Heidi Böhm

PAH growth and soot formation in the pyrolysis of acetylene and benzene at high temperatures and pressures: Modeling and experiment

The growth of high molecular polycyclic aromatic hydrocarbons (PAHs) before soot inception is modeled during C 2 H 2 and C 6 H 6 pyrolysis at temperatures between 1600 and 2400 K, p =60 bar, and C-atom concentrations of (3.8–4.0)×10 −6 mol/cm 3 . The formation of high molecular PAH is mainly computed by two different reaction pathways: (1) successive H-abstraction C 2 H 2 additions and (2) a combinative ring-ring condensation of aromatics. The calculations are compared with measurements obtained from C 2 H 2 and C 6 H 6 pyrolysis by the shock wave method. From reaction flux analysis, it is deduced that the combinative route plays a major role in forming PAHs, especially at early reaction times. The calculated induction times of higher PAHs reasonably reflect the experimental trends of soot inception. The slope of the computed and measured induction times in dependence on the temperature resulted in a similar activation energy for high molecular PAH and soot formation. It is concluded that soot mass growth rates during the pyrolysis of C 2 H 2 and C 6 H 6 are strongly related to PAH formation. Thus, the decrease of the soot mass growth rates during C 2 H 2 pyrolysis for T >2000 K seems to be substantially influenced by a change in the nucleation process of soot precursors.

On PAH formation in strained counterflow diffusion flames

The structural response of methane/air and methane-nitrogen/air counterflow diffusion flames to strain was investigated by measurements and computations. The numerical predictions were found to be in reasonably good agreement with the experiments. Different reaction pathways leading to PAH formation are examined computationally to obtain a deeper understanding of the process of soot precursor formation in strained diffusion flames. Both experimental and computational results indicate that the concentration of C2H2 and C3H3, as well as that of the PAH, leading candidates for soot precursor formation, diminish with increasing strain rates. The decrease of the PAH is caused by a depletion of the benzene precursors. In looking to find control parameters for strained reactive flows, it is suggested to image strain rates based on the CH2O, respectively CHO, to C2H2 ratio.

Modelling of a fuel-rich premixed propene–oxygen–argon flame and comparison with experiments

The chemical structure of a fuel-rich, non-sooting (C:O = 0.773) premixed propene–oxygen–argon flame at 50 mbar was studied and compared with experimental results. The chemical kinetic pathways leading to polycyclic aromatic hydrocarbons (PAHs) are identified. The reaction pathway for aromatic growth includes successive growth by small hydrocarbons, combinative reaction sequences and the cyclopentadienyl pathway. Additionally, the influence of experimental errors of the temperature profile, used as an input for the calculations, on the computed species concentrations is demonstrated. The model shows satisfactory agreement with the measured results. Benzene was predicted to be formed primarily by the recombination of propargyl. It was found that the growth of aromatic compounds is caused mainly by the reaction of side-chains of the PAH with propargyl, the cyclopentadienyl pathway and combinative steps, whereas the H abstraction C2H2 addition channel cannot account for the early reaction steps of PAH growth in the flame investigated here.

On extinction limits and polycyclic aromatic hydrocarbon formation in strained counterflow diffusion flames from 1 to 6 bar

The extinction limits of methane/nitrogen/air diffusion flames in counterflow configuration were investigated by measurements and computations from 0.1 to 0.6 MPa. The numerical predictions were found to be in satisfactory agreement with the experiments for these conditions. The accuracy of the extinction limits at 0.1 MPa was better than 10%: at 0.3 MPa, it was better than 20%. Different reaction pathways leading to polycyclic aromatic hydrocarbon (PAH) formation close to the extinction limits and in fuel-rich conditions were examined computationally in order to contribute to the understanding of the process of soot precursor formation at high pressures in more detail. UV laser fluorescence measurements were performed to measure relative PAH concentrations at high pressures and a constant global strain rate of 200 s −1 . Profiles of selected species were computed. In the experiments and computations, the concentrations of PAHs, leading candidates for soot precursor formation, increased with increasing pressure. From the reaction flux analysis with respect to PAHs, it can be concluded that the strong incline of the aromatics with pressure is caused by the influence of combinative reaction steps. The calculated maximum concentration of the PAH C 32 H x indicates non-sooting conditions for a methane/air flame burning at 0.3 MPa and a global strain rate of 200 s −1 , in agreement with the experiments.

Experimental and modelling study of 1-pentene combustion at fuel-rich conditions

A fuel-rich, non-sooting (C/O = 0.773) 1-pentene/oxygen/argon flame is studied experimentally at 50 mbar. The results are compared to computations. The calculations show satisfactory agreement with the data obtained from the measurements. The main channels for 1-pentene decomposition are presented and discussed in view of previous data for a propene flame at similar experimental conditions. Special emphasis is directed towards the formation of the first aromatic ring and the further growth of small aromatic hydrocarbons. By reaction flow analysis, it is found that the major reaction channel for benzene formation results from the recombination of propargyl radicals. Furthermore, the major reaction channels for naphthalene formation are presented.

Comparative modelling study on the inhibiting effect of TAME, ETBE and MTBE at low temperature

The oxidation of tert-amyl methyl ether (TAME) at low temperature and low pressure is investigated computationally in a static reactor. The results are compared to measurements. The observed satisfactory agreement between the model predictions and the experimental data confirms the validity of the proposed reaction scheme. The results for TAME oxidation are related to those for ethyl tert-butyl ether (ETBE) and methyl tert-butyl ether (MTBE), previously obtained in the literature. For all three ethers, it is found that their addition to n-pentane increases the induction period of the first cool flame of the latter. The calculations indicate that the amount of alkene production which can form resonance-stabilized ‘‘dead radicals’’ correlates with the efficiency of octane boosters.

Evaluating the strain and fuel concentration of counterflow diffusion flames by selected species

In the present investigation, the response of methane-nitrogen/air and methane/air counterflow diffusion flames to the strain rate and to the initial fuel concentration was studied at atmospheric pressure by computations. The reaction model used was related to experimental data from the literature. After the successful check of the model, the study concentrated on finding suitable species for the characterisation of strained reactive flows. It suggests the evaluation of strain rates and initial fuel concentrations based on the concentrations of only a few key species which are also able to indicate the limit at which the pollutions of polyaromatic hydrocarbons (PAHs) can be minimised.