MODELING OF CATALYTIC COMBUSTION ANDCONVERSION OF METHANE

MODELIZATION DE LA COMBUSTION CATALYTIQUE ETDE LA CONVERSION CATALYTIQUE DU MÉTHANE

O. Deutschmann and J. WarnatzInterdisciplinary Center of Scientific Computing, Heidelberg University, Germany

ABSTRACTCatalytic combustion and conversion of methane are characterized by a complex interactionof transport and chemical reactions on the catalytic surface and in the gas phase.Computational tools have been developed in which the catalytic processes are coupled to thesurrounding flow. These tools are applied to study catalytic combustion of methane onplatinum and catalytic conversion of methane to syngas (H2, CO) on rhodium. The transientbehavior of catalytic and homogeneous ignition of methane combustion on a platinum foil issimulated using a time-dependent one-dimensional code. A two-dimensional simulation isused to investigate syngas formation in a short contact time reactor at elevated pressure.

RESUMELa combustion catalytique et la conversion du méthane sont caractérisées par de complexesinteractions entre des transfers de masse et des réactions chimiques sur la surface catalytiqueet dans la phase gazeuse. Des programmes informatiques, dans lesquels les processuscatalytiques sont couplés au fluide environnant, ont été développé. Ces programmes sontutilisés pour étudier la combustion catalytique du méthane sur le platine et la conversioncatalytique du méthane au gaz H2 et CO (syngas) sur la rhodium. Le comportement transitoirede l’ignition catalytique et de l’ignition homogène de la combustion du méthane sur unefeuille de platine est simulé avec un code uni-dimensionel qui dépend du temps. Unesimulation en deux dimensions est utilisée pour étudier la formation de syngas dans unréacteur ou le temps de contact est court, et on la pression est élevée.

INTRODUCTION

The application of heterogeneous catalysis has been shown to extend the range ofnatural gas utilization. The use of catalysts such as platinum and palladium leads to adecrease of pollutant emissions in gas burners and turbines [1-3]. For instance, the catalystallows running of the combustion process at a lower temperature resulting in lower thermalNO formation. Noble metal catalysts such as rhodium and platinum have been found toconvert natural gas to more useful chemicals, so that natural gas can be used as chemicalfeedstock. Here, for example, the direct partial oxidation of light alkanes in a monolithiccatalyst at very short contact times has recently been shown to offer a promising route toconvert these natural gas components to syngas (CO, H2), higher hydrocarbons, andoxygenates [4-7].

Both catalytic combustion as well as conversion of natural gas are characterized by acomplex interaction of transport and chemical kinetics. The chemistry may include surface aswell as gas phase reactions. Therefore, the description of these heterogeneous processesrequires the coupling of reactive flow with gas-surface interactions. Computational tools havebeen developed that provide opportunities to analyze the elementary chemical processesoccurring at the gas-surface interface and couple them to the surrounding gas phase. The aimis to achieve a quantitative understanding of catalytic combustion and conversion. Thisknowledge will finally help to optimize processes of natural gas utilization.

In this paper, we focus on the application of the computational tools to studyheterogeneous reactions of methane, the main component of natural gas, under differentconditions and in different devices. An example of both the catalytic combustion the catalyticconversion of methane are discussed. The overall chemical processes considered can beglobally written as

CH4 + 2 O2 -> CO2 + 2 H2O

for complete methane oxidation (combustion) and as

CH4 + 1/2 O2 -> CO + 2 H2

for partial methane oxidation (conversion) to syngas. The combustion reaction is highly exo-thermic (-890 kJ/mol) while the syngas formation is only slightly exothermic (-36 kJ/mol).

Catalytic methane combustion is investigated in a stagnation point flow configuration.A stoichiometric methane/air mixture flows slowly on a platinum foil where methane isoxidized. This configuration is often used for experimental and numerical studies because, onone side, the experiment can easily be set up, and on the other side, the configuration allows aone-dimensional analysis. We use a time-dependent one-dimensional model to simulate thetransient behavior of catalytic ignition and the transition from catalytic combustion tohomogeneous combustion. Understanding of ignition is crucial to characterize the range ofconditions at which a catalytic combustion device can be run. Furthermore, a comparison ofcalculated and experimentally observed ignition processes is used to validate the establishedmodels, and so the simulation elucidates the physical and chemical elementary processes.Additionally, homogeneous ignition is of special interest for safety in design of catalyticburners and gas turbines.

We study the partial oxidation of methane in a short contact time reactor with arhodium coated foam monolith as an example of natural gas conversion. The desired productis syngas that can subsequently be converted to higher alkanes or methanol. The short

residence time leads to a tremendous throughput using a small amount of catalytic material.The reactor can be run autothermal and nearly adiabatic and is characterized by fast variationof temperature, velocity and transport coefficients of the reactive mixture at the catalystentrance. The numerical simulation is carried out using a two-dimensional flow fielddescription that is coupled with detailed reaction mechanisms for surface and gas phasechemistry. The competition between partial and complete oxidation is studied and explainedby elementary steps at the gas-surface interface. The interaction of surface and gas phasechemistry at elevated reactor pressure is considered in more detail because the industrialapplication of short contact time reactors depends on the possibility of running the reaction athigher pressure.

MODELING APPROACH

Our approach to model catalytic combustion and conversion of methane consists incoupling of the fluid flow and the chemical processes in the gas phase and at the gas-surfaceinterface. The fluid flow is described by the Navier-Stokes equations, an energy governingequation, and one further governing equation for each chemical species. The governingequation system is closed by the ideal gas law. In the cases studied in this paper, the flowfield is laminar.

The surface boundary conditions are rather complex in the presence of heterogeneousreactions. The species mass fluxes at the surface are determined by diffusive and convectiveprocesses as well as by the creation and depletion rate of species by surface reactions. Thetemperature at the catalytic boundary is often calculated from an energy balance. Thecoverage of the surface with adsorbed species represents a further set of variables that ismodeled and calculated.

Depending on the external conditions, different processes are rate-limiting for theglobal reaction system. Furthermore, the transport properties of the gases vary significantlybetween inlet and reactor conditions. Therefore, detailed chemistry models (elementaryreactions) and detailed transport models are included into the governing equations.

Two configurations particularly amenable to experiment and simulation weremodeled: the stagnation flow field over a catalytic flat plate and a tubular catalytic reactor. Inboth systems, elementary-step reaction mechanisms are used to model gas phase chemistry aswell as reactions on the catalytic surface.

Stagnation Flow ConfigurationA stagnation flow configuration is used to study catalytic combustion of methane on

noble metal catalysts. Methane/air mixtures flow slowly at atmospheric pressure upward on acatalytic foil which is heated resistively, Figure 1 (left). The temperature of the foil iscontrolled by the current sent through the foil and can be monitored either by the voltage dropover the foil or by a thermocouple. Details of the experimental setup and procedure can befound elsewhere [8]. Methane can be oxidized on the catalytic foil or in the gas phasedepending on the ambient conditions.

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Figure 1. Experimental setup to study catalytic combustion in a stagnation flow configuration(left) and sketch of the model (right).

Our stagnation flow model is shown in Fig. 1 (right) where a flat plate serves as themodel of the foil. If edge effects are neglected, a one-dimensional analysis of the Navier-Stokes equations can be used to model this system [9]. So, the independent variables are thedistance from the foil (x) and time. The surface of the catalytic foil is described by itscoverage with adsorbed species and its temperature. Here, the catalyst temperature is derivedfrom an energy balance at the gas-surface interface including conductive, convective anddiffusive energy transport from/to the gas phase, chemical heat release by surface reactions,thermal radiation, catalyst heat capacity, and external energy sources (e.g. resistive heating).The variation of the surface coverage is calculated from surface reaction rates. A transientone-dimensional computer program has been developed to solve the resulting equationsystem using an implicit solver [10]. The time-dependent profiles of density, velocity,temperature, and species concentrations as a function of the distance from the catalyst as wellas the surface coverage and the catalyst temperature are calculated.

Tubular Catalytic ReactorThe short contact time reactor used for partial oxidation of methane consists of a well

insulated quartz tube reactor 18 mm in diameter with an α-alumina ceramic foam monolith, 1cm in length and coated with 5% rhodium. The typical catalyst porosity is 20-80 ppi (poresper linear inch) which corresponds to a pore diameter of 1-0.25 mm. Methane/oxygenmixtures, diluted by nitrogen, are fed to the reactor at flow rates which result in a residencetime of a few milliseconds. The catalyst is insulated on each side with an inert monolith to act

as a radiation shield. A sketch of the reactor is shown in Fig. 2 (top). We refer to [4, 11] forfurther details of the experimental setup and procedure.

Thermal Insulation

Catalyst

Radiation Shields

CH4, O2 H2, CO,CO2, H2O, ...

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Figure 2. Sketch of a laboratory-scale reactor (top) and the tube reactor model (bottom)representing a single pore of the monolithic reactor catalyst.

A small diameter tube with a catalytic inner wall serves as a model of a pore of thecatalytic monolith, Fig. 2 (bottom). Two chemically inert sections, each 1 cm in length, areadded as models for the radiation shields in front of and behind the catalytic monolith. Flowin the pore is described by the two-dimensional governing equations for mass, momentum,energy, and for each species in cylindrical coordinates. The independent variables are theaxial (z) and the radial (r) direction. The problem is solved using the computational fluiddynamics code FLUENT [12] which is coupled to newly developed external subroutines thatmodel detailed gas phase and surface chemistry [13, 14]. These subroutines calculate thechemical source terms, the surface coverage, and surface mass fluxes using a point-implicitmethod.

In experimental studies, axial temperature gradients have not been observed along thecatalytic monolith at conditions used in the present study [11]. Therefore, an isothermal wallis assumed as temperature boundary condition in the simulation. An adiabatic boundarycondition is used for the chemically inert heat shields.

CHEMISTRY MODELS

The typical operating temperature of catalytic burners/reactors for methane com-bustion/conversion is over 1000 K. For these temperatures, gas phase reactions can signifi-cantly influence conversion and selectivity, and a complex interaction between gas phase andsurface chemistry occurs. Therefore, the chemistry is modeled by elementary reactions on amolecular level in the gas phase and on the surface as well. The temperature dependence ofthe reaction rate is described by modified Arrhenius expressions.

Gas Phase ReactionsThe gas-phase reaction scheme is taken from modeling work on homogeneous

reactive flow including reactions of C1 and C2 species (species with one/two carbon atom(s))[15]. 162 reactions among 22 species are taken into account. The decisive steps of the gasphase reactions of methane under conditions used in this study are as follows: Hydrogenabstraction from the methane molecule, mainly by H, O, and OH radicals, leads to CH3

radicals. CH3 will either be oxidized to CO and CO2 via intermediates such as CH3OH,CH2OH, CH3O, CH2O, CH, CH2, and CHO, or they will recombine to C2H6. Ethane cansubsequently be dehydrogenated to ethylene. The formation of C2 species, here mainly C2H6

and C2H4, occurs in methane rich regions. Therefore, the C2 reaction channel becomes moreimportant for partial oxidation of methane.

A radical pool has to be established in order to have a consumption rate of methaneby gas phase reactions that is fast enough to compete with the catalytic consumption rate. Inthe stagnation flow configuration, on one side, the probability of radical formation in the gasphase is high near the foil due to its higher temperature. But, on the other side, the catalyticsurface also acts as a radical sink because radicals adsorb easily on the surface and recombineto stable molecules.

Inside the tubular reactor, the incoming fluid reaches the operating reactortemperature (over 1000 K) inside a very short entrance region being smaller than a millimeter[13]. In this reactor, the residence time becomes the limiting step for gas phase reaction. Thesurface/volume ratio is so large that oxygen may already be completely consumed before aradical pool is established, that means the residence time of the reactive mixture is shorterthan the ignition delay time of the gas phase oxidation. However, for increased mass flowrates (elevated pressure), the surface/volume ratio may become too small for completeoxygen consumption inside the ignition delay time leading to a significant amount of gasphase conversion.

Surface Reactions The chemical processes on the surface are treated by a procedure very similar to thatfor gas-phase reactions. The surface chemistry is resolved into elementary reactions on amolecular level. The problem here is to find proper reaction rate coefficients, which alsodepend strongly on the catalyst used. Furthermore, the reaction rate of adsorption, desorption,and surface reactions generally depend on the surface coverage.

Based on previous research on catalytic methane reactions, heterogeneous reactionmechanisms with associated rate expressions have been established for complete and partialoxidation of methane on platinum [10, 4] and for partial oxidation of methane on rhodium[4]. The typical reaction scheme of methane oxidation on noble metals is shown in Table 1where X(s) denotes the adsorbed species X and (s) is an uncovered surface site. The mainreaction steps are: fast and complete decomposition of the methane molecule duringirreversible adsorption, dissociative oxygen and hydrogen adsorption and their associativedesorption, water formation via OH in two different channels, formation of CO via adsorbedcarbon and oxygen, desorption of CO and its complete oxidation to CO2. A notable differencebetween the proposed mechanisms consists in the treatment of oxygen adsorption, aside fromvariation in the kinetic data sets and in reaction orders. In the catalytic combustionmechanism [10], oxygen is assumed to adsorb competitively with all other species, and it wasfound that this competition in adsorption explains the dependence of the catalytic ignitiontemperature on fuel type and fuel/oxygen ratio. In the partial oxidation mechanism [4],oxygen is assumed to adsorb non-competitively with other species. In this work, we apply themodel of CH4 oxidation on Pt [10] for the combustion simulation and the model of CH4

oxidation on Rh [4] for the conversion simulation. We refer to the original literature fordetails.

TABLE 1. Surface Reaction Scheme

Adsorption:CH4 + 2 (s) -> CH3(s) + H(s)H2 + 2 (s) -> 2 H(s)H + (s) -> H(s)O2 + 2 (s) -> 2 O(s)O + (s) -> O(s)H2O + (s) -> H2O(s)OH + (s) -> OH(s)CO + (s) -> CO(s)Surface Reactions:CH3(s) + (s) -> CH2(s) + H(s)CH2(s) + (s) -> CH(s) + H(s)CH(s) + (s) -> C(s) + H(s)H(s) + O(s) <-> OH(s) + (s)OH(s) + OH(s) <-> H2O(s) + O(s)H(s) + OH(s) <-> H2O(s) + (s)C(s) + O(s) <-> CO(s) + (s)CO(s) + O(s) -> CO2(s) + (s)Desorption:2 H(s) -> H2 + 2 (s)2 O(s) -> O2 + 2 (s)H2O(s) -> H2O + (s)OH(s) -> OH + (s)CO(s) -> CO + (s)CO2(s) -> CO2 + (s)

CATALYTIC COMBUSTION OF METHANE

The heterogeneous oxidation of stoichiometric methane/air mixtures in a stagnation-point flow onto a platinum foil is investigated as an example of the catalytic combustion ofmethane. Initially, the mixture is at room temperature. The temperature of the catalytic foil isincreased by a stepwise rise of the electric power supplied to the catalyst (corresponding to atemperature increase of 3 - 5 K). The system is allowed to reach its steady-state temperatureafter each electric power increase. This procedure is also applied in the simulation, in whichthe resistive heating is a term in the heat balance equation at the catalytic surface [10]. Thecatalyst temperature is too low to achieve significant methane conversion for a power under0.8 W/cm2 as shown in Figure 3 (left).

When reaching the ignition temperature of the heterogeneous reaction, thetemperature of the catalyst rises rapidly because of heat release by the exothermic surfacereaction. The ignition process can be understood by studying the elementary processes at thecatalytic surface. Figure 3 (right) reveals the transient behavior of surface coverage andtemperature during ignition. Before ignition, the surface is primarily covered with oxygen

because the sticking probability of oxygen is higher than that of methane. An increase of thesurface temperature by power supplied to the catalyst (at t = 0 s) leads to a point at which theadsorption/desorption equilibrium of oxygen shifts to desorption, resulting in bare surfacesites (denoted by Pt(s)) where CH4 can be adsorbed. H abstraction leads to adsorbed C(s) andH(s) atoms reacting immediately with the surrounding O atoms to form CO(s) and OH(s).This leads to a relatively fast formation of H2O and CO2 which desorb. So, more and moresurface sites become available for CH4 adsorption and further O2 adsorption leading to anincreased oxidation rate. The chemical heat released by this exothermic reaction causes anincrease of surface temperature accelerating the reaction rate, that is, ignition occurs. A fewseconds after ignition, a new steady state is established. Now, the global process is controlledby diffusion of reactants to the catalyst and of products away from the catalyst. The numberof uncovered surface sites (Pt(s)) is sufficiently large to keep the system ignited. Steepgradients are formed at the catalyst during ignition as shown in Figure 4 (left) for CO2.

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Figure 3. Consumption of CH4 (surface mass flux) on the catalyst and catalyst temperature asa function of power supplied to the foil (left); circle is experimental data. Surface coverage

and catalyst temperature during catalytic ignition (right); the power is increased at time zero.

It is important to understand the transition from catalytic combustion to purehomogeneous combustion for safety reasons in the design of catalytic gas burners andturbines. If, after catalytic ignition, the catalyst temperature is increased to high enoughvalues (Fig. 3 left), the establishment of a flame in front of the catalytic foil is observedexperimentally. This complex transient behavior is simulated numerically. Figure 4 (right)describes how the CO2 formation zone moves into the gas phase where a flame front isformed. Now, only the hot exhaust gases reach the catalytic foil. All methane is consumed inthe flame, so the methane mass flux on the catalytic surface becomes zero, Fig. 3 (left).

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Figure 4. CO2 mass fraction in the gas phase during catalytic ignition (left) and homogeneousignition (right). The r-axes denote the distance from the catalytic foil, so the catalyst is at the

wall at r = 0 cm; the lines in r-direction are the CO2 profiles at a certain time.

CATALYTIC CONVERSION OF METHANE

The industrial application of short contact time reactors depends on the possibility ofrunning the reaction at elevated pressure. In laboratory experiments, measurements of thepressure dependence are very limited due to safety and costs. At atmospheric pressure, theresidence time of the mixture inside the reactor is too short to experience significant gasphase reactions; methane is almost completely converted before a radical pool in the gasphase is established [14]. An effect of gas phase reactions on syngas selectivity and methaneconversion becomes more likely at higher pressure. A former study, in which gas phasereactions were neglected, demonstrates the decrease of methane conversion with increasingpressure while the syngas selectivity decreases only slightly with rising pressure [13]. Thisstudy is now extended using also a gas phase reaction model.

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Figure 5. Gas phase profiles of H2 (left) and H2O (right) as a function of the position in thereactor. The axial position (z) is zero at the catalyst entrance; the catalytic wall is

at the radial position (r) of 0.125 mm, that is at the back wall in the graph;the centerline of the tubular reactor is at r = 0.

The following reactor conditions are used to study the interactions of transport as wellas gas phase and surface chemistry in the short contact time reactor at the elevated pressure of10 bar. The methane/oxygen mixture (CH4/O2 ratio of 2, 20% nitrogen dilution) flows at 298K into the tubular catalytic reactor with an inlet velocity of 3.7 m/s. The pore diameter is 0.25mm and the temperature of the Rh/α-Al 2O3 catalyst is 1188 K

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Figure 6. Surface coverage as a function of the axial position (z) in the reactor.The axial position is zero at the catalyst entrance.

Figure 5 shows the gas phase profiles of hydrogen, a desired partial oxidation product,and water, an undesired complete oxidation product. Both species can be producedcatalytically on the rhodium surface and homogeneously in the gas phase. The reaction startsimmediately at the catalyst entrance (z = 0) where a strong competition between partial andcomplete oxidation occurs. Here, strong radial concentration profiles are formed and theprocess is limited by mass transport. In the entrance region, the oxygen concentration is stillhigh enough to produce a significant amount of water. The surface is mainly covered byCO(s), as shown in Fig. 6. In the first part of the reactor, adsorbed oxygen reacts not onlywith C(s) to form CO(s), which can desorb, but also with CO(s) and H(s) to form theundesired complete oxidation products CO2 and H2O. The oxygen coverage stronglydecreases farther downstream. Therefore, newly adsorbed oxygen forms CO(s) almostexclusively. However, water formation continues along the catalytic reactor as shown in Fig.5 (right), but water is now made by gas phase reactions.

A comparison between this simulation and one that neglects gas phase reactionsreveals approximately 2% variation in the product composition. Gas phase reactions becomemore and more significant at increasing pressure and lead to a decrease in syngas selectivity.

CONCLUSIONS

Computational tools were developed which allow for studying of the intrinsicelementary physical and chemical processes of heterogeneous reactive flows. The numericalsimulation is based on the solution of the governing equations that include detailed modelsfor transport and chemical reactions in the gas phase and on catalytic surfaces. Using the

most modern computer facilities and recently achieved knowledge on surface reactionmechanisms, practical problems can be simulated.

A transient one-dimensional computer program was applied to study catalyticcombustion of stoichiometric methane/air mixtures on a platinum catalyst in a stagnationflow configuration. Increasing the catalyst temperature by resistive heating of the catalyticfoil leads to ignition of the heterogeneous methane oxidation. The ignition process itself iscontrolled by adsorption/desorption of reactants and products. After catalytic ignition, thesystem is limited by mass transport. At high enough catalyst temperatures, a transition fromcatalytic to homogeneous combustion occurs and a flame is formed in front of the catalyst.

After the elementary steps of methane oxidation on platinum have been elucidated,the models can now be used to study catalytic methane oxidation in other flowconfigurations. The simulation of a honeycomb catalyst, such as proposed for catalyticallyworking gas turbines, is in progress.

A two-dimensional computer code was used to investigate syngas formation frommethane at short contact times in a monolith coated with rhodium. The competition betweenpartial and complete oxidation of methane on the catalytic surface is understood using thecalculated surface coverage. The undesired, complete oxidation products CO2 and H2O aremainly formed at the catalyst entrance where the oxygen concentration is still high. Atelevated pressure, however, more and more CO2 and H2O are also formed by gas phasereactions leading to a decrease in syngas selectivity. This point has to be addressed for theindustrial application of syngas formation in short contact time reactors.

Though the used tubular reactor model is obviously a simplification of a pore or achannel in a monolithic catalyst, it can be used to achieve a better insight into elementaryprocesses on the catalytic surface and in the surrounding reactive flow. In the case studied inthis paper, temperature gradients along the catalytic wall could be neglected. However,models of heat balances in the solid have to be included in order to apply the computerprogram to chemical systems where a large temperature variation along the catalyst wasobserved, such as in a platinum coated monolith used for methane coupling to acetylene [6].This problem will be addressed in future studies.

In the present paper, the surface properties are described by values averaged over thetotal surface, that is, the mean field approximation has been applied. However, a detailedstudy is necessary to see if lateral processes on the surface, e. g. species diffusion and islandformation of adsorbed species, have to be included in the models. One possible approach isthe coupling of averaged results of Monte Carlo calculations for the surface reactions to theNavier-Stokes equations describing the gas phase [16].

ACKNOWLEDGMENTS

The authors would like to thank Professor L. D. Schmidt, University of Minnesota,for his support of our studies of short contact time reactors. O.D. gratefully acknowledges agrant from the DFG (Deutsche Forschungsgemeinschaft) for a one-year stay at University ofMinnesota, Department of Chemical Engineering and Materials Science.

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