P. Sabia

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.

Numerical investigation of the ignition and annihilation of CH4/N2/O2 mixtures under MILD operative conditions with detailed chemistry

The identification of controlling processes on the micro-scale level is critical in the elaboration of effective models with particular regard to MILD (moderate or intense low-oxygen dilution) combustion modelling. The objective of this study is to determine relevant features and controlling mechanisms in the ignition, interference and annihilation of diffusion layers in MILD combustion conditions. The two interfaces between the mixing layers are sufficiently close to cause interference phenomena. The inner region between the two interfaces is assumed to be air to mimic the engulfed air in a fuel jet. The temporal evolution has been analysed in dependence on the initial oxidant layer width and temperature; evidencing possible interactions between the two mixing layers. Interactions can be categorised according to the ignition and annihilation stages. Concerning the ignition, the minimum ignition delay for interfering double diffusive layers is significantly shorter than the corresponding isolated one. However, the ignition exhibits the same most reactive mixture fraction, and the ignition delay dependence on the oxidant layer width behaves similar to a stratified charge condition for the same most reactive mixture fraction. In the ignition region, the scalar dissipation rate is reduced due to expansion toward fuel sides due to heat release. The annihilation time delay scales with the initial layer thickness according to the canonical diffusion equation and does not significantly depend on the initial oxidant temperature. Oxygen depletion during annihilation limits oxidation in the rich side mixture, which produces a final oxidation level near equilibrium values.

Thermochemical oscillation of methane MILD combustion diluted with N2/CO2/H2O

ABSTRACT Strict environmental rules endorse moderate or intense low oxygen dilution (MILD) combustion as a promising technology to increase efficiency while reducing pollutants emission. The experimental and theoretical investigation of oscillatory behaviours in methane MILD combustion is of interest to prevent undesired unstable combustion regimes. In this study new speciation measurements were obtained in a jet-stirred flow reactor (JSR) for stoichiometric mixtures of CH4 and O2, diluted in N2, CO2 and N2-H2O, at p = 1.1 atm and T = 720–1200 K. Oscillations were experimentally detected under specific temperature ranges, where system reactivity is sufficient to promote ignition, but not high enough to sustain complete methane conversion. A thorough kinetic discussion highlights reasons for the observed phenomena, mostly focusing on the effects of different dilutions.

Distributed combustion in a cyclonic burner

Distributed combustion regime occurs in several combustion technologies were efficient and environmentally cleaner energy conversion are primary tasks. For such technologies (MILD, LTC, etc…), 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. However, 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, leading to significant features such as uniformity and distributed ignition. The present study numerically characterized the turbulence-chemistry and combustion regimes of propane/oxygen mixtures, highly diluted in nitrogen, at atmospheric pressure, in a cyclonic combustor under MILD Combustion operating conditions. The velocity and mixing fields were obtained using CFD with focus on mean and fluctuating quantities. The flow-field information helped differentiate between the impact of turbulence levels and dilution ones. The integral length scale along with the fluctuating velocity is critical to determine Damkohler and Karlovitz numbers. Together these numbers identify the combustion regime at which the combustor is operating. This information clearly distinguishes between conventional flames and distributed combustion. The results revealed that major controllers of the reaction regime are dilution and mixing levels; both are significantly impacted by lowering oxygen concentration through entrainment of hot reactive species from within the combustor, which is important in distributed combustion. Understanding the controlling factors of distributed regime is critical for the development and deployment of these novel combustion technologies for near zero emissions from high intensity combustors and energy savings using fossil and biofuels for sustainable energy conversion.Distributed combustion regime occurs in several combustion technologies were efficient and environmentally cleaner energy conversion are primary tasks. For such technologies (MILD, LTC, etc…), 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. However, 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, leading to significant features such as uniformity and distributed ignition. The present study numerically characterized the turbulence-chemistry and combustion regimes of propane/oxygen mixtures, highly diluted in nitrogen, at atmospheric pressure, in a cyclonic combustor...