MILD Combustion Conditions Research Articles

MILD combustion stabilization issues through the analysis of hysteresis behaviors: The case of new energy carriers

MILD combustion processes are renewed to reveal a strong resilience to extinction phenomena and/or instabilities, whereas the oxidation process is stabilized trough ignition phenomena. Under MILD conditions, igni-diffusive and/or perfectly mixed kernels, forming during the mixing process between hot products and fresh reactants, are so much diluted and pre-heated to escape classical feed-back flammable flames stabilization mechanisms, while ignition and extinction events merge in a unique condition through “anhysteretic” behaviors. So far, considering methane as reference fuel, it has been largely demonstrated the mentioned “anhysteretic” condition is very conservative and defines a sub-domain of MILD combustion processes, following Cavaliere and de Joannon's definition. Furthermore, the coincidence of ignition and extinction phenomena can occur also preserving hysteresis phenomena. In turns, this condition strongly enlarges the stabilization domain of MILD combustion processes, starting from the upper branch of the hysteresis behaviors to the real extinction, with characteristic unstable loci to consider as further/last opportunity to promote stable operative conditions through the formation of local thermo-kinetic conditions in the combustion chamber during hot products/fresh reactants mixing process (injection configuration/burner design), or by forced ignition events. The hysteresis behaviors of renewable/alternative fuels, relevant within the decarbonization policies of several energy sectors, are thoroughly discussed under MILD conditions through numerical studies in model reactors in order to shed light on common and/or different features, and outline practical rules towards the definition of stable MILD combustion domains. Results show that, as MILD combustion is a chemical kinetics-driven processes, stability issues have to be discussed in relation to fuel nature, albeit with common behavior can be derived. The coincidence between extinction/ignition phenomena is reached for extremely diluted conditions, already ascribable to MILD combustion conditions, thus defining a small sub-domain of the process. This condition can be reached through “hysteretic” or “anhysteretic” behaviors.

Enhancing pure NH3 combustion: Impacts of O2 enrichment under MILD conditions in a 20-kW semi-industrial scale furnace

This experimental work provides insights into the effects of oxygen (O2) enrichment on pure ammonia (NH3) combustion in a 20-kW semi-industrial scale furnace featuring an industrial-grade burner operated under MILD/Flameless conditions. The impact of O2 enrichment on the NH3 combustion stabilization, the flame and furnace temperature, the exhaust emissions, and the OH* chemiluminescence were explored. The tests were conducted by varying O2 concentration in the oxidizer stream from 21 % to 50 % and using two different oxidizer injectors to achieve average inlet velocities of 80 m/s and 150 m/s while keeping the inlet power and the total oxidizer mass flow rate constant. Stable flameless combustion with a distributed reaction zone and a consistent and uniform temperature profile throughout the combustion chamber was achieved with O2 enrichments higher than 34 %. Nevertheless, high levels of NH3 slips were observed, which were drastically reduced to negligible concentrations at higher O2 enrichments. On the contrary, NO emissions increased with the O2 fraction with an opposite non-linear trend compared to that of NH3 slip. An optimum trade-off between NOx emissions and NH3 slip was achieved by co-firing NH3 with O2=37 % with ID16 and 36 % with ID20 injector. This study also compares the behavior of pure O2-enriched NH3 combustion under flame and MILD combustion conditions to highlight the advantages of oxygen-enhanced NH3 flameless combustion. While operating in flame mode ensured a stable ammonia oxidation process already at low O2 enrichment=30 %, NO emissions and NH3 slips were significantly higher than those measured under MILD conditions. Finally, the impact of varying industrial load and the related heat balance on the NH3 slip was assessed at different levels of O2 enrichments at a fixed equivalence ratio. The overall results show that O2-enriched flameless ammonia combustion can simultaneously expand the range of stable combustion and reduce NO and NH3 emissions.

Effect of the primary oxidizer stream jet velocities on the ignition and combustion characteristics of pulverized coal under MILD oxy-coal combustion conditions

MILD oxy-coal combustion technology is a prospective technology with the potential for CO2 capture and storage, and it is characterized by the intense flue gas recirculation to preheat the fuel and to dilute the oxidizer due to the oxidizer or fuel jet with high velocity. The effect of the primary oxidizer stream jet velocities ranging from 1.7 m/s to 11.2 m/s on the coal ignition and combustion characteristics were experimentally investigated on a flat flame pulverized coal burner at various oxygen concentrations of 5%-21% and gas temperatures of 1473–1873 K under O2/CO2 and O2/N2 atmospheres. The coal flame images were recorded by the digital camera, and the flame structure evolution and ignition delay times of coal particles were analyzed at different primary oxidizer stream jet velocities and hot coflow conditions. The coal combustion radiation spectrum was monitored by an optical fiber spectrometer and the coal particle temperatures were derived with the two-color pyrometry. The results showed that the bright yellow soot flame gradually disappeared with the decrease of O2 concentration when CO2 replaced N2. The difference of ignition delay times between O2/CO2 atmosphere and O2/N2 atmosphere decreased from 2.8 ms to 0.2 ms when the primary air jet velocity increased from 1.7 m/s to 11.2 m/s at 1473 K and 5% O2. This meant the elevated jet velocity was beneficial to offset the delayed effect of the physicochemical properties of CO2 on coal ignition. The peak coal particle temperature increased by 68 K at 1873 K and 5% O2 under O2/CO2 atmosphere with the highest jet velocity owing to the accelerated diffusion of O2 into coal particle stream. Moreover, a dimensionless coal particle temperature parameter η was proposed to characterize the variation of the peak coal particle temperature caused by the improved heat transfer and mass transport between the coflow and coal particles under high jet velocity. The maximum value of η was 1.053 at 1873 K and 21% O2 under O2/CO2 atmosphere with the jet velocity of 11.2 m/s, indicating the increased jet velocity had the predominant effect on the increase of peak particle temperature at high gas temperature and O2 concentration.

Ammonia/Methane combustion: Stability and NOx emissions

In the framework of the energy shift toward Renewable Energy Sources, ammonia is considered a valuable energy vector, due to its very high hydrogen-density and well-established production processes. Despite the many potential advantages, its combustion features (narrow flammability limits, high auto-ignition temperature, potential high fuel-NOx emissions) may hinder its wide utilization. To overcome these issues in conventional combustion, “fuel enhancers” are also used. Differently from conventional systems, MILD Combustion already proved to be very effective in oxidizing ammonia in terms of stability and NOx emission. Nevertheless, a “fuel enhancer” can be also useful in MILD Combustion conditions to further improve the process characteristics. The present study focuses on the ammonia/methane combustion characteristics under MILD Combustion conditions in a lab-scale burner. Gaseous pollutant emissions (NOx, NH3, H2, CO) and process stability limits were analysed as a function of the equivalence ratio and NH3/CH4 fuel composition. Results showed that the use of NH3/CH4 blends extends the stable operational range of the system, in terms of both working temperatures and equivalence ratios, with respect to pure NH3. On the other hand, blends produce higher NOx emissions, with respect to both the pure NH3 and CH4 cases. Experimental data were compared with chemical kinetics modelling results. Chemical pathways and rate of production of main intermediate products highlighted that oxidation pathways of carbon and nitrogen-based species are essentially decoupled as experiments suggested. In contrast, the interplay of the methane and ammonia main chemical pathways affects the DeNOx channel in correspondence of the observed NOx emission increase.

MILD combustion of co-firing biomass and pulverized coal fuel blend for heterogeneous fuel NO and PM2.5 emission reduction

A sawdust and bituminous coal blend is studied experimentally under MILD combustion conditions, and the PM2.5 emissions of MILD combustion are measured. MILD combustion is established by low-temperature preheated air in a 200-kW pilot-scale furnace. The pilot tests consist of the conventional flame combustion through a swirl nozzle, MILD combustion through symmetrical double straight nozzles, and MILD combustion through an asymmetrical single straight nozzle. Compared to conventional swirl flame combustion, the MILD combustion of the sawdust blend can reduce the fuel-NO emission by at least 45.5%. The results further reveal that the PM2.5 emissions are decreased by over 37%. The burnout rates of the sawdust blend MILD combustion are higher than 95%. Considering the in-furnace distribution, low NO and PM2.5 emissions, and high burnout rate, MILD combustion is an effective way to utilize biomass fuel blends on a large scale. This work extends fuel adaptation of MILD combustion to biomass blends. Compared with previous MILD combustion studies, this investigation further confirms the inhibitory effect of MILD combustion on the formation of fine particles.

Temperature and NOx distribution characteristics of coal particles under high-temperature and low-oxygen environments simulating MILD oxy-coal combustion conditions

A diffusion-flamelet-based Hencken burner was used to produce moderate and intense low-oxygen dilution oxy-coal combustion (MILD-OCC) of coal particles under O2/CO2 environments with the coflow temperatures of 1000–1400 K and oxygen concentrations of 2.86–8.75%. The flame temperature distributions were measured by a R-type thermocouple coupled with temperature continuous recorder. The NOx was captured by a self-made air extraction sampling device and measured by UV spectrophotometer. Three influencing factors including fuel-air ratio of methane, secondary air and particle size of pulverized coal on temperature and NOx distributions were experimentally researched. It is found that increasing the fuel-air ratio and the volume of secondary air leads to lower peak temperature, more uniform temperature distribution, lower NOx and stronger NO-reburning effect in flame. To a certain extent, decreasing the particle size of pulverized coal causes lower ignition delay, higher release rate of volatiles and little change in NOx emissions, further reducing the particle size of pulverized coal results in higher NOx in flame. Moreover, the mathematical criterion of MILD-OCC state is deduced based on criterion of MILD coal combustion. The research has important reference value for in-depth understanding of MILD-OCC regime and the generation mechanism of fuel NOx.

Numerical analysis of NOx formation mechanisms and emission characteristics with different types of reactants dilution during MILD combustion of methane and coke oven gas

A numerical study is conducted to emulate the MILD combustion conditions in actual industrial furnaces and to explore the effects of different types of reactants dilution on NOx formation mechanisms and emission characteristics of methane and coke oven gas (COG) combustion. A well-stirred reactor (WSR) is utilized to generate high-temperature flue gas that dilutes and preheats the fuel and the oxidizer at different dilution ratios. The model with two plug-flow reactors and one opposed-flow diffusion flame reactor is selected for the simulations. The OH radical profile analysis indicates that COG combustion maintains a larger reaction zone due to high hydrogen concentration as compared to methane combustion. The results show that there exists a critical oxygen percentage that divides MILD combustion into two different combustion regimes based on the inlet temperature Tin of the reactants. And the critical oxygen percentage to achieve MILD combustion is about 9.5% and 7.5% for methane and COG, respectively. Moreover, the effect of different dilution types on the NOx emissions indicates that the NOx emission index (EINO) value for the fuel dilution case is about 0.2 g-NO/kg-fuel whereas the value for the oxidizer dilution case is around 0.4 g-NO/kg-fuel at the dilution ratio of 0.9. The comparison between the EINO values for fuel dilution and oxidizer dilution cases suggests that the fuel dilution is more effective to reduce NOx emission than the oxidizer dilution. The relative contribution analysis of different NOx formation routes reveals that the dominant NOx formation route is the prompt route for methane combustion whereas the N2O intermediate and NNH routes are also important for COG combustion. The results provide a good insight into the NOx formation mechanism and assist in the design of a low NOx burner of MILD combustion.

Structures of Ethanol Spray Flames under CO2 Dilution of the Oxidizer in the Counterflow Configuration under MILD Combustion Conditions

Structures of both gaseous and liquid ethanol flames in different oxidizing gas environments in the axisymmetric counterflow configuration at atmospheric pressure are studied. Initially, ethanol/air gas flames are considered where pure ethanol is directed against air at initial temperatures of 400 K, and N2 is successively removed to obtain structures of ethanol/O2 gas flames. Furthermore, the addition of CO2 to the oxidizer side is carried out. Then, an ethanol spray is carried by air and directed against an air stream, and the same procedure is performed as described for the gas flames. The gas strain rate at the fuel side of the configuration is increased from low values of 55/s up to extinction, and the initial droplet diameter is varied. For the combustion of gaseous ethanol in air and in pure oxygen, the nitrogen removal results in an increase in the maximum flame temperature from 2010 K to 2920 K at a gas strain rate of 55/s on the fuel side of the configuration, and the extinction strain rates are 630/s and 26,000/s, respectively. It is confirmed that ethanol spray flames in air show two reaction zones at low strain whereas the lean ethanol spray flames in pure oxygen exhibit a single reaction zone in all situations studied. For increased liquid fuel mass flow rate to a global equivalence ratio of unity, two reaction zones are retrieved. An analysis regarding the addition of CO2 in both the ethanol/oxygen gas and spray flames is also discussed and is found that CO2 dilution of the carrier gas the spray is much more efficient than diluting the opposed gas stream in the counterflow configuration for the generation of MILD combustion conditions in oxy-fuel flames.

Effect of hydrogen addition on NOx formation mechanism and pathways in MILD combustion of H2-rich low calorific value fuels

Hydrogen is a major component of low heating value (LHV) fuels like syngas, coke oven gas, and blast furnace gas. These fuels can potentially replace methane in the industrial process under MILD combustion conditions. NOx formation mechanism in MILD combustion regime is different from conventional air combustion due to uniform temperature distribution and low oxygen concentration. Hence, it is important to investigate the effect of hydrogen addition inmethane (CH4) on different NOx formation mechanism and pathways under MILD combustion conditions. A well-stirred reactor (WSR) model with atmospheric pressure and long residence time was employed to simulate the ideal MILD combustion conditions with Chemkin Pro software and GRI Mech 2.11. H2 and CH4 blends ranging from 0% H2 (100% CH4) to 60% H2 (B60 = 40%CH4/60%H2) were selected as fuels to emulate MILD combustion of LHV fuels. The oxygen mass fraction was kept from 3% to 9%, and the fuel blends were altered. The results showed that hydrogen addition enhanced NNH route while decreasing prompt route and N2O intermediate route whereas thermal NOx route was insignificant for lower O2 mass fraction of 3% and 6%. It was observed that H2 addition reduced NO-reburning which led to more NOx emission. Moreover, the sensitivity analysis was conducted to analyze the pathway of NOx formation and to quantitively separate the relative contribution of the individual route on the total NOx formation, which was beneficial to control the NOx emission of LHV fuels during MILD combustion.

Burning characteristics of grape marc under mild combustion conditions

In this experimental study, the burning characteristics of grape marc, as a biomass fuel, under MILD (Moderate or Intense Low-oxygen Dilution) combustion conditions are explored. Specifically, the impact of grape marc particle size on flame structure, stability and pollutant emissions, under MILD combustion, is investigated. Three different particle sizes of the grape marc are considered, in the range of 150–250 μm, 250–355 μm and 355–500 μm. The particles are injected through a central insulated jet into a hot and vitiated coflow inside a vertical MILD combustion furnace. All other operating conditions remain the same. CO2 is employed as a carrier gas and the central jet bulk Reynolds number is set to the value of Rejet = 13,040. The vitiated coflow had ~6% oxygen molar fraction and a flow bulk temperature of about Tco-flow = 1330 K. The heat input from the grape marc fuel is kept constant at 10 kW, corresponding to a mass flow rate of 1.63 kg/h. Stable MILD combustion conditions have been successfully achieved for all cases. The temperature gradient along the vertical and the radial direction of the furnace is limited to below 100 K for all the cases. The variation of particle size does not significantly impact on the furnace temperature and exhaust emissions. Albeit, the case of largest particle size exhibits a higher temperature by ~40 K as compared to the other two cases, and the highest NO production is identified for the middle particle size's case showing peak value of approximately 478 ppm at the exhaust at 3% excess O2. No remarkable NO-reburning process is observed for any cases, which points towards the intense volatile combustion of grape marc under MILD combustion conditions.

110th Anniversary: MILD Combustion of Liquid Hydrocarbon–Alcohol Blends

Among the various low NOx combustion technologies, MILD combustion combines low pollutant emissions, combustion stability and efficiency, fuel flexibility, and noise reduction through a high preheating of the combustion chamber coupled with a massive exhaust gas recirculation at high turbulence level. In this work, the sustainability of MILD combustion for liquid hydrocarbon–alcohol blends (as possible constituents of surrogate fuels representing the behavior of blends of fossil fuels and biofuels) has been investigated experimentally using a dual-nozzle laboratory-scale burner. Several hydrocarbon–alcohol blends have been investigated to identify the regions (in the furnace temperature vs dilution ratio space) where MILD combustion can be sustained with low NOx and CO emissions. It has been found that the MILD combustion conditions of the various liquid fuel blends investigated differ slightly one to each other and are quite similar to that of the hydrocarbons without any oxygenated species. This means t...

Numerical investigation on NOX formation in oxygen-CH4 MILD combustion with air leakage

In this paper, we focus on formation of NOX under oxy-fuel MILD combustion using JHC (jet-in hot co-flow) burner. The effect of the different oxygen dilution ratio in the co-flow is analyzed for MILD combustion formation and also for characterizing NOX emission. RANS (Reynolds averaged Navier Stokes) with the modified k-e equation and EDC (eddy dissipation concept) model were applied for 2D-axisymmetric computational domain using the Ansys Fluent. It is found that the effect of the oxygen concentration rather than the composition of dilution gas is more significant for the formation of the MILD combustion and as the oxygen concentration of the dilution gas increases under MILD combustion conditions, the region with Da < 1 expressed as MILD regime tends to be decreased. The effect of a diluted oxygen concentration in the oxy-fuel combustion on NOx emission under MILD condition is found to be much greater than the amount of air leakage into co-flow, and NO emissions are almost unaffected by the increase of N2 concentration in case of MILD combustion condition within practical air leak ranges.

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.

Impact of external operating parameters on the performance of a cyclonic burner with high level of internal recirculation under MILD combustion conditions

Internal exhaust gases recirculation represents a promising strategy to stabilize the oxidation process for new technologies (such as MILD, LTC) that limit system temperatures to reduce pollutants emissions while ensuring high process efficiency.A cyclonic flow configuration may represent a proper way to enhance the mixing process in very short time while allowing for residence times long enough to achieve complete oxidation of diluted and pre-heated mixtures.To this aim, a cyclonic burner was built and used to assess potentials of this peculiar configuration. Experimental tests have been performed varying independently the external operating parameters, namely the overall dilution level, the mixture composition and the average residence time. Temperature measurements inside the combustor and gas sampling analysis at the stack were carried out to evaluate cyclonic burner performance and pollutant emissions.Distributed combustion regimes have been proved to occur when a sufficient entrainment of hot species in the fresh oxidant and fuel jets is attained. The experimental results obtained for reactive conditions were ascribable to some peculiar characteristics of the cyclonic flow field.They confirmed that the cyclonic configuration is a valuable solution to stabilize the oxidation process while containing the pollutants emission for a wide range of operational parameter.

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

The present study investigates the characteristics of MILD/flameless combustion 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 incoming 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 efficient 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 investigated 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 combustion 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 simulations 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.

Reduced NO formation models for CFD simulations of MILD combustion

Two reduced kinetic models, incorporating thermal, N2O, NNH as well as HNO/NO2 intermediate routes, are proposed for the quick evaluation of NO emissions from MILD combustion of H2-enriched fuels through post-processing of Computational Fluid Dynamics simulations. The models were derived from a Rate Of Production Analysis carried out with two different detailed kinetic schemes. The models were tested using data from the Adelaide Jet in Hot Coflow burner fed with CH4/H2 mixture and operated with three different O2 contents. Very satisfactory predictions of in-flame NO measurements were achieved for the three cases, indicating a good applicability of the models across a wide range of MILD combustion conditions. Significant impact of the NNH intermediate path was observed.

Numerical Analysis of Methane Ignition Phenomenon Under Mild Combustion Condition

Abstract This work presents the results of a modeling study of the ignition process of methane under Mild Combustion conditions. A mathematical model was formulated to predict the dependence of the equivalence ratio Φ and the initial temperature of oxidizer t on the temperature increment ΔT=t max -t. The increment of temperature was assumed as parameter characterizing the process of ignition. The model incorporates the basic principles of the energy and mass balance. The results of calculation were compared with the experiments which have been done on constant volume bomb (CVB) reactor. It is shown that in order to achieve the effective reaction of ignition it is not necessary to maximize the initial oxidizer temperature. There are optimal values of temperature oxidizer (t≈830 o C) in which parameters mentioned above reaches its extreme values.

Visualization of MILD combustion from jets in cross-flow

The behaviour of methane jet flames autoigniting in a turbulent cross-flow at MILD combustion conditions was examined experimentally. The methane jet was injected orthogonally into a turbulent, low oxygen (XO2x<8%) cross-flow at high temperatures in a cylindrical quartz tube. Regardless of cross-flow conditions, the methane jet autoignited and a flame was stabilized in a wide region clearly separated from the injection nozzle. OH∗ chemiluminescence images show that OH∗ intensity was not significantly diminished by changing XO2x, but the size of the reacting region, as well as its location, was noticeably sensitive to XO2x. Cross-flows enriched with additional air sustained methane jet flames with larger reactive regions than those diluted with N2 or composed of pure hot products. Additional cross-flow dilution with N2 shifted the reaction further downstream, an observation supported by the imaging of individual autoignition kernels. Mean OH-PLIF shows OH radical presence in areas devoid of heat release, as indicated by the time-averaged OH∗ chemiluminescence images. In all cases, OH-PLIF instantaneous images show regions with wide OH distributions, but sharp gradients, similar to those evident in traditional combustion regimes. The data interpretation is supported by laminar non-premixed flame calculations that demonstrate some differences and similarities between MILD and conventional flames.

Autoignition delay times of propane mixtures under MILD conditions at atmospheric pressure

The aim of the present work was to obtain experimental reference data in controlled, simple systems collected under MILD combustion. The combustion processes evolving under such conditions show behaviors specific to unique ranges of operating conditions that are not predictable using the available kinetic mechanisms.Experimental tests were conducted in a tubular flow reactor for propane/oxygen mixtures diluted in nitrogen under MILD combustion conditions by varying the mixture composition (from fuel-lean to fuel-rich conditions) and the dilution level over a wide range of temperatures (850-1250K) at atmospheric pressure. Several combustion regimes were identified as a function of these external parameters. Auto-ignition delay times were evaluated, and they showed different levels of dependence on the system inlet temperature for intermediate to high temperatures.Under MILD combustion conditions, numerical simulations based on several available kinetic models predicted results with weak correlations to the experimental data, especially under fuel-rich, highly diluted conditions.To identify the controlling reaction pathways that should be tuned to extend the validity of models to wider operating conditions, sensitivity and reaction flux analyses were used. However, this topic is outside of the scope of the present work. This study provides reproducible experimental data for a reference system under novel conditions.

Methane auto-ignition delay times and oxidation regimes in MILD combustion at atmospheric pressure

MILD/oxy-fuel combustion is proving to be a reliable technology for clean and efficient energy production systems. It is by now established that the related operative conditions drastically change the combustion kinetics and the relative weight of kinetics and fluid-dynamics involved in the process. Thus a fundamental analysis of single sub-processes involved in fuel oxidation under MILD combustion conditions is needed to make easier the tuning and control of real systems in a wider range of working conditions, thus allowing for the spread of such a technology over a broader industrial scale.Such a work is devoted to the characterization of combustion regimes and the evaluation of auto-ignition delay times of methane in highly diluted and pre-heated conditions at atmospheric pressure in a one-dimensional flow reactor.Experimental tests have been carried out changing the mixture inlet temperature from 1100 up to 1400K, as well as the mixture carbon/oxygen feed ratios from fuel ultra-lean up to rich conditions. Mixtures are diluted in nitrogen from 85% up to 95%. At the same time, numerical simulations have been performed by means of a commercial software and a kinetic mechanism available in literature to value its reliability in predicting experimental results under MILD oxidation conditions.Results clearly show the occurrence of some discrepancies between numerical and experimental auto-ignition delay time data for fuel rich mixtures at high inlet temperatures.In addition, a richness of combustion regimes (slow combustion, pyrolysis, dynamic behavior, transitional combustion, combustion) was experimentally found. These unique behaviors as well as the dependence of ignition process on environmental parameters outline that modest temperature gradients due to high mixture dilution levels typical of MILD oxidation conditions, coupled with heat exchange to the surroundings, slow down the changeover among kinetic routes (namely oxidation and recombination channels) promoted by temperature thus stressing their competition. Such an aspect is generally hidden in traditional systems by a strong heat release that implies high temperature gradients.Finally, the data reported in this paper have an intrinsic value for the identification of stable operative conditions and for either tuning or reducing detailed kinetic schemes because few data are present in literature on methane ignition and oxidation in a simple reactor at atmospheric pressure.