M. de Joannon

The relation between ultraviolet-excited fluorescence spectroscopy and aromatic species formed in rich laminar ethylene flames

Abstract The exploitation of fluorescence techniques for the characterization of aromatic pollutants formed in combustion processes needs a reliable interpretation for the assignment of fluorescence emission to specific products. To this aim, ultraviolet-excited LIF (laser-induced fluorescence) spectra were measured in premixed rich ethylene/oxygen flames having a different PAH mass loading as verified by sampling and chomatographic analysis of the condensed species produced along the flames. Fluorescence emission in the ultraviolet was mainly found in the flames where PAH formation is relatively low indicating that ultraviolet-fluorescence emission is not related to PAH species. On the opposite, broad visible emission features became prevalent in PAH-rich flames suggesting that the fluorescence of PAH species could be shifted toward the visible for effect of the high-temperature flame environment. In alternative to this hypothesis the visible fluorescence could be due to the heavier unidentified part of the condensed species whose fluorescence emission is shifted toward the visible.

Mild combustion : Process features and technological constrains

Abstract The Mild Combustion is a relatively novel combustion technique characicrized by both an elevated temperature of reactants and an adiabatic flame temperature not higher than 1600K. These features are the results of several technological demands coming form different application fields. The main advantages derived by Mild Combustion concern both the combustion process itself and its applications. In the former case the pollution reduction, the increase of efficiency process and fuel flexibility has to be considered whereas the latter one is principally related to heat treatment process and wall confinement. In this paper, characteristics of Mild Combustion arc discussed with particular regards towards the aspects of plant design. A general classification of the processes, where initial and adiabatic temperature values are different from characteristic ones of classical combustion system, is given. This classification leads to a definition of Mild Combustion process as a direct consequence of technological evolution and practical constrains, especially related to environmental problems. In this sense, the high temperature of reactants and the low adiabatic temperature, principally obtained using exhausts for both heat recovery and dilution, can not be considered independently. Application of Mild Combustion in practical systems infers the use of particular solutions because of high temperature of reactants involved. In the final part of the paper, specific solutions utilized in a laboratory mild combustor are presented.

Pyrolitic and Oxidative Structures in Hot Oxidant Diluted Oxidant (HODO) MILD Combustion

The typical reactive structure stabilized in a diffusion layer in standard conditions can be significantly modified whether injected flows are diluted and/or preheated. The flow high initial enthalpy and the low fuel and/or oxygen concentration can drastically modify the structure of the oxidative and pyrolytic region due to change of the physical and chemical kinetics respect to conventional diffusion flame. Such operative conditions are typical of MILD (Moderate or Intense Low-oxygen Dilution)combustion processes. The effect of inlet conditions on the stabilized reactive structure has been studied by analyzing the behavior of a steady, one-dimensional diffusive layer. The change of the structures of the reactive region induced by a hot oxidant and diluted oxidant flow (HODO) fed towards a fuel jet at environmental temperature was numerically analyzed by means of temperature and heat release profiles, that are key parameters to understand the main features of the reactive region. In addition, the effect of diluent nature was studied by comparing the reactive structures also obtained with steam and carbon dioxide.

DYNAMIC BEHAVIOR OF METHANE OXIDATION IN PREMIXED FLOW REACTOR

Thermokinetic temperature oscillations related to oxidation of small hydrocarbons have not been extensively studied yet, because they occur in a temperature and pressure range not relevant for practical applications. Exploitation of new combustion methodologies such as Mild Combustion also indicated such a phenomenology in small-hydrocarbons oxidation. In this paper, experimental characterization of dynamic behavior occurring in methane mild combustion in premixed flow conditions reported in a previous work was extended and a comparative analysis with data obtained by means of numerical studies was performed. The experimental study was carried out in an atmospheric jet-stirred flow reactor at different inlet temperature and mixture compositions. Several typologies of temperature oscillations were identified whose amplitudes and frequencies strongly depend on the temperature and carbon/oxygen ratio considered. These dynamic behaviors were tentatively explained by means of a rate of production analysis performed by using different methane oxidation kinetic models available in the literature. It was shown that the CH3 recombination path present in the methane oxidation mechanism plays a key role in modulation of temperature oscillations.

A Comprehensive Kinetic Modeling of Ignition of Syngas–Air Mixtures at Low Temperatures and High Pressures

Syngas has gained increasing attention due to the possibility that it is produced by coal and biomass. In order to develop more efficient processes suitable for different combustion conditions, a study of the kinetic of syngas ignition was carried out. Several kinetic mechanisms present in the literature were compared with experimental data. Different conditions were investigated, including various parameters such as temperature, pressure, and composition. H2O2 + M = OH + OH + M and H + H2O2 = HO2 + H2 have been found to be dominant in the prediction of ignition times by a sensitivity analysis. Enhancements to the kinetic mechanism have been carried out by splitting the reactions in forward and backward reactions, and by adjusting values of the rate constants in the range of confidence of their evaluation. The new mechanism is able to predict quite well the behavior of syngas in all conditions examined, particularly in gas turbine conditions. Moreover, the influence of pressure and of CO concentration have been investigated with the new enhanced kinetic mechanism.

Highly Preheated Lean Combustion

Publisher Summary This chapter discusses the concepts and application of combustion in these highly preheated, highly diluted, and excess enthalpy systems, including their taxonomy, based on whether the dilution and/or preheating occurs in the fuel and/or oxidizer stream. It describes the simple processes that define and characterize MILD combustion systems, and compares them to the properties of more traditional premixed and diffusion flames. It also includes a conceptual application of MILD combustion to a gas turbine engine. MILD combustion is controlled by different factors according to the simple processes it undergoes. Simple processes are those that evolve in simple reactors like a well-stirred reactor, batch reactor, plug flow reactor, or counter-diffusion reactor. In each of these simple processes, elementary phenomena like convection, diffusion, and reactions develop in a specific way. The general feature is that the heat released by the MILD combustion is comparable to or less than the heat content of the reactants in each of these simple processes, characterizing as well as differentiating them from standard feedback combustion and HiTeCo processes. It is anticipated that the main difference consists of the fact that in standard combustion, there is always an internal flame structure that has relatively small scales with respect to the fluid dynamic scales in which the reaction heat is released.

Laser excited emission and chemiluminescence from autoigniting spray

The work presented in this paper deals with the synchronous multiwavelength detection of thermal, chemiluminescent and laser-excited (λ0=266nm) emission from spray of a large saturated paraffin (tetradecane) in auto-igniting conditions. Two of these have been selected by varying oxygen molar fraction (0.13 and 0.21) of high pressure (2MPa), high temperature oxidant (900K) stream yielding non-sooting combustion The main measured spectroscopic feature was an isolated broadband fluorescence signal detected around λ=330nm. This signal is here attributed to aldehyde functionality on the basis of both calibration tests, performed on liquid aldehydes with the same optical set-up. and literature data relative lo fluorescence and chemiluminescence measurements in simpler conditions. Furthermore, temporal evolution of this signal, in the conditions reported here, is consistent with two-step ignition kinetics of paraffins to which a large production of aldehydes and ketones is related. Chemiluminescence maximum, due to HCO and C2 radicals, is slightly delayed with respect to the fluorescence maximum and is in synchrony with a minimum of the aldehydic species fluorescence signal. These maxima and minima mark unambiguously the time interval in which combustion evolves in premixed and diffusion controlled conditions, when pyrolitic species formation occurs.