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This study investigated the effect of the H 2 fraction on reaction dynamics and NO x formation in a premixed NH 3 /H 2 /air jet flame within a hot coflow (JHC) through numerical simulations. We found that even a small mole fraction of H 2 (e.g., X H2, F = 5%) significantly enhances the ignition of the premixed NH 3 /air flame. However, the presence of H 2 markedly intensifies the main combustion reactions only when X H2, F ≥ 30%, resulting in a sharp increase in temperature and heat release rate. Moreover, as X H2, F increases, the NO x emission first increases until X H2, F = 75% and then decreases. Without any H 2 addition, a high emission of unburned NH 3 occurs in the pure NH 3 JHC flame. On the other hand, when X H2, F ≥ 50%, the emissions of unburned H 2 and OH increase rapidly due to extremely high temperatures. Notably, maintaining X H2, F between 5% and 20% minimizes the emissions of NO x , NH 3 , H 2 , and OH. Furthermore, the preferential diffusion of H 2 plays a key role in enhancing the production of active radicals (OH, O, and H), thereby significantly boosting the initiation and combustion reactions of NH 3 /H 2 blended fuel. [Display omitted] • Effect of H 2 addition on the premixed NH 3 /H 2 /air jet flames is investigated. • A small amount of H 2 (5%) in the fuel notably enhances the ignition of NH 3 flame. • H 2 significantly boosts major combustion reactions only when its proportion ≥30%. • Optimal H 2 proportion is 5%–20% to reduce emissions of NO x , unburned NH 3 and H 2. • Preferential diffusion of H 2 greatly enhances the ignition and combustion reactions. [ABSTRACT FROM AUTHOR]

• LES-SWF well predicts oxy-flame characteristics across a wide Re range. • Higher Re reduces radial flame size, enhancing effects of vortex on reaction zone. • Increasing Re weakens the beneficial effect of preferential diffusion of H 2. • Adverse effect of CO 2 on combustion increases with increasing Re. Due to differences in the physical and chemical properties of CO 2 and N 2 , along with a reduction in the momentum ratio between oxidant and fuel streams, the local extinction of oxy-fuel flames is significantly more pronounced compared to conventional air–fuel flames, which poses challenges in the design and operation of oxy-fuel burners. This study further validates the species-weighted flamelet/progress variable (FPV) model proposed in previous work [Jiang et al., 2023], particularly in oxy-fuel flames characterized by highly local extinction. Large eddy simulations were conducted on the Sandia oxy-fuel jet diffusion flame at various Reynolds numbers. The predictions are systematically compared with experimental data, and the influence of Reynolds number on local extinction is thoroughly analyzed. The results demonstrate that the numerical simulation effectively predicts mean temperature, species mass fractions, differential diffusion parameters (Z HC), and local extinction in oxy-fuel jet flames across a wide range of Reynolds numbers. The errors in predicted mean temperature and mass fractions exhibit a slight increase with rising Reynolds numbers, yet remain below 15 %. As the Reynolds number increases from 12,000 to 18,000, the predicted peak Z HC decreases by 30 %, and the beneficial effect of preferential diffusion of H 2 weakens, while the adverse effect of CO 2 on combustion becomes stronger. [ABSTRACT FROM AUTHOR]

• Ignition process is controlled by both mixing and chemistry for all jet flames. • Flow mixing greatly affects both ignition and stabilization of low-oxygen flames. • Rich-side reactions are much slower than lean-side and are far from equilibrium. • Premixing of fuel jet and coflow before ignition is crucial for low-oxygen flames. • Low-oxygen flames develop by autoignition versus high-oxygen ones by propagation. This study numerically investigates the ignition and evolution of CH 4 jet flames in a hot coflow (JHC) using large eddy simulation (LES). Three coflow oxygen fractions are carefully selected to compare moderate or intense low-oxygen dilution (MILD, Y O2,C = 6 % and 9 %) and high temperature combustion (HTC, Y O2,C = 23.3 %). Systematic investigations are performed on flame ignition, lift-off, reaction progress, and stabilization mechanisms. The results show that for the MILD cases, both the ignition process and final stable flame are controlled by turbulent mixing and chemistry in a state far from chemical equilibrium. In contrast, the reactions are much stronger for HTC, and the final flame is controlled by chemistry through equilibrium reactions. Moreover, for all three cases, the reactions always progress more rapidly at the lean side than at the rich side; thus, autoignition dominates the former, whereas flame propagation is more important for the latter. In addition, the premixing of fuel and oxidant prior to main reactions is crucial for realizing MILD combustion; therefore, the flame is lifted and stabilized by the autoignition of partially premixed reactants in the fuel-lean region. In contrast, the final HTC flame attaches to the burner exit and is stabilized by both the lean-side autoignition and rich-side flame propagation. For all three cases, the lean-side and stoichiometric flames adopt the nonpremixed mode, whereas the rich-side reactions are in the premixed mode, except for the flame base. [ABSTRACT FROM AUTHOR]

• NH 3 flame has much lower temperature rise and larger reaction zone than CH 4. • NO x emission of NH 3 flame is two orders higher than CH 4 at equivalence ratio < 1. • NO x emissions of both flames increase rapidly with coflow oxygen concentration. • NO x emission mainly comes from NO while N 2 O is important at low temperature. • MILD combustion can reduce the NO x emission of NH 3 flame by 1–2 orders. Moderate or intense low-oxygen dilution (MILD) combustion is suitable for solving the problems of unstable flames and high NO x emissions (E NO x) of ammonia fuels; however, studies on this are rare. This paper numerically investigates the MILD combustion characteristics of a premixed NH 3 /air jet flame in hot coflow (JHC) under different jet equivalence ratios (Φ J), coflow temperatures (T C), and oxygen levels (X O2,C). For comparison, similar CH 4 /air MILD-JHC flame characteristics are calculated. The results show that the NH 3 flame generally has a lower temperature increase and heat release, as well as a larger reaction zone, than CH 4. This suggests that the NH 3 flame can develop into a MILD regime more easily than CH 4. E NO x of the NH 3 flame is two orders of magnitude higher than that of CH 4 flame at all T C and X O2,C values for Φ J < 1. At Φ J > 1, E NO x of the NH 3 flame rapidly decreases to nearly zero, but unburned NH 3 emissions (E NH3) and H 2 emissions (E H2) are significantly high (∼1000 ppm) and the combustion efficiency is low. Moreover, E NO x increases rapidly (gradually) with X O2,C (T C), whereas E NH3 and E H2 are considerably low at T C ≥ 1300 K and X O2,C ≥ 1% for the NH 3 flame. E NO x primarily originates from NO; however, N 2 O is also important at low Φ J and T C values. Therefore, considering E NO x , E NH3 , E H2 and the burning efficiency, the optimal conditions for NH 3 MILD combustion are Φ J = 1, T C ≥ 1500 K, and X O2,C = 1%–3%. In addition, compared with conventional flames, NH 3 MILD combustion can reduce E NO x by one to two orders of magnitude. [ABSTRACT FROM AUTHOR]

NO-reburning technology can effectively decrease NO x emissions during combustion. Despite extensive studies have been conducted on NO reburning under traditional combustion, homogeneous NO-reburning characteristics have not been systematically examined in flameless combustion (FLC). This study performs a systematic comparative investigation of homogeneous NO-reburning for the first time via experiments and simulations in a 20-kW furnace. The influences of combustion mode, air-preheating temperature (T a), and equivalence ratio (Φ) are studied in detail. The experimental results show that FLC can improve NO-reduction efficiency by more than 35% compared to swirl flame combustion (SFC). Moreover, the results also show that reducing T a from 723 K to 573 K can enhance NO-reburning during FLC, while the NO-reburning is insensitive to Φ in the range of 0.65–0.89. Furthermore, a detailed numerical simulation that couples the eddy-dissipation concept model and a well-verified skeletal mechanism for NO-reburning is validated and performed, to further reveal the reduction pathways of NO. By combining flameless combustion with NO-reburning, this study confirms the preponderance of flameless combustion under low-temperature air conditions (T a ≤ 573 K) for considerable NO reduction by reburning. The results offer novel fundamental insights into the homogeneous NO-reburning characteristics. • Homogeneous NO-reburning in flameless combustion is studied for the first time. • Flameless combustion can improve the NO reduction efficiency by more than 35%. • Reducing the air preheat-temperature enhances NO-reburning of flameless combustion. • Reaction pathways of NO reduction for swirl and flameless combustion are revealed. [ABSTRACT FROM AUTHOR]

• Pilot-test of MILD combustion of solid waste blend is realized for the first time. • The burnout rates of solid waste blend MILD combustion are higher than 95%. • The fuel-NO emission of MILDC-AS combustion is reduced by 54%. • MILD combustion reduces the PM 2.5 emissions of biomass blend fuel by 50%. The treatment of solid waste through incineration has become an important development trend. A solid waste blend (sawdust and residual char) is studied experimentally in a tubular-furnace reactor and a 200-kW pilot-scale furnace under moderate or intense low-oxygen dilution (MILD) combustion conditions. The effects of temperature and oxygen concentration on NO release characteristics are examined in the tubular-furnace reactor. The pilot-scale furnace experiments include the conventional flame combustion using a swirl nozzle (CFC-S), MILD combustion using symmetrical dual straight nozzles (MILDC-SD), and MILD combustion using an asymmetrical single straight nozzle (MILDC-AS). The tubular-furnace reactor results indicate that the NO released from solid waste blend mainly originates from fuel-NO under medium temperature and low oxygen environments. The pilot-scale furnace results demonstrate that stable combustion of the solid waste blend can be achieved under three co-combustion conditions. Compared to CFC-S, the MILDC-AS combustion of solid waste blend inhibits the fuel-NO and PM 2.5 emissions by 54% and 50%, respectively. The burnout rates of solid waste blend MILD combustion exceed 95%. This study not only extends the fuel adaptability of MILD combustion to solid waste blend, but also proves the suppression influence of MILD combustion on fuel nitrogen and PM 2.5 emissions. [ABSTRACT FROM AUTHOR]

A flamelet/progress variable (FPV) model accounting for the differential diffusion (DD) effects is proposed and applied to large eddy simulation (LES) of turbulent oxy-fuel flames (Sevault et al. 2012). Based on tabulations with the detailed molecular diffusion and the equal diffusion (ED) assumption, a species-weighted flamelet (SWF) model is developed to represent the DD effects. The influences of combustion progress, mixture fraction, and turbulence on variable Lewis numbers are incorporated into the model. The model assessments are first conducted on a laminar coflow flame, showing that the effect of DD on temperature and species is accurately captured, in that the importance of DD is seen in the laminar flame. Subsequently the model is implemented in a fully coupled LES of oxy-fuel jet flames. The LES results show that DD plays an important role in the reaction zone and regions near the fuel nozzle, and its effect decreases farther downstream, consistent with the experimental observations. The LES with the SWF model yields good predictions on the mean temperature and major species at the fuel-rich side while slight deviation (less than 6%) at the fuel-lean side. In comparison, LES with the original unity Lewis numbers flamelet model (Pierce et al. 2004) predicts a full extinction because of neglecting the DD characteristics, while the variable Lewis number flamelet model provides the maximum deviation of 26% because of ignoring the ED characteristics produced by turbulent disturbance. The trend and location of localized extinction of oxy-fuel flame are also well predicted using the SWF model. [ABSTRACT FROM AUTHOR]

In pulverized coal combustion, the participating radiative media includes gas, soot, and coal/char particles; a fundamental understanding of their radiation behavior is essential for the prediction of radiation heat transfer and control of radiation in the furnace. In this study, the individual contributions of gas, soot, and coal particles to the total radiation were examined based on the large eddy simulation of the CRIEPI pulverized coal jet flame. The radiation transfer equation was solved using the discrete ordinate method with state-of-the-art non-gray radiative property models of multi-phase media. The full spectrum correlated k -distributions (FSCK) model was used to calculate the radiative property of the gas-soot mixture. A burnout-based particle radiative property model was applied for coal/char particles. The results showed that the predicted spatial distributions of coal particles, tar, and soot were in good agreement with the experimental data. The gray gas radiation model predicted temperatures approximately 50 K lower in this flame, and the peak soot volume fraction was predicted at approximately 22% lower. For the laboratory scale burner under examination, the individual radiative contributions of gas, soot, and coal/char were found to be 35%, 40%, and 25%, respectively, so that it was necessary to consider all components for accurate prediction. Moreover, these contribution ratios varied with the flame position. Because of the high particle concentration, the individual contribution of coal/char particles accounted for up to 65% in the devolatilization region. Soot radiation played an important role in the pulverized coal flame. In the region where the ensemble-averaged soot volume fraction reached the maximum of 2.7 ppm, the individual contribution of soot was 50% of the total radiation, which accounted for 70% of the total particles (soot and char) radiation. [ABSTRACT FROM AUTHOR]

A sawdust and bituminous coal blend is studied experimentally under MILD combustion conditions, and the PM 2.5 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 PM 2.5 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 PM 2.5 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. • Pilot-test of MILD combustion of sawdust fuel blend is realized successfully. • MILD combustion of the sawdust fuel blend can reduce the fuel-NO emission by 45.5%. • MILD combustion can distinctly reduce the PM 2.5 emissions by at least 37%. • MILD combustion is an effective way for large-scale utilization of biomass blend. [ABSTRACT FROM AUTHOR]

Co-firing residual char and pulverized coal can apply the residual char in pulverized coal combustion equipment, whose difficulties lie in poor ignition performance and low burnout efficiency. For the first time, residual char and bituminous coal fuel blend is experimentally co-fired under moderate or intense low-oxygen dilution (MILD) conditions. MILD co-combustion is achieved with low-temperature preheating air in a pilot-scale furnace. The co-firing experiments include swirl low-NO flame combustion, MILD combustion through symmetrical secondary air jets, and MILD combustion through an asymmetrical secondary air jet. The pilot-scale tests reveal that MILD co-combustion through the asymmetrical single nozzle achieves the lowest peak temperature and the most uniform distributions of temperature, heat flux, and gas compositions in the furnace. The NO emission of MILD co-combustion through the asymmetrical air jet is decreased by 57% compared with swirl low-NO flame co-combustion. The burnout rate of MILD co-combustion through the asymmetrical air jet reaches 94%. Considering more uniform in-furnace distribution, lower NO emission, and higher burnout rate, MILD combustion through the asymmetrical single nozzle shows more obvious advantages than that through symmetrical air nozzles. Overall, the present study further widens the fuel flexibility of MILD combustion even to residual char fuel blend. • Co-firing residual char under MILD combustion is realized for the first time. • MILD co-combustion of residual char can reduce fuel-NO emission by at least 54%. • Fuel flexibility of MILD combustion is even expanded to residual char fuel blend. [ABSTRACT FROM AUTHOR]

• A WSGG model for arbitrary concentrations of CO 2 and H 2 O is developed. • Two modeling methods (mixture modeling and superposition method) for WSGGM are compared. • WSGG models for gas-soot mixture are assessed in flame simulations. The global radiative property models are widely used in combustion simulation due to its high computational efficiency. This study assesses the weighted sum of gray gases (WSGG) models for gas-soot mixture in the simulations of turbulent jet diffusion flame at atmospheric pressure with fully coupled combustion and radiation heat transfer. Non-adiabatic tabulated chemistry approach and the method of moments were employed for combustion modeling and soot particle dynamics. The radiative transfer equations are solved by the discrete ordinate model. Two kinds of WSGG model based on mixture modeling (MM) and superposition method (SM), and their combination with the gray and non-gray soot radiative property models are involved in the comparison. First, the radiative property models are evaluated on the one-dimensional parallel plate system using line-by-line model as the benchmark, and then the WSGG models are evaluated on the basis of the FSCK model in the combustion simulation. The results show that MM and SM provide similar prediction accuracy, while MM has much lower computational cost. The MM combined with the non-gray soot radiative property model (MMNS) provides the best accuracy for gas-soot mixture. Moreover, the relative error of MM combined with gray soot radiative property model (MMGS) is less than 10% for radiation source term, which is acceptable in engineering simulation. The gray implementation of WSGG model has low accuracy, which indicates that it is necessary to use the non-gray implementation of WSGG model in CFD simulations. [ABSTRACT FROM AUTHOR]

• CFD results of a swirl oxy-coal flame are compared with detailed experiment. • Nonlinear variation particle radiative property is first used in oxy-coal modeling. • Contributions of gas and particle radiation are illustrated in oxy-coal flame. • Constant model ten times over-predicts the particle absorption after burnout. This paper reports a comprehensive computational fluid dynamics simulation of the IFRF's swirling oxy-fuel flame with pulverized coal using wet flue gas recycle. Special attention is given to the influence of particle radiative property model. Based on the burnout rate of pulverized coal, a recent proposed nonlinear conversion-dependent particle radiative property model (Guo et al., 2018) is used in the simulations. The influences of the high CO 2 concentration on the sub-models are also considered in detail. The gas-phase global reaction mechanism, kinetic parameters of char oxidation and gasification, and gas radiative property model are modified for oxy-fuel condition. The results show the prediction results agree well with the measurements, in terms of the temperature and composition of the flue gas. The particle radiation has an important effect on the flame temperature. Even in the small furnace studied here (optical length is approximately 1.6 m), the radiation in the flame zone is still dominated by particle radiation. Linear variation assumption (Yin et al., 2015) under-predicts the particle absorption in combustion zone. Ignoring the effect of burnout rate on particle radiative property will 10 times over-predict the particle absorption after burnout. [ABSTRACT FROM AUTHOR]