Experimental and numerical investigations of the chemical kinetics of fuel mixtures in homogeneous low-temperature reactors

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During self-ignition of fuel mixtures in the low-temperature regime between 500 K and 1200 K, interactions between the different reaction chains of the fuels may occur. We aim to investigate such details of the low-temperature kinetics in homogeneous reactors for characteristic binary mixtures of the long-chain aliphatic hydrocarbons iso-octane and n heptane with ethanol or dimethyl ether (DME), respectively. For iso-octane and n-heptane, a slow chain-branching mechanism exists in the low-temperature regime, which is not the case for ethanol, however. DME presents a slow branching mechanism where additional formaldehyde is produced which may act as an inhibitor. In the above binary mixtures it is therefore to be expected that the slow chain branching of the aliphatic fuel will be influenced by the presence of the other fuel, as both fuels compete for radicals. Since mixtures of different chemical components are used in nearly all technical applications, the investigation of low-temperature kinetics of such mixtures as discussed here with respect to self-ignition is of primary importance. In the present joint project of the principal investigators Kohse-Höinghaus (Bielefeld) and Peters (Aachen), the interaction of the reaction chains of kinetically different fuels will be investigated in the low-temperature regime. Several binary mixtures will be analyzed by numerical calculations on the basis of experiments. In Aachen the emphasis will be on the numerical calculations and in Bielefeld, the emphasis will be on experiments. The kinetic mechanisms will be progressively reduced in order to identify the rate-determining reactions. The experiments will be conducted at two different test benches: in the homogeneous well-stirred reactor in Aachen as well as in the homogeneous flow reactor in Bielefeld. In the well-stirred reactor, oscillations will be investigated, since these are very sensitive to the details of chemical kinetics. The stable species will be analyzed by means of gas chromatography and by a fast hydrocarbon sensor. In addition, a time-of-flight mass spectrometer will be used to measure the concentrations of semi-stable species such as ketohydroperoxides that may be formed during the oscillations in relatively large concentrations. The flow reactor, on the contrary, operates at stationary conditions. Here the reaction proceeds along a prescribed temperature ramp. In this set-up, all species including radicals can in principle be detected along the reaction path by means of molecular-beam mass spectroscopy (MBMS) and coupled gas chromatography. The measurements will be supported by absorption spectroscopy using quantum cascade lasers and MBMS experiments with photoionization. On the basis of all these experimental results, the respective detailed kinetic reaction models will be improved.