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Pollutant formation and reduction mechanisms (NOx, CO, unburned hydrocarbons, PAHs) in stationary and non-stationary combustion processes



 Abb. 1: Polyaromatische Kohlenwasserstoffe (PAK)
zum Bild Abb. 1: Polyaromatische Kohlenwasserstoffe (PAK)

In almost all technical combustion processes, polyaromatic hydrocarbons (PAHs) are formed as precursors of soot, both of which substances are considered hazardous to health. In order to minimise their concentrations, it is essential to have detailed knowledge about both formation and destruction processes. Chemical kinetic mechanisms are therefore required which include all these key reactions and species and which thus permit reliable prediction of PAH and soot concentrations across the entire range of operating parameters for such combustion processes.

Computational Fluid Dynamics (CFD) tools are frequently used nowadays for combustion process design. Since currently available CFD tools will be unable to handle detailed chemical kinetic models in the foreseeable future, there is a need for reduced chemical models which nevertheless allow sufficiently precise and reliable prediction of the concentrations of the molecules mentioned above.

The department of Chemical Kinetics is currently developing a sub-mechanism for PAH formation for application in gas turbine combustion chambers. This mechanism entails a detailed description of the growth of PAH molecules with up to five aromatic rings (see Fig. 1). This sub-model has been verified and optimised on the basis of experimental data (soot volume fraction, molecule concentrations of CH4-, C2H4-, C3H6, C3H8, C7H8 und n-C7H16-mixtures) from 14 internationally well-known working groups. The data covers experiments with ambient pressure laminar premixed flames as well as high pressure shock tubes (see Fig. 2). A comparison of experimental data with modelling results shows good agreement over the entire range of temperatures, mixture ratios and pressures.

Fig. 3 shows an example of modelled soot concentrations predicted by applying a modified Frenklach/Wang soot model (see soot modelling) with an incorporated PAH chemistry model for describing the coagulation and growth of soot particles.

 Fig. 2: Mole fractions of different PAH molecules of a premixed laminar flame of a CH4/O2/Ar-mixture at 1 bar and Φ = 2.6.
zum Bild Fig. 2: Mole fractions of different PAH molecules of a premixed laminar flame of a CH4/O2/Ar-mixture at 1 bar and Φ = 2.6.
 Fig. 3: Soot volume fraction of a laminar propene/air flame at different pressures and equivalence ratios.
zum Bild Fig. 3: Soot volume fraction of a laminar propene/air flame at different pressures and equivalence ratios.

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