Hydrogen assisted catalytic combustion of methane on platinum

doi:10.1016/S0920-5861(00)00279-0

Catalysis Today

Volume 59, Issues 1–2, 10 June 2000, Pages 141-150

https://doi.org/10.1016/S0920-5861(00)00279-0 Get rights and content

Abstract

The objective of this paper is to study hydrogen assisted catalytic combustion of methane on platinum experimentally and numerically. In the experiment, we measure the exit temperatures of methane/hydrogen/air mixtures flowing at atmospheric pressure through platinum coated honeycomb channels. A single channel of this monolith is investigated numerically by a two-dimensional Navier–Stokes simulation including an elementary-step surface reaction mechanism. Furthermore, a one-dimensional time-dependent simulation of a stagnation flow configuration is performed to elucidate the elementary processes occurring during catalytic ignition in the mixtures studied. The dependence of the hydrogen assisted light-off of methane on hydrogen and on methane concentrations is discussed. The light-off is primarily determined by the catalyst temperature that is a result of the heat release due to catalytic hydrogen oxidation. Increasing hydrogen addition ensures light-off, decreasing hydrogen addition requires an increasing methane feed for light-off.

Introduction

The application of catalytic combustion in gas turbines critically depends on the feasibility of a convenient light-off mechanism of the catalytic combustor, because ignition of methane on platinum and palladium occurs at relatively high temperatures at lean conditions [1], [2]. The addition of hydrogen to the initial mixture may help to reduce the ignition temperature, because ignition of hydrogen on platinum occurs at almost room temperature. In the present work, the hydrogen assisted catalytic ignition of lean methane/air mixtures on platinum is studied experimentally and numerically.

In the experiment, methane/hydrogen/air mixtures flow through platinum coated honeycomb monoliths. The exit temperature is monitored as a function of methane and hydrogen concentration. Each experimental run starts with a pure hydrogen/air flow that is ignited catalytically. Then, the methane feed is slowly increased. Depending on hydrogen as well as methane inlet concentrations, hydrogen or both hydrogen and methane are oxidized catalytically.

Numerical simulations are performed for two configurations, the stagnation point flow onto a platinum foil and the flow through a single channel of the honeycomb monolith. In both systems, detailed models for transport and surface chemistry are used. The stagnation flow system is described by a one-dimensional simulation, where the distance from the foil and the time are the independent variables. Since this code is able to simulate transient problems, it is used to understand the behavior during ignition processes. The single channel flow is studied by a two-dimensional, axi-symmetric simulation of the Navier–Stokes equations. This approach permits to study the interaction of transport, surface chemistry, and heat transfer in the honeycomb channels.

Section snippets

Experimental setup

A sketch of the experimental setup is shown in Fig. 1. A 5.0 cm diameter flow reactor contains two thermally insulated honeycomb monoliths, 2.5 cm in length each. The features of the monoliths are: alumina washcoat of 0.16 g alumina per 1 g cordierite substrate, platinum loading of 0.01 g per 1 g substrate, 200 cells per square inch with 20% blockage resulting in a single channel diameter of approximately 1.8 mm. The inflow is varied between 66 and 216 slpm air with various hydrogen and methane

Modeling approach

Our approach to model catalytic combustion consists in coupling of the fluid flow and the chemical processes in the gas phase and at the gas–surface interface. The fluid flow is described by the Navier–Stokes equations, an energy conservation equation, and additional conservation equations for each chemical species. This equation system is closed by the ideal gas law. In the cases studied in this paper, the flow field is laminar.

Two flow configurations, the stagnation point flow onto a

Stagnation flow simulations

Methane/hydrogen/air mixtures flow slowly at atmospheric pressure onto the platinum foil. The inlet velocity is 1 m s⁻¹, and the inlet temperature is always 300 K. The temperature of the catalytic foil (T_cat) is derived from the energy balance at the foil, so the heat release by the exothermic combustion reaction leads to an increase of foil temperature T_cat. Because this energy balance also includes heat conduction into the gas phase, inlet velocity variation leads to variation of the foil

Summary and conclusions

The addition of hydrogen to the initial mixture offers a way to light-off catalytic combustion of methane on platinum. It seems that the main effect of hydrogen addition is to provide enough heat to reach a catalyst temperature at which methane oxidation will light-off. A consequence is that hydrogen addition must be over a certain minimum, 3 vol.% in our case, in order to be able to use lean methane/air mixtures. The temperature of light-off of methane combustion also depends on methane

Acknowledgements

Two of the authors (OD and UR) would like to thank Professor J. Warnatz, University of Heidelberg, for his continuous support. L.I. Maier gratefully acknowledges a grant from the Otto–Benecke–Stiftung eV for a 1-year stay at the Interdisciplinary Center of Scientific Computing (IWR) at University of Heidelberg.

References (11)

J. Warnatz et al.
Combust. Flame
(1994)
O. Deutschmann et al.
Catal. Today
(1994)
M. Rinnemo et al.
Combust. Flame
(1997)
O. Deutschmann, R. Schmidt, F. Behrendt, J. Warnatz, in: Proceedings of the 26th International Symposium on Combustion,...
G. Veser et al.
AIChE J.
(1996)

There are more references available in the full text version of this article.

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