fig. 9) and nearly par-allel to the walls via supply of fresh unburned gas from the channelcore at �673 K preheat. As shown in Fig. 16, the mass burning ratesfirstly increased with rising pressure, peaked at a transitionpressure ptr, then decreased with increasing pressure, and finallybecame modestly dependent on pressure. Peak burning rates ofH2-rich fuels are always higher than those of CO-rich fuels: thepeak mass burning rate for H2:CO = 4:1 is the highest and de-creases with dropping H2:CO ratio for each equivalence ratio.Moreover, the transition pressure ptr for H2:CO = 4:1 is the lowestand a reduction in H2:CO ratio drives ptr to higher values. Increas-ing the pressure above the transition point leads to burning rates ofCO-rich fuels crossing over those of H2-rich fuels due to the combi-nation of higher ptr and weaker pressure dependence of CO richfuels than those of H2 rich fuels. For instance, the mass burningrate for H2:CO = 1:4 (u = 0.25) at 15 bar is the highest among allexamined H2:CO molar ratios at this pressure, as evidenced in

Fig. 16b. A rise of the equivalence ratio to 0.3 increased the abso-lute mass burning rates and shifted the transition pressure andthe crossover points to higher values or even above the presentupper pressure limit of 15 bar as shown in Fig. 16a. Conversely,lowering the equivalence to 0.2 (Fig. 16c) significantly decreasedthe mass burning rates and shifted the transition pressures andcross-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 thecylindrical reactor (u = 0.30) and the optically accessible planarreactor (u 6 0.30), since the upstream heterogeneous consump-tion reduced the availability of fuel for gaseous reactions at thelocation of homogeneous ignition. Moreover, the unequal diffusiv-ities of the two fuel components could reduce the local H2:CO ratiowhen compared to this ratio in the initial feed. The profiles ofH2: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 barwere much lower than those of 4 bar. This was due to the dropof the mass burning rates at these higher pressures (Fig. 16b) thatin turn pushed the gaseous combustion zones closer to the walls,such that larger amounts of H2 and CO leaked through the flamezones to be subsequently consumed on the catalytic surface.Reducing the H2:CO ratio resulted in decreased burning rates atlow pressures. The shift of the transition pressure to higher valuesand the weaker pressure dependence at low H2:CO ratios enhancedthe burning rates at high pressures compared to those of mixtureswith higher H2:CO ratios. The resulting behavior in Fig. 16 wasthen consistent with the findings in Fig. 9b and c. It is also notedthat an asymptotic study of gaseous combustion over stagnationflow catalytic surfaces has reported a similar behavior [40]. Thisstudy has shown that a progressive increase of the strain ratepushed the flame towards the wall, leading to catalytic conversionof the leaked fuel and thus to weaker gaseous combustion. In thechannel-low geometry of Fig. 9, the aerodynamic strain (character-ized by the imposed radial fuel fluxes) was the same at all pres-sures since the mass throughput was maintained constant for allconditions; however, the laminar burning rates dropped apprecia-bly at the highest examined pressures, leading to the same resulti.e. a push of the flames towards the catalytic wall and increasedfuel leakage through the gaseous combustion zones.

The profiles for u = 0.20 in Fig. 16c could be used to interpretthe observations in the optically accessible reactor (Figs. 3–6),whereby measurements were performed mostly with u around0.2 for all studied pressures (except for 2 bar). The observed in-creased drop of gas-phase oxidation with rising pressure couldbe partially attributed to the reduced equivalence ratio, but pre-dominantly to the lower mass burning rates, even though the pre-heat increased from approximately 400 to �600 K as the pressureincreased. The additional CO2 dilutions with rising pressure (im-posed to avoid autoignition in the mixing section) further intensi-fied the aforementioned restraint of gaseous combustion. The

Table A1Heterogeneous chemical reaction mechanism.a

Reactions A

S1 H2 þ 2PtðsÞ ) 2HðsÞ 4S2 2HðsÞ ) H2 þ 2PtðsÞ 3S3 Hþ PtðsÞ ) HðsÞ 1S4 O2 þ 2PtðsÞ ) 2OðsÞ 1S5 O2 þ 2PtðsÞ ) 2OðsÞ 2S6 2OðsÞ ) O2 þ 2PtðsÞ 3S7 Oþ PtðsÞ ) OðsÞ 1S8 H2Oþ PtðsÞ ) H2OðsÞ 7S9 H2OðsÞ ) H2Oþ PtðsÞ 1S10 OHþ PtðsÞ ) OHðsÞ 1S11 OHðsÞ ) OHþ PtðsÞ 1S12 HðsÞ þ OðsÞ ) OHðsÞ þ PtðsÞ 3S13 OHðsÞ þ PtðsÞ ) HðsÞ þ OðsÞ 1S14 HðsÞ þ OHðsÞ () H2OðsÞ þ PtðsÞ 3S15 OHðsÞ þ OHðsÞ () H2OðsÞ þ OðsÞ 3S16 COþ PtðsÞ ) COðsÞ 8S17 COðsÞ ) COþ PtðsÞ 2S18 CO2ðsÞ ) CO2 þ PtðsÞ 1S19 COðsÞ þ OðsÞ ) CO2ðsÞ þ PtðsÞ 3S20 CðsÞ þ OðsÞ ) COðsÞ þ PtðsÞ 3S21 COðsÞ þ PtðsÞ ) CðsÞ þ OðsÞ 1S22 OHðsÞ þ COðsÞ ) HCOOðsÞ þ PtðsÞ 3S23 HCOOðsÞ þ PtðsÞ ) OHðsÞ þ COðsÞ 1S24 HCOOðsÞ þ OðsÞ ) OHðsÞ þ CO2ðsÞ 3S25 OHðsÞ þ CO2ðsÞ ) HCOOðsÞ þ OðsÞ 2S26 HCOOðsÞ þ PtðsÞ ) CO2ðsÞ þHðsÞ 3S27 CO2ðsÞ þ HðsÞ ) HCOOðsÞ þ PtðsÞ 2

a From [28,29]. The surface site density is C = 2.7 � 10-9 mol/cm2. In the surface and deand E [J/mol]. In all adsorption reactions, except S1 and S4, A denotes a sticking coefficienS16 has an order of two with respect to the platinum. The suffix (s) denotes a surface s

complete suppression of gaseous combustion at 14 bar forCases 15–17 (shown in Figs. 3 and 6) had also an analog in theaforementioned stagnation flow studies over catalytic surfaces[40]; therein a sufficient increase of the strain rate could pushthe flame against the stagnation surface and eventually extinguishit. In terms of channel-geometries, flame extinguishment has alsobeen 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 pressuredependence was evident in Fig. 16 as the transition pressure ptr

shifted to higher values with dropping H2:CO ratio. This influencewas attributed to the reduced global reaction order with risingCO content in syngas [12]. In [12] the mass burning rates _mo weremodeled as:

_mo ¼ qoSL / pn2ffiffiffiffiffiffiffiffiffiqoa

pexp � Ea=R

2Trz

� �; ð4Þ

where a is the mixture thermal diffusivity, qo the unburned gasdensity, n the overall reaction order, Ea the effective activation en-ergy, R the gas constant and Trz the reaction zone temperature. Var-iation of H2:CO ratio did not change the monotonic drop of thereaction order n with respect to pressure but changed its rate ofdrop. It was shown in [12] that the reaction order for H2:CO = 1:4decreased much slower with pressure than the one forH2:CO = 4:1, such that the reaction order of the former mixtureovertook the reaction order of the latter at about 2 bar. At 15 barthis led to a reaction order n � 1.1 for H2:CO = 1:4, which wasnearly twice the value for H2:CO = 4:1 (n � 0.6). Such a compositiondependence of the mass burning rate exemplified the importance ofthe CO reaction COþHO2 () CO2 þ OH (R26), which produced OHto compensate the termination reaction (R11) at high pressures andhigh CO contents.

The foregoing discussion of mass burning rates could clarify thecombustion behavior in the cylindrical channel (Figs. 9–11) andthe planar reactor (Figs. 3–6) with the variation of pressure at

b Ea

.46E+10 0.5 0

.70E+21 0 67,400–6000hH

.00E+00 0 0

.80E+21 �0.5 0

.30E�02 0 0

.70E+21 0 213,200–60,000hO

.00E+00 0 0

.50E�01 0 0

.00E+13 0 40,300

.00E+00 0 0

.00E+13 0 192,800

.70E+20 0 70,500

.00E+21 0 130,690

.70E+21 0 17,400

.70E+21 0 48,200

.40E�01 0 0

.13E+13 0 136,190–33,000hCO

.00E+13 0 20,500

.70E+20 0 108,000–33,000hCO

.70E+21 0 62,800

.00E+18 0 184,000

.70E+21 0 94,200

.33E+21 0 870

.70E+21 0 0

.79E+21 0 151,050

.70E+21 0 0

.79E+21 0 90,050

sorption reactions, the reaction rate coefficient is k = ATbexp(�Ea/RT), A [mol cm K s]t. Reactions S4 and S5 are duplicate. S1 has an order of one with respect to platinum.pecies.

Table A2Homogeneous chemical reaction mechanism.a

Reactions A b Ea

R1 H + O2 = O + OH 1.04E+14 0.00 15,286R2 O + H2 = H + OH 3.82E+12 0.00 7948R3 O + H2 = H + OH 8.79E+14 0.00 19,170R4 H2 + OH = H2O+H 2.16E+08 1.51 3430R5 OH + OH = O + H2O 3.34E+04 2.42 �1930R6 H2 + M = H + H + M 4.58E+19 �1.40 104,380R7 O + O + M = O2 + M 6.16E+15 �0.50 0R8 O + H + M = OH + M 4.71E+18 �1.00 0R9 H2O + M = H + OH + M 6.06E+27 �3.32 120,790R10 H2O + H2O = H + OH + H2O 1.01E+26 �2.44 120,180R11 H+O2(+M) = HO2(+M) 4.65E+12 0.44 0

H + O2(+M) = HO2(+M) 6.37E+20 �1.72 525R12 HO2+H = H2 + O2 2.75E+06 2.09 �1451R13 HO2 + H = OH + OH 7.08E+13 0.00 295R14 HO2 + O = O2 + OH 2.85E+10 1.00 �723.9R15 HO2 + OH = H2O+O2 2.89E+13 0.00 �497R16 HO2 + HO2 = H2O2 + O2 4.20E+14 0.00 11,982R17 HO2 + HO2 = H2O2 + O2 1.30E+11 0.00 �1629.3R18 H2O2(+M) = OH + OH(+M) 2.00E+12 0.90 48,749

H2O2(+M) = OH + OH(+M) 2.49E+24 –2.30 48,749R19 H2O2 + H = H2O + OH 2.41E+13 0.00 3970R20 H2O2 + H = HO2 + H2 4.82E+13 0.00 7950R21 H2O2 + O = OH + HO2 9.55E+06 2.00 3970R22 H2O2 + OH = HO2 + H2O 1.74E+12 0.00 318R23 H2O2 + OH = HO2 + H2O 7.59E+13 0.00 7270R24 CO + O(+M) = CO2(+M) 1.80E+10 0.00 2380

CO + O(+M) = CO2(+M) 1.55E+24 �2.79 4190R25 CO + O2 = CO2 + O 2.53E+12 0.00 47,700R26 CO + HO2 = CO2 + OH 3.01E+13 0.00 23,000R27 CO + OH = CO2 + H 2.23E+05 1.90 �1160R28 HCO + M = H + CO + M 4.75E+11 0.70 14,900R29 HCO + O2 = CO + HO2 7.58E+12 0.00 410R30 HCO + H = CO + H2 7.23E+13 0.00 0R31 HCO + O = CO + OH 3.02E+13 0.00 0R32 HCO + O = CO2 + H 3.00E+13 0.00 0R33 HCO + OH = CO + H2O 3.02E+13 0.00 0R34 HCO + HO2 = CO2 + OH + H 3.00E+13 0.00 0R35 HCO + HCO = H2 + CO + CO 3.00E+12 0.00 0R36 HCO + HCO = CH2O + CO 3.13E+13 0.00 0

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 and0.42, respectively, and R24 a pressure-dependent reaction (second entries are thelow pressure limits).

168 X. Zheng et al. / Combustion and Flame 160 (2013) 155–169

p P 4 bar and of H2:CO ratio. The transition pressure ptr was a crit-ical parameter in determining the combustion efficiency and thusgreat attention should be paid on its effects when designing andoperating hetero-/homogeneous combustion systems.

5. Conclusions

The hetero-/homogeneous combustion of fuel-lean H2/CO mix-tures over Pt was investigated experimentally at H2:CO molar ra-tios 0.47–4.54 and turbine-relevant conditions, specificallypressures up to 14 bar and preheat temperatures up to 736 K. Com-plementary 2-D simulations were performed to evaluate the per-formance of hetero-/homogeneous kinetic models in terms oftheir capacity to reproduce the homogeneous combustion charac-teristics. Assessment of the pressure and composition effects onthe gaseous oxidation processes was further achieved with de-tailed simulations in a practical cylindrical channel with a diame-ter of 1 mm. The key results of this study are summarized below.

(1) Predictions with the employed detailed hetero-/homoge-neous kinetic schemes satisfactorily reproduced the LIF-OHmeasured gas-phase ignition distances and the flame shapesfor all examined conditions. Mass transport limited catalyticconversion of CO and H2 was attested by both the Ramanmeasurements and model predictions. The results indicatedthat enhanced preheats were required to sustain gaseouscombustion at elevated pressures. At 14 bar, gaseous com-bustion was fully suppressed at 400 K preheat and onlymildly restored at 658 K preheat. Measured and predictedflame shapes for all investigated H2:CO ratios were open(i.e. formed two separate branches) and this was largely anoutcome of the diffusional imbalance of the H2 fuel compo-nent (LeH2 < 1). The addition of CO up to an H2:CO ratio of 0.5did not alter this particular feature of hydrogen gas-phasecombustion.

(2) For a given pressure, a stronger quenching of homogeneousreactions was observed with increasing CO content, leadingto increased CO and H2 leakage through the gaseous com-bustion zone to be subsequently converted catalytically onthe channel walls. This increased fuel leakage through thegaseous combustion zone led to a noticeable increase in cat-alytic conversion (by 10% for H2 and 20% for CO) for theH2:CO ratio of 0.5 when compared to the H2:CO ratio of 4.Moreover, this composition effect was negligible for ratiosH2:CO P 1.0.

(3) Gas-phase ignition of H2 was achieved slightly upstreamthat of CO, despite the fact that over the gaseous inductionzone the catalytic depletion of H2 was higher than the corre-sponding one of CO due to the larger molecular diffusivity ofthe former species. Simulations in the optically accessiblechannel geometry as well in a tubular channel with 1 mmdiameter indicated that the initiation of gas-phase CO oxida-tion reactions necessitated H2 gas-phase oxidation reactionsfor the supply of crucial OH radicals. However, at the rear ofthe CO gaseous oxidation zones whereby H2 was fully con-verted, the upstream produced H2O (via the catalytic andgaseous pathways) facilitated the radical pool buildup forthe CO gaseous oxidation.

(4) The OH radical adsorption/desorption reactions promotedmildly homogeneous ignition at p P 2 bar due to the net-desorptive OH flux over the gaseous induction zone. How-ever, the effect of these reactions on the downstream gas-eous combustion zone was determined by the sign of theOH flux (net desorptive or adsorptive). The hetero-/homoge-neous radical coupling was insensitive to the specific H2:CO

ratio. The kinetic impact of the catalytically-produced H2Oon the gaseous oxidation of H2 and CO strongly varied withH2:CO ratio. CO gas-phase oxidation was inhibited by cata-lytically-produced H2O at high H2:CO ratios and was pro-moted at low H2:CO ratios. On the other hand, theheterogeneous production of CO2 always inhibited gas-phase oxidation of H2 and CO but its effect was weaker whencompared to that of H2O.

(5) Computations in a tubular channel with 1 mm diameterwere carried out under a constant mass flux and preheat rel-evant to large scale gas turbine applications. The gaseousoxidation was suppressed at 1 bar, preheat of 673 K, andconstant wall temperature of 1350 K. This was due to theincreased catalytic fuel consumption over the extended gas-eous pre-ignition period, which in turn competitivelydeprived fuel from the homogeneous pathway. At elevatedpressures, homogeneous ignitions occurred fartherupstream (compared to 1 bar) and flame propagation char-acteristics gained importance in determining the strength

X. Zheng et al. / Combustion and Flame 160 (2013) 155–169 169

of the established flames. The reduced mass burning rates atp > 4 bar confined the combustion zones near the walls andsubsequently led to a leakage of fuel through the near-wallflames, thus appreciably reducing the homogeneous fuelconversion.

(6) Gaseous ignition and propagation characteristics were quan-titatively analyzed with the aid of homogeneous batch reac-tor and 1-D freely propagating flame codes, respectively. Thepredictions could explain the observations in the opticallyaccessible reactor and the results in the cylindrical channelgeometry regarding the variation of gaseous fuel conversionunder a wide range of pressures, temperatures, and H2:COratios. In contrast to the well-known monotonic increaseof ignition delays with rising CO fuel fraction, the mass burn-ing rates could either increase or decrease with increased COfuel fraction, depending on the pressure. This was attributedto the weakening pressure dependence of the effective glo-bal reaction order used to describe the mass burning rateswhen reducing the H2:CO ratio.

Acknowledgments

Support was provided by Swiss-Electric Research, the SwissCenter of Energy and Mobility (CCEM) project Carbon Managementand the EU under project H2-IGCC. We thank Mr. Rene Kaufmannfor help in the experiments.

Appendix A

Chemical reaction mechanisms. See Tables A1 and A2.

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