Combustion kinetic characteristics of smart energy carriers in model reactors

Abstract

The use of advanced combustion technologies is among the most promising methods to reduce emission of pollutants. For such technologies, working temperatures are enough low to boost the formation of several classes of pollutants, such as NOx and soot. To access this temperature range, a significant dilution as well as preheating of reactants is required. Such conditions are usually achieved by a strong recirculation of exhaust gases that simultaneously dilute and pre-heat the fresh reactants. These peculiar operative conditions also imply strong fuel flexibility, thus allowing the use of low calorific values (LCV) energy carriers with high efficiency. Coupling these innovative combustion technologies with the energy carriers, define the Smart Energy Carriers. The intersection of low combustion temperatures and highly diluted mixtures with intense pre-heating alters the evolution of the combustion process with respect to traditional flames, thereby affecting the kinetics involved during fuel ignition and oxidation. Furthermore, the high content of diluent species, namely CO2 and H2O, deriving either from the presence of diluent in LCV fuels or from the recirculation of flue gases, makes the role of these species relevant in the oxidation chemistry in such a non-standard condition. Such issues are currently largely unexplored. The effects of high dilution and pre-heating levels, along with the significant presence of non-conventional diluents on the ignition and oxidation kinetics were studied in two model reactors. More specifically, ignition delay times have been experimentally evaluated in a Plug Flow Reactor using propane and a model gas surrogating the gaseous fraction of biomass pyrolysis products containing C1-C2 species, CO and CO2. Experimental tests have also been carried out in a Jet Stirred Flow Reactor, using methane, propane and n-pentane as reference fuels. The experimental analysis has been carried out at atmospheric pressure, in a wide range of inlet temperatures and equivalence ratios, for mixtures highly diluted in He, N2, CO2, H2O, or mixtures of them. Temperature and species concentration measurements obtained in the Jet Stirred Flow Reactor, along with the ignition delay times obtained in the Plug Flow Reactor, suggest that the ignition and oxidation processes of simple fuels are significantly altered by CO2 and H2O in dependence of mixture inlet temperatures and equivalence ratios. Furthermore, the exploitation of the ignition and oxidation processes under diluted conditions leads to the identification of peculiar phenomena and combustion regimes, such as the existence of an NTC-like behaviour for propane mixtures at intermediate temperature, strong and weak ignitions, and oscillatory regimes for all the tested fuel mixtures. Numerical simulations for studying the ignition and oxidation processes in the same working conditions of experimental tests have been carried out by means of twelve kinetic models available in the literature. It has been shown that kinetic models are not always able to correctly reproduce the experimental results, particularly when CO2 and H2O are used as diluents. In addition, large variations can be observed among models themselves. Further analysis were performed to identify the controlling reaction pathways in these non-standard conditions and to explore the interaction of CO2 and H2O with the ignition and oxidation processes. Results suggested that, for diluted conditions and lower adiabatic flame temperatures, the competition among several pathways, i.e. intermediate- and high-temperature branching, branching and recombination channels, oxidation and recombination/pyrolysis pathways, is enhanced, thus permitting the onset of phenomena that are generally hidden during conventional combustion processes. Moreover, CO2 and H2O participate in this competition, influencing termolecular reactions as third body species with high collisional efficiencies, or directly participating in bimolecular reactions.