Moderate Or Intense Low Oxygen Dilution Combustion Regime Research Articles

Effects of oxidant stream composition on non-premixed laminar flames with heated and diluted coflows

The moderate or intense low oxygen dilution (MILD) combustion regime offers reductions in pollutant emissions and improvements in efficiency. The implementation of MILD combustion in non-premixed systems, however, still requires a significantly improved understanding of the effects of the oxidant stream composition in both the MILD combustion regime, and in the transition from MILD combustion to conventional spontaneous-ignition. This paper presents a numerical study of two-dimensional flames near the transition between MILD combustion and conventional spontaneous-ignition. The numerical study is performed with a detailed kinetic mechanism, using the laminarSMOKE code to provide insight into the chemical structure of flames with a hot and diluted coflow. This study is complemented by new experimental observations of laminar flames in similar coflows, which demonstrate a non-monotonic change in lift-off height with changing oxidant O2 concentration. This change occurs in conjunction with a transition from spatially gradual ignition to well-defined flame bases, with increasing coflow O2 level. The simulated flames are compared to a previous definition of MILD combustion in a hot coflow as an edge flame without a tribrachial flame structure. The different structures of simulated CH4 flames are consistent with the observed experimental behaviour under similar conditions, however comparisons between experimental observations and simulations of C2H4 flames highlight the importance of the flow-field, even in a simple streaming flow. The simulations show that equilibrium levels of the OH radical (< 10 ppm) in the oxidant stream significantly intensifies a MILD CH4 reaction zone, by increasing CH3 oxidation, however such levels have little effect on tribrachial, spontaneously-igniting flames. Conversely, increasing the ratio of CO2 to H2O in the coflow reduces the intensity of a MILD CH4 reaction zone. Neither the inclusion of equilibrium concentrations of OH, nor the change in CO2 to H2O ratio, in the oxidant results in a transition between MILD reaction zones to tribrachial spontaneously-igniting flames, despite significantly affecting the temperature of reaction zones in the MILD combustion regime. The results show that the intensity of MILD combustion is strongly dependent on the different chemical species in the oxidant stream. Tribrachial spontaneously-igniting flames are, in contrast, relatively resilient against changes other than temperature and O2 level.

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

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

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.

MILD Combustion under Different Premixing Patterns and Characteristics of the Reaction Regime

Through experiment and numerical modeling, this study investigated the establishment of moderate or intense low-oxygen dilution (MILD) combustion in a laboratory-scale furnace when fuel and air are fully premixed (FP), partially premixed (PP), or non-premixed (NP). Experiments were carried out at firing rates from 7.5 to 15 kW and equivalence ratios (Φ) ranging from 0.5 to 1. The furnace thermal fields and exhaust NOx emissions for the three mixing patterns were compared. Validated computational fluid dynamics was used to aid in better understanding the flow and compositional structures in the furnace. Natural gas was used as the fuel. The eddy dissipation concept (EDC) model and the GRI-Mech 3.0 mechanism were used. Additional chemical kinetics calculations were also performed to examine reaction pathways under the MILD combustion regime. Moreover, the characteristics of the reaction regime of MILD combustion were examined and are discussed in detail. Estimation of the initial jet momentum rate (J) showe...

MILD combustion in diffusion-controlled regimes of Hot Diluted Fuel

Reactive structures have been characterized in a steady laminar, unidimensional mixing layer on a dense grid of parameters in moderate or intense low-oxygen dilution (MILD) combustion conditions with hot and diluted fuel. The structures have been studied in terms of temperature and heat release profiles in a mixture fraction space for various ranges of stretch rates and for two reference pressures (1 and 10bar) using a standard code and standard kinetic scheme.In the analysis of reactive structure three synthetic characteristics have been pointed out in previous works as discriminative for the occurrence of different combustion regimes. Such characteristics are the thickness of the oxidation structures, the presence/absence of a pyrolysis region and the correlation/no correlation of the regions of maximum heat release with those at which the mixture is stoichiometric. Following the same criteria, maps of regimes have been built up on a inlet fuel temperature – temperature increase plane for fixed stretch rates and different pressures.It has been pointed out that in diffusion controlled regimes of Hot Diluted Fuel, analysed in this paper, MILD combustion conditions are characterized by flame thickening and pyrolysis depression, which are also typical of flameless combustion. In addition, the region of maximum heat release has a generally high level of correlation with the stoichiometric regions. Thus, MILD combustion regime differs from MILD combustion regime found in other diffusion controlled regimes where the region of maximum heat release is generally not correlated with the stoichiometric regions. In the case presented in this paper, only a second solution, corresponding to a very low conversion occurring in a very wide stretch-rate range, shows no correlation between heat release and stoichiometric mixture fraction. This behavior has been attributed to the oxidative pyrolysis of methane.At atmospheric pressure, the flameless characteristics extend to a low level of preheating, provided that the fuel is diluted, with a consequently low level of temperature increase, thus confirming that MILD and flameless regimes are not coincident in all possible feeding conditions.In general, the results obtained in these Hot-Fuel-Diluted-Fuel conditions are consistent with and extend those reported in the literature for Hot-Oxidant-Diluted-Fuel, Hot-Oxidant and Diluted-Fuel conditions, supporting the assertion that these inlet parameters are a suitable choice for the definition of MILD combustion.

Influence of turbulent mixing intensity on the MILD combustion and the pollutant formation

Moderate or intense low-oxygen dilution (MILD) combustion has been acknowledged as one of the most interesting combustion technologies to meet both the targets of high process efficiency and low pollutant emissions. Therefore, the potential for its implementation in gas turbine combustors is investigated in recent research activities. In the MILD combustion regime, we find that the characteristic times of kinetics and turbulence become comparable and the two phenomena are coupled with each other. In this work, a partially stirred reactor is applied to investigate the influence of the turbulent mixing intensity on the combustion process as well as its impact on NOx formation. The proposed model is based on a combination of simple turbulent mixing approach with a detailed chemistry sub-model for methane oxidation and NOx formation. The computational results demonstrate that the combustion processes as well as the pollutant formation are very sensitive to the mixing intensity. However, the total NOx emissions after complete combustion are seen to be only slightly influenced by the mixing intensity.

Key modeling issues in prediction of minor species in diluted-preheated combustion conditions

Abstract A numerical investigation of a jet in hot coflow (JHC) burner emulating Moderate or Intense Low-oxygen Dilution (MILD) combustion is carried out in order to understand key modeling issues for such three stream problem. Favre-averaged Navier–Stokes (FANS) equations were solved in a finite volume scheme with the Eddy Dissipation Concept (EDC) model for the turbulence-chemistry interaction and the skeletal KEE mechanism (consisting 17 species and 58 reactions) for the combustion reactions. It was found that mixing in shear layers has a significant impact on the temperature, species and flow fields and could be predicted only with a proper selection of the turbulence levels at the inlet boundaries. Analyzing the minor species reaction path together with the mean and fluctuating temperature fields elucidated the great influence of temperature fluctuations on the net rate of CO and OH production in MILD combustion regime.

Investigation of the MILD combustion regime via Principal Component Analysis

Moderate or Intense Low oxygen Dilution (MILD) combustion is particularly appealing for coupling very high combustion efficiencies with very low pollutant emissions. Experimental and numerical studies have been devoted to MILD combustion. However, fundamental aspects of such combustion regime are still not completely understood and this generates uncertainties, especially from the modeling perspective. A novel methodology based on Principal Component Analysis (PCA) is proposed for the investigation of the main features characterizing the MILD combustion regime. High fidelity experimental datasets of CH 4/H 2 flames propagating into a hot and diluted co-flow are employed to this purpose. Results indicate that the proposed methodology can provide progress variables which coherently follow the modification of the flame structure experimentally observed at different levels of oxygen dilution in the co-flow. A local PCA approach is also proposed, based on the partition of the data sets into cluster, followed by the local application of PCA. Such approach ensures that the low-dimensional projection is characterized by the minimum parameterization error. Results indicate that local PCA leads to the identification of regions of the flow characterized by different physical processes, thus allowing the determination of optimal progress variables in each of them.