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.     

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.     

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.     

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 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.     

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.     

Comparative study of flame structures and NOx emission characteristics in fuel injection recirculation and fuel gas recirculation combustion system

A numerical study with momentum‐balanced boundary conditions has been conducted to grasp the chemical effects of added CO₂ to fuel‐ and oxidizer‐sides on flame structure and NO emission behaviour in H₂–O₂ diffusion flames with varying flame location. A reaction mechanism is proposed to show better agreements with experimental results in CO₂‐added hydrogen flames. Oxidizer‐side dilution results in significantly higher flame temperatures and NO emission. Flame location is dramatically changed due to high diffusivity of hydrogen according to variation of the composition of fuel‐ and oxidizer‐sides. This affects flame structure and NO emission considerably especially the chemical effects of added CO₂. The present work also displays separately thermal contribution and prompt NO emission due to the chemical effects caused by thermal dissociation of added CO₂ in NO emission behaviour. It is found that flame temperature and the flame location affect the contribution of thermal and prompt NO due to chemical effects considerably in NO emission behaviour. Copyright © 2004 John Wiley & Sons, Ltd.     

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.     

Effect of steam addition on flame structure and NO formation in H2–O2–N2 diffusion flame

Numerical analysis is conducted to clarify chemical effects of added steam to either fuel‐ or oxidizer‐side on flame structure and NO emission behaviour with detailed chemistry in hydrogen–oxygen–nitrogen diffusion flames. An artificial species, which has the same thermodynamic, transport, and radiation properties to added H₂O, is introduced to feasibly isolate chemical effects of added H₂O. It is found that the reaction step (‐R23) is the starting point to induce chemical effects of added steam. Special concern is, thus, focused on the impact of OH radical on flame structure and NO emission behaviour. A strong dependency of the amount of steam addition on OH radical behaviour is clearly displayed, and this modifies flame structure sufficiently to produce higher flame temperature at more than a certain mole fraction of added steam in comparison to that diluted with artificial species. It is also shown that the reaction step (‐R23) is closely related to flame temperature and thereby the location of maximum flame temperature. The behaviour of NO emission index is shown to be greatly influenced by the competition between the reaction steps of (R63) and (R65) in addition to Zeldovich NO. It is, consequently, seen that the intermediate active species, HNO, affects NO emission behaviour remarkably. These results may be helpful to understand the role of recirculated steam in the combustion systems with flue gas recirculation to either fuel‐ or oxidizer‐side. Copyright © 2004 John Wiley & Sons, Ltd.     

Numerical study on effect of CO2 addition in flame structure and NOx formation of CH4‐air counterflow diffusion flames

Numerical study with detailed chemistry has been conducted to investigate the effect of CO₂ addition on flame structure and NO_x formation in CH₄–air counterflow diffusion flame. Radiation effect is found to be dominant especially at low strain rates. The addition of CO₂ makes radiation effect more remarkable even at high‐strain rates. It is, as a result, seen that flame structure is determined by the competition between the radiation and strain rate effects. The important role of CO₂ addition is addressed to thermal and chemical reaction effects, which can be precisely specified through the introduction of an imaginary species. Thermal effect contributes to the changes in flame structure and NO formation mainly, but the effect of chemical reaction cannot be neglected. It is noted that flame structure is changed considerably due to the addition of CO₂, in such a manner, that the path of methane oxidation prefers to take CH₄→CH₃→C₂H₆→C₂H₅ instead of CH₄→CH₃→CH₂→CH. Copyright © 2001 John Wiley & Sons, Ltd.     

Hydrogen utilization as a fuel: hydrogen-blending effects in flame structure and NO emission behaviour of CH4–air flame

Hydrogen-blending effects in flame structure and NO emission behaviour are numerically studied with detailed chemistry in methane–air counterflow diffusion flames. The composition of fuel is systematically changed from pure methane to the blending fuel of methane–hydrogen through H₂ molar addition up to 30%. Flame structure, which can be described representatively as a fuel consumption layer and a H₂–CO consumption layer, is shown to be changed considerably in hydrogen-blending methane flames, compared to pure methane flames. The differences are displayed through maximum flame temperature, the overlap of fuel and oxygen, and the behaviours of the production rates of major species. Hydrogen-blending into hydrocarbon fuel can be a promising technology to reduce both the CO and CO₂ emissions supposing that NO_x emission should be reduced through some technologies in industrial burners. These drastic changes of flame structure affect NO emission behaviour considerably. The changes of thermal NO and prompt NO are also provided according to hydrogen-blending. Importantly contributing reaction steps to prompt NO are addressed in pure methane and hydrogen-blending methane flames. Copyright © 2006 John Wiley & Sons, Ltd.     

Numerical study on NO formation in CH4–O2–N2 diffusion flame diluted with CO2

Numerical study with momentum‐balanced boundary conditions has been conducted to grasp chemical effects of added CO₂, to either fuel‐ or oxidizer‐side on flame structure and NO emission behaviour in CH₄–O₂–N₂ diffusion flames. Cautious investigation is made for the comparison among the behaviours of principal chain branching and important H‐removal key reactions. This describes successfully the reason why flame temperatures for fuel‐side dilution are higher than those for oxidizer‐side dilution. The role of the principal chain branching reaction is also recognized to be important even in the change of major flame structure caused by chemical effects. The importantly contributing reaction steps to NO production are examined. The reduced production rates of thermal NO and prompt NO due to chemical effects are much more remarkable for fuel‐side dilution. It is also found that the reaction step, H+NO+M=HNO+M plays a decisive role of the formation and destruction of prompt NO. Copyright © 2005 John Wiley & Sons, Ltd.     

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.     

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.     

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.     

NO emission from non-premixed MILD combustion of biogas-syngas mixtures in opposed jet configuration

In this paper NO emission from MILD combustion of the mixture biogas-syngas is deeply elucidated, five NO routes were considered, specifically: thermal, prompt, NNH, N₂O and reburning. Several operating conditions are studied namely: fuel mixture composition, oxygen concentration in the oxidizer and injection velocity or strain rate. Biogas is modeled by a mixture of methane and carbon dioxide; while, syngas is considered to be composed by hydrogen and carbon monoxide, this gives a fuel mixture of CH₄/CO₂/H₂/CO. Volume of methane and hydrogen are varied alternatively from 0 to 50% in fuel mixture. Oxidizer is composed by O₂/N₂ mixture where oxygen volume is increased from 4 to 21%. Finally, injection strain rate is varied from apparition to vanishment of combustion. Atmospheric pressure is considered with constant fuel and oxidizer injection temperatures of 300 K and 1200 K respectively. Chemical kinetics of such complicated system is handled by a composed mechanism from the USC C₁–C₄ and the Gri 2.11 N-sub mechanism. It is found that under MILD regime, temperature intervals and levels are enhanced by hydrogen compared to methane. Furthermore, temperature levels keep relatively low which guarantees MILD regime. Contrariwise, when oxygen increases in oxidizer, temperature grows up rapidly and the MILD regime disappears. However, if strain rate augments, temperature shows a steep increase then reduces monotonically. It is observed that for low methane volume in the fuel mixture, NNH route dominates NO production. Whereas, when CH₄ increases, the prompt route is enhanced and exceeds NNH one at a methane volume of 12%. When hydrogen increases, prompt and NNH routes are enhanced with a domination of the prompt route until 44% of hydrogen volume. Oxygen increasing in the oxidizer improves thermal mechanism which surpasses prompt one at 17% of oxygen volume and governs NO production. Globally, the third most important route in NO production is the reburning one which is enhanced by all parameters except strain rate.     

HCl emission and capture characteristics during PVC and food waste combustion in CO2/O2 atmosphere

The emission and capture characteristics of HCl during PVC and food waste combustion in CO₂/O₂ atmospheres were studied. Replacement of N₂ by CO₂ decreased the dechlorination rate of limestone at 600–700 °C but increased dechlorination rate at 800–900 °C. The chlorine species and temperature highly influenced the HCl emission and capture efficiency of limestone for HCl in CO₂/O₂ atmospheres. Compared with inorganic chloride in food waste, organic chlorine in PVC had much greater Cl–HCl conversion percent (75.0–93.9%), and higher dechlorination rate (20.4–44.9%) with 10% limestone in 80CO₂/20O₂ atmosphere. The increment of O₂ partial pressure in CO₂/O₂ atmospheres promoted Cl–HCl conversion. Sulphur in the fuel suppressed the formation of HCl but decreased the dechlorination rate at 700–1000 °C in CO₂/O₂ atmospheres. The dechlorination efficiency of limestone was better than magnesium based additive and could be improved by modification with NaOH. This research helps control HCl and manages MSW oxy-fuel incineration.     

Numerical study on flame structure in H2–O2/CO2 laminar flames

Numerical study, aimed at the understanding of the flame structure in O₂/CO₂ recycling combustion system, has been conducted with detailed chemistry. Special concern is focused on addition effect of carbon dioxide on flame structure in H₂–O₂ counterflow diffusion flame as a simulating configuration. To clarify chemical and thermal effects on flame structure, the comparison between predicted results with a virtual species X to displace the real carbon dioxide and with added carbon dioxide in oxidizer stream is made according to strain rate and the concentration of added CO₂. From the systematical comparison of a dominant radical producing reaction with a chain termination reaction the effects of strain rate and composition control of oxidizer stream on flame structure are estimated. It is found that the behaviours of C₁‐ and C₂‐branch species are a direct outcome of that of produced CO due to the breakdown of added CO₂. There exists a temperature dependency in the behaviour of produced CO and this competes for the behaviour of the produced CO with chemical effects due to the backward reaction of CO+OH=CO₂+H. Copyright © 2003 John Wiley & Sons, Ltd.