1021

Proceedings of the Combustion Institute, Volume 29, 2002/pp. 1021–1029

HOMOGENEOUS IGNITION IN HIGH-PRESSURE COMBUSTION OFMETHANE/AIR OVER PLATINUM: COMPARISON OF MEASUREMENTS AND

DETAILED NUMERICAL PREDICTIONS

MICHAEL REINKE, JOHN MANTZARAS, ROLF SCHAEREN, ROLF BOMBACH,WOLFGANG KREUTNER and ANDREAS INAUEN

Paul Scherrer InstituteCombustion Research

CH-5232 Villigen-PSI, Switzerland

The gas-phase ignition of fuel-lean methane/air premixtures over Pt was investigated experimentallyand numerically in laminar channel-flow configurations at pressures of up to 10 bar. Experiments wereperformed in an optically accessible catalytic channel reactor established by two Pt-coated ceramic plates,300 mm long (streamwise direction) and placed 7 mm apart (transverse direction). Planar laser-inducedfluorescence (PLIF) of the OH radical along the streamwise plane of symmetry was used to monitor theonset of homogeneous (gas-phase) ignition, and thermocouples embedded beneath the catalyst providedthe surface temperature distribution. Computations were carried out with a two-dimensional elliptic nu-merical code, which included the elementary heterogeneous (catalytic) reaction scheme for methane onPt from Deutschmann and two different elementary homogeneous reaction schemes, Warnatz and GRI-3.0. Following homogeneous ignition, very stable V-shaped flames were established in the reactor. Atpressures of up to 6 bar, the measured and predicted (Deutschmann/Warnatz schemes) flame sweep anglesand OH levels were in good agreement with each other, while the homogeneous ignition distances werepredicted within 10%. However, at pressures greater than or equal to 8 bar, a marked overprediction ofthe homogeneous ignition distances was evident (�25%). The Deutschmann/GRI-3.0 schemes yieldedmuch shorter (�55%–65%) homogeneous ignition distances at all pressures. Sensitivity analysis indicatedthat the latter discrepancies were ascribed to the homogeneous reaction pathway. GRI-3.0 yielded a muchfaster radical pool buildup than the scheme of Warnatz, clearly showing its inapplicability under catalyticallystabilized combustion (CST) relevant conditions. The heterogeneous reactivity was enhanced with increas-ing pressure. Although the increase in pressure inhibited the adsorption of methane due to the resultinghigher oxygen surface coverage, this effect was overtaken by the corresponding increase of the methanegas-phase concentration.

Introduction

The advancement of catalytically stabilized com-bustion (CST) to practical devices requires the de-velopment of catalysts with enhanced activity andthermal stability, the knowledge of heterogeneous(catalytic) kinetics and their coupling with low-tem-perature homogeneous (gas-phase) kinetics, and theavailability of improved numerical models. In partic-ular, multidimensional CFD codes [1,2] supple-mented with validated hetero/homogeneous chem-ical reaction schemes can aid considerably the designof CST reactors. Methane is the main fuel compo-nent of CST-based power systems and the under-standing of its heterogeneous kinetics on preciousmetals has progressed significantly, for both the total[3] and partial [1,4] oxidation routes. Complement-ing the previous heterogeneous kinetic studies, com-bined hetero/homogeneous chemical reactionschemes for CH4/air CST over Pt were validatedrecently [5] in their capacity to reproduce measured

homogeneous ignition characteristics at atmosphericpressure. Homogeneous ignition is detrimental tothe catalyst integrity and, therefore, the delineationof safe operating conditions that ensure no gas-phasecombustion inside the catalytic reactor is of primeimportance. To this direction, homogeneous ignitioncriteria for channel-flow CST were developed re-cently [6], elucidating the dependence of the ho-mogeneous ignition distance on the relevant flow,transport, chemical, and geometrical channel-flowparameters; they were further adapted to atmo-spheric-pressure CH4/air CST over Pt in Mantzaraset al. [7], using the hetero/homogeneous schemesthat were validated in Dogwiler et al. [5].

High-pressure operation, typical of many practicalsystems, exacerbates the potential of homogeneousignition due to the enhanced reactivity of hydrocar-bons with increasing pressure [8]. Therefore, in par-allel with surface science studies of heterogeneousprocesses under realistic pressures [9], the under-standing of high-pressure hetero/homogeneous

1022 NEW CONCEPTS IN COMBUSTION TECHNOLOGY—Catalytic Combustion

Fig. 1. Schematic of the high-pressure catalytic com-bustion test rig and the OH PLIF arrangement. All dis-tances are in millimeters. The reactor volume is defined bythe 300 � 104 � 7 mm3 enclosure.

combustion is cardinal for CST. The present studyundertakes an experimental and numerical investi-gation of high-pressure CH4/air CST over Pt, withthe main objective to validate the CST applicabilityof various hetero/homogeneous reaction schemes. Aparticular objective was to investigate the effect ofpressure on the underlying hetero/homogeneousprocesses. Experiments were performed in an opti-cally accessible, laminar channel-flow catalytic re-actor with Pt-coated walls, at pressures of up to 10bar. Planar laser-induced fluorescence (PLIF) of theOH radical was used to monitor the onset of ho-mogeneous ignition and thermocouples embeddedbeneath the catalyst yielded the surface temperaturedistribution. The numerical predictions included atwo-dimensional elliptic fluid mechanical model,which included the heterogeneous scheme for thetotal oxidation of CH4 over Pt by Deutschmann etal. [3] and the homogeneous schemes of Warnatzand Maas [10] and GRI-3.0 [11].

Experimental

High-Pressure Burner

The test-rig (see Fig. 1) consisted of an opticallyaccessible catalytic channel reactor, which formed aliner inside a 1.8-m-long and 0.28-m-in-diameterhigh-pressure cylindrical vessel. The reactor itselfcomprised two horizontal Si[SiC] ceramic plates300 mm long and placed 7 mm apart, supported onan inconel-steel frame. Two 3-mm-thick quartz win-dows (spring pressed against the ceramic plates)

formed the other two channel surfaces. Plasma va-por deposition was used to coat the inner Si[SiC]surfaces with a 1.5-lm-thick non-porous Al2O3 layerfollowed by a 2.2-lm-thick Pt layer. The surfacetemperature along the streamwise plane of symme-try was measured by S-type thermocouples (12 foreach plate) embedded 0.9 mm beneath the catalystthrough holes eroded from the outer, inert Si[SiC]surfaces. The catalyst plate temperatures were con-trolled independently by two resistive heaters posi-tioned above the outer ceramic surfaces. The air waspreheated, mixed with CH4 in two static mixers,passed through a 40-mm-long section filled with ce-ramic spheres 2 mm in diameter, and then througha 50-mm-long inert honeycomb placed just up-stream of the channel entrance to attain a uniforminlet velocity profile. A thermocouple positioned atthe downstream end of the honeycomb monitoredthe reactor inlet temperature. The air and methaneflow rates were regulated with two BROOKS massflow controllers. A small amount of secondary air(supplied by a distribution ring) flushed the volumebetween the liner and the vessel from unwantedcombustion products. To facilitate optical accessibil-ity, the high-pressure vessel had two 350-mm-longand 35-mm-thick quartz side windows at the sameaxial locations as the windows of the reactor. Twoadditional quartz windows located at the reactor’sexhaust section and the rear flange of the high-pres-sure vessel, respectively, provided a streamwise op-tical access through which the PLIF laser sheet wasintroduced into the reactor (Fig. 1). All thermocou-ples and power inputs were driven to the reactor viahigh-pressure feedthroughs.

Laser Diagnostics

A frequency-doubled Nd:YAG pulsed laser(Quantel YG781C20) pumped a tunable dye laser(Quantel TDL50). Its frequency-doubled radiation(285 nm) was transformed into a laser sheet by acylindrical lens telescope and a 1-mm slit mask. Thelaser sheet propagated counterflow, along the x-ysymmetry plane from the exhaust side (Fig. 1). Thepulse energy was low enough (0.5 mJ) to avoid sat-uration of the A(v � 1) r X(v� � 0) transition. Thefluorescence of both OH (1-1) and (0-0) transitionsat 308 and 314 nm, respectively, was collected at 90�through one of the vessel’s side windows with an in-tensified CCD camera (LaVision FlameStar 2F,576 � 384 pixels) equipped with a 105-mm focallength f/4.5 lens (Nikon UV-Nikkor) and a band-passfilter centered at 310 nm. A 67 � 7 mm2 section ofthe combustor was imaged on a 576 � 60 pixel area:the camera was traversed axially to map the entire300-mm channel extent. As the flow conditions werelaminar, 100 images were averaged to increase thesignal-to-noise ratio. The OH PLIF was calibratedwith absorption measurements performed with the

HIGH-PRESSURE HOMOGENEOUS IGNITION OF METHANE 1023

vertical laser sheet crossing laterally (z direction) thereactor through the side windows. This arrangementprovided the only possible through-access neededfor the calibration and was the same as in earlieratmospheric-pressure CH4/air CST studies [5].Given the absorption path length and the Einsteinfactor of the Q1(7) OH line (including pressurebroadening), the concentration at the location of theabsorption measurement could be deduced at eachparticular pressure. The concentrations in the otherparts of the flame were calculated by scaling the fluo-rescence signal to that of the absorption location.This was a very good approximation since thequenching was nearly constant everywhere given thevery lean premixed conditions and the small tem-perature variations across the flame. In that, mod-eling of the quenching was not necessary.

Numerical

A two-dimensional elliptic CFD code based on afinite-volume approach was used (see Refs. [2,12]).Gaseous and surface reaction rates were evaluatedwith CHEMKIN [13] and Surface-CHEMKIN [14].Gaseous and surface thermodynamic data weretaken from CHEMKIN [15] and Warnatz et al. [16],respectively. Mixture-average diffusion, includingthermal diffusion for the light species [17], was usedin the species transport. For surface chemistry, theCH4/O2 scheme on Pt by Deutschmann et al. [3]was employed (11 surface species, 9 gaseous species,and 24 reactions). The surface site density was 2.7 �10�9 mol/cm2, simulating polycrystalline platinum[2,3]; the 2.2-lm-thick platinum layer on top of anon-porous Al2O3 layer closely resembled such asurface. For the very lean mixtures of this study, twoC1/H/O gaseous mechanisms were used: Warnatzand Maas [10], further denoted as Warnatz (106 re-actions and 19 species including N2) and GRI-3.0[11] (242 reactions and 22 species). The pressuredependence in Warnatz’s scheme was modeled ac-cording to Warnatz et al. [18].

Computations were performed over half the chan-nel domain (300 � 3.5 mm2). An orthogonalstaggered grid of 420 � 80 grid points (in x and y,respectively) was sufficient to produce a grid-inde-pendent solution. The inlet boundary conditionswere uniform profiles for the temperature, the axialvelocity, and the species mass fractions. Fittedcurves through the individual thermocouple mea-surements provided the wall-temperature profiles.The profiles of both plates differed, at any axial lo-cation, by a maximum of �8 K. The average tem-perature profile between both plates was used as theenergy wall boundary condition; computations haveshown that the aforementioned local random plate-temperature differences resulted in ignition distancedifferences of the order of �3%, a factor well within

the overall experimental uncertainty. No-slip condi-tions were used for both velocity components at thewall (y � 0) and zero-Neumann conditions at theplane of symmetry (y � 3.5 mm) and the channeloutlet (x � 300 mm) for all scalars.

Results and Discussion

Comparisons between measured and predicteddistributions of the OH radical are illustrated in Fig.2; the predictions refer to the Deutschmann/War-natz schemes. All flow conditions were laminar, withincoming Reynolds numbers (ReIN) ranging from632 to 1268 (see legend of Fig. 2). Once established,the stability and reproducibility of all V-flames withregard to their shapes and anchoring positions wereexcellent over extended measuring times; hence, thePLIF images of Fig. 2 were constructed by joiningsuccessive 67-mm-long camera records. The homo-geneous ignition distance (xig) was defined by theintersection with the wall of lines (at an angle equalto the average flame sweep angle) fitted through theflame tails of Fig. 2. With increasing pressure, theinlet velocity was reduced (see legend of Fig. 2) tomaintain a laminar flow. As the homogeneous igni-tion distance in channel-flow CST scaled with ReIN(see discussion in Ref. [7]), an increase in ReINthrough an increase in pressure would have inhib-ited homogeneous ignition if the gaseous pathwayremained unaffected by pressure. However, not-withstanding the twofold increase in ReIN when thepressure was increased from 3 to 10 bar, all mea-sured flames of Fig. 2 ignited at about the same axiallocations, clearly indicating an increase of the gase-ous reactivity with increasing pressure. As seen fromFig. 2, the agreement between measured and pre-dicted homogeneous ignition distances was, irre-spective of u, particularly good (within �6%) atpressures of up to 4 bar with a somewhat increaseddeviation (�10%) at 6 bar; the good agreement wasalso extended to the sweep angles of the V flamesand in the absolute OH concentrations. At evenhigher pressures, the deviations became larger(�25%, cases 6 and 7). In case 5, the predictions didnot capture the formation of a V-shaped flame; con-ditions similar to case 5 were common at pressures�8 bar. Fig. 3 illustrates the computed average (overthe transverse direction) mole fractions of CH4, CO,OH, the fractional CH4 conversions, and the mea-sured wall temperature profiles. The CH4 consump-tion was accompanied by the buildup of CO; in cases1 and 4, both CH4 and CO were consumed shortlyafter homogeneous ignition, with practically no CH4or unburned CO escaping at the channel exit. At theignition point, both reaction pathways had alreadyconverted 82% and 76% of CH4 in cases 1 and 4(Fig. 3), respectively; the corresponding values were

1024 NEW CONCEPTS IN COMBUSTION TECHNOLOGY—Catalytic Combustion

Fig. 2. PLIF-measured (1a–7a) and -predicted (1b–7b) OH concentrations (ppmv). The predictions refer to theDeutschmann/Warnatz chemical reaction schemes. The rear section of the catalytic channel is shown (120 � x � 300mm). The green arrows illustrate the onset of homogeneous ignition. The experimental conditions are: (1) p � 3 bar,u � 0.36, UIN � 0.75 m/s, TIN � 560 K, ReIN � 640; (2) p � 4 bar, u � 0.31, UIN � 0.56 m/s, TIN � 562 K, ReIN �

632; (3) p � 4 bar, u � 0.4, UIN � 0.55 m/s, TIN � 546 K, ReIN � 645; (4) p � 6 bar, u � 0.36, UIN � 0.47 m/s,TIN � 563 K, ReIN � 790; (5) p � 8 bar, u � 0.4, UIN � 0.57 m/s, TIN � 567 K, ReIN � 1263; (6) p � 8 bar, u �

0.4, UIN � 0.52 m/s, TIN � 566 K, ReIN � 1149; (7) p � 10 bar, u � 0.36, UIN � 0.47 m/s, TIN � 571 K, ReIN �

1268. The inlet Reynolds numbers were based on the channel hydraulic diameter (� 13.1 mm). The levels of OH foreach pair of images (measured and computed) are indicated by the color-coded bars.

81% and 88% for cases 2 and 3. Such large preig-nition CH4 conversions were much desired to ex-emplify the degree of hetero/homogeneous cou-pling.

The underlying hetero/homogeneous processesare further elucidated in Fig. 4 (case 4), which illus-trates the computed heterogeneous and homoge-neous production rates of four species. Throughout

the entire channel length, CH4 was consumed byboth reaction pathways, whereas throughout thepreignition zone CO and H2 were produced by thegaseous and consumed by the catalytic pathway. Inthe early preignition zone, OH was produced by thesurface and consumed by the gaseous pathway; how-ever, at the latest preignition stages (x � 145 mm)the heterogeneous pathway became a sink of OH

HIGH-PRESSURE HOMOGENEOUS IGNITION OF METHANE 1025

Fig. 3. Profiles of the computed average (over the chan-nel transverse direction) species mole fractions (CH4,dashed-double dotted; CO, dotted; and OH, solid), frac-tional CH4 conversion summed over the heterogeneousand homogeneous pathways (dashed-dotted lines), andmeasured wall temperature averaged over top and bottomcatalyst plates (dashed lines), cases 1 (3 bar), 4 (6 bar), and7 (10 bar). All computations refer to the hetero/homoge-neous schemes of Deutschmann/Warnatz. The predictedhomogeneous ignition distances are shown by the verticalarrows.

Fig. 5. Transverse profiles of the CH4 mole fraction andstreamwise profiles of oxygen coverage (Inset). Case 1 (3bar), solid lines; case 4 (6 bar), dotted lines; and case 7 (10bar), dashed lines. The case notation is the same as in Fig.2. The catalytic wall is located at y � 0. The transverseCH4 profiles refer to three selected axial locations: x � 38,96, and 154 mm. The arrow points to the direction of in-creasing axial distance. The Inset provides the streamwiseprofiles of the oxygen surface coverage (HO) for cases 1, 4,and 7; the line notation is the same as in the main figure.

Fig. 4. Streamwise profiles of predicted (Deutschmann/Warnatz schemes) species production rates for case 4 (6bar). The solid lines denote the heterogeneous pathwayand the dashed lines the homogeneous pathway. The con-tribution of the homogeneous pathway has been integratedover the transverse direction. The vertical arrow indicatesthe computed location of homogeneous ignition. In thepreignition zone, CO and H2 were produced by the gaseousand consumed by the surface pathway, their net rates ofproduction being positive.

radicals that resulted in an overall mild homoge-neous ignition inhibition. This was confirmed by re-moving both OH adsorption/desorption reactionsand computing anew; the xig was reduced by about10%. In case 7 (10 bar) of Fig. 2, the numericalpredictions indicated a weak ignition occurring�50% farther downstream of the measured location.At the ignition location, 98% of the methane wasalready consumed (Fig. 3); moreover, the in-channelflame residence time was not sufficient, so that con-siderable amounts of unburned CO were computedat the reactor exit (1050 ppmv). This was even morepronounced in the predictions of case 5, which didnot capture the formation of the V-shaped flame;large amounts of CH4 and CO exited the reactor(e.g., 8% unburned CH4 and 3650 ppmv CO). Theforegoing have exemplified the importance of thehybrid CST combustion approach [19], since theheterogeneous pathway itself, without a subsequentpostcatalyst flame, cannot provide complete COburnout. The important conclusion reached fromthe comparisons of Fig. 2 is the validation of theDeutschmann/Warnatz schemes at pressures of upto 6 bar, a range of particular interest in microtur-bine applications.

The purely surface processes are investigated inFig. 5 with the transverse CH4 profiles (at three se-lected preignition axial locations) and the streamwisecoverage of oxygen (HO), the dominant surface spe-cies. With increasing pressure (and hence ReIN), the

1026 NEW CONCEPTS IN COMBUSTION TECHNOLOGY—Catalytic Combustion

Fig. 6. Streamwise profiles of computed average (overthe channel transverse direction) CH4 and OH mole frac-tions for cases 1 (3 bar) and 7 (10 bar). Computations withtwo different gaseous schemes (and the heterogeneousscheme of Deutschmann), Warnatz and GRI-3.0. Dashed-double dotted lines, CH4; solid lines, OH. The vertical solidarrows indicate the predicted homogeneous ignition loca-tions with the schemes of Warnatz (xig,W), GRI-3.0 (xig,GRI)and the vertical dashed arrows the measured ones (xig,m).

fractional CH4 catalytic conversion decreased asmanifested by the corresponding higher CH4 levelsnear the channel center. Although the transversefluid mechanical transport was augmented at higherpressures due to the increase in ReIN, the low CH4wall levels of Fig. 5 indicated that the surface reac-tions were still able to cope with this increase; themethane conversion was practically mass-transport-limited in all cases and, moreover, the wall CH4 lev-els were somewhat lower in the higher-pressurecases. This was due to the increased heterogeneousreactivity at higher pressures, for the reasons ex-plained next. The inset of Fig. 5 indicated higheroxygen coverage at higher pressures due to the cor-responding increase of the oxygen partial pressure.The HO profiles had a minimum at the location ofmaximum wall temperature (see Fig. 3) due to thenegative temperature dependence of the O2-stickingcoefficient [3]; the minimum HO levels in all profilesof Fig. 5, however, reflected the O2 partial pressureeffect, as the maximum wall temperatures of thethree cases differed by only 9 K. The initiation stepof the surface mechanism was CH4 adsorption,which had an overall order of 2.3 with respect to thefree site coverage [3], the free coverage being prac-tically HPt � 1 � HO. The resulting CH4 adsorptionrate was Rads � cCH4HPt

2.3 [CH4], with cCH4 themethane-sticking coefficient and [CH4] the wall con-centration of methane. Therefore, the increase in

HO with increasing pressure inhibited the hetero-geneous reactivity due to the reduced availability offree sites. The factor HPt

2.3 decreased by a maximumof 60% when the pressure increased from 3 to 10bar; however, the nearly 3-fold increase in methanenear-wall concentration ([CH4] � p) overtook thisreduction, resulting in an overall increased high-pressure catalytic reactivity.

Homogeneous ignition predictions with theDeutschmann/GRI-3.0 schemes are illustrated inFig. 6. GRI-3.0 underpredicted homogeneous igni-tion substantially (by �55%–65%) over the entirepressure range, notwithstanding its more detailed ki-netic description. Computations with the C2/H/OGRI-3.0 mechanism yielded the same ignition pre-dictions with those of the C1/H/O mechanism.Comparisons of atmospheric-pressure predictionswith the Deutschmann/Warnatz schemes (validatedin Ref. [5]) and Deutschmann/GRI-3.0 schemeshave also revealed similar discrepancies. Therefore,GRI-3.0 has strong deficiencies in CST at pressuresof 1–10 bar. It is emphasized that the heterogeneouspathway cannot be held responsible for the perfor-mance of GRI-3.0, for the following reasons. If aheterogeneous scheme faster than Deutschmann’sscheme was used, it would not influence significantlythe location of homogeneous ignition, since the cat-alytic conversion is already close to its mass-trans-port limit (see Fig. 5), and hence, the near-wall CH4levels (which are crucial for homogeneous ignition)are not dictated by kinetics. On the other hand, aslower heterogeneous scheme would drive the cat-alytic conversion toward the kinetically controlledregime leading to a near-wall CH4 excess that would,in turn, result in an even earlier homogeneous ig-nition. This is also shown in the surface sensitivityanalysis (SA) of Fig. 7a; the pre-exponential constantof every reaction in Deutschmann’s scheme was mul-tiplied (divided) by a given factor K and the com-putations were repeated. The most significant sur-face reactions for homogeneous ignition were thesame, irrespective of pressure, gaseous mechanism(Warnatz or GRI-3.0), factor K (K � 10 and 20 wereused) and the particular test case. As seen, the sur-face pathway had a moderate influence on homo-geneous ignition. The most sensitive reaction wasCH4 adsorption; the highly skewed bars of this re-action reflected the transport limitations discussedabove. In addition, the effect of radical (OH) ad-sorption/desorption reactions on homogeneous ig-nition was particularly weak. The overall weak cou-pling of the heterogeneous and homogeneouspathways at conditions close to mass-transport-lim-ited operation allowed for independent validation ofthe gas-phase mechanisms, as also shown in our pre-vious atmospheric-pressure H2/air CST studies [20].

A gas-phase SA was performed by multiplying thepre-exponential constants of each gaseous reactionby a factor K (factors of 1.2, 1.5, and 2 produced the

HIGH-PRESSURE HOMOGENEOUS IGNITION OF METHANE 1027

Fig. 7. Sensitivity analysis of surface and gaseous path-ways on homogeneous ignition. The percentage decreasein the homogeneous ignition distance is shown for the mostsignificant reactions; a multiplication (black bars) or divi-sion (gray bars) factor K of the pre-exponential constantwas used. The sensitivity analysis is qualitatively the samefor pressures up to 10 bar (the results of Fig. 7 refer to 6bar). (a) Surface sensitivity analysis, K � 10. The five mostsignificant reactions are given: reaction, 1, methane ad-sorption; reaction 2, oxygen adsorption; reaction 3, oxygendesorption; reaction 4, OH adsorption; reaction 5, OH de-sorption. The suffix (s) denotes a surface species. (b) Sen-sitivity analysis on the gaseous scheme of Warnatz (K �

2). The six most significant reactions are given. The reac-tion numbering is: reaction 1, O2 � H � OH � O; re-action 2, H � O2 � M � HO2 � M; reaction 3, HO2 �

OH � H2O � O2; reaction 4, CH3 � O2 � CH2O � O� H; reaction 5, CH4 � OH � CH3 � H2O; reaction 6,CH3 � OH � CH2O � H2. Sensitivity analysis on gaseousscheme of GRI-3.0 (K � 2). The six most significant re-actions are given. The reaction numbering is: reaction 1,O2 � H � OH � O; reaction 2, H � O2 � M � HO2

� M; reaction 3, HO2 � OH � H2O � O2; reaction 4,CH4 � OH � CH3 � H2O; reaction 5, CH3 � HO2 �

CH3O � OH; reaction 6, CH2O� HO2 � CHO � H2O2.

same set of significant reactions) while keeping un-altered the heterogeneous scheme. The origin of thedifferences between the Warnatz and GRI-3.0schemes was investigated with the SA of Fig. 7b, cand with a radical reaction flux analysis. Over theentire preignition zone, the OH, H, and O radicalconcentrations in GRI-3.0 were about three timeshigher than in Warnatz’s scheme. The H/O reactionsubset was not responsible for these differences as

shown by interchanging the H/O subsets of bothschemes and computing anew. Moreover, this is con-sistent with our recent H2/air CST comparative in-vestigation of various hetero/homogeneous schemes[20]. The main fuel-depletion step was, in bothschemes, CH4 � OH � CH3 � H2O; although therate constant of this reaction was 30%–40% higherin GRI-3.0 over the temperature range 1000–1400 K(representative of the near-wall gas, see Fig. 3), thiswas not the main reason for the ignition differencesof the two gaseous schemes. The CH3 consumptionin GRI-3.0 proceeded fast through CH3 � HO2 �CH3O � OH, which was the main production stepfor OH, and subsequently via CH3O � M � H �CH2O � M, which was the main production stepfor H; O was produced via the branching step H �O2 � OH � O. In the scheme of Warnatz, however,the slower route CH3 � O2 � CH2O � O � Hprovided the main production mechanism for bothH and O, whereas OH was produced primarily viaH2O2 � M � 2OH � M. Furthermore, of the sig-nificant C-containing reactions of Warnatz’s scheme,reaction 4 (CH3 � O2 � CH2O � O � H) hadthe strongest pressure dependence: a factor-of-twoincrease in the pre-exponential constant of this re-action resulted in xig agreement within 14% to themeasurements and good prediction of the V-shapedflames at pressures � 8 bar, while still maintainingan xig agreement within 6% at lower pressures.Therefore, reaction 4 is one possible source for thediscrepancies of Warnatz’s scheme at pressures �8bar. Finally, the differences between the Warnatzand GRI-3.0 schemes were verified with indepen-dent, gaseous studies: the SENKIN [21] packagewas used to evaluate ignition delay times of purelygaseous CH4/air mixtures having 0.3 � u � 0.4, 1 �p � 10 bar and 1000 � TIN � 1400 K. GRI-3.0yielded always ignition delay times three to five timesshorter compared with those of Warnatz’s scheme.The present experiments have thus shown that, un-der CST-relevant conditions, GRI-3.0 is inadequate.

Conclusions

The homogeneous ignition of fuel-lean CH4/airmixtures over Pt was investigated at pressures up to10 bar. Predictions with the hetero/homogeneousschemes of Deutschmann/Warnatz were in goodagreement (within �10%) with the measured ho-mogeneous ignition distances (xig) and the estab-lished V-shaped flames at pressures � 6 bar, ren-dering the above schemes of particular interest tomicroturbine catalytic combustion applications; atpressures � 8 bar, however, a marked overpredictionof xig was evident (� 25%) and in some cases thepredictions did not reproduce the measured V-shaped flames. An increase in pressure inhibited theadsorption of CH4 due to the enhanced surface cov-erage by oxygen, but this effect was overtaken by the

1028 NEW CONCEPTS IN COMBUSTION TECHNOLOGY—Catalytic Combustion

corresponding increase of the methane concentra-tion. The scheme of GRI-3.0 underpredicted sub-stantially the ignition distances (by �55%–65%) dueto its significantly faster radical pool buildup in theinduction zone.

Acknowledgments

Support was provided by the Swiss Federal Office ofEnergy (BFE), Office of Education and Technology (BBT),and Alstom Power of Switzerland.

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18. Warnatz, J., Maas, U., and Dibble, R. W., Combustion,2nd ed., Springer-Verlag, Berlin, 1999, p. 78.

19. Dalla Betta, R. A., and Rostrup-Nielsen, T., Catal. To-day 47:369 (1999).

20. Appel, C., Mantzaras, J., Schaeren, R., Bombach, R.,Inauen, A., Kaeppeli, B., Hemmerling, B., and Stam-panoni, A., Combust. Flame 128:340 (2002).

21. Lutz, A. E., Kee, R. J., and Miller, J. A., Sandia reportSAND87-8248, July 1996.

COMMENTS

A. M. Dean, Colorado School of Mines, USA. Your slideshows the reaction CH3 � O2 � CH2O � O � H asbeing very sensitive. I am surprised at this product state. Iwould have expected perhaps: CH3 � O2 � CH2O � OHand CH3 � O2 � CH3O � O. Can you comment on yourchoice of products?

Author’s Reply. The reaction with the relevant productsis not our choice but is given in the original mechanism ofWarnatz (Ref. [10] in paper) with specific reaction param-eters A (cm-mol�s), b, and E (kJ/mol) of 5 � 1013, 0.0,and 120, respectively.

We have also tested the more recent C1 mechanism ofWarnatz (Ref. [18] in paper) where the reaction CH3 �

O2 � CH2O � OH is used instead. Although the 1999mechanism is more detailed in number of species and re-actions and it worked well at high pressures (�8 bar), itperformed poorly at low pressures: it underpredicted thehomogeneous ignition distance by about 50% at atmo-spheric pressure. We opted to use the Warnatz mechanismas it had shown very good performance in our previousatmospheric pressure studies (Ref. [5] in paper). We willreport in future publications the differences between these

two mechanisms, as well as other additional gaseous mech-anisms.

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Michael Forsth, Chalmers University of Technology,Sweden. Different surface reactions seem to affect the ho-mogeneous ignition differently; some reactions promote ig-nition, while others inhibit ignition. So, finally, from a ho-mogeneous ignition point of view, what is the total effectof the catalytic surface as compared to an inert surface? Inaddition, how was the cleanliness of the catalytic surfacechecked?

Author’s Reply. We have to distinguish between purelychemical effects induced primarily by the heterogeneousradical adsorption/desorption reactions (and secondarilyvia the heterogeneous product formation) and the trans-port effects induced by the near-wall fuel depletion due tothe catalyst, which acts as a sink for the fuel. The near-wallfuel depletion is, in all cases, the strongest inhibitor of ho-mogeneous ignition since it deprives fuel from the near-wall hot ignitable mixture. This strong inhibition effect has

HIGH-PRESSURE HOMOGENEOUS IGNITION OF METHANE 1029

been elaborated, for example, in our previous study (Ref.[5] in paper) with a comparative study between inert andcatalytic cases.

The radical adsorption/desorption reactions have a netinhibiting effect on homogeneous ignition as discussed inthe second paragraph of the Results and Discussion sec-tion: ‘‘In the early preignition zone, OH was produced bythe surface and consumed by the gaseous pathway; how-ever, at the latest preignition stages (x � 145 mm) the het-erogeneous pathway became a sink of OH radicals thatresulted in an overall mild homogeneous ignition inhibi-tion. This was confirmed by removing both OH adsorption/desorption reactions and computing anew; the xig was re-duced by about 10%.’’ This behavior has been found in ouratmospheric-pressure methane/air study (Ref. [5] in paper)and the hydrogen/air study (Ref. [20] in paper).

The catalytic surfaces have been examined with XPS(Ref. [20] in paper) after the PVD Pt deposition and haveassessed the presence of pure platinum on the surface(which is not surprising given very thick, 2.2 lm, Pt layercoating). During experimental runs, the catalytic activity ismonitored in order to guarantee that it remains the samewith that of the fresh sample.

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Robert Kee, Colorado School of Mines, USA. Your ob-servations on the gas-phase ignition are very interesting,

especially from the scientific viewpoint. From the view-point of application, for example, in microturbines, is itdesirable to have homogeneous combustion within thechannel.

Author’s Reply. The interest for homogeneous ignitionstudies in the catalytic combustion of gas turbines stemsprimarily from safety considerations. Gas-phase combus-tion can cause catalyst meltdown, and in this respect, thedelineation of safe operating conditions that ensure no ho-mogeneous ignition inside the catalytic channel is an im-portant reactor design issue. This is precisely the objectiveof this work, that is, to provide validated heterogeneous/homogeneous reaction schemes for design purposes.

There could be, however, cases where gas-phase com-bustion inside the catalytic reactor in indeed desirable. Thishas to be seen in conjunction with the degree of non-adiabaticity that the reactor is designed. In gas-turbine ap-plications, for example, alternate channel coating providesa high degree of non-adiabaticity and certainly, in thesecases, gas-phase combustion is undesirable, as it would leadto channel temperatures well above their design values. Incases with no substantial degree of non-adiabaticity, how-ever, the gaseous and catalytic combustion temperaturescan be roughly the same and gas-phase combustion can betolerated.