High-pressure catalytic combustion of methane over platinum: In situ experiments and detailed numerical predictions

Abstract

The catalytic combustion of fuel-lean methane/air premixtures over platinum was investigated experimentally and numerically in the pressure range 4 to 16 bar. Experiments were performed in an optically accessible, laminar channel-flow catalytic reactor. In situ, spatially resolved Raman measurements of major species and temperature over the reactor boundary layer were used to assess the heterogeneous (catalytic) reactivity and planar laser-induced fluorescence (LIF) of the OH radical confirmed the absence of homogeneous (gas-phase) ignition. Numerical predictions were carried out with a two-dimensional elliptic code that included elementary heterogeneous and homogeneous chemical reaction schemes. Comparisons between measurements and numerical predictions have led to the assessment of the high-pressure validity of two different elementary heterogeneous chemical reaction schemes for the complete oxidation of methane over platinum. It was shown that the catalytic reactivity increased with increasing pressure and that crucial in the performance of the heterogeneous reaction schemes was the capture of the decrease in surface free-site availability with increasing pressure. Even in the absence of homogeneous ignition, the contribution of the gaseous reaction pathway to the conversion of methane could not be ignored at high pressures. The delineation of the regimes of significance for both heterogeneous and homogeneous pathways has exemplified the importance of the preignition gaseous chemistry in many practical high-pressure catalytic combustion systems. Sensitivity and reaction flux analyses were carried out on a validated elementary heterogeneous reaction scheme and led to the construction of reduced catalytic reaction schemes capable of reproducing accurately the catalytic methane conversion in the channel-flow configuration as well as in a surface perfectly stirred reactor (SPSR) and, when coupled to a homogeneous reaction scheme, the combined heterogeneous and homogeneous methane conversions. A global catalytic step could not reproduce the measured catalytic reactivity over the entire pressure range 4 to 16 bar.

Introduction

The complete and partial catalytic oxidation of lower hydrocarbons over noble metals is of prime interest in many industrial applications ranging from power generation and microreactors to pollutant abatement and chemical synthesis. In particular, the complete catalytic oxidation of natural gas has received increased attention in decentralized heat and power systems and in stationary gas turbines; the latter employ the catalytically stabilized combustion (CST) technology [1], [2], whereby ultralow NO_x emissions can be achieved in a sequential heterogeneous (catalytic) and homogeneous (gas phase) hybrid combustion concept. The advancement of catalytic combustion in power systems requires the development of catalysts with increased activity toward the complete oxidation of methane in fuel-lean air-fed combustion (methane is the main constituent of natural gas), the understanding of the heterogeneous and of the low-temperature homogeneous chemical kinetics of methane, and the availability of multidimensional numerical codes that can be used for reactor design. Moreover, the hetero/homogeneous kinetics and their interactions should be investigated under the high-pressure operating conditions of the aforementioned combustion systems. Validation of different elementary homogeneous reaction schemes in CST over platinum and investigation of the underlying hetero/homogeneous chemistry coupling were reported recently in Reinke et al. [3] for CH₄/air mixtures at pressures up to 10 bar and in Appel et al. [4] for H₂/air mixtures at atmospheric pressure; they assessed homogeneous ignition in a channel-flow catalytic reactor using planar laser-induced fluorescence (LIF) of the OH radical and—in conjunction with detailed numerical predictions—they clearly demonstrated substantial differences in the performance of various elementary gaseous reaction schemes.

Existing elementary heterogeneous chemical reaction schemes for the complete [5], [6], [7] and partial [6], [7], [8], [9] oxidation of methane over platinum (the catalyst of interest in the present study) have relied primarily on ultra high vacuum (UHV) surface science data, notwithstanding recent advances of in situ surface science diagnostics [10]. The extension of the developed reaction schemes to realistic pressures and technical catalysts has necessitated appropriate kinetic rate modifications in order to bridge the well-known “pressure and materials gap.” These modifications were aided by measurements of catalytic ignition, catalytic extinction, steady fuel conversion, and product selectivity in two basic reactor configurations: the nearly isothermal, low-temperature (T≲600 °C), gradientless tubular or annular flow reactor [11], [12] (fed with highly diluted fuel/oxidizer mixtures in order to maintain a minimal temperature rise) and the stagnation flow reactor [5], [7], [13], [14]. Although the extraction of kinetic data is straightforward in the former configuration, the nearly isothermal operation poses inherent limitations in the description of processes that can be—for certain fuels—thermally controlled (for example, catalytic ignition). Stagnation flow configurations, on the other hand, have provided a wealth of data on catalytic ignition/extinction, steady fuel conversion and product selectivity under realistic temperatures and mixture compositions. The measurements, in conjunction with numerical predictions from well-established one-dimensional codes [13], [14], have aided considerably the refinement of surface reaction mechanisms. Most experiments involved mainly measurements of global quantities (total fuel conversion, surface temperature, etc.) and were, therefore, better suited for the description of abrupt ignition/extinction transient phenomena rather than of steady processes where local experimental variations in the gas and/or on the surface could potentially affect the data interpretation.

In our recent atmospheric-pressure catalytic combustion studies of H₂/air over Pt [4], we introduced the methodology of in situ spatially resolved Raman measurements of gas-phase species concentrations in the boundary layer formed over a catalyst, as a direct way to assess the catalytic reactivity—as well as the gaseous reactivity when combined with OH LIF—under steady operating conditions. Other researchers have further adopted similar approaches: Sidwell et al. [15] studied the catalytic combustion of CH₄/air mixtures over Pd-substituted hexaluminates at atmospheric pressure with spatially resolved gas sampling over a stagnation-flow boundary layer. The validity of various heterogeneous reaction schemes for the total oxidation of methane at high pressures relevant to practical systems has not been addressed in the literature. In the present study we apply the aforesaid established methodology to investigate the catalytic combustion of CH₄/air over Pt at pressures up to 16 bar. Experiments were performed in an optically accessible catalytic channel-flow reactor, which was operated at sufficiently low temperatures that ensured kinetically controlled methane conversion away from the mass-transport limit. Fuel-lean CH₄/air premixtures (ϕ=0.35 to 0.40) were investigated under laminar flow conditions, with surface temperatures and pressures in the ranges $\text{780}\text{K}\text{⩽T⩽1250}$ K and $\text{4}\text{bar}\text{⩽p⩽16}$ bar, respectively. One-dimensional Raman measurements (across the channel transverse distance) provided the boundary layer profiles of major species and temperature, planar LIF of the OH radical along the streamwise plane of symmetry confirmed the absence of homogeneous ignition and the ensuing formation of a flame, and, finally, thermocouples embedded beneath the catalyst yielded the surface temperature distribution. Computations were carried out with an elliptic two-dimensional CFD code that included elementary heterogeneous and homogeneous chemical reaction schemes and detailed transport. Two recent heterogeneous reaction schemes for the total oxidation of methane over Pt [5], [7] were investigated, with the main objectives of validating their applicability under high pressures and, subsequently, based on the validated elementary heterogeneous scheme(s), of developing reduced and—if possible—one-step surface reaction mechanisms. Particular objectives were to investigate the effect of pressure on catalytic reactivity, to elucidate the influence of gaseous chemistry during high-pressure catalytic combustion, and to address issues of high-pressure reactor performance in light of recent analytical studies that have identified the controlling parameters in channel-flow CST [16], [17].

First the test rig, the Raman/LIF measuring techniques, and the numerical model are presented. The contribution of the homogeneous pathway to the conversion of methane at high pressures is then elaborated, followed by comparisons between measurements and numerical predictions that result in the assessment of the validity of the tested schemes. Sensitivity and reaction flux analyses elucidate the differences among the heterogeneous schemes and lead to the development of reduced and global catalytic reaction mechanisms. Finally, the effect of pressure on catalytic reactor performance is addressed.