Catalytic Combustion

Methane Catalytic Combustion Modeling



The implementation of a noble-metal catalytic combustor in a natural-gas fired turbine for NOx (nitrogen oxides) reduction has drawn great attention in recent years (Dalla Betta et al. 1994). Currently NOx emissions from stationary gas turbine systems are controlled either by lowering the combustion temperature with water injection or by removing NOx through exhaust gas treatment such as selective catalytic reduction. In a catalytic combustor, a major portion of fuel conversion takes place on the catalyst surface; consequently, the gas phase NOx production route via the prompt (or Fenimore) pathway is avoided (Schlegel et al. 1994). In addition, the peak gas phase combustion temperature is substantially reduced leading to low thermal (or Zeldovich) NOx formation rate.

Catalytic combustion includes several essential processes: (1) diffusion of the reactants from the gas phase to the catalytic surface, (2) adsorption of the reactants onto the catalytic surface, (3) movement of adsorbed species, (4) reaction on the surface of the catalyst, (5) desorption of the products from the surface, and (6) diffusion of the products from the catalytic surface to the gas phase. Depending on the conditions, each of these processes can be rate limiting. Since measuring chemical activities near or on a catalyst surface is difficult, experimental data of surface kinetics, temperature, or concentrations of gas phase species near the catalyst surface are scarce. As a result, catalytic combustors have conventionally been modeled as a “black box” that produces a desired amount of fuel conversion. While this approach has been useful for proof-of-concept studies, we expect practical applications to emerge from a greater understanding of the details of the catalytic combustion process.

In the present study, a numerical model simulating a honeycomb catalytic combustor is developed. Modeling of the chemical interaction between the gas phase and the surface is accomplished by an improved multistep surface reaction mechanism for methane oxidation on platinum. The performance of the chemical model is assessed by comparing the numerical predictions with available experimental measurements. First, a series of calculations of a perfectly-stirred reactor with catalytically active surface are performed to determine the apparent activation energy at several methane-air equivalence ratios. The results are compared with the measurements by Trimm and Lam (1980) and by Griffin and Pfefferle (1990). Second, the surface ignition temperatures of various methane-air compositions are computed by using the proposed surface chemistry model. The predicted surface ignition temperatures are assessed with the measured data by Griffin and Pfefferle (1990). Third, following the satisfactory predictions of the essential features of the methane catalytic combustion, the monolith honeycomb catalytic reactor experiment by Bond et al. (1996) is simulated by a two-dimensional flow code with active catalyst surfaces. The computational domain contains two regions - a gas phase reactor channel and a solid phase substrate wall. The energy conservation equation is solved for the solid wall so that the conductive heat transfer within the substrate can be properly determined. The predicted gas phase temperatures, methane percentage conversions, and carbon monoxide (CO) mole fractions are compared with the measurements by Bond et al. (1996). Fourth and finally, the pressure effects on methane conversion in a catalytic flow reactor are explored by a parametric study using the proposed surface chemistry and the two-dimensional numerical model.

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Surface Chemistry Models

One-Step Surface Reaction

Due to limited knowledge of the elementary surface reaction kinetics, numerical studies of methane catalytic combustion were often performed with a single step global surface reaction. Song et al. (1991) used a single step surface reaction for predictions of methane catalytic combustion in a stagnation flow. With this simple chemical kinetic model, their calculations showed success in predicting surface ignition/extinction temperatures of lean methane-air mixtures. Since surface ignition temperatures of methane-air catalytic combustion are low, ca. 600C (Williams et al. 1991), the heterogeneous ignition process is dominated by the surface reaction due to its much lower activation energy compared to those of gas phase reactions. As calculations with a single-step reaction do not involve radicals, the well predicted ignition temperatures of lean methane-air mixture by Song et al. suggest that, under fuel- lean conditions, the interaction between catalytic and gas phase reactions is not important because the heterogeneous ignition is driven by the heat release of surface reaction. However, for high temperature conditions (> 1200 K), the interaction between catalytic and homogeneous reactions via radicals such as hydroxyl (OH) and O atom may potentially affect the ignition process (Pfefferle et al. 1989, Griffin et al. 1989). In order to include the radical interaction between surface and gas phase at high temperatures, Markatou et al. (1993) modified the single step surface reaction model by introducing a coefficient to regulate the amount of OH desorbing from surface. The value of this coefficient was determined from experimental data. Their results show that the OH desorbed from the surface enhances the gas phase reactions and, hence, the generation of radicals in the boundary layer for surface temperatures above 1300 K. Markatou et al. (1993) suggested the need of detailed surface kinetics and gas-surface energy balance to properly couple phase and surface processes in catalytic combustion calculations.

Multi-Step Surface Mechanisms

Previous numerical studies of catalytic methane oxidation by using multiple-step surface reactions have been reported by Hickman and Schmidt (1993), by Deutschmann et al.(1994), and by Behredt et al. (1995). These multi-step surface reaction mechanisms were developed with available kinetic and thermal data along with several assumptions, such as Langmuir- Hinshelwood type surface reaction mechanism, dissociative adsorption of O2 and CH4, perfect catalyst surface, no substrate diffusion, and monolayer surface coverage. Following the framework of these existing surface reaction mechanisms, an improved surface mechanism is developed in this study by optimizing pre-exponential factors/activation energies and by adding one new reaction. The resulting mechanism consists of several basic reactions: adsorption of reactants (O2 and CH4) and intermediate species (CO, H2, and OH), surface reactions of adsorbed species, and desorption of products (CO2 and H2O) and intermediate species. Details of these surface reactions are tabulated in Table 1. The surface reaction rates are described by an Arrhenius expression or by an initial sticking coefficient for adsorption processes. The new reaction included in the present mechanism is

       CH4 + O(*) + PT(*) => CH3(*) + OH(*),      (A3)
where PT(*) denotes a free surface site and species with a label, (*), are those adsorbed at the surface. Since there is no kinetic data available for reaction (A3), the sticking coefficient and activation energy used in this study are chosen such that at low surface temperatures, the surface mechanism predicts an apparent activation energy of 188 kJ/mole as measured by Griffin and Pfefferle (1990). With the assumption of Langmuir-Hinshelwood surface reaction mechanism, the onset of surface ignition is determined by the competition between O2 and CH4 for surface sites. Based on their experimental observation, Trimm and Lam (1980) concluded that the dominant limiting process changes from oxygen desorption to methane adsorption as the surface temperature increases. When the surface temperature is low, the O2 adsorption process, reaction (A1), dominates and the catalyst surface is entirely covered by adsorbed O atom, O(*). The newly-added reaction (A3) is important for methane conversion at low surface temperatures (or high oxygen atom surface coverage O(*)) because it allows the direct reaction between the gas phase methane and adsorbed O atom. Moreover, it provides a thermal mechanism for initiating surface ignition via the self-heating process. Bond et al. (1996) observed surface ignition (i.e., light-off) in their experiments without external heat addition to the catalyst. Through reaction (A3), conversion of methane may proceed on surfaces with high coverage of O(*). The heat generated by this reaction raises the surface temperature. As the surface temperature increases, the surface O(*) coverage drops and the methane adsorption process via reaction (A2) starts to increase. When the surface temperature reaches a certain point, the methane adsorption rate exceeds the oxygen adsorption rate such that reaction (A2) becomes dominant. Consequently, more heat is generated and ignition soon takes place on the catalyst surface. Similar surface ignition processes have been postulated by Behrendt et al. (1995). In the following sections, we will use the proposed multi-step surface mechanism to model various catalytic combustion systems. Results of these numerical simulations will be compared with available experimental measurements.

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Apparent Activation Energy

Using the present surface mechanism, calculations with the SPSR model are repeated to determine the apparent activation energies of various mixture equivalence ratios. The predicted Arrhenius plot for stoichiometric CH4-air mixture is presented in Figure 1. As seen in the figure, the numerical model exhibits two regimes of different kinetic behaviors as observed by Trimm and Lam (1980). An activation energy of 172 kJ/mole is predicted when the surface temperatures are below ca. 800 K and a value of 102 kJ/mole is obtained for higher surface temperatures. These activation energies and the transition temperature agree reasonably well with the experimental observation. In addition, surface species information predicted by the model shows that the surface is mainly covered by oxygen for surface temperatures below 800 K. When the surface temperature is around 800 K, the surface oxygen coverage decreases rapidly and CO becomes the major species formed at the surface thereafter. These features obtained by the present model are consistent with the postulation offered by Trimm and Lam (1980).

Griffin and Pfefferle (1990) studied gas phase and catalytic ignition of methane and ethane over platinum. They measured ignition temperatures of various mixture equivalence ratios and deduced the apparent activation energies at the surface ignition temperatures for both fuels. Their results show that the activation energy varies with the equivalence ratio, f. The activation energy is about 188 kJ/mole for f > 0.4 and 88 kJ/mole for 0.2 < f < 0.4. Since the surface ignition temperature varies with the equivalence ratio, the dependence of activation energy on equivalence ratio can be used to provide a relationship between activation energy and surface temperature. The activation energy plot for methane obtained by Griffin and Pfefferle (1990) also shows two kinetic regimes with a transition temperature ca. 870 K corresponding to the surface ignition temperature for a mixture with f = 0.4. Because this mixture is lean, it is unlikely that CO would be the major species formed on the catalyst surface even at high temperatures. This change in activation energy might not be caused by the change in oxygen adsorption as seen in Trimm and Lam's experiments. Griffin and Pfefferle (1990) suggested that the higher activation energy at high surface temperatures may be the result of the greater reactivity of surface oxygen when its surface coverage is low.

Figure 2 shows the Arrhenius plots of the overall reaction rate predicted by the SPSR model for equivalence ratios of 0.3, 0.4, and 0.5. The Arrhenius curves in Figure 2 show a smooth transition in the predicted activation energy from low values of 66-88 kJ/mole to high values of 166-212 kJ/mole. These apparent activation energies agree well with those determined by Griffin and Pfefferle (1990). The numerical model also predicts the transition of activation energy occurring at surface temperatures between 900 and 1000 K. Griffin and Pfefferle (1990), however, observed the transition in activation energy taking place at slightly lower surface temperatures between 800 and 873 K. Curve-fittings of the Arrhenius relationship for CH4-air mixture of equivalence ratio 0.4 are shown in Figure 3. The predicted apparent activation energy is seen to decrease from 186 kJ/mole to 77 kJ/mole when the surface temperature increases from 750 K to 1100 K. Note that the predicted overall activation energy by fitting the calculations over the entire temperature range is 136 kJ/mole which is close to the value used by the one-step surface reaction numerical models (Song et al. 1991, Markatou et al. 1993).

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Prediction of Surface Ignition Temperatures

Surface ignition temperature is another important feature which should be properly predicted by the surface reaction mechanism. Griffin and Pfefferle (1990) measured methane surface ignition temperatures on a platinum wire. Their experimental results show the surface ignition temperature decreases with mixture equivalence ratio. By using the present surface mechanism and a two- dimensional tube flow model to be described in the next section, the surface ignition temperatures of lean methane-air mixtures are determined numerically. The predicted surface ignition temperatures along with the measurements by Griffin and Pfefferle (1990) are plotted in Figure 4 for 0.3 < f < 0.8. The predicted surface ignition temperatures are seen to agree reasonably well with the experimental results.

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Modeling of A 2-D Monolith Catalytic Combustor

Bond et al. (1996) measured gas phase temperatures, CH4 conversion, and CO concentrations in their honeycomb catalytic combustor. In the reactor, the catalyst honeycomb is segmented into wafers with gaps between each wafer permitting sampling access. A schematic of Bond's reactor is shown in Figure 5a. The objective here is to investigate the catalytic combustion by using the newly-developed surface mechanism. In the present study, the CURRENT code developed by Winters et al. (1996) is modified to include an energy balance equation for the gas-catalyst interface. Similar to the flamelet approach described in the previous chapter, the CURRENT code solves the two- dimensional, low Mach number, variable-property Navier-Stokes equations along with energy and species conservation equations in general non-orthogonal curvilinear coordinates. The flow code is coupled with the CHEMKIN software libraries (Kee et al. 1996), providing generality for treating chemically reacting mixtures of gases including multicomponent diffusion and thermal diffusion. Surface chemistry is incorporated into the code using SURFACE CHEMKIN (Coltrin et al. 1996).

In the present study, a single channel in the honeycomb reactor is modeled by assuming that conditions in all channels are identical. Since the catalyst is deposited on a washcoat of aluminum that was applied to the inner surface of the honeycomb channel, corners of the square channel are rounded off by the thick washcoat deposit. Great simplification is achieved when, instead of modeling the actual round-cornered square channel in the honeycomb reactor, a circular tube with an equivalent hydraulic diameter is used. Detailed dimensions are shown in the sketch presented in Figure 5b. The computational domain contains both the gas phase (area enclosed by B-A-F-E-B) and the wall surrounding it (shaded area C-B-E-D-C). In order to describe the heat conduction in the solid wall, the following equation is solved for the wall

. (1)

In the gas phase region, a non-uniform mesh with grid points clustered near the catalyst surface and the channel entrance is used. The mesh system consists of 201 grid points in the streamwise direction and 31 grid points in the cross-stream direction and will provide sufficient resolution for the computational domain (see Fig. 5b). The inlet profiles of velocity, gas temperature, and species mass fractions are assumed to be uniform (i.e., "plug flow" inlet conditions). The boundary conditions of species and energy conservation equations at the gas-solid interface are described next.

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An improved surface mechanism of methane oxidation on platinum surface is proposed in the present study. Based on the multistep surface mechanisms developed by Hickman and Schmidt (1993) and by Deutschmann et al. (1994), an additional methane adsorption step (reaction A3) is included in the present surface mechanism. The newly- developed mechanism is applied to determine the apparent activation energy of CH4-air catalytic combustion showing good agreement with the measurements. The numerical model indicates that the change in activation energy corresponds to the change in oxygen desorption for the stoichiometric mixture as suggested by Trimm and Lam (1980). In addition, the model qualitatively predicts the dependence of the activation energy on the equivalence ratio as observed by Griffin and Pfefferle (1990). The surface ignition temperatures of lean methane-air mixtures are also determined and found to agree well with the experimental data by Griffin and Pfefferle (1990).

Another objective of the current study is to develop a two-dimensional numerical model for simulation of a monolith honeycomb catalytic reactor. With detailed gas phase and surface chemistry models, a two-dimensional code, CURRENT, is modified to include the energy balance on the gas-catalyst interface. In order to properly model the thermal conduction inside the substrate, the numerical model is expanded to solve the two-dimensional heat conduction equation for the solid phase. The predicted gas temperature, CH4 conversion, and CO mole fraction compare favorably with the measurements by Bond et al. (1996). The numerical model qualitatively captures the characteristics of the catalytic combustor. The model shows the surface ignition occurring in the second honeycomb section of the catalytic combustor as the experimental data indicated. Although the model overpredicts the gas temperature and the methane conversion after surface chemistry is ignited, the predicted trends of gas temperature and CO emission agree well with the experimental observations. In addition to the gas phase properties, the numerical model provides critical information on the catalyst surface, such as surface temperature and O atom fractional coverage. Moreover, the surface temperature prediction provides useful engineering information, such as the maximum temperature and temperature gradients, for better honeycomb designs.

The numerical model is further applied to study the pressure effect on the catalytic combustion. The model predicts that the surface temperature becomes higher and more evenly distributed at higher pressures. However, despite the gas diffusivity decreases monotonically with pressure, the predicted fuel conversion does not show the same trend. The predicted methane conversion rate first increases slightly, then it starts decreasing monotonically after the pressure reaches 2 atm.

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