Dilution effect of air stream on NO emission characteristic in H2/Ar counterflow diffusion flame

The dilution effect of air stream according to agent type on flame structure and NO emission behaviour is numerically analysed with detailed chemistry. The adopted fuel is hydrogen diluted with the argon of volume percentage 50 per cent and the volume percentage of diluents (H₂O, CO₂ and N₂) in air stream is systematically changed from 10 to 50. The radiative heat loss term, based on an optically thin model, is included to clearly describe the flame structure and NO emission behaviour, especially at low strain rates. The effect of dilution of air stream on the decrease of maximum flame temperature varies as CO₂>H₂O>N₂. The qualitative tendency of the numerically predicted mole fractions of H, O and OH is well described using a simplified formula, based on a partial equilibrium concept. It is seen that the H₂O addition to air stream is the most effective for reducing NO emission. In the case of the addition of H₂O and N₂ the NO emission behaviour is governed by the thermal effect and in the case of CO₂ addition it is governed by both the thermal effect and the chemical effect. But the chemical effect, which is mainly attributed by the Fenimore mechanism to the breakdown of CO₂, is much more predominant in comparison with the thermal effect. Copyright © 2002 John Wiley & Sons, Ltd.     

Effects of CO2 addition on flame structure in counterflow diffusion flame of H2/CO2/N2 fuel

Numerical analysis on flame structure in a counterflow diffusion flame has been conducted for understanding the effects of CO₂ addition to fuel, systematically varying initial concentration of CO₂ and axial velocity gradient. The effects of CO₂ addition to fuel side in a counterflow diffusion flame are globally divided into two categories: diluent effects due to the relative reduction in the concentrations of the reactive species, and direct chemical effects caused by the breakdown of CO₂ through the reactions of third‐body collision and thermal dissociation. The deflection of CO₂ mole fraction profile with mixture fraction clarifies that the converted CO quantity from CO₂ is not negligible at low axial velocity gradients. It is also known that the addition of CO₂ does not alter the basic skeleton of the H₂–O₂ reaction mechanism, but contributes to the formation and destruction of hydrocarbon products such as HCO. At high axial velocity gradients the CO converted reaction is suppressed and then CO₂ plays the role of a diluent at these conditions. Copyright © 2001 John Wiley & Sons, Ltd.     

Effect of radiation emission and reabsorption on flame temperature and NO formation in H2/CO/air counterflow diffusion flames

The radiation effect on flame temperature and NO emission of H₂-lean (0.2H₂ + 0.8CO) and H₂-rich (0.8H₂ + 0.2CO) syngas/air counterflow diffusion flames was numerically investigated using OPPDIF code incorporated with the optical thin model, statistical narrow band model and adiabatic condition. Firstly, the coupled effect of strain rate and radiation was studied. Disparate tendencies of NO emission with an increasing strain rate between H₂-lean and H₂-rich syngas flames were found at very small strain rate, and the effect of radiation reabsorption on NO formation can be neglected when the strain rate was greater than 100 s^?1 for both H₂-lean and H₂-rich syngas flames. Because the radiation effect is vital to flames with small strain rate, its impact on flame temperature and NO emission was investigated in detail at a strain rate of 10 s^?1. The results indicated that NO formation is more sensitive to radiation reabsorption than flame temperature, especially for the H₂-rich syngas flame. The underlying mechanism was discovered by using reaction pathway analysis. Furthermore, the radiation effect under CO₂ dilution of the syngas fuel was examined. It was demonstrated that the radiation effect on flame temperature became more prominent with the increase of CO₂ concentration for both H₂-lean and H₂-rich syngas. The radiation effect on NO emission increased first and then decreased with an increasing CO₂ content for H₂-lean syngas, whereas for H₂-rich syngas the radiation effect is monotonic.     

Thermal and chemical contributions of added H2O and CO2 to major flame structures and NO emission characteristics in H2/N2 laminar diffusion flame

Numerical simulation with detailed chemistry has been carried out to clearly discriminate the thermal and chemical contributions of added diluents (H₂O and CO₂) to major flame structures and NO emission characteristics in H₂/N₂ counterflow diffusion flame. The pertinence of GRI, Miller–Bowman, and their recent modified mechanisms are estimated for the combined fuel of H₂, CO₂, and N₂. A virtual species X, which displaces the individual CO₂ and H₂O in the fuel sides, is introduced to separate chemical effects from thermal effects. In the case of H₂O addition the chain branching reaction, H + O₂ → O + OH is considerably augmented in comparison with that in the case of CO₂ addition. It is also seen that there exists a chemically super‐adiabatic effect in flame temperature due to the breakdown of H₂O. The reaction path of CH₂O→CH₂OH→CH₃ and the C1‐branch reactions become predominant due to the breakdown of CO₂. In NO emission behaviour super‐equilibrium effects caused by the surplus chain carrier radicals due to the breakdown of added H₂O are more superior to the enhanced effects of prompt NO with the breakdown of added CO₂. Especially, it is noted that thermal NO emission is directly influenced by the chemical super‐equilibrium effects of chain carrier radicals in the case of H₂O addition. As a result the overall NO emission in the case of the addition of H₂O is higher than that in the case of CO₂ addition. Copyright © 2002 John Wiley & Sons, Ltd.     

NOx emission from high-temperature air/methane counterflow diffusion flame

The objectives of the present study are to measure NO_x emission of counterflow diffusion flame, to compare the findings with numerical results, and finally to demonstrate efficacious effect of high-temperature air with low concentration of oxygen on NO_x emission. Recently, high-temperature air with low concentration of oxygen is used for various industrial furnaces, resulting high efficiency and low emission of pollutants. Since high-temperature air increases NO_x emission and air with low concentration of oxygen decreases it, these effects are competitive. Measurement and computation were conducted to clarify these two effects by use of counterflow diffusion flame. Since it is difficult to employ very high temperature over 1100 K in a laboratory-scale apparatus, a quantitative agreement between experimental and numerical results was confirmed first, and then a numerical approach was used to obtain a larger effect of low oxygen to reduce NO_x emission. In the experiments, the methane concentration is changed from 10 to 30 vol% diluted by nitrogen, oxygen from 10 to 21 vol%, and air temperature from room temperature to 1100 K. The total amount of NO_x sufficiently agreed between experimental and numerical results, although NO and NO₂ could not be separated. By the numerical method, it was found that NO_x emission from the counterflow diffusion flame of high-temperature low-oxygen air of 1500 K and 5% oxygen is comparable with that of room-temperature air of 21% oxygen.     

Computed extinction limits and flame structures of H2/O2 counterflow diffusion flames with CO2 dilution

A narrowband radiation model is coupled to the OPPDIF program, which uses detailed chemical kinetics and thermal and transport properties to enable the study of one-dimensional counterflow H₂/O₂ diffusion flames with CO₂ as dilution gas over the entire range of flammable strain rates. The effects of carbon dioxide dilution, ambient pressure and inlet temperature of opposed jets on the extinction limits and flame structures are compared and discussed. The extinction limits are presented using maximum flame temperature and strain rate as coordinates. Both high-stretch blowoff and the low-stretch quenching limits are computed. When the CO₂ dilution percentage is higher, the flame is thinner and flame temperature is lower. The combustible range of strain rates is decreased with increasing CO₂ percentage due to the effects of CO₂ dilution, which is categorized as dilute effect, chemical effect and radiation effect. In addition, the flame temperature of low-stretch diffusion flame with radiation loss is substantially lower than that computed with the non-radiation model. This large temperature drop results from the combined effect of flame radiation and chemical kinetics. The extinction limits and flame temperature are increasing with increasing atmospheric pressure and temperature, but the flame thickness is decreased with the pressure. At higher pressure and temperature, the extinction limits are extended more on the high-stretch blowoff limits, indicating the influence of the ambient pressure and temperature on the chemical reaction.     

Chemical effects of CO2 addition to oxidizer and fuel streams on flame structure in H2–O2 counterflow diffusion flames

Numerical simulation of CO₂ addition effects to fuel and oxidizer streams on flame structure has been conducted with detailed chemistry in H₂–O₂ diffusion flames of a counterflow configuration. An artificial species, which displaces added CO₂ in the fuel- and oxidizer-sides and has the same thermochemical, transport, and radiation properties to that of added CO₂, is introduced to extract pure chemical effects in flame structure. Chemical effects due to thermal dissociation of added CO₂ causes the reduction flame temperature in addition to some thermal effects. The reason why flame temperature due to chemical effects is larger in cases of CO₂ addition to oxidizer stream is well explained though a defined characteristic strain rate. The produced CO is responsible for the reaction, CO₂+H=CO+OH and takes its origin from chemical effects due to thermal dissociation. It is also found that the behavior of produced CO mole fraction is closely related to added CO₂ mole fraction, maximum H mole fraction and its position, and maximum flame temperature and its position. Copyright © 2003 John Wiley & Sons, Ltd.     

Chemical effect of diluents on flame structure and NO emission characteristic in methane‐air counterflow diffusion flame

The dilution effect of air stream according to agent type on flame structure and NO emission behaviour is numerically simulated with detailed chemistry in CH₄/air counterflow diffusion flame. The volume percentage of diluents (H₂O, CO₂, and N₂) in air stream is systematically changed from 0 to 10. The radiative heat loss term, based on an optically thin model, is included to clearly describe the flame structure and NO emission behaviour especially at low strain rates. The effect of dilution of air stream on the decrease of maximum flame temperature varies as CO₂>H₂O>N₂, even if heat capacity of H₂O is the highest. It is also found that the addition of CO₂ shows the tendency towards the reduction of flame temperature in both the thermal and chemical sides, while the addition of H₂O enhances the reaction chemically and restrains it thermally due to a super‐equilibrium effect of the chain carrier radicals caused by the breakdown of H₂O in high‐temperature region. The comparison of the nitrogen chemical reaction pathway between the cases of the addition of CO₂ and H₂O clearly displays that the addition of CO₂ is much more effective to reduce NO emission. Copyright © 2002 John Wiley & Sons, Ltd.     

Effects of Lewis number and preferential diffusion on flame characteristics in 80%H2/20%CO syngas counterflow diffusion flames diluted with He and Ar

Numerical study is conducted to grasp flame characteristics in H₂/CO syngas counterflow diffusion flames diluted with He and Ar. An effective fuel Lewis number, applicable to premixed burning regime and even to moderately stretched diffusion flames, is suggested through the comparison among fuel Lewis number, effective Lewis number, and effective fuel Lewis number. Flame characteristics with and without the suppression of the diffusivities of H, H₂, and He are compared in order to clarify the important role of preferential diffusion effects through them. It is found that the scarcity of H and He in reaction zone increases flame temperature whereas that of H₂ deteriorates flame temperature. Impact of preferential diffusion of H, H₂, and He in flame characteristics is also addressed to reaction pathways for the purpose of displaying chemical effects.     

Numerical study on flame structure and NO formation in CH4–O2–N2 counterflow diffusion flame diluted with H2O

Numerical study on flame structure and NO emission behaviour has been conducted to grasp chemical effects of added H₂O on either fuel‐ or oxidizer‐side in CH₄–O₂–N₂ counterflow diffusion flames. An artificial species, which has the same thermodynamic, transport, and radiation properties of added H₂O, is introduced to feasibly isolate the chemical effects. Special concern is focused on the important role of remarkably produced OH radicals due to chemical effects of added H₂O on flame structure and NO emission. The reason why the difference of behaviours between the principal chain branching reaction rate and flame temperature appear is attributed to the drastic change of reaction step (R120) from the production to the consumption of OH. It is also, however, seen that the most important contribution of produced OH due to chemical effects of added H₂O is through reaction step (R127). The importantly contributing reaction steps to NO production are also examined. The production rates of thermal NO and prompt NO are suppressed by chemical effects of added H₂O. The contribution of the reaction steps related to HNO intermediate species to the production of prompt NO is also stressed. Copyright © 2004 John Wiley & Sons, Ltd.     

Effects of CO2 and N2 dilution on the characteristics and NOX emission of H2/CH4/CO/air partially premixed flame

For the combustion of the mixture of blast furnace gas, natural gas, and coke oven gas in industrial burners, how to improve combustion efficiency and reduce pollutant emission are of significance. To accomplish this, an industrial partially premixed burner with a combustion diagnostic system is used to experimentally reveal the characteristics and NO_X emission of H₂/CH₄/CO/air flame under CO₂, N₂, and CO₂/N₂ (replacing half of N₂ with CO₂) dilution. NO_X emission and flame length, temperature profile, along with CO, CH₄, and CO₂ concentration profiles are analyzed with the three diluents in the fuel stream under different dilution rates (0–32% by volume). Experimental results show that for lean H₂/CH₄/CO combustion, greater proportions of CO₂ in the diluent affect flame characteristics in various ways. These effects include longer flame length, lower highest flame temperature, the highest flame temperature being located farther away from the nozzle, and the highest CO₂ concentration being located nearer the nozzle. Furthermore, results of CO, CH₄, and CO₂ concentrations indicate that chemical reactions in the flame are significantly affected by CO₂ owing to the series reaction CH₄?CH₃→CO?CO₂. Finally, increasing diluents or the ratio of CO₂ in diluents has the benefit of reducing NO_X emission.     

Edge flame instability in low-strain-rate counterflow diffusion flames

Experiments in low-strain-rate methane-air counterflow diffusion flames diluted with nitrogen have been conducted to study flame extinction behavior and edge flame oscillation in which flame length is less than the burner diameter and thus lateral conductive heat loss, in addition to radiative loss, could be high at low global strain rates. The critical mole fraction at flame extinction is examined in terms of velocity ratio and global strain rate. Onset conditions of the edge flame oscillation and the relevant modes are also provided with global strain rate and nitrogen mole fraction in the fuel stream or in terms of fuel Lewis number. It is observed that flame length is intimately relevant to lateral heat loss, and this affects flame extinction and edge flame oscillation considerably. Lateral heat loss causes flame oscillation even at fuel Lewis number less than unity. Edge flame oscillations, which result from the advancing and retreating edge flame motion of the outer flame edge of low-strain-rate flames, are categorized into three modes: a growing, a decaying, and a harmonic-oscillation mode. A flame stability map based on the flame oscillation modes is also provided for low-strain-rate flames. The important contribution of lateral heat loss even to edge flame oscillation is clarified finally.     

Influence of fuel composition on chemiluminescence emission in premixed flames of CH4/CO2/H2/CO blends

The growing concern about pollutant emissions and depletion of fossil fuels has been a strong motivator for the development of cleaner and more efficient combustion strategies, such as the gasification of coal, biomass or waste, which have increased the interest in using a new type of fuels, mainly composed of CH₄, H₂, CO and CO₂.These new fuels, commonly called syngas, display a wide range of compositions, which affects their combustion characteristics and, in some cases, are more prone to instabilities or flashback. Since flame properties have been demonstrated to be strongly related to equivalence ratio, a precise measurement of the flame stoichiometry is a key pre-requisite for combustion optimization and prevention of unstable regimes. In particular, chemiluminescence emission from flames has been largely tested for stoichiometry monitoring for methane flames, but its use in syngas flames has been far less studied. Consequently, the main goal of this work is analyzing the effect of fuel composition on the chemiluminescence vs. equivalence ratio curves for different fuel blends, as a first approach for a wide range of syngas compositions. The experimental results revealed that the ratio OH*/CH*, which had been widely demonstrated to be the best option for methane, may not be suitable for monitoring with certain fuels, such as those with a high percent of hydrogen. Alternatively, other signals, in particular the ratio OH*/CO₂*, appear as viable stoichiometry indicators in those cases.The analysis was also completed by numerical predictions with CHEMKIN. The comparisons of calculations with different flame models and experimental data reveals differences in the chemiluminescence vs. equivalence ratio curves for the different combustion regimes, depending on the range of the equivalence ratio ranges and fuel compositions. This finding, which confirms previous observations for a much narrower range of fuels, could have important practical consequences for the application of the technique in real combustors.     

Preferential diffusion effects on flame characteristics in H2/CO syngas diffusion flames diluted with CO2

Numerical study is conducted to clarify preferential diffusion effects of H₂ and H on flame characteristics in synthetic diffusion flames of the compositions of 80% H₂/20% CO and 20% H₂/80% CO as representatively H₂-enriched and CO-enriched H₂/CO flames. Impacts of CO₂ addition to the flames are also examined through the variation of added CO₂ mole fraction from 0 to 0.5. A comparison was made by employing a mixture-averaged diffusivity and the suppression of the diffusivities of H and H₂. It is found that preferential diffusion effects on maximum flame temperature cannot be explained by the well-known behavior between maximum flame temperature and scalar dissipation rate but by chemical processes. The concrete evidence is also presented through the examination of the behavior of maximum H mole fraction and the behavior of importantly-contributing reaction steps to overall heat release rate.     

Determination of particle temperatures in a silica-generating counterflow flame via flame emission measurements

We propose a simple technique to measure particle temperatures in a particle generating counterflow flame. The silica particle temperature was derived from flame light emission measurements. This technique allows the non-intrusive measurement of particle temperatures over 2000 K. In addition, the OH concentration distribution in the hydrogen–oxygen flame was estimated from flame emission spectra in the ultraviolet region. A numerical model of the combustion processes, which included the reactions of SiCl₄ leading to the formation of silica particles, verified that the measured particle temperatures and OH concentration were close to the theoretical values.     

The effects of N2 and steam dilution on NO emission for a H2/Air micromix flame

The different effects of N₂ and water steam dilution on the NO emission from a micromix H₂/air flame were experimentally and numerically studied. NO emission was measured using Fourier transform infrared spectroscopy gas analyzer (FTIR). Numerical simulation of the H₂/air flame was carried out with realizable k-ε model and the Eddy Dissipation Concept (EDC) model. Two pseudo species were introduced in the simulations to numerically isolate the thermal-dilution, direct chemical and third-body effect of the steam. The experimental and numerical results show that NO emission with steam dilution is about 20%～50% of the emission with N₂ dilution at the same equivalence ratio (φ) and dilution ratio (D). The present simulation results show that the thermal-dilution and direct chemical effects of steam account for 50.8%～92.1% and 5.8%～15.8% of the total NO emission, respectively, while the third-body effect of steam shows less impact on NO emission. Specifically, the third-body effect slightly promotes the formation of H and NO at lower φ (0.6–0.8) and suppresses the formation of H and NO at higher φ (0.9–1.0) through inhibiting the third-body reaction of H + O₂ + M ? HO₂ + M. In addition, the direct chemical and third-body effects of steam always decrease the formation of NO from the NNH route.     

The evolution of a soot aerosol in a counterflow diffusion flame

The dynamics of a soot aerosol in a stagnation point diffusion flame have been simulated numerically. The impact of the flow velocity gradient on soot loading which was observed experimentally was found to arise simply from variations in aerosol residence times; the computed effect was consistent with constant particle formation and growth rates. The aerosol size distribution adopted a self-preserving form following the particle inception stage in spite of intense gas to solid conversion by surface growth; the inclusion of a retarded van der Waals interaction did not give rise to significant differences in the moments of the size distribution.     

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.     

Thermal and emission characteristics of a turbulent swirling inverse diffusion flame

The thermal and emission characteristics of a swirl-stabilized turbulent inverse diffusion flame (IDF) burning liquefied petroleum gas (LPG) were studied experimentally and the results of visible flame lengths, flame temperatures, in-flame gaseous species concentrations and global pollutant emissions were reported.The flame shape and length of the swirling IDF and a non-swirling IDF were compared. The swirling IDF is featured by a large internal recirculation zone (IRZ), which plays an important role in stabilizing and shortening the flame. Compared with the non-swirling IDF, the swirling IDF is shorter, wider and more stable. For the swirling IDF, both temperature and species distributions are uniform in the IRZ. Comparison of the radial NO_x/temperature distributions indicates that the thermal NO mechanism plays a leading role in NO_x formation, since the high-temperature IRZ favors thermal NO production. The effects of air jet Reynolds number (Re) and overall equivalence ratio (Ф) on centerline temperature and emission index were examined. The main finding is that the IRZ which is large in size and high in temperature dominates the thermal and emission characteristics of the swirling flame.Efforts were made to compare the global NO_x and CO emissions of the swirling and non-swirling IDFs. It was found that strong swirl and lean combustion are two key factors for reducing NO_x emission. However, the decreasing NO_x emission is compromised by increasing CO. Under stoichiometric and rich conditions, EINO_x of the swirling IDFs is slightly higher, but the EICO is significantly lower. Further comparison of EINO_x with other studies indicates that the swirling IDF can achieve low NO_x emission.