Critical Issues of Chemical Kinetics in Diluted Combustion Processes

The control of system temperature enhancing the global heat capacity of the reactant mixtures by diluting the system with exhausted gases is becoming a common strategy in several combustion technologies, involving MILD-Oxyfuel processes for furnaces, boiler and turbines, or EGR and HCCI systems for engines, to minimize pollutants formation without renouncing to a high thermal and energetic efficiency. Some problems rise when the chemical kinetic aspects are attempted to be modeled. The performance of kinetic schemes at high temperatures is insured by the proper description of the fast high temperature branching reactions of the sub-system H2/O2, supported by the availability of reliable and exhaustive experiments. They hide the elementary reaction constants uncertainties of slow reactions, reducing the impact of their relative weight in the prediction of combustion features. On the contrary, at low-intermediate temperatures, the oxidation processes comes through a larger number of species and slow elementary reactions whose uncertainties have a huge impact in the predictive performance of detailed models. In addition, the kinetic scheme validation procedure under these operating conditions suffers the lack of experimental results in simple facilities. Under diluted conditions, the role of third molecular reactions in the oxidation chemistry at low-intermediate temperatures becomes crucial. Thus a proper description of the temperature and pressure dependence of third molecular reactions, along with the quantification of third body collisional efficiencies for H2O and CO2 becomes a crucial point for the predicting performances of kinetic models. Furthermore the "mixing rule" commonly employed in kinetic schemes have to be discussed when H2O and CO2 dilute the fresh reactants, because of the formation of complex interaction among species not responding to a linear rule. In this perspective, a set of experiments realized in model reactors, namely a Jet Stirred Flow Reactor (JSFR) and a Tubular Flow Reactor (TFR), by changing the inlet temperatures, the equivalence ratios of the mixtures and the diluent species, are considered. The autoignition delay times and the oxidation regimes of CH4/O2 and C3H8/O2 mixtures diluted in N2, CO2 and H2O are reported [1, 2]. Under diluted conditions, the oxidation process occurs through several regimes. In particular temperature oscillations are detected during experiments in the JSFR reactor for both the considered fuels. This behavior is reflected in the data obtained in the TFR, where a NTC phenomenology is experimentally detected for both the simple hydrocarbons. The numerical analyses suggested that such phenomenologies comes trough the competitions between oxidation and recombination/pyrolytic channels [3-4]. Under diluted conditions, the modest system working temperatures do not promote any dominating oxidation kinetic mechanism, thus oxidation pathway and recombination routes have comparable global reaction rates and interact in a complex-non linear manner in dependence of system operating conditions and heat exchange mechanisms to the surroundings. By considering the results obtained with mixtures highly diluted in N2, the role of CO2 and H2O as "bath gas" is highlighted. Fig. 1 shows the variation of the oxidation regimes identified for CH4/O2 mixtures diluted at 90% in N2, in N2 and H2O with a relative steam concentration of 10% and 20%.