Moderate And Intense Low Oxygen Dilution Research Articles

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.

Incremental sampling methods for multi-fidelity surrogate modeling: Application on a furnace operating in MILD combustion conditions

This study introduces a framework for developing a multi-fidelity reduced order model (MF-ROM) of a combustion furnace operating under Moderate and Intense Low-oxygen Dilution (MILD) conditions. It integrates Proper Orthogonal Decomposition for data compression, Procrustes manifold alignment for fidelity transfer, and CoKriging for interpolation. Design parameters such as air injector diameter, fuel composition, and equivalence ratio were used to generate two- and three-dimensional simulations and build the MF-ROM. Additionally, the key question concerning the optimal number of high-fidelity simulations to balance accuracy and training cost is addressed when building the MF-ROM. Through incremental sampling strategies, it is demonstrated that around half of the training cost can be conserved while maintaining comparable error values.

Experimental investigation on the NO formation of pulverized coal combustion under high-temperature and low-oxygen environments simulating MILD oxy-fuel combustion conditions

The NO formation experiments simulating moderate and intense low-oxygen dilution (MILD) oxy-coal combustion conditions were conducted on a laminar diffusion flame burner with the coflow temperatures of 1473–1873 K and the oxygen volume fractions of 5 %–20 % in O2/CO2, O2/Ar and O2/N2 atmospheres. The flame images of pulverized coal combustion were captured to obtain the ignition delay distances, and the axial species concentrations were measured to obtain the variation of NO formation and reduction. The NO yield in O2/Ar atmosphere decreased by nearly 0.2 when the oxygen volume fraction decreased from 20 % to 5 % and by about 0.05 when the coflow temperature decreased from 1873 K to 1473 K. The NO yield in O2/CO2 atmosphere was 0.1–0.15 lower than that in O2/Ar atmosphere. The optimal kinetic parameters of thermal NO and fuel NO formation rate were obtained by a nonlinear fit of nth-order Arrhenius expression. Finally, the relative contribution rates of thermal NO to total NO (Rth) and NO reduction to fuel NO (Rre) were quantitatively separated. Rth decreases with the increase of oxygen volume fraction, below 6 % at 1800 K, 25 % at 2000 K. Rre is almost unaffected by the coflow temperature and affected by the oxygen volume fraction, reaching 30 % at 5 % O2.

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.

Investigations of the extended Eddy Dissipation Concept formulation for weakly turbulent and slow chemistry flames

Recent extensions of the Eddy Dissipation Concept (EDC) are investigated with the purpose of analysing the importance of model limiters to the EDC performance when predicting Moderate and Intense Low-oxygen Dilution (MILD) flames. These limiters are associated with the mass fraction of fine structure regions, turbulence Damköhler number (Da), and turbulence Reynolds number (Re). The method is Reynolds Averaged Navier Stokes computation using OpenFOAM v.7 combined with the modified steady-state solver edcSimpleSMOKE. The results show that increasing the upper limit of fine structure region close to unity influences flame temperature, which could critically affect the turbulence Da and Re fields. The minimum constraint of turbulence Da plays a significant role in distributing reaction, thus imitating the behaviour of MILD flames. Tuning this constraint is also crucial for the accuracy of the model extension since it can allow nullifying or maximising modification effects for weakly turbulent and slow chemistry flames. The limit of turbulence Re is analysed in relation to turbulence modelling. Evaluation against a conventional turbulent flame demonstrates that the extended EDC underpredicts turbulence Da field, allowing similar modification to that applied in MILD condition.

Tabulated Chemistry Approach for the Simulation of MILD Combustion: Effects of Scalar Mixing and Chemistry Tabulation.

MILD (moderate and intense low-oxygen dilution) combustion is a highly promising technology to deliver clean and efficient thermal energy. However, because of its unconventional reaction nature, the optimization of the MILD combustion in various industrial burners is still challenging, for which the design tool based on accurate and cost-effective numerical simulation is most desirable. To this end, the tabulated chemistry approach (TCA) is thoroughly assessed for the modeling of MILD combustion by simulations of the Adelaide Jet in Hot Coflow (JHC) burner. The sensitivities to the submodel accounting for the scalar micromixing and the canonical flame configurations (i.e., flamelet and PSR-based reactors), being relevant for MILD regime characterization in TCA, are studied. It is found that the scalar mixing enhanced through the dynamic adjustment of model parameter C s leads to an improved prediction, and the optimal value of C s = 8 is identified for the current flames. The proper parametrization of the detailed chemical structures in TCA is found to affect the accurate prediction of the MILD flame profiles, especially the mass fraction of minor species (e.g., OH, CO). Furthermore, the endothermic reaction path of O + C2H2 ⇒ CO + CH2 is indicated as the main contributing step to the disparities in the CO predictions. This implies that the multiple reaction regimes in complex MILD burning should be accounted for by the use of either flamelet or PSR structures, depending on the local microscale diffusion/chemistry competitions. Overall, the results highlight the influential role of multiscale mixing and its intercoupling with the finite-rate chemistry, the accurate determination of which is important for MILD combustion modeling.

Uncertainty quantification of the premixed combustion characteristics of NH3/H2/N2 fuel blends

Green ammonia is a candidate fuel to decarbonise shipping and other industries. However, ammonia features a lower reactivity compared to conventional fuels and is therefore difficult to burn. To resolve this issue, thermo-catalytic cracking of ammonia using waste heat is often employed to produce NH3/H2/N2 blends as fuel. However, on-site operational variations in this process can become sources of uncertainty in the fuel composition, causing randomness of the flame's physicochemical properties and challenging flame stability. In the present work, a surrogate model is built using the polynomial chaos expansion (PCE) method to investigate the impact of fuel composition variability on combustion characteristics at different operating conditions. Impacts of 1.5% deviation in the fuel composition on the flame properties for different initial pressures (Pi) and unburnt fuel temperatures (Tu) are investigated for a wide range of equivalence ratios covering lean and rich mixtures. The uncertainty effects defined by the coefficient of variation (COV) fluctuate for equivalence ratios greater than 1.1, while no fluctuation is observed in COV for near stoichiometric combustion conditions. It is shown that H2 variation in the fuel blend has the strongest effect (over 80%) on the uncertainty of all investigated physicochemical properties of the flame. The least affected property is the adiabatic flame temperature with variations of about 2.5% in richer fuel conditions. The results further show that preheating of the reactants can significantly reduce the COV of laminar flame speed. The consequences of these uncertainties upon different combustion technologies are then discussed and it is argued that moderate and intense low oxygen dilution (MILD) and colourless distributed combustion (CDC) technology may remain resilient.

The effects of the initial NO volume fractions on the NOx generation and reduction routes under natural gas MILD combustion conditions

• The thermal and N 2 O-intermediate routes are dominant in NOx generation under MILD combustion. • Contributions of each NO generation and reduction route to NO emission are quantitatively determined. • The EINO in exhaust gas increases with the increase of initial NO volume fraction. Nitrogen oxides (NO x ) generated from fossil fuel combustion are the main sources of air pollutants and photochemical smog. Moderate and Intense Low-oxygen Dilution (MILD) combustion has been regarded as a novel combustion technology which can significantly improve combustion efficiency and reduce pollutant emissions. The natural gas MILD combustion in the International Flame Research Foundation (IFRF) semi-industrial test furnace were simulated by using standard k-ε model and Eddy Dissipation Concept (EDC) model with GRI-Mech 2.11 mechanism. The predicted results of flue gas velocity, temperature and species volume fractions were compared with the experimental data to validate better performance of the present model. The relative contributions of various NO generation and reduction routes to total NO x emissions were quantitatively determined to obtain the dominant NO formation pathway under MILD combustion conditions. The NO in the preheated oxidizer and the in-furnace recycled flue gas may have an important effect on the NO x generation and reduction during the combustion process. The effects of the initial NO volume fraction on the key reactions of individual NO x generation and reduction route were studied by adjusting the NO volume fraction in the oxidizer. The results showed that the net NO x emission and CO emission in the IFRF furnace were as low as 13.6 mg/m 3 and 3.7 mg/m 3 referring to 15% O 2 volume fraction, respectively, which were much less than the NO x and CO emission thresholds (40 mg/m 3 for NO and 67 mg/m 3 for CO) for natural gas MILD combustion. Moreover, the thermal route and N 2 O-intermediate route played the dominant roles in NO x generation and their contributions to the total NO x emission in the exhaust gas were 48% and 34% for IFRF experiment case, respectively. Although the kinetic rates of NO-reburning reactions in the furnace continued to increase, the net emission index of NO ( EINO ) in the exhaust gas increased by 27% due to the enhanced thermal route when the initial NO volume fraction in the oxidizer increased from 0 ppm to 500 ppm.

Moderate and Intense Low-Oxygen Dilution (MILD) Combustion of Liquid Fuels: A Review

Moderate and Intense Low-Oxygen Dilution (MILD) Combustion of Liquid Fuels: A Review

Numerical study on self-ignition temperature of biomass gasified gas for the application of MILD combustion

Moderate and intense low-oxygen dilution (MILD) combustion of biomass gasified gas (BGG) has great potential. However, self-ignition temperature (Tsi) of BGG is not clear, which limits the establishment of BGG MILD combustion. In this study, Tsi of BGG was calculated in a perfectly stirred reactor using NUIG-NGM-2010 mechanism. Tsi of BGG/O2/N2 mixture was identified by the S-shape curve. Very wide ranges of the residence time τ (10−5 s–10 s), oxygen concentration XO2 (0.1%–35%) and equivalence ratio Φ (0.4–2.8) were considered. Results show that τ = 1 s was reasonable for MILD combustion. Tsi decreased with the increase of XO2. Tsi first decreases and then increases with the increases of Φ. BGG is easier to establish MILD combustion than CH4 because the critical XO2 is 11.7%, which is larger than CH4 (9.7%). Sensitivity analysis shows that R1 (H + O2 = O + OH) and R4 (O + H2O = OH + OH) are the most sensitive reaction for promoting ignition. R3 (OH + H2 = H + H2O), R21 (H2O2 + OH = H2O + HO2) and R13 (HO2 + OH = H2O + O2) were key reactions for prohibiting ignition under designed conditions.

Interpretation and characterization of MILD combustion data using unsupervised clustering informed by physics-based, domain expertise

In this study we use an unsupervised clustering algorithm called Vector Quantization Principal Component Analysis (VQPCA) to characterize Moderate and Intense Low-oxygen Dilution (MILD) combustion. Two direct numerical simulation (DNS) datasets of non-premixed MILD combustion are used and they differ primarily in the oxygen (O2) dilution levels. Maximum volume fraction of O2 for Case-AZ1 is 3.5% (lower dilution) and for Case-BZ1 it is 2.0% (higher dilution). DNS grid points with thermo-chemical information are fed to VQPCA which partitions them into a few clusters. Qualitative and quantitative comparison between VQPCA clusters and structures of physical variables such as mixture-fraction, Z, and heat-release rate, HRR, are used to identify variable for system-level characterization. While qualitative comparison is based on visualization, quantitative comparison is newly developed in this study and is based on Boolean logic. Subsequently, a new physics-based method is proposed to rank important features (or variables) for each cluster. This is useful for a fine-grained characterization of the system. The method is based on calculating the Hellinger distance between the probability density functions (PDF) of each feature conditioned over a cluster and the region outside it. The outcome of physics-based feature ranking method is compared with several feature selection methods based on PCA. Finally, we also demonstrate that through careful selection of features we can tailor clustering outcomes in systems with high dilution, such as like BZ1, where initially no system-level variable was recognized.

Review on MILD Combustion of Gaseous Fuel: Its Definition, Ignition, Evolution, and Emissions

Combustion of fossil fuels has been continuously generating over 70% of the total energy for our human society, concurrently emitting vastly greenhouse gases and ecologically harmful pollutants. To mitigate the adverse issue, combustion technologies of a new type were successfully developed and debuted around 1990, associated with various terms of “flameless oxidation (FLOX)”, “flameless combustion (FLC)”, “high-temperature air combustion (HiTAC)”, and “moderate and intense low-oxygen dilution (MILD) combustion”. These combustion processes may be regarded as similar or the same class of combustion, because they all involve high preheat and deep dilution of individual reactants (non-premixed) or the reactant mixture (premixed), prior to main combustion reactions, with high-temperature flue gases (burnt products) internally and/or externally. Very significantly, all of these combustion technologies can simultaneously achieve high efficiency and ultralow emissions of nitric oxides (NOx), light, and noise. This class of combustion has been frequently unified as “MILD combustion”. Highly worthy to note, as a particular type of combustion, the MILD combustion has thus far been most extensively explored, resulting in many hundreds to 1000 academic articles published by various energy-related journals and conferences. The present review summarizes the research progress in the gaseous-fuel MILD combustion on its definition, ignition, evolution, and NOx emissions, specifically, through its aspects associated with open jet flames in hot coflow and industrial flames in combustion chamber or furnace. Also, discussions are provided on the relevant terminologies, existing problems, and further fundamental issues.

Evaluation of ignition process and NOx reduction of coal under moderate and intensive low-oxygen dilution combustion by implementing fuel-rich/lean technology

This paper numerically investigated the coal ignition, burnout, char consumption, and NOx conversion behaviors under moderate and intensive low-oxygen dilution (MILD) combustion with the implementation of fuel-rich/lean technology in a semi-industrial scale furnace. For the ignition behavior, the results show that the fuel-rich/lean technology can reduce the lift-off distance compared to the original MILD combustion burner due to the higher equivalence ratio in the fuel-rich jet. Moreover, the improvement of the coal ignition is more pronounced by adjusting primary air supply between the lean and rich jets. For coal burnout at the furnace exit, the impact of fuel-rich/lean technology is negligible, but it significantly affects the coal consumption rate near the burner exit region (x ≤ 1 m), which subsequently results in different devolatilization, char combustion, and char gasification rates. The adjustment of primary air supply has a much more significant impact on the burnout profile than the adjustment of fuel supply due to the different devolatilization processes and flue gas recirculation intensity. Moreover, the coal consumption rate as well as ignition process are highly dependent on the relative mixing sequences among the inner stream, the outer stream, the secondary air stream and the recirculated flue gas. In addition, the NOx formation and reduction mechanisms have been revealed with the help of a user defined function (UDF). It is found that the implementation of fuel-rich/lean technology can further mitigate NOx emissions mainly from the hindered thermal NOx route.

Design and analysis of moderate or intense low-oxygen dilute combustion burner

Analysis of the Moderate and Intense Low-Oxygen Dilution (MILD) combustion burner is performed using commercial CFD code ANSYS. The burner is designed in such a way to be a non-premixed and open burner. Four exhaust gas recirculation (EGR) pipes are used to transport part of the exhaust gas from exhaust to the inlet where it is mixed with the fresh oxidant. The chamber temperature distribution of MILD combustion chamber is found to be reduced in comparison with normal burner with conventional combustion. This leads to reduction in the exhaust gas temperature at the MILD combustion burner outlet and reduces the NOx emissions.

MILD combustion versus conventional bluff-body flame of a premixed CH4/air jet in hot coflow

The present work is to investigate the MILD (moderate and intense low-oxygen dilution) combustion of a premixed methane jet in hot coflow against its conventional counterpart, i.e., a typical bluff-body flame, under identical inlet and boundary conditions. This paper demonstrates that the MILD combustion develops as a stable ‘flame’ lifting far downstream from the nozzle while the conventional flame evolves immediately behind the bluff body. More specifically, all chemical reactions are found to occur far more slowly over a greatly larger reaction zone for the MILD combustion than for the conventional one. Also, for the MILD combustion, the laminar flame speed (SL) is very small, far below the local flow speed (Ux), whereas chemical and mixing times are compatible so that the Damköhler number Da ∼1.0. In contrast, the conventional combustion takes place with SL ≥ Ux and Da = 10–1000. Moreover, the MILD combustion eventually emits little NOx, only less than 3% of the emission from the conventional counterpart. Fundamentally, the MILD combustion produces the NOx emission mainly through the N2O-intermediate and NNH routes while the thermal NOx mechanism dominates the conventional flame. In addition, this paper provides a comprehensive explanation to each of the above differences.

A multiscale combustion model formulation for NOx predictions in hydrogen enriched jet flames

The present paper investigates the role of combustion models and kinetic mechanisms on the prediction of NOx emissions in a turbulent combustion system where conventional and unconventional routes are equally important for NOx formation. To this end, a lab-scale combustion system working in Moderate and Intense Low-oxygen Dilution (MILD) conditions, namely the Adelaide Jet in Hot Co-flow (JHC) burner, is targeted. The Eddy Dissipation Concept (EDC) and the Partially-Stirred Reactor (PaSR) models are used for turbulence-chemistry interactions. The KEE and GRI2.11 chemical mechanisms are employed. The results show that the choice of the combustion model has a higher impact than the selection of the kinetic mechanism for the investigated cases, indicating that biases in the turbulent reactive flow closure are as important, if not more, as the level of the accuracy of the chemical scheme employed. Moreover, the sensitivity of the NO emissions to the uncertain kinetic parameters of the rate-limiting reactions of the NNH pathway is found to be significant when a detailed kinetic mechanism is used. An engineering modification of the PaSR combustion model is proposed to account for the different chemical time scales of fuel oxidation reactions and NOx formation pathways. It shows an equivalent impact on the emissions of NO than the uncertainty in the NNH pathway kinetics. At the cost of introducing a negligible mass imbalance, the adjustment leads to improved predictions of NO. The investigation establishes a possibility for the engineering modeling of NO formation in turbulent flames with a finite-rate chemistry combustion model that can incorporate a detailed mechanism at an affordable computational cost.

Experimental and Numerical Investigation of a MILD Combustion Chamber for Micro Gas Turbine Applications

In the field of energy production, cogeneration systems based on micro gas turbine cycles appear particularly suitable to reach the goals of improving efficiency and reducing pollutants. Moderate and Intense Low-Oxygen Dilution (MILD) combustion represents a promising technology to increase efficiency and to further reduce the emissions of those systems. The present work aims at describing the behavior of a combustion chamber for a micro gas turbine operating in MILD regime. The performances of the combustion chamber are discussed for two cases: methane and biogas combustion. The combustor performed very well in terms of emissions, especially CO and NO x , for various air inlet temperatures and air-to-fuel ratios, proving the benefits of MILD combustion. The chamber proved to be fuel flexible, since both ignition and stable combustion could be achieved by also burning biogas. Finally, the numerical model used to design the combustor was validated against the experimental data collected. The model performs quite well both for methane and biogas. In particular, for methane the Partially Stirred Reactor (PaSR) combustion model proved to be the best choice to predict both minor species, such as CO, more accurately and cases with lower reactivity that were not possible to model using the Eddy Dissipation Concept (EDC). For the biogas, the most appropriate kinetic mechanism to properly model the behavior of the chamber was selected.

Chemical Explosive Mode Analysis for a Jet-in-Hot-Coflow burner operating in MILD combustion

Large Eddy Simulations (LES) of Moderate and Intense Low oxygen Dilution (MILD) combustion of a Jet-in-Hot-Coflow (JHC) burner were performed using detailed chemistry. On the contrary to traditional flames, where heat release is occurring in very thin fronts, MILD combustion occurs in the distributed reaction regime where the reaction zone is broad, thus, this paper applies a direct Arrhenius closure with detailed chemistry to resolve important details of the fuel oxidation reactions. Comparisons of LES results are in good agreement with experiments, demonstrating that the simulations capture the intermediate species and finite reaction rate effects. A Chemical Explosive Mode Analysis (CEMA) was used to determine the flame structure and to detect the pre- and post-ignition regions, including the contributions to the CEMs analyzing the Explosion Index (EI) and Participation Index (PI). To the best of our knowledge, a detailed study of CEMA on MILD or flameless regime has never been reported. The flame structure was clearly visualized with CEMA, as well as the lean and the rich flame fronts. Different flame zones close to the anchoring points of these turbulent lifted flames were selected and the analysis demonstrates the contributions of dominant chemical species, such as HO2 and O. The reactions related to the dominant local CEM were obtained to highlight the nature of the stabilization in these highly diluted operating conditions.

Analysis of the Eddy Dissipation Concept formulation for MILD combustion modelling

Performance of the Eddy Dissipation Concept (EDC) in the regime of Moderate and Intense Low-oxygen Dilution (MILD) combustion is investigated. The special MILD features, where chemical and turbulence time scales are comparable (Damköhler number close to unity), have led several researchers to suggest modifications of EDC, mainly by changing model constants. EDC with standard and modified constants are compared, and the importance of each effect is outlined. Different fine-structure reactor models and their inflow/initial conditions are discussed and found to play a significant role. The reacting fraction of fine structures, which in virtually all other numerical studies is set to unity, is also discussed and found to be important. We observe better agreement with experiment when the reacting fraction is reduced below unity, which is also described by the original EDC. The results obtained with the variable reacting fraction are found to improve both the temperature distributions and the lift-off height predictions. The calculations are carried out with the use of open source software OpenFOAM. The main test case was the Delft Jet-in-Hot-Coflow burner emulating MILD regime at three different flow conditions (jet Reynolds numbers of 2500, 4100 and 8800).

Global reaction mechanisms for MILD oxy-combustion of methane

This paper optimizes global reaction mechanisms under the MILD (Moderate and Intensive Low-oxygen Dilution) oxy-combustion combustion. Seven global mechanisms are compared and validated with two detailed mechanisms under the oxy-fuel combustion, MILD air-combustion, and MILD oxy-combustion conditions. The ability of these global models to capture the combustion process under different conditions is compared first using computational fluid dynamics (CFD) simulations of both the MILD oxy-combustion in a laboratory-scale furnace and non-premixed turbulent jet open flames, and later using a plug flow reactor (PFR) approach. Experiments of the MILD oxy-combustion are also carried out for the mechanism validation. The detailed comparison shows that the present optimized global mechanism significantly improves the prediction of temperatures, equilibrium concentrations of major species, and the peak CO concentration, relative to other global mechanisms, for the MILD oxy-combustion. The present refined mechanism is an appropriate global reaction mechanism for methane MILD oxy-combustion, if the computational cost of detailed reaction mechanism is unaffordable.