Critical Issues of Chemical Kinetics in MILD Combustion

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Effects of Bath Gas and NO x Addition on n -Pentane Low-Temperature Oxidation in a Jet-Stirred Reactor

International audienceThe oxidation of n-pentane (C5H12) in different bath gases (He, Ar, and CO2) and in Ar with NO2 or NO addition has been studied in a jet-stirred reactor at 107 kPa, temperatures between 500 and 1100 K, with a fixed residence time of 2.0 s, under stoichiometric conditions. Four different quantification diagnostics were used: gas chromatography, a chemiluminescence NOx analyzer, continuous wave cavity ring-down spectroscopy, and Fourier transform infrared spectroscopy. The results showed that the onset temperature of the fuel reactivity was the same (575 K) regardless of the type of bath gases. Although the low-temperature fuel oxidation window was not affected by the type of bath gas, the n-pentane conversion was slightly larger when diluted in Ar through the negative temperature coefficient (NTC) region (625-725 K). Above 800 K, the reactivity according to the diluent was in the order CO2 > Ar > He. In the presence of NO2 or NO, it was found that the consumption rate of n-pentane exhibited a different trend below 700 K. The presence of NO2 did not modify the fuel conversion below 675 K. On the contrary, NO addition increased the onset temperature of the fuel reactivity by 75 K and almost no NTC zone was observed. This clearly indicated that NO addition inhibited n-pentane oxidation below 675 K. Above 700 K, n-pentane conversion was promoted by the presence of both NOx additives. The intermediate species HONO was quantified, and a search for HCN and CH3NO2 species was also attempted. A new detailed kinetic mechanism was developed, which allowed a good prediction of the experimental data. Reaction rate and sensitivity analyses were conducted to illustrate the different kinetic regimes induced by the NOx addition. The inhibition by NO of the n-pentane oxidation below 675 K can be explained by its direct reaction with C5H11O2 radicals disfavoring the classical promoting channels via isomerizations, second O2 addition, and formation of ketohydroperoxides, the well-known branching agents during alkane oxidation. With respect to NO2 addition, the major consumption route is via NO2 + CH3 = NO + CH3O, which is not directly related to the direct fuel consumption. HONO formation mainly derives from NO2 reacting with CHiO (i = 2, 3). The reaction, HONO + M = OH + NO + M, is one of the most sensitive reactions for HONO depletion.

2023 Roadmap on ammonia as a carbon-free fuel

The 15 short chapters that form this 2023 ammonia-for-energy roadmap provide a comprehensive assessment of the current worldwide ammonia landscape and the future opportunities and associated challenges facing the use of ammonia, not only in the part that it can play in terms of the future displacement of fossil-fuel reserves towards massive, long-term, carbon-free energy storage and heat and power provision, but also in its broader holistic impacts that touch all three components of the future global food-water-energy nexus

Design and development of a lab-scale burner for MILD/flameless combustion

This paper discusses the design and development of a burner for MILD/Flameless Combustion processes. Computational Fluid Dynamics was used to simulate preliminary designs for the burner and to obtain a characterization of the combustor for non-reactive conditions. MILD or Flameless combustion is a stable combustion process without the presence of visible flame, defined by the recirculation of hot products of combustion inside the chamber volume. The combustor presented in this work was built in vermiculite and it has a prismatic shape to ensure the optical accessibility. A preheated main flow of diluent and oxygen and the fuel flow are fed inside the combustion chamber from one side. Diametrically opposed the feeding configuration is reproduced, thus realizing a spiral flow field inside the combustion chamber. The system is provided with a quartz window. The oxidation process of fuels/oxygen mixtures diluted in N2 can be studied varying external parameters of the system, namely inlet temperatures (up to 1200K), equivalence ratio (lean to reach mixtures), residence times and mixture dilution levels. Temperature measurements inside the chamber are realized. Rapid mixing between the injected fuel and hot oxidizer has been carefully explored for autoignition of the mixture to achieve volumetric combustion reactions. Distributed reactions can be achieved for a non-premixed configuration with sufficient entrainment of hot species present in the flame and their rapid turbulent mixing with the reactants. Experimental tests, realized for C3H8/O2 mixtures diluted in N2, showed that for inlet temperatures higher than 900 K, MILD combustion condition is established for an overall dilution level of 90 % in nitrogen. Increasing the preheating temperature, MILD occurs and combustion becomes invisible and homogeneous

The Effect of Diluent on the Sustainability of MILD Combustion in a Cyclonic Burner

The present study investigates the characteristics of MILD/flameless combus- tion in a cyclonic lab-scale burner. Such a configuration is effective for achieving turbulent mixing in a very short time while allowing for a reasonably long residence time for the development of combustion reactions. These two constraints are mandatory in the case of MILD combustion processes (high inlet temperatures and diluted mixtures). Such operating conditions are achieved through massive heat/mass recirculation towards the fresh incom- ing mixtures by recycling the exhausted gases, featuring a process where chemical kinetics times are elongated because of the dilution levels. Thus, long residence times are needed to achieve a satisfying reaction progress, and the high inlet temperatures result in fast and effi- cient mixing between disproportionate flows to avoid the onset of oxidation reactions before achieving diluted conditions. Under these constraints, a lab-scale facility was designed and built. The oxidation processes of C3H8/O2 mixtures highly diluted in N2 or CO2 were inves- tigated by varying the external parameters of the system, namely, the inlet temperature (up to 1300 K) and the mixture composition (from lean to rich mixtures). Several combus- tion regimes were experimentally identified. When the MILD regime was established, the combustion process became homogeneous within the burner without luminous emissions. To investigate the distributed nature of the MILD combustion processes, chemical simula- tions were performed under the assumption of a well-stirred reactor. For both the diluents, good agreement between the experimental and numerical results was obtained for MILD combustion conditions

Diffusion Ignition Processes in MILD Combustion: A Mini-Review

MILD combustion processes belong to new combustion technologies developed to achieve efficient and clean fuel conversion. The basic concept behind its implementation is the use of high levels of hot exhausted gas recirculation within the combustion chamber. They simultaneously dilute fresh reactants, to control system temperatures and pollutants emission, while promoting fuel complete oxidation. The combination of low maximum system working temperatures and high diluted mixtures with intense pre-heating delineates an oxidation process with unique chemical and physical features, such as uniformity of scalars at macroscale related to distributed reacting regions at microscale, extremely different from conventional flames. In turn, this requires the definition and characterization of new elementary processes, not ascribable to traditional deflagration or diffusive flame structures, which, in literature have been identified as “diffusion ignition.” The present mini-review reports on several literature characterizations of such reactive structures under steady and unsteady conditions combining evidences from numerical, experimental, and/or theoretical studies. Both premixed and non-premixed configurations were analyzed in terms of system temperature, heat release, and species distributions as key parameters to describe the intrinsic nature of such new elementary processes. Analyses were realized changing the main system external parameters (mixture pre-heating temperature, dilution level in several feeding configurations) moving from traditional to MILD conditions. Results highlighted the “distributed ignition” nature of igni-diffusive structures, with implication on the thickness of the oxidation structures in the mixture fraction space, the presence/absence of a pyrolysis region, and the correlation of the maximum heat release with the mixture stoichiometric

Influence of preheating and thermal power on cyclonic burner characteristics under mild combustion

Autoignition and stabilization of distributed combustion regimes have been proved to occur when a sufficient entrainment of hot species in the fresh reactants jets is reached, thus providing simultaneously for the sensible enthalpy to promote the auto-ignition process and the mass to dilute the incoming fresh reactants. The present study investigates the stabilization process along with the performance of the combustion process in a cyclonic burner operated under MILD combustion conditions. The cyclonic flow has been achieved by means of two pairs of oxidant/fuel jets injected using an anti-symmetric configuration in a prismatic combustion chamber thus realizing a centripetal cyclonic flow field directed toward the top-central gas outlet. Propane/air combustion experimental campaigns without external dilution, on a detailed grid of equivalence ratio and preheating temperature values, at different average residence time and nominal thermal power values were made. In each test condition, temperature measurements inside the chamber and gas sampling analyses have been carried out in order to evaluate the operability range of the cyclonic burner and its performances. These tests allowed to demonstrate the feasibility of stable MILD Combustion regimes in a wide range of operating conditions even when feeding the cyclonic burner with undiluted air. The residence time of the streams inside the burner plays an important role for both reactive structure stabilization and combustion performances/emissions. Significantly, fuel-lean conditions correspond, in the considered cases, to simultaneously low CO and NOx emissions. Furthermore, it has been demonstrated that stable combustion can be sustained in absence of any preheating in a considerable thermal load range and that it is possible, in this condition, to achieve a complete fuel conversion, with a remarkably low pollutant emission for thermal loads up to 8 kW