Computed NOx emission characteristics of opposed-jet syngas diffusion flames

This paper reported a numerical study on the NO_x emission characteristics of opposed-jet syngas diffusion flames. A narrowband radiation model was coupled to the OPPDIF program, which used detailed chemical kinetics and thermal and transport properties to enable the study of 1-D counterflow syngas diffusion flames with flame radiation. The effects of syngas composition, pressure and dilution gases on the NO_x emission of H₂/CO synthetic mixture flames were examined. The analyses of detailed flame structures, chemical kinetics, and nitrogen reaction pathways indicate NO_x are formed through Zeldovich (or thermal), NNH and N₂O routes both in the hydrogen-lean and hydrogen-rich syngas flames at normal pressure. Zeldovich route is the main NO formation route. Therefore, the hydrogen-rich syngas flames produce more NO due to higher flame temperatures compared to that for hydrogen-lean syngas flames. Although NNH and N₂O routes also are the primary NO formation paths, a large amount of N₂ will be reformed from NNH and N₂O species. For hydrogen-rich syngas flames, the NO formation from NNH and N₂O routes are lesser, where NO can be dissipated through the reactions of NH + NO → N₂ + OH and NH + NO → N₂O + H more actively. At a rather low pressure (0.01 atm), NNH-intermediate route is the only formation path of NO. Increasing pressure then enhances NO formation reactions, especially through Zeldovich mechanisms. However, at higher pressures (5–10 atm), NO is then converted back to N₂ through reversed N₂O route for hydrogen-lean syngas flames, and through NNH as well for hydrogen-rich syngas flames. In addition, the dilution effects from CO₂, H₂O, and N₂ on NO emissions for H₂/CO syngas flames were studied. The hydrogen-lean syngas flames with H₂O dilution have the lowest NO production rate among them, due to a reduced reaction rate of NNH + O → NH + NO. But for hydrogen-rich syngas flames with CO₂ dilution, the flame temperatures decrease significantly, which leads to a reduction of NO formation from Zeldovich route.     

A detailed numerical study of NOx kinetics in low calorific value H2/CO syngas flames

The present study provides an extensive and detailed numerical analysis of NO_x chemical kinetics in low calorific value H₂/CO syngas flames utilizing predictions by five chemical kinetic mechanisms available out of which four deal with H₂/CO while the fifth mechanism (GRI 3.0) additionally accounts for hydrocarbon chemistry. Comparison of predicted axial NO profiles in premixed flat flames with measurements at 1 bar, 3.05 bar and 9.15 bar shows considerably large quantitative differences among the various mechanisms. However, at each pressure, the quantitative reaction path diagrams show similar NO formation pathways for most of the mechanisms. Interestingly, in counterflow diffusion flames, the quantitative reaction path diagrams and sensitivity analyses using the various mechanisms reveal major differences in the NO formation pathways and reaction rates of important reactions. The NNH and N₂O intermediate pathways are found to be the major contributors for NO formation in all the reaction mechanisms except GRI 3.0 in syngas diffusion flames. The GRI 3.0 mechanism is observed to predict prompt NO pathway as the major contributing pathway to NO formation. This is attributed to prediction of a large concentration of CH radical by the GRI 3.0 as opposed to a relatively negligible value predicted by all other mechanisms. Also, the back-conversion of NNH into N₂O at lower pressures (2–4 bar) was uniquely observed for one of the five mechanisms. The net reaction rates and peak flame temperatures are used to correlate and explain the differences observed in the peak [NO] at different pressures. This study identifies key reactions needing assessment and also highlights the need for experimental data in syngas diffusion flames in order to assess and optimize H₂/CO and nitrogen chemistry.     

Exergy analysis and NOx emission of the H2-fueled micro burner with slinky projection shape channel for micro-thermophotovoltaic system

In order to improve the energy output of the MTPV system and reducing the NO_x emission, a novel micro burner with a slinky projection shape channel for the MTPV system is proposed. To conduct the numerical simulation, 3-D models with detailed H₂/air reaction mechanisms and the extended Zeldovich mechanism of NO_x generation are employed. The influence of the slinky projection amplitude, slinky projection fins number, and the basic oscillating channel radius on the energy conversion characteristics and the NOx emission is investigated. The increase of the slinky projection amplitude and slinky projection fins number can improve the energy output and exergy efficiency, as well as reduce the NOx emission. When the slinky projection amplitude is 0.4 mm and the slinky projection fins number is 42, the exergy efficiency reaches the maximum value of 70.3%, while the energy output of MTPV reaches the maximum value of 4.99 W at 10 m/s. Meanwhile, the decrease of the basic oscillating channel radius can significantly decrease the NO mole fraction at the outlet. Generally, an efficient technique to increase energy output and reduce NOx emission for the MTPV system is to introduce a micro burner with a slinky projection shape channel.     

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., X_O2 < 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.     

Ammonia conversion and NOx formation in laminar coflowing nonpremixed methane-air flames

This paper reports on a combined experimental and modeling investigation of NO_x formation in laminar, ammonia-seeded, nitrogen-diluted, methane diffusion flames. The methane-ammonia mixture is a surrogate for biomass fuels, which contain significant fuel-bound nitrogen. The experiments use flue-gas sampling to measure the concentration of stable species in the exhaust gas. The computations use adaptive mesh refinement (AMR) to capture fine-scale features of the flame. The model includes a detailed chemical mechanism, differential diffusion, buoyancy, and radiative losses. The model shows good agreement with the measurements over the full range of experimental NH₃ seeding amounts. As more NH₃ is added, a greater percentage is converted to N₂ rather than to NO. The simulation results are analyzed to trace the changes in NO formation mechanisms with increasing amounts of ammonia in the fuel.     

An analytic model for the effects of nitrogen dilution and premixing characteristics on NOx formation in turbulent premixed hydrogen flames

Advanced hydrogen gas turbine is a promising technology to achieve near-zero emission of carbon dioxide and higher cycle efficiency. With the increased firing temperature and pressure ratio, nitrogen reinjection combined with dry premixed combustion is promising to achieve the challenging low NOx emission. In this study, the effects of nitrogen dilution and fuel/air premixing characteristics on the flame characteristics and NOx emission are investigated first through simulating one-dimensional premixed flames with a 13-species and 39-reaction mechanism at the elevated engine operation conditions. The variation of flame thicknesses and laminar flame speeds with nitrogen dilution is investigated. The NOx formation is characterized by the flame-front NOx and the constant NOx formation rates in the post-flame region. It is shown that the flame-front NOx is an order of 1 ppm and does not change significantly (within 20%) with nitrogen dilution. In contrast, the NOx formation rates in the post-flame region decrease monotonically with nitrogen dilution due to the decrease of oxygen concentration. A detailed analysis of NOx formation reveals that the N₂O pathway is significant and it can account for at least 20% of the NOx formation in the post-flame region. Then an analytic model considering both the extended Zeldovich mechanism and the N₂O pathway is constructed by assuming the involved radicals being in chemical equilibrium. The model can be employed to efficiently estimate the NOx formation in fully premixed hydrogen gas turbines. Next, the effects of fuel/air premixing characteristics on the mean NOx formation rate in the post-flame region are quantified by reconstructing the PDF of mixture fraction. It is shown that without the nitrogen dilution, the NOx formation rate increases dramatically with fuel/air unmixedness due to the existence of local hot spots. Nitrogen dilution can dramatically reduce the NOx formation rate at the same level of unmixedness through reducing the local hot spots. Moreover, nitrogen dilution reduces the sensitivity of the NOx formation rate to fuel/air unmixedness, which greatly alleviates the mixing requirement for the premixing nozzles in gas turbines. Finally, a model for the estimation of NOx emission is constructed, which builds the connection between NOx emission, nitrogen dilution, unmixedness and flow residence time in combustors.     

A numerical study on NOx formation in laminar counterflow CH4/air triple flames

Formation of NO_x in counterflow methane/air triple flames at atmospheric pressure was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. Results indicate that in a triple flame, the appearance of the diffusion flame branch and the interaction between the diffusion flame branch and the premixed flame branches can significantly affect the formation of NO_x, compared to the corresponding premixed flames. A triple flame produces more NO and NO₂ than the corresponding premixed flames due to the appearance of the diffusion flame branch where NO is mainly produced by the thermal mechanism. The contribution of the N₂O intermediate route to the total NO production in a triple flame is much smaller than those of the thermal and prompt routes. The variation in the equivalence ratio of the lean or rich premixed mixture affects the amount of NO formation in a triple flame. The interaction between the diffusion and the premixed flame branches causes the NO and NO₂ formation in a triple flame to be higher than in the corresponding premixed flames, not only in the diffusion flame branch region but also in the premixed flame branch regions. However, this interaction reduces the N₂O formation in a triple flame to a certain extent. The interaction is caused by the heat transfer and the radical diffusion from the diffusion flame branch to the premixed flame branches. With the decrease in the distance between the diffusion flame branch and the premixed flame branches, the interaction is intensified.     

Detailed investigation of NO mechanism in non-premixed oxy-fuel jet flames with CH4/H2 fuel blends

This study systematically investigates the detailed mechanism of nitrogen oxides (NO_x) in CH₄ and CH₄/H₂ jet flames with O₂/CO₂ hot coflow. After comprehensive validation of the modeling by experiments of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154]; the effects of CO₂ replacement of N₂, mass fraction of oxygen in the coflow (Y_O2), and mass fraction of hydrogen in the fuel jet (Y_H2) on NO formation and destruction are investigated in detail. For methane oxy-fuel combustion, the NNH route is found to control the NO formation at Y_O2 ≤ 3%, while both NNH and N₂O-intermediate routes dominate the NO production at 3% < Y_O2 < 10%. When Y_O2 ≥ 10%, NO is obtained mainly from thermal mechanism. Moreover, in the oxy-combustion of methane and hydrogen fuel blends with Y_O2 = 3%, with hydrogen addition the contribution of the NNH and prompt routes increases, while that of the N₂O-intermediate route decreases. Furthermore, the chemical effect of CO₂ is significant in reducing NO in both oxy-combustion of methane with Y_O2 ≤ 3% and combustion of methane and hydrogen fuel blends with Y_H2 ≤ 10%.     

Nitric oxide destruction during coal and char oxidation under pulverized-coal combustion conditions

A modified drop-tube reactor that allows particle distribution over the reactor cross-sectional area, and oxidation of chars produced in situ, was used to study the conversion efficiency of char nitrogen to nitric oxide (α_NO). The results confirm previous findings by other investigators that α_NO decreases as the weight of char burned increases. α_NO for coal was the same as (at 4% O₂) or lower than (at 20% O₂) that for an equal mass of char during oxidation. Since coal will yield approximately half its mass as fixed carbon, these results suggest that the local stoichiometry surrounding the particle is responsible for the observed reduction in α_NO as sample size increases. The analysis of the exhaust gases showed increases in HCN concentration and a decrease in CO₂/CO ratio as sample size increased, suggesting that local stoichiometry influences α_NO. Additional experiments showed that α_NO decreased as the background NO concentration was increased, at rates that diminished as the oxygen concentration increased, independent of particle size. The steep reduction in NO production as the background NO concentration increased was explained by the destruction of NO in the gas phase.     

An investigation into the heat release and emissions from counterflow diffusion flames of methane/dimethyl ether/hydrogen blends in air

In the present work, the effects of blending dimethyl ether (DME) and hydrogen (H₂) with methane (CH₄) have been numerically studied in the context of counterflow diffusion flames. In order to do so, a reaction mechanism consisting of 974 reaction steps among 146 species with updated thermodynamic and transport properties has been developed. This mechanism has been validated against the experimental data on laminar burning velocity, ignition delay time and species profiles in counterflow diffusion flames. The present study suggests that the heat release pattern of the CH₄ counter flow diffusion flame shows major changes when DME and H₂ are present in the fuel stream. Furthermore, the results show that the presence of low volume fractions of DME in CH₄ increases the formation of benzene (C₆H₆) in the flame. This fact can be negated by the presence of H₂ together with CH₄ and DME in the fuel stream. Moreover, the present study suggests that H₂ mitigates the C₆H₆ formation in the CH₄ diffusion flame with greater effectiveness compared to DME. Contrary to the popular belief, the main reason behind such efficacy of H₂ has been found to be physical rather than chemical. On the other hand, the NO production routes are primarily dominated by the Zeldovich mechanism in the flames involving CH₄, DME and H₂ blends. In this regard, the present analysis suggests that the simultaneous presence of DME and H₂ in CH₄ effectively prevents the formation of NO in the flame.     

Pulse combustion: The mechanisms of NOx production

A study has been performed to elucidate the fundamental mechanisms controlling the production of NO in a Helmholtz-type pulse combustor. Cycle-resolved measurements of velocity and temperature in the combustion chamber were made. These measurements were combined with the Zeldovich NO formation mechanism to explain the mechanism responsible for the low NO formation found in pulse combustors.

The mechanism responsible for low NO formation found in pulsating flow as compared to nonpulsating flow was found to be a short residence time at high temperature. Three different possible mechanisms for this short residence time were investigated: (1) hot products from combustion are cooled by mixing with the cooler exhaust gases entering the combustion chamber from the tail pipe, thus quenching the NO formation reaction (“Automatic Exhaust Gas Recirculation”), (2) hot combustion products are quickly cooled by mixing with the incoming cold reactants, and (3) residual products with a lower overall temperature (due to an increased rate of heat transfer in the combustion chamber) readily mix with hot products producing a short residence time at high temperature. It is shown that the mechanism responsible for the low NO emission in pulse combustors is a result of item 3. A short residence time at high temperature is caused by rapid mixing with cooler residual gases that are lower in temperature due to increased rates of heat transfer in the combustion chamber.     

Chemiluminescence of BO2 to map the creation of thermal NO in flames

The aim of this study is to detect and map the local conditions that generate thermal NO in flames. According to the Zeldovich mechanism, the formation of NO comes from the local conjunction of a high concentration of atomic oxygen and a temperature above a critical high level imposed by the high activation energy of the rate-limiting reaction. The green light emitted when a flame is seeded with boron salts is a chemiluminescence from the BO^∗₂ that is chemically formed in its excited state when BO reacts with atomic oxygen. As the rate of this oxidation is also strongly increasing with temperature, the chemiluminescence of BO₂ depends on the concentration of atomic oxygen and on the temperature in a way similar to the formation rate of thermal NO. This double analogy suggests the possibility of an experimental in situ simulation of the formation rate of thermal NO or at least the use of the chemiluminescence of BO₂ to map the sites where thermal NO is being created. Spectroscopic experiments and comparisons with numerical simulations have been performed to test the feasibility of this technique in laminar premixed and diffusion methane/air flames. The agreement is good except in the burnt gases of fuel-rich flames. Imaging strategies with different spectral filters have been developed in the same flames to overcome the problem of interference from soot radiation in diffusion flames.     

Numerical investigation on NO formation in laminar counterflow methane/n-heptane dual fuel flames

To understand the fundamental mechanisms of NO formation in natural gas-diesel dual fuel combustion, a numerical study on NO formation in laminar counterflow methane (CH₄)/n-heptane (n-C₇H₁₆) dual fuel flames is conducted. The results reveal that the flame structure and NO formation vary with the fuel equivalence ratio. For a given n-C₇H₁₆/air mixture, the NO emission index decreases with increasing the equivalence ratio of the CH₄/air mixture (φ(CH₄/air)). The NO formation route analysis suggests that the prompt and thermal routes dominate the NO formation. The increase in φ(CH₄/air) causes the decrease in the contribution of the prompt route to overall NO formation. NO formation by prompt route is mainly caused by rich n-C₇H₁₆ combustion. As φ(CH₄/air) increases, the mole fractions of the radicals (OH, O and H) related to CH formation in the reaction zone of rich n-C₇H₁₆/air flame branch are decreased, which reduces the formation of NO by prompt route.     

NOx reduction and NO2 emission characteristics in rich-lean combustion of hydrogen

Hydrogen is a clean alternative to conventional hydrocarbon fuels, but it is very important to reduce the nitrogen oxides (NO_x) emissions generated by hydrogen combustion. The rich-lean combustion or staged combustion is known to reduce NO_x emissions from continuous combustion burners such as gas turbines and boilers, and NO_x reduction effects have been demonstrated for hydrocarbon fuels. The authors applied rich-lean combustion to a hydrogen gas turbine and showed its NO_x reduction effect in previous research. The present study focused on experimental measurements of NO and NO₂ emissions from a coaxial rich-lean burner fueled with hydrogen. The results were compared with diffusion combustion and methane rich-lean combustion. Significant reductions in NO and NO₂ were achieved with rich-lean combustion. The NO and NO₂ reduction effects by rich-lean combustion relative to conventional diffusion combustion were higher with hydrogen than with methane.     

Nitrogen transfer of fuel-N in chemical looping combustion

Chemical-looping combustion (CLC) is a novel technique used for CO₂ separation that has been investigated for gaseous fuel and solid fuel. The nitrogen transfer of fuel-N in the coal is experimentally investigated with a NiO/Al₂O₃ oxygen carrier under a continuous operation in a 1 kW_th interconnected fluidized bed prototype. The effects of the fuel reactor temperature, coal type and operation conditions on the release of gaseous products of nitrogen species in the air reactor and the fuel reactor are carried out. Results show that the nitrogen transfer direction of fuel-N is toward N₂ formation in the fuel reactor independent of fuel type. In the fuel reactor N₂ is the sole product of nitrogen transfer of fuel-N. The concentration of N₂ in the fuel reactor exit gas increases with the fuel reactor temperature. The NO_x precursor of HCN can be oxidized by the oxygen carrier to form NO or N₂ in the fuel reactor. However, in the fuel reactor NO from coal devolatilization and HCN oxidization by oxygen carrier is completely reduced to N₂. The other NO_x precursor of NH₃ is completely converted to N₂ due to oxidization by NiO and the catalytic effect of Ni on the decomposition of NH₃. After coal devolatilization, char-N conversion in the fuel reactor is toward N₂ formation according to the investigation of solid–solid reaction between char and oxygen carrier. The amount of residual char has a potential to cause formation of nitrogen contaminants in the air reactor. In the air reactor, NO is the only nitrogen contaminant, and there is no NO₂ formation. The high fuel reactor temperature results in little residual char coming into the air reactor. The proportion of char-N converted to NO in the air reactor increases from 16.98% to 18.85% when the fuel reactor temperature changes from 850 to 950 °C. For the fuels containing more volatile matter, the possibility of NO formation in the air reactor is smaller than the fuels containing less volatile matter. For the fuels containing less volatile matter, char gasification rate is still a significant factor both for the carbon capture efficiency and NO formation.     

煤粉燃烧过程中NOx生成的数值模拟

本文提出用有限反应速率２阶矩封闭模型来模拟湍流对煤燃烧过程中ＮＯｘ生成的影响。     

Ammonia chemistry below 1400 K under fuel-rich conditions in a flow reactor

The oxidation of NH₃ under fuel-rich conditions and moderate temperatures has been studied in terms of a chemical kinetic model over a wide range of conditions, based on the measurements of Hasegawa and Sato. Their experiments covered the fuels hydrogen (0 to 80 vol%), carbon monoxide (0 to 95 vol%), and methane (0 to 1.5 vol%), stoichiometries ranging from slightly lean to very fuel rich, temperatures from 300 to 1330 K, and NO levels from 0 to 2500 ppm. A detailed reaction mechanism has been established, based on earlier work on ammonia oxidation in flames and on selective noncatalytic reduction of NO by NH₃. The kinetic model reproduces the experimental trends qualitatively over the full range of conditions covered, and often the predictions are in quantitative agreement with the observations. Using reaction path analysis and sensitivity studies, the major reaction paths have been identified. The comparatively low temperatures in the present study, as well as the presence of NO, promote the reaction path NH₃→NH₂→N₂ (directly or via NNH), rather than the sequence NH₃→NH₂→NH→N important in flames. The major conversion of fuel-N species to N₂ occurs by reaction of amine radicals with NO, in particular NH₂+NO. In the presence of CH₄, NO is partly converted to cyanides by reaction with CH₃. The mechanism is recommended for modeling the reduction of NO by primary measures in the combustion of biomass, since it has been validated under conditions resembling the conversion of early nitrogenous volatile species in a staged combustion process. It is also appropriate for studies of NO formation in the combustion of gas from gasifying coal.     

Dilution effects analysis on NOx emissions of opposed-jet H2/CO syngas diffusion flames

Syngas is a promising alternative fuel for stationary power generation due to cleaner combustion than convectional fossil fuels. During the gasification processes, the by-products of CO₂, H₂O, or N₂ may be present in the syngas mixture to control the flame temperature and emissions. Several studies indicated that syngas with dilutions is capable of reducing pollutant emissions such as NO_x emissions. This work applied a numerical model of opposed-jet diffusion fames to explore the dilution effects on NO_x formation and differentiate the inert effect, thermal/diffusion effect, chemical effect, and radiation effect from CO₂, H₂O, or N₂ dilutions. The numerical study was performed by a revised OPPDIF program coupling with narrowband radiation model and detail chemical mechanism. The dilution effects on NO_x formation were analyzed by comparing the realistic and hypothetical cases. Regardless the diluent types, the inert effect is the main cause to reduce NO production, followed by chemical effect and radiation effect. The thermal/diffusion effect may promote NO formation because the preferential diffusion due to different diffusivities between diluents and syngas magnifies the reaction rate locally. CO₂ dilution reduces NO by radiation effect at low strain rate, and contributes NO reduction by chemical effect at high strain rate. At the same dilution percentage, CO₂ dilution reduces NO production the most, followed by H₂O and N₂. Besides the thermal/diffusion effect, the chemical effect of H₂O enhances NO production through thermal route and reburn route.     

Modeling of NOx formation in diesel engines using finite-rate chemical kinetics

A fast, physics-based model to predict the temporal evolution of NO_x in diesel engines is investigated using finite-rate chemical kinetics. The temporal variation of temperature required for the computation of the reaction rate constants is obtained from the solution of the energy equation. NO_x formation is modeled by using a six step mechanism with eight species instead of the traditional equilibrium calculations based on the Zeldovich mechanism. Fuel combustion chemistry is modeled by a single-step global reaction. Effects of various stages of combustion on NO_x formation is included using a phenomenological burning rate model. The effects of composition and temperature on the thermophysical properties of the working fluid are included in the computations. Comparison with measured single-cylinder engine-out NO shows good agreement with experimental data. The validated model is then used to demonstrate the impact of various operating parameters such as injection timing and EGR on engine-out NO_x. This fast, robust model has potential applications in model-based real-time control strategies seeking to reduce feed gas NO_x emissions from diesel engines.     

Combustion characteristics of a swirling inverse diffusion flame upon oxygen content variation

The combustion characteristics of a swirling inverse diffusion flame (IDF) upon variation of the oxygen content in the oxidizer were experimentally studied. The oxidizer jet was a mixture mainly composed of oxygen and nitrogen gases, with a volumetric oxygen fraction of 20%, 21% and 26%, and liquefied petroleum gas (LPG) was used as the fuel. Each set of experiment was conducted with constant oxygen content in the oxidizer. When the oxygen was varied, the changes in flame appearance, flame temperature, overall pollutant emission and heating behaviors of the swirling IDF were investigated. The swirling IDFs with different oxygen content in the oxidizer have similar flame structure involving a large-size and high-temperature internal recirculation zone (IRZ) which favors for thermal NO formation, and the thermal mechanism dominates the NO production for the swirling IDFs. The use of nitrogen-diluted air (with 20% oxygen) allowed the IDFs to operate at lower temperature with reduced NO_x formation, compared to the case of air/LPG combustion (with 21% oxygen). Meanwhile, an increase in CO emission is observed. With oxygen-enriched air (26% oxygen), the increase in temperature and EINO_x under lean conditions is more significant than under rich conditions. With 26% oxygen in the oxidizer stream, the IDF produces: (1) a shorter and narrowed navy-blue flame ring located closer to the burner exit, (2) highly luminous yellow flame extending into the central IRZ and above the blue flame ring, (3) a low CO emission, especially under lean conditions, (4) an increase in temperature at low Ф while a decrease in temperature at high Ф, and (5) an increase in EINO_x at all Ф. The heating test using the swirling IDFs in flame impingement heat transfer reveals that the heating rate can be monotonically increased as oxygen content in the oxidizer jet increases under the lean condition (Ф = 1.0). The oxygen enrichment does not contribute to the heating rate under the rich condition (Ф = 2.0), because for the non-premixed combustion of an IDF, the enrichment in oxygen means a lower oxidizer jet Reynolds number and thus less complete combustion occurs as a result of reduced amount of entrained ambient air.