Low-oxygen Dilution Combustion Research Articles

Dynamics of regime transition of autoignitive jet flames from conventional to MILD combustion

Turbulent combustion of jet flames in a hot diluted coflow of combustion products has been studied across a range of jet Reynolds numbers for propane, ethylene, and an ethylene-propane blend as fuels. The study revealed a transition from conventional autoignitive combustion to a regime of Moderate or Intense Low-oxygen Dilution (MILD) combustion, which is characterized by a nearly invisible flame. The flames studied are luminous at low jet velocities and become MILD at higher jet velocities. Planar Laser-Induced Fluorescence (PLIF) of formaldehyde (CH2O), a key intermediate species in hydrocarbon combustion, is combined with CH*-chemiluminescence imaging and Rayleigh scattering to investigate the phenomena. The transition to MILD combustion is found to accompany a broadening of the formaldehyde region, indicating a broader low-temperature reaction zone. As the transition is approached, the appearance of holes in the chemiluminescent front is also observed. Correspondence between the location and structure of these holes with regions of formaldehyde in the flame are illustrated. These regions are proposed to be signatures that precede the transition to a fully MILD flame. In other words, the transition to MILD combustion begins at a local level and the MILD region spreads to engulf the entire combusting mixture. Scalar gradients were imaged to further unravel the local turbulence/chemistry interactions that yield the MILD inception. The measured scalar gradients were found to be commensurate with scalar gradients obtained from chemical kinetic simulations at the stoichiometric mixture fractions, while being an order of magnitude larger at other locations; this suggests that the inception of MILD combustion occurs near the stoichiometric region.

Furnace MILD combustion versus its open counterpart in hot coflow

The open jet flame in hot co-flow (JHC) has been frequently utilized for fundamental investigations of Moderate or Intense Low-oxygen Dilution (MILD) combustion due to its controllable conditions and relatively easy measurement capabilities. However, practical MILD combustion must take place within a combustor that is enclosed. Therefore, it is necessary to examine the similarity and disparity of combustion characteristics between two flame configurations. This issue is addressed currently. Specifically, we investigate the flow mixing, ignition, and combustion, as well as emission characteristics of non-premixed and premixed JHC and cylindrical furnace (FUR) flames at various values of the environmental temperature (Te) and central jet Reynolds number (Re). For the open non-premixed flames, we employ both previous single-tube JHC (SJHC) combustor and presently modified JHC (MJHC) one that uses the same nozzle configuration as the FUR burner. It is revealed that significant differences occur in flow, combustion and emission characteristics between the SJHC and FUR cases. On the other hand, both non-premixed and premixed MJHC configurations exhibit high similarity to the corresponding FUR cases in terms of upstream flow mixing and combustion features. Moreover, different CO and NOx emissions result from open JHC and close furnace flames due to different post-combustion configuration and residence time. Accordingly, future experiments on non-premixed MJHC and premixed JHC flames are highly recommended for better understanding practical MILD combustion.

Research on characteristic chemical time scales for MILD combustion based on partition PCA method

MILD combustion represents a more moderated reaction process in contrast to traditional combustion. The employment of hydrogen-containing syngas as fuel introduces a spectrum of intricate and diverse chemical reactions. This complexity renders traditional reliance on characteristic chemical timescales less appropriate for the analysis and modeling of Moderate or Intense Low-oxygen Dilution (MILD) combustion phenomena. In response to this inadequacy, a new technique named the partition Principal Component Analysis (PCA) method was developed. This method begins by segmenting combustion regions based on the reaction intensity, then extracts characteristic chemical time scales for each of these individual regions through the application of the PCA method. The innovative method was subsequently applied to investigate the chemical traits of the CH4/H2 mixture in both MILD and standard combustion processes with respect to the time scale. The insights obtained from this novel approach demonstrate that the characteristic chemical time scales (τc) deduced through this methodology exhibit a more accurate representation of the intrinsic characteristics of MILD combustion within the reaction region. This approach enhances the comprehension of the interplay between turbulence and chemistry in MILD combustion contexts. Furthermore, it establishes a solid groundwork for forthcoming research endeavors in the field of MILD combustion. Furthermore, the τc calculations presented in this paper have been derived mathematically, confirming their widespread applicability.

Effect of hydrogen-blending ratio and wall temperature on establishment, NO formation, and heat transfer of hydrogen-enriched methane MILD combustion

Hydrogen-enriched methane moderate or intense low-oxygen dilution (MILD) combustion is a promising technology that offers an attractive pathway to achieve low NOx and CO2 emissions in furnaces and boilers. This paper numerically investigates the influence of hydrogen-blending ratios (XH2 = 0–100 %) and wall temperature (Twall = 1500–300 K) on establishment, NO formation/reduction mechanisms, and heat transfer behaviors of hydrogen-enriched methane MILD combustion with non-preheated air in a laboratory-scale furnace. Results show that hydrogen addition increases the peak Damköhler number from 7.5 to 20.3, which could cause departure from MILD combustion at high wall temperatures. However, lowering Twall can slow down the MILD reaction rate and thus reduce Damköhler number closer to unity, which is beneficial for achieving hydrogen-enriched methane MILD combustion. Interestingly, as XH2 is increased from 0 to 100 %, the threshold wall temperature for achieving MILD combustion is extended from 1170 to 690 K. In other words, MILD combustion could be sustained/stabilized under low Twall conditions (encountered in boilers) when burning pure hydrogen or fuels with a high percentage of hydrogen. Moreover, as Twall is lowered, the major NO formation pathway is changed from thermal-NO to NNH and N2O-intermediate in pure hydrogen MILD combustion; also, H reburning via the pathway of NO→+HHNO→+HNH→+NON2 becomes more important in reducing NO at a lower Twall level. As XH2 is increased from 0 to 100 %, the radiative heat transfer is enhanced by 29.4 % while the convective heat transfer is reduced by 16.8 %, ultimately resulting in an increase of 15.0 % in the total heat transfer (Twall = 1300 K). Along the furnace sidewall, the distribution of heat flux has a low value near the burner exit, peaks in the downstream furnace, and declines further downstream close to the furnace exit in MILD combustion with non-preheated air.

Understanding of MILD Combustion Characteristics of NH3 Air Flames in N2 And H2O Steam Diluted Environment at Atmospheric Pressure

Ammonia is becoming increasingly popular as a carbon-neutral fuel with zero carbon dioxide emissions. However, a significant hurdle lies in its combustion, which leads to substantial emissions of NOx. The current research involves conducting a chemical kinetic investigation to examine the characteristics of Intense Low oxygen Dilution (MILD) or Moderate combustion in ammonia (NH3)/air flames. This study is carried out under specific conditions, such as oxygen concentrations ranging from 11to 23%, premixed reactant temperatures between 1300 and 1700 K, and a pressure of one atmosphere. The study focuses on investigating the combustion characteristics of MILD using dilution with H2O and N2.With the rise in the inlet temperature of the premixed reactant, the peak temperature of the flame also rises. Moreover, flames diluted with H2O exhibit lower peak temperatures compared to flames diluted with N2.Flames diluted with H2O result in lower NOx emissions compared to flames diluted with N2. Additionally, for N2diluted flames, the exit NOx emissions rise as the oxygen concentration increases.Despite this, NOx emissions from H2Odiluted flames demonstrate non-monotonic behaviour.This means that the exit NOx increases initially as the oxygen concentration reaches 21%, but then begins to decrease. In contrast to N2and H2Odiluted flames exhibits a wider regime of no-ignition.Moreover, the rise in peak temperature in H2Odiluted flames is less apparent than in N2diluted flames, corresponding to broader ranges MILD combustion ranges.Furthermore, to attain MILD combustion in H2O diluted flames at a specific O2 concentration, the temperature of reactant needs to be higher than that required for N2diluted flames.

On the Definition of Reaction Progress Variable in Exhaust Gas Recirculation Type Turbulent MILD Combustion of Methane and n-Heptane

Three-dimensional Direct Numerical Simulations of Exhaust Gas Recirculation (EGR)-type Moderate or Intense Low Oxygen Dilution (MILD) combustion of homogeneous mixtures of methane- and n-heptane–air have been conducted with skeletal chemical mechanisms. The suitability of different choices of reaction progress variable (which is supposed to increase monotonically from zero in the unburned gas to one in fully burned products) based on the mass fractions of different major species and non-dimensional temperature have been analysed in detail. It has been found that reaction progress variable definitions based on oxygen mass fraction, and linear combination of CO, CO2, H2 and H2O mass fractions (i.e. cO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${c}_{O2}$$\\end{document} and cc\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${c}_{c}$$\\end{document}) capture all the extreme values of the major species in the range between zero and one under MILD conditions. A reaction progress variable based on fuel mass fraction is found to be unsuitable for heavy hydrocarbons, such as n-heptane, since the fuel breaks down to smaller molecules before the major reactants (products) are completely consumed (formed). Moreover, it has been found that the reaction rates of cO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${c}_{O2}$$\\end{document} and cc\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${c}_{c}$$\\end{document} exhibit approximate linear behaviours with the heat release rate in both methane and n-heptane MILD combustion. The interdependence of different mass fractions in the EGR-type homogeneous mixture combustion is considerably different from the corresponding 1D unstretched premixed flames. The current findings indicate that the tabulated chemistry approach based on premixed laminar flames may need to be modified to account for EGR-type MILD combustion. Furthermore, both the reaction rate and scalar dissipation rate of cO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${c}_{O2}$$\\end{document} and cc\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${c}_{c}$$\\end{document} are found to be non-linearly related in both methane and n-heptane MILD combustion cases but the qualitative nature of this correlation for n-heptane is different from that in methane. This suggests that the range of validity of SDR-based turbulent combustion models can be different for homogeneous MILD combustion of different fuels.

Modified Reacting Solver: A Simplified Approach for Capturing the Molecular and Flow Diffusivities for the Nonpremixed MILD Flames.

The present study investigates the applicability of the inlet boundary species Lewis number (combined effect of molecular and flow diffusion) for the nonpremixed moderate and intense low oxygen dilution (MILD) flames. A modified reactive solver named modifiedReactingFoam is developed by including the enthalpy flux in the energy equation and using modified model constants in OpenFOAM. The present solver is tested on the delft-jet-in-hot-coflow burner operating under a moderate and intense low oxygen dilution combustion environment. Along with the flame with Reynolds number 4100, eight other jet-in-hot-coflow flames are simulated to test the capability of the present proposed solver. The main aim of the current work is to investigate the efficacy of the proposed solver in predicting the velocity field, temperatures, and flame lift-off height for the considered flames with a significant reduction in computational time. The predictions with the modified eddy dissipation concept model are improved. However, a significant deviation is still observed in the downstream direction of the burner. The numerical simulations are performed with methane Lewis numbers of 0.9-1.14 by keeping the respective constant Lewis numbers for the inlet boundary species. The modifiedReactingFoam predictions at a methane Lewis number of 1.12 are in very close agreement with the experimental results. The maximum deviation in lift-off heights is within ±3% of the experimental results. The present modified solver outperformed the other combustion models in the literature and reduced the computational time up to 10 times with a combination of DLBFoam compared to the inbuilt solver.

Flame self-interactions in MILD combustion of homogeneous and inhomogeneous mixtures

Flame self-interaction (FSI) events in Moderate or Intense Low-Oxygen Dilution (MILD) combustion of homogeneous and inhomogeneous mixtures of methane and oxidiser have been analysed using three-dimensional Direct Numerical Simulations (DNS). The simulations have been conducted at the same global equivalence ratio (⟨ϕ⟩=0.8\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle \\phi \\rangle = 0.8$$\\end{document}) for different levels of O2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm {O_2}$$\\end{document} concentration (dilution) and initial turbulence intensities. It has been reported that both homogeneous and inhomogeneous mixture MILD combustion cases exhibit significant occurrences of FSI events, with the peak frequency of FSI events occurring towards the burned gas side in all cases. Moreover, the frequency of FSI events increases with increasing dilution level and turbulence intensity, but the presence of mixture inhomogeneity leads to a reduction in total FSI events. In all cases, the cylindrical FSI topologies (i.e. tunnel formation and tunnel closure) were found to have a higher likelihood of occurrence compared to spherical FSI topologies (i.e. unburned and burned gas pockets). The geometries of FSI topologies were also analysed using the mean and Gaussian curvatures. It has been shown that the inward propagating spherical FSI topologies (i.e. unburned gas pockets) are associated with negative mean curvature, while outward propagating spherical FSI topologies (i.e. burned gas pockets) are associated with positive mean curvature. Moreover, tunnel formation (tunnel closure) FSI topologies predominantly exhibit either elliptic geometries with positive (negative) mean curvature or hyperbolic saddle geometries with negative (positive) mean curvature. It has been shown for the first time in MILD combustion that the mean values of kinematic restoration and dissipation terms in the transport equation of the magnitude of the reaction progress variable conditional upon the reaction progress variable tend to cancel each other in the vicinity of the critical points associated with cylindrical topologies. Thus, the singular contributions in these terms, which are obtained from analytical descriptions in the vicinity of tunnel formation and tunnel closure topologies, do not affect the balance equation of the magnitude of the gradient of the reaction progress variable. Consequently, there is no need for a separate model treatment for singularities in modelling approaches based on the magnitude of the gradient of the reaction progress variable. The FSI events in the reaction dominated and propagating flame regions of MILD combustion have also been analysed for the first time. It has been found that more FSI events occur in the reaction dominated region, particularly towards the burned gas side. However, the majority of spherical FSI topologies are found in the propagating flame region. The findings from this study indicate that turbulence intensity, dilution level and mixture inhomogeneity effects need to be considered in any attempt to extend flame surface-based modelling approaches to MILD combustion.

Exploring the Impact of CO2 Atmosphere on Propane Moderate or Intense Low-Oxygen Dilution Combustion: A Numerical Simulation Study

Abstract Moderate or intense low-oxygen dilution (MILD) combustion is a promising combustion technology widely recognized by the international combustion community. In this study, numerical simulation was used to investigate the effects of CO2 atmosphere on MILD combustion of propane in a 20-KW furnace.The results show that the O2/CO2 atmosphere leads to a lower average temperature in the furnace, better temperature uniformity, and more uniform distribution of OH and CH2O compared to MILD combustion in the N2/O2 atmosphere. Propane MILD combustion is established well under the physical and chemical effects of CO2. An analytical approach is proposed to describe the physical and chemical effects of CO2 on MILD combustion. The physical effect of CO2 shortens the ignition delay time and advances the pyrolysis and ignition of propane, which causes a high-temperature zone in the front furnace and reduces the temperature uniformity in MILD combustion. However, the chemical effect of CO2 dominates the establishment of the MILD combustion by increasing the ignition delay time and reducing burning rates, with the help of the physical effects of CO2 by intensifying the entrainment in the furnace. Thus, the overall effects of CO2 lead to enhanced temperature uniformity by enlarging the area and evening the temperature of both the ignition zone and combustion zone. These findings provide valuable insights into the physical and chemical mechanisms of CO2 in MILD combustion and have important implications for optimizing combustion processes for improved efficiency and reduced emissions.

An Assessment of the Validity of Damköhler’s Hypotheses for Different Choices of Reaction Progress Variable in Homogenous Mixture Moderate or Intense Low-Oxygen Dilution (MILD) Combustion

The applicability of Damköhler’s hypotheses for homogenous mixture (i.e. constant equivalence ratio) moderate or intense low-oxygen dilution (MILD) combustion processes (with methane as the fuel) has been assessed using three-dimensional direct numerical simulation data with a skeletal mechanism. Two homogeneous MILD combustion cases with different levels of O2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\ ext{O}}}_{2}$$\\end{document} concentration (4.8% and 3.5% by volume) and different turbulence intensities have been investigated to analyse the influence of dilution level, turbulence intensity and the choice of the reaction progress variable definition (i.e. different choices of major species for turbulent burning velocity and flame surface area evaluations) on the applicability of Damköhler’s hypotheses in MILD combustion. It has been found that the normalized volume-integrated burning rate remains of the same order of magnitude as that of the normalized flame surface area only for the reaction progress variable definition based on a species mass fraction which has a Lewis number close to unity (e.g. CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\ ext{CH}}}_{4}$$\\end{document}) but the level of applicability deteriorates when the Lewis number of the species mass fraction, based on which the reaction progress variable is defined, deviates significantly from unity (e.g. CO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\ ext{CO}}}_{2}$$\\end{document}). Moreover, it has been demonstrated that the flame surface area calculation from the OH mole fraction-based information can lead to significant departures from Damköhler’s first hypothesis. It is also found that the relative magnitudes of normalised volume-integrated burning rate and normalised flame surface area are significantly affected by the level of dilution and the choice of the reaction progress variable definition. Damköhler’s second hypothesis, which provides a relation between the normalised turbulent burning velocity and the ratio of turbulent to molecular diffusivities, has been found to hold in an order of magnitude sense in homogeneous mixture MILD combustion only for the reaction progress variable definition based on species that has a Lewis number close to unity (e.g. CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\ ext{CH}}}_{4}$$\\end{document}) but the level of disagreement increases as the Lewis number of the reaction progress variable deviates significantly from unity (e.g. CO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\ ext{CO}}}_{2}$$\\end{document}).

Enhanced mitigation of inhalable particles and fine particle-bound PAHs from a novel hazardous waste-power plant candidate

Emissions of the inhalable particle (dp < 10 μm, PM10) and their harmful compositions from combustion sources have high potential on health risk with nearly no regulation. This study investigates the particle size distribution (PSD), as well as the removal mechanism of PM10 and fine particle (FP)-bound polycyclic aromatic hydrocarbons (PAHs) from the flue gas of a hazardous waste thermal treatment system. It has ultralow regulated emission and becomes a candidate of power generation module. A series of the advanced scrubbers, cyclonic demister, and baghouse was equipped for multi-pollutant control. The moderate or intense low oxygen dilution (MILD) combustion effectively inhibited the PM2.5 generation by volumetric oxidation. Advanced scrubbers removed PM1, PM2.5, and PM10 by 85.24, 68.68, and 97.60%, respectively, which achieved by local supersaturation, heterogeneous condensation of water vapor, and the growth of fine PM. Moreover, the scrubbers effectively scavenged the course PM10 containing the high-molecular-weight PAH homologs onto the water phase but promoted the condensation and absorption of the lighter homologs onto the fine particle surface (dp ∼5.3 μm). The size window (dp = 0.3–1.0 μm) of the minimum efficiency reporting value of a BH filtration led to the peak of FP-PAH mass and BaP equivalent (BaPeq) toxicity at dp = 0.1–0.4 and 0.1–0.8 μm, respectively. Consequently, the synergy of MILD combustion and the SCB-CYC-BH system effectively inhibited the PM2.5, PM10, PM2.5-PAHs, and FP-PAH levels from a waste thermal treatment process and further mitigated the potential health risk.

Ignition characteristics of the high-velocity pulverized coal jet in MILD combustion mode: Experiments and prediction improvements

The influence of high jet velocity on the ignition characteristics of pulverized coal is very important for the Moderate or Intense Low-oxygen Dilution (MILD) combustion technology of pulverized coal. This paper conducts experimental and numerical simulation research on the ignition characteristics of the pulverized coal jet in a high-temperature environment based on the Hencken-type flat flame burner. The results show that when the ambient temperature is 1873 K and the ambient oxygen mole fraction is 5 %, the ignition delay distance of the pulverized coal jet decreases as the jet velocity increases from 15 m/s to 100 m/s for the formation of MILD combustion. The adoption of the turbulent particle heat transfer model and the non-spherical particle motion model effectively improves the prediction of particle dispersion, ignition delay, and reaction zone in the high-velocity coal jet flame. The enhancement of gas–solid heat transfer by turbulent fluctuation plays a dominant role in advancing the coal ignition in the high-velocity jet, while the non-spherical shape of coal particles mainly improves the uniformity of reaction distribution. The influence of ignition characteristics in the high-velocity jet on the MILD combustion formation of pulverized coal has been also analyzed in detail.

Estimation and testing of single-step oxidation reactions for hydrogen and methane in low-oxygen, elevated pressure conditions

Advanced combustion concepts, such as moderate or intense low-oxygen dilution (MILD) combustion, offer a reduction in NOx emissions and increased thermal efficiency. MILD is characterised by low-oxygen, high-temperature conditions, where finite-rate chemistry effects are significant. Modelling this regime using reduced or single-step chemistry remains a challenge in part due to the finite-rate chemistry. This work proposes a generalised method for assigning Arrhenius coefficients of a single-step reaction using the outputs of a detailed mechanism. Assigning the typical chemical conservation equation to a progress variable, activation energy and pre-exponential factor for an Arrhenius kinetics global reaction are determined for hydrogen and methane across a number of conditions, with the proposed method extended to n-heptane as well. The temperature exponent in the modified Arrhenius equation is determined by minimising the ignition delay error between detailed and single-step simulations. Functional forms of each coefficient are calculated from a multivariate regression, dependent on initial temperature, pressure, and oxidant mole fraction. The predicted mechanisms are compared against the detailed kinetics in closed homogeneous batch reactors, with comparisons for hydrogen extended to both laminar opposed flow diffusion flames, and computational fluid dynamics (CFD) simulations. Ignition delay and equilibrium temperature are both well predicted for all three fuels in the batch reactors. Notably, the negative temperature coefficient behaviour of n-heptane is successfully recreated with the single-step mechanism. Temperature and heat release of hydrogen flames are well captured in both opposed flow laminar flames, and in turbulent CFD simulations. The computational time was also significantly reduced through the single-step mechanisms, resulting in ∼100 times reduction in compute time for CFD simulations. The function form of the Arrhenius coefficients shows promise for extension outside of the ranges and fuels analysed herein, and presents interesting phenomena for exploring how initial reactant temperature and pressure influence the effective activation energy of an oxidation process.

Numerical analysis of the enrichment of CH4/H2 in ammonia combustion in a hot co-flow environment

A numerical investigation is conducted using OpenFOAM to study the characteristics of ammonia flames enriched with methane and hydrogen, influenced by a diluted co-flow (exhaust gas). The study aims to explore various flame characteristics, including fuel consumption, ignition delay time, lift-off height and position, temperature, and CO and NO emissions while maintaining a constant energy input of 5 kW. Non-stabilized flames are identified by analyzing the OH peak in the computational domain among ammonia flames enriched with CH4/H2 for different dilutions of O2 (6 %, 9 %, and 12 %). The study is structured into three stages of numerical simulations corresponding to stabilized flames: (1) no reaction, (2) reaction in the absence of O2, and (3) reaction with O2 to see the sole impact of the hot co-flow and local turbulence mixing on the primary cracking of ammonia. Furthermore, the rates of H2 and N2 formation resulting from the utilization of ammonia in the presence of hot co-flow are thoroughly examined. Additionally, the behavior of ammonia flames and CH4/H2-enriched ammonia flames under preheated air conditions (23 % O2) is analysed. Stable flames are observed in the co-flow at 12 % O2, displaying characteristics of Moderate or Intense Low-oxygen Dilution (MILD) combustion. Conversely, ammonia flames under preheated air conditions exhibit conventional flame behavior. Pure ammonia flames exhibit stronger temperature gradients and higher levels of CO and NO emissions. As the enrichment of NH3 with CH4/H2 increases, peak temperatures decrease while lift-off heights increase, indicating a transition towards more distributed flame structures. Notably, the instance with 40 % H2 enrichment exhibits lower NO emissions and a higher rate of ammonia consumption along with more distributive flame characteristics.

Assessment of MILD combustion in co/counter-swirl configuration using syngas as a fuel

Moderate or intense low oxygen dilution (MILD) combustion is investigated in a co/counter-swirl using OH∗-chemiluminescence, species measurements, OH planar laser-induced fluorescence (OH-PLIF), two-dimensional particle image velocimetry (2D-PIV), and microphone measurements. Experiments are performed in a two-stage combustor, where the first stage of combustor is catalytic stage and the second stage is the swirl stage (co/counter-swirl configuration). A fuel-rich syngas (20 % H2, 20 % CO, 12 % CO2, 2 % CH4, and 46 % N2) reacts with air in the catalytic stage. Then, the mixture of unconsumed syngas and products of catalytic combustion is supplied to the swirl stage where the mixture is burnt with oxidizer with varied oxygen concentration. The flame in the swirl stage is established by two concentric swirling streams where the inner stream is the hot gases from the catalytic stage, and the outer stream is the oxidizer. Co/counter-swirl flames are generated by changing the swirl direction of the inner stream. After achieving a stable flame, the macrostructure, flame steadiness, CO and NOx emissions, reaction zone distribution, and sound pressure level (SPL) are investigated at several oxygen concentrations (O2 = 13.13 %–21 %) with an aim to assess the MILD combustion mode in co/counter-swirl configuration.As the oxygen percentage decreases, the flame luminosity decreases for both co/counter-configuration. However, the reduction in luminosity is profound for co-swirl configuration. Clear distinctions between co and counter-swirl configurations are observed regarding flame height and stand-off height. Two-dimensional particle image velocimetry (2D-PIV) is utilized to understand these trends. The steadiness of the flame is investigated using standard deviations (SD) of OH∗-chemiluminescence images and global luminosity (I(t)). The flame steadiness is found to be improved as the oxygen concentration decreases. The OH-PLIF indicates the distributed nature of combustion. The NOx emission is found to be extremely low in all studied cases; however, the CO emission shows an increasing trend when O2 reduces. Finally, the sound pressure level and the dynamics stability are investigated using microphone measurements. The SPL decreases by ∼3 dB and ∼7 dB for the counter-swirl and co-swirl configuration, respectively. Furthermore, the frequency domain analysis suggests that the fundamental axial mode of the combustor is excited at high oxygen concentration. However, the unsteady combustion and chamber acoustics become decoupled at lower O2 concentrations. Thus, the present paper, for the first time, confirms that MILD combustion can be achieved in co/counter-swirl configuration, provided the oxygen concentration is low (∼13 %). The present study also establishes that the co-swirl configuration is more suitable than the counter-swirl for achieving the MILD combustion mode.

Sustainability of hydrogen-enriched methane MILD combustion over a wide range of hydrogen-blending ratios in a strongly heat-extracted well-stirred reactor

Moderate or intense low-oxygen dilution (MILD) combustion of methane blended with hydrogen offers a potential solution for gas-fired industrial furnaces and even boilers to realize rapid decarbonization while maintaining ultra-low NO emissions. In this work, the combustion mode map for identifying MILD combustion when burning CH4/H2 mixtures from pure methane to pure hydrogen is proposed, and then the sustainability of hydrogen-enriched methane MILD combustion over a wide range of hydrogen-blending ratios from 0 to 100% under strongly heat-extracted conditions is revealed. Results show that, there exists a threshold oxygen mole fraction (XO2*) below which hydrogen-enriched methane MILD combustion can be achieved as long as the inlet temperature (Tin) is above the self-ignition temperature of reactants. As the hydrogen-blending ratio increases from 0 to 100%, XO2* is reduced from 9.3 to 6.5% and thus the unconditional MILD combustion region is concentrated, implying that hydrogen addition requires a larger oxygen dilution level to sustain MILD combustion. Interestingly, the sustainable operating range of oxygen concentration to sustain hydrogen-enriched methane MILD combustion can be extended by increasing the heat extraction ratio (HER) properly, e.g., XO2* up to 13.2% at HER = 30% for pure hydrogen fuel. However, methane MILD combustion would become unstable and even extinguished once HER exceeds about 49%, whereas MILD combustion of methane blended with a high percentage of hydrogen could be sustained even under highly heat-extracted conditions, e.g., a maximum allowable HER of about 91% for pure hydrogen fuel. In pure hydrogen MILD combustion, heat extraction can further reduce NO emissions, mainly due to less NO production via the N2O-intermediate, NNH, and thermal pathways, and NO reduction by H radicals tends to be weakened with heat extraction. In conclusion, high-percentage hydrogen co-firing makes it possible to achieve stable MILD operation with low NO emissions under strongly heat-extracted conditions encountered in boilers.

Numerical study on the ignition and combustion of a CH4 jet flame in diluted and undiluted coflows

This study numerically investigates the ignition and evolution of CH4 jet flames in a hot coflow (JHC) using large eddy simulation (LES). Three coflow oxygen fractions are carefully selected to compare moderate or intense low-oxygen dilution (MILD, YO2,C = 6 % and 9 %) and high temperature combustion (HTC, YO2,C = 23.3 %). Systematic investigations are performed on flame ignition, lift-off, reaction progress, and stabilization mechanisms. The results show that for the MILD cases, both the ignition process and final stable flame are controlled by turbulent mixing and chemistry in a state far from chemical equilibrium. In contrast, the reactions are much stronger for HTC, and the final flame is controlled by chemistry through equilibrium reactions. Moreover, for all three cases, the reactions always progress more rapidly at the lean side than at the rich side; thus, autoignition dominates the former, whereas flame propagation is more important for the latter. In addition, the premixing of fuel and oxidant prior to main reactions is crucial for realizing MILD combustion; therefore, the flame is lifted and stabilized by the autoignition of partially premixed reactants in the fuel-lean region. In contrast, the final HTC flame attaches to the burner exit and is stabilized by both the lean-side autoignition and rich-side flame propagation. For all three cases, the lean-side and stoichiometric flames adopt the nonpremixed mode, whereas the rich-side reactions are in the premixed mode, except for the flame base.

The reactor-based perspective on finite-rate chemistry in turbulent reacting flows: A review from traditional to low-emission combustion

In flames, turbulence can either limit or enhance combustion efficiency by means of strain and mixing. The interactions between turbulent motions and chemistry are crucial to the behaviour of combustion processes. In particular, it is essential to correctly capture non-equilibrium phenomena such as localised ignition and extinction to faithfully predict pollutant formation. Reactor-based combustion models — such as the Eddy Dissipation Concept (EDC) or Partially Stirred Reactor (PaSR) — may account for turbulence-chemistry interactions at an affordable computational cost by calculating combustion rates relying upon canonical reactors of small fluid size and timescale. The models may include multiscale mixing, detailed chemical kinetic schemes and high-fidelity multispecies diffusion treatments. Although originally derived for conventional, highly turbulent combustion, numerous recent efforts have sought to generalise beyond simple empirical correlations using more sophisticated relationships. More recent models incorporate the estimation of scales based on local variables such as turbulent Reynolds and Damköhler numbers, phenomenological descriptions of turbulence based on fractal theory or specific events such as extinction. These modifications significantly broaden the effective range of operating conditions and combustion regimes these models can be applied to, as in the particular case of Moderate or Intense Low-oxygen Dilution (MILD) combustion. MILD combustion is renown for its ability to deliver appealing features such as abated pollutant emissions, elevated thermal efficiency and fuel flexibility. This review describes the development and current state-of-the-art in finite-rate, reactor-based combustion approaches. Recently investigated model improvements and adaptations will be discussed, with specific focus on the MILD combustion regime. Finally, to bridge the gap between laboratory-scale canonical burners and industrial combustion systems, the current directions and the future outlook for development are discussed.

Prediction model of NOx emissions in the heavy-duty gas turbine combustor based on MILD combustion

A prediction model of NOx emissions in heavy-duty gas turbine combustors based on the moderate and intense low-oxygen dilution (MILD) combustion was proposed, and the inlet parameter sensitivity analysis was also investigated to identify the impact weight for the practical operation of heavy-duty gas turbines. The optimal space-filling (OSF) design was first adopted to determine the optimal combination of gas temperature (Tgas), primary air temperature (Tfirst), secondary air temperature (Tsecondary), gas mass flow (mgas), primary air mass flow (mfirst), and secondary air mass flow (msecondary) for minimum NOx emissions. Based on the results of the OSF design, the minimum NOx emissions of 24.11 mg/m3 were obtained, and the corresponding Tgas, Tfirst, Tsecondary, mgas, mfirst, and msecondary were 769.67 K, 825.10 K, 722.61 K, 1.71 kg/s, 4.29 kg/s, and 7.69 kg/s within the range of 10% fluctuation of rated input parameters. The sensitivity ranking of the six inlet parameters was secondly achieved as mfirst > Tfirst > Tsecondary > msecondary > mgas > Tgas with the base of NOx emissions. Finally, a novel prediction model with six inlet parameters was established to quickly and accurately calculate the NOx emissions influenced by complicated boundary conditions in MILD combustion of heavy-duty gas turbines.

MILD combustion of a premixed NH3/air jet flame in hot coflow versus its CH4/air counterpart

Moderate or intense low-oxygen dilution (MILD) combustion is suitable for solving the problems of unstable flames and high NOx emissions (ENOx) of ammonia fuels; however, studies on this are rare. This paper numerically investigates the MILD combustion characteristics of a premixed NH3/air jet flame in hot coflow (JHC) under different jet equivalence ratios (ΦJ), coflow temperatures (TC), and oxygen levels (XO2,C). For comparison, similar CH4/air MILD-JHC flame characteristics are calculated. The results show that the NH3 flame generally has a lower temperature increase and heat release, as well as a larger reaction zone, than CH4. This suggests that the NH3 flame can develop into a MILD regime more easily than CH4. ENOx of the NH3 flame is two orders of magnitude higher than that of CH4 flame at all TC and XO2,C values for ΦJ < 1. At ΦJ > 1, ENOx of the NH3 flame rapidly decreases to nearly zero, but unburned NH3 emissions (ENH3) and H2 emissions (EH2) are significantly high (∼1000 ppm) and the combustion efficiency is low. Moreover, ENOx increases rapidly (gradually) with XO2,C (TC), whereas ENH3 and EH2 are considerably low at TC ≥ 1300 K and XO2,C ≥ 1% for the NH3 flame. ENOx primarily originates from NO; however, N2O is also important at low ΦJ and TC values. Therefore, considering ENOx, ENH3, EH2 and the burning efficiency, the optimal conditions for NH3 MILD combustion are ΦJ = 1, TC ≥ 1500 K, and XO2,C = 1%–3%. In addition, compared with conventional flames, NH3 MILD combustion can reduce ENOx by one to two orders of magnitude.