Moderate Or Intense Low-oxygen Dilution Regime Research Articles

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.

Air-blast atomization and ignition of a kerosene spray in hot vitiated crossflow

Increasingly stringent regulations of pollutant emissions from aviation require rapid implementation of novel combustion technologies. Promising concepts based on moderate or intense low-oxygen dilution (MILD) combustion have been investigated in academia and industry. This MILD regime can be obtained from the recirculation of the hot vitiated combustion products to raise the temperature of the reactants, resulting in distributed reaction regions and lower flame temperatures. In the present work, we consider the air-blast atomization of a kerosene spray in crossflow, which enables efficient mixing between fuel and oxidizer. We investigate experimentally and numerically the effect of the spray air-to-liquid mass-flow ratio (ALR) variation on the reaction front and flame topology of a kerosene spray flame. The spray is injected transversely into a turbulent vitiated crossflow composed of the products of a lean CH4-H2 flame. The spray flame thermal power is varied between 2.5 and 5 kW, along with the atomizer ALR between 2 and 6. The experimental characterization of the reaction zone is performed using OH* chemiluminescence and OH and fuel planar laser-induced fluorescence (PLIF). The Large Eddy Simulations (LES) of the multiphase reactive flow provide good agreement with the experimental observations. Experiments and simulations show that the ALR governs mixing, resulting in different flame stabilization mechanisms and combustion regimes. Low ALR results in a relatively small jet-to-crossflow momentum ratio and a large spray Sauter mean diameter (SMD). A thick windward reaction region is formed due to inefficient shear layer mixing between the fuel spray and the crossflow. Meanwhile, the correspondingly large spray SMD leads to isolated penetration and localized combustion of fuel clusters. At high ALR, the higher penetration and the faster droplet evaporation due to the lower spray SMD result in an efficient entrainment-induced mixing between the two streams, forming more distributed reaction regions.

Predictions of NOx and SOx in MILD regime based on thermal conversion of solid sewage sludge surrogates

Thermal treatment of sewage sludge using a combination of gasification and combustion of the producer gas is an attractive option to produce sterilized high-notorious bio-char and recover energy from the feedstock. Given the high temperature of the producer gas after gasification, employing the MILD (Moderate or Intense Low Oxygen Dilution) combustion regime in the second stage might benefit the heat balance of the process and result in low emissions. The current work employs multi-component surrogates derived from solid fuel samples of various ultimate elemental compositions and moisture content, a stochastic reactor model for gasification, and a 1D counterflow diffusion flame configuration for combustion with a dedicated, detailed chemical mechanism. Using an improved methodology for emission measurement, we analyzed the combustion emissions of the wet producer gas from gasification, which includes fuel-NO precursors, sulfur components, and tar species up to a size of C11. The analysis confirms that thermal and fuel-NO emissions are reduced in the MILD regime. While high moisture content in the feedstock has been shown to enhance the formation of the pollutant via identified chemical pathways, different representations of fuel samples (ultimate composition) are found to have a significantly more considerable impact on the overall SOx and NOx emissions. The more sulfur is contained in the ultimate composition of the solid surrogate fuel, the more SOX is formed in the combustion stage. This direct relation does not hold for NO. The NO emissions in MILD combustion are the lowest in the performed simulations for the highest amount of fuel-bonded nitrogen. This is caused by the key role of the fuel-NO precursor ammonia (NH3) and its radicals that act in NO formation but also are a driving force in NO reduction. Given this key role of specific fuel-NO precursors, it is further highlighted that there is a significant difference in predicting NO, depending on the complexity of the fuel-NO precursors in the boundary condition of the flame calculation.

MILD combustion of methane in a model combustor with an inverse-diffusion flame configuration

MILD (moderate or intense low-oxygen dilution) combustion is a promising technology for mitigating NOx emission from fuel combustion, but the combustion stability becomes an issue when using low-preheat air due to the slower reaction at the burner exit. In this study, an inverse-diffusion flame (IDF) burner configuration is proposed to enhance the combustion stability of MILD regime. The burner features an earlier preheat of fuel stream instead of usually adopted air stream, and can be also operated in traditional diffusion swirling combustion mode. Experiments are conducted in a model combustor to examine the effect of varying burner load (P) and equivalence ratio (φ) on flame topology, pollutant (CO and NO) emission performance, as well as flame emitting spectrum for this burner. It shows that increasing P can enhance the combustion stabilities of both swirling mode and MILD mode, and MILD combustion is found unable to be sustained when P is lower than 2 kW. Increasing φ enlarges the flame liftoff distance under swirling combustion, but has a much smaller effect on MILD combustion. Emission data shows a nearly 50 % reduction of NO by MILD combustion comparing to swirling combustion in all operating cases. Spectra analysis suggests a higher sensitivity of the flame response to the operating parameters under swirling combustion in respect to MILD combustion. A longer combustion chamber is found to produce a lower CO emission but has negligible effect on NO emission. Flow field and temperature distributions by further numerical simulation clearly demonstrate the effectiveness of fuel preheat by adopting IDF burner configuration for enhancing the flame stability when adopting normal temperature air in MILD combustion.

NO mechanisms of syngas MILD combustion diluted with N2, CO2, and H2O

The NO mechanism under the moderate or intense low-oxygen dilution (MILD) combustion of syngas has not been systematically examined. This paper investigates the NO mechanism in the syngas MILD regime under the dilution of N2, CO2, and H2O through counterflow combustion simulation. The syngas reaction mechanism and the counterflow combustion simulation are comprehensively validated under different CO/H2 ratios and strain rates. The effects of oxygen volume fraction, CO/H2 ratio, pressure, strain rate, and dilution atmosphere are systematically investigated. For all the MILD cases, the contribution of the prompt and NO-reburning routes to the overall NO emission is less than 0.1% due to the lack of CH4 in fuel. At atmospheric pressure, the thermal route only accounts for less than 20% of the total NO emission because of the low reaction temperature. Moreover, at atmospheric pressure, the contribution of the NNH route to NO emission is always larger than 55% in the N2 atmosphere. The N2O-intermediate route is enhanced in CO2 and H2O atmospheres due to the increased third-body effects of CO2 and H2O through the reaction N2 + O (+M) ↔ N2O (+M). Especially in the H2O atmosphere, the N2O-intermediate route contributes to 60% NO at most. NO production is reduced with increasing CO/H2 ratio or pressure, mainly due to decreased NO formation from the NNH route. Importantly, a high reaction temperature and low NO emission are simultaneously achieved at high pressure. To minimize NO emission, the reactions should be operated at high values of CO/H2 ratios (i.e., >4) and pressures (e.g., P > 10 atm), low oxygen volume fractions (e.g., XO2 < 15%), and using H2O as a diluent. This study provides a new fundamental understanding of the NO mechanism of syngas MILD combustion in N2, CO2, and H2O atmospheres.

Bluff-body MILD combustion of large-proportion semicoke-blend under various secondary air velocities

Secondary air jet velocity is strongly related to flue gas entrainment in semicoke-blend MILD combustion, which exerts a critical influence on the turbulence–chemistry interaction and combustion characteristics. To extend the application of moderate or intense low oxygen dilution (MILD) combustion to tangential fired boilers, this paper numerically investigates the effect of secondary air velocity (Vsec) on the turbulent flow and chemical reaction interaction behaviors of large-proportion semicoke and bituminous coal co-firing to establish the bluff-body MILD combustion regime. The results show that increasing Vsec abates the peak temperature by enlarging radiative heat transfer and enhances flue gas recirculation. The MILD regime first occurs at Vsec = 51.8 m/s and then extends quickly from 0.4 m to 1.2 m with further increasing Vsec. Increasing Vsec decreases the maximum of the turbulent Damköhler number Dat and increases the minimum of Karlovitz number Ka, implying that the predominant role of turbulent mixing gradually is transferred into the comparable role. The local-MILD regime (1 < Dat < 10) is enhanced significantly and an ideal MILD combustion regime occurs at a velocity of 158.6 m/s, i.e., Dat≈1, and Ka≫1, denoting a slow-chemistry regime. Meanwhile, the maximum heterogeneous Damköhler number Da,C-O2 decreases from 1.09 to 0.86, while the maximum Da,C-CO2 and Da,C-H2O decrease from 0.013 to 0.007 and from 0.003 to 0.0018, respectively, hinting that the char-O2 reaction is controlled by diffusion/kinetics, char-CO2 and char-H2O reactions are determined by kinetics, and they all proceed toward the kinetics-controlled regime with increasing Vsec. Low NOx emissions can be captured and coupled with small Dat by increasing Vsec, and weakening the transitional MILD regime (10 < Dat < 30) is of great importance when establishing the MILD regime.

A Review of the Numerical Investigations of Jet-In-Hot-Coflow Burner With Reactor-Based Models

Moderate or Intense Low-oxygen Dilution (MILD) combustion is considered as one of the most promising novel combustion technologies, as it ensures high efficiency and very low emissions (NOx and CO). Because of the high level of dilution, the system reactivity is reduced and the chemical time scale is increased compared to conventional flames. Therefore, combustion models accounting for finite-rate chemistry are needed to study the characteristics of such flames. Reactor based models, such as the Partially Stirred Reactor and the Eddy Dissipation Concept models have been successfully used to model MILD combustion. This article describes recent progress and developments in the application of reactor based models for the simulation of the jet-in-hot-coflow burners that emulate MILD combustion. The main objective is to provide an overview about the current state of the art of reactor based models for turbulence-chemistry interactions in MILD regime and outline future prospects for the further development of such models. The literature acknowledges both Reynolds Averaged Navier Stokes and Large Eddy Simulations studies, with various operating conditions as well as different fuels. The results indicate that it is necessary to include both the mixing and chemical time scales explicitly in the combustion model formulation. Because of the distributed reaction area, according to recent investigations, Large Eddy Simulation grid can be sufficient to resolve the MILD combustion reacting structures. The present review underlines the importance of finite rate chemistry in MILD combustion simulations, as well as of providing reliable estimation of the characteristic time scales.

Additional criteria for MILD coal combustion

We proposed a theoretical basis for Moderate or Intense Low-oxygen Dilution (MILD) coal combustion based on the turbulent scalar energy spectra. This is motivated by the hypothesis that smallest scalar mixing length scales should be on the order of the particle size or smaller to ensure that mixing can occur to prevent formation of diffusion flames. Our proposed criterion is evaluated using several experimental datasets from the literature for coal combustion in both MILD and traditional combustion regimes. The experimental results confirm that the smallest mixing length scales should be of the order of or smaller than the particle diameter, ηmix⪅dp, to breakup the heat and mass transfer boundary layers around particles in MILD coal combustion. Results indicate that poor mixing of species with small Schmidt numbers around small particles leads to the high luminous intensity in the reactor. The effects of inlet velocity and jet diameter on the mixing length scales are analyzed. Higher inlet velocity and smaller jet diameter are expected to reach MILD regime. The proposed criterion can be used to guide experimental design to achieve MILD conditions for coal combustion.

NO x Formation in MILD Combustion: Potential and Limitations of Existing Approaches in CFD

Emissions of nitrogen oxides (NOx) from combustion systems remain a lingering environmental issue, being these species either greenhouse gases or acid rain precursors. Moderate or Intense Low-oxygen Dilution (MILD) combustion can reduce the emissions of nitrogen oxides thanks to its characteristic features (i.e., homogeneous reaction zones, reduced temperature peaks, diluted mixtures of reactants) that influence and change the main chemical pathways of NOx formation. A summary of the relevant routes of formation and destruction of nitrogen oxides in MILD combustion is presented in this review, along with the identification of the sources of uncertainty that prevent reaching an overall consensus in the literature about the dominant NOx chemical pathway in MILD regime. Computational Fluid Dynamics (CFD) approaches are essential tools for investigating the critical phenomena occurring in MILD combustion and the design of pollutant-free turbulent combustion systems. This paper provides an outline of the modeling approaches employed in CFD simulations of turbulent combustion systems to predict NOx emissions in MILD conditions. An assessment of the performances of selected models in estimating NOx formation in a lab-scale MILD combustion burner is then presented, followed by a discussion about relevant modeling issues, perspectives and opportunities for future research.

Modeling Temperature Variations in MILD Combustion Using MuSt-FGM

The energy demand in the world is ever increasing, and for some applications combustion is still the only reliable source, and will remain as such in the foreseeable future. To be able to mitigate the environmental effects of combustion, we need to move to cleaner technologies. Moderate or intense low oxygen dilution (MILD) combustion is one of these technologies, which offer less harmful emissions, especially nitric oxide and nitrogen dioxide (NOx). It is achieved by the recirculation of the flue gases into the fresh reactants, reducing the oxygen content, and thereby causing the oxidation reactions to occur at a milder pace, as the acronym suggests. This results in a flameless combustion process and reduces the harmful emissions to negligible amounts. To assist in the design and development of combustors that work in the MILD regime, reliable and efficient models are required. In this study, modeling of the effects of temperature variation in the oxidizer of a MILD combustion case is tackled. The turbulent scales are fully resolved by performing direct numerical simulations (DNS), and chemistry is modeled using multistage flamelet generated manifolds (MuSt-FGM). In order to model the temperature variations, a passive scalar which is created by normalizing the initial temperature in the oxidizer is defined as a new control variable. During flamelet creation, it was observed that not all the compositions are autoigniting. Several approaches are proposed to solve this issue. The results from these cases are compared against the ones performed using detailed chemistry. With the best performing approach, the ignition delay is predicted fairly well, but the average heat release rate is over-predicted. Some possible causes of this mismatch are also given in the discussion.

Finite-rate chemistry modelling of non-conventional combustion regimes

The present work deals with the numerical simulation of Adelaide Jet in Hot Co-flow burner under Moderate or Intense Low oxygen Dilution (MILD) conditions, using the Partially Stirred Reactor (PaSR) model for turbulence-chemistry interactions. The PaSR model assumes that reactions are confined in specific regions of the computational cells, whose mass fractions depend both on the mixing and the chemical time scales. Therefore, the appropriate choice of mixing and chemical time scales becomes crucial to ensure the predictivity of the numerical simulations. Results show that the most appropriate choice for the evaluation of the mixing time scale in MILD regime consists in using the geometric mean of the Kolmogorov and the integral mixing time, rather than Kolmogorov or integral scales. This is supported by the validation of the numerical results against experimental profiles of temperature, major and minor species mass fractions. Moreover, the identification of the key species in the oxidation process allows to determine a chemical time scale that improves the prediction of centerline and upstream temperature predictions, compared to the results obtained by including all species in the evaluation of the chemical scale.

Methane combustion in MILD oxyfuel regime: Influences of dilution atmosphere in co-flow configuration

MILD (moderate or intense low oxygen dilution) oxyfuel combustion is a recently proposed clean combustion mode which can remedy the shortcomings of the standard oxyfuel combustion technology. Nowadays most available studies on MILD oxyfuel combustion focus on how to realize this new combustion regime in O2/CO2 atmosphere. The open research on methane MILD oxyfuel combustion in O2/H2O atmosphere is quite sparse. In the present work, we carry out a comprehensive comparison study on methane MILD oxyfuel combustion in different dilution atmosphere for the first time. The JHC (jet in hot co-flow) burner is adopted as a research prototype. The investigation is based on numerical simulation, so firstly the adopted numerical approach is validated by some experimental data in open literature. The numerical comparison is conducted by varying the mass fraction of oxygen in the co-flow and the temperature of the hot co-flow, two key parameters affecting fine reaction structures in JHC. Through the present investigation, a number of findings are reported for the first time and some conclusions presented in previous publications are checked with analyses, especially on some conflicted claims between the previous publications. In addition, several new questions are raised, which may inspire further research activities in future.

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.

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 flame structure in the mild combustion regime

In this paper, turbulent non-premixed CH4+H2 jet flame issuing into a hot and diluted co-flow air is studied numerically. This flame is under condition of the moderate or intense low-oxygen dilution (MILD) combustion regime and related to published experimental data. The modelling is carried out using the EDC model to describe turbulence-chemistry interaction. The DRM-22 reduced mechanism and the GRI2.11 full mechanism are used to represent the chemical reactions of H2/methane jet flame. The flame structure for various O2 levels and jet Reynolds numbers are investigated. The results show that the flame entrainment increases by a decrease in O2 concentration at air side or jet Reynolds number. Local extinction is seen in the upstream and close to the fuel injection nozzle at the shear layer. It leads to the higher flame entertainment in MILD regime. The turbulence kinetic energy decay at centre line of jet decreases by an increase in O2 concentration at hot Co-flow. Also, increase in jet Reynolds or O2 level increases the mixing rate and rate of reactions.

Coal-derived syngas MILD combustion in parallel jet forward flow combustor

The effect of air-fuel momentum flux ratio on MILD (Moderate or Intense Low-oxygen Dilution) combustion of coal-derived syngas was examined in parallel jet forward flow combustor. The results were presented on flow field using non-reactive numerical simulations and on OH∗ radicals distribution and exhaust emissions using experiments. The predicted gas recirculation ratios in the combustor are high enough to establish the reaction condition of MILD scheme. Lower air-fuel momentum flux ratio associated with higher heat load benefits the drop of peak flame temperature and the increase of reaction zone volume. The critical air-fuel momentum flux ratios below which MILD combustion occurred were identified for three MILD configurations. The MILD configuration equipped with larger air nozzles and smaller fuel nozzles was observed to achieve MILD combustion at leaner condition. The MILD regime was established for syngas fuel with lean operational limit and ultra-low NOx and CO emissions.

Effect of air preheat temperature on the MILD combustion of syngas

The effect of air preheat temperature on MILD (Moderate or Intense Low-oxygen Dilution) combustion of coal-derived syngas was examined in parallel jet forward flow combustor. The results were presented on flow field using numerical simulations and on global flame signatures, OH∗ radicals distribution and exhaust emissions using experiments. The discrete and high speed air/fuel injections into the combustor is necessary for the establishment of MILD conditions, because they cause strong gas recirculation and form large mixing region between the air and fuel jets. The critical equivalence ratio above which MILD combustion occurred was identified. The MILD regime was established for syngas fuel under air preheating conditions with lean operational limit and suppressed NOx and CO emissions. In the MILD combustion regime, the air preheating resulted in higher NOx but lower CO emissions, while the increase of equivalence ratio led to the increase of NOx and the decrease of CO emissions.

Characteristics and mechanistic analysis of CO formation in MILD regime with simultaneously diluted and preheated oxidant and fuel

The innovative combustion modality of moderate or intense low-oxygen dilution (MILD) combustion has high combustion efficiency and low pollutant emissions. In this paper, the characteristics and mechanisms of CO formation in the MILD combustion regime were studied using opposed diffusion flame (OPPDIF) with simultaneously diluted and preheated oxidant and fuel. The GRI 3.0 mechanism was adopted to represent the chemical reactions. The effects of the entrainment ratio (Kv), the equivalence ratio (ϕ), and the inlet temperature (Tin) on CO formation were studied under three calculation conditions: (1) the range of Kv from 1 to 8 at Tin=1300K, (2) the range of ϕ from 0.6 to 1.6 at Tin=1300K, and (3) the range of Tin from 1000 to 1300K at Kv=3 and ϕ=1. The results show that in the MILD combustion regime, increasing entrainment ratio and equivalence ratio can reduce the emission of CO. However, their effects become weak when entrainment ratio exceeds 6. From the threshold states to MILD combustion regime, the EICO sharply decreases with increasing entrainment ratio and equivalence. Increasing inlet temperature can gently enhance the emission of CO in MILD combustion regime.The main differences of the mechanisms of CO formation between in MILD combustion regime and in threshold state are two: (1) in the threshold states, C2H2 makes contribution comparable with CH3 to the CO formation in the threshold state; due to oxidative reforming process, the path from CH3 to C2H2 (CH3→C2H6→C2H5→C2H4→C2H3→C2H2→CO) is greatly weakened in MILD combustion regime, causing that the contribution of CH3 is up to be comparable with HCCO and the contribution of C2H2 is down to the lowest; (2) in threshold states, the ROP of CO of the elementary reaction, OH+CO⇔H+CO2, changes from positive value to negative value due to the increase in [OH]/[H]; the direct reaction of this elementary reaction predominate in MILD combustion regime, because its direct reaction prefers the lower temperature compared to its reverse-direction reaction. According to the sensitivity analysis of CO formation, the CO formation is mainly controlled by the direct reaction of the elementary reaction, OH+CO⇔H+CO2 , in the MILD combustion regime. The importance of the role of this elementary reaction increases with increasing the dilution and the equivalence ratio and decreases with increasing the inlet temperature.

Effect of fuel injection velocity on MILD combustion of syngas in axially-staged combustor

The role of fuel injection velocity on MILD (Moderate or Intense Low-oxygen Dilution) combustion of coal-derived syngas was examined in an axially staged combustor, where the secondary air was mixed with the flue gases from the gas generation zone to produce hot and diluted oxidant prior to its mixing with the secondary fuel. The global flame signatures, OH∗ radicals distribution, and exhaust emissions were obtained through experimental measurements, while the mixing behavior between the secondary fuel and oxidant was numerically studied. Higher secondary fuel injection velocity within 199–299 m/s facilitated the earlier entrainment of oxidizer into the secondary fuel and increased the flame lift-off height, resulting in a lower flame temperature, a more distributed reaction zone and reduced NOx emissions, but higher pressure loss and CO formation. The MILD regime yields lower NOx emissions compared to the traditional diffusion combustion mode, and the N2O-intermediate mechanism dominates the NO production in the syngas MILD flame with adiabatic flame temperature lower than 1565 K according to the prediction of the chemical reactor network model.

Influence of Inlet Dilution of Reactants on Premixed Combustion in a Recuperative Furnace

This paper presents a numerical study by RANS modeling that investigates the effect of external dilution on the premixed combustion occurring in a recuperative furnace. Calculations are performed using the detailed GRI-Mech 3.0 mechanism to ensure the accuracy of the modeling. Results of the in-furnace flow, temperature, and concentrations of OH, O2, CO2 and NOx are provided. It is found that the external dilution with the inert gas CO2 plays a significant role in establishing the premixed MILD (Moderate or Intense Low-oxygen Dilution) combustion. Externally diluting the reactant mixture not only reduces the initial concentration of O2 but also ensures a stronger internal dilution by recirculation of more hot combustion products. Importantly, the latter effect is more significant for achieving the MILD regime. There is a critical mass fraction of the diluent CO2 present, below which MILD combustion cannot occur. While the traditional premixed flame produces much more NOx than the MILD combustion, the emission of NOx appears to result most from the thermal-NO route and least from the N2O route no matter which mode occurs. Moreover, the present simulation demonstrates that the MILD mode occurs over a wider range of initial reactant conditions for premixed combustion than for diffusion combustion.