oxy-MILD Combustion Research Articles

Performance Improvement of Methane oxy-MILD Combustion with High Initial Oxygen Contents Using a Novel Non-premixed Oxygen/Recycled Flue Gas Jet Burner

Performance Improvement of Methane oxy-MILD Combustion with High Initial Oxygen Contents Using a Novel Non-premixed Oxygen/Recycled Flue Gas Jet Burner

Effect of H2 and CO in syngas on oxy-MILD combustion

This paper investigates the effect of adding H2 and CO to a syngas on oxy-MILD combustion. This work employs a Perfectly Stirred Reactor (PSR) model to calculate the autoignition temperature and the critical oxygen concentration for different component fuels under oxy-MILD conditions. Our investigation obtains the critical O2 concentration for oxy-MILD combustion of various H2/CO/CH4 component fuels. The results indicate that adding H2 to CH4 significantly reduces the minimum O2 concentration required for achieving the MILD state while adding CO has little effect. Furthermore, using the novel PCA method, we analyze characteristic chemical time scales for oxy-MILD combustion for different fuel ratios. We conclude that the mixture containing H2 and CO can be used as fuel for oxy-MILD combustion, as its characteristic chemical time scale is comparable to the turbulent time scale, resulting in Da number close to unity. However, achieving a steady MILD combustion state is more challenging than methane due to reduced ignition delay times. Moreover, syngas with various ratios of H2/CO presents the potential for controlling the MILD combustion process.

Reaction behaviors in pulverized coal MILD-oxy combustion affected by physical and chemical effects of diluents

In pulverized coal MILD-oxy combustion process, the oxidizer diluents are switched from N2 to CO2 and H2O. Hence, the remarkable discrepancies in combustion characteristics directly originate from the differences in physical and chemical properties of diluents, namely, the physical and chemical effects. This work utilizes the fictious materials, i.e., FCO2 and FH2O, which have the same physical properties with the real ones but are completely non-reactive, to isolate these two dilution effects. Their individual and combined impacts on the global reaction behaviors are numerically evaluated. Results reveal that the physical effect of CO2 and H2O plays an important role in enhancing the internal flue gas recirculation. Compared with H2O-dilution, CO2-dilution better inhibits the temperature rise and promotes the in-furnace thermal uniformity, which predominantly originates from its physical effect. Hence, the MILD reaction regime with high temperature and strong dilution levels occur in a wider region. The homogeneous diffusion/kinetics-controlled regime (0.2 < Dat < 2) is more susceptible to the chemical effect of CO2 and H2O. The kinetics-controlled regime of heterogeneous reaction is facilitated under CO2- and H2O-dilutions, where the physical effect is dominant in the former case while the physical and chemical effects act jointly in the latter case. The chemical effect of CO2 and H2O is responsible for greatly strengthening gasification reaction by direct participating in the reaction process, which in turn slows down the burnout of char. Importantly, H2O-dilution shows greater potential to lower NO emission level than CO2-dilution. The physical effect is revealed to play a dominant role in reducing the global NO formation rate, while the chemical effect enhances the reburning of NO in a wide lean-O2 reduction region. The findings of this work are expected to help gain a more comprehensive understanding in MILD-oxy combustion technology.

MILD-Oxy Combustion Characteristics of Pulverized Coal Affected by Oxidizer O2 Concentrations under Dry and Wet Recycle Modes

MILD-Oxy Combustion Characteristics of Pulverized Coal Affected by Oxidizer O₂ Concentrations under Dry and Wet Recycle Modes

Combustion behaviors of pulverized coal MILD-oxy combustion under different steam injection parameters

MILD-oxy combustion under the wet flue gas recirculation mode is of potential in achieving a synergetic control of C/N emissions, whereas its practical realization is restricted by how to efficiently form an in-furnace H2O-rich atmosphere under MILD regime. Towards this issue, this work numerically evaluates the feasibility of applying the direct steam injection method by arranging the individual steam nozzles with different diameters (Dste) on both sides of the primary stream nozzle (i.e., the inner- and outer-steam injections). The effects of incident position and velocity on combustion behaviors are studied. Results show that compared with the advanced confluence of primary and secondary streams in the inner injection cases, outer-steam injection delays the confluence of inlet jets to enhance the in-furnace dilution level. Inner-steam injection is better in inhibiting temperature rise and fluctuation, which facilitates a more uniform distribution of temperature. MILD combustion with diffusion/kinetics-controlled regime is better maintained in the inner injection cases under a small Damkӧhler number. The share of gasification reaction on char consumption in the inner injection cases is higher than that in the outer injection cases, and the burnout time of char is also increased. The increased Dste further enhances the contribution of gasification reaction and prolongs char burnout. Inner-steam injection inhibits the oxidation of fuel-N and enhances NO-reburning in a large low-O2 region as Dste increases, which leads to the lower NO emission concentration. Meanwhile, interaction between NO and fuel-N intermediates is found to play an inevitable role in reducing the overall NO emission.

Internal association between combustion behavior and NOx emissions of pulverized coal MILD-oxy combustion affected by adding H2O

In practical pulverized coal Moderate or Intense Low-oxygen Dilution oxy-fuel combustion (MILD-oxy) process, the accumulation of steam results in the recycled flue gas consisting of highly concentrated H2O, particularly under wet flue gas recirculation. This work numerically studies the individual effect of H2O on the turbulence-chemistry interaction, aiming at revealing the internal association between combustion behavior and NOx emissions. Results show that H2O addition reduces the flame temperature by enhancing the radiative heat transfer. The extremum values of turbulent Damkӧhler number (Dat,max) and Karlovitz number (Kamin) are reduced and increased, respectively, denoting the slow-chemistry feature (Dat < 10 and Ka >> 1) in the reaction zone. With the promoted H2O level, the heterogeneous reactions on the char surface tend to be governed by kinetics due to the reduced reaction rate under low flame temperature. The gasification reaction is enhanced to produce more synthesis gas, and the burnout of char particle is prolonged. The in-furnace NO formation is inhibited under low Dat,max, while the NOx emissions are reduced directly due to the enhanced gasification reaction under low maximum surface Damkӧhler number (Das,max). The addition of H2O essentially establishes a moderate diffusion and reaction process under the kinetics-controlled regime to obtain the uniform thermal field and low-NOx emissions.

Fuel-NO formation mechanism in MILD-oxy combustion of CH4/NH3 fuel blend

MILD-oxy combustion operates conventional oxy-combustion processes under moderate or intense low-oxygen dilution (MILD) condition, which offers a practical approach towards the synergetic control of CO2 and NOx emissions. This numerical work investigates the MILD combustion characteristics and NO emission of CH4/NH3 fuel blend under N2, CO2 and H2O dilutions, aiming at revealing the fuel-NO formation mechanism under MILD-oxy condition. It is found that relative to MILD-N2 combustion, the flame temperature is reduced under MILD-CO2 and MILD-H2O combustion, while the reaction zone is expanded and reduced, respectively, due to the altered co-flow properties. CO2 dilution lowers the radical pool level, while the formation of OH radical is boosted under H2O dilution. Overall NO emission (EINO) under MILD-N2 combustion is the largest while that under MILD-H2O generates the lowest value, showing its low-NOx emissions superiority. NH3 is converted into NO via routes NH3 → NH2 → HNO → NO and NH3 → NH2 → NH → HNO → NO under MILD-CO2 combustion. By contrast, the importance of route NH3 → NH2 → NH → N → NO in producing NO is largely strengthened under MILD-H2O combustion, wherein the direct formation of NO via HNO is inhibited with the enhanced availability of reactive N-containing species, contributing to lower NO emission under MILD-H2O combustion over MILD-CO2 combustion.

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.

Investigation of air-MILD and oxy-MILD combustion characteristics of semicoke and bituminous coal mixtures in a 0.3 MW fuel-rich/lean fired furnace

To study the combination of fuel-rich/lean combustion with oxy-MILD (moderate and intense low-oxygen dilution) combustion for the reduction of NOx emissions by adopting carbon capture and storage, this paper numerically investigates the turbulence−chemistry interactions and NOx emission behaviours of semicoke mixtures under different secondary velocities Vsec (from air-fuel combustion to air-MILD combustion) and under different CO2 dilution levels (oxy-MILD combustion). The bias concentration ratio (BCR) hardly affects the establishment of air-MILD combustion, which is obtained at Vsec = 158.6 m/s. The oxy-MILD combustion, which is composed of small flamelets diffusing in large-scale eddies (turbulent Damköhler number Dat = 0.1–1), is enhanced, and the ignition and char burnout times are prolonged by increasing CO2 from 0 vol% to 79 vol%. The char oxidation reaction in the diffusion/kinetics-controlled regime and the char gasification reactions in the kinetics-controlled regime both shift towards the kinetics-controlled regime with increasing velocities and CO2 dilution, thus leading to lower burnout rates. The heterogeneous MILD regime of the fuel-lean side is more easily established for the air-MILD combustion, while that of the fuel-rich side is more easily established for oxy-MILD combustion. This is because the intensive turbulence increases the O2 dilution level in the fuel-lean jet, while a high concentration of CO2 reduces the oxygen diffusivity in the fuel-rich jet, which also leads to a larger XNO in the fuel-lean jet compared to the fuel-lean jet. The NOx concentration at the furnace outlet decreases significantly from air-fuel combustion to air-MILD combustion and further decreases as the oxy-MILD combustion is enhanced, along with relatively lower homogeneous and heterogeneous Da.

Combustion regimes and fuel-NO mechanism of CH4/NH3 jet diffusion flames in hot O2/CO2 co-flow

Moderate or Intense Low-oxygen Dilution oxy-fuel (MILD-oxy) combustion presents a practical approach toward achieving carbon neutrality with low-NOx emissions, and its combustion process is highly affected by O2 and temperature levels in the reactant mixture. This paper numerically evaluates the effects of co-flow O2 (XO2) and temperature (Tcof) on the combustion regimes and fuel-NO mechanism of CH4/NH3 jet diffusion flames in hot O2/CO2 co-flow, in which XO2 and Tcof vary from 3% to 30% and from 1300 K to 2100 K, respectively. Results show that MILD-oxy combustion can be fully established for all Tcof levels when XO2 ≤ 21%, whereas a higher Tcof is required for maintaining MILD regime at XO2 ≥ 24%. Otherwise, the maximum temperature rise exceeds the auto-ignition temperature and combustion regime will shift to high temperature combustion (HTC). From the physical perspective, the promoted Tcof level facilitates the achievement of MILD regime in a large reaction zone by greatly enhancing the specific heat of co-flow and weakening the entrainment of O2 into the mixing layer. Meanwhile, reducing XO2 in co-flow remarkably inhibits the fuel combustion and hence fulfils MILD combustion in the chemical view. Increasing XO2 and Tcof levels facilitate NH2 + OH ↔ NH + H2O because of high content of OH radical, while depleting O radical and blocking NH2 + O ↔ HNO + H. Fuel-NO formation routes with HNO as the N-containing intermediate via NH3 → NH2 → HNO → NO and NH3 → NH2 → NH → HNO → NO are weakened and lead to the decrease in NO emission index.

Investigation of Air-MILD and Oxy-MILD Combustion Characteristics of Semicoke and Bituminous Coal Mixtures in a 0.3 WM Fuel-Rich/Lean Fired Furnace

Investigation of Air-MILD and Oxy-MILD Combustion Characteristics of Semicoke and Bituminous Coal Mixtures in a 0.3 WM Fuel-Rich/Lean Fired Furnace

NO formation mechanism of CH4/NH3 jet flames in hot co-flow under MILD-oxy condition: Effects of co-flow CO2 and H2O

MILD-oxy combustion is an advanced technology for achieving carbon capture and low NOx emissions simultaneously. In its combustion process, the atmosphere comprises CO2 and H2O due to intense flue gas recirculation. This work numerically investigates the chemical effects of co-flow CO2 and H2O on the homogeneous fuel-NO formation mechanism of CH4/NH3 jet flames in hot co-flow under MILD-oxy condition. The results reveal that compared with H2O dilution, CO2 dilution alters the radical pool and thus lowers the overall oxidation rate, which favors achieving the MILD regime with a notably enlarged reaction zone. The increase in CO2 and H2O contents facilitates the formation of NO, wherein NH3 conversion into NO primarily occurs via four routes with HNO as the major N-containing intermediate. The addition of CO2 significantly promotes NH2 + O ↔ HNO + H and NH + CO2 ↔ HNO + CO, which enhances the oxidation of NH3 through the routes NH3 → NH2 → HNO → NO and NH3 → NH2 → NH → HNO → NO. The enhancement of H2O concentration accelerates NH + OH ↔ HNO + H and NH + OH ↔ N + H2O through the formation of abundant OH radical. The conversion of NH radical to HNO and N radical is consequently facilitated and strengthens the routes NH3 → NH2 → NH → HNO → NO and NH3 → NH2 → NH → N → NO.

Effect of steam addition on turbulence-chemistry interaction behaviors of pulverized coal MILD-oxy combustion

The enrichment of steam is closely associated with flue gas recirculation in the pulverized coal MILD-oxy combustion, which exerts significant impacts on the turbulence-chemistry interaction and combustion behaviors. The present work numerically evaluates the effect of steam addition on the interaction behaviors between turbulent mixing and chemical reaction of pulverized coal MILD-oxy combustion through investigating the Damköhler number Da in the main reaction zone. The results show that the addition of steam affects the flame temperature by enhancing thermal radiation and weakens the internal flue gas recirculation. While maintaining the slow chemistry regime in main reaction zone, the turbulent Damköhler number Dat increases and Karlovitz number Ka decreases with an increase in steam concentration, respectively, which reveal that high concentration steam dilution governs MILD-oxy combustion by turbulent mixing rather than chemical kinetics. Meanwhile, the ranges of the surface Damköhler number Das-O2, Das-CO2 and Das-H2O are within 15–45, 0.06–0.65, 0.02–0.20, respectively, which indicate that the Cs-O2 reaction is governed by diffusion while the Cs-CO2 reaction and Cs-H2O reaction are determined by diffusion-kinetics. As the steam concentration increases, the heterogeneous reactions proceed towards the diffusion-controlled regime. The proportion of char consumed by oxidation reactions increases due to the corresponding weakening of gasification reactions, which leads to the less formation of synthesis gas. The findings can provide deep insight into the interaction between turbulence and chemistry of pulverized coal MILD-oxy combustion under wet recycle.

CFD modeling on char surface reaction behavior of pulverized coal MILD-oxy combustion: Effects of oxygen and steam

Gasification reaction is not supposed to be omitted for pulverized coal MILD-oxy combustion regime, especially for the wet recycle (steam up to 40 vol%). This paper numerically investigates the individual effects of steam and oxygen on pulverized coal MILD-oxy combustion to have better understanding of char reaction behavior under this combustion mode. Results show that, the flame temperature affected by steam is because of thermal radiation, whereas that of O2 is because of chemical reactivity. The maximum value of the turbulent Damköhler number (Da,t) is within 0.25–1.9 for all cases, implying that the rate of chemical reaction is comparable with that of turbulent flow. For 30% O2 and 40% steam cases, gasification reaction accounts for up to ~62%. With a given O2 level, the reaction of char-H2O competes the active sites with char-O2 and char-CO2, whereas for a certain level of steam, a positive synergy exists between char-H2O and char-CO2. The effect of CO + H2O → CO2 + H2 on generating H2 becomes increasingly significant when O2 content is >21%, and easy to occur under low steam concentration. Increasing the O2 and steam is conducive for the decrease in overall char burnout rate. As the steam content increases, the reduction on char burnout rate is insignificant for 18% and 21% O2 cases, but it is serious for 24%–30% O2 cases.

Effects of oxygen concentration and inlet velocity on pulverized coal MILD combustion

The inlet velocity is an important parameter in the Moderate or Intense Low-Oxygen Dilution (MILD) combustion. Different inlet velocities not only change the flow of the flue gas, but also affect the dilution effect of the circulating flue gas and the temperature distribution of the furnace. In coal MILD-oxy combustion, different oxygen concentrations change the amount of flue gas and inlet velocity when the excess oxygen coefficient is fixed. Therefore, the change of inlet velocity is worthy of attention in MILD-oxy combustion. In this work, the MILD combustion characteristics with different inlet velocities at different oxygen concentrations are discussed by changing the burner diameter. The results show that increasing the inlet velocity suppresses the temperature peaks in both MILD-air and MILD-oxy combustion. When the inlet velocity is fixed, the flame temperature is increased with the O2 concentration. The facilitation of char gasification reaction with increasing oxygen concentration is also partly due to the increase of temperature and char gasification is also partially affected by the O2 concentration. In terms of heat transfer, the inlet velocity has a large effect on convective heat transfer.

Numerical simulation of pulverized coal MILD-oxy combustion under different oxygen concentrations

As an emerging clean coal combustion technology, Moderate or Intense Low-Oxygen Dilution (MILD) combustion or oxy-fuel combustion, compared with traditional coal combustion, has many advantages. However, compared with MILD combustion and oxy-fuel combustion, MILD-oxy combustion is believed more attractive. In this work, MILD-oxy combustion characteristics with oxygen concentrations from 10% to 50% are studied numerically. The results show that within a certain range, increasing the oxygen concentration is in favor of MILD-oxy combustion performance close to that of MILD-air combustion. When the oxygen concentration is higher enough, the momentum reduced by the increase of oxygen concentration has a great influence on the furnace temperature. With the increase of oxygen concentration, the radiation heat transfer is enhanced and the convective heat transfer is weakened. The increase of oxygen concentration can promote the occurrence of char gasification reaction with CO2. In addition, MILD-oxy combustion has a large impact on CO emission.

Numerical study of biomass Co-firing under Oxy-MILD mode

Oxy-MILD (Oxygen Moderate and Intense Low-Oxygen Dilution) combustion is one of the most promising technologies for the mitigation of CO2 emissions from coal-fired furnaces, benefiting from its good performance in flame-temperature controlling and NOx reduction. Under oxy-MILD mode, the combustion or co-firing of biomass (CO2-neutral) can achieve “negative CO2 emissions”. In this paper, oxy-MILD biomass co-firing is numerically studied by CFD modeling for the IFRF furnace NO.1, where Guasare coal and Olive waste are co-fired under air-MILD and oxy-MILD conditions, respectively. The effects of biomass co-firing ratio (0–30%, energy basis) and atmosphere on the temperature and heat flux distribution, and NOx emissions are discussed. The modeling results show that under MILD combustion mode, both oxy-combustion and biomass co-firing can generate a more moderate temperature distribution and lower NOx emissions than air-combustion and coal combustion, respectively. When biomass co-firing ratio increases from 0% to 30%, under oxy-MILD combustion mode, the peak temperature linearly decreases by 28 K and the NOx emissions decrease by 141 ppm; while under air-MILD combustion mode, the peak temperature increases by 15 K and the NOx emissions decrease by only 73 ppm. This modeling work suggests that oxy-MILD biomass co-firing is a more promising technology to achieve “negative CO2 emissions” in coal combustion, with lower furnace temperatures as well as NOx emissions.

Effects of Preheating and CO2 Dilution on Oxy-MILD Combustion of Natural Gas

Oxy-moderate or intense low-oxygen dilution (MILD) combustion, which is a novel combination of oxy-fuel technology and MILD regime, is numerically studied in the present work. The effects of external preheating and CO2 dilution level on the combustion field, emission, and CO formation mechanisms are investigated in a recuperative laboratory-scale furnace with a recirculating cross-flow. Reynolds-averaged Navier–Stokes (RANS) equations with eddy dissipation concept (EDC) model are employed to perform a 3-D simulation of the combustion field and the turbulence–chemistry interactions. In addition, a well-stirred reactor (WSR) analysis is conducted to further examine the chemical kinetics of this combination when varying the target parameters. The simulations used the skeletal USC-Mech II, which has been shown to perform well in the oxy-fuel combustion modeling. Results show that with more preheating, the uniformity of temperature distribution is noticeably enhanced at the cost of higher CO emission. Also as inlet temperature increases, the concentration of minor species rises and CO formation through the main path (CH4→CH3→CH2O→HCO→CO→CO2) is strengthened, while heavier hydrocarbons path (C2H2→CO) is suppressed. Meanwhile, greater CO2 addition notably closes the gap between maximum and exhaust temperatures. In a highly CO2-diluted mixture, chain-branching reactions releasing CH2O are strengthened, while chain-terminating reactions are weakened. CH2O production through CH3O is accelerated compared with the straight conversion of methyl to formaldehyde. When diluting the oxidant, methylene CH2(s) plays a more influential role in CO formation than when pure oxygen is used, contributing to higher CO emission.

A comparative study of methane MILD combustion in O2/N2, O2/CO2 and O2/H2O

Abstract Moderate or intense low-oxygen dilution (MILD) combustion is a promising technology for reducing pollutant emissions while maintaining high thermal efficiency. Recently, MILD combustion has been expected to be applied in combination with oxy-fuel combustion for carbon capturing and storage (CCS). This paper presents a numerical study of methane MILD combustion in O2/N2, O2/CO2 and O2/H2O, in order to deepen the knowledge to the combined form, namely oxy-MILD combustion. Firstly, steady computational fluid dynamics (CFD) simulation was carried out inside a closed lab-scale MILD combustion furnace following the previous experiment conducted in O2/N2. Detailed in-furnace temperature and species data as well as laminar flame speed were used to validate the CFD models and the chemical reaction mechanism. Subsequently, flame structure and turbulence/reaction interaction were examined under the three atmosphere conditions. The results suggest that oxy-MILD (diluted with either CO2 or H2O) combustion exhibits larger reaction zone and higher likelihood to be operated under distributed reaction regime in comparison with air-MILD (diluted with N2). Specifically, CO2 is the most preferable diluent among N2, CO2 and H2O to achieve MILD combustion regime.

Effect of H2O/CO2 mixture on heat transfer characteristics of pulverized coal MILD-oxy combustion

The integration of moderate or intense low-oxygen dilution (MILD) combustion technology with oxy-fuel combustion, that is, MILD-oxy combustion, has a favorable compatibility to obtain high thermal efficiency and extra low CO2 emissions for pulverized coal combustion. MILD-oxy combustion is characterized by high levels of H2O/CO2 dilution, whose heat transfer characteristics are affected by the physical and chemical properties (i.e., gasification reactivity, heat capacity, and heat radiation) of H2O/CO2 mixture. In this work, dependences of the heat transfer characteristics of MILD oxy-coal combustion on the physical and chemical properties are qualitatively analyzed. Results show that high CO2 levels is favorable for the establishment of pulverized coal MILD combustion for a given 30 vol% O2. An increase in H2O concentration reduces transferred heat while the enhancing the share of heat radiation. In addition, the effect of gasification reactivity on flame temperature is remarkable at a narrow region and gradually weakens with the increase in H2O concentration. The heat capacity makes a slight reduction on overall furnace temperature. As the H2O is substitute for CO2, the influence of heat capacity progressively declines. Moreover, the effect of heat capacity on strengthening convective heat transfer or weakening radiative heat transfer gradually decreases. As the H2O concentration increases, the reduction of flame temperature affected by heat radiation ranges from 13 K to 34 K. Notably, 20% and 30% H2O, and 40% and 50% steam cases have similar radiative emissivity, but the intensity of radiation of the steam cases is increasingly intense, thereby resulting in a significant reduction on flame temperature.