MILD Combustion Regime Research Articles

Large Eddy Simulation of Hydrogen/Air MILD combustion in a cyclonic burner

In this work Large Eddy Simulation (LES) of a Hydrogen/Air cyclonic burner operating in the MILD combustion regime at 1 atm is performed. The cyclonic flow configuration may represent a proper way to enhance the mixing process in a very short time while allowing for residence times long enough to achieve complete oxidation of diluted and preheated mixtures. Accurate molecular transport properties and a reduced chemical mechanism for hydrogen-air combustion, consisting of 18 transported species and 244 elementary reactions are considered. Results show that at the center of the combustor, where MILD combustion occurs, temperature exhibits a statistically steady trend (a mean of 1450 K) with peak values lower than 1700 K. This implies that the Zeldovich mechanism for thermal NO formation is negligible, so NO levels are low and statistically steady with a maximum peak of 20 ppm. These values are lower than those observed in standard combustion, confirming the ability of MILD to reduce NOx pollutants also with hydrogen as fuel. Finally, the simulations outcomes are in very good agreement with the experimental results, in particular radiative heat transfer must be taken into account to better capture the preheating of the reactants and consequently fit the experimental temperature profiles in the combustion chamber.

Design and testing of a Multi-Fuel industrial burner suitable for syn-gases, flare gas and pure hydrogen

The complete design and validation procedure of a novel Multi-Fuel heavy-duty burner is illustrated, starting from the conception and CFD analysis, up to an extensive experimental campaign assessing the actual performance of an industrial scale prototype.The need for an innovative burner to be employed in utility boilers comes from the context of energy transition with still increasing steam demand for process/industrial applications. The capability to burn a wide range of fuels, going from natural gas to flare-gases, syn-gases and fuels with even lower heating values, up to pure hydrogen, with high efficiency and low emissions is mandatory: it is shown and explained how all the above tasks are pursued starting from the early stage of conceptual design, leading to the development of a 2.5 MW prototype; then the burner operation is studied more in depth by CFD analysis, with particular attention to the implementation of MILD combustion regime: internal recirculation along with mixing, namely fuel dilution in flue gases leads to distributed volumetric reaction with oxygen, involving low peak temperatures with high conversion efficiency. Furthermore, numerical analysis helps to investigate the effectiveness of a central stabilizing flame, the coupling between furnace and burner in view of the tests, and also helps to evaluate different injection patterns for the fuel jets.As a final step, a 1.5 MW prototype is tested in a refractory lined furnace: the performance is assessed with natural gas, either pure or diluted with nitrogen to mimic a low heating value syn-gas, and with increasing percentage of hydrogen up to 95% as thermal input. Different levels of input to the central stabilizing flame are tested, and excellent flame stability of burner operation is proven. NOx emissions decrease for lower heating value fuels, increase for hydrogen/natural gas mixtures up to 35% H2, and remain almost constant above this threshold. Though, keeping a very low excess of air and almost no carbon monoxide in the exit flue gases, the absolute NOx levels compare very well with state of the art Ultra-Low-NOx industrial burners developed for standard fossil fuels. The presented data set can be relevant both for industrial designers, as a reference for heavy-duty burners operating with a wide range of fuels, up to pure hydrogen, and for academic researchers investigating on combustion, even in MILD regime.

Bluff-body MILD combustion regime for semicoke and bituminous coal mixtures with effect of oxidizer O2 concentration in O2/N2 and oxy-fuel (O2/CO2) atmospheres

The present study numerically investigates the effects of the O2 content (XO2) in the oxidizer on the MILD combustion behaviours of large-proportion semicoke mixtures, such as those involving temperature, MILD regime recognition, char surface consumption, homo/heterogeneous Damkhöler number, and NOx emission, under O2/N2 and O2/CO2 atmospheres. The results indicate that by increasing XO2 from 15 % to 30 %, the peak temperature increases significantly due to the increased char consumption rate, the high temperature region shrinks, and the axial length of the MILD regime region slightly increases from ∼1 to ∼1.4 m because of enhanced flue gas recirculation under O2/N2 or O2/CO2 atmosphere. The reduction effects of the char-O2 reaction proportions and the promoted effects of the char-CO2 reaction proportions due to the increased XO2 are both obviously weakened when N2 is substituted for CO2. A high O2 content and an O2/N2 atmosphere reduce the char burnout time. With increasing XO2, the maximum value of the turbulent Damkhöler number Dat increases from 6.79 to 10.16 under the O2/N2 atmosphere and from 3.99 to 10.6 under the O2/CO2 atmosphere, implying that a low O2 dilution and a CO2 environment promote the establishment of the homo-MILD regime. The maximum values of the surface Damkhöler number (Das-CO2 and Das-H2O) of the two gasification reactions (Char-CO2 and Char-H2O) have the same order of magnitude and are much lower than those of the surface Damkhöler number (Das-O2) of the Char-O2 oxidation reaction, i.e., Das-H2O < Das-CO2 ≪ Das-O2 < 1. The maximum values of the three surface Damkhöler numbers all increase with increasing XO2, while they decrease with the replacement of N2 by CO2, indicating that a high O2 content and an O2/N2 atmosphere could facilitate the establishment of an ideal hetero-MILD combustion regime to some extent. Low NOx emissions can be realized under moderate O2 and CO2 dilution conditions by regulating the oxy-MILD combustion regime of semicoke blends.

Structure of the ethanol spray flame under conventional and MILD conditions

A numerical examination is carried out on the experimental MILD turbulent spray burner of Delft Spray in Hot Co-flow, DSHC. The RANS approach is combined with the Lagrangian tracking of droplets for modeling the pressure-swirl atomizer. The EDC combustion model is employed, to take into account detailed chemical mechanisms. Predictions show that the structure of the triple-flame of liquid fuel in the atmospheric condition is different from the conventional gaseous triple-flames. Furthermore, the structure of the MILD spray flame is compared to the non-MILD spray flame. The flame index shows that the triple-flame structure in the conventional spray flame that consists of two outer and middle diffusion branches, and a premixed inner branch converts to a double-flame structure under MILD condition, including an outer diffusion and a premixed inner branch. In another step, sensitivity analysis relative to the temperature, and oxygen concentration of the co-flow stream is performed independently, which was not possible in the experiment. The parameter of ignition delay time is used for the selection of parametric study cases. Moderate preheating could lead to deviation from the MILD combustion regime, while high preheating is beneficial. NO pollutant generation shows more sensitivity to dilution rather than preheating in MILD spray flame. This study showed that increasing or decreasing each of the temperature and oxygen concentration doesn’t improve all the effective aspects toward achieving the MILD condition. So, the effective parameters should be changed according to a specific and more precise goal.

Dilute spray flames of ethanol and [formula omitted]-heptane in the transition to mild combustion

The structure and stabilisation mechanisms of ethanol and n-heptane spray flames are investigated in this study. The burner configuration involves a dilute spray of dispersed droplets which is produced and transported via a carrier gas stream of air to the reaction zone, where the flames are stabilised by a hot coflow of combustion products. A range of coflow conditions were implemented for the different flame cases, allowing the effects of the coflow oxygen (O2) concentration and temperature to be examined independently. The resulting flames were analysed using three simultaneous laser diagnostic techniques, enabling the combined planar imaging of the hydroxyl (OH) and formaldehyde (CH2O) radicals, along with the location of droplets. For both fuel types, a noticeable shift in stabilisation behaviour was observed with a variation in the coflow O2 concentration from 11% to 3%, while the coflow temperature was not seen to have a significant impact. These flames also show an interesting departure from the typical behaviour observed for gaseous and prevaporised flames in a similar configuration, particularly for coflow conditions that are typically associated with the transition to the mild combustion regime.

Effect of bluff body addition in fuel-rich stream on reaction behaviours of large-proportion semicoke rich/lean blended combustion

Fuel-rich/lean combustion is considered as a novel combustion technology due to its enhanced ignition performance and low NOx emissions. To further improve the poor performance of large-proportion semicoke and bituminous coal blended combustion and reduce NOx emissions, under a bias concentration ratio (BCR) of 2:1, an addition of bluff bodies at the fuel-rich side is proposed to numerically and experimentally investigate the turbulence-chemistry interaction behaviours of the fuel-rich/lean combustion regime in a 0.3 kW pilot-scale bias combustion furnace for various bluff body blockage ratios (BR = 0%, 11% and 21%). The results show that, increasing the blockage ratio evidently shortens the ignition delay time and burnout time of bituminous coal and semicoke, and the promotive effect of triangular bluff-body (BR = 21%) is greater. The turbulent Damköhler number Dat maximums decrease from 194 to 113 with increasing BR, denoting the regime is traditional combustion mode controlled by mixing. On the fuel-rich side, a local homogeneous MILD combustion zone (Dat <10) extends with increasing BR. For the heterogeneous combustion, the char-O2 reaction determined by diffusion/kinetics and the char gasification reaction controlled by kinetics both show a kinetically controlled trend with increasing BR. Moreover, with increasing BR, a decreasing in the NO concentration in the main reaction zone can be attributed to an enhanced recirculation rate Kv. There is a strong correlation between the maximum NO and the local homogeneous/heterogeneous Damkhöler number on the fuel-rich/lean side in the plane of maximum Kv. Increasing BR can evidently decrease the maximum NO concentration on the fuel-rich side due to the enhanced local MILD combustion regime, and thus reducing NOx exit emissions. Considering temperature, ignition, burnout, turbulence-chemistry interactions and NOx emissions, a triangular-shaped bluff body on the fuel-rich side (BR = 21%) is recommended rather than a cubic bluff body in the fuel-rich/lean co-combustion of semicoke.

Effect of thermal input, excess air coefficient and combustion mode on natural gas MILD combustion in industrial-scale furnace

To promote the application of MILD scheme in furnace, the present work was performed to evaluate the effect of thermal input, excess air coefficient and combustion mode on the MILD combustion at high thermal level (thermal input varies from 600 to 786 kW); meanwhile, the heat from the utilized furnace was extracted by an annular water jacket and a corundum tube was installed in the furnace tube to create the high temperature atmosphere. The results were presented on flow field using numerical simulations and on global flame signatures, temperature/species distribution and exhaust emissions using experiments. Better combustion performance of MILD mode than conventional swirling diffusion combustion mode was verified in the present work. The positive effect of increased thermal input and excess air coefficient on gas recirculation, thermal uniformity and exhaust emissions reduction was observed. In the MILD combustion regime, the switching from nonpremixed to premixed combustion resulted in lower oxygen level and reduced reaction temperature in the combustion region, also much lower NOx emissions and slightly higher CO emission were observed. At 786 kW thermal input, the reaction zone covers the entire combustion chamber; meanwhile, low NOx emission level of 32 ppm@3%O2 are achieved for premixed MILD combustion mode. The MILD regime was established for natural gas fueled furnace with colorless distributed combustion and suppressed NOx and CO emissions.

Characterization of temperature criteria using gas-phase fuel streams for MILD coal combustion

MILD combustion is achieved when the reactants’ inlet temperature (Tinlet) is higher than the self-ignition temperature of the reactants (Tself-ignition) and the temperature increase upon reaction (ΔT=Tmax-Tinlet) is lower than Tself-ignition. The method to get Tinlet and Tself-ignition is ambiguous for coal combustion because the multiple fuel streams are released from coal particles through vaporization, devolatilization, tar/soot reaction and char oxidation/gasification at different temperatures and different times as the particles heat up. We propose a method to determine the gas-phase fuel streams in coal combustion using the reaction rate of each subprocess and gas and particle temperature profiles in the reactor before ignition. The mixture of fuel streams with oxidizer is used to get Tinlet and Tself-ignition for the temperature criteria for MILD coal combustion. By way of example, this method is applied to identify the required dilution rate needed to reach MILD coal combustion. The MILD coal combustion regime does not shift significantly for the two types of coal used in this work (Illinois #6 and Guizhou), even with the temperature and composition of the gas fuel mixture affected by the rank of coal. It is observed that the required dilution rate, Kv, to reach MILD regime increases for lower oxidizer temperature, with the decrease of both inlet and maximum temperatures. The required dilution rate to achieve MILD coal combustion is not sensitive to the model for tar/soot reactions, even for a simplified model that uses C2H2 for tar while neglecting the formation of soot.

An Experimental Study on Kerosene Spray Combustion Under Conventional and Hot-Diluted Conditions

ABSTRACT The combustion of kerosene spray under hot-diluted conditions and conventional conditions was experimentally investigated. By examining flame photographs, chemiluminescence images, and in-field temperature measurements, the separate effect of different variables including oxygen concentration, temperature and velocity of the co-flowing air, fuel flow rate and injection pressure, and eventually the type of spray nozzle on multiple parameters such as flame stability, structure, luminosity, temperature field, and qualitative CH radical distribution, as well as HCO and NO2 with lower precision, in the reaction region, have been studied. It was observed that an increment in injection pressure and co-flow temperature enhances the spray flame stability, while dilution exacerbates it. Also, a solid cone spray pattern with a lower spray angle has better stability than hollow cone ones with a higher spray angle. Moreover, it was noted that liquid fuels, compared to gaseous fuels, require higher preheating temperatures, for the same dilution level, to engender a stable flame. For combustion of spray in conventional conditions, a double-flame structure was observed consisting of a bluish section at the leading edge emerging into a yellowish sooting trail. An increase in co-flow velocity, as well as injection pressure, strengthens the inner flame front, whereas raising the co-flow temperature or diluting the oxidant, deteriorates the inner flame front. In the case of highly preheated air (without dilution), the flame liftoff height is reduced to as close to the atomizer as a few millimeters, forming a single flame structure similar to gaseous flames. In this case, the peak temperature was considerably higher than the conventional combustion, yet the gain was much lower than the preheating level. Combined effects of preheating and dilution alter the spray flame structure in a way that the flame volume is reduced, the temperature field has become more homogeneous, the peak temperature is limited to less than 1500 K, and temperature fluctuations have significantly decreased, seemingly approaching MILD combustion regime conditions.

Modeling curvature effects in turbulent autoigniting non-premixed flames using tabulated chemistry

Turbulent flames are intrinsically curved. In the presence of preferential diffusion, curvature effects either enhance or suppress molecular diffusion, depending on the diffusivity of the species and the direction of the flame curvature. When a tabulated chemistry type of modeling is employed, curvature-preferential diffusion interactions have to be taken into consideration in the construction of manifolds. In this study, we employ multistage stage flamelet generated manifolds (MuSt-FGM) method to model autoigniting non-premixed turbulent flames with preferential diffusion effects included. The conditions for the modeled flame are in MILD combustion regime. To model the above-mentioned curvature-preferential diffusion interactions, a new mixture fraction which has a non-unity Lewis number is defined and used as a new control variable in the manifold generation. 1D curved flames are simulated to create the necessary flamelets. The resulting MuSt-FGM tables are used in the simulation of 1D laminar flames, and then also applied to turbulent flames using 2D direct numerical simulations (DNS). It was observed that when the curvature effects are included in the manifold, the MuSt-FGM results agree well with the detailed chemistry results; whereas the results become unsatisfactory when the curvature effects are ignored.

Effect of hydrogen addition on NOx formation mechanism and pathways in MILD combustion of H2-rich low calorific value fuels

Hydrogen is a major component of low heating value (LHV) fuels like syngas, coke oven gas, and blast furnace gas. These fuels can potentially replace methane in the industrial process under MILD combustion conditions. NOx formation mechanism in MILD combustion regime is different from conventional air combustion due to uniform temperature distribution and low oxygen concentration. Hence, it is important to investigate the effect of hydrogen addition inmethane (CH4) on different NOx formation mechanism and pathways under MILD combustion conditions. A well-stirred reactor (WSR) model with atmospheric pressure and long residence time was employed to simulate the ideal MILD combustion conditions with Chemkin Pro software and GRI Mech 2.11. H2 and CH4 blends ranging from 0% H2 (100% CH4) to 60% H2 (B60 = 40%CH4/60%H2) were selected as fuels to emulate MILD combustion of LHV fuels. The oxygen mass fraction was kept from 3% to 9%, and the fuel blends were altered. The results showed that hydrogen addition enhanced NNH route while decreasing prompt route and N2O intermediate route whereas thermal NOx route was insignificant for lower O2 mass fraction of 3% and 6%. It was observed that H2 addition reduced NO-reburning which led to more NOx emission. Moreover, the sensitivity analysis was conducted to analyze the pathway of NOx formation and to quantitively separate the relative contribution of the individual route on the total NOx formation, which was beneficial to control the NOx emission of LHV fuels during MILD combustion.

The role of dilution level and canonical configuration in the modeling of MILD combustion systems with internal recirculation

MILD combustion regime emerged as one of the most promising technology to ensure low pollutant emissions and large fuel flexibility.Large-scale use of such technology requires important developments of its numerical modelling. The difficulty of this task lies in the recirculation of mass and heat that results in coupling chemistry and turbulence phenomena.In this work, the effect of the internal exhaust gas recirculation on the performances of a tabulated chemistry model was investigated by carrying out an experimental and numerical study on a cyclonic burner. FGM (Flamelet Generated Manifold) model was adopted. It makes two main assumptions: a turbulent flame can be considered as an ensemble of 1D laminar flames (Flamelets), and the space of compositions can be represented by a lower-dimensional manifold by defining few controlling variables. In this work, two different 1D flame configurations and two controlling variables (a mixture fraction and a progress variable) were used. The comparisons between model and experiments were reported for different dilution conditions at inlet and two diluent species.Results showed that the role of the internal EGR is crucial in the establishment of MILD regimes. In fact, for air conditions the model is not able to predict the low and more distributed reactivity of the system. On the other hand, inlet diluted conditions show a good agreement with the experiments due to burned product effects. CO2-dilution instead of N2, improves the model results due to the lower reaction rates. Finally, the Flamelet configuration does not strongly affect the simulation results.

Computational study on the effect of gasification reaction on pulverized coal MILD combustion diluted by N2 and CO2

The behavior of pulverized coal gasification reaction under MILD combustion regime will be significant, and then it affects pulverized coal combustion. This work performs a computational study on the effects of gasification reaction on pulverized coal MILD combustion with N2 and CO2 dilution. Results show that, regardless of at air or O2/CO2 atmospheres, the proportion of char consumed by gasification reaction exceeds 50%. The gasification reaction makes a decrease in flame temperature about 80 K at O2/CO2 and about 50 K at air. The delayed ignition time affected by the gasification reaction is 1.38 ms at air while 3.29 ms at O2/CO2 due the formation of CO. The presence of gasification reaction shortens the char burnout time at air and O2/CO2 atmospheres. The reactions of H2O and CO2 with char destroy the char surface structures and create additional porosity, which prompts the char burnout. The significant decrease in the NOx emission affected by gasification reaction at air and O2/CO2 atmospheres can be attributed to the enhancement of NO reburning and the suppression of fuel-NOx conversion, i.e., NH3 + O2 → NO + products.

Numerical investigation behavior of reacting flow for flameless oxidation technology of MILD combustion: Effect of fluctuating temperature of inlet co-flow

To grow the knowledge to the moderate or intense low-oxygen dilution combustion, namely MILD combustion, the present paper examines the CH4H2 MILD combustion under various working conditions in a jet in hot coflow burner which called Dally burner using a finite volume code. This burner can produce MILD combustion regime in the upstream and conventional combustion regime in the downstream. For this end, detailed reaction mechanism (GRI-Mech 2.11), the k-ε RNG by the kinetic energy of turbulence of 53 m2/s2 for fuel inlet and 3 m2/s2 for hot coflow inlet as well as the eddy dissipation concept model for treating the interaction between the turbulence and the chemistry in flames are used. At first, the results are compared with the experimental data and after validating the results of the solution obtained, the MILD combustion is investigated by using sinusoidal pattern temperature between 1580 K and 1180 K, respectively. The results show that by using fluctuation temperature of inlet co-flow, the MILD combustion regime develop in a wider area. reaction zone and the release energy have increased during the initial oscillation at the incoming temperature, and along with this amount of Water vapor and CO2 have increased, while the amount of CO has also been slightly reduced.

Structural differences of ethanol and DME jet flames in a hot diluted coflow

This study compares the flame structure of ethanol and dimethyl ether (DME) in a hot and diluted oxidiser experimentally and computationally. Experiments were conducted on a Jet in Hot Coflow (JHC) burner, with the fuel jet issuing into a 1250-K coflow at three oxygen levels. Planar measurements using OH-LIF, CH2O-LIF, and Rayleigh scattering images reveal that the overall spatial distribution and evolution of OH, CH2O, and temperature were quite similar for the two fuels. For both the ethanol and the DME flames, a transitional flame structure occurred as the coflow oxygen level increased from 3% to 9%. This indicates that the flames shift away from the MILD combustion regime. Reaction flux analyses of ethanol and DME were performed with the OPPDIF code, and ethane (C2H6) was also included in the analyses for comparison. These analyses reveal that the H2/O2 pathways are very important for both ethanol and DME in the 3% O2 cases. In contrast, the importance of fuel-specific reactions overtakes that of H2/O2 reactions when fuels are burnt in the cold air or in the vitiated oxidant stream with 9% O2. Unsteady laminar flamelet analyses were also performed to investigate the ignition processes and help interpret experimental results. Flamelet equations were solved in time and mixture fraction field, which was provided by non-reactive Large-Eddy Simulation (LES).

MILD combustion of hydrogenated biogas under several operating conditions in an opposed jet configuration

MILD combustion of biogas takes its importance firstly from the combustion process that diminishes significantly fuel consumption and reduces emissions and secondly from the use of biogas which is a renewable fuel. In this paper, the influence of several operating conditions (namely biogas composition, hydrogen enrichment and oxidizer dilution) is studied on flame structure and emissions. The investigation is conducted in MILD regime with a special focus on chemical effects of CO2 in the oxidizer. Opposed jet diffusion combustion configuration is adopted. The combustion kinetics is described by the Gri 3.0 mechanism and the Chemkin code is used to solve the problem.It is found that oxygen reduction has a significant effect on flame temperature and emissions while less sensitivity corresponds to hydrogen enrichment in MILD combustion regime. Temperature and species are considerably reduced by oxygen decrease in the oxidizer and augmented by hydrogen addition to the fuel. The maximum values of temperature and species are not influenced by the composition of the biogas in MILD regime. Blending biogas with hydrogen can be used to sustain MILD combustion at very low oxygen concentration in the fuel.In MILD combustion regime, the chemical effect of CO2 in the oxidizer stream reduces considerably the flame temperature and species production, except CO which is enhanced. For high amounts of CO2 in the oxidizer, the chemical effect of CO2 becomes negligible.

Methane combustion in various regimes: First and second thermodynamic-law comparison between air-firing and oxyfuel condition

MILD oxyfuel combustion has been attracting increasing attention as a promising clean combustion technology. How to design a pathway to reach MILD oxyfuel combustion regime and what can provide a theoretical guide to design such a pathway are two critical questions that need to be answered. So far there has been no open literature on these issues. A type of combustion regime classification map proposed in our previous work, based on the so-called ”Hot Diluted Diffusion Ignition” (HDDI) configuration, is adopted here as a simple but useful tool to solve these problems. Firstly, we analyze comprehensively the influences of various dilution atmosphere and fuel type on combustion regimes. The combustion regime classification maps are made out according to the analyses. In succession, we conduct a comparison between the map in air-firing condition and its oxyfuel counterpart. With the aid of the second thermodynamic-law analysis on the maps, it is easy to identify the major contributors to entropy generation in various combustion regimes in advance, which is crucial for combustion system optimization. Moreover, we find that, for the first time, a combustion regime classification map also may be used as a safety indicator. With the aid of these maps, some conclusions in previous publications can be explained more straightforwardly.

The transition of ethanol flames from conventional to MILD combustion

The present paper is focused on prevaporised ethanol flames in the transition from conventional combustion to the MILD combustion regime. Photographs and imaging of OH* chemiluminescence reveal a distinctive flame structure when ethanol carried by air burns in a 3% O2 coflow (typical of MILD combustion), differing from that at higher oxygen levels. In comparison to flames carried by air in a 9% O2 coflow, the spatial gradient and the peak in the OH* signal profile are significantly reduced in the 3% O2 coflow, indicating a more uniform distribution of heat release and temperature. The use of N2 as a carrier gas renders the OH* profile for the 6% O2 coflow case similar to that of flames carried by air in the 3% O2 coflow. The experimental results indicate a transition from conventional combustion to MILD combustion with the decrease of coflow O2 level and/or the use of N2 as a carrier gas. Calculations reveal that a substantial drop in the peak heat release rate and/or overall net heat release rate might contribute to the lack of luminosity of flames in the 3% O2 coflow, suggesting a need for threshold values of these two in defining MILD combustion. A series of laminar flame calculations are performed to identify the MILD combustion regime based on the absence of a negative heat release region. The absence of a negative heat release region is found to be strain rate dependent at a given temperature and O2 level of the oxidant stream. This is mainly a result of the enhanced transportation of O2 across the reaction zone at a higher strain rate. At low strain rates, the negative heat release region is more likely to disappear in a 3% O2 oxidant flow due to a combination of low flame temperature and high availability of O2.

Experimental characterization of pulverized coal MILD flameless combustion from detailed measurements in a pilot-scale facility

This paper presents a series of measurements on pulverized coal MILD flameless combustion in a pilot-scale facility for nitrogen oxides (NOx) emissions reduction. Local measurements of gaseous species concentrations, gas temperature and velocity associated to reaction zone imaging by OH* chemiluminescence highlight specific features of this MILD flameless combustion regime. The flameless burner used during the investigation could abate significantly NOx emission levels.Specific aerodynamics of the flow in the furnace induced by the burner geometry are discussed. High momentum turbulent air jets favor a large recirculation of hot flue gas in the combustion chamber. Such a recirculation induces a large diluted combustion regime from the entrainment in the air jets of devolatilized species from the pulverized coal jet and of recirculating hot inert combustion products. The main reaction zone is lifted from the burner exit as it starts in the mixing layers of separated pulverized coal and air jets. As is typical of a flameless combustion regime, such a large dilution of reactants induces low local heat release and temperature increase in the reaction zone. Nitrogen oxides are generated from the fuel NO route at quite a limited rate due to the low temperature environment.The effect of the carrier gas of the pulverized coal jet is also analyzed. The change from carbon dioxide to air as carrier gas generates a first reaction zone attached to the burner exit. This reduces the dilution and increases the heat release in the main lifted reaction zone, leading to higher NOx emissions.

Simulations of laminar non-premixed flames of methane with hot combustion products as oxidiser

A numerical study has been performed to investigate the behaviour of laminar strained non-premixed flames of CH4 with hot combustion products as an oxidiser with a focus on various characteristics of MILD combustion. The oxidiser stream was hot combustion products from a Φ = 0.6, 0.7, 0.8, and 0.9 laminar premixed flame, therefore having temperatures above 1650 K and oxygen content, XO2, below 8%, placing the flames studied in the MILD combustion regime. The simulations used the GRI-Mech 3.0 chemical scheme with a chemiluminescence kinetic sub-mechanism. Heat release rate profiles in MILD flames were spread over a wide region in mixture fraction space. The location of this region and its peak is discussed in comparison to conventional experimental markers for heat release such as the OH* chemiluminescence and pixel-by-pixel product of OH- and CH2O-PLIF signals. These experimental markers cannot predict the location of peak heat release rate in MILD flames as accurately as in conventional flames due to the extended nature of MILD heat release rate, in contrast to the thin regions of both XOH* and XOH, presented in both real and mixture fraction space. The effect of strain rate on temperature rise during combustion, ΔT, XOH*,XOH, and heat release rate is discussed with an analysis of extinction behaviour. MILD flames produced NO through both thermal and prompt NO mechanisms, with both the high temperatures and initial NO concentrations in the oxidiser significantly affecting reaction rate behaviour. The simulations provide insights to some experimental observations usually associated with MILD combustion.