Molecular Beam Mass Spectrometry (MBMS) is an analytical technique for determining the molecular weight of molecules sampled from a reactive environment. Gases are probed in situ and the sample is expanded into a molecular beam. Thus, chemical reactions are quenched immediately and the sample composition is "frozen". Even highly reactive species, such as radicals, are preserved by this sampling technique and can be detected. The subsequent mass spectrometric analysis allows for the separation of species by their individual molecular weight.
The combination of molecular beam sampling with a time-of-flight mass spectrometer (TOF) provides multiple advantages:
In addition to the fundamental validation experiments for reaction kinetics at DLR-VT (laminar flame speeds, ignition delay times), speciation data provides an ambitious test case for reaction mechanism. For this purpose, a modern high-temperature flow reactor is used for mass spectrometric studies of gas phase reactions under well-controlled conditions.
High diluted, cold reactants are fed premixed to the reactor in order to suppress a self-sustaining combustion reaction while gas composition is determined by MBMS at the reactors exhaust. This composition can be used for validation and development of kinetic reaction models in combination with the known (measured) temperature and velocity profiles. Variation of the residence time (and the reaction rate) is realized by changing the reactor temperature. Temperatures up to 1800 K are applicable by three individually controlled temperature zones with this atmospheric pressure flow reactor. Detailed speciation data is obtained using the sensitive MBMS technique, providing in situ access to almost all chemical species involved in the combustion process, including highly reactive species such as radicals. The superior range of operation conditions gives access to extraordinary combustion applications, which are typically not assessable by flame experiments. These include super rich conditions at high temperature, important for gasification processes, or the peroxy chemistry governing the low-temperature oxidation regime. The experiment allows for quantitative speciation data for a systematic understanding of combustion, gasification and pyrolysis processes.
Typically, validation of kinetic reaction models is performed by investigating combustion processes of pure compounds. The experiment has been enhanced to be used for real fuels (e.g., multi-component mixtures) as well to allow for phenomenological analysis of occurring combustion intermediates like soot precursors or pollutants. The controlled and comparable boundary settings enable predictions of pollutant formation tendencies.
Current research areas involve larger linear and branched aliphatics, naphthenes and aromatics. In the project “Future Fuels” these classes are investigated as model components for liquid storage substances from the Fischer-Tropsch process as well as for alternative fuels in the aviation industry (e.g. projects „Power Fuel“ and „Kopernikus P2X“). Alternative ground transportation fuels are in investigated within the projects „Solare Kraftstoffe“ or „Future Fuels“. Within the framework of the EU project SOPRANO (SOot Processes and Radiation in Aeronautical inNOvative Combustors), oxidation tests are also carried out on young soot particles. In case of gasification, the conditions of partial oxidation (very rich compositions) are of particular interest.
Selected publications: Oßwald, P.; Köhler, M., An atmospheric pressure high-temperature laminar flow reactor for investigation of combustion and related gas phase reaction systems. Review of Scientific Instruments 2015, 86, (10), 105-109. Oßwald, P.; Whitside, R.; Schäffer, J.; Köhler, M., An experimental flow reactor study of the combustion kinetics of terpenoid jet fuel compounds: Farnesane, p-menthane and p-cymene. Fuel 2017, 187, 43-50. Bierkandt, T.; Oßwald, P.; Schripp, T.; Köhler, M., Experimental Investigation of Soot Oxidation under Well-Controlled Conditions in a High-Temperature Flow Reactor. Combustion Science and Technology 2019, 1-21.