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•MILD combustion hysteresis behavior strongly depends on fuel oxidation chemistry.•MILD combustion includes hysteresis behaviors, irrespective of literature acceptance.•Anhysteretic conditions define only a small sub-domain of MILD combustion.•MILD combustion fuel-flexibility/resilience to extinction rely on these behavior.

Biomass for energy production has been extensively studied in the recent years. To overcome some constraints imposed by the chemical–physical properties of the biomass, several pretreatments have been proposed. Torrefaction is one of the most interesting pretreatments because torrefied biomass holds a wide range of advantages over raw biomass. The devolatilization of water and some oxygenated compounds influences the increase in the calorific value on both a mass and volumetric basis. The increase in the density reduces the transportation costs. Moreover, the decreased moisture content increases the resistance of biomass to biological degradation, thus facilitating its storage for long periods. Under torrefaction conditions, approximately 10–40 wt % of the initial biomass is converted into volatile matter, including liquid and non-condensable combustible gases. The energy efficiency of the process could greatly benefit the exploitation of the energy content of these products. Recent studies and technological solutions have demonstrated the possibility to realize polygeneration systems that integrate torrefaction/pyrolysis to a combustion process with the aim of obtaining torrefied material/biochar and/or energy from biomass. Some examples include Pyreg, Pyreg-Aactor GT, TorPlant, and Top Process. The identification of the main volatiles produced under the torrefaction regime is useful for the optimization of the operating conditions of the integrated system. The integrated process raises some concerns when biomass from phytoremediation and wood from demolition and construction activities are used as feedstock because they could contain potential toxic elements (PTEs). During the torrefaction treatment, the fate of PTEs should be controlled to avoid their release in the gas phase and to evaluate the extent of their concentration in the torrefied biomass. The present work aims at studying torrefaction as an eco-sustainable process for the combined production of a solid biofuel with improved characteristics with respect to the starting material and a combustible vapor phase, embedded in the gas carrier flow, to be directly burned for energy recovery. Herein, torrefaction tests on Populus nigraL. branches from phytoremediation and demolition wood were conducted at three temperatures, 250, 270, and 300 °C, at a holding time of 15 min. The energetic content of torrefied materials was determined. At the same time, the fate of the heavy metals (Cd, Pb, and Zn) in the raw biomass at different torrefaction temperatures was studied, and their mobility in the torrefied biomass was investigated and compared to the mobility in the raw biomass.

Innovative solutions in terms of energy efficiency and pollutant emission abatement require the development of new combustion technologies. In particular, several solutions imply a process based on mixture dilution and preheating that can lead to very peculiar combustion regimes. New combustion concepts, such as moderate or intense low-oxygen dilution (MILD) combustion, rely on a local self-ignition mechanism as a result of the obtainment of burned gas/fresh reactant mixtures that lead to a process mainly stabilized by means of a distributed autoignition. Despite the very interesting features related to such a concept, several modeling issues have to be properly investigated, to permit the development of MILD combustion concepts through a computationally driven design. The evaluation of characteristic times, on both micro- and macroscales, is strongly influenced by the emerging characteristics related to a strong coupling between mixing and kinetic times as a result of the high dilution levels of such technologies. To include detailed chemistry in computational fluid dynamics simulations, the flamelet-generated manifold seems to be a promising choice. Specifically, the aim of this work is to prove the reliability of the tabulated chemistry method with respect to a cyclonic burner that was used as a test case for validation purposes. Reynolds-averaged Navier–Stokes simulations were realized, and a chemistry tabulation approach was used to take into account detailed chemistry effects. Finally, an assessment of the heat transfer mode was carried out by including both convection and radiation heat exchange in the modeling and comparing experimental and numerical results to temperature and gas concentration measurements obtained in several locations of the experimental apparatus. The computational tool was able to catch in a satisfactory manner the main features of the combustion regime in the cyclonic burner, and the validation was strongly improved when radiative heat transfer is included in the numerical model.

Flow structures and gas-mixing induced in the splash zone of a fluidized bed reactor by bubble bursting at the bed surface were investigated. A planar laser light scattering technique based on a nondiffusive gas tracer and coupled with computer-assisted imaging was used to characterize the macroscopic and microscopic features of flow structures. Bursting of either isolated bubbles or of chains of closely time-delayed bubbles was considered. The basic flow structure associated with bursting of isolated bubbles is a toroidal vortex ring. Multiple vortex rings are generated by bubble chains, eventually interacting with each other to give rise to more complex hydrodynamic patterns in a way that depends on the time-delay between successive bubbles. Quantitative assessment of the flow structures included axial displacement, rise velocity, angular velocity, volume growth, and intermaterial specific surface area. The influence of bubble size, of bed material size, and of the time-delay between subsequent bubble eruptions was investigated. The properties of the flow structures provided the basis for an assessment of the extent of gas macro- and micromixing in the splash zone. The issues associated with volatile matter burn-out in the splash zone of fluidized bed combustors of high-volatile solid fuels were analyzed in the light of the findings of the present study.

New combustion technologies aim to increase the conversion efficiency and reduce the pollutant formation of combustion processes by extending the operating reaction conditions to new domains. The mild combustion mode represents one of the most promising technologies in view of its many potential advantages and numerous fields of application. The large-scale diffusion of such systems in practical applications is limited because of the lack of confidence of the system reliability, due mainly to the lack of studies carried out on the fundamentals of mild combustion processes. This paper can be considered a contribution to set up tuning and control tools for the application of the mild operating mode in practical combustion devices. Based on the definition of the conditions required to operate a combustion system in a mild mode, the specification of some requirements for the operation of high-compression ignition engines, like the homogeneous charge compression ignition engine or mild gas turbine, in mild conditions are analyzed. The role of oxygen dilution on the autoignition delay (@t) of heavy paraffins in temperature (600-1200 K) and pressure (1.3-4.0 MPa) ranges of interest in such applications is discussed on the basis of both experimental and numerical approaches. The very good agreement of results obtained by means of the two methodologies leads to the identification of a power law dependence of @t on oxygen molar fraction in the environment (@t ~ X"O"""2^-^n) where the exponent is a function of temperature. Three main regimes with respect to ignition delay dependence on oxygen concentration at high pressure have been identified. The different functional dependencies found both in the modeling and experimental results have been correlated to the different reaction pathways followed in the different temperature ranges.

Inelastic light scattering, excited at λ=266 nm, has been measured from 300 to 600 nm in tetradecane pulsed spray injected into a high-temperature (900 K), high-pressure (4 MPa), nearly quiescent environment.

Two-dimensional laser light scattering has been used in the study of atomization and dispersion processes of unsteady diesel sprays. Main features, calibrations and digital procedures of the technique have been presented and its diagnostics limits for dense sprays have been discussed in relation to test experiments performed on single droplets and dilute sprays. It has been preliminarily shown that the technique, taking advantage of its quantitative recording of calibrated scattering intensity, yields the evaluation or the signature of total surface area of the liquid according as the spray is dilute or dense.

A unique experimental method for analysing mass transfer between gaseous flows has been presented. It consists of the measurement of Lagrangian quantities, linked to intermaterial surfaces evolution generated by means of a pulsed smoke wire technique. Stretch intensity, stretch rate, mixing-layer thickness, separation distance between neighbouring segments of the interface, for a purposely designed prototypical quasi-two-dimensional flow configuration, have been measured against time and presented in terms of their statistical distribution and averages. A peculiar regime named ‘isolated mixing-layer regime’ has been identified by means of comparison of the mixing-layer thickness with the interface separation distance. Further, the direct experimental evaluation of the stretch rate of the intermaterial surface allows the evaluation of the role of the kinematics on reactive mixing-layers, in particular in the case of the stretched diffusion flame.

Diesel combustion has been studied under simplified experimental conditions in a nearly quiescent, high-temperature (900 K), high-pressure (4 MPa) environment by means of two-dimensional (2D) laser light scattering techniques and chemical analysis of gaseous and condensible material sampled by a fastacting valve.