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The strategy to stabilize distributed combustion regimes adopting cyclonic flow fields has been proven to be challenging. In fact, the establishment of a toroidal flow field within a combustion chamber may ensure the recirculation of mass and sensible enthalpy required to simultaneously dilute the fresh reactants and increase the temperature above the autoignition one. The combination of reactants dilution and preheating may greatly increase system energy efficiency and lower pollutants production producing very peculiar combustion regime (named MILD Combustion). At the same time this strategy can be compromised if the sensible enthalpy is not high enough to promote the auto-ignition process of diluted mixtures. This may happen either because of an inefficient recirculation system or due to a too low calorific fuel. The paper aims at exploiting the performance of a small-size cyclonic burner for a conventional fuel (CH4) and a low calorific fuel (synthetic biogas) through the characterization of the process stabilization and pollutant emissions as a function of the mixture equivalence ratio and the nominal thermal power of the inlet mixture (from 2 to 10 kW), with the aim of identifying the optimal operating condition of the system. Results suggest that the system has to be exercised with mixtures with compositions slightly under the stoichiometric conditions and in a well identified temperature range to minimize both NOx and CO emission. The burner can be easily exercised also with low calorific fuels for higher thermal powers according to the low LHV. However, it results that an efficient recirculation of the exhausts produces a robust MILD combustion condition also when low calorific fuels are used.

New scenarios for energy systems pointed out the importance of designing innovative combustion systems. In this context, high levels of internal dilution and preheating show interesting features related to low emissions, smooth temperature gradients, absence of visible flame and large fuel and load flexibility. Those characteristics are very difficult to obtain simultaneously with conventional combustion processes in the same device. The large-scale utilization of such novel concepts relies on the developments of proper modeling tools that should consider the multiple physical phenomena involved under distributed ignition. A challenging modeling aspect is related to the strong coupling between fluid-dynamics and kinetic time scales that implies the use of detailed mechanisms. Moreover, the heat transfer mechanisms and the heat loss at walls play key roles. In this context, tabulated chemistry methods are viable solutions to represent the thermo-chemical pattern in combustion systems with internal recirculation. However, the identification of adequate controlling variables for these systems is not trivial. In fact, in addition to mixture fraction and progress variable, an internal dilution and a heat loss parameter must be considered, leading to a 4-dimensional thermo-chemical manifold, with an inherent increase of computational costs. In this work a novel tabulation procedure is proposed in order to represent such comprehensive manifold taking into account the primary role of the internal recirculation on system reactivity. Moreover, a reduction of the thermo-chemical manifold was carried out by exploiting active interconnections between experiments and computations and embedding physical and process constraints based on measurable quantities obtained from experiments. These constrains are related to minimum ignition and maximum attainable process temperatures, heat loss through the surroundings and recirculation rate. The reliability of the proposed approach was assessed by comparing the reduced manifolds to the measured data for a cyclonic burner operating under massive internal dilution levels.

The hydrogen effect as a “fuel enhancer” on ammonia oxidation features is a relevant topic for ammonia-based energy conversion systems. For the most, scientific literature is focused on high temperature ammonia-hydrogen oxidation chemistry, whereas few works focus on low-intermediate temperatures (900–1350 K), relevant for non-conventional low-temperature combustion processes. Recently, low-intermediate and the shift to high-temperature ammonia oxidation chemistry has been characterized through experimental tests in a Jet Stirred Flow Reactor (JSFR) by the same authors, with the identification of thermo-kinetic instabilities. In addition, the ammonia effect on hydrogen oxidation chemistry has been addressed through a mutual inhibiting interaction for low-intermediate temperatures. Given this background, this work investigates the hydrogen effects on ammonia oxidation and thermo-kinetic instabilities from low-intermediate to high temperatures in a JSFR, parametrically varying the H2 inlet concentration. Maps of combustion behaviours (Tin- ϕ) are then drawn up, on the basis of experimental evidences, in the range 1200K

MILD combustion has a wide potential in enhancing thermal efficiency with nearly zero emissions. It has no visible flame since the radiation from the reacting zones is attenuated due to both the intermediate species at reduced temperatures, induced by intensely burned gas recirculation, and the absence of particulate emitters. Beyond these main features, there are other characteristics such as temperature uniformity and distributed ignition that have to be addressed and analyzed looking at the peculiar role of the heat transfer for such reactors. First, the category of combustion systems object of the study is described. Afterwards an analysis on the heat transfer mechanisms under MILD combustion of gaseous fuels is carried out. Therefore, in this Mini-Review, several literature findings highlighting the role of the heat transfer on the combustion peculiarities of MILD reactors (i.e., temperature uniformity, distributed ignition, low pollutant emissions) are reported and discussed. Heat exchange modes, in fact, contribute to providing MILD macroscopic characteristics by means of the strong interplay between wall and gas heat transfer, instead of the reactive structure. In particular, the thermal behavior of these systems is analyzed in order to stress the distinctive role of the heat loss and the relative contributions of the convective and radiative terms. Heat transfer mechanisms between gas and walls and their interactions, in fact, favor the wide temperature distribution within the chamber. In order to better understand the different effects of the heat transfer under MILD regime, the mechanisms regarding walls and recirculating gas are separately investigated.

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.

This work focuses on the characterization of the oxidation process of NH3/O2/N2 mixtures in a quartz Jet Stirred Flow Reactor (JSFR). Experimental tests are run as a function of mixture pre-heating temperatures (900–1350 K) and different equivalence ratios (Φ = 0.8, 1, 1.2), while keeping constant pressure, mixture residence time and mixture dilution level (d = 86%). Three different oxidation characteristic regimes are experimentally recognized as a function of the mixture pre-heating temperature (Tin), namely low (Tin < 1100 K), intermediate (1100 < Tin < 1225 K) and high (Tin > 1225 K) temperatures. Ammonia reactivity exhibits no-dependency on the mixture equivalence ratio for low-intermediate temperatures, while it is strongly dependent on it at higher ones. Some effort is devoted to evaluate the heterogeneous surface effects on the homogeneous ammonia oxidation process comparing the previous results with further tests obtained passivating the inner surface of the JSFR with water and comparing experimental results from two tubular Laminar Flow Reactors (LFR) made of different materials (quartz and alumina). Results suggest surface reactions mainly affect NO profiles, with no remarkable influence of key species concentrations. Numerical simulations show that tested models are not able to carefully reproduce the experimental results, specially within the low-intermediate temperature oxidation regimes. The analysis of the Reaction Rates highlights that ammonia oxidation is strongly dependent on the NH2 reaction routes at different temperatures. The discrepancy between the considered kinetic schemes can be ascribed to the different description of NH2 chemistry

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.

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. Two model fuels, constituted of a simple alkane hydrocarbon (tetradecane (TD)) and an aromatic/aliphatic mixture ( a -methylnaphthalene/tetradecane), have been used in order to study the effect of the fuel specificity on the combustion process. Temporal profiles of the scattering intensity and of the evolution of the oxidation progress evaluated byCO and CO 2 determinations, have shown the same behaviour independently on the fuel type and on the sampling location. For both fuels, the combustion proceeds through the formation of CO and subsequent oxidation to CO 2 , which is anticipated with respect to the appearance of a scattering signal due to soot formation and is almost completed in correspondence of the beginning of soot oxidation. The early phase of the latter process takes place in correspondence of the CO depletion when OH radicals become available. The importance of this result consists in the recognition that mechanisms of soot oxidation by an OH attack can realistically occur in diesel engine combustion and that this is the only pathway through which soot oxidation takes place in characteristics timescales comparable to those of its formation.

Combustion processes evolving in mild conditions are correcterized by working temperature lower than the flame temperature of the traditional combustion systems, allowing for a drastic reduction of pollutant formation. Such operating conditions' need of specific plant configurations related to the recovery of flue gases generally used for diluting and preheating the reacting stream. The internal or external exhaust recirculation and the high mixing level with fresh feed needed in order to gain the "mild combustion" mode, present several technological problems. These problems are considered in this paper in relation to the feasibility of a reliable plant. It is demonstrated that the power consumption for the treatment of high temperature flue gases is in an acceptable range for working conditions characteristic of mild combustion. Complete and reduced models of methane oxidation have been considered in order to study the feasibility of batch and Well-Stirred Reactor (WSR) configurations. It is shown that a WSR is not only desirable for mild combustion but, also, that this reactor configuration is technologically interesting for a combustion process only if a high initial temperature (greater than 1000 K) and a very diluted condition (oxygen molar fraction of 0.05) are considered. Only in this case, residence times of practical interest are related to the suitable conversion level of reactants. The end of this paper discusses a fluid-dynamic configuration for a WSR operating in mild combustion mode.