Moderate And Intense Low Oxygen Dilution Combustion Research Articles

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.

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.

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.

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.

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.

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.

Thermo-ecological evaluation of an integrated MILD oxy-fuel combustion power plant with CO2 capture, utilisation, and storage – A case study in Poland

This study investigated the environmental benefits of a new boiler design for a fossil fuel-based power plant with CO2 capture, utilisation, and storage (CCUS) through thermo-ecological cost (TEC) analysis. MILD oxy-fuel combustion (MOFC) combines the advantages of the moderate and intense low-oxygen dilution (MILD) combustion and oxy-fuel combustion (OFC) to achieve efficient and environmentally justified CO2 capture from fossil fuel-based power generation. The advantages of MOFC application are: (i) it increases the efficiency of the coal-fired boiler, (ii) it increases the purity of the CO2 in the flue gases, (iii) it reduces the oxygen consumption of the boiler by using lower oxidiser excess, and (iv) it reduces the energy consumption associated with CO2 recirculation. Therefore, using MOFC decreases the penalty of the overall net energy efficiency associated with the CO2 capture from coal-fired power plants.The environmental analysis in this study considered the TEC, which measures the depletion of non-renewable natural resources by estimating the cumulative exergy consumed by the production processes. Moreover, the additional exergy consumption that compensates for the negative impact of harmful emissions has also been considered. The data for the new boiler design were obtained by CFD modelling, while the other technological modules of the integrated MOFC power plant were modelled by process modelling software. The data concerning the CO2 transport, utilisation, and storage were obtained from the available databases and literature.Three configurations of the power plant were modelled and analysed from the environmental point of view. The first two were the reference coal-fired power plants (the conventional power plant and the oxy-fuel combustion power plant with CO2 capture), while the third case used the same power plant configuration but with an MOFC boiler. The results were compared to other CO2 capture options, such as post-combustion, using the electro-energy system in Poland as the reference. The results emphasised the importance of the emissions and energy consumption occurring upstream (e.g. coal extraction and transport) and downstream (e.g. CO2 transport and storage) of the project when assessing the environmental impact factors. Additionally, two industrial options for CO2 utilisation were proposed, because CO2 storage is generally opposed by the public.

FGM-based non-adiabatic multi-environment PDF modeling for MILD combustion processes with the strong flow reversal

A Multi-environment probability density function (MEPDF) approach coupled with a tabulated chemistry has been developed to numerically investigate the flame structure under Moderate and intense low oxygen dilution (MILD) combustion with strong flow reversal. The tabulated chemistry is represented by Flamelet generated manifolds (FGM). To realistically represent the highly recirculated flameless combustion, we employed the chemistry database using laminar premixed flamelets rather than laminar diffusion flamelets. Moreover, the present FGM-based MEPDF realistically accounts for the heat loss effects caused by convective heat transfer at the wall. Numerical results in this study were validated against experimental data. Based on numerical results, detailed discussions were made for the essential features of the highly recirculated flameless combustion.

Experimental and numerical study of MILD combustion in a bench-scale natural gas partial oxidation gasifier

Moderate and intense low-oxygen dilution (MILD) is a promising combustion technology for the non-catalytic partial oxidation (POX) process of natural gas. This study investigates the establishment of MILD combustion in a bench-scale non-catalytic partial oxidation gasifier. Experiments under both inverse diffusion configuration (IDC) and normal diffusion configuration (NDC) are carried out using methane and pure oxygen as reactants. The flame appearances and temperature profiles are recorded using an imaging system and thermocouples, respectively. Results show that no visible flame can be observed in IDC case while a flame can be observed near the burner exit in NDC case. The effects of jet velocity and O2/CH4 ratio on the IDC flame are further studied. Numerical simulations of the experimental cases are also performed and the results agree well with the experimental results. It is found that the recirculation rate inside the furnace is sufficiently high to fully dilute the reactants. Combustion regime classification on grid scale shows that MILD combustion is established in the gasifier in IDC case. Analysis on the flame structure in IDC case shows that dilution of oxygen before ignition is the reason for the establishment of MILD combustion and the barrier effect of fuel between oxygen and hot syngas in the necessary condition for this process.

Reactivity and Structural Changes of Coal during Its Combustion in a Low-Oxygen Environment

The aim of this study is to improve the understanding of phenomena that occur when a solid carbonaceous material is burned in a high temperature with a low-oxygen-content (<10%, v/v) environment similar to that found in the moderate and intense low-oxygen dilution (MILD) combustion. The morphology, reactivity, and physicochemical properties of partially reacted coal samples extracted from an environment emulating MILD combustion conditions were investigated through different analytical techniques, including elemental and thermogravimetric analyses, scanning electron microscopy, surface area, and Raman spectroscopy. The early stage of coal burnout was characterized by some changes in the organic constituents of coal because the low-molecular-weight compounds react quickly and are the first to be removed. After the devolatilization process under low oxygen combustion, an increase in surface area and porosity was observed simultaneously with a reduction in carbonaceous material reactivity as a result of the ...

Assessment of the potential benefits and constraints of a hybrid solar receiver and combustor operated in the MILD combustion regime

A novel configuration of a Hybrid Solar Receiver and Combustor (HSRC) operated in the Moderate and Intense Low oxygen Dilution (MILD) combustion regime, termed the MILD HSRC is reported. This combustion regime is chosen because of its potential to lower NOx emissions, while increasing the magnitude and uniformity of heat transfer relative to alternative combustion systems, but has not previously been assessed with the HSRC concept. An analytical model is used to identify configurations of the MILD HSRC that achieve the conditions required for MILD combustion. The preferred configuration was then incorporated into a multiple time-step, piecewise-continuous model to evaluate the potential savings in fuel and hence, overall Levelized Cost of Electricity (LCOE) for the MILD HSRC in an electrical power plant. This revealed that there is potential to reduce fuel consumption and LCOE by up to 41% and 4% respectively, relative to the HSRC operating with conventional combustion for a receiver size of 30MWth. The reduction in LCOE increases up to 6% for a receiver size of 100MWth due to economies of scale. This justifies further work to develop detailed designs and evaluate specific technical and economic performance through demonstration and scale-up.

Flame-seed structures: Original structures of nonpremixed flames in mixing layers of methane, ethane, propane and DME

The ultimate structure of an edge-flame near the extinction limit was experimentally investigated using a fuel–air mixing layer within a narrow channel. The experimental parameters were the fuel dilution ratio, fuel concentration gradient, and average flow velocity. As a result, an interesting flame structure was discovered; that is, a nonpremixed-flame branch enclosed by two distinctive asymmetric premixed-flame branches. Experimental conditions that could most clearly distinguish the three flame branches were examined for the following fuels: methane, ethane, propane, and dimethyl ether (DME). Flame morphologies and their sensitivities to the fuel concentration gradient were investigated, and chemiluminescence trajectories along the three flame branches for various fuels were compared on a ternary diagram. Conclusively, it was found that each fuel has a unique morphological and chemiluminescent flame structure near its extinction limit. This structure can provide insight into various flame structures for premixed, partially premixed, nonpremixed, and MILD (moderate and intense low-oxygen dilution) combustion. Since this unique flame structure can bridge the gap between various laminar flame structures, it is called a flame-seed. If insight into the flame-seed structure can be gained, it will extend our understanding of the inter-connections among various laminar flame structures.

Pilot급 산소 MILD 연소에 관한 실험 및 수치해석적 연구

MILD (Moderate and Intense Low-oxygen Dilution) combustion using oxygen as an oxidizer is considered as one of the most promising combustion technologies for high energy efficiency and for reducing nitrogen oxide and carbon dioxide emissions. In order to investigate the effects of nozzle angle and oxygen velocity conditions on the formation of oxygen-MILD combustion, numerical and experimental approaches were performed in this study. The numerical results showed that the recirculation ratio (<TEX>$K_V$</TEX>), which is an important parameter for performing MILD combustion, was increased in the main reaction zone when the nozzle angle was changed from 0 degrees to 15 degrees. Also, it was observed that a low and uniform temperature distribution was achieved at an oxygen velocity of 400 m/s. The perfectly invisible oxy-MILD flame was observed experimentally under the condition of a nozzle angle of <TEX>$10^{\circ}$</TEX> and an oxygen velocity of 400 m/s. Moreover, the NOx emission limit was satisfied with NOx regulation of less than 80 ppm.

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

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

Inverse diffusion flame of CH4–O2 in hot syngas coflow

The structure and combustion mode of inverse diffusion flame of CH4 and O2 in hot syngas coflow are numerically studied to gain a fundamental understanding of the flame in non-catalytic partial oxidation (NC-POX) reformer. The configuration is modified based on the burner system of Cabra et al. [Combust. Flame2005, 143 (4), 491–506] to make the flame representative of that in NC-POX reformer. The Eddy Dissipation Concept (EDC) model with the detailed GRI 3.0 mechanism is used to model the turbulence–reaction interactions. Results of the study show that the flame is stabilized by autoignition with a wide reaction zone located far away from the stoichiometric line. Analyses on combustion mode show that the flame is established in Moderate and Intense Low-oxygen Dilution (MILD) mode. The inverse diffusion flame configuration which ensures a fully dilution of oxygen plays a key role in achieving MILD combustion in fuel rich coflow. The Increase of coflow temperature or decrease of jet velocity within the range of this study can lead to an early autoignition, but doesn't change the combustion mode.