Ignition Behavior of Methane-Oxygen Mixtures at Elevated Conditions

New processes for hydrocarbon oxidation aim at increasingly severe operating conditions. Examples are ethylene production by oxidative coupling of methane (OCM) and the production of synthesis gas by methane partial oxidation (POM), with process temperatures ranging from 700°C-1000°C [1,2]. Air or pure oxygen are generally favored as oxidizing agents for these heterogeneously catalyzed gas phase reactions. Mixing of methane and oxygen is needed at some point in the process which comprises a potential explosion hazard. The knowledge on explosion properties at these elevated conditions is sparse. This is mainly attributed to experimental difficulties at high temperatures and pressures [3]. Most studies treat conditions below 600°C, only air as oxidizing agent or lean gas mixtures [4-6].

One of the determining process variables is the molar ratio of methane to oxygen, R:

Operation at high R values with rich gas mixtures is aimed to suppress ignition at these conditions. This increases process safety but limits the possible per-pass conversion in a reactor. A detailed understanding is therefore needed to determine safe and at the same time economically viable process conditions.

In a first approach, a high-temperature autoclave was set up for investigations of the explosion properties. Measurements of the auto-ignition behavior of methane-oxygen mixtures were conducted in a temperature range from 600 to 850 °C. The influence of variations in feed rate, gas mixture composition R and inert gas addition is investigated. The tests show that considerably high R-values are needed to suppress an ignition or to limit its outcome. Heating-up tests show that ignition is possible at temperatures that are significantly lower than the values published in literature. This can be attributed to pre-ignition reactions that produce more instable intermediates.

This work is part of the Cluster of Excellence “Unifying Concepts in Catalysis” coordinated by the Technische Universität Berlin. Financial support by the Deutsche Forschungsgemeinschaft (DFG) within the framework of the German Initiative for Excellence is gratefully acknowledged (EXC 314).

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[4] Norman, F. Dissertation Leuven (2008) ISBN: 978-90-5682-960-5.

[5] Caron, M. et al.  J. Hazard. Mater., Vol. 65, No. 3, pp. 233-244 Elsevier Verlag (1999).

[6] SAFEKINEX Deliverable No. 5, Contract  No. EVG1-CT-2002-00072 Berlin (2005).