MILD Combustion Research Articles

On the effects of NH3 addition to a reacting mixture of H2/CH4 under MILD combustion regime: Numerical modeling with a modified EDC combustion model

This paper examines the behavior of reacting NH3/H2/CH4 mixtures in moderate or intense low oxygen dilution (MILD) condition. A series of axisymmetric, turbulent reacting flow simulations are carried out incorporating a modified version of eddy dissipation concept and a few reaction mechanisms. The effects of adding a progressively increasing amount of NH3 to a reacting H2/CH4 mixture in moderate condition are investigated. It is observed that addition of NH3 to MILD combustion leads to markedly different behaviors compared to that in conventional combustion. Most notably, the inherently strong preheating of reactants in MILD combustion causes thermal cracking of NH3 prior to ignition. The resultant production of H2 profoundly affects the reacting flow as such increasing the NH3 mass fraction in the fuel blend decreases the flame lift-off. Further, unlike that in conventional combustion, adding NH3 to MILD combustion increases the process reactivity. In addition, the usual flame thickening typically seen in NH3 flames is not observed here, while in keeping with the thermodynamic predictions, NH3 addition lowers the temperature of combustion products. The results also show that in sharp contrast to that reported for conventional combustion, addition of NH3 in MILD condition does not increase the emission of NO, while the mass fraction of NO2 drops slightly. Overall, it is concluded that MILD combustion could be a promising route to NH3 combustion.

Influence of wall thermal conditions on the ignition, flame structure, and temperature behaviors in air-fuel, oxygen-enhanced, and oxy-fuel combustion under the MILD and high-temperature regimes

In this work, variations of wall thermal conditions and oxidant compositions are considered to investigate the temperature distribution, ignition process, and flame structure under MILD and high-temperature combustion (HTC) regimes. Studies are performed by the computational fluid dynamics (CFD) approach with OpenFOAM open-source code. Also, kinetic calculations by well-stirred-reactor (WSR) is used to quantitatively describe results. Results show that by the transition from HTC to the MILD regime, Tmax and Tave have intense (more than 15%) and uniform (less than 3%) drops, respectively. This indicates that moving from HTC to the MILD regime can be considered as an effective way to maintain the thermal conditions while reducing the emission of pollutants. The MILD regime is formed at Tw = 1500 K and Tw = 1700 K under air-fuel and oxy-fuel combustion, respectively, indicating that oxy-fuel MILD combustion is associated with fewer heat losses. Enhancement of heat loss creates an intense variation of reaction pathways under the MILD regime, while it is constant for HTC regime. Higher heat loss leads to diminishing the role of reactions O + H2O ⇌ 2OH and O2+CH2OH ⇌ HO2+CH2O on the ignition delay time (IDT) under insulation reactor and increases the contribution of reactions H + OH + M ⇌ H2O + M and O + CH3 ⇌ H + CH2O.

Identifying the skeleton mechanism for oscillatory combustion with functional weight analysis

In this work, a functional weight (FW) method is proposed to identify the primary mechanism of oscillatory combustion in MILD combustors. Inter-species interactions are quantitatively measured by analyzing combustion oscillation data to recognize the importance of each link. Detailed statistics of the functional weight in the oscillatory combustion mechanism revealed that the weight distributions are highly heterogeneous, with a very limited part of the inter-species links dominating. This allowed us to easily identify important processes and build a skeleton mechanism for oscillation prediction. Case studies are conducted on methane- and propane-fueled MILD combustion systems, where oscillatory dynamic behavior is reported. It is found that the skeleton mechanism obtained can eliminate more than 90% of the reactions, keeping good oscillation prediction ability, and the recognized important reactions are in perfect agreement with the related kinetic investigations.

Numerical Research for MILD Burners with Different configurations

ABSTRACT In the experiment or simulation study of Moderate or Intense Low-Oxygen Dilution (MILD) combustion, scholars used different burner structures. In order to evaluate the effects of MILD burner configurations on combustion, five different burners are numerically studied. In addition, the effects of furnace outlet position are also investigated. The results show that the structure of the burner has a great influence on MILD combustion. The burners with high momentum nozzles deviating from the outlet has a more uniform temperature distribution, a lower Damköhler number (Da) and a higher flue gas recirculation rate , so it has a higher MILD combustion performance. When the oxidant inlet is at the center, more fuel inlets disperse the initial momentum of the fuel, which is not conducive to MILD combustion. When the fuel inlet is at the center, the oxidant inlet should be reasonably arranged to avoid hindering the dilution performance of the circulating flue gas. In addition, the outlet position also has a great influence on the flow in the furnace, and a reasonable outlet also has the effect of optimizing the MILD combustion.

Reaction zone characteristics, thermal performance and NOx/N2O emissions analyses of ammonia MILD combustion

Pure ammonia MILD combustion is studied numerically in a furnace. The effects of furnace wall temperature and dilution of inlet air with nitrogen on the characteristics of reaction zone (RZ), temperature distribution and exhaust emissions are investigated. The results show that lowering the furnace wall temperature and adding diluent to the inlet air, result in expansion of the RZ, reduction of fuel and oxidizer consumption rates, lower Damkohler, increased turbulent Reynolds and more similar RZ properties to the distributed combustion regime in the combustion regimes diagram. At a lower concentration of oxygen in the oxidizer, a higher threshold wall temperature is required for stable MILD combustion. Reducing the wall temperature and oxygen mole fraction in the oxidizer, increases the uniformity of temperature distribution. Analysis of exhaust emissions shows that reducing wall temperature and oxygen level in the oxidizer stream can significantly reduce NOx pollution, however, N2O emission rises.

Quantitative evaluation of NO formation and destruction routes during methane MILD combustion using an improved calculation method

Moderate or intense low-oxygen dilution (MILD) combustion has become increasingly attractive because of its ultra-low nitric oxide (NO) emission. Although many efforts have been made to investigate the NO formation and destruction pathways of MILD combustion, their relative contribution still fails to reach an agreement when using previously-developed NO calculation methods. For this reason, these NO calculation methods are first evaluated for methane MILD combustion by the kinetic modeling of a well-stirred reactor (WSR). Then, an improved calculation method for quantifying the contribution of NO sub-routes is proposed, which involves thermal, prompt, N2O-intermediate, NNH, selective non-catalytic reduction of NO, and NO-reburning routes. Also, this improved method allows the application of a more comprehensive mechanism (Progress in Energy and Combustion Science, 2018, 67: 31–68) other than GRI-Mech 2.11. Using the improved method with the more comprehensive mechanism, the NO formation and destruction routes during methane MILD combustion are quantitatively investigated. The results show that in the WSR at 1300 K with 5% O2, the contribution of the N2O-intermediate route accounts for about 80% at an equivalent ratio (Φ) of 0.8, and it gradually decreases as Φ increases. Once Φ is >1.0, the prompt route surpasses the N2O-intermediate route to dominate the NO generation entirely. The NNH mechanism is found to be of little importance at Φ ranging from 0.5 to 1.5. Additionally, the NO-reburning reaction reduces <10% of the total NO production when Φ < 1.0, and its contribution increases abruptly at 1.0 < Φ < 1.1.

Analysis of EDC constants for predictions of methane MILD combustion

The indisputable wide use of the Eddy Dissipation Concept (EDC) indicates that the mean reaction rate is reasonably modeled for MILD (Moderate or Intense Low-oxygen Dilution) and conventional combustions. However, RANS (Reynolds-Averaged Navier-Stokes) modelling with the standard EDC constants tends to over-predict maximum temperatures when applied to the MILD combustion regime. Therefore, there is a modification need of the EDC constants to predict properly the mean reaction rate, and then the maximum temperatures. The relation of the mean reaction rate and the EDC constants, including the primary and the secondary, is presented. A lab-scale methane MILD combustion furnace (MCF) with non-preheated air from literature is chosen for the validation of the EDC constants. The modification is done with one of the secondary EDC constants ranging from 1 to 28 where the primary constant is kept at CD1=0.1357. Predictions with the secondary EDC constant of 4 agree quite well with the measurements of the MILD combustion in this furnace. The modified EDC constant of 2 are suggested for the predictions of the conventional combustion in this furnace. Finally, based on the excess temperature or the normalized root mean square excess temperature (NRET), a criterion for the attainment of MILD combustion in this furnace is suggested.

Numerical study of fuel-NO formation and reduction in a reversed flow MILD combustion furnace firing ammonia-doped methane

Moderate or intense low-oxygen dilution (MILD) combustion is well-known for its high potential of reducing thermal-NO formation during combustion, but its capability of mitigating fuel-NO formation is still elusive. To provide new insights into this aspect, this paper numerically studies the fuel-NO formation characteristics under CH4/NH3 MILD combustion in a lab-scale reversed flow furnace by computational fluid dynamics (CFD) modeling with NH3 volume fraction varying from 0% to 4%. It is found that, the overall NO emission at the furnace outlet increases as the initial NH3 concentration rises under both MILD combustion and traditional bluff-body stabilized combustion mode, however, fuel-NO conversion ratio gradually drops. In the present reversed flow furnace, there exists a critical value (1.4%) for the initial NH3 volume fraction, above which MILD combustion loses the advantage of generating lower NO emission in comparison with the traditional combustion operation. However, this critical value would be case-dependent and should be further re-evaluated for different combustors. In the current combustion system, the lower NO emission under traditional combustion when firing higher NH3 doping fuels is caused by the enhanced NO reduction via both CHi reburning and NO2 interconversion routes, which occurs on the outer and inner sides of the flame front, respectively. For MILD combustion, the weakening of CHi reburning in the outer side of reaction zone causes the higher NO emission when initial NH3 concentration exceeds the critical value.

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.

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

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

Effects of hydrogen addition on combustion characteristics of a methane fueled MILD model combustor

Combined with need of the carbon emissions, the feasibility of Moderate or Intense Low-oxygen Dilution (MILD) combustion fueled with hydrogen/methane blends needs to be investigated. This paper discusses the pollutant emissions, the stable operating range and the flame morphology for a jet-induced MILD model combustor. The hydrogen/methane volume ratios range 0:10 to 5:5. The NOx emissions are less than 5 ppm@15%O2 when the hydrogen content is less than 50% by volume in the atmospheric conditions. The calculation using chemical reactor network (CRN) model demonstrates that the effect of heat loss on NOx emissions increases as the adiabatic combustion temperature increases, which is consistent with the experimental results. The maximum OH∗ signal intensity increased at higher hydrogen content, especially when the hydrogen content exceeds 30% by volume. Due to the increase in turbulent burning velocity and the enhancement in the reaction intensity, the reaction zones shrink with increasing hydrogen content. In addition, with increasing hydrogen content, the stable operation range of the combustor becomes narrower, and the stable combustion is not maintained when the hydrogen content exceeds 50% by volume. The findings of the paper help to further understand the effect of hydrogen content on the formation of MILD combustion in the jet-induced combustor.

Heat and mass transfer mechanism and control strategy of clean low carbon combustion technology in the novel-type coke oven flue with MILD combustion

In the field of emission reduction, considering the limitations of single clean combustion technology and joint control of carbon emission, the application of Technology of Mixed-Gas (TMG), Technology of Low-calorific Value Gas (TLVG) and Technology of Low Oxygen Combustion (TLOC) in the Novel-type Coke Oven Flue with MILD Combustion (MILD-NCOF) was realized. By adjusting the ratio of coke oven gas to blast furnace gas (α), the ratio of flue gas to gas (β) and the ratio of flue gas to air (γ), the heat and mass transfer mechanism of combustion process in MILD-NCOF was researched, the dual emission control system and the regulation strategy of pollutant and carbon emissions were established. The results showed that with the increase of α, β and γ, the fluidity of gas and the reaction rate of combustible components in the fire path were improved, the heat and mass transfer was enhanced, and the effect of dual emission reduction of pollutants and carbon emissions was improved. When the mixing ratio α was 0.6, Tave was reached at 1594.93 K. The NOx concentration at the outlet was 116.34 mg/m3, and the sum of the molar concentrations of CO and CO2 at the outlet was reduced to 1.440 × 10−1. When the mixing ratio β was 2.4, Tave was reached at 1518.73 K, the NOx concentration at the outlet was 128.42 mg/m3, and the sum of the molar concentrations of CO and CO2 at the outlet was 1.127 × 10−1. When the mixing ratio γ was 0.6, Tave was reached at 1627.28 K, the NOx concentration at the outlet was 142.73 mg/m3, and the sum of the molar concentrations of CO and CO2 was reduced to 1.128 × 10−1, which could meet the dual standards of clean production and low-carbon environmental protection of MILD-NCOF.

Ammonia/Methane combustion: Stability and NOx emissions

In the framework of the energy shift toward Renewable Energy Sources, ammonia is considered a valuable energy vector, due to its very high hydrogen-density and well-established production processes. Despite the many potential advantages, its combustion features (narrow flammability limits, high auto-ignition temperature, potential high fuel-NOx emissions) may hinder its wide utilization. To overcome these issues in conventional combustion, “fuel enhancers” are also used. Differently from conventional systems, MILD Combustion already proved to be very effective in oxidizing ammonia in terms of stability and NOx emission. Nevertheless, a “fuel enhancer” can be also useful in MILD Combustion conditions to further improve the process characteristics. The present study focuses on the ammonia/methane combustion characteristics under MILD Combustion conditions in a lab-scale burner. Gaseous pollutant emissions (NOx, NH3, H2, CO) and process stability limits were analysed as a function of the equivalence ratio and NH3/CH4 fuel composition. Results showed that the use of NH3/CH4 blends extends the stable operational range of the system, in terms of both working temperatures and equivalence ratios, with respect to pure NH3. On the other hand, blends produce higher NOx emissions, with respect to both the pure NH3 and CH4 cases. Experimental data were compared with chemical kinetics modelling results. Chemical pathways and rate of production of main intermediate products highlighted that oxidation pathways of carbon and nitrogen-based species are essentially decoupled as experiments suggested. In contrast, the interplay of the methane and ammonia main chemical pathways affects the DeNOx channel in correspondence of the observed NOx emission increase.

Dynamic adjustment of the Eddy Dissipation Concept model for turbulent/combustion interactions in mixed combustion regimes

The Eddy Dissipation Concept (EDC) combustion model, in comparison with some other combustion models, has drawn attention, especially for the Moderate or Intense low oxygen Dilution (MILD) combustion. The original formulation of the EDC combustion model is not developed for the MILD combustion regime, and a revision of the model could be considered. In this study, the effect of the characteristic frequency on the EDC combustion model has been investigated, and some parametric studies on the ratios of length and time scales of the fine structures to the Kolmogorov scales have been performed. Results revealed that finding optimum model constants for all combustion field with a wide range of Damköhler numbers seems to be complicated. The variations in scales ratio of fine structure to the Kolmogorov eddies shows different consequence at low and high Damköhler number combustion regions which leads to a conclusion on difficulties for EDC model global optimization. As a solution, a localized modulation method based on some local variables such as the turbulence Reynolds number and Damköhler number has been developed and applied on the Adelaide Jet in Hot Coflow(JHC) and Delft Jet in Hot Coflow (DJHC), and in addition in supplementary material on Sandia/TUD piloted flame and Sydney bluff-body burners. These burners demonstrate combustion regimes with both low and high Damköhler numbers. Results have shown the better performance of the local approach in the simulation of the burners. The improvement is more perceptible at the sections far from the inlet, where Damköhler number is increased, in comparison with the standard (default) EDC model. The average of deviations from the measurements is decreased by more than 10% and even more by a decrement in oxidizer oxygen level.

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

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

Mixture fraction measurement in turbulent non-premixed MILD jet flame using Rayleigh scattering.

Turbulent combustion of jet flames in a hot diluted coflow of combustion products is conducive to the transition from conventional flamelet combustion to a regime of moderate or intense low oxygen dilution (MILD) combustion, which is commonly characterized by a very low emission and noise. MILD combustion is also characterized by distributed combustion where the net heat release is positive across the entire combustion domain. The turbulence/chemistry interactions in this regime that determine the flame structure, local temperature, and species distribution critically depend on the mixture fraction and scalar dissipation fields. However, there are no experimental tools to measure the mixture fraction field in a distributed (MILD) combustion regime. The present work offsets this limitation by demonstrating a Rayleigh scattering-based approach to measure mixture fraction in a turbulent ethylene MILD combustion zone. 1D counterflow flame simulations enabled mapping the locally calibrated Rayleigh scattering fields to mixture fractions in the fuel-rich regions. This approach also shows very low sensitivity to the local temperature and composition. Overall, the results provide compelling evidence that the distributed heat release does not significantly impact the turbulent processes of the flow-field for the conditions examined. The measurement uncertainty from this approach and its extension to more complex fuels are also discussed. The present technique is limited to mildly turbulent, fully MILD/distributed flame with representative scalar dissipation rates.

Experimental studies on soot in MILD oxy-coal combustion flame: Sampling, microscopic characteristics, chemical compositions and formation mechanism

Based on a diffusion-flamelet-based Hencken burner, the effect of temperature on the volume fraction of soot was explored firstly. The temperature of MILD oxy-coal (MILD-OCC) flame was measured by temperature measuring device and the volume fraction of soot was obtained by the thermocouple deposition method. Then the mass concentration, microstructure, nanostructure parameters (the distribution of particle size, the length of crystallite, the curvature of crystallite, the layer spacing of crystallite) and chemical compositions of soot were studied. The soot particles generated from MILD-OCC flame were harvested by thermophoretic sampling system and capillary sampling system, and measured with the combination of field emission transmission electron microscopy (TEM) and gas chromatography-mass spectrometry analyzer (GC-MS). The results showed that as the height of MILD-OCC flame increased, the degree of agglomeration of soot particles increased. The surface growth and oxidation existed at the same time and were in a competitive relationship, which manifested in the mass concentration of soot, the average distribution of particle size and the content of aromatic compounds. With the rise of flame height, the average length of the crystallite increased while the average curvature of the crystallite and the average crystalline layer spacing showed a decreasing trend, which showed that the microstructure of soot became more mature and orderly. These results are valuable for further understanding the formation mechanism of soot.

MILD combustion of co-firing biomass and pulverized coal fuel blend for heterogeneous fuel NO and PM2.5 emission reduction

A sawdust and bituminous coal blend is studied experimentally under MILD combustion conditions, and the PM2.5 emissions of MILD combustion are measured. MILD combustion is established by low-temperature preheated air in a 200-kW pilot-scale furnace. The pilot tests consist of the conventional flame combustion through a swirl nozzle, MILD combustion through symmetrical double straight nozzles, and MILD combustion through an asymmetrical single straight nozzle. Compared to conventional swirl flame combustion, the MILD combustion of the sawdust blend can reduce the fuel-NO emission by at least 45.5%. The results further reveal that the PM2.5 emissions are decreased by over 37%. The burnout rates of the sawdust blend MILD combustion are higher than 95%. Considering the in-furnace distribution, low NO and PM2.5 emissions, and high burnout rate, MILD combustion is an effective way to utilize biomass fuel blends on a large scale. This work extends fuel adaptation of MILD combustion to biomass blends. Compared with previous MILD combustion studies, this investigation further confirms the inhibitory effect of MILD combustion on the formation of fine particles.

Numerical study on combustion performance of propane non-premixed mild in O2/CO2 atmosphere

ABSTRACT Moderate or intense low-oxygen dilution (MILD) combustion is one of novel clean combustion technologies. Propane MILD combustion in CO2/O2 atmosphere is investigated in a 20 KW MILD combustion furnace by numerical simulation. The results indicate propane MILD combustion is established and stable in O2/CO2 atmosphere instead of N2/O2 atmosphere, and O2/CO2 atmosphere leads to lower average temperature in the furnace, better temperature uniformity, as well as higher CO emission. The dilution concentration of CO2 affects the thermal and chemical performance in the propane MILD combustion, such as temperature distribution, propane pyrolysis, exothermic rate, free radical OH and CO distributions in the furnace. CO emission always keeps several thousand ppm in all CO2 cases, which indicates that CO emission and burnout are the key problem in propane MILD combustion in O2/CO2 atmosphere. It is concluded by analyzing these factors that in the range of 73%~85% of CO2 dilution concentration, the 79% Case presents best, which is closely related to the multiple effects of CO2 physical dilution, jet entrainment, jet penetration, and chemical reaction.

A three-step reduced mechanism for MILD combustion

ABSTRACT A previously derived two-step mechanism describing high-temperature autoignition in alkanes is augmented by addition of a CO2-production step to account for subsequent heat release, producing a three-step reduced mechanism that can facilitate computational investigations at MILD conditions. Comparisons of ignition-times and well-stirred-reactor S-shaped temperature/residence-time curves for propane mixtures suggest that this greatly simplified chemical-kinetic scheme may produce predictions of sufficient accuracy for many applications. A distinctive aspect of the proposed simplified description is that, unlike other available reduced mechanisms, it is free from empiricism, in that it is systematically derived from detailed chemistry by incorporating steady-state approximations for chemical intermediates.