Fuel 113 (2013) 467–480

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Fuel

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A sensitivity study of the oxidation of compressed naturalgas on platinum

0016-2361/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.fuel.2013.06.007

⇑ Corresponding author. Tel.: +966 544 700 233; fax: +61 2 9351 7060.E-mail address: jihad.badra@kaust.edu.sa (J.A. Badra).

Jihad A. Badra a,⇑, Assaad R. Masri b, Aamir Farooq a

a Clean Combustion Research Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabiab School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, NSW 2006, Australia

h i g h l i g h t s

� Five different surface chemistries are tested and compared to our experimental data.� Significant discrepancies are observed with the original mechanisms.� Sensitivity analysis is performed and an optimal surface chemistry for methane is proposed.� The updated reaction rates compare well with the experimental data for a wide range of operating conditions.� Compressed natural gas shows higher selectivity towards CO and H2.

a r t i c l e i n f o

Article history:Received 25 April 2013Received in revised form 4 June 2013Accepted 5 June 2013Available online 19 June 2013

Keywords:Catalytic combustionHomogeneous and heterogeneous reactionsSensitivity analysis

a b s t r a c t

This paper presents a sensitivity study for the oxidation of methane (CH4) over platinum (Pt). Some dom-inant reactions in the CH4–Pt surface chemistry were identified and the rates of these reactions were sub-sequently modified to enhance the calculations. Initially, a range of CH4–Pt surface mechanisms availablein the literature are used, along with the relevant detailed gaseous chemistry to compute the structure ofpremixed compressed natural gas (CNG)/air flames co-flowing around a flat, vertical, unconfined, rectan-gular, and platinum plate. Comparison with existing measurements of surface temperature and speciesconcentrations revealed significant discrepancies for all mechanisms. Sensitivity analysis has identifiednine key reactions which dominate the heterogeneous chemistry of methane over platinum. The ratesof these reactions were modified over a reasonable range and in different combinations leading to an‘‘optimal’’ mechanism for methane/air surface chemistry on platinum. The new mechanism is then usedwith the same flow geometry for different cases varying the temperature of the incoming mixture (Tjet),its equivalence ratio (U) and the Reynolds number (Re). Results from the modified surface mechanismdemonstrate reasonably good agreement with the experimental data for a wide range of operatingconditions.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction oxides [1]. The catalytic monolith and packed-bed reactors enjoyed

Catalytic surface oxidation of gaseous hydrocarbon species hasreceived considerable attention due to its relevance in a wide rangeof applications, which include: (i) reduction of pollutants, with themost common example being the catalytic convertors used forvehicle exhaust [1–3], (ii) fuel conversion with typical applicationsbeing catalytic reforming [4,5], synthesis of methanol from naturalgas [6] or the catalytic partial oxidation of hydrocarbons to syngas[7,8], and (iii) energy production through the use of catalytic mate-rials in fuel cells. A range of catalysts are used in these processesranging from the noble-metals such as platinum (Pt), palladium(Pd), and rhodium (Rh) to the less expensive transition metal

extensive research including both experiments and simulations[9–12] but models developed remain limited to relatively narrowrange of operating conditions.

An obvious but complex feature of catalytic surface oxidation isthe presence of significant physical and chemical interactions atthe solid–fluid interface. While difficulties exist at both levels, dis-cussion here focuses on the chemical kinetics of the surface cata-lyst and the neighboring fluid. Mechanisms for the gas-phaseoxidation are generally well-developed for most common hydro-carbon fuels such as methane, propane and n-heptane [13,14].The same is not true for the surface chemistry of such fuels evenon well-known catalysts such as platinum. Moreover, the experi-mental and validation processes that are well-established in thegeneration of gaseous mechanisms are not directly applicable tosurface chemistry due to the added difficulty of molecularinteraction with surfaces.

468 J.A. Badra et al. / Fuel 113 (2013) 467–480

Detailed surface mechanism for methane over platinum alreadyexist in literature but these seem to be applicable for a specific setof conditions or a narrow range of operating conditions [15–24].This is because the rates of many surface reactions [16,17,21–25]are based on empirical fits from measurements and only a fewrates are generated from theoretical models [18–20]. The theoret-ical calculations make use of the transition state theory to estimatethe pre-exponential factor and the semi-empirical bond order con-servation technique to determine the activation energy. Campbellet al. [26] investigated the interaction of O2 with Pt(111) usingmolecular beam/surface scattering techniques in combination withAuger Electron Spectroscopy (AES) and Low-Energy Electron Dif-fraction (LEED) for surface characterization.

The uncertainty lies also in the thermo-chemical data of surfacespecies and this is the reason why many surface mechanisms listboth the forward as well as reverse reaction rates separately. Forthe simple case of hydrogen over platinum, some reliable ther-mo-chemistry exists [20] where the enthalpies of the surface spe-cies were derived from the heats of adsorption of H2, O2, OH, andH2O taken from experiments. Also, the entropies were obtainedby estimating surface vibrational frequencies and applying stan-dard statistical mechanics formulae. There is an urgent need, how-ever, to develop a more reliable chemical kinetics database for abroad range of gaseous species interacting with catalysts.

Combustion over platinum has been studied using various mi-cro-reactor configurations and with fuels such as H2 [27–31], CO[32], CH4 [33–38], and C3H8 [33,38–41], dimethyl ether [38], eth-ane [41], propylene [42], and butane [38,41]. The group of Mantz-aras employed a range of diagnostic methods including Ramanscattering [39] and LIF [24] to measure the product species fromthe combustion of fuels such as hydrogen [27], methane [33],and ethane [24] on platinum. Smyth et al. [43,44] studied the inter-action of methane and propane with platinum on a simple config-uration of a flat plate positioned in a co-flowing fuel/air mixture.They reported measurements of temperature as well as selectedspecies sampled from close to the plate and analyzed using gaschromatography and mass spectroscopy (GC–MS). They concludedthat the reactive layer along the plate can be nominally split intothree zones [43,44].

This paper sheds light on the need to develop more reliable sur-face chemistry by surveying existing mechanisms for methaneover platinum and performing a sensitivity analysis to identifythe dominant reactions. Attempts are then made at optimizingthese reactions using existing data for compressed natural gasflowing over a platinum plate. The configuration adopted herehas been used extensively to investigate the surface–gas chemistryinteractions for a range of fuels at different equivalence ratios, mix-ture temperature, and Reynolds numbers [38,41,42]. Measure-ments used for comparison include surface temperature usingtwo-color infrared pyrometer and the species analysis using gaschromatography (GC). The experimental data discussed hereinprovides a detailed map of various reactive scalars over a broadrange of operating conditions and hence forms a platform for fur-ther refinement of CNG surface chemistry on platinum. Also, thesignificant differences noted in the calculations resulting from var-ious existing surface chemistries highlight the need to for furtherimprovements in this area.

2. Experimental and numerical considerations

2.1. Experimental set-up

The geometry used here is intentionally simple and consists of avertical platinum plate (6 mm � 20 mm and a thickness of0.25 mm) positioned in a co-flowing fuel/air mixture issuing from

a porous plug, 23.5 mm in diameter. The plate remains fully im-mersed within the co-flowing mixture that has controlled temper-ature, equivalence ratio and velocity. The surface temperature ofthe platinum is measured using a two-color infrared pyrometer(Mikron M90R) that covers a range between 700 �C and 2000 �C.Gas samples are extracted from sixteen different locations alongthe streamwise and transverse directions of the platinum plate.The gas sample (without water) is then delivered to a micro-GasChromatogram (GC) (Varian CP-4900) where the concentrationsof CH4, C2H6, O2, CO2, CO, H2, and He are determined. Further de-tails about the experimental configuration and measurementstechniques may be found elsewhere [38,41,42].

2.2. Numerical issues

The commercial package FLUENT-13 [45] is used in all calcula-tions shown here. The numerical domain is modeled in a 2D geom-etry; further details about the grid as well as boundary conditionscan be found in [38,42]. Since the flow is laminar, non-unity Lewisnumber is adopted with full thermal and multi-component diffu-sion. Radiation is accounted for in all calculations presented herewith the platinum plate having a fixed value of the emissivity,e = 0.15. This yields a platinum surface temperature of 1200 �Cwhich is close to the average measured temperature. The energyequation is solved within the solid plate so that heat conductioncan be modeled in both x and y directions (2D). The platinum platehas thermal conductivity of 71.6 W/m K. To identify the impact ofgravity on the results presented here, separate calculations withand without buoyancy revealed very little differences which arepresented in subsequent sections. More detailed descriptions ofthe models, solvers, and numerical processes used to perform thecalculations may be found in [38].

2.3. Chemical mechanisms

The fuel used in the experiments is CNG which consists of 90%methane, 6% ethane and 2% carbon dioxide. Ethane is substantiallymore reactive than methane so its contribution needs to be ac-counted for in the calculations. Zheng et al. [24] compared the cat-alytic reactivity of C2H6 on platinum to that of CH4 using detailedethane and methane surface kinetic schemes. This study showedthat C2H6 can have appreciable impact on CH4 catalytic oxidationimplying that surface as well as gaseous chemistry of ethane shouldbe included in the current calculations. For volumetric reactions,DRM22 [46] mechanism which has 24 species including ethaneand helium is adopted. The GRI 2.11 transport database file is usedalong with a comprehensive thermodynamic database accountingfor the relevant gaseous and surface species.

A survey of the literature revealed that the following surfacemechanisms are available for methane on platinum:

(a) Deustchmann et al. [16]. This mechanism was developed forthe catalytic ignition of methane, carbon monoxide andhydrogen on platinum. It consists of 11 surface species, 20forward, and 3 reverse surface reactions. It has been vali-dated for a range of fuel/oxygen ratios and reproduced theignition temperatures of different mixtures over platinumwith very good agreement. However, this mechanism wasdeveloped for catalytic ignition which usually takes placeat relatively low temperatures such as 600–800 K for CH4,500–700 K for CO, and 300–500 K for H2.

(b) Chou et al. [22] consists of 11 surface species and 23 forwardsurface reactions. This chemistry was developed for leanmethane/air oxidation on platinum in a catalytic honeycombreactor. It was used to determine the activation energy ofmethane/air catalytic combustion and the outcome

Table 1Flow conditions of the five selected CNG/air cases.

Case # Tjet (�C) Re Velocity (m/s) U He (%) in reactants

1 21 500 0.347 1.08 4.252 21 500 0.346 1.41 4.323 21 500 0.35 3.22 4.334 21 1000 0.693 1.36 4.535 200 500 0.85 1.41 4.49

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compared well with the experimental data. Also, the predic-tions of gas phase CO profiles and methane conversion atlow surface temperatures were improved.

(c) Quiceno et al. [17] contains of 11 surface species and 36reverse surface reactions. It was developed for the high-tem-perature catalytic partial oxidation (CPO) of methane over aplatinum gauze reactor. The model results have shown goodagreement with experimental data. The selectivity to CO lin-early increases with temperature and H2 was observedabove 1200 K whereas H2O was the only hydrogen-containing product below 1200 K. This chemistry was vali-dated for very rich methane/oxygen diluted mixtures suchas CH4/O2 = 2.5 diluted with 80% helium (by volume).

(d) Shahamiri et al. [23] consists of 11 surface adsorbed speciesand 36 reverse elementary reactions. This mechanism wasdeveloped for simulating the combustion of preheated leanhomogeneous methane/air mixtures in an adiabatic catalyticpacked bed reactor. Very good agreement was achieved withexperimental data in terms of methane conversion andthe chemistry was tested for various inlet temperatures(600–1250 K) and a range of lean methane/air mixtures(equivalence ratio of 0.15–0.5).

(e) Zheng et al. [24] contains 13 surface species, 21 reverse, and3 reversible surface reactions. This chemistry was developedto investigate the combustion of fuel-lean ethane/air mix-tures over platinum at pressures of 1–14 bar, equivalenceratios of 0.1–0.5, and surface temperatures ranging from700 to 1300 K. They validated their scheme against experi-mental data of ethane, water, and temperature profileswithin the domain. Their chemistry included the elementaryreactions for methane, carbon monoxide, and hydrogen oxi-dation over platinum. They showed that the pressure-dependence of ethane catalytic reactivity was significantlystronger than that of methane at temperatures up to 1000 K.

The surface chemistry of ethane is accounted for using themechanism of Donsì et al. [47] which includes 20 reversible reac-tions among 11 surface species and is based on published reactionsteps for hydrogen and methane oxidation combined with lumpedsteps for ethane surface chemistry. This scheme was developed tostudy ethane oxidative dehydration to ethylene on Pt- and Pt/Sn-coated monoliths for a relatively wide range of operating condi-tions. The outcome from the calculations agreed well with theirexperimental data and the selectivity of various species was repro-duced well for rich ethane/oxygen mixtures with ethane/oxygenratios from 1.5 to 2.

2.4. Cases selected for calculations

Five different cases are selected for product analysis and theseare listed in Table 1. The cases are selected to study the effects ofvarying equivalence ratio (U), Reynolds number (Re) and tempera-ture of reactants (Tjet) on the product species. For each of thesecases sampling is performed in a non-reacting mixture to deter-mine the mixture’s equivalence ratio. Three different equivalenceratios ranging from stoichiometric to very rich (Cases 1, 2, and 3)are tested at the reference Re (500) and Tjet (21 �C). A very richCNG/air mixture (Case 3) is tested here because earlier work [38]showed that CNG oxidation on platinum goes through an interest-ing transition from high to intermediate temperature regime atmoderately rich mixtures (U � 1.7). Therefore, analyzing a mixturethat falls within this intermediate temperature regime is of greatimportance to test the robustness of the current surface chemis-tries. The remaining two cases (4 and 5) test the effects of doublingthe Re number (500–1000) and increasing Tjet from 21 �C to 200 �C.

3. Preliminary calculations

Calculations are performed for Case 1 in Table 1 using the exist-ing mechanisms described earlier and the results are comparedwith available experimental data. The mole fractions (percentage)of the reactants of Case 1 in Table 1 as measured by the GC are18.37% O2, 9.57% CH4, 0.1668% CO2, 0.2187% C2H6, and 4.245% Hebalanced in nitrogen. The same mole fractions are fed into thenumerical domain at the specified Re and Tjet. The flow is laminarwith low velocities (0.347 m/s) and the platinum plate is placedvertically and is at relatively high temperatures. Thus natural con-vection can affect the results and hence cases with and withoutgravity are tested. The gravity is in the y-direction and has a valueof gy = �9.81 m/s2.

The surface mechanisms of Deustchmann et al. [48], Quicenoet al. [17] with and without gravity, and Shahamiri et al. [23] areused for initial calculations. The outcome from the mechanismsof Chou et al. [22] and Zheng et al. [24] are not presented as the cal-culations could not sustain reactions for the stoichiometric caseconsidered here. The mechanisms by Chou et al. [22] and Zhenget al. [24] are developed for lean methane/air mixtures and there-fore a relatively rate for methane adsorption reaction is utilized inboth schemes. With a stoichiometric mixture such as Case 1 or ri-cher methane/air mixtures, this high reaction rate results in a plat-inum surface saturated by carbon site species (C(S)) and methylsite species (CH3(S)) which are not desorbing or reacting fast en-ough and leaving no vacant sites for further adsorption to takeplace. This causes the reactions on the surface to stop and the plat-inum plate to be deactivated.

Fig. 1 below shows the streamwise profiles of O2, CH4, CO, CO2,H2, H2O, and temperature 200 lm away from the plate for Case 1using CNG/air mixtures. Fig. 2 presents the same profiles in thetransverse direction. Few observations can be made:

� The temperature along the plate increases for the stoichiometricmixture (Case 1) compared to the moderately rich mixture(Case 2) reported earlier [41]. This is in agreement with theobservation discussed in an earlier reported work [38] aboutthe reactivity limits of CNG over platinum. The temperatureprofile along the plate is nearly constant with the decay in thetemperature along the 20 mm plate being about 100–150 �C.The calculated plate temperatures are in reasonable agreementwith the experimental data when using Deutschmann’s andShahamiri’s chemical models. Deutschmann’s mechanismreproduces the plate temperature to within 3% of the experi-mental profile. However, larger deviation is noticed with Quic-eno’s mechanism. The temperature of the plate can be related tothe consumption of the reactants (O2 and CH4) on the plate.Fig. 2 shows that the consumption of O2 and CH4 is best repro-duced by Deutschmann’s mechanism followed by Shahamiriand Quiceno. This explains the better reproducibility of theplate temperature with Deutschmann et al. [16] chemistry.� This reasonable agreement between calculations and experi-

ments regarding the plate temperatures is not reflected onother species where all tested surface mechanisms fail to repro-duce the experimental species profiles.

Fig. 1. Streamwise profiles (wet-based mole fractions) of O2, CH4, CO, CO2, H2, H2O, and temperature at 200 lm from the plate for Case 1 using CNG/air mixtures. Theexperimental profiles are compared with numerical calculations using different surface chemistry mechanisms.

470 J.A. Badra et al. / Fuel 113 (2013) 467–480

� The structure of the boundary layer along the plate is consistentwith that noted earlier [43,44] and consists of two nominalzones defined as: (i) the leading edge zone (0 mm to about5 mm from the leading edge of the plate) where gradients ofspecies are observed and (ii) the trailing edge zone (5–10 mmfrom the leading edge) where the streamwise profiles of speciesare somewhat flat and more uniform. These two distinct zonesare common for all the cases studied here. Note that the molefractions of the reactants (O2, CH4, and C2H6) increase with dis-tance along the plate and away from the leading edge. This maybe due to the slight decrease in maximum temperature which ismeasured downstream of the leading edge. This behavior issimilar to what is reported by others [43,44] albeit with differ-ent magnitudes.

� Oxygen, CH4 and CO are over-predicted by all mechanismswhile CO2 and H2O are under-predicted. Hydrogen is under-pre-dicted when using Deutschmann’s chemistry while it is over-predicted when using Quiceno and Shahamiri’s mechanisms.� Gravity has very minimal effect on the distribution of almost all

species near the plate, however it starts affecting the solutionfarther away from the plate (see Fig. 2). Therefore, gravity isincluded in all subsequent calculations.

A fair conclusion from these observations is that the tempera-ture of the plate can be considered as a global parameter and caneasily mask kinetic effects even if the agreement between mea-surements and predictions is better than 20–40 �C shown herewith Deutschmann’s chemistry. Also, all the tested surface

Fig. 2. Transverse profiles (wet-based mole fractions) of O2, CH4, CO, CO2, H2, H2O, and temperature at 5 mm from the leading edge of the plate for Case 1 using CNG/airmixtures. The experimental profiles are compared with numerical calculations using different surface chemistry mechanisms.

J.A. Badra et al. / Fuel 113 (2013) 467–480 471

mechanisms suffer from serious shortcomings and fail to repro-duce the product species distributions quantitatively and some-times qualitatively. While results are shown here for Case 1 only,the discrepancy noted here applies for all cases listed in Table 1.Therefore, more work is needed to refine these surface mecha-nisms even for a well-studied fuel such as methane.

4. Sensitivity analysis

The mechanism by Quiceno et al. [17] is picked for refinementin this study. The reason this mechanism was chosen is becauseit is an extension and more detailed work of Deutschmann’s chem-istry. Also, Shahamiri’s mechanism is built based on Deutschmannand Quiceno’s chemistries. CHEMKIN-PRO package [49,50] is uti-lized to perform sensitivity analysis and relevant parametric stud-ies. A plug flow reactor with inlet mole fractions similar to thoseused in the CFD calculations is employed here for sensitivity anal-ysis. The inlet mixture is at 1500 K and so is the wall temperature.The plug flow reactor is 10 cm in length and has a 10 cm diameter.A typical result from sensitivity analysis is shown in Fig. 3 for Case2 where the impact of various reactions on O2, CH4, CO, CO2, H2,and H2O are shown. The motivation behind performing the sensi-tivity analysis is not to reduce the chemical mechanisms but toidentify the key reactions that control the reactivity of the CNG-Pt system and then modify their rates to improve the predictionsover a wide range of operating conditions. From this analysis, nine

reactions are identified as most influential in shaping the composi-tional structure of methane–air flames on platinum:

� R1: H2 + 2PT(S)) 2H(S)� R3: O2 + 2PT(S)) 2O(S)� R10: OH(S) + OH(S)) H2O(S) + O(S)� R12: CO + PT(S)) CO(S)� R13: CO(S)) CO + PT(S)� R16: CO(S) + O(S)) CO2(S) + PT(S)� R18: CO(S) + OH(S)) CO2(S) + H(S)� R21: CH4 + O(S) + PT(S)) CH3(S) + OH(S)� R22: CH4 + OH(S) + PT(S)) CH3(S) + H2O(S)

In addition, the temperature of the plate is related to the con-sumption of fuel and oxidizer. Thus, if the depletion of methaneand oxygen increases, the temperature of the plate will increase.Reactions 12 and 13 are very important reactions regarding theconsumption of CO2 and production of CO and they also affectthe reactants (O2 and CH4). Reaction 12 is the CO adsorption reac-tion and this is very well-studied. The sticking coefficient (S = 0.84)of this reaction is same for all reported mechanisms, therefore therate of this reaction is not altered. However, the reported rate ofreaction 13 has quite a bit of variation, where Chou et al. [22] used8.5E12 for the pre-exponential factor (A) of R13, Deutschmann et al.[16] and Mhadeshwar et al. [19] used 1E13, and Quiceno et al. [17]and Shahamiri et al. [23] used 1E15. It is clear that R13 has huge

Fig. 3. Sensitivity analysis for O2, CH4, CO, CO2, H2, and H2O for Case 2 using Quiceno et al. [17] surface chemistry.

472 J.A. Badra et al. / Fuel 113 (2013) 467–480

uncertainty where the pre-exponential factor changes by two or-ders of magnitude between different published surface mecha-nisms. Hence, a refinement of this reaction rate is employed inthis current work to match the reported experimental data.

Reactions 21 and 22 are extremely important reactions for theconsumption of CH4, O2, and H2 and are playing a minor role inthe production of CO, CO2, and H2O. In order to improve the agree-ment of measurements and simulation, the consumption of fuel/oxygen and the production of CO2 should increase. Reaction 22has the highest sticking coefficient (S = 1) and low activation en-ergy (Ea = 10 kJ/mole) and hence the rate is not altered in the sub-sequent calculations. Reaction 21 has reaction rates that arescattered in the literature, where Quiceno et al. [17] and Shahamiriet al. [23] used 5E18 for A of R21, Chou et al. [22] used 5.3E19, andDeutschmann et al. [16] and Zheng et al. [24] did not include thisreaction. Thus, the rate of this reaction has been modified to obtainbetter agreement with the experimental profiles.

Reaction 10 also has very different reaction rates reported in liter-ature. Deutschmann et al. [16] and Zheng et al. [24] used 3.7E21 for itspre-exponential factor,Chouetal. [22]used1E20,Shahamirietal. [23]used 3.68E20, and Quiceno et al. [17] used 7.4E20. Therefore, the reac-tion rate of R10 is also modified in the subsequent calculations.

The sticking coefficient of Reaction 3 is scattered in literatureeven though it is very important reaction in surface chemistry asit handles the adsorption of the main oxidizer (oxygen) on plati-num. The sticking coefficient of R3 is reported by Chou et al. [22]to be 0.003, Shahamiri et al. [23] to be 0.014, Deutschmann et al.[16] to be 0.07, Zheng et al. [24] to be 0.023, and the equivalentpre-exponential factor used by Quiceno et al. [17] is 1.89E21. Thus,the sticking coefficient of R3 is slightly altered to refine this surfacechemistry.

Reactions 16 and 18 have minimal effects and therefore theirreaction rates have not been modified herein. Finally, R1 is the pri-mary reaction that affects the hydrogen distribution near the plate

J.A. Badra et al. / Fuel 113 (2013) 467–480 473

and its sticking coefficient (0.046) is almost same in allmechanisms since the adsorption of hydrogen on platinum iswell-studied. Therefore, the rate of this reaction is not modifiedin the current study, however a slight change might be needed tofit the experimental data for certain operating condition as willbe explained in subsequent sections.

5. Improved calculations

A manual optimization method is followed with the purpose ofgetting an optimized set of reaction rates that result in best agree-ment of simulations with the experimental data for a wide range ofoperating conditions. The authors acknowledge at the outset theexistence of more effective methods of optimization. However,the current strategy is sufficient to highlight the need for improvedand well-founded surface chemistry for hydrocarbon fuels on plat-inum. The modified surface chemistry implemented throughoutthe subsequent calculations is presented in Table 2 that lists therates and the corresponding references. These rates are used inthe DRM22 mechanism to simulate Case 2 in Table 1 using thesame numerical procedure and parameters described earlier. Case2 is selected as a basis for optimization because it is an intermedi-ate fuel-rich case among the cases listed in Table 1.

Table 2Methane and ethane surface reaction mechanism with improved reaction rates.

Reaction # Reaction A (cm, mol, s) b

R1 H2 + 2PT(S)) 2H(S) 0.046b 0R2 2H(S)) H2 + 2PT(S) 3.7 � 1021 0R3 O2 + 2PT(S)) 2O(S) 5 � 1021 �R4 2O(S)) O2 + 2PT(S) 3.7 � 1021 0R5 H2O + PT(S)) H2O(S) 0.75b 0R6 H2O(S)) H2O + PT(S) 4.5 � 1012 0R7 H(S) + O(S)) OH(S) + PT(S) 1.28 � 1021 0R8 OH(S) + PT(S)) H(S) + O(S) 7.39 � 1019 0R9 H2O(S) + PT(S)) H(S) + OH(S) 1.15 � 1019 0R10 OH(S) + OH(S)) H2O(S) + O(S) 7.4 � 1022 0R11 H2O(S) + O(S)) OH(S) + OH(S) 1.0 � 1020 0R12 CO + PT(S)) CO(S) 0.84b 0R13 CO(S)) CO + PT(S) 1 � 1014 0R14 CO2 + PT(S)) CO2(S) 0.005b 0R15 CO2(S)) CO2 + PT(S) 1.0 � 1013 0R16 CO(S) + O(S)) CO2(S) + PT(S) 3.7 � 1019 0R17 CO2(S) + PT(S)) CO(S) + O(S) 3.7 � 1019 0R18 CO(S) + OH(S)) CO2(S) + H(S) 1.0 � 1019 0R19 CO2(S) + H(S)) CO(S) + OH(S) 1.0 � 1019 0R20 CH4 + 2PT(S)) CH3(S) + H(S) 0.0009b 0R21 CH4 + O(S) + PT(S)) CH3(S) + OH(S) 5 � 1020 0R22 CH4 + OH(S) + PT(S)) CH3(S) + H2O(S) 1b 0R23 CH3(S) + PT(S)) CH2(S) + H(S) 1.26 � 1022 0R24 CH2(S) + H(S)) CH3(S) + PT(S) 3.09 � 1022 0R25 CH2(S) + PT(S)) CH(S) + H(S) 7.31 � 1022 0R26 CH(S) + H(S)) CH2(S) + PT(S) 3.09 � 1022 0R27 CH(S) + PT(S)) C(S) + H(S) 3.09 � 1022 0R28 C(S) + H(S)) CH(S) + PT(S) 1.25 � 1022 0R29 C(S) + O(S)) CO(S) + PT(S) 3.7 � 1019 0R30 CO(S) + PT(S)) C(S) + O(S) 3.7 � 1019 0R31 H2 + C(S)) CH2(S) 0.04b 0R32 CH2(S)) H2 + C(S) 7.69 � 1013 0R33 H(S) + OH(S)) H2O(S) + PT(S) 2.04 � 1021 0R34 CH3(S) + H(S)) CH4 + 2PT(S) 3.3 � 1021 0R35 CH3(S) + H2O(S)) CH4 + OH(S) + PT(S) 3.7 � 1021 0R36 CH3(S) + OH(S)) CH4 + O(S) + PT(S) 3.7 � 1021 0R37 C2H6 + 2PT(S)) C2H6(S) 0.003b 0R38 C2H6(S)) C2H6 + 2PT(S) 1 � 1013 0R39 C2H6(S)) C2H5(S) + H(S) 1 � 1013 0R40 C2H6(S) + O(S)) C2H5(S) + OH(S) + PT(S) 1.0 � 1022 0R41 C2H5(S) + 6PT(S)) 2C(S) + 5H(S) 5.0 � 1021 0

The surface site density, U, is 2.72 � 10�5 mol/m2.a Denotes the species i, to which the parameter lik and eik refer to, see Eq. (12) in [17b Sticking coefficient.

Fig. 4 presents the streamwise profiles of plate temperature,C2H6, O2, CH4, CO, CO2, H2, and H2O wet-based mole fractions200 lm away from the plate for Case 2 using CNG/air mixtures.The experimental profiles are compared with the numerical pro-files based on the modified surface chemistry. Fig. 5 comparesthe same profiles for the transverse direction 5 mm from the lead-ing edge of the plate. The following observations are made:

� The plate temperature is in very good agreement with theexperimental profile and almost overlapping for the entire dis-tance of the plate. With the original chemistry, the plate tem-perature was under-predicted numerically by about 100 �C(Fig. 1) and this was due to the slower depletion of reactantsin the vicinity of the plate. The adsorption reaction rates of bothreactants have been increased in the modified chemistry wherethe pre-exponential factor of R3 is raised from 1.89 � 1021 to5 � 1021 and that of R21 is increased from 5 � 1018 to5 � 1020. This alteration led to increased depletion of O2 andCH4 on the plate and hence higher reactant conversion let tohigher plate temperatures.� Oxygen is slightly over-predicted close to the leading edge of

the plate but compares very well with the experimental profilefurther downstream as can be clearly seen from the transverseprofile presented in Fig. 5. Qualitatively, leading edge effects,including the higher gradients of O2, are well reproduced.

Ea (kJ/mol) lik eik a Reference

.0 0.0 �1.0 0.0 PT(S) [17] & This work

.0 67.4 0.0 10.0 H(S) [17]0.5 0.0 This work.0 235.5 0.0 188.3 O(S) [17].0 0.0 [17].0 41.8 [17].0 11.2 [17].0 77.3 0.0 73.2 O(S) [17].0 101.4 0.0 �167.3 O(S) [17].0 74.0 This work.0 43.1 0.0 �240.6 O(S) [17].0 0.0 [17].0 146.0 0.0 33.0 CO(S) This work.0 0.0 [17].0 27.1 [17].0 117.6 0.0 33.0 CO(S) [17].0 173.3 0.0 �94.1 O(S) [17].0 38.7 0.0 30.0 CO(S) [17].0 8.4 [17].0 72.0 [17].7 42.0 0.0 �8.0 O(S) This work.0 10.0 [17].0 70.3 [17].0 0.0 0.0 2.8 H(S) [17].0 58.9 0.0 �50.0 C(S) [17].0 0.0 0.0 2.8 H(S) [17].0 0.0 0.0 2.8 H(S) [17].0 138.0 [17].0 0.0 [17].0 236.5 0.0 33.0 CO(S) [17].0 29.7 0.0 �4.6 C(S) [17].0 25.1 0.0 �50 C(S) [17].0 66.22 [17].0 50.0 0.0 2.8 H(S) [17].0 110.6 [17].0 87.9 [17].0 0.0 [47].0 20.9 [47].0 57.7 [47].0 25.1 [47].0 20.0 [47]

].

474 J.A. Badra et al. / Fuel 113 (2013) 467–480

� Methane is over-predicted along the length of the plate. Effortsto reduce this discrepancy by altering the relevant reactionrates without disturbing the good agreement obtained for otherspecies were not successful. However, this difference is withinthe experimental uncertainty of 15%. The same trend of CH4

over-prediction is observed for other cases. Qualitatively, theleading edge effect is successfully reproduced as can be seenfrom Fig. 4.� Carbon monoxide and carbon dioxide are seen to be in much

better agreement with the experimental data and almost over-lap the transverse profiles (Fig. 5). This is mainly due to the sig-nificant decrease in the pre-exponential factor of R13 from1 � 1015 to 1 � 1014. The same can be expressed about H2 wherethe profiles are in very good agreement.

Fig. 4. Streamwise profiles (wet-based mole fractions) of O2, CH4, CO, CO2, H2, H2O, aexperimental profiles are compared with numerical calculations using refined surface c

� Ethane compares well with the transverse experimental profile(Fig. 5). However, the streamwise profiles (Fig. 4) reveal thatC2H6 is over-predicted near the leading edge of the plate.� Water is slightly under-predicted numerically but it is still

within the experimental error when comparing the number ofmoles of oxygen and hydrogen in the reactants and products.

Cases 4 and 5, where the effects of Re and Tjet are examined, aresimulated using the same modified surface chemistry. The trans-verse profiles of O2, CH4, CO, CO2, H2, H2O, and C2H6 wet-basedmole fractions are compared 2 mm and 5 mm from the leadingedge of the plate in Figs. 6 and 7, respectively. The general agree-ment is quite reasonable and the following points are noted:

nd temperature at 200 lm from the plate for Case 2 using CNG/air mixtures. Thehemistry.

Fig. 5. Transverse profiles (wet-based mole fractions) of O2, CH4, CO, CO2, H2, H2O, and temperature at 5 mm from the leading edge of the plate for Case 2 using CNG/airmixtures. The experimental profiles are compared with numerical calculations using refined surface chemistry.

J.A. Badra et al. / Fuel 113 (2013) 467–480 475

� Temperature along the plate increases when Re and Tjet

increase. This increase in plate temperature is expected sinceincreasing Re means more fuel being pumped near the plateand hence more energy conversion results in higher tempera-tures. In addition, as Re increases, the ratio of heat generationto heat loss increases since the heat loss does not scale linearlywith the mass throughput. Also, heating the mixture (higherTjet) results in higher plate temperature as expected from theenergy balance. This is in agreement with the observation dis-cussed in an earlier reported work [38]. This increase in platetemperature is reproduced numerically (not shown here).

� When Re and Tjet increase, the CO and H2 mole fractions increase.However, the CO2 percentage increases as Tjet increases butdecreases when Re increases. Water behaves like CO but at a dif-ferent rate. The CO transverse profiles peak away from the platefor Cases 2 and 4 at the three locations (2 mm (Fig. 6), 5 mm(Fig. 7), and 10 mm (not shown here)); however, for Case 5 theCO transverse profile decays as it moves away from the plateat 2 mm (Fig. 6). This profile flattens out near the plate at5 mm (Fig. 7) and peaks away from the plate at 10 mm (notshown here). The CO2 transverse profiles show a peak at 2 mm(0.2 mm away from the plate) for Case 4 and a more distinct

Fig. 6. Transverse profiles (wet-based mole fractions) of O2, CH4, CO, CO2, H2, H2O, and temperature at 2 mm from the leading edge of the plate for Cases 4 and 5 using CNG/air mixtures. The experimental profiles are compared with numerical calculations using refined surface chemistry.

476 J.A. Badra et al. / Fuel 113 (2013) 467–480

peak farther away from the plate (0.5–1 mm away from theplate) at the three locations (2 mm, 5 mm, and 10 mm) for Case5. The code fails to reproduce the local peaks of CO and CO2 awayfrom the plate which are believed to be formed by the gaseousreactions. Therefore, no attempts are made to modify the gas-eous chemistry with the aim of replicating the local peaks awayfrom the plate.� Hydrogen behaves in a similar manner across the various cases

(Cases 2, 4, and 5) where it decays slightly 200 lm away fromthe plate, begins to increase farther away from the plate, andthen decreases farther out (2 mm, Fig. 6), flattens out (5 mm,Fig. 7), or keeps increasing (10 mm). Hydrogen seems to indi-cate the reactive boundary layer thickness and the peaks away

from the plate are purely due to gaseous reactions where a hotfuel/air mixture is partially reacting to produce H2. The regionwhere CO2 peaks for Case 5 shows decay in H2 unlike Cases 2and 4 where H2 increases away from the plate to present a dis-tinct peak. These local H2 peaks away from the plate are also notreproduced numerically.

Cases 1 and 3 represent stoichiometric and very rich CNG/airmixtures, respectively. The same chemistry is tested for thesetwo cases without altering the optimized reaction rates presentedin Table 2. The transverse profiles of various species 5 mm from theleading edge of the plate for Cases 1 and 3 are show in Fig. 8. Fewobservations can be made here,

Fig. 7. Transverse profiles (wet-based mole fractions) of O2, CH4, CO, CO2, H2, H2O, and temperature at 5 mm from the leading edge of the plate for Cases 4 and 5 using CNG/air mixtures. The experimental profiles are compared with numerical calculations using refined surface chemistry.

J.A. Badra et al. / Fuel 113 (2013) 467–480 477

� Oxygen decreases as the equivalence ratio increases from 1.08(Case 1) to 1.41 (Case 2), but it increases for very rich mixtures(U = 3.22 (Case 3)). Methane decreases slightly when Udecreases from 1.41 to 1.08, but it increases significantly whenU goes up to 3.22. The higher O2 percentage for the very richmixture can be explained by the low reactivity on platinum,where the temperature of the platinum plate is notably lowerthan that of the nearly stoichiometric case (U = 1.08).� Carbon monoxide and H2 decrease considerably as the equiva-

lence ratio decreases from 1.41 to 1.08 and these speciesdecrease when U increases to 3.22. On the other hand, CO2

increases drastically as U decreases from 1.41 to 1.08 but thenit starts increasing again as U rises from 1.41 to 3.22.

� It is worth noting that CO peaks away from the plate forU = 1.08 and U = 1.41 while CO2 peaks away from the plate forU = 3.22. H2 almost always peaks away from the plate, as canbe seen from Figs. 5–8.� The agreements between the calculations and experiments for

various species are not the same when the equivalence ratiochanges from moderately rich mixtures (Case 2) to stoichiome-tric (Case 1) and rich (Case 3) mixtures. Oxygen, H2O, CH4 andC2H6 are in relatively good agreement with the experimentaldata where the observed discrepancies are similar tothose mentioned for Case 2. Carbon monoxide and H2 areover-predicted and CO2 is under-predicted numerically for bothCases 1 and 3. Also, C2H6 is under-predicted numerically for

Fig. 8. Transverse profiles (wet-based mole fractions) of O2, CH4, CO, CO2, H2, H2O, and temperature at 5 mm from the leading edge of the plate for Cases 1 and 3 using CNG/air mixtures. The experimental profiles are compared with numerical calculations using refined surface chemistry.

478 J.A. Badra et al. / Fuel 113 (2013) 467–480

Case 3 in the vicinity of the plate highlighting the need of fur-ther improvements of the rates of Donsì et al. [47] reactionsto obtain better agreement with such varying operating condi-tions. However, this is outside the scope of this work wherethe focus has been on improving the methane/platinum surfacechemistry. Ethane/platinum surface mechanism is an ongoingwork that will be reported separately elsewhere.� The discrepancies observed in CO, CO2, and H2 for Cases 1 and 3

can be explained by the changes in the operating conditions,particularly equivalence ratio. The optimized set of reactionrates for moderately rich mixtures (Cases 2, 4, and 5) need tobe altered slightly for other equivalence ratios (Cases 1 and 3).R1 and R13 are the two reactions that need to be modified to

fit the experimental data. A sticking coefficient of 0.12 for R1and a pre-exponential factor of 2 � 1013 for R13 are requiredto obtain good agreement between calculations and experi-ments for Case 1 (not shown here). Regarding Case 3, apre-exponential factor of 5 � 1013 for R13 is required to obtaingood agreement between calculations and experiments (notshown here).� Qualitatively, H2 also differs between the experimental and

numerical profiles especially away from the plate where localH2 peaks are observed experimentally 0.5–1 mm away fromthe plate. These local peaks in H2 are believed to be producedby the gaseous chemistry where volumetric reactions mightbe taking place away from the plate. Again, the modification

J.A. Badra et al. / Fuel 113 (2013) 467–480 479

of the gaseous chemistry is outside the scope of this work andhence the deficiencies away from the plate are reported as theyare.

While the overall agreement for the stoichiometric (Case 1) andrich (Case 3) mixtures is marginal at best, it highlights the generaldifficulty associated with the current surface mechanism for acommon and simple fuel like methane. Surface mechanisms aregenerally optimized for a narrow range of equivalence ratios andtend to fail, as shown here, at other operating regimes. Admittedly,species such as CO and H2 show the highest discrepancy but theneed arises for a more universal surface chemistry particularly ifnon-premixed combustion applications are sought. The work pre-sented in this paper makes a relevant step in this direction.

6. Conclusions

Detailed surface and volumetric chemistries are used to com-pute the structure of premixed compressed natural gas (CNG)/airflames co-flowing around a flat, vertical, unconfined, rectangular,platinum plate. Ethane surface chemistry is included in the calcu-lation because of the presence of 6% ethane in CNG. The followingconclusions are drawn:

1. All five surface mechanisms tested here exhibit significant dis-crepancies when compared with measurements over a range ofequivalence ratios.

2. Sensitivity analysis was used to identify nine key reactionswhich dominate the heterogeneous chemistry of methane overplatinum. Some of these reactions were subsequently modifiedto produce an improved surface mechanism for CH4–Pt.

3. Results from the modified surface mechanism demonstrate rea-sonably good agreement with the experimental data for a widerange of operating conditions (temperature of incoming mix-ture (Tjet), equivalence ratio (U), and Reynolds number (Re)).

Acknowledgments

The experimental part of this work was supported by the Aus-tralian Research Council. Additional funding was provided by theClean Combustion Research Center and King Abdullah Universityof Science and Technology.

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