Intense Low-oxygen Dilution Combustion Research Articles

TRENDS REGARDING THE DEVELOPMENT OF HIGH THERMIC EFFICIENCY AND LOW LEVEL OF POLLUTING EMISSIONS COMBUSTION SYSTEMS

Flameless combustion, or moderate or intense low oxygen dilution combustion, is an advanced technology that meets all the conditions for being an important actor in the researchers’ efforts to develop low level of polluting emissions and high thermic efficiency combustion systems. Because all the factors that contribute to the emerge of this kind of combustion are not completely understood, there is the suspicion that this type of combustion could be achieved only by overheating the combustion air, which generates the limitation of this technology’s application area. After all the last years conducted research, it came to the conclusion that the requirements for this combustion are less severe than they were believed to be. The recent research is conducted towards the use of this type of combustion for all type of fuels, gas, liquid and solid.

Simulation of MILD combustion using Perfectly Stirred Reactor model

A simple model based on a Perfectly Stirred Reactor (PSR) is proposed for moderate or intense low-oxygen dilution (MILD) combustion. The PSR calculation is performed covering the entire flammability range and the tabulated chemistry approach is used with a presumed joint probability density function (PDF). The jet, in hot and diluted coflow experimental set-up under MILD conditions, is simulated using this reactor model for two oxygen dilution levels. The computed results for mean temperature, major and minor species mass fractions are compared with the experimental data and simulation results obtained recently using a multi-environment transported PDF approach. Overall, a good agreement is observed at three different axial locations for these comparisons despite the over-predicted peak value of CO formation. This suggests that MILD combustion can be effectively modelled by the proposed PSR model with lower computational cost.

Modeling MILD combustion using a novel multistage FGM method

Moderate or intense low-oxygen dilution (MILD) combustion is a promising concept to reduce emissions and increase efficiency. It requires high levels of dilution and preheating of reactants, which is realized by mixing reactants with products. In order to extend the application areas of MILD combustion, adequate computational models must be developed. In this study, one of the candidate models, flamelet generated manifolds (FGM), has been assessed in terms of applicability to MILD combustion. Based on the results of this assessment, a novel multistage (MuSt) FGM method has been developed. The need for developing such a method mainly stems from the existence of different combustion stages in the MILD regime, which cannot be represented by a single progress variable. In the MuSt-FGM approach, each stage of combustion is modeled using a different progress variable, without increasing the dimension of the lookup table. The MuSt-FGM approach has been tested by conducting a priori study, and simulating both 1D laminar and 2D turbulent flames. Proving successful in all three tests, the MuSt-FGM method emerges as a promising tool for modeling not only MILD combustion, but also other systems where combustion is characterized by different stages.

Ignition Characteristics in Spatially Zero-, One- and Two-Dimensional Laminar Ethylene Flames

In the continual effort to reduce emissions and improve efficiency, moderate or intense low-oxygen dilution combustion has been suggested for aeroengine applications. This new application of moderate or intense low-oxygen dilution combustion requires further insight in applying the knowledge from conventional analyses of well-mixed systems to non-premixed flames. The ignition of ethylene, a key species in hydrocarbon oxidation, is simulated in simplified combustion systems with three different hot oxidants using detailed chemical kinetics. Zero-dimensional batch reactors, one-dimensional opposed-flow flame simulations, and planar two-dimensional laminar coflowing slot flame simulations are used to compare different ignition metrics across the autoignitive and moderate or intense low-oxygen dilution combustion regimes. It is found that the autoignition of ethylene with hot air may be described in two dimensions as the intersection of a critical hydroxyl fraction and the most reactive mixture fraction. Although this provides a reasonable prediction of the flame base for ignition with hot air, this becomes less reliable in the approach to the moderate or intense low-oxygen dilution combustion regime. For the cases in, and in the transition to, the moderate or intense low-oxygen dilution combustion regime, a good agreement is seen between a 10 K rise above the oxidant temperature and the onset of strong chemiluminescence seen experimentally.

Classification and lift-off height prediction of non-premixed MILD and autoignitive flames

Moderate or intense low oxygen dilution (MILD) combustion has been the subject of numerous studies in recent years. An issue remains, however, in the definition of the boundaries of the MILD combustion regime with respect to non-premixed configurations without predefined reference temperatures. A flamelet definition is applied to non-premixed configurations to better understand the MILD combustion regime and classify previous experimental investigations. Through a simplified analysis, a new definition for the non-premixed MILD combustion regime is derived. This new definition is a function of initial and final temperatures, and the effective activation energy of an equivalent one-step chemical reaction. This inherently agrees with the features of the premixed flamelet definition and provides insight into previous attempts to reconcile premixed and non-premixed classifications of MILD combustion. Previously studied turbulent flames are classified using the new definition of MILD combustion and are compared to experimental observations. The new definition of MILD combustion is subsequently compared to the ignition characteristics of opposed-flow ethylene flames, showing good agreement. Finally, transient flamelets are solved for a modelled flow-field to successfully reproduce non-monotonic trends in the lift-off that is observed experimentally in a series of MILD and autoignitive, turbulent ethylene flames in hot, diluted coflows.

Impact of ignition temperature on particle size and magnetic properties of CoFe2O4 nanoparticles prepared by self-propagated MILD combustion technique

We prepared nanocrystalline CoFe2O4 by changing its ignition temperatures, using moderate and intense low-oxygen dilution (MILD) combustion technique. The effect of ignition temperature on the particle size and its magnetic behavior was investigated by HR-TEM and VSM respectively. We observed a vast change in the structural behavior and the magnetic properties of the prepared samples. X-ray diffraction studies revealed that the resultant samples had single phase with different grain sizes from 23±5nm to 16±5nm, which was understood by observing the growth of the grains through heat released from the combustion reaction. FE-SEM analysis showed high porosity with heterogeneous distribution of the pore size based on the adiabatic temperature and EPMA analysis, which confirmed the elemental compositions of the prepared samples. The saturation magnetization values measured at room temperature, employing vibrating sample magnetometer (VSM) decreased gradually from 50 to 34emu/g when the ignition temperature was increased from 243°C to 400°C. Some of Fe ions on the B sites moved periodically to the A sites because of quenching treatment. The presence of Fe2+ ions in the existing ferrite structure ruled the magnetic behavior of the sample, as confirmed by the Mössbauer analysis.

Numerical study of oxy-fuel MILD (moderate or intense low-oxygen dilution combustion) combustion for CH4–H2 fuel

This paper demonstrates a numerical study on the combination of Oxy-Fuel and MILD (moderate or intense low-oxygen dilution combustion) combustions, i.e. OXY-MILD. The N2 of a hot oxidizer was replaced with CO2 and H2O in a MILD combustion test case. The study was conducted using a CFD analysis, a zero-dimensional well-stirred reactor analysis, and a reactors network analysis. In the CFD analysis, RANS equations with modified k−ε equations were solved for a 2D-axisymmetric computational domain. Results showed a decrease in temperature gradient, reaction rate, and Damköhler number under the OXY-MILD condition in comparison with the MILD one. It seems the higher the oxygen level in the preheated oxidizer, the more effective the removal of N2 from the hot oxidizer was on the uniformity and extending of the reaction zone. Under OXY-MILD condition, an increment of inlet oxygen level decreased concentrations of CO, NO, CH2O, and HCO and increased temperature in the reaction zone. Furthermore, the effect of fuel hydrogen content on the reaction zone was investigated. A reduction of fuel hydrogen content led to an increase in both the uniformity of temperature field and the area of the reaction zone, and a decrease in formations of NO and CO for the OXY-MILD combustion.

Physical and Chemical Effects of CO2Addition on CH4/H2Flames on a Jet in Hot Coflow (JHC) Burner

Moderate or intense low-oxygen dilution (MILD) combustion is a promising technology for energy savings and pollutants emission reduction, and it has been experimentally shown that the MILD regime can be achieved more easily when using CO2 as diluent with respect to N2 [Li, P.; Combust. Flame 2013, 160 (5), 933−946]. Since CO2 is different from N2 in both physical and chemical properties, the present study aims at distinguishing the physical and chemical effects of CO2 addition on the establishment of MILD combustion. A jet in hot coflow (JHC) burner firing CH4/H2 blended fuel is numerically modeled coupled with a detailed chemical kinetics mechanism. Following the examination of grid independency and model validation, the differences in combustion temperature, minor and major species formations as well as the CH4 oxidation pathway are compared by, respectively, replacing N2 with CO2 or X (artificial CO2) in low-oxygen coflow. An interesting phenomenon is presented that the chemical effects of CO2 play a comparable role in the suppression of temperature rise to the physical effects. However, with the replacement of N2 by CO2, the contribution of chemical effects to temperature reduction is gradually weakened. Moreover, the chemical effects of CO2 are responsible for the ignition delay as well as the enhanced CO emission when is CO2 replacing N2, which is due to the inhibition of CH4 oxidation through Routes I (CH4 → CH3 → CH2(S)/CH2 → (CH → CH2O →) HCO → CO → CO2) and II (CH4 → CH3 → CH2O → HCO → CO → CO2) together with the enhanced CO2 dissociation by R99 (OH + CO ↔ H + CO2) and R153 (CO2 + CH2(S) ↔ CO + CH2O).

Numerical Study of Pulverized Coal MILD Combustion in a Self-Recuperative Furnace

A numerical study of pulverized coal combustion under Moderate or Intense Low oxygen Dilution (MILD) combustion conditions is presented in a parallel jet self-recuperative MILD combustion furnace. The Reynolds-Averaged Navier–Stokes equations, in a three-dimensional axisymmetric furnace domain, were solved using the Eddy Dissipation Concept model for the turbulence–chemistry interaction. The main aim of this study is to understand the influence of devolatilization models on the prediction accuracy of pulverized coal combustion under MILD combustion conditions. In particular, three devolatilization models are analyzed: a conventional single-rate model, a two-competing-rates model, and an advanced chemical percolation devolatilization (CPD) model based upon structural networks of the coal with a global kinetics mechanism. In addition, a new simplified numerical model is developed for Australian black and/or hard coal and optimized for the MILD combustion conditions. The modeling cases are validated with the experimental results of MILD combustion of high volatile Australian brown coal from Kingston using N2 as a carrier gas and low volatile Australian black coal from the Bowen basin using CO2 as a carrier gas in a MILD combustion furnace. From the comparison, the advanced CPD devolatilization model with a three-step global kinetic mechanism gives, as expected, the best agreement with the experimental measurements for the Kingston brown coal case, while all the models tested provide similar results for the Bowen basin black coal case. The heterogeneous reactions on the char burnout rate for pulverized coal combustion under MILD combustion conditions are presented and discussed. The characteristics of three NO formation mechanisms, namely, thermal-NO, prompt-NO, and fuel-NO, together with the NO reduction mechanism, named NO-reburning, are evaluated using the postprocessing modeling approach. The results reveal that the sum of contributions from the thermal-NO and prompt-NO routes only accounts for less than 3.8% to the total NO emissions, while the fuel-NO route dominates the NO emissions under MILD combustion conditions. It is also found that the total NO emission is reduced by up to 47% for the brown coal case and up to 39% for the black coal case via the reburning mechanisms.

Extended EDC local extinction model accounting finite-rate chemistry for MILD combustion

An extended Eddy Dissipation Concept (EDC) local extinction model is proposed to take into account the effects of finite-rate chemistry, normally occurring in Moderate to Intense Low oxygen Dilution (MILD) combustion, on the extinction limits. Local extinction is predicted when the local fine structure residence time is below a local critical value that is determined theoretically in the present study. The proposed model has been evaluated against experimental data reported for CH4/H2 jet-in-hot and diluted coflow flames. Comparison with the standard EDC extinction model is also presented. Results show that prediction of extinction threshold in MILD combustion conditions is attainable only through the application of the extended EDC extinction model on a well-resolved turbulence–chemistry interaction field. The effect of penetrating surrounding air into the reaction zone with subsequent flame cooling at downstream is also captured by the proposed extinction model. Despite its simplicity, the extended EDC extinction model is able to describe many features of localized extinction under MILD combustion as well as conventional combustion conditions.

Extension of the Eddy Dissipation Concept for turbulence/chemistry interactions to MILD combustion

Over the past 30years, the Eddy Dissipation Concept (EDC) has been widely applied in the industry for the numerical simulations of turbulent combustion problems. The success of the EDC is mainly due to its ability to incorporate detailed chemical mechanisms at an affordable computational cost compared to some other models. Detailed kinetic schemes are necessary in order to capture turbulent flames where there is strong coupling between the turbulence and chemical kinetics. Such flames are found in Moderate and Intense Low-oxygen Dilution (MILD) combustion, where chemical time scales are increased compared with conventional combustion, mainly because of slower reactions (due to the dilution of reactants). Recent modelling studies have highlighted limitations of the standard EDC model when applied to the simulation of MILD systems, noticeably a significant overestimation of temperature levels. Modifications of the model coefficients were proposed to account for the specific features of MILD combustion, i.e. an extension of the reaction region and the reduction of maximum temperatures. The purpose of the present paper is to provide functional expressions showing the dependency of the EDC coefficients on dimensionless flow parameters such as the Reynolds and Damköhler numbers, taking into account the specific features of the MILD combustion regime, where the presence of hot diluent and its influence on the flow and mixing fields impacts on the reaction rate and thermal field. The approach is validated using detailed experimental data from flames stabilized on the Adelaide Jet in Hot Co-flow (JHC) burner at different co-flow compositions (3%, 6% and 9% O2 mass fraction) and fuel-jet Reynolds numbers (5000, 10,000 and 20,000). Results show promising improvement with respect to the standard EDC formulation, especially at diluted conditions and medium to low Reynolds numbers.

3D DNS of MILD combustion: A detailed analysis of heat loss effects, preferential diffusion, and flame formation mechanisms

Moderate or intense low oxygen dilution (MILD) combustion is a relatively new technology which combines low emissions with high efficiency. As the name suggests, it requires high degrees of dilution in addition to preheating of the reactants using the combustion products. It has been applied to industrial burners for some time, but the complex physical mechanisms are yet to be resolved to extend the application to other areas. In this study, our aim is to analyze MILD combustion conditions to reveal the mentioned physical phenomena and their interactions. To this end, direct numerical simulations (DNS) in the form of autoigniting mixing layers are conducted, and the results are thoroughly analyzed. Conditions used in the simulations are taken from the jet in hot coflow experiments, which are designed to mimic MILD conditions. Detailed chemistry and transport models are employed in the numerical tools to fully understand the interactions of turbulence, molecular diffusion and chemical reactions. In addition, temperature variations due to heat loss effects in the experiments are taken into account. We have found that the heat loss and preferential diffusion effects are crucial in predicting not only the ignition delay, but also the flame structures and heat release rates. In addition, it is found that the flame formation is initiated by autoignition with different ignition delays along the most reactive mixture fraction, instead of a flame propagation following an initial autoignition spot. The findings of this study will broaden the knowledge on MILD combustion, and provide useful insight in developing reduced turbulence and chemistry models in the future.

Moderate or Intense Low-Oxygen Dilution Combustion of Methane Diluted by CO2 and N2

Moderate or intense low-oxygen dilution (MILD) combustion is regarded as one of the most effective technologies to achieve extremely low NOx emissions of combustion. MILD combustion diluted by CO2 and by N2 are herein termed “MILD oxy-combustion” and “MILD air combustion”, respectively. The present study is to investigate the difference of the two by experimental observation in a furnace of 20 kW and chemical kinetics calculation of a well-stirred reactor (WSR). Also, to identify their difference in mechanism, reaction paths of combustion diluted with N2 and CO2 are examined. It is revealed that the region of MILD oxy-combustion is notably larger than that of MILD air combustion for gaseous fuels, which suggests that the requirement for establishing MILD combustion is less stringent with dilution by CO2 than by N2. The key reason is that the CO2 dilution substantially lowers the temperature rise because of combustion, delays the ignition, and slows the overall reaction rate, thus facilitating the occurren...

Transported PDF Modeling of Ethanol Spray in Hot-Diluted Coflow Flame

This paper presents a numerical modeling study of one ethanol spray flame from the Delft Spray-in-Hot-Coflow (DSHC) database, which has been used to study Moderate or Intense Low-oxygen Dilution (MILD) combustion of liquid fuels (Correia Rodrigues et al. Combust. Flame 162(3), 759–773, 2015). A “Lagrangian-Lagrangian” approach is adopted where both the joint velocity-scalar Probability Density Function (PDF) for the continuous phase and the joint PDF of droplet properties are modeled and solved. The evolution of the gas phase composition is described by a Flamelet Generated Manifold (FGM) and the interaction by exchange with the mean (IEM) micro-mixing model. Effects of finite conductivity on droplet heating and evaporation are accounted for. The inlet boundary conditions starting in the dilute spray region are obtained from the available experimental data together with the results of a calculation of the spray including the dense region using ANSYS Fluent 15. A method is developed to determine a good estimation for the initial droplet temperature. The inclusion of the “1/3” rule for droplet evaporation and dispersion models is shown to be very important. The current modeling approach is capable of accurately predicting main properties, including mean velocity, droplet mean diameter and number density. The gas temperature is under-predicted in the region where the enthalpy loss due to droplet evaporation is important. The flame structure analysis reveals the existence of two heat release regions, respectively having the characteristics of a premixed and a diffusion flame. The experimental and modeled temperature PDFs are compared, highlighting the capabilities and limitations of the proposed model.

Numerical study of H2O addition effects on pulverized coal oxy-MILD combustion

This paper numerically investigates the effects of H2O addition on pulverized coal oxy-MILD (moderate or intense low oxygen dilution) combustion based on our previous study, in which only O2/CO2 was treated as oxidizer. While in the present study, the initial H2O mole fraction in oxidizer is varied at a constant oxygen level (30% by volume) to represent four typical oxy-fuel operations, i.e. ideal dry recycle (0% H2O+70% CO2), practical dry recycle (5% H2O+65% CO2), wet recycle (15% H2O+55% CO2) and oxy-steam (70% H2O+0% CO2). It is revealed that, with the addition of H2O, the internal recirculation rate (KV) as well as the coal ignition is improved, however, due to the difference in physical effects between CO2 and H2O, the temperature peak is likewise promoted. CO2 is preferred than H2O to moderate the flame temperature and realize uniform temperature distribution under MILD combustion. Higher H2 and lower CO formations are obtained by adding H2O because of the competing char gasification reactions between CO2 and H2O under in-furnace low oxygen condition of MILD combustion. H2O addition tends to suppress the volatiles-N conversion while accelerate the char-N conversion in the upper furnace, respectively, and reduce the final NO emission by enhanced H2 formation. Moreover, H2O addition is beneficial to increasing heat transfer in both radiative and convective forms.

An experimental study on MILD combustion of prevaporised liquid fuels

This paper presents an experimental study on moderate or intense low oxygen dilution (MILD) combustion of prevaporised liquid fuels burning in a reverse-flow MILD combustor under elevated pressures. The influence of fuel type, equivalence ratio, carrier gas, operating pressure and air jet velocity on the combustion stability and emissions are investigated. Ethanol, acetone and n-heptane are vaporised and carried to the combustor using either nitrogen or air. It is found that the combustion stability is highly dependent on fuel type, with n-heptane being the most unstable due to its fast ignition under all high-pressure conditions studied. Measured CO emissions emitted from all fuels are very low except when the equivalence ratio approaches the lean extinction limit, and this effect is not dependent on the pressure. The joint regime of low CO and NOx emission becomes narrower under elevated pressure as NOx emissions emitted from all fuels increased with pressure. The enhanced NOx formation rate via the nitrous oxide mechanism, the slower mixing, the increased flame temperature and residence time are believed to cause higher NOx emissions as pressure increases. The NOx emissions are reduced by increasing the air jet velocity, which is attributed to a lower peak temperature. The NOx emissions are also reduced when the fuel is carried by nitrogen instead of air. Further research is required to understand this trend which will help in reducing NOx emissions under these conditions, especially at elevated pressures.

Numerical Analysis on Microscopic Characteristics of Pulverized Coal Moderate and Intense Low-Oxygen Dilution Combustion

Moderate and intense low-oxygen dilution (MILD) combustion is regarded as a new clean combustion mode, which can simultaneously improve combustion efficiency and decease NOx emission. The computati...

Dynamic Behaviors in Methane MILD and Oxy-Fuel Combustion. Chemical Effect of CO2

The oxidation process of CH4/O2 mixtures diluted in CO2 under moderate or intense low-oxygen dilution (MILD) and oxy-fuel combustion conditions was numerically investigated in a perfectly stirred flow reactor at atmospheric pressure. The analysis aimed to investigate the kinetics involved in fuel oxidation in systems highly diluted and strongly preheated. Particular attention was focused on the effects of CO2 on oxidation routes because it can significantly alter the kinetic pathways participating directly in key reactions or indirectly in termolecular reactions as a third body species. Furthermore, adiabatic flame temperatures are lower with respect to air conditions because of the higher thermal heat capacity of CO2 in comparison to that of N2, thus modifying the kinetics promoted by temperature in combustion processes. The analyses were realized as a function of main system parameters, systematically changing inlet temperatures and mixture compositions. Results showed the establishment of complex dynam...

Numerical and Experimental Studies of NO Formation Mechanisms under Methane Moderate or Intense Low-Oxygen Dilution (MILD) Combustion without Heated Air

moderate or intense low-oxygen dilution (MILD) combustion a newly developing and innovative combustion technology. A MILD combustion without preheated air was achieved with inner flow recirculation and simulated by a computational fluid dynamics (CFD) model with GRI-Mech 3.0. Comparisons between the experimental and predicted results, including temperature and O2, CO, and NO concentration fields, demonstrate that the model closely predicted MILD combustion without preheated air and is suitable for studying the NO formation mechanism under these conditions. According to the distribution of NO and NO2, there are three zones in the furnace: a central zone (I), a transition zone (II), and a recirculation zone (III). The NO concentration is lowest in the central zone, highest in the recirculation zone, and intermediate in the transition zone. The NO2 concentration is high in regions where the NO concentration is low and low in regions where the NO concentration is high. The NO formation mechanism is that NH3, ...

Modeling Lifted Jet Flames in a Heated Coflow Using an Optimized Eddy Dissipation Concept Model

Moderate or intense low oxygen dilution (MILD) combustion has been established as a combustion regime with improved thermal efficiency and decreased pollutant emissions, including NOx and soot. MILD combustion has been the subject of numerous experimental studies, and presents a challenge for computational modeling due to the strong turbulence–chemistry coupling within the homogeneous reaction zone. Models of flames in the jet in hot coflow (JHC) burner have typically had limited success using the eddy dissipation concept (EDC) combustion model, which incorporates finite-rate kinetics at low computational expense. A modified EDC model is presented, which successfully simulates an ethylene-nitrogen flame in a 9% O2 coflow. It is found by means of a systematic study in which adjusting the parameters and from the default 0.4082 and 2.1377 to 3.0 and 1.0 gives significantly improved performance of the EDC model under these conditions. This modified EDC model has subsequently been applied to other ethylene- and methane-based fuel jets in a range of coflow oxidant stream conditions. The modified EDC offers results comparable to the more sophisticated, and computationally expensive, transport probability density function (PDF) approach. The optimized EDC models give better agreement with experimental measurements of temperature, hydroxyl (OH), and formaldehyde (CH2O) profiles. The visual boundary of a chosen flame is subsequently defined using a kinetic mechanism for OH* and CH*, showing good agreement with experimental observations. This model also appears more robust to variations in the fuel jet inlet temperature and turbulence intensity than the standard EDC model trialed in previous studies. The sensitivity of the newly modified model to the chemical composition of the heated coflow boundary also demonstrates robustness and qualitative agreement with previous works. The presented modified EDC model offers improved agreement with experimental data profiles than has been achieved previously, and offers a viable alternative to significantly more computationally expensive modeling methods for lifted flames in a heated and vitiated coflow. Finally, the visually lifted flame behavior observed experimentally in this configuration is replicated, a phenomenon that has not been successfully reproduced using the EDC model in the past.