NO emission characteristics in counterflow diffusion flame of blended fuel of H2/CO2/Ar

Flame structure and NO emission characteristics in counterflow diffusion flame of blended fuel of H₂/CO₂/Ar have been numerically simulated with detailed chemistry. The combination of H₂, CO₂ and Ar as fuel is selected to clearly display the contribution of hydrocarbon products to flame structure and NO emission characteristics due to the breakdown of CO₂. A radiative heat loss term is involved to correctly describe the flame dynamics especially at low strain rates. The detailed chemistry adopts the reaction mechanism of GRI 2.11, which consists of 49 species and 279 elementary reactions. All mechanisms including thermal, NO₂, N₂O and Fenimore are taken into account to separately evaluate the effects of CO₂ addition on NO emission characteristics. The increase of added CO₂ quantity causes flame temperature to fall since at high strain rates a diluent effect is prevailing and at low strain rates the breakdown of CO₂ produces relatively populous hydrocarbon products and thus the existence of hydrocarbon products inhibits chain branching. It is also found that the contribution of NO production by N₂O and NO₂ mechanisms are negligible and that thermal mechanism is concentrated on only the reaction zone. As strain rate and CO₂ quantity increase, NO production is remarkably augmented. Copyright © 2002 John Wiley & Sons, Ltd.     

Influences of different diluents on NO emission characteristics of syngas opposed-flow flame

This paper used the opposed-flow flame model and GRI 3.0 mechanism to investigate NO emission characteristics of H₂-rich and H₂-lean syngas under diffusion and premixed conditions, respectively, and analyzed influences of adding H₂O, CO₂ and N₂ on NO formation from the standpoint of thermodynamics and reaction kinetics. For diffusion flames, thermal route is the dominant pathway to produce NO, and adding N₂, H₂O and CO₂ shows a decreasing manner in lowering NO emission. The phenomenon above is more obvious for H₂-rich syngas because it has higher flame temperature. For premixed flames, adding CO₂ causes higher NO concentration than adding H₂O, because adding CO₂ produces more O radical, which promotes formation of NO through NNH + O = NH + NO, NH + O = NO + H and reversed N + NO = N₂ + O. And in burnout gas, thermal route is the dominant way for NO formation. Under this paper's conditions, adding N₂ increases the formation source of NO as well as decreases the flame temperature, and it reduces the NO formation as a whole. In addition, for H₂-lean syngas and H₂-rich syngas with CO₂ as the diluent, N + CO₂ = NO + CO plays as an important role in thermal route of NO formation.     

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.     

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.     

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.     

Evaluation of chemical effects of added CO2 according flame location

A numerical study has been conducted to clearly grasp the impact of chemical effects caused by added CO₂ and of flame location on flame structure and NO emission behaviour. Flame location affects the major source reaction of CO formation, CO₂+H→CO+OH and the H‐removal reaction, CH₄+H→CH₃+H₂. It is, as a result, seen that the reduction of maximum flame temperature due to chemical effects for fuel‐side dilution is mainly caused by the competition of the principal chain branching reaction with the reaction, CH₄+H→CH₃+H₂, while that for fuel‐side dilution is attributed to the competition of the principal chain branching reaction with the reaction, CO₂+H→CO+OH. The importance of the NNH mechanism for NO production, where the reaction pathway is NNH→NH→HNO, is recognized. In C‐related reactions most of NO is the direct outcome of (R171) and the contribution of (R171) becomes more and more important with increasing amount of added CO₂ as much as the reaction step (R171) competes with the key reaction of thermal mechanism, (R237), for N atom. This indicates a possibility that NO emission in hydrogen flames diluted with CO₂ shows less dependent behaviour upon flame temperature. Copyright © 2004 John Wiley & Sons, Ltd.     

Modeling of combustion characteristics and NOx emission in highly preheated and diluted air combustion

The gas diffusion combustion in a regenerative furnace with highly preheated and diluted air has been numerically investigated in this paper. The highly preheated air combustion possesses high combustion intensity and high level temperature, but the NO_x emission also has an unwanted high level. Decreasing the oxygen concentration in the highly preheated air could decrease the NO_x emission and improve the uniformity of temperature distribution in the furnace. The combustion characteristics of highly preheated and diluted air combustion have been studied, including temperature distribution, soot formation, OH radical distribution, as well as NO_x emission. The influence of the preheated air temperature, the oxygen concentration, and the air diluent has also been investigated. The optimal combinations of the preheated air temperature and the oxygen concentration have been predicted in the case of flue gas recirculation, which could provide the highest possible temperature in the furnace while keeping the NO_x emission lower than the permitted value. © 1998 by John Wiley & Sons, Ltd.     

Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging

Low NOx burner and air staged combustion are widely applied to control NOx emission in coal-fired power plants. The gas-solid two-phase flow, pulverized coal combustion and NOx emission characteristics of a single low NOx swirl burner in an existing coal-fired boiler was numerically simulated to analyze the mechanisms of flame stability and in-flame NOx reduction. And the detailed NOx formation and reduction model under fuel rich conditions was employed to optimize NOx emissions for the low NOx burner with air staged combustion of different burner stoichiometric ratios. The results show that the specially-designed swirl burner structures including the pulverized coal concentrator, flame stabilizing ring and baffle plate create an ignition region of high gas temperature, proper oxygen concentration and high pulverized coal concentration near the annular recirculation zone at the burner outlet for flame stability. At the same time, the annular recirculation zone is generated between the primary and secondary air jets to promote the rapid ignition and combustion of pulverized coal particles to consume oxygen, and then a reducing region is formed as fuel-rich environment to contribute to in-flame NO_X reduction. Moreover, the NOx concentration at the outlet of the combustion chamber is greatly reduced when the deep air staged combustion with the burner stoichiometric ratio of 0.75 is adopted, and the CO concentration at the outlet of the combustion chamber can be maintained simultaneously at a low level through the over-fired air injection of high velocity to enhance the mixing of the fresh air with the flue gas, which can provide the optimal solution for lower NOx emission in the existing coal-fired boilers.     

Flame structure and NO emissions in gas combustion of low calorific heating value

Numerical study on addition effects of CO and CO₂ in fuel side (H₂/Ar) on flame structure and NO emission behaviour in counterflow diffusion flame has been conducted with detailed chemistry to fundamentally understand gas combustion of low calorific heating value. A modified Miller–Bowman reaction scheme including a complementary C₂-reaction subset is adopted. The radiative heat loss term, which is based on an optically thin model and it especially important at low strain rates, is included to cover the importance of the temperature dependence on NO emission. Special interest is taken to estimate the roles of added CO and CO₂ in fuel side on flame structure and NO emission characteristics. Increasing CO concentration in fuel side contributes to the enhancement of combustion due to the increase effect of the concentration of reactive species. The increase of added CO₂ concentration in fuel side suppresses overall reaction rate due to the high heat capacity. It is seen that chemical effects due to the breakdown of added CO₂ in fuel side make C₂-branch chemical species be remarkably formed and the prevailing contribution of prompt NO is a direct outcome of these effects. It is found that in the combined forms of H₂/CO/CO₂/Ar fuels the effects of added CO and CO₂ concentrations in fuel side compete contrarily to each other in NO emission behaviour. Particularly the role of added CO is stressed in the side of restraining prompt NO. Copyright © 2003 John Wiley & Sons, Ltd.     

Experimental investigation of combustion characteristics and NOx emission of a turbocharged hydrogen internal combustion engine

In order to alleviate the contradictions of increasingly prominent environmental pollution, greenhouse gas emissions and oil resource security issues, the search for renewable and clean alternative energy sources is getting more and more attention. Hydrogen energy is known as a future energy source because of its safety, reliability, wide range of resources and non-polluting products. Hydrogen internal combustion engine combines the technical advantages of traditional internal combustion engines and has comprehensive comparative advantages in terms of manufacturing cost, fuel adaptability and reliability. It is one of the practical ways to realize hydrogen energy utilization. In this paper, the combustion characteristics and NOx emission of a turbocharged hydrogen engine were investigated using the test data. The results showed the combustion duration (the crank angle of 10%–90% fuel burned) at 1500 rpm and 2000 rpm was equal and the combustion duration is much bigger than the other loads when the BMEP is 0.27 MPa. The reason is the effect of the turbocharger on the gas exchange process, which will influence the combustion process. The cylinder pressure and pressure rise rate were also investigated and the peak pressure rise rate was lower than 0.25 MPa/°CA at all working conditions. Moreover, the NOx emission changed from 300 ppm to 1200 ppm with engine speed increasing and the maximum value can reach to 7000 ppm when the equivalence ratio is 0.88 at 2500 rpm, maximum brake torque. The NOx emission shows different changing tendencies with different working conditions. Finally, these conclusions can be used to develop controlling strategies to solve the contradictions among power, brake thermal efficiency and NOx emission for the turbocharged hydrogen internal combustion engines.     

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.     

Low NOx emission characteristics of refuse-recovered low BTU gases as fuel for high efficiency gas turbines

This paper discusses the problems of using refuse-recovered low Btu gases as fuel of high-temperature regenerative gas turbines. First, it is briefly described that the system using refuse-recovered fuels has a great possibility of being superior both in energy conservation and in solving refuse disposal problems in urban areas. Next, by taking a fermentation gas of the sewage sludge and a pyrolysis gas of the municipal refuse as examples, it is confirmed that a combustion gas with the high flame temperature required for a high efficiency gas turbine can sufficiently be obtained. Finally, through a case study in which no special condition is assumed, the system using these refuse-recovered low Btu gases whose flame temperatures are low is shown to be excellent in the thermal NO_x formation characteristics as compared to that using the conventional fuel with a high calorific value.     

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.     

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.     

Experimental study on velocity characteristics of recirculation zone in humid air non-premixed flame

To examine the effect of the flow field within the recirculation zone on flame structure, the characteristic velocity fields of methane/humid air flame in non-premixed combustion behind a disc bluff-body burner were experimentally studied by particle image velocimeter (PIV).The results show that two stagnation points exist on the centerline in the recirculation zone flame. However, the distance of the two stagnation points in humid air combustion shortens, and the minimal dimensionless velocity increases compared with the conventional nonhumid air combustion. In addition, the positional curves of the minimal velocities can be partitioned into three phases representing three different flame patterns. The analysis of axial minimal velocities on the centerline and their positions under different co-flow air velocity conditions reveals that fuel-to-air velocity ratio is the crucial parameter that governs humid air combustion flame characteristics. __________ Translated from Journal of Shanghai Jiaotong University, 2007, 41(3): 357–360, 365 [译自: 上海交通大学学报]     

The mechanisms of flame stabilization and low NOx emission in an eccentric jet pulverized coal combustor

The mechanisms of flame stabilization and low NO_x emission features of an eccentric jet pulverized coal combustor were studied through numerical modelling and experimental investigation. The results show that the formation of the unique flowfield structure is closely related to the interaction among combustor configuration, the primary jet and the control jet; and that certain rules should be followed in order to obtain the optimum condition for flame stabilization. The distributions of temperature and concentrations of NO, O₂, CO and CO₂ inside the combustor were experimentally measured. The effects of structural and operational parameters on combustion and NO formation were studied. It was found that reduction of primary air, suitable use of control jet and reasonable uptilt angle of the primary jet all contributed to the reduction of NO_x at the combustor exit. A new hypothesis, that reasonable separation of oxygen and fuel within the fuel-rich zone is beneficial to further reduction of NO_x emission, is given. The study showed that good compatibility existed between the capability of flame stabilization and low NO_x emission for this type of combustor. This project was supported by the National Natural Science Foundation of China     

Research of boiler combustion regulation for reducing NOx emission and its effect on boiler efficiency

The effect of boiler combustion regulation on NO_x emission of two 1025t/h boilers has been studied.The re-searches show that NOx emission is influenced by coal species,operation conditions,etc,and can be reduced byregulating the combustion conditions.The effect of combustion regulation on boiler efficiency has also beenchecked.     

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.     

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.