Studying key elementary reactions in the kinetics in fuel pyrolysis and oxidation, enables the description of the formation and destruction pathways of pollutants, such as CO, NOx, UHCs, soot precursors (PAHs) and soot. The prerequisite for generating detailed chemical-kinetic reaction mechanisms is the knowledge of the elementary reactions and their reaction rate coefficients the simulation of a chemical kinetic system is based on. The goal of experiments in the field of elementary kinetic reactions, e.g. behind shock waves, and of quantum chemical computations, is to describe how the rate coefficient k is dependent on temperature, k = k(T), or on temperature and pressure, k = k(T, p), within the relevant parameter range.
To investigate the kinetics of an elementary reaction, the reaction system must be started in a controlled way, at a high level of dilution with inert gas, to minimize the influence of subsequent reaction steps. The shock tube technique applied, combined with a sensitive detection method, is the ideal tool for these investigations. The shock wave’s characteristic of compressing a gas within a microsecond thus raising it to a high temperature state is exploited in order to initialize the reaction system under well-defined initial conditions. Radicals are playing a dominant role in elementary reactions. Their kinetics can be studied by selecting appropriate radical precursor molecules that lead to the generation of the radical of interest after a thermally induced bond fracture, due to the high temperatures generated almost instantaneously by the shock wave. Reaction progress is tracked, e.g. by analyzing the reactants or transient (generally radical) intermediate products. Spectroscopic methods are preferably used for this purpose as they allow detection in the sub ppm range, such as atomic/molecular resonance absorption spectrometry (ARAS / MRAS) or laser absorption spectrometry.
Once the absorption profiles have been measured under the respective boundary conditions, and converted into concentration profiles, by calibration or computation, the reaction model design work begins. The first step is developing a small reaction model for describing the main reaction steps: the experimental profiles are compared with the profiles predicted by the reaction model, and the reaction(s) being studied are adjusted by fitting the rate coefficient(s) to the profiles, temperature by temperature and experiment by experiment, until a coherent picture emerges and k = k(T) can be derived.